EQGLOGY MD POPULATION DYNAMECS OF PAMPA r

RODENTS NEAR BAtCARCE, ARGENTINA

Dissertation for the Degree of Ph. D.
WOMAN STATE UNIVERSITY
PETER LENN DALBY
1974

 

 

 

A\4

llflllllllllwflllj

lfllljfllmflflllcllllllvfllfll ““

Michigan State
University

 

Qt _'
. «n...

This is to certify that the
thesis entitled
ECOLOGY ANO POPULATION DYNAMICS OF PAMPA RODENTS NEAR

BALCARCE , ARGENTINA

presented by

 

PETER LENN DA LBY

has been accepted towards fulfillment
of the requirements for

Ph . D. degree in Zoo | 0qu

Major professor

 

0-7 639

 

 

populationc yha nics of ThIee abundanT C

ECOLCCY ARC POP' iIlU“ “’“‘.IC 3? PAHPA IVH‘NTS NEAR
[12C ‘RCE. ‘«{.vil A

d \,

PeTer Lehn Lalby

A sevenTeenumonTh Tield sTudy of Tle compw uraiive ecology and

.J

rass land mycmorphs was conducted

fit

an several one~hocTare siTes in The viciniTv of Balcarce, Province of
Buanos Aires, ArgenTina. Thes o Thr especies, namely, The herbivorous

flkcdcn 9"“3’ The granivorous Cryzonvs nigripos, and The insecTivorous

.- ....——-,-..._ . - . V- -’....._..._ ___L

' 'wycTerIs ruTilans were conpared with Their "ecologicals o‘uiva lerI. s'

I

2 - s.-,'+'.. . r-‘,.., .,.l. , e- - ,. ' 0.‘ - -.°° .‘I. ,,r° . ,I -L ‘J-
Inha;I.Inp temporaae g! sslands Ir NOIrn ”mollca. IT was founc lhuli

’5

n
b C

Pcouiafions of A. a;:arae durino periods of hi h ans. T cxrr‘jrd LOO
r H I

animals/hecTare, comparable in densiTy and raTe of increase T3 ThaT

found in NorTh ‘mt“ rIcan gras s—eaTeIs.. By laTe winTer The densiTv was
reeuced To approx unaTely 50 animals/hecTare. AlThough spora Tdic

populaTion irrupTions like Those of NorTh American Siqmodc n and

possibly 0 y'o-ys occur ,cyciic pepulaTion fluctuations like Those

a.-- v-y

of microTin s have noT be:2n roporTed. Typical of granivores, O.

OlPFIU’u reached 50 animals/hecTare The firsT summer and fall when
The seed crop was abundanT. By The following winTer and early summer,
The sensity we 5 so reduced IhaT several Three—day Trapping periods

would pass before There were furTher captures. OxymycTeris ruTilans

 

had The lowest buT wosT srable peauiaTions, wiTh densiTies flucTuaTing
hefween 3'lb mice/hectar e.

_L' ”- I

r.. -L-tm .' .---\.\ .'.,..< . .. ' .- .. ...-
sex {daluJ or A. diulun Quagfiolrn ~hgl, like In many oTher grtss~

(4’

a.

m
‘0

Peter L. Dolby

eaters, females make up a higher proportion of the total catch than
males during the breeding (summer) season than in the non-breeding
seaSOT. This was particularly true in resident versus recruit animals.

erzomys nigripes consistently had a much smaller proportion of

 

females, a feature common To the social organization of other seed-
eaters. The proportion of female 9. :uTilans was higher, perhaps a

function of its continual reproductive activity. Ratios of_A. azarae

 

coming into one of the study plots indicated that adult females dis-
persed less often than males during The breeding season, and about
equally during The winter (non-breeding) season.

Reproduction of A. azarae, unlike other grass-eaters, from similar
latitudes, was strongly seasonal, with litters born from the months
of November to April. Litter size averaged 4.6, and gestation length,
22.7 days. Delayed implantation may occur. Young were successfully
weaned at l4-l5 days of age. The growth rate of the young did not
attain the level found in the North American grass-eating microtines.
Sexual maturity occurred at two months of age. Young born late in
the year did not become sexually mature until the following breeding

season. Oryzomys nigripos were noted To breed from January to May.

 

Litter size averaged 3.6, but gestation period was not established.
Young were successfully weaned at l4-l5 days of age. Sexual maturity

may not be reached until the following reproductive season. Oxymycteris

 

rutil§n§_bred in all seasons of The year. Litter size averaged 3.l,
but gestation period was not established. Young Iere successfully
weaned at l4 days of age. Sexual maturity occurred near three months
of age. Although A, gzarge_is a seasonal breeder, the species realizes

a much higher reproductive potential than the seed—eating Q, “l“rlPfifi

PeTer L. Dalby

or The insecT-eaTing Q, ruTilans. This agrees well wlTh our knowledge
of myomorph grass-, seed—, and inverTebraTe—eaTers in NorTh America.
The minimum survival raTes for A, aggra§_averaged sligthy higher
during The winTer (80-83 percenT) Than The summer (59~79.5 percenT).

ATTriTion during The winTer season indicaTed ThaT The survival raTe

was closer To 93-95 percenT per fourTeen days. Oryzomys nigrlpes had

 

minimum survival raTes near 60 percenT for The Time when They were

mosT abundanT. 9. ruTilans was calculaTed To have a survival raTe of
70-76 percenT during The summers, buT in The winTers iT was closer To

92 percenT. Dispersal, especially in summer, seemed To be a prime facTor
in lowering The minimum survival raTes. PredaTion appeared To be insig-
nificanT. These findings make iT diniculT To esTablish wheTher A.

azarae, Q. nigripes, and O. ruTilans have survival raTes comparable To

Their NorTh American ecological equivalenTs.

« The spaTial disTribuTion (cenTer of acTiviTy) of The mice on The sTudy

ploTs was influenced by vegeTaTive cover. A shifT in disTribuTion for
.A. azarae and Q, nigripes was noTed in one ploT when increasing

numbers of guinea pigs (Cavia aperea) caused a deTerioraTion in

 

habiTaT qualiTy. Orygomys nigripes was capTured raTher homogeneously,

 

buT by The heighT of The habiTaT deTerioraTion, had disappeared. On
anoTher ploT where The vegeTaTion consisTed of a greaTer number of
dominanT grasses, A, azarae_and 9, ruTilans were generally capTured
in The mosT heTerogeneous habiTaT, whereas 9: gigrip§§_was found in
The porTion where vegeTaTive cover was mosT dense and highesT.
MovemenTs (radii of acTiviTy) of A, 953593.Typified graminivorous
rodenTs, being The leasT of The Three species. WinTer and male WOVL“

menTs were only sligthy longer Than summer and female movemenTs,

PeTer L. Dalby

respecTively. Female movemenTs during a summer of hlgh reproducTive
inTensiTy were greale resTrlcTed when compared To a summer when
breeding was less inTense. The vegeTaTive desTrucTion by Qayig seemed
To be a facTor favoring The increase in The radius of acTiviTy of A,
azarae as The sTudy progressed. LimlTed daTa for Q, nlgripes and 9,
ruTilans showed only ThaT Their movemenTs were 2-4 Times more
exTensive Than ThaT for A. azarae and in general agreemenT wiTh Their
NorTh American counTerparTs. Dispersal informaTion for A. azarae
lndicaTed ThaT nearly 75 percenT of The mice moved IOO m or less

from Their previous cenTer of acTiviTy. This species homed readily
considering The shorT disTances TesTed (less Than 200 m).

The diel acTiviTy paTTerns of A, azarae and Q, nigripes, buT noT The
insecTivorous Q, ruTilans, agree wiTh whaT is known of ecologically~
similar Species in NorTh America. Specifically, The grass—eaTing

A, azarae was acTive aT nearly all Times, wiTh peaks during The
crepuscular hours. The seed-eaTing 9, nigripes was acTive-exclusively
aT nighT. Unlike The nocTurnal insecTivorous NorTh American grass-

hopper mice (Onychomys) which inhabiT arid grasslands or The mesic-

lnhabiTing shrew Blarina brevicauda, 9, ruTilans appeared To be

 

exclusively diurnal. Energy demands and The Type of food (grasses,
grains, or insecTs) play a major role in dicTaTlng The acTiviTy
paTTern for each species. AlThough A, azarae is The primary myomorph
herbivore, iT appears To consume more lnverTebraTes Than NorTh
American grass-eaTers. High meTabolic raTes for A, azarae are
comparable To Those of microTlnes. Burrowing and runway-making
behavlors were less developed in A, aggrae_Than The microTines.

The above ieaTures lndlcaTe ThaT The grass-eaTlng A. azarae resembles

op-“-

PeTer L. Dalby

The NorTh American grass—eaTing myomorphs in many respecTs. Oryzomys
nieribes, in as much as The daTa will allow, Typifies a long-Tailed
scansorial seed-eaTer, such as NorTh American grassland forms of

Peromyscus or ReiThrodonTomys. Only The insecTivorous O. ruTilans is

 

 

a form noT Truly represenTed in NorTh America and appears To have an
ecological posiTion overlapping Those of boTh grasshOpper mice, Onychomys,
and shrews such as Blarina. The number of grass-eaTing and seed-eaTing
myomorph Species is comparable To ThaT found aT similar laTiTudes in

Tall grass areas of NorTh America. A ToTal of Two myomorph grass-eaTers

(one uncommon), five seed—eaTers (includes Mus musculus) and'one insecT—

 

eaTer inhabiTed The sTudy siTes. The number of principal myomorph grass-
eaTing species (one) To seed-eaTers (four To five) agrees wiTh The
hypoThesis suggesTing ThaT seed—eaTers divide Their consumer niche more
efficienle Than The grass-eaTers. The seed~eaTers of NorTh American
grasslands, unlike Those found in This sTudy, are augmenTed sligthy in
species numbers by The presence of The zapodids (jumping mice) and
heTeromyids (kangaroo raTs, pockeT mice). The insecTivore niche may

also allow several species To inhabiT one area. OxymycTeris ruTilans,

 

and The marsupials, Monodelphis dimidiaTa and possibly LuTreolina

 

 

crassicaudaTa, appear To fill The insecTivore niche normally filled by

 

Onychomys and shrews in NorTh America.

ECOLOGY AND POPULATION DYNAMICS OF PAMPA RODENTS NEAR

BALCARCE, ARGENTINA

by

PeTer Lenn Dalby

A DISSERTATlON

SubmiTTed To
Michigan sTaie UniversiTy
in parTial fulfillmenT of The requiremenTs
for The degree of

DOCTOR CT PH LOSOFHY

-'

DeparTmenT of Leoiogy

I974

ACKNOWLEDGMENTS

A Ford FoundaTion granT adminisTered by The MidwesT UniversiTies
ConsorTium for lnTernaTional AcTIviTies, Inc. (MUCIA) aT Michigan STaTe
UnlversiTy was responsible for financing The firsT year of research,
November I968-l969. A six-monTh exTension (To May I970) was covered by
funds from The lnsTiTuTe of InTernaTional AgriculTure, The LaTin American
STudles CenTer and The Museum, all aT Michigan STaTe UnlversiTy. The
DeparTmenT of Zoology and The Museum generously provided financial supporT
before and afTer The granT by means of Teaching and research assisTanT—
ships.

Dean Miguel Goni of The FacuITad and DirecTor Domingo Pasquali of
INTA (lnsTiTuTo Nacional de Technologia Agropecuaria) were cooperaTive
in every respecT, supplying areas for field sTudies, IaboraTory space,
and Technical help and equipmenT. I am also graTeful To AugusTin CroveTTo
for allowing me To use his land for some of The sTudies. OTher people and
Thelr families on The sTaTlon who assisTed in various ways were Enrique
Gil, Adolpho Casaro, Rodolfo Camino, Carlos CaTTaneo, Dosindo DosanTos,
Jorge Doze, AnTonio Garcia, Cesar lnduni, Guillermo JoandeT, Paul Lewis,
Tomas Lopez, Laura San MarTin, AlberTo Regoni, Carlos Lopez SoubldeT,
Fernando QuinTana, as well as many oThers.

Several MSU faculTy involved In an AID projecT wiTh INTA and The
FaculTad were also Invaluable sources for assisTance of any naTure. My

deepesf graTlTude ls exTended To The CllnTon Bournes, Oscar Toboadas,

RoberT Wilkinsons and Garland Woods. The Rollo Ehrichs, a U.S. family
also working wiTh lNTA, exTended Their kindnesses many Times.

Very special Thanks go To several field assisTanTs who worked hard,
ofTen under condiTions of irregular hours and inclemenT weaTher. JusT as
ImporTanle, They gave me my mosT inTimaTe and memorable conTacT wiTh
ArgenTine culTure: Jorge Esponda, Jorge Grave, Eduardo Lorenz and VicTor
ZITarosa.

Rollin H. Baker, chairman of my docToral guidance commiTTee, has my
greaTesT appreciaTion for The conTinuous guidance and enThusiasm provided
me during my graduaTe years, and parTicularly This research. OTher members,
John Beaman, Max Hensley, John King, and Oscar Toboada also conTribuTed
To many aspecTs of This sTudy. Thanks go To Karel Rogers and especially
WaiTer Conley for much needed sTaTisTical and programming assisTance and
advice. CompuTer (CDC 6500) Time was available Through The courTesy of
The DeparTmenT of Zoology and William Cooper. Barbara Barker and Jackie
Church assisTed wiTh key punching. RoberT Robbins is graTefulIy acknowl-
edged fer his phoTographic knowledge and many hours in The dark room.
Deborah Cool and Sarah PaTTen spenT many hours on a predaTor scaT conTenT
analysis. Fellow graduaTe sTudenTs were an invaluable source of helpful-
ness and encouragemenf, and my graTiTude is exTended To Them.

Jorge Morello of INTA, CasTelar, graciously idenTified mosT of The
planT species coilecTed; STephen STephenson and Jesse Saylor of MSU
ldenTifled oThers from voucher specimens. Voucher specimens for all The
planTs IisTed herein are deposiTed In The collecTion of The Beal-DarlingTon
Herbarium, MSU.

Lasle, my appreclaTlon and graTiTude ls exTended To my wife, Barbara

who shared In my research endeavors whenever she could, and adjusTed well

III

To ArgenTine life and my long hours In The field. Her many hours of
Typing assisTance associaTed wiTh This manuscrlpT are also graTefully

acknowledged.

TABLE OF CONTENTS

 

 

 

 

 

 

 

 

Page

I. INTRODUCTION .......... ..... ..... . ......... ..... .............. l
Purpose .................................... . ............. 3

STUdy Area ........... . ..... ..... ....... ....... ........ ... 4
Biogeography_ ..................................... .... 4
Physiograpfiy .......................... . .............. 4

Cllmafe... ...... ...... ..... .. ............ ..... ...... . 7

ll. METHODS ................................... . .................. l0
STudy PloTs and Trapping Schedules ....................... l0
Trapping Procedure and DaTa Recording .............. . ..... l4
VegeTaTion Analysis .............. . ........... . ........... l6

III. RESULTS.... .................................................. 28
PopuIaTion DensiTy and STrucTure .................... ..... 28
EsTimaTion gi_populaTion densiTies ............. ...... 28
TrappabiilTy.. ........ ........... ..... ............... 3O

RaTe gj_increase ......................... ..... ....... 30

RecruiTmenT ........... ..................... ...... .... 33

Sex raTios ..... ...................................... 36
ReproducTion ............................. ................ 42
Lgigfli_gi_reproducTive animals ..... ....... ..... ...... 44

 

Leanh of breeding season............................ 5|
--.—J” -..—-

 

JET-engifxgj‘breedinq. 00000000 OOOOOOOOOOOOOOOOOOOOIOOO 57
surVi‘Ja| ..... .....OOOOOOOIOIOOOOOOOOOOIOOOOOOOOOOOOOOOOOO 62

 

Minimum survival raTes....... ..... .............. ..... 62
True survival raTes ................ . .......... ....... 64

 

[dovemen-TSOOOOOOOO...0....I.O...0.00.0.0.........OOOOOOOIO 66

 

 

 

 

 

RecapTures per Trapping period ...... .. ......... ...... 66

SpaTlai disTribuTion~conTers 9:_acTiviTy .......... ... 7O
Bagll,2j_achviTy .............. ........... ...... ..... 72

Dispersal 2:_Akodofl azarae..... ...................... 76
.figmlflg.gj_Akodon azarae .............. . ........ ....... 8|

HabiTaT QuaIiTy ..... . .................. ............. ..... 83
influence 2i_gavi§_gfl_Akodon pgpulaTions ............. 83

IV. DISCUSSION...... ............................................. 88

 

 

PhylogeneTic HisTory of Akodon, Oryzomys, and OxymycTeris 88

 

 

 

 

 

PopulaTion Changes ....................................... 90
Seasonal densiTy changes .................... . ........ 9O
lrrupTions and gyclic flucTuaTions.... ...... ......... 9i
Densljy esTimaTes and seasonal variaTions ............ 9i
TrappabiliTy .................... ..................... 92
RecruiTmenT ...... ....................... ..... ........ 93

Sex RaTios .................. ............... ..... ......... 96

ReproducTion ..... . .............. ....... .................. IOI

surVivaI RafeSo O O O I O O O O C O O O O O 0 O O O 0 O 0 O O O O O O O O O O O O O O O O O O O O O '05
MovemenTSo I O O O O O O O O O O I O O O O O O O I O O O ..... O O O O O O O O O O O ........ '07
Dispersal and Homing. I O O O O O O O O O O O 0 O O O O O O O O O O O I O O I I O O O O O O O IIO

AC+TVTTY and FOOd HabiTSOOOOOOOOOOO......OOOOOOOOOOOOOOOO ||3

 

AchvlTy paTTerns and eaTing habiTs... ..... . ....... .. ll5
EaTing habiTs and meTabolic raTes... ..... ....... ..... Il8

 

Runwayjmaklng and burrowing achvlTle§,.............. I2l

 

vl

V

VI

VIII

X.

SUMMARY AND CONCLUSIONSooooooooooc0.0000000000000000... oooooo
LIST OF REFERENCES ............ . .......... . ......... . ....... ..
APPENDIX A - COMPARATIVE POSTNATAL DEVELOPMENT OF AKODON

AZARAE, ORYZOMYS NJGRIPES, AND OXYMYCTERIS RUTILANS ..........

 

 

 

INTRODUCTION. 0 O O O O O O 0 O O 00000000000000 O O ........... O ......... 0
Purpose..... ..... . .......................................
REPRODUCTION - LABORATORY RESULTS ............................

PosTnaTal DevelopmenT of Akodon azarae .......... . ..... ...

 

MeThods ............................. . ................

GesTaTion pgfiod .....................................

 

LiTTer size ......................... . ........... .....

 

Physical develoPmenT ............. . ...... . ............

 

 

GrowTh - weighT gain and maTuraTion ..................

 

GeomeTric growTh raTe ................................

 

GEOWTh — boqxpmeasuremenTSoooo ooooooooooo o ooooo 000000

 

PosTnaTal DevelopmenT of Oryzomys nigripe: ...............

 

MeThods ........................ . ............... ......

LiTTer size and gesTaTion period ..... ................

 

GrowTh and maTuraTiop_ ............ . ...... . ............

 

Physical deyelopmenT ............ .....................

 

PosTnaTal DevelopmenT of ngmycferls ruTilans... ..... ....

 

WThOdSOOOOOOOOOOOOOOOO.......IOOOCOOOOOOOOOOO .......

LiTTer size and gesTaTlon period.....................

 

 

 

 

 

GFOWTh and EETUFBTLQQ. ooooooooooooo 090000000 ..........
PDYSICBI OOVCIOPWGHT oooooooooo ooooooooooooooooocooooo
DISCUSSION ccccc cococoo-0000000000000000.0000.00.0.0000..0000.

vii

Page
l24

I29

I40
I40
I40
l4l
I4I
l4l
I4I
I42

I44

I60
I60
|60
I6I
|6I
I64
I69
I69
I69
I70
I70

I76

Page

GesTaTIon Period..................................... I76

PosTparTum EsTrus ........... .. ....... . ........... .... I78
LiTTer Slze ......... . ........... . ....... ..... ...... .. I79
Reproduchve Efficiency .............. . ............ ... I79
GrowTh — WelghT .................. .................... I79
Physical DeveIOpmenT ...... . ............. ... ....... ... l8I
Sexual MaTuriTy........... ..... ...................... I83
XI . SUMMARY ............................. . ..... . .................. I86

viii

Table

LIST OF TABLES

Page
Ecological parameTers of myomorphs which dlfferenTiaTe
TemperaTe grass—eaTers from seed— and insecTweaTers. BesT
comparisons are made among sympaTric species ........ . ..... ..... 2
The record of TemperaTure, reIaTive humidiTy, and rainfall
aT INTA, Balcarce ...... ........................................ 8

The common grasses and forbs of PloT l and Hill PIoT. Those
species marked wiTh an * are characTerisTic of The AusTrai

Pampan DisTricT, presenTed by Cabrera (I953, I968)............. l5
PercenT cover found in PIoT | and Hill PIoT, summer I969-i970.. I5
PercenT deviafion of The ToTal caTch (firsT column) from The

minimum number known alive (middle column) for Akodon azarae,

 

Oryzomys nLgripes, and OxymycTeris :pTilans. CalculaTed as

 

 

minimum number known alive - OOOIOOOOOOOOOOOOOOOOO... 29
(. ToTaI caTch I00% |OO%

TrappabiliTy of Akodon azarae, Ogyzomys nigripes, and

 

ngmycTeris ruTilans. Given is The ToTal number (N) of animals

 

capTured, summed from The ToTaIs of six Trapping periods, and
The average percenT caTch of Those known alive over ThaT span.. 3i

RaTe of increase for Akodon azarae, Ogyzomys nlgrlpes, and

 

Oxymycferls ruTilans, calcuIaTed as The percenT change In

 

popuIaTlon size from one Trapping period To The nexT, based

on fhe mInImljm nU‘nber known aliVeOOOOOOCOOOIO......OOOOOOQOOOOO 32

Ix

Table

I4.

Page

RecruiTmenT of new individuals of Akodon azarae, Oryzomys

 

nigripes, and Oxymycferis ruTilans inTo PloT I and Hill PIoT
aT each Trapping period, as based on ToTal capTured each
periOdOOOOOOOOOOOOOOOOI.0..0.0.0.0000...OOOOOOOOOOOOOOOOOOOOOO 34

RecruiTmenT of new individuals of Akodon azarae, Ogyzomys

 

nigripes, and ngmycTeris ruTilans inTo PIoT I and Hill PloT

 

aT each Trapping period, as based on minimum number known alive.35

ProporTion of female Akodon azarae, Oryzomy§_nigripes, and

 

OxymycTeris ruTilans capTured in each Trapping period.. ..... ... 38

 

ProporTion of female Akodon azarae of various age and sTaTus

 

(recruiTs, residenTs) classes, and seasons found in CroveTTo
PasTure and Hill PIoT. Female sample sizes (in parenTheses)

and probabiliTies for null hypoTheses of l:l sex raTios are

also given ........ . ......... ...... ..... ........................ 39

Sex raTio daTa on Oryzomys nigripes and OxymycTeris ruTilans,

 

 

PioT l and Hill PloT pooled, shown wiTh proporTion of females,
sample sizes (in parenTheses), and iasTiy, probabiliTy for
null hypoTheses of I:I sex raTios ..... ......................... 43

InTensiTy of breeding in Akodon azarae, PIoT I and Hill PloT

 

(in parenTheses), as measured by The number of pregnanT females
deTermined by palpaTion, The percenTage of pregnancy of The

aduiT females and all age class females, and The number of

liTTers and average liTTer size counTed by palpaTion........... 53

InTensiTy of breeding In Oryzomys nigripes, PloT I and Hill

 

PIoT (daTa combined), as measured by The number of pregnanT

females defermined by palpaTion, The percenTage of pregnancy

Table

Page
of The aduiT females and all age class females, and The
nUmber of IiTTers and average liTTer size as counTed by
palpaTion.... ............ .. ...... ........ ......... ............. 54

InTensiTy of breeding in OxymycTeris :pTilans, PIoT I and

 

Hill PloT (daTa combined), as measured by The number of

pregnanT females deTermined by palpaTion, The percenTage of
pregnancy of The aduiT females and all age class females,

and The number of iiTTers and average liTTer size as counTed

by palpaTion... ........... . ........ ...... ....... ..... ..... ..... 55

InTensiTy of breeding in Akodon azarae, PIoT I, as measured

 

by The presence of a well~deveioped scroTum in males and a
perforaTe vagina in females. Under each age heading and

Trapping period are given The number of animals capTured,

The number perforaTe or scroTal, and The percenTage of The

group which show ThaT condiTion...... ............ .............. 59

inTensiTy of breeding in Akodon azarae, Hill PloT, as measured

 

by The presence of a well-developed scroTum In males and a
perforaTe vagina in females. Under each age heading and.

Trapping period are given The number of animals capTured, The .
number perforaTe or scroTal, and The percenTage of The age

group which show ThaT conlelon................................ 6O

InTensiTy of breeding in Oryzomys nigripe§_and OxymycTeris

 

 

ruTilans, PIoT l and Hill PIoT (daTa combined), as lndicaTed

by The number of young animals capTured (SA=subadulTs) and
sexual condiTion (Tesies scroTal, vagina perforaTe). The firsT

column in each class gives The ToTaI number capTured; The

xi

Table

20.

2i.

22.

23.

24.

Page
second, in parenTheses, The number showing The sexual
condiTion; and in The Third column, The percenT showing The
sexual condiTion ..... ....... ...... . ....... . ..... .............. 6i

Minimum reTenTion raTes per l4 days for Akodon azarae,

 

Oryzomys nigeres, and OxymycTeris ruTilans. ToTaI caTch

 

 

released is in parenTheses, followed by The ToTaI number
of The sane animals capTured In subsequenT Trapping periods... 63

True survival raTes for Akodon azarae during The non-

 

reproducTive (winTer) season.................................. 65

PercenTage of Akodon azarae which were capTured more Than once

 

during each Trapping period, arranged according To sex, age
class, and sTudy ploT ...... . ...... . ...... ..................... 67

PercenTage of Onyzomys nigripes and Oxymycferis ruTilans,

 

 

which were capTured more Than once during each Trapping
period, and arranged wiTh sex and age classes pooled for
each sTudy ploT ......... ........ ....... . ....... .... ...... .... 68

The proporTion of recapTured aduiT Akodon azarae segregaTed

 

by sex, pIoT, and season. ProporTion is The average of
appropriaTe daTa shown In Table 2i. Numbers in parenTheses
show average number of individuals capTured each period...... 69

The proporTion of recapTured Oryzomy§_nigripes and

 

OxymycTeris ruTilans, sexes and ages pooled, and segregaTed

 

by ploT and season. ProporTion is The average of appropriaTe
daTa shown in Table 22. Numbers In parenTheses show average
number of Individuals capTured each period.................. 69

Average number of Akodon azarae, Oryzomys nlgripes, and

 

 

xii

Table

26.

27.

28.

29.

30.

3|.

32.

Page

OxymycTeris ruTilans capTured each Trapping period for

 

Individual quadranTs in PIoT | and Hill PIoT.................. 7|
Average disTance, in meTers, from The cenTer of acTiviTy,

wiTh N, for Akodon azarap, DaTa grouped by sex, age class,

 

and sTudy ploT, for each Trapping period...................... 73
Seasonal average disTance, in meTers, moved from The cenTer

of acTiviTy, wiTh N, for Akodon azarae, by sex, age class,

 

and sTudy ploT.................... ............. ............... 74
Three-way analysis of variance for Table 27 aduITs.... ..... ... 75
Seasonal average disTance, in meTers, moved from The cenTer

of acTiviTy, wiTh N, for Opyzomy§_nigripes and OxymycTeris

 

 

ruTilans, by sex and sTudy ploT............................... 75

PrOporTion of female Akodon azarae of various age classes,

 

seasons, and Trapping areas found in CroveTTo PasTure. Female
sample sizes (in parenTheses), and probabiliTies for null
hypoTheses of l:l sex raTios are also given................... 78

Dispersal of Akodon azarae from PIoT I, calculaTed as The

 

nearesT disTance in meTers To PioT l - Removal PloT exclosure
wall, aT The Time of firsT capTure away from PIoT I. Numbers

noT In parenTheses are Those for males; in parenTheses, females.
The percenT of The ToTaI which are in each disTance caTegory is
also glven........... ........... .............................. 80

Dispersal of Akodon pzarae from PioT I, calculaTed as The

 

nearesT disTance, In meTers, from The poinT of IasT capTure
In PIoT I To The firsT poinT of capTure wiThin Removal PloTs

I and II. Numbers noT In parenTheses are Those for males; In

xlll

Table

33.

34.

IA-So

A-4.

Page
parenTheses, females. The percenT of The ToTal which are In

each disTance caTegory Is also glven....... ...... .. ........... 80

The monTth raTe of dispersal, In percenT, for PloT l Akodon

azarae, based on The number dispersing To Removal PIoT I and

Ii, and The minimum number known alive in PloT I aT The
beginning of each new monTh of dispersal ....... . ....... ....... 82

Homing success aT various disTances for Akodon azarae wiTh:

 

(A) giving The number of animals which had aT IeasT one

cpporTuniTy To home, (B) The number which acTually did home,

and (D) The number of Times homing Took place for (8)

individuals. DaTa were TabuiaTed for each Trapping period,

Then summed over all periods To presenT This compilaTion..... 82
APPENDIX A

ReiaTionship of liTTer size To mean weighT, and weighT

variaTion wiThin, compared To beTween IiTTers in Akodon azarae, I43

 

Frequency disTribuTion aT age when The developmenTaI characTer-
isTics of incisor erupTIon, hearing, and eye opening firsT

appeared in Akodon azarae .................. . ..... ............ I43

 

roporTionaI growTh (in percenT), means, ranges, daily
InsTanTaneous raTe of growTh (I.R.G.), and approximaTe age

aT one-half growTh for ToTal, Tail, body, hind fooT, and ear
lengThs, and weighT for A, 333522, All prOporTions based on
84~day~old aduiT measuremenTs.. ..... ..... ....... ............. I58
ProporTionai growTh (in percenT), means, ranges, daily
insTanTaneous raTe of growTh (I.R.G.), and approximaTe age

aT one~half growTh for ToTal, Tail, body, hind fooT, and

xiv

Table

A-8.

Page

weighT for Oryzomys pigripes and OxymycTeris ruTilans. All

 

caiculaTions based on 84-day-old aduiT measuremenTs ...... ..... I62
GesTaTion period, average neonaTe and aduiT weighTs, average
liTTer size, percenT of neonaTe and liTTer weighT To ThaT of

The aduiT for some grassland InhabITing NorTh American and
ArgenTine grass-, seed-, and insecT-eaTing myomorphs.... ...... I77
Daily insTanTaneous raTes of growTh for approximaTer The

firsT week for some grassland lnhabiTing NorTh American

and ArgenTine grass—, seed-, and insecT-eaTing myomorphs...... I80
Time of firsT observance of several developmenTal

characTerisTics in some grassland inhabiTing NorTh American

and ArgenTine grass-, seed-, and insecT-eaTing myomorphs.

Time expressed in days ....................................... . I82
Age of sexual maTuriTy, in weeks, in some grassland inhabiTing
NorTh American and ArgenTine grass-, seed-, and insecT-eaTing

myomorphs. .......... .......................................... I85

XV

Figure

U'i

LIST OF FIGURES

Buenos Aires Province, ArgenTine, showing iTs zoogeographic
and phyTogeographic divisions, and The locaTion of Balcarce...
CroveTTo PasTure, showing The approximaTe locaTIons of The
sTudy ploTs. PIoT I is divided lnTo four QuadranTs ....... .....

PIoT l maTure growTh of Paspalum elongaTum, summer I968—I969..

 

Removal PIoT l (righT) exclosure fence, Taken near The back
side of The PIoT I, Removal PIoT corner, summer I968-l969.

Major vegeTaTion is Paspalpm_elongafum... ........ .............

PioT I, winTer I969. DominanT vegeTaTion is Paspalum eiongaTum

 

RBII'IOVSI PIC-I- I,WInTeI" [9690.000 0000000000000000 00.00.000.000.

PloT l maTure growTh of Ppspalum elongaTum, summer l969-I970.

 

NoTe deTeroriaTion of dominanT vegeTaTion, and increase in
forbs, parTiculariy The ThIsTle, Cirslum. PhoTo Taken from
The same approximaTe locaTion as Figure 5. ........ ...... ..... .
Close—up Taken from The approximaTe area shown In Figure 7.
The cane debris covering The ground was produced bY.9§Xl§:°°°'
DiagramaTic map of Hill PloT. DoTs show pure sTands of

Paspalum quadrifarlum, scaTTered paTTern ThaT of Eryngium

 

Page

I7

2|

2|

23

23

paniculaTum, and unTouched areas, mixed sTands of ElpfochaeTlum,

 

 

STipa, and oTher grasses and forbs. The ploT Is divided InTo

four quadranTs, named A, B, C, and D, sTarTing from The lower

va

Figure

Page

IefT—hand corner and going clockwise ....... . ....... ........... 25

Hill PloT, summer I969-l970, showing Paspalum_quadrifarium To

 

The IefT, Ecyngium panlculafum mosle in The background, and

 

PipTochaeTium and STlpa, eTc. in The foreground............... 27

 

Hill PloT, summer I969-l970, looking down The genTie slope.

PipTochaeTium, STlpa, eTc. are in The foreground, Paspalum

 

quadrifarium In The background. Several kilomeTers from This

 

siTe and locaTed on The farmland in The background is CroveTTo
Pas+ureIOOOOOO0.0.0....OOOOOOOOOOOOOOOOO......OOOOOOOOOOOOO... 27

Minimum number esTimaTes of Akodon azarae in PloT I and Hill

 

PIoT ............... ....... ..... .. ..... ..... ................... 46

Minimum number esTimaTes of Ofiyzomys niggipes (Top) and

 

OxymycTeris ruTilans (boTTom) in PIoT l and Hill PioT......... 48

 

Removal PIoT I adjacenT To PIoT III - Removal PIoT I exclosure.

NoTe The cane on The ground, The lack of inflorescences, and

The generally grazed appearance of each grass (Paspalum

eiongaTum) clump................... ...... ........... .......... 86

Same posiTion and daTe as ThaT for Figure I4, buT wiTh a Turn

of l80°, and now looking InTo PloT Ill. There is somewhaT less

cane on The ground, The inflorescences are abundanT, and The

grass clumps are much denser and less disTurbed............... 86
APPENDIX A

NeonaTe Akodon azarae ............. .. ......... ..... ...... ...... I47

 

FOUF'dBY"OId AkOdOfl azaraE—ooOOOOQOOocoo.oooooooooooooooooooooo '47

 

Seven—day-old Akodon azarae..................... .......... .... I49
FourTeen—day-old Akodon azarae ........ ......... ..... ... ..... I49

Mean weighT, range, and i 2 SE for pooled sexes of Akodon

xvii

252522, Numbers above The mean designaTe female (Top) and male

(boTTom) sample sizes. Numbers below The mean wiThouT

parenTheses give The proporTion of perforaTe females To ToTaI

females; Those wiTh parenTheses give The proporTion of pregnanT

females To ToTal non~nursing females........................... I53
A-6. Weekly average raTe of growTh (weighT) for pooled sexes of

Akodon azarae. The sTraighT lines roughly indicaTe The

 

insTanTaneous raTe of growTh. Sample sizes are given above
eaCh pOIn-f 0000000000 O ..... ... ........ O ..... 00...... ......... .0. '57

A-7. Neonafe Oryzomys nigripes. NoTe folded ears.................... I66

 

A-8. Four—day-old Oryzomy§_nigripes.. ........ .... ............ ....... I66

 

A-go seven-day-Old oryzomys ningPeSOOOOOO000000000000...0000.00.00. I68

 

A-lO. FourTeen-day-old Oryzomys nigripes.... ............... .. ...... .. I68

 

A-Il. NeonaTe OxymycTeris ruTilans. NoTe The darkened planTar

 

 

Tubercles.......................... ............... ............. i73
A_l20 Four-daY‘OId oxmeCTerL: T'U'I'I IanS................... oooooooo coo '73
A-I3. Seven-day-old OxymycTeris rpTilans......... ....... ....... ..... . l75

 

A-l4. FourTeen-day-old OxymycTeris ruTilans.......................... I75

 

xviII

INTRODUCTION

While ecosysTems are being examined more inTenTIy Through compuTer
models and field experimenTaTion, demographic analyses of various
organisms in The environmenT and sTudies of causal mechanisms To
explain changes are necessiTies in applicable sysTems programs. WiThin
several major biomes, a number of organisms, from differenT Taxa and
consumer levels are being sTudied. Likewise, similar biomes are being
sTudied in differenT regions of The world To deTermine if The same
ecological principles hold True. if noT, Then modeling and sysTems
analyses are of limiTed use since Their predicTive values are diminished.

Grassland—lnhabiTing rodenTs are imporTanT componenTs of pasTorai
ecosysTems, noT only economically, buT also in Their role as primary
consumers. One meThod of sTudying ecological sysTems is Through
energeTics, i,e,, according To each species consumer level. Baker (i97l)
discussed The nuTriTionaI aspecTs of grass-eaTing and seed-eaTing
myomorphs found in NorTh American grassland environs. These Two groups
have disTincTive specializaTions as expressed by Their morphological,
physiological and behavioral characTerisTics. Ecologically, The Two
groups may be separaTed by cerfain disTincTive feaTures (Table l).

The posiTion of myomorph insecT-eaTers In such a Table is
uncerTain. NorTh American species are few (§,g,, boTh species of
_Qflygflgmy§). HershkoviTz (I962) believed ThaT early myomorphs probably
were foresT dwellers, had low crowned and cuspidaTe TeeTh, and aTe,

In adleion To insecTs and OTher inverTebraTes, seed, fruIT, and oTher

Table I. Ecological parameTers of myomorphs which dlfferenTiaTe
TemperaTe grass—eaTers from seed— and insecT—eaTers.
BesT comparisons are made among sympaTric species.

 

GRASS—EATERS

SEED-EATERS

 

I. Wide populaTion flucTuaTions.
2. FasT popuiaTion Turnover.

3. TemperaTure, nuTriTion primary

facTors reguiaTing reproducTion.

4. Early sexual maTuriTy.
5. No delayed implanTaTion.
6. ShorT gesTaTion periods.
7. Large liTTers.

8. Many IiTTers per reproducTive
season.

9. Young parle precocious.

IO. Diurnal or crepuscular acTiviTy
paTTern.

II. HibernaTion or TorpidiTy
unknown.

l2. PredaTors ofTen diurnal or
crepuscular.

l3. Generally one species per area.
I4. Small home ranges.

I5. Food seiecTion resTrlcTed.

l6. High annual biomass.

I7. Much of ToTaI energy budgeT

uTilized in food gaThering,
assimilaTien.

Narrow dramaTic populaTion
flucTuaTions.
Slow populaTion Turnover.

PhoToperiod primary facTor
regulaTing reproducTion.

LaTe sexual maTuriTy.
Delayed implanTaTion if
IacTaTing.

Long gesTaTion periods.
Small liTTers.

Few liTTers per reproducTive
season.

Young alTricial.

NocTurnal acTiviTy paTTern.
HibernaTion or TorpidiTy

common.

PredaTors ofTen nocTurnaI.
Generally Two or more species
per area.

Large home ranges.

Food selechon reIaTively broad.
Low annual biomass, singularly or
collecTively.

LITTie ToTal energy budgeT uTilized
in food gaThering, assimilaTlon.

 

planT producTs. PresenT—day seed—eaTers and some grass—eaTers conTinue
To consume raTher large quanTlTies of animal food (Landry, i970),
especially during seasons when nuTrlenT demands are greaTer (FleharTy
and Olson, I969). Since seed—eaTers generally eaT larger percenTages of
lnverTebraTes Than grass-eaTers, iT is besT To place insecTivorous
species such as Onychomys wiTh The former. The low densITies of
Onychomys as well as oTher characTerisTics (Bailey and Sperry, I929;
Horner _T _J., i964) also indicaTe ThaT They are mosT similar To seed~
eaTers.

Compared To NearcTic grassland rodenTs, less is known abouT Their
counTerparTs in SouTh America. Crespo (I966, I970) and Pearson (I967)
have been The main conTribuTors To our knowledge of ArgenTine grassland
rodenT biology Through snap-Trapping Techniques; Barlow (I969) has done
likewise for adjacenT Uruguay.

Purpose

This sTudy summarizes an eighTeen—monTh demographic analysis of a
pampan myomorph rodenT communiTy on Two sTudy siTes near Balcarce,
Buenos Aires Province, ArgenTIna. The principal myomorphs lnhabiTing

These live-Trapped siTes included The field mouse, Akodon azarae, The

 

peromyscine-like rice raT, Oryzomys nigripes, and The insecTivorous

 

burrowing mouse, OAymycTeris ruTilans. Less comnon species were Calomys

 

Iaucha, C. musculinus, Cabreramys obscurus, ReiThrodon auriTus, and Mus

 

 

musculus. A hysTricomorph, The guinea pig, Cavia aperea, and a small

marsupial, Monodelphis dimidiaTa, also were presenT.

 

The objecTives were Two~foldz I) To compare and conTrasT The
demographic changes In a yearly cycle of rodenT populaTions (The Three

principle species) lnhabiTing Two differenT planT cunmunITles; and

2) compare These daTa, Including perTinenT ecological and behavioral

observaTions, wiTh ThaT presenle known abouT TemperaTe grassland

rodenTs in oTher parTs of SouTh America and especially NorTh America.
STudy Area

Biogeography

 

The Balcarce area, locaTed approximaTely 37°45'S, 58°|8'W, is parT
of a grassland sTeppe which covers all The province of Buenos Aires,
parTs of SanTa Fe, Cordoba, La Pampa, and EnTre Rios, and a large
secTor of Uruguay. Basically, This sTeppe is a TransiTionaI area,
TundamenTally and hisToricaliy a subTropical faunisTic reTracTion
(since The Pliocene) wiTh The addiTion and persisTence of elemenTs from
The souTh and wesT (RingueleT, l960). Various workers, 239: Cabrera
and Yepes (I940), RingueleT (I96l) and FiTTkau (I969), using mammals
and oTher faunal elemenTs, and Cabrera (I953, I968) uTilizing knowledge
concerning general relief, soils, climaTe and vegeTaTion, have aTTempTed
To delimiT iTs boundaries. Figure l delimiTs a porTion of The pampan
sTeppe in The Province of Buenos Aires, as given by RingueleT (i960)
and Cabrera (l968), and denoTes The IocaTion of Balcarce.

Physiography

The physiography (afTer Bourne, per. comm.; Borrello, l968) of The
Balcarce area, approximaTeiy ll2 m above sea level, involves Two
disTincT geological feaTures. The firsT is The ancienT (Precambrian _
and lower Paleozoic) mesa—like buTTes or hills which consTiTuTe a parT
of The Sierras de Tandli and are scaTTered aT irregular inTervals. The
second feaTure consisTs of The bgoad, level To genTIy roiling plains
beTween such prominences.

The more rounded, somewhaT lower hills are underlaid by graniTe

Figure l. Buenos Aires Province, ArgenTine, shcwing iTs zoogeographic

and phyTogeographic divisions, and The iocaTion of Balcarce.

 

 

 

 

P‘MPAN DOMINION
(ZOOGEOGRAPHK)

 

 

 

 

 

RINGUELET—l960
40- A 0 ~40
‘ USTRAL ISTRICT
(PHYTOGEOGRAPHIC)
o \\\\\\\\ CAMERA—I963
0 BALCARCE
5.0 ‘90 290 Km
I I
60 57

 

 

Figure I

which may be exposed. The more prominenT meses, composed mainly of
quarTziTe, presenT abrupT slopes wiTh sTeep colluvial slopes below,
and are of liTTle agriculTural value. The plains Themselves are a
complex of eiTher TlaT lowlands having poor exTernal drainage, or
sligthy sloping To genle rolling uplands.

The soils in The area have developed from QuaTernary sedimenTs
composed of waTer—worked loessal maTerial wiTh a high percenTage of
volcanic ash. OTher Than in The viciniTy of The rock ouTcroppings and
mesas, There are no soils developed from rock sTraTa. In general, The
soils of The well-drained uplands are of Brunizems. They have black
(moisT), granular, loamy, sligthy acid surface soil wiTh 4-8 percenT
organic maTTer. The subsoils, beginning aT l5-40 cm, are clay loams
wiTh a sTrongly developed prismaTic biocky sTrucTure. The subsTraTa
are loams To fine sandy loams. Lime hardpans and weakly consolidaTed
loamy formulaTions begin aT depThs of 50-300 cm.

in The lowland areas wiTh resTricTed drainage, The soils are
dominanle soloneTz and solodized soloneTz wiTh some solods in The
beTTer drained parTs. These soils are characTerized by very slowly
permeable columnar or prismaTic clay loam To clay subsoils wiTh high
sodium saTuraTion and high pH.

‘CiimaTe

 

The Balcarce climaTe is summarized in Table 2. Rainfall, which
averages 836 mm annually, is mosT abundanT In February and March, and
leasT during The winTer monThs (May~AugusT). in The growing season,
which runs for annual crops from abouT November Through March, January
is likely To be The driesT monTh. Rain may be expecTed abouT every five

or six days during any monTh of The year, wiTh 75 PercenT of The rains

 

 

 

 

A_m+0kv m.emm en 0.0 e.m e.om m.n_ m_mmm
>_Lmo>
o.ow so k.m _.N_ e.km N.a_ Lmsemomo
...k we N._ _.o_ ..em m.o_ Losem>oz
o._o we o.~- e.k m.om k.n_ Lmno+oo
k.Mk me e.m- m.m m.o_ v.0. Lms2m+amm
m.me we e.e- o.n m.m_ _.m +msms<
k._o me o.e- m.n N.~_ o.k >_ss
0.0m em w.m- m.e o.m_ e.m mess
..mm mm e.m- m.e m.m_ a.o_ >gs
m.mk wk m.os N.m m.om e.e_ __La<
m.m__ wk m.m k... m.mm ..k. goumz
0.0m mo e.m k.n_ o.mm _.om >Lmscsom
m.©e no m.m m.m_ m.mm m.om >LM::Me

es m Oo.asek 0o.aEwh 0o.asme Qo.ceek
.a_uoL¢ >+_e_EsI .c_s .c_z .sz coo: £+coz

cmmz e>_+m_em o+:_oma< awe: cmoz

_ .Aomm_-_em_v

oocmo_mm .<Hz_ +m __m+cmmc ocm ~>+_om23c m>_+m_mc .oL:+mLane+ +0 oLoooL 05k .N o_omh

being under l0 mm. ExTended droughT is rare.

Average monThiy TemperaTures range from a low of 7.6°C in July To
a high of 20.3°C in January. Nigthy TemperaTures are frequenfly below
freezing during winTer monThs, buT due To mariTime influences, prolonged
freezing TemperaTures and winTer precipiTaTion in The form of snow are
very rare.

The relaTive humidiTy averages 74 percenT for The year and ranges
from 63 percenT in January (summer) To 84 percenT in June (winTer) when
evaporaTion and insolaTion are reduced. Wind velociTy is generally
moderaTe, varying around an average of l5 km/hr. The wind movemenT is
greaTesT during December and January (summer) and leasT during May and
June (winTer).

During This sTudy, increased rainfall (995.l mm from January I969
To December l969) Through all four seasons of The year was The mosT

significanT climaTic change from The average.

iETHDDS

STudy PloTs and Trapping Schedules

The sTudy ploTs were locaTed on lands of The ExperimenT STaTion
(lNTA), approximaTely I5 km NW of Balcarce, and on The AugusTin CroveTTo
properTy, siTuaTed abouT 6 km N of Balcarce.

Five of The six ploTs esTablished To sTudy The small mammal
communiTy were placed in CroveTTo PasTure (Figure 2). This area, a l7
hecTare lowland (l00 m above sea level) fallow pasTure wiTh poor.
exTernal drainage, was locaTed adjacenT To Highway 226 and a dirT road.
A small creek, The Arroyo PanTanoso, bisecTed The pasTure on iTs

souThernmosT corner. AlThough seeded in i965 wiTh Agropyron elongaTum,

 

FesTuca arundinacea, Phalaris Tuberosa, Lolium perenne and Trifolium

 

 

praTense, caTTle grazing There for The nexT Three years, up To and
including The early parT of The winTer (June—July) of l968 kepT growTh
reduced. By The beginning of summer (November and December), The
vegeTaTional cover, alThough lacking ground liTTer, was well—developed,
and a subsTanTial rodenT populaTion quickly inhabiTed This unpasTured
land.

Because of The possibiliTy, alThough remoTe, ThaT parT of The
pasTure mighT be grazed during The forThcoming year, The five sTudy
ploTs were esTablished along The norThwesTern edge. Following is a
descrlpTion of Them (Figures 3-8).

I. PloT l. Unenclosed, i.2 hecTares in area,
a ID x l0 Trapping grid wiTh l2 m Spacing.
l0

2-3.

Three nighTs of live-Trapping wiTh Sherman
live—Traps scheduled every oTher week aT
9—l4 day (average, l4 days from middle of
one Trapping period To The middle of The
nexT) inTervals,depending.on weaTher
condiTions. Basic demographic daTa described
herein concerning The rodenTs in This
pasTure were gaThered from This ploT from
December l968 To April I970.

Removal PloTs I and Il. Like PloT l in

size buT wiTh each enclosed by an opaque
plasTic barrier (cleared for 50 cm on each
side). The barrier, 40 cm high, 5 cm buried
depTh, 6 mil Thickness was locaTed 6 m from
The nearesT Trap row of PloT l, and The
perimeTer Trapping sTaTions which They
enclosed. BoTh Removal PloTs were designed
To TesT The effecT of removal of The

prominenT mouse (Akodon azarae) on The

 

populaTion numbers of The remaining species.
Live-Trapping wiTh removal of 5599923 buT
merely examining, marking, and releasing
The oTher species was conducTed for four
consecuTive days each monTh, from February
l969 (June for PloT II) To March I970. Like
The AcTiviTy PloT, daTa are given only in

relaTien To dispersal movemenTs.

4. PloT lll. Enclosed, conTiguous wiTh Removal
PioT l, a .l3 hecTares Trapping grid of
3 x 3, wiTh l2 m spacing, wiTh perimeTer
Trap sTaTions 6 m fron The exclosure. InsTead
of plasTic, 50 x |20 cm inTerlocking Tin
sheeTing, held uprighT by sTeeI sTakes,
surrounded The ploT. Because The guinea pig

(Cavia aperea) increased in subsTanTial

 

numbers during The sTudy and any dramaTic
change in small rodenT pepulaTion composiTion
and sTrucTure which mighT have occurred in
The Removal PloTs was desTroyed, PloT III was
consTrucTed in The spring (early December) of
l969 To deTermine whaT effecT Cavia was having
in depressing The local small rodenT populaTion
by effecTively excluding The former. Plans To
replicaTe This experimenT did noT maTerialize
when The remaining supply of sheeTing became
unavailable. The Trapping schedule was four
consecuTive days each monTh from December l969
To April l970.

5. AcTiviTy PloT. Unenclosed, buT conTiguous
wiTh PIoT l, .70 hecTares in area, a 7 x 7
Trapping grid, wiTh l2 m spacing. This ploT
was live-Trapped for Three consecuTive days
every monTh (February 1969 To March I970)

wiTh Traps lnspocTed aT The end of every

l3

Three hour period, day and nighT. DaTa
are uTilized here only To describe general
acTiviTy paTTerns for each of The Three
species.

6. Hill PloT. Like PloT l, unenclosed, l.2
hecTares in area, a l0 x l0 Trapping grid,
wiTh l2 m spacing. Live-Trapping iniTiaTed
in March l969 conTinued To April I970, and
followed The same schedule (generally
Trapped back-To-back or on The same days)
as PloT i.

The laTTer sTudy area, Hill PloT (Figures 9, l0, II), was siTuaTed
on a small hill wiThin one km of INTA headquarTers. The bill (5-l0
percenT overall slope, l50 m above sea level) was grazed or cuiTivaTed
excepT for several hecTares on The norTheasTern slepe where rock ouT-
croppings and Thin soils predominaTed. This depauperaTe porTion, on
which Hill PloT was locaTed, was undisTurbed during INTA's 20 years of
managemenT. CaTTle, preferring The surrounding areas of inTrcduced
pasTures, only infrequenle grazed on Hill PloT.

in conTrasT To The raTher homogeneous vegeTaTive cover and few
major species found in The CroveTTo PasTure, Hill PloT was quiTe
heTerogeneous, being covered by a correspondingly higher number of
dominanT planT species (Tables 3, 4). Dense paTches of Tall (l-2 m)

Paspalum quadrifarium, a common grass of low areas and The slopes of

 

The pampan sierras (Cabrera, I953, I968), covered abouT 4O percenT of

The area. The grasses PipTochaeTium monTevidense, P. bicolor, SeTaria

 

 

genlculafa, and The pineapple-like (vegeTaTively, noT reproducTively)

 

l4

umbellifer, Eryngium paniculaTum dominaTed The remainder.

 

Trapping Procedure and DaTa Recording

Sherman aluminum folding live-Traps (75 x 75 x 227 mm), baiTed
wiTh rolled oaTs, were seT wiThin an hour or Two of dusk and checked
by early morning. AfTer each 3-4 day Trapping period, The Traps were
washed in a deTergenT-disinfecTanT soluTion, Then rinsed and dried
before furTher use. During The colder monThs, ToileT paper was placed
In The Traps each nighT for nesTing maTerial. If The TemperaTure was
likely To approach freezing,addiTional exTernal coverings, consisTing
of one or Two cloThs draped over each Trap and Then a l5 x 25 cm
plywood sheeT seT on Top, were applied in an efforT To decrease Trap
morTaliTy. Such efforTs were especially necessary aT The CroveTTo
PasTure where evening TemperaTures were nearly always lower Than aT
The Hill PloT.

Traps conTaining animals were collecTed and broughT To a TenT
shelTer near each ploT for processing. Individuals were Toe-clipped,
and weighed in a known-weighT conTainer placed on a 500 g Ohaus spring
balance accuraTe To I.0 g. The Trap sTaTion, species, sex, individual
number, weighT, reproducTive condiTion, and presence of exTernal

parasiTes (rodenT boT flies, CuTerebridae) were recorded aT every

 

capTure. Concerning reproducTive condiTion, male mice were noTed as
scroTal or non-scroTal, females perforaTe or imperforaTe, and by
abdominal palpaTion, pregnanT or non-pregnanT. Embryos were counTed

by palpaTlon when They were of an easily deTecTable size. Age classes,
consisTing of juveniles, sub—adulTs, and adulTs, were designaTed,
based when possible on The characTerisTics of known—age laboraTory

sTock mainTained for The sTudy of comparaTive posTnaTal developmenT.

l5

Table 3. The common grasses and forbs of PloT l and Hill PloT. These
species marked wiih an * are characTerisTic of The AusTral
Pampan DisTricT, presenTed by Cabrera (I953, I968).

 

PLOT l HILL PLOT
Grasses
*Paspalum quadrifarium Lam.
*PipTochaeTium monTevidense (Spr.)
Parodi
TE, bicolor (Vahl.) Desv.
*STipa bonariensis Henr. eT_Parodi
S. papposa Nees

*Panicum bergii Arech.

Grasses
Paspalum elongaTum Griseb.
E. vaginaTum_Siv.
FesTuca arundinacea Shreb.
Forbs
Trifolium prafense L.
§pi|anThes STolenifera DC.
Slda lepresa (OrT.) K. Sch.

 

 

 

 

 

 

 

 

 

 

 

 

Phyla canescens (H.B.K.) Greene SeTaria geniEUTaTa (Lam.) Beauvois
Ambrosia Tenuifolia Spr. BouTeloua sp.
Cirsium vulgare (Savi) Airy—Shaw Forbs

 

Eryngium paniculaTum Cav. ei Dreeb.
Oxalis monTevidensis Prog.
Senecio—burchelliifisC.

STevia mulTiarisTaTa Spr.
ChapTaliE'sp.

Cirsium sp.

Hyppochoeris sp.

iephyranThes sp.

Solanum sp.

Physalis sp.

 

 

 

 

 

 

 

Table 4. PercenT cover found in PloT I and Hill PloT, summer i969-l970.

 

 

 

 

 

 

 

 

 

 

 

PLOT l HILL PLOT
Grasses Grasses
_Agrgpyron elongaTum ..... .29.6 Paspalum quadrifarium.....44.5
FesTuca arundinacea ...... l0.0 PipTochaeTium bicolor.....l2.4
Forbs P. monTevidense.....Tl....IO.2
Phyla canescens.... ..... . 4.l SeTaria geniculaTa........ l.l
CTTSium vulgare .......... 3.9 Forbs
Slda Ieprosa.. ......... .. 2.8 Eryngium paniculafum.......4.4
OThers ChapTalia Eb.............. 2.6
Misc. grasses and forbs...3 9 Cirsium sp................ l.2
LiTTer...................28.0 OThers
Bare ground..............l7 9 Misc. grasses and forbs... .

LlTTer......

.4 I
...OOOOOOOOOOOI405
Bare ground............... 5 0

 

l6

General weaTher condiTions and presence of predaTors were noTed.
PredaTor feces found on each ploT during The Trapping period were
collecTed for iaTer analyses. dammalian nomenclafure follows Cabrera
(I96l) excepT where more recenT revisions perTain (HershkoviTz, I959;
Massoia and Fornes, l967; Massoia ”i a_:, I968). Avian predaTors were
IdenTified according To Olrog (I959).

VegeTaTion Analysis

A 50 cm square wire frame was placed over undisTurbed vegeTaTion
aT a 50 cm disTance from 30 randomly chosen Trapping sTaTion sTakes.
An esTimaTe of The percenT vegeTaTive cover in PloT l and Hill PloT
was Then made (Table 4).

An analysis of The Hill PIoT cover during The summer of l970
varied IiTTle in resulTs from ThaT Taken in l969. A parTial analysis
of PIoT l during l969 indicaTes ThaT raTher dramaTic changes occurred
beTween The summers of l969 and i970. Much of The change, resulTing
in an approximaTely 50 percenT increase in ground liTTer cover and a

300 percenT increase in bare ground, is direchy aTTribuTable To an

increase in densiTy of The guinea pig, Cavia aperea, during This period.

 

More abouT oTher quaIiTaTive and quanTiTaTive changes which occurred

will be menTioned in a laTer secTion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I l I
I I PasTure I
I race .. ...—... .. _ .. ._ .. -___-__1'__.__
I . .
I N SCALE I
I ' L . '
I \ F l00 IIe-I-ers
I I i
I
' I Am” 8"} ________ | '
I
, i PLOT l EACTIVITYE
I I
u: 8 I ----- '
:05 :53! PLOT
«2 It | REMOVAL III
a. 25! PLOT l
i I CROVETTO PASTURE
I
I I
. Z
. i REMOVAL :E
I PLOT II ‘2:
I CL.
i I s
' E
I | .<
| I
' I
i ' .
I
.....I L - - ._ _. _. _ _. .. _ -J
....2. i

 

 

 

 

 

 

 

 

 

   

 

 

 

Figure 2. CroveTTo PasTure, showing The approximaTe locaTIons of

HIGHWAY 226

ploTs. PloT | Is divided inTo four quadranTs.

A

’II‘
he

sTudy

Figure 3. PloT l maTure growTh of Paspalum elongaTum, summer l968—I969.

 

Figure 4. Removal PloT I (righT) exclosure fence, Taken near The back
side of The PloT I, Removal PloT corner, summer l968-I969.

Major vegeTaTion is Paspalum elongaTum.

 

l9

 

Figure 3

 

Figure 4

Figure 5. PloT I, winTer l969. DominanT vegeTaTion is Paspalum elongaTum.

 

Figure 6. Removal PloT I, winTer l969.

2|

 

 

Figure 5

 

Figure 6

22

Figure 7. PloT l maTure growTh of Paspalum elongaTum, summer I969—

 

l970. NoTe deTerioraTion of dominanT vegeTaTion, and
increase in forbs, parTicularly The ThisTle, Cirsium.

PhoTo Taken from The same approximaTe locaTion as Figure 5.

Figure 8. Close-up Taken from The approximaTe area shown In Figure 7.

The cane debris covering The ground was produced by Cavia.

23

 

Figure 7

 

Figure 8

24

Figure 9. DiagramaTic map of Hill PloT. DoTs show pure sTands of

Paspalum quadrifarium, scaTTered paTTern ThaT of Eryneium

paniculaTum, and unTouched areas, mixed sTands of PipTochaeTium,

 

~ STIpa, and oTher grasses and forbs. The ploT is divided inTo

four quadranTs, named A, B, C, and D, sTarTing from The lower

IefT-hand corner and going clockwise.

25

 

 

 

 

 

 

 

 

 

 

 

SCALE

I2 MeTers

Figure 9

26

Figure I0. Hill PloT, summer I969-I970, showing Paspalum quadrifarium

 

To The lefT, Eryngjum paniculafum mosTIy in The background,

 

and PipTochaeTium and STipa, eTc. In The foreground.

 

Figure ll. Hill PloT, summer i969-l970, looking down The genTle slope.

PipTochaeTium, STipa, eTc. are in The foreground, Paspalum

 

quadrifarium in The background. Several kilomeTers from

 

This siTe and locaTed on The farmland in The background is

The CroveTTo PasTure.

27

 

Figure l0

 

Figure ll

RESULTS
PopulaTion DenslTy and STrucTure

EsTimaTion gj_populaTion densiTies

 

 

Random sampling was noT aTTained eiTher beTween The marked and
unmarked segmenTs of boTh populaTions or wiThin The marked segmenT of
each (see Leslie ej aj., I953, and Krebs, I966, for TheoreTical and
pracTicaI discussions). The alTernaTive meThod of populaTion esTimaTion,
based on a raTher inTensive live—Trapping program, Is a direcT enumera-
Tion, calculaTed by means of The MeThod B Grouping, given by Leslie
_I_§l: (I953).

The populaTion esTimaTes of A: §E§£§§.and Q, nigripes (Figures l2,
I3) ThroughouT The sTudy were highesT in PloT I. The 03 ruTilans, however,
mainTained slighTIy higher numbers on Hill PloT. The 9: nigripes
populaTion in PloT l, alThough iT increased dramaTicaliy during IaTe
summer and early fall l969, compleTely disappeared by The sTarT of The
following spring (afTer period 25), and never reappeared. Similarly,

A, EEEIEE.”9V9F reached Their previous year's level for equivalenT
periods in PloT i.

The percenT deviaTion (Table 5) of The ToTal caTch for each
Trapping period from The minimum number known alive esTimaTe is high,
regardless of The species or season. No clear differences were observed
due To These facTors, however, and IT is besT jusT To summarize The daTa
by noTIng ThaT The minimum number generally ranged from 30-50 percenT

higher Than The ToTaI caTch.

28

29

Table 5. PercenT deviaTion of The ToTaI caTch (firsT column) from The
minimum number known alive (middle column) for Akodon azarae,
Oryzomys nigripes, and OxymycTeris ruIIligi. CalculaTed as
(minimum number known all\e)lOOpM - I0()}T'

 

 

 

ToTal caTch

 

 

 

 

       

 

 

 

 

 

 

TRAP ORYZOLYS OXYIYCTERIS
FERIOD 3cm. A7120: NIFRIPLL __ we). ______
PLOT l HIIL PLOT PLOIS IC’lli NIJIEZ F LL23
l Dec. '68 43 43 0.0 2 2 0.0
2 " " 66 76 l5.2 2 2 0.0 3 4 33.3
3 Jan. '69 il9 l3l l0.l 5 5 0.0 3 5 66.7
4 " " 86 ll4 32.6 2 4 5 0.0
5 " " 8| l09 34.6 2 4 6 20.0
6 Feb. l23 I48 20.3 i 3 200.0 5 7 l6. 7
7 " " li8 I68 42.4 52 52 0 0 I 7 600.0 I 7 600. 0
8 Mar. I46 l92 3l.5 89 l02 i4.6 I3 l7 30.8 6 8 33.3
9 " " l29 I99 54.3 83 I2l 45.8 34 40 l7.6 6 I0 66.7
I0 Apr. I46 2l4 46.6 l03 I45 40.8 20 34 70.0 6 l0 66.7
II " " l27 I87 47.2 I03 I45 40.8 33 5| 54.5 8 i2 50.0
l2 May Ill I85 66.7 Ii6 l53 3|.9 63 74 l7.5 7 I3 85.7
I3 " " 84 I80* Il4.3 IOI I43 4I.6 46 64 39.l 7 l2 7|.4
l4 June Ill II79 6l.3 ll4 I42 24.6 22 54 l45.5 6 i3 II6.7
l5 V " 99 I62 63.6 I27 I60 26.0 4| 59 43.9 II l3 l8.2
I6 July 85 ISO 76.5 l2l l60 32.2 46 63 37.0 6 I2 I00.0
l7 " " 93 I46 57.0 Il2 I4I 25.9 53 59 ll.3 8 i4 75.0
l8 " " I09 I39 27.5 78 l28 64.I IS 22 46.7 l2 I6 33.3
I9 Aug. l07 I43 33.6 l00 l29 29.0 I7 20 Il7.6 I5 l7 l3.3
20 “ " l02 l27 24.5 79 l06 34.2 6 8 33.3 I| l5 36.4
2| SepT. 6| 83 36.l 63 90 42.9 3 5 66.7 l4 l7 2|.4
22 " " ~ 7I 94 32.4 70 83 I8.6 3 4 33.3 l3 I6 23.!
23 OCT. 8i 99 22.2 74 80 8.I 4 5 25.0 l5 i6 6.7
24 " " 75 97 29.3 66 74 l2.l 3 3 0.0 I4 l7 2|.4
25 Nov. 78 93 I9.2 36 42 l6.7 2 2 0.0 6 l4 l33.3
26 " " 59 78 32.2 3i 38 22.6 I l 0.0 8 l3 62.5
27 Dec. 57 69 2I.l I4 23 64.3 « 7 l2 7|.4
28 " " 40 52 30.0 I5 23 53.3 l5 l8 20.0
29 Jan '70 37 48 29.7 I2 2| 75.0 I3 I9 46.2
30 " " 30 47 56.7 I7 25 47.| 9 I8 I00.0
3| " " 40 53 32.5 24 30 25.0 l6 2| 3l.3
32 Feb. 35 55 57.I 28 33 l7.9 3 3 0.0 I7 22 29.4
33 " " 58 70 20.7 58 67 l5.5 2 2 0.0 l4 2i 50.0
34 Mar. 92 I04 I3.0 7l 97 36.6 l3 I9 46.2
35 " " 68 86 26.5 82 l03 25.6 I l 0.0 I7 24 4|.2
36 Apr. 77 77 0.0 86 86 0.0 4 4 0.0 I6 l6 0.0
* This percenT deviaTiIen is probably relaTed To a very heavy rainfall abouT

a week subs equenT To This period. STanding wafer remained in PloT I up To
This daTe and several days more.

 

3O

IiappabiliTy

 

The "TrappabiliTy" (Krebs 23.213’ l969) of The Three species
(Table 6) seems To vary wiTh The season, ploT and species involved.

Akodon azarae was less Trappable during high summer populaTion levels

 

and more Trappable aT low populaTion levels in spring (during The
SepT.—Dec. period when cover and food have been aT a premium). Hill
PloT A, a;g:ae_were sligthy more Trappable Than Those aT PloT l,
buT wheTher This is a funcTion of differences in densiTy or oTher
facTors is unknown.

The diel acTiviTy paTTerns of The Three species cerTainly affecTs

The TrappabiliTy of each. Akodon azarae acTiviTy is mainly crepuscular,

 

ThaT of Q. nigripes sTricTiy nocTurnal, and ThaT of g, ruTilans diurnal
(personal daTa). Since The Traps were opened in The laTe afTernoon, A:
azarae, in accordance wiTh Their densiTy, would enTer The Traps, Thus
excluding The oTher Two species somewhaT.

LimlTed quaniTies of Q, nigripes preclude inTerpreTing The
TrappabiliTy daTa, while daTa for O, ruTilans suggesT ThaT They were
mosT Trappabie on The Hill PloT. This perhaps was due To sligthy
lower A, azarae densiTies on The Hill PloT or The more dispersed
paTTern of habiTaTion of O, nigripes on The laTTer ploT, resulTing in
higher probabiliTies of finding an open Trap.

Rafe of increase

 

The raTe of increase flucTuaTed considerably from one Trapping
period and ploT To anoTher for each species (Table 7). ParT of This is
due To differing Trapping successes every period, while The major parT
reflecTs demographic changes in The populaTions. In A: azarae, The raTe

of Increase lasTed sligthy longer aT Hill PloT Than PloT I in summer

3|

 

 

 

0.Ne me N..k 05 -I- 0 --- 0 5.00 00. p.00 0mm 0mm. .L02-.CMs
0.0» 04 0.0a 0e 0.00 0 0.00. k 0.00 own n.0k 0M0 = .000-.+aa0
0.00 N0 «.me 00 «.0e he _ 0.00 00. 0.0m 40k 0.k0 00k = .+a00->.ss
0.0k 0e 0.N_ am 0.00 .0. 0.km 00m 0.00 000 0.00 50.. = mess-.La<
-- - 0.me 0N -- -- 0.0. 00 k.N0 0mm 000. .Lm.-.cms
deck 2 .qmch z .mmch z deck 2 .amck z deck 2

+0.1 ...: . +0.1 +o.a ...: _ +o.a +o.a ...: _ +o.a

 

 

 

 

 

 

 

 

mcm__+:L m_ce+oxs>xo

 

000_Lm_c wxsomxuo

 

emLmNm couox<

 

 

.cmam +mg+ ce>o 0>__m czocx 0005+ +0 zo+mo +c00ceaI
0mmL0>m 05+ 000 .mno_Lma mc_aamc+ x_m +0 m_m+o+ 05+ soc+ 008230 .00L:+amo m_mmkcm +0 “20 cmneac

 

 

_m+o+ 05+ 0. c0>_o .mcm__+ac mwcewmxsxmm 0:0 .mem.cm_c m>so~>co .mmcmwm couox< +0 >+___nmaamck .0 0.00%

32

Table 7. RaTe of increase for Akodon azarae, Oryzomys nigripes, and
QAymycTeris ruTilans, calculaTed as The percenT change in
populaTion sT2e from one Trapping period To The nexT, based
on The minimum number known alive.

 

 

 

 

_—

 

 

 

 

 

 

 

Akodon azarae Oryzomys OxymycTeris
IUiflLUEEi EEiJJEEEi
RaTe of Min. No. RaTe of Min. No. RaTe of Min. No.
Increase Alive Increase Alive increase Alive
loT Hill PloT Hill PloT Hill PloT Hill PloT Hill PloT Hill
I P Lof L P lei l Plnf 1 P101 4 Piaf I Lint
I Dec. '68 43 0 2
2 " " 77 76 200 2 I00 4
3 Jan. '69 72 I3I 250 5 25 5
4 " " -I3 ll4 — 60 2 0 5
5 " " - 4 I09 0 2 20 6
6 Feb. " 36 l48 50 3 l7 7
7 " " I4 I68 52 0 3 4 - 43 4 3
8 Mar. " I4 96 I92 I02 33 225 4 I3 - 25 67 3 5
9 " " 4 I9 I99 I2I 500 23 24 I6 0 40 3 7
l0 Apr. " 8 20 2l4 I45 _ 4 - 3I 23 II 0 0 3 7
II " " -I3 0 l87 I45 48 55 34 I7 33 I4 4 8
I2 May " - I 6 I85 I53 56 24 53 2| 25 0 5 8
l3 " " - 3 — 7 I80 I43 - I3 - I4 46 IB — 20 0 4 8
i4 June " 0 - I I79 I42 _ 20 - 6 37 I7 0 I3 4 9
l5 " " -l0 I3 I62 I60 l4 0 42 I7 0 o 4 9
I6 July " - 7 0 I50 I60 i0 0 46 I7 0 -II 4 8
I7 " " - 3 —I2 I46 I4I 0 _ 24 46 I3 50 0 6 8
I8 " " - 5 - 9 I39 I28 _ 65 - 54 I6 6 I7 l3 7 9
I9 Aug. " 3 I I43 I29 - I3 0 I4 6 I4 0 8 9
20 " " -ll -I8 l27 l06 _ 64 - 50 5 3 - I3 -ll 7 8
2| SepT. " -35 -I5 83 90 - 40 - 33 3 2 0 25 7 I0
22 " " I3 - 8 94 83 - 33 50 I 3 0 -IO 7 9
23 OCT. " 5 - 4 99 80 200 - 33 3 2 o 0 7 9
24 " " - 2 - 8 97 74 - 33 _ 50 2 I l4 0 8 9
25 Nov. " - 4 ~43 93 42 - 50 0 I l - 25 -II 6 8
26 " " -I6 —IO 78 38 .I00 0 0 I — I7 0 5 8
27 Dec. " -l2 -40 69 23 0 I00 0 0 40 -38 7 5
28 " " -25 o 52 23 0 o 0 0 7I 20 I2 6
29 Jan. '70 - 8 - 9 48 2| 0 0 0 0 25 -33 IS 4
30 " " - 2 I9 47 25 o 0 0 0 - 27 75 ll 7
3| " " I3 20 53 30 0 0 0 0 9 22 I2 9
32 Feb. " 4 IO 55 33 0 300 0 3 0 II I2 I0
33 " " 27 I03 70 67 o - 33 0 2 - 33 3o 8 I3
34 Mar. " 49 45 l04 97 0 .I00 0 O - I3 _ 8 7 I2
35 " " ~l7 6 86 I03 0 I00 0 I I4 33 8 I6
36 Apr. " -ll —l7 77 86 0 300 O 4 - 38 -3l 5 ll

 

 

 

 

 

33

I969. The summer I970 increase in boTh pepuIaTions apparenle was noT
as large as ThaT for The previous summer, lndicaTed by The very slow
and raTher laTe increase (see especially PloT I) The second summer,
followed by an earlier sTarT of The pepulaTion decrease.

Oryzomys nigripgs, alThough in generally low numbers, shows The

 

mosT varied raTe, parTicularly in PloT I. On The oTher hand, 0, ruTilans
remained The mosT sTabIe of The Three species. ConsisTenT wiTh Their
reproducTive habiTs, discussed laTer, These animals showed no definiTe
cycle of increases or decreases; raTher, small increases and decreases
could be expecTed aT any Time of The year. However, during The second
summer boTh field populaTions experienced a shorT buT definiTe period

of increase, followed by a decrease To more or less The previous level.

RecruiTmenT

 

RecruiTmenT (Table 8) varied in each of The Three species. The)

recruiTmenT raTe for A. azarae, based in percenT (number of new animals

 

recruiTed over The ToTal individuals capTured in each Three day
Trapping period), remained high (PloT I, periods 2—I0, 43.2 percenT)
unTll near The end of The firsT summer (breeding) season. WiTh few
excepTions, The winTer (non—breeding) season recruiTmenT was much lower
and averaged for boTh ploTs, periods II'24, only l3.5 percenT, and did
noT reach iTs former high levels unTil The beginning of The second
breeding season. AT ThaT Time (periods 28—36), recruiTmenT again
climbed To iTs former levels, averaging 40.3 percenT, ploTs pooled.
Much of This jump can be aTTribuTed To Hill PloT, as PloT I alone
averaged 28.8 percenT, much lower Than in The previous year.

Oryzomys nigripes and Q: ruTilags showed differenT Trends Than

 

ThaT found for A. azarae. The recruiTmenT raTe for 0. nigripes

om...

. 34

 

 

 

 

 

 

 

Table 8. RecruiTmenT of new individuals of Akodon azarae, Oryzomys nl—
gripes and OxymycTeris ruTilans lnTo PloT l and Hill PloT aT
each Trapping period, as based on Tofal capTured each period.

Oryzomys OxymycTeris
Akodon azarae nigripes ruTilans

'8 PloT l Hill PloT Pooled PloTs Pooled PloTs

I U) U) U) 0') U) (I) U) U)

0 -e ++ +- ++ -e ++ +- ++

Q .... c... .— c... .... c... .— c..-

-- I} Q): r- I) 0):) —° 3 (DD -—0 D 0):)

C. 0+ L UL I‘D-I- L UL (D-I- L UL. I'D-I- L. UL

8 88 8 88 88 8 88 88- 8 88 88- 8 88

F HQ c: am hi) m am EU a am hi) m am

I Dec I968 43 (43) loo 2 (2) l00
2 " " 66 (45) 68 2 (2) I00 3 (3) I00
3 Jan. I969 Il9 (77) 65 5 (5) IOO 3 (I) 33
4 " " 86 (42) 49 5 (I) 20
5 " " BI (25) 3i 5 (3) 60
6 Feb. " l23 (66) 54 I (I) loo 6 (3) 50
7 " " ll8 (42) 36 52 (52) I00 I* (0) 0 l* (0) 0
8 Mar. " I46 (48) 33 89 (59) 66 I3 (ll) 85 6 (2) 33
9 " " l29 (27) 2| 83 (4|) 49 34 (29) 85 6 (2) 33

I0 Apr. " I46 (47) 32 l03 (39) 38 20 (I6) 80 6 (I) I7

ll " " I27 (29) 23 I03 (25) 24 33 (20) 6i 8 (2) 25

l2 May " IIl (I2) ll ll7 (27) 23 63 (34) 54 7 (2) 29

I3 " " 84 (I2) l4 I0l (I2) l2 46 (2|) 46 7 (0) 0

l4 June " Ill (9) 8 ll4 (II) IO 22 (6) 27 6 (I) l7

l5 " " 99 (8) 8 l27 (32) 25 4| (IO) 24 ll (0) 0

l6 July " 84 (7) 8 l2l (23) I9 46 (I6) 35 6 (0) 0

I7 " " 92 (6) 7 ll2 (2) 2 53 (I9) 36 8 (2) 25

IB " " I09 (7) 6 78 (I) l l5 (5) 33 I2 (2) l7

l9 Aug. " l07 (I5) l4 I00 (I2) l2 l7 (8) 47 I5 (2) I3

20 " " l02 (l5) IS 79 (5) 6 6 (O) 0 II (0) 0

2| SepT. " 6| (I8) 30 63 (6) I0 3 (2) 67 I4 (4) 29

22 " " 7| (2|) 30 70 (4) 6 3 (l) 33 I3 (2) I5

23 OcT. " 8i (l4) I7 74 (7) 9 4 (2) 50 IS (I) 7

24 " " 75 (8) II 66 (II) l7 3 (l) 33 I4 (3) 2I

25 Nov. " 78 (6) 8 36 (6) l7 2 (l) 50 6 (0) 0

26 " " 59 (3) 5 32 (7) 22 l (I) IOO 8 (0) 0

27 Dec. " 57 (5) 9 l4 (l) 7 7 (3) 43

28 " " 40 (5) l3 l5 (7) 47 I5 (7) 47

29 Jan. I970 37 (IO) 27 l2 (6) 50 I3 (7) 54

30 " " 30 (9) 30 I7 (8) 47 9 (3) 33

3| " " 40 (l4) 35 24 (I3) 54 I6 (6) 38

32 Feb. " 35 (l0) 29 28 (l3) 46 3 (3) I00 I7 (5) 29

33 " " 58 (23) 40 58 (44) 76 2 (2) l00 I4 (5) 36

34 Mar. " 92 (45) 49 7| (45) 63 I3 (3) 23

35 " " 68 (IS) 22 82 (39) 48 l (I) IOO l7 (8) 47

36 Apr. " 77 (ll) l4 86 (32) 37 4 (4) I00 l6 (2) I3

 

 

 

 

 

*Correcfed To accounT for firsT Trapping period In Hill PloT.

 

35

 

 

 

 

 

 

 

 

 

 

Table 9. RecruiTmenT of new individuals of Akodon azarae, Oryzomys ni-
gripes and OxymycTeris ruTilans lnTo PloT l and HiIT'PIOT aT
- -each Trapping period as based on minimum number known alive.
Oryzomys OxymycTeris
Akodon azarae nigripes ruTilans
‘8 PloT I Hill PloT Pooled PloTs Pooled PloTs
: 0 U) U) 0 U) U) 0 U) U) 0 U) U)
0 o + ++ o + +4- 0 + ++- p + ++
o. 2: ._ c:— 2: -— Cu- 2: .- o— 2: .- Cw-
‘ 3 (DD -° :3 (DD 3 (DZ) :3 (DD
8 88 8 88 as 8 88 .28 8 88 .28. 8 88
L ”-(D (I) (DO) °—'(U (I) (DO) --(U 0) (DO) --(U (1) 00)
F 20 m am 5:) 0: am 20 a em :1) a em
I Dec i968 43 (43) I00 2 (2) loo
2 " " 76 (45) 59 2 (2) i00 4 (3) 75
3 Jan I969 l3l (77) 59 5 (5) l00 5 (I) 20
4 " " Il4 (42) 37 2 (0) O 5 (l) 20
5 " " l09 (25) 23 2 (0) 0 6 (3) 50
6 Feb. " I48 (66) 45 3 (I) 33 7 (3) 43
7 " " l68 (42) 25 52 (52) loo 7* (0) 0 7* (0) 0
8 Mar. " I92 (48) 25 i02 (59) 58 I7 (II) 65 8 (2) 25
9 " " l99 (27) I4 l2l (4|) 34 40 (29) 73 IO (2) 20
IO Apr. " 2l4 (47) 22 l45 (39) 27 34 (I6) 47 ID (I) I0
ll " " i87 (29) I6 I45 (25) l7 SI (20) 39 l2 (2) I7
l2 May " I85 (l2) 6 I53 (27) l8 74 (34) 46 I3 (2) l5
l3 " " I80 (I2) 7 I43 (I2) 8 64 (2|) 33 l2 (0) 0
l4 June " l79 (9) 5 I42 (II) 8 54 (6) ll l3 (I) 8
I5 " " I62 (8) 5 I60 (32) 20 59 (I0) I7 l3 (0) 0
l6 July " ISO (7) 5 I60 (23) I4 63 (i6) 25 l2 (0) 0
I7 " " l46 (6) 4 I4I (2) I 59 (l9) 32 I4 (2) I4
l8 " " I39 (7) 5 l28 (I) l 22 (5) 23 I6 (2) I3
I9 Aug. " I43 (l5) l0 l29 (I2) 9 20 (8) 40 I7 (2) l2
20 " " I27 (I5) l2 l06 (5) 5 8 (0) 0 i5 (0) 0
2i SepT. " 83 (I8) 22 90 (6) 7 5 (2) 4O l7 (4) 24
22 " " 94 (2|) 22 83 (4) 5 4 (I) 25 l6 (2) I3
23 OcT. " 99 (I4) I4 80 (7) 9 5 (2) 40 I6 (I) 6
24 " " 97 (8) 8 74 (II) l5 3 (l) 33 I7 (3) i8
25 Nov. " 93 (6) 6 42 (6) I4 2 (I) 50 I4 (0) 0
26 " " 78 (3) 4 38 (7) l8 l (I) I00 I3 (0) 0
27 Dec. " 69 (5) 7 23 (I) 4 I2 (3) 25
28 " " 52 (5) I0 23 (7) 30 l8 (7) 39
29 Jan. i970 48 (IO) 2| 2| (6) 29 i9 (7) 37
30 " " 47 (9) I9 25 (8) 32 IB (3) I7
3| " " 53 (I4) 26 30 (I3) 43 2| (6) 29
32 Feb. " 55 (I0) I8 33 (I3) 39 3 (3) i00 22 (5) 23
33 " " 70 (23) 33 67 (44) 66 2 (2) IOO 2| (5) 24
34 Mar. " l04 (45) 43 97 (45) 46 I9 (3) I6
35 " " 86 (i5) i7 l03 (39) 38 l (I) IOO 24 (8) 47
36 Apr. " 77 (ll) I4 86 (32) 37 4 (4) I00 i6 (2) l3
,

 

 

 

*CorrecTed To accounT for firsT Trapping period in Hill PloT.

 

36

remained very high aT all seasons, averaging 6|.O percenT for periods
l—36, ploTs pooled (daTa correcTed To accounT for firsT Trapping period
of each ploT). The Q, ruTilans daTa, examined in The same manner,
presenTed a more sTable condiTion Than The former species, averaging
25.9 percenT. AlThough reproducTion was found To occur in This species
ThroughouT The year, There was a high recruiTmenT for The Two summer
seasons (average, 38.4, 30.3 percenT, respecTively) and less Than half
This during The winTer season (average, l4.l percenT).

If The recruiTmenT raTe is based on The minimum number of known
alive insTead of The ToTal individuals capTured, The resulTs, alThough
essenTially The same, show lower and less variable raTes (Table 9). Then
recruiTmenT for 5, agarae_for The firsT breeding season dropped from
43.2 To 34.3 percenT; for The non-breeding season, boTh ploTs pooled,
formerly l3.5, now 9.9 percenT, and for The second breeding season,
from 40.3 To 3|.2 percenT. Hill PloT, as before, conTribuTed To much of
The laTTer since PloT l averaged only 22.3 percenT.

RecruiTmenT raTes for Q: nigripes now averaged 47.3 insTead of
6I.O percenT for periods l-36, ploTs pooled. For 9, ruTilans during The
same period, recruiTmenT dropped from 25.9 To 2I.5 percenT. The Two
summer seasons were now 29.2 and 27.2 insTead of 38.4 and 30.3 percenT,
respecTively, and The winTer season raTe dropped To I0.0 from l4.l
percenT.

Sex raTios

 

Sex raTios of A: azarae born in capTiviTy do noT deviaTe sTaTisTI-
cally from The expecTed l:l (Appendix A). LaboraTory neonaTe daTa for
.9; nigripes and Q; ruTilans were meager, since few were sexed aT birTh,

and laTer deaTh precluded obTalning These daTa. This secTion examines,

37

Through several meThods, The changes in sex raTios which occur under
field condiTions. Field daTa were firsT TabuIaTed for The Three species
To deTermine The percenTage of females found in each Trapping period
(Table IO).

Sex raTios in A, azarae favor a higher preporTion of females during

 

The breeding season, alThough noT necessarily over .500. During The
firsT breeding season (periods l—IO) in PloT I, an average 54.6 percenT
of The mice were females. During The second breeding season (periods 25-
34), The percenTages were 48.9 and 5|.6 percenT, respecTively, for PloT l
and Hill PloT. In conTrasT, The non-breeding season (periods ll-24)
percenTages averaged lower for The Two ploTs, 45.6 and 43.| percenT,
respecTiver. Sex raTio daTa (pooled ploTs) for 9, nigripes and 0,
ruTilans showed ThaT female 9, nigripes_were consisTenTIy below 50.0
percenT (average, 28.4) while female 9: ruTilans averaged higher, or
59.2 percenT.

Goodness of fiT X2 analyses, according To age class, season, and
sTaTus (residenT or recruiT), were also compIeTed for A: azarae (Table
II). Since This species was snap—Trapped in reasonable numbers in oTher
areas of CroveTTo PasTure, These daTa also are included. EiTher because
of small sample sizes, and/or The non-seasonaliTy of reproducTion,
similar daTa (Table l2) for 9, nigripes and 9, ruTilans are less
compleTe.

The sTaTus of an individual was deTermined as defined by Meyers
and Krebs (l97l). Specifically, residenT mice were Those capTured
during any one Three—day Trapping period. Originally, however, They
had been firsT capTured in a previous Trapping period. These mice, Then,

have been ploT residenTs for a minimum of 9-I4 days, The normal Time

38

Table l0. PreporTion of female Akodon azarae, gfyzomys nigripes,and
.‘QxymycTeris ruTilans capTured in each Trapping period.

 

 

 

 

 

 

 

Trap A. azarae _O__. nigripes (_)_. ruTilans
Period PloT I Hill PloT PloTs Pooled
l Dec. l968 .5I2 — — .000
2 " " .606 - .500 .667
3 Jan. I969 .664 - .333 .667
4 " " - .686 - - .600
5 " " .5l9 - ~ .800
6 Feb. " .5l2 — .000 .333
7 " " .44! .250 .400 .500
8 Mar. " .500 .348 .I54 .500
9 " " .488 .325 .265 .667
ID Apr. " .534 .456 .400 .667
II " " .4I7** .476** .242 .625
l2 day " .45|** .393** .349 .57I
l3 " " .464** .456** .370 .7l4
l4 June " .496** .447** .409 .667
I5 " " .495** .433** .463 .727
I6 July " .47|** .438** .4l3 .667
I7 " " .452** .384** .453 .625
l8 " " .459** .372** .267 .583
I9 Aug. " .505** .420** .4I2 .667
20 " " .5l0** .405** .500 .636
.2l SepT. " .4l0** .444** .000 .57l
22 " " .380** .47I** .333 .6I5
23 OcT. " .444** .446** .500 .533
24 " " .427** .455** - .000 .643
25 Nov. " .5l3 .360 .000 .500
26 " " .407 .406 .000 .625
27 Dec. " .42I .57I - .857
28 " " .500 .533 - .733
29 Jan. I970 .460 .667 - .462
30 " " .533 .765 - .444
3| " " .550 .542 - .563
32 Feb. " .457 .60I .667 .588
33 " " .552 .362 .000 .7l4
34 Mar. " .500 .352 - .538
35 " " .44l** .427** .000 .47l
36 Apr. " .494** .430** .250 .563

 

**lndicaTes

non-breeding season for A. azarae.

 

39

 

 

 

 

 

 

 

 

 

 

 

.commom mc_noecn m:o_>eLa +0 +Lma +mm_ 508+ m_me_cm mmu:_oc.*
moo.V8 0..88 08.88 o..88 om.w8 om.88 o_.88
.086 .mec .mc Ammo .8V ..V .0c m+c00.888
808. 888. 0mm. mvm. mmm. mmm. |wmo. . +0.8
mo.w8xo..
“mm. --- --- --- --- --- --- 8+808.888
mm0. +0.8X848:
m00.va 0..A8 0..A8 0m.w8 om.w8
.0ms .m.c .m.v --- .m.c .N_c --- 8+.38008
mme. 8.8. 8.8. N04. N08. +0.8 ...:
moo.V8 o..x8 o..A8 0m.A8 om.w8
.800 .8.0 .0.v --- .N.c .8.0 II. .+880 ..<
008. 8N8. mme. N08. 8N0e. +0.8 ...:
.0.A8Ammo. m0.88~0.. om.mm .888+-8888.
.mmc .8.V --- --- .8V --- --- 8838888
|800. 80m. mme. 0++8>08o
o..x8 .0.A8Amwo. 0..A8 .0.A8Ammo. 0..A8 0m.88 80.8880_.
.m.e .0... .880 ..0c .mmc .m. .omc m+_:8088
8mm. N88. mom. ||88m. man. 888. mm0. _ +0.8
moo.V8 .0.A8Ammo. 0..A8 mNo.Aaxmo. o..A8 0..A8 muo.w8xmo.
.mmc .00.c .omc .om.c .046 .06 .08. .+880 ..<
0m8. mem. 8mm. 08m. 088. 008. mm0. . +0.8
commmm Um _OOn_ cOmmmw cOmmow Um_OOn. cOmmom COmmmm
mc.umm8m chmmmm mc_vmmLm mc_vmoem mcommmm m:_nmmcm mc_nmecm
icoz mc_neeLm ecoomm +m8_m mc_nomcm ncoomm +m8_m
messo<mam mm..zm>:s

 

 

.cm>_m em_m 08m mo_+m8 xmm _
.i__nmnoLa new Amommc+cmLma cmv mmN_m o_asmm e_memm

umem 0cm .mmmmm_o Am+cmn_moc .m+_:LoeLv mz+m+m vcm 0mm m:o_Lm> +0 omcmum .< m_oeo+ +0 co_+LoaoLm

 

_ +0 memec+oQ>s ..zc 80+ mo_+_
.+o_a ___I can mca+mmm.m++e>080 c. n::0+ mcom

... 0.90%

 

4O

.e__ce>:a oco mm0:_oc_**
.mL:+amo m:o_>080 +0 eocmco o: euc_m 0003.00. +0: £o+mo mxmez +0888 *

 

 

 

 

 

 

 

 

 

 

800.V8 880.88880. 08.88 .0.888880. 800.V8 80.8880.. 08.88 .0.888880.
.8.8. .8.8. .008. .8.8. .008. .808. .80.. ..88. 0+800.008
.88. 888. 808. 888. .88. 888. 808. 8888. . 80.8
800.88 08.88 08.88 800.V8 08.88 08.88
.808. .08. .08. --- .888. .08. .08. -.. 0+c00.008
008. 008. 008. 008. 008. 008. II80.8|8..:
0..88 .0.888880. .0.888880. 0..88 880.88880. 880.88880.
.80. .08. .08. --- .88. .08. .08. --- 0+.08008
.88. 008. 008. 888. 808. 808. +0.8 ..88
800.88 0..88 0..88 800.V8 0..88 0..88
.008. .08.. .08.. --- .008. .8... .8... --- .+800 ..8
.88. 088. 088. 0.8. 008. 008. +0.8 .8.0
om.80 0..80 oo._ua 0..80. o_.Aa om.Aa om.Aa om.Aq 8008+iamcm.
.08.. .08. ..8. .88. .00. .88. .0.. .88. 080+008
,I808. 088. 008. 008. 888. .08. 8.8. 008. 0++0>000
08.88 08.88 0..88 0..88 08.88 880.88880. 880.88880. 0..88
..0. .808. .08. .888. .08. .88.. .08. .8... 8+.08008
008. 0.8. 888. 088. 808. 8888 808. .808. 8 8888
800.V8 80.8880.. 08.88 800.888.0. 800.V8 08.88 08.88 0. 88
.800. .080. .088. ..88. .808. .880. .888. ..08. .+800 ..8
88088. 888. 808. 088. 888. 008. 808. 888. . +0.8
cOm mmm Um _ OOQ cOmmmm cOmmmm cOmmmm U0 _ OOn_ cOm 0mm GOmmmm
mc.0mmLm chmmmm mc.0emLm mc.vmeLm mc.nmmLm chmmem mc_umm8m mc_nmeLm
IcOZ m: _ UmmLm Ucoomm .5... _u_ IcOZ mc _ vomLm ucoomm +9.. 7..
8008 .08 08.008

 

 

 

..00sc.+0008 .. 0.008

4|

beTween The end of one Trapping period and The sTarT of anoTher. Re-
cruiTs were Those capTured on The grid for The firsT Time during any
one of The Three—day Trapping periods. Meyers and Krebs have sTaTed
ThaT residenT sex raTios are indicaTive of The effecTive sex raTio of a
more or less sTable "residenT" populaTion, while recruiTs give an
esTimaTe of The poTenTial on which selecTion can acT To arrive aT
residenT sex raTios.

On examinaTion of Table II, juvenile A, azarae did noT differ
significanle from The expecTed l:l sex raTio excepT aT one period. Sub—
aduITs were somewhaT mixed in sex raTios wiTh significanT Trends (p4.05)
sTrongesT during The non—breeding season, where a high proporTion of
females were favored. Very likely a major conTribuTing facTor for This
phenomenon is The facT ThaT adulT female weighTs average less Than males,
resulTing in a greaTer number of females falling inTo The subadulT class.
This is reflecTed in The aduiT class where suddenly The proporTion of
females during The non—breeding season was less Than .500 in all cases.
OTher Than wiTh non—breeding season adulTs, in all buT Two cases breed-
ing season females were noT significanle differenT from The expecTed
lzl sex raTio. When all ages are pooled, The resulTs, as before, suggesT
ThaT females during The breeding season consTiTuTe a larger parT of The
Trappable pepulaTion Than during The non—breeding season. This Trend was
especially influenced by residenT raTher Than recruiTed animals.

The generalizaTions menTioned above for A: azarae hold True for
boTh sTudy siTes, wiTh The excepTion of The snap—Trap resulTs. When
differences did occur of a raTher large magniTude, iT could be
aTTribuTed To small sample sizes (933., PloT | subaduIT all capTures,

firsT breeding season effecT on pooled breeding season resulTs).

42

The significance of The snap-Trap, live—Trap dispariTy should be ex-
amined more closely in The fuTure. Yang gi_ai. (I970) found no differ-
ences beTween sexes when Museum Special snap—Traps were TesTed againsT
LongsworTh (prebaiTed) live—Trapping of Indiana MicroTus populaTions.

PloT l and Hill PloT sex raTio daTa were pooled for Q. nigripes
and 9, ruTilans To increase The sample size and Then analyzed (Table l2).
The resulTs indicaTe ThaT There were no significanT differences in The
expecTed lzl sex raTio of The juvenile-subadulT groups. However, The
proporTion of adulT female 9, nigripes was significanle lower Than
expecTed, and This resulT held even when all age classes were pooled.

The same Type of analysis for adulT females of Q: ruTilans showed jusT

The opposiTe since adulT females represenTed a significanle higher

proporTion of The Trappable populaTion Than expecTed. When sex raTios

of recruiTs were compared To Those of residenTs in The same Table,

9, nigripes residenTs and recruiTs boTh showed significanle fewer

females Than expecTed. On The oTher hand, 9, ruTilans recruiTs did noT

differ significanle from The expecTed l:l raTio, buT The residenTs did.
ReproducTion

AlThough seasonal reproducTive changes in a mouse populaTion are
besT analyzed by an inTensive snap—Trapping and auTopsy program, iT also
necessiTaTes sacrificing animals from The communiTy under sTudy or aT
leasT from an area in close proximiTy.

The reproducTive field daTa presenTed here, Therefore, are based on
finger palpaTion (Godfrey, I953; Ranson, l94l) of sexually maTure females.
This Technique is criTlcized for being inaccuraTe, and possibly TraumaTic
To boTh The embryos and The female. WiTh pracTice, however, palpaTion can

be considered very reliable, excepT when a large number of embryos may

 

.1 ‘I‘ll I] I'll! II III III.... II .. l

.43

 

 

 

 

 

 

800.v8 800.v8
.80. ---- ..8.. ---- .00.008 0+0.8.
.08. 8.0. 0+:00.008
800 .v8 0. .88
.08. ---- .88. ---- .00.008 0+0.8.
888. 888. 0+.08008
800.88 0..88 800.88 0..88
.80.. .8. .008. .08. .00.008 0+0.8.
008. 888. 808. 800. 0080+800 ..<
00.000 00.000 00.000 00.000
80880.0 0m< <m + .>:0 00880.0 0m< <m + .>:0

 

80.3.5 .0 8.88.0850

 

mcm_.+:L 8.80+0x&>xo

 

 

 

 

.mo.+mc xmm
.8. +0 80808+00>£ ..zc 80+ >+...000080 .>_+mm. ucm ..mommc+cmcma 0..
808.8 0.0808 .me.mse+ +0 co.+800080 c+.3 czocm .00.000 +0.0 ...I new
. +0.0 .mcm..+:L m.Le+0>s>xo 0:0 mem.cm.c m>aowxco c0 m+mn 0.+mL xom .N. 0.008

44

make accuraTe counTing difficulT. If The embryos are small and easily
discernible from each oTher, palpaTion appears To have liTTIe effecT on
The developmenT of The embryos or The healTh of The female.

WhaT is known abouT The reproducTive biology of A. azarae, 9.
[flifljifiii' and Q. ruTilans has been gaThered To daTe Through snap-
Trapping sTudies (Crespo, I966, I970; Barlow, I969). More informaTion,
gaThered under laboraTory and field condiTions, is necessary To fully
undersTand The reproducTive paTTerns of These species. Since liTTle has
been previously published on reproducTive periods, liTTer sizes, posT-
naTal developmenT, eTc., These daTa are presenTed in Appendix A.

WeighT gi_raproducTive animals

 

In order To express reproducTive acTiviTy as a funcTion of weighT,
iT is imperaTive To remember several poinTs. FirsT, if old animals are
The sole members of The Trappable populaTion during spring and early
summer, They will Tend To make weighT minimums high. Second, weighTs
of reproducTive animals midway in The season may be low because of The
large proporTion of young individuals. Third, young born laTe in The
reproducTive season will aTTain The weighTs commonly found for repro-
ducTive animals, buT may noT develop sexually unTil The following
season.

The average weighT of reproducTive animals for each Trapping
period was esTablished by summing The weighTs of all animals showing
a parTicular sexual TraiT (e.g., scroTal TesTes). Depending on The Type
of analyses involved, a number of periods were grouped To give average
weighTs. By doing so, iT was possible for an animal To be recorded more
Than once, alThough The populaTion's average weighT should noT be far

from ThaT expecTed if one sampled from The middle of The grouped

45

Figure l2. Minimum number esTimaTes of Akodon azarae in PloT | and

 

Hill PloT.

 

 

/‘
I/
\’
\\\\
\
\\\
W
J
/
K
/J
,
l’
)
/
93 x
3 K
0 A
O I
I /
/
F‘
53
\\ __ O.
I- .1
o .J
__1 ..
. 0- 0
< I r
l
L.
\\
\
\
\
\\
‘\\\
N
4.4 illiilgiLi lvll L
O O O C) o C) o o O O O
a 9 t if) '2 : 0‘ “ L“ m '-

UBIIL’IHN l‘ll'l."l l N I H

Pox udv
u H
v JET/l
m
H II
N 'qed
m
H H
0 0L; UQF
m
H U
00 '08:]
N
II II
\o AON
N
II II
a '+OO
U H
g °+des
II N
o 'an 8
N .—
(r
H II IJJ
a.
29 MM 92
H N a:
a.
:9 ATM ,9?

l4
0)
c
:1
.3

g flew
II II
9 'Jdv
H ll
00 'JGW
II N
‘0 ° Clad
H II
V 69. 'U9 F
II
II
N 89A .390

 

2' ..
(”
\\\ "‘
\\\\\ '1
,1 1
/
,1 .1
r//
r '4
q
) ~
/ '4
N (I
E (
:3 A -
(D I
I 4
/
[...
3 _.
\\ — £1.
1-— ...J ‘
o _1
—J — a-I
, 0- :9
< 1 t ‘
1 .1
.J
\x
\ J
\
\
\N“\\ -
\N.
N .1
.1
d
.1
J _1 1 1 1 1 1 1 1 1 1 1 J 1 1 L
O O O O C.) ’) O O O O O
a g: t 12 rd 0\ 1\ Ln M '—

no.

711113115" L'Jfl.'J l N IH

 

'9‘ 'Jdv
N H
v ’JGN
M
N H
N 'qad
M
II M
0 CL: 'U91‘
[1‘
H H
a) '38:]
N
II II
@- 'AON
N
H It
<31 ‘43:)
II II
a: was
II H
O °6nv 8
N .—
a:
II II LLJ
0.
3 AIM £2
11 u E
0.
\o A nr 5
_ | ...
H II
3 aunr
H II
2 Aew
II ll
9 udv
H u
00 “JEN
H H
‘0 ’qed
H N
v 691'U9F
II
N
N 891.330

47

Figure l3. Minimum number esTimaies of Oryzomys nigriEes (Top) and

OxymyciOfiig ruTilans (boifom) in PloT l and Hill PloT.

 

I3

FIGURE

48

HILL PLOT
-f-..1

*-* PLOT l

*4

 

l 1 t

 

 

 

 

m1
UWTHHN HHNININ

35 r
5
I5 -

L
N

(‘4

34

32

30

28

26

24

22

20

I8

I6

I4

I2

IO

696|
H

896I

'Jdv
ll

'JQW

'qad

°J9w
ll

'QGJ
ll

-uer

'Deg

TRAPPING PERIOD

' I. ill! ||nllv ll" I If} IIIII Ill-II'II: III-II

49
collecTing period. In order To deTermine if acTively reproducing females
weighed slgnificanle differenT from oThers merely perforaTe, weighTs
for The former were Taken, when possible, wiThin one or Two periods
(someTimes more if The animal was an adulT whose weighT appeared To
change liTTle Through Time) before or afTer The pregnanT recording.

In Appendix A, weighTs aT which laboraTory reproducTive maTuriTy
occurred for each of The Three species are given. In review, adulT A,
azarae were Those weighing l8 9 or more, for Q. nigripes, II g or more,
and for 9:.EflilLEEE’ 48 g or more. However, examinaTion of The daTa
showed ThaT very few of The animals in The field reached reproducTive
condiTion in The same weighT class as deTermined for The laboraTory
colony.

Whereas sexual maTuriTy may resulT aT a relaTively early age
(lighT weighT), older (heavier) animals are The mosT reproducTively
acTive. The average weighT found for scroTal fl. azarae in PloT I was
32.5 i 5.5 g (N=258) wiTh a range of l7—45, and for Hill PloT, 33.7 i
6.7 g (N=60) wiTh a range of l6-45 g. PerforaTe A. azarae females for
PloT l averaged 24.2 i 5.2 g (N=227) wiTh a range of lO-4O g, while
Hill PloT females averaged 24.9 i 6.9 g (N=45) wiTh a range of l5-42 9.
Females which had experienced recenT pregnancies averaged several grams
heavier Than perforaTe individuals. PloT l females averaged 26.7 i
3.7 g (N=l27) wiTh a range of l5-37 9, while Those from Hill PloT
weighed 27.4 i 4.8 g (N=36), wiTh a range of l9~36 9. There were no
significanT differences (p>.05) in The means of like sex beTween sTudy

siTes. Similarly, male—male and female-female mean comparisons wiThin

PloT I for The Two reproducTive periods (I-6; The second period was

sligthy longer, 24—32, To increase sample size) showed no significanT

50

differences. When The mean weighTs of scroTal males were compared To
Those of perforaTe or recenle pregnanT females, The former were found
To be significanle heavier (p<.05), alThough boTh seTs of females were
noT significanle differenT from each oTher.

The reproducTive fl. azarae in The field siTes were obviously much
larger, heavier and older Than The I2—week—old animals from The
laboraTory (for lab daTa, see Table A—3). iice snap-Trapped in surround-
ing areas showed ThaT male A, azarae (N=57) in The 3l-34 g (near The
average weighT found for scroTal males in live—Trapped ploTs) weighT
class averaged (To nearesT mm), in body measuremenTs: ToTal lengTh,
l85; Tail lengTh, 74; head and body lengTh, Ill; fooT lengTh, 2|; and
ear lengTh, l3. For females (N=65) in The 23—26 g weighT class, These
measuremenTs respecTively are l7l, 69, iO3, 20 and I3.

The smaller populaTions of Q: nigripes and g. ruTilans did noT
lend Themselves well To some of The above analyses. For The former
Species, scroTai males (N=26, boTh ploTs pooled) weighed 2|.2 i 4.0 g
wiTh a range or l3-27 9 while perforaTe females (N=8, boTh ploTs pooled,
and including four capTured elsewhere on CroveTTo Field) weighed i7.6 i
4.4 g wiTh a range of 9-22 9. No weighTs for pregnanT mice were recorded.
in conTrasT To The significanT differences in average weighTs beTween
scroTal and perforaTe A, azarae, no difference (p7.05) was found in
9. nigripes.

ScroTal males of 9, ruTilans, pooled from boTh sTudy siTes, weighed
92.4.i |4.i g (N=82), wiTh a range of 62-I25 9 while perforaTe females
were less, 76.2 i l6.0 g (N=39), wiTh a range of 46-il0 9. However, This
difference was noT significanT aT The .05 level, nor was There any

difference beTween The samples from PloT l and Hill PloT. Pooled females

51

which were known To have been pregnanT averaged heavier (85.8 i.l3.9 g)
Than perforaTe females, buT again This difference was noT significanT.

As wiTh The reproducTive fl, azarae, reproducTively acTive 93
nigr'pes and 9. ruTilans in The field siTes were obviously much larger
and older Than The |2-week—old animals from The laboraTory (for lab daTa,

see Table A—4). Oryzomys nigripes snap—Trapped in surrounding areas show

 

ThaT males (N=6) in The 20-23 g weighT class average (To nearesT mm),
in body measuremenTs: ToTaI iengTh 2l6; Tail lengTh, li8; body lengTh,
97; fooT lengTh, 26; and ear lengTh, l3. Females (N=8) in The l6—l9 g

weighT class measure, respecTively, 204, |09, 94, 25, and I3. OxymycTeris

 

ruTilans males (N=ll, 3 wiTh incompleTe Tails) in The 85-99 g weighT
class averaged 256, IOO, l57, 30, and l8, while females (N=5, 4 wiTh
incompleTe Tails) in The 79-84 g class averaged 255, lOO, l58, 30, and
I7, respecTively.

LenoTh of breedino season
_—._¥.._.._.. H

 

The annual cycles of breeding for A, azarae, 9, nigripes and 9,
ruTilans were measured by presence or absence of females wiTh palpable-
sized embryos and/or perforaTe vaginas, and in The case of males, scroTal
TesTes.

Akodon azarae showed a definiTe reproducTive seasonal Trend

 

(Tables i3, i6, i7). The l968-l969 breeding season was underway before
The Two field siTes were esTablished. The presence of embryos, marking
The onseT of breeding The following season (I969-I970), was firsT noTed
on Trap period 25 (firsT week of November) in boTh ploTs. However,
perforaTe females were firsT deTecTed seven periods (buT noT To any
degree unTil Two periods) earlier in PloT l, and Two earlier in Hill

PloT. Scrofal males were firsT observed in boTh siTes Three periods

52

before The firsT embryos were deTecTed.

The firsT breeding season for A. azarae ended aT period l0 (9-l4
April, l969) for boTh siTes, as no embryos were found afTer This daTe.
The presence of scroTal males ended ThaT same period in PloT l, buT
conTinued for several more Trapping periods in Hill PloT. The lasT
embryos palpaTed in The second breeding season were found in period 36
(4-9 April, I970) and period 34 (l2—l4 March, l970) for Hill PloT. The

percenT of perforaTe females aT boTh sTudy siTes was much less Than

during The comparable period of The previous breeding season. A few

females were sTill pregnanT while The female populaTion as a whole was
ToTally imperforaTe. ScroTal males were sTill presenT in PloT l aT The
lasT Trapping period, while none was found in Hill PloT afTer period
34.

Embryo deTecTion Then, appears To be much more reliable Than
scroTal and vaginal changes. The approximaTe lengTh of The breeding
season for A. azarae in The Balcarce area exTended from The lasT of
OcTober and firsT week of November To The middle of April, a period of
abouT 5.5 monThs.

The number of Times a female can produce liTTers per breeding
season, and The conTribuTion made by The young of The year during ThaT
breeding season are difficulT parameTers of reproducTion To measure.
Some lnformaTion is supplied by daTa from The Hill PloT.

From November l969 To March l970 (Trapping periods 25-34), 28
females produced young 42 Times. NineTeen were found pregnanT only one
Time, eighT were pregnanT Two Times, one was found pregnanT Three Times,
and one was recorded pregnanT four Times. Based upon paipaTion daTa,

reproducTive animals gave berh To approximaTely 200 young during The

53

Table I3. InTensiTy of breeding ln Akodon azarae, PloT l and Hill PloT
(In parenTheses), as measured by The number of pregnanT females
deTermined by palpaTion, The percenTage of pregnancy of The
adulT females and all age class females, and The number of
liTTers and average liTTer size counTed by palpaTion.

 

 

NO.

TRAP N0. % ADULT Z TOTAL LITTERS AVG. LITTER
PERIOD REG. PPEG. LPREG. IVJIIIED, SIZE

I Dec. l968 9 52.9 40.9 9 4.2

2 " " l6 50.0 40.0 I4 4.7

3 Jan. l969 23 43.4 29.l 22 4.3

4 " " I0 28.6 |7.0 8 4.0

5 " " 8 29.6 l9.l 7 4.6

6 Feb. " 8 23.5 l2.7 5 3.2

7 " " I2(3) 30.0(60.0) 23.|(23.l) 8(3) 4.9(3.7)
8 Mar. " 3(3) 6.0(27.3) 4.l(9.7) l(|) 5.0(5.0)
9 " " 2(l) 4.3(4.6) 3.2(3.7) 2(I) 3.0(4.0)
l0 Apr. " l(l) l.5(3.l) I.3(2.l) l 3.0

I I H H

l2 May "

'3 H H

I4 June "

'5 n 11

I6 July "

'7 n w No Pregnancies DeTecTed Here.

l8 n n

l9 Aug. "
20 H H
2| SepT. "
22 n n
23 OcT. "
24 n n
25 Nov. " 4(2) l0.0(l5.4) l0.0(l5.4) 2(l) 5.0(4.0)
26 " " 5(5) 20.8(38.5) 20.8(38.5) |(4) 5.0(4.8)
27 Dec. " II(4) 45.8(50.0) 45.8(50.0) 6(2) 4.5(5.5)
28 " " 7(3) 35.0(42.9) 3|.8(37.5) 2(l) 4.5(6.0)
29 Jan. I970 4(2) 26.7(33.3) 23.5(25.0) 2(2) 3.5(4.0)
30 " " i(7) 8.3(53.9) 6.3(53.9) (4) (5.5)
3| " " II(3) 64.7(23.l) 50.0(23.|) 5(2) 3.8(5.5)
32 Feb. " 7(8) 46.7(57.I) 43.8(47.l) 3(3) 4.3(5.0)
33 " " 9(3) 36.0(30.0) 28.I(I4.3) 6(l) 3.8(5.0)
34 Mar. " 8(6) 25.8(35.3) l7.4(24.0) 3(2) 5.0(3.5)
35 " " 2 8.7 6.7 l I.O
36 Apr. " l 3.2 2.6

 

 

 

Table I4. InTensiTy of breeding in Orygggy§_nieripes, PloT | and Hill
PloT (daTa combined), as measured by The number of pregnanT
females deTermined by palpaTion, The percenTage of pregnancy
of The adulT females and all age class females, and The
number of IiTTers as counTed by palpaTien. '

 

 

 

.TRAP NO. % ADULT* % TOTAL NO. LITTERS AVG. LITTER
PERIOD PREG. PREG. PREG. PALPATED SIZE
I Dec. '68

2 I! ll

3 Jan. '69 I 50.0 50.0 I 4.0
4 H H

5 n n

6 Feb. "

7 H " 2 100.0 100.0 2 4.0
8 Mar. " I 50.0 50.0

9 " " I l2.5 lI.|

l0 Apr. " I l6.7 ’ l4.3 I 6.0
II " " I l6.7 l6.7

I2 May " 2 ll.8 ll.8 I 3.0
'3 I! H

I4 Jun. "

'5 H II

I6 July "
[7 n n

'8 H H

I9 Aug. "
20 " " I 50.0 50.0 I l.0
2| SepT. "
22 I! II
23 OcT. "
24 u u
25 Nov. " LasT week Oryzomys nigripes recorded for PloT l.
26 H II
27 Dec. "
28 n n
29 Jan. '70
30 u 11
3| 1! n
32 Feb " 2 l00.0 I00.0 I 3.0
33 H H
34 Mar "
35 II N
36 Apr. "

 

 

YNe subadulTs were found pregnanT.

55

Table l5. InTensiTy of breeding in OxymycTeris ruTilans, PloT l and Hill
PloT (daTa combined), as measured by The number of pregnanT
females deTermined by palpaTion, The percenTage of pregnancy
of The adulT females and all age class females,and The number
of IiTTers and average liTTer size as counTed by_palpaTien.

 

 

 

 

 

 

TRAP NO. % ADULT* % TOTAL NO. LITTERS AVG. LITTER
PERIOD PREG. PREG. PREG. PALPATED SIZE
l Dec. '68
2 " H 2 IO0.00 100.0 2 3.0
3 Jan. '69 I 50.00 50.0 I 4.0
4 " " 2 66.7 66.7 2 4.5
5 " " I 50.0 33.3
6 Feb. "
7 I! II
8 Mar. "
9 H II
l0 Apr. " I 25.0 25.0 I 4.0
| I I! H
I2 May " 2 50.0 50.0 2 3.0
'3 H H
l4 June "
I5 " " I 50.0 50.0
I6 July "
l7 " " I 50.0 50.0 I 3.0
I8 " " I .33.3 33.3
l9 Aug. " 2 33.3 33.3 I 3.0
20 II II
2| SepT. "
22 " " 4 50.0 40.0 2 3.0
23 06+. " 2 40.0 40.0 I 3.0
24 " " I l6.7 l6.7 I 4.0
25 Nov. " 2 66.7 66.7 I 4.0
26 " " 2 40.0 40.0
27 Dec. " 3 75.0 42.9 I 3.0
28 " " 6 60.0 54.5 3 3.7
29 Jan. '70 I 33.3 l6.7
30 " " I I00.0 33.3
3| " " 3 50.0 33.3 I 6.0
32 Feb. " 3 33.3 27.3
33 " " 3 42.9 30.0
34 Mar. " I 25.0 25.0 I 2.0
35 " " I 50.0 l2.5
36 Apr. " 2 33.3 28.6 I 2.0
¥Ne subaduITs were found pregnanT, and only Three females from Hill PloT

were discovered To be pregnanT.

56

5—6 monTh breeding season. PloT l A, azarae during This same season
produced abouT I30 young, compared To over 330 produced The high densiTy
season, noT ceunTing The breeding prior To early December when The sTudy

began.

Oryzomys nigripes did noT produce young unTil iaTer in The summer

 

season, Then remained in reproducTive condiTion during early fall
(Table I4, l8). No young were palpaTed unTil Trap period 3, (l-3 Jan.,
I969) during The firsT summer, and period 32 (II~I3 Feb., I970), The
second summer. ScreTaI males were common aT The onseT of The sTudy The
firsT summer and were firsT evidenT in period 26 (20-22 Nov., I969) of
The second summer. PerforaTe females were Too few To allow any conclu-
sions. The lasT of The season were palpaTed in period l2 (9-ll May,
I969). One female was TheughT To be pregnanT during laTe winTer (lasT
of AugusT), wiTh one embryo. When brOUghT To The lab, Then checked
several days laTer, There was no evidence of pregnancy. ScreTal males
were noT found afTer Trap period I3 (25—27 May, I969). The reproducTive
season iT appears, exTended for aT leasT 4.5 monThs, and possibly
longer.

Unlike The former Two species, 93 ruTilans were found in reproduc-
Tive condiTion in all seasons of The year (Tables I5, l8). Embryos were
deTecTed in nearly every monTh, as were screTal males and perforaTe
females.

An analysis of Hill PloT 93 ruTilans naTaIiTy showed ThaT l0
females were discovered pregnanT 26 Times during a l2 monTh period
sTarTing wiTh period I0. One of These animals was found pregnanT seven
Times; one, four Times; Two, Three Times; one, Two Times; and four, one

Time. WITh an average liTTer size of approximaTely 3.|, This resuITed

57

In The preducTien of abouT 8| young during ThaT year. SIigthy fewer,
76, were recorded during The Time PloT I was in operaTien.
InTensiTy-gi_breeding

Observing changes in pregnancy raTes, percenTage of perforaTe
(or if a male, screTaI) animals in The populaTion are meThods of
deTermining aT whaT period of The annual breeding cycle reproducTien
is mosT inTense. In addiTion, species specific breeding sTraTegies, and
mulTi—annual populaTion qucuaTiens can also be examined by means of
These parameTers.

Akodon azarae, judging only from PIeT I, did noT undergo any change

 

during The Two breeding seasons in The percenT of pregnanT females
(Table I6). The l968-l969 breeding season mighT be expecTed To produce
a larger proporTion of pregnanT females in PloT I because of The
plenTifui cover and abundanT forage. However, if The I968~l969 breeding
season is compared wiTh The same Time The following year (27—32), There
is no difference in The average percenT (appreximaTely 38 percenT) of
pregnanT females in The adulT pepulaTion. YeT, comparing These same Two
periods, ever Twice as many females were pregnanT The firsT season,
suggesTing ThaT merTaliTy and/or emigraTien are having an effecT (dis-
cussed laTer) in reducing The populaTion densiTy from iTs firsT season
high. This is also suggesTed by The facT ThaT The inTensiTy of breeding
among The TeTal female populaTion decreased for a Time (periods l-6,
Table I3) as young animals enTered The Trappable populaTion, buT did
noT reach reproducTive condiTion unTil somewhaT laTer. This is In

conTrasT To comparable periods in The second season (periods 27-32)

where There Is IiTTIe difference beTween The percenT of adulT females

pregnanT and The percenT of TeTal females pregnanT, Thus demensTraTing

ThaT young animals made up only a very small porTion of The ToTal
populaTion The second season.

Based on The above observaTions, The firsT breeding season for
A, azarae may have begun somewhaT earlier in PloT I since There was
already approximaTely a IO percenT difference beTween The Two female
groups by The firsT period, buT only a 3 percenT difference by The
comparable daTe (period 27) in The second season. UnforTunaTer, The
sTudy siTe was noT begun early enough To verify The commencemenT of
breeding. IT can only be sTaTed ThaT breeding TerminaTed The same Time
in boTh seasons.

The inTensiTy of breeding in A: azarae females, as deTecTed by The
proporTion of perforaTe individuals in The populaTion (Table I6, I7),
definiTely shows a higher reproducTive inTensiTy aT a high pepulaTion
(firsT season). NoT only are more adulT females perforaTe buT a number
of subadulTs and juveniles also were perforaTe, a condiTion which did
noT manifesT iTself The second season in The laTTer Two age groups. Male
scroTaI developmenT showed liTTIe change during The Two seasons. During
The Two reproducTive seasons (periods I—6 and 27-32) in which The females
were examined, The proporTion of ToTaI adulT scroTal males in each season
did noT differ more Than an average four percenT (63, 67 percenT, PloT l).

LimiTed daTa are available for The oTher Two species. Oryzomys
nigripes repreducTion (Tables l4, l8) only occurred in PloT | The firsT
year, afTer which The populaTion sTeadin declined and never reappeared.
The daTa also suggesT ThaT The main peak of reproducTion is sligthy
laTer in The year Than ThaT found for A: azarae, The inTensiTy of breed-
Ing by Q, ruTilans (Tables I5, l8) showed liTTIe seasonal difference in

reproducTive performance. However, of 29 periods (ouT of a possible 30)

59

Table I6. InTensiTy of breeding in Akodog azarae, PloT I, as measured by
The presence of a well—developed scroTum in males and a
perforaTe vagina in females. Under each age heading and Trapping
period are given The number of animals capTured, The number
perforaTe or scroTal, and The preporTion of The age group which

Show ThaT condiTion.

 

 

 

 

Males Females
Trap Juv. SA AduIT Juv. SA AdulT
Period
lDec. '68 3(0) 3(0) I5(l4).93 4(0) I(0) I7( 5).29
2 " " 2(0) 4(0) 20(I8).9O 3(l).33 5(3).6O 32(23).72
3Jan. '69 2(0) |5(O) 23( 7).30 I0(I) I0 I6(3).|9 53(40).75
4 " " 2(0) 9(0) I6(I0).63 3(0) 2I(4).l9 35(3I).89
5 " " 9(0) II(I).O9 l9(I2).63 7(0) 8(0) 27(25).93
6Feb. H 2I(0) 39(I4).36 7(0) 22(3).I4 34(22).65
7 " " I(0) 9(0) 56( 9).l6 I2(0) 40(24).60
8Mar. " 2(0) 13(0) 58( 8).l4 23(2).09 50(24).48
9 " " 2(0) 6(0) 58( 5).09 4(0) I2(2).I7 47( 7).I5
IOApr- " 7(0) 6I( 2).03 2(0) 10(0) 66( I).02
II I " 5(0) 69( 0) I(0) 6(0) 46( 0)
I2May " 2(0) 59( 0) 7(0) 43( 0)
l3 " " 2(0) 43( 0) 2(0) 37( 0)
I4June " 56( 0) 2(0) 53( I).02
I5 " " 50( I).02 3(0) 46( 0)
I6July " I(0) 44( 0) 40( 0)
I; 3 x 4(0) 47( 0) 3(0) 39( 0)
59( I).02 2(l).50 48( I).02
I9Aug. " 53( 0) 4(2).50 50( I).02
20 " " 50( 0) 2(0) 50( 3).06
2|Sepf- " 36( 0) 25( 0)
22 " " 44( 7).I6 4(l).25 23( I).04
23OCT. " 45( I).02 4(2).50 32(I2).38
24 " " 43( 6).l4 32(l9).59
25Nov. " 38(l2).32 40(I7).43
26 " " I(0) 34(22).65 24( 8).33
2706C. " I(0) I(0) 3I(I9).6I 24( 4).I7
28 " " I8(I2).67 2(0) 20( I).05
29Jan. '70 4(0) I(0) l5(l2).80 I(0) I(0) 15( 2).l3
3o " " 2(0) 3(0) 9( 4).44 4(0) I2( 0)
3| " " 4(0) I4(II).79 I(0) 4(I) I7( 4).24
32Feb. " I(0) I8(I3).72 I(0) l5( I).07
33 " " 4(I).25 22(I5).68 I(0) 6(0) 25( 3).I2
34Mar. " I(0) 7(0) 38(l0).26 I(0) I4(0) 3I( 0)
35 " " I(0) 4(0) 33( 7).2I 7(0) 23( 3).I3
BGAPr- " I(0) 2(0) 36( 8).22 7(0) 3I( 0)

 

 

 

 

6O

 

 

 

 

 

Table I7. inTensiTy of breeding in Akodon_azarae, Hill PloT, as measured
by The presence of a well-develeped scroTum in males and a
perforaTe vagina in females. Under each age heading and Trapping
period are given The number of animals capTured, The number
perforaTe or scroTaI, and The percenTage of The age group which
show ThaT condiTion.

MALES FETALES

TRAP Juvenile SA AdulT Juvenile SA AdulT
PERIOD

I Dec. '68

2 H H

3 Jan. '69

4 n n

5 u n

6 Feb. "

7 " " 3(0) I7(O) I9(4) 2| i(O) 7(2)29 5(5)IOO

8 Mar. " 2I(2)l0 37(2) 05 3(I)33 l7(4)24 II(5) 45

9 " " 8(0) 48(0) 5(0) 22(3) l4

I0 Apr. " I0(O) 46(I) 02 2(0) |3(O) 32(4) l3

II " " I(0) 5(0) 48(I) 02 |3(O) 36(0)

I2 May " 2(0) 8(|)I3 60(i) 02 II(O) 35(0)

I3 " " 3(0) 53(0) 6(0) 39(0)

I4 June " I(0) 62(2) 03 I(0) 50(0)

I5 " " 3(0) 69(0) 7(0) 48(0)

I6 July " 4(0) 64(0) 8(0) 45(0)

l7 " " 4(0) 65(0) 5(0) 38(0)

I8 " " 49(0) 29(0)

I9 Aug. " I(0) 57(0) lO(O) 32(0)

20 " " I(0) 46(0) 2(0) 30(I)

2| SepT. " I(0) 34(0) I(0) 27(0)

22 " " 37(I) O3 I(0) 32(0)

23 OcT. " 4|(l) 02 3(0) 30(l) O3

24 " " 36(4) Il I(0) 29(9) 3|

25 Nov. " 23(7) 30 |3(3) 23

26 " " |8(7) 39 |3(O)

27 Dec. " 6(5) 83 8(I) I3

28 n n I(0) 6(2) 33 I(0) 7(l) l4

29 Jan. '70 4(4)IOO i(O) I(0) 6(l) I7

30 " " i(O) 3(3)IOO |3(I) 08

3| " " I(O) 4(0) 6(3) 50 |3(I) 08

32 Feb. " 2(0) 9(4) 44 2(0) i(O) I4(2) I4

33 " " 5(0) |3(0) I9(4) 2| 2(0) 9(0) IO(O)

34 Mar. " I(0) 5(0) 40(2) 05 I(0) 7(0) I7(O)

35 " " I6(0) 3I(O) I6(O) I9(O)

36 Apr. " I(0) II(O) 37(0) I6(O) 2I(0)

 

 

 

 

6|

Table I8. InTensiTy of breeding in Oryzomys nigripes and OXymycTeris
ruTilans, PloT l and Hill PloT (daTa combined),as lndicaTed
by The number of young animals capTured (SA = subadulTs) and
sexual condiTion (TesTes scroTal, vagina perforaTe). The
firsT column in each class gives The ToTaI number capTured;
The second (in parenTheses) The number showing The sexual
condiTion; and in The Third column, The percenT showing The

sexual condiTion.

 

 

 

Oryzomys nigrlpes

 

OxymycTeris ruTilans

 

 

 

 

 

TRAP MALES FEMALES MALES FENALES

PERIOD SA AdulTs SA AdulTs SA AdulTs SA AdulTs
T’Dec. T6 2(2)IOO

2 u n l(l)|OO I(0) I(I)IOO 2(l) 50
3 Jan. '69 4(3) 75 2(0) I(I)IOO .2(2)I00
4 n u 2(l) 50 3(3)l00
5 n n 2(2)IOO 2(I)50 2(I) 50
6 Feb. " I(I)IOO I(0) 3(l) 33 2(2)IOO
7 " " I(0) 2(I) 50 2(0) 2(I) 50 2(2)IOO
8 Mar. " l2(l) 9 2(l) 50 3(3)IOO 3(3)IOO
9 " " i(O) 24(8) 33 I(I)IOO 9(l) II 2(2)|00 I(0) 4(0)

I0 Apr. " l0(i) IO I(0) I0(O) 2(I) 50 4(0)

ll " " 2(0) 25(5) 20 I(0) 8(0) 3(2) 67 5(2) 40
l2 May " 43(5) l2 I(0) 22(I) 5 I(0) 2(I) 50 4(0)

I3 " " I(0) 34(I) 3 I5(I) 7 2(I) 50 5(0)

l4 June " I4(0) I0(O) 2(2)IOO 4(I) 25
IS " " 23(0) I9(O) 3(3)IOO 9(2) 22
I6 July " 28(0) 20(0) 2(I) 50 4(0)

I7 " " 3I(O) 24(0) 3(3)l00 5(2) 40
I8 " " ll(0) 4(0) 5(5)l00 8(4) 50
I9 Aug. " l0(0) 7(0) 5(5)l00 II(2) l8
20 " " 3(0) 2(l) 50 4(3) 75 8(2) 25

2| SepT. " 3(0) I(0) 5(5)IOO ** 8(2) 25
22 n u 2(0) I(0) 4(4)|00 2(0) 8(0)

23 OCT. " 2(0) 2(0) I(0) 6(5) 83 8(4) 50
24 n n 3(0) 5(2) 40 9(4) 44

25 Nov. " *2(0) 3(3)IOO 3(0)

26 " " I(I)|OO 3(3)l00 5(0)

27 Dec. " 3(0) 4(2) 50
28 " u 4(I) 25 I(0) l0(0)

29 Jan. ‘70 3(0) 5(0) 3(0) 3(2) 67
30 n n 5(I) 20 2(0) 2(0)

3i " " I(0) 4(4)IOO 3(0) 6(0)

32 Feb. " I(i)lOO 2(I) 50 I(0) 7(4) 57 2(0) 9(I) Ii
33 n n 2(I) 50 I(0) 3(2) 67 3(0) 7(3) 43
34 Mar. " 6(4) 67 I(0) 6(0)

35 " " I(0) 3(0) 6(2) 33 2(0) 6(l) I7
36 Apr. 3(0) I(0) 1(0) 6(2) 33 110) 811) 131

 

 

 

 

7N0 9. nigripes in PloT I afTer This daTe.
**Juvenile capTured here.

 

I ‘Jil'll'll-I'I (II .I' III

62

In which PloT ll adulT females were found, only i4 percenT were in The
perforaTe condiTion, compared To 39 percenT for 20 periods In PloT I
during The same 30 period Time span. In direcT conTrasT To This, The
number of pregnanT females was 2.4 Times (I2) higher in Hill PloT Than
ThaT (5) for PloT I.

Survival

Minimum survival raTes

 

The minimum survival raTe is here defined as ThaT proporTion of
The Trappable populaTion, which when marked aT Time T will sTill be in
The sTudy ploT aT T + I. If capTured afTer, buT noT aT T + I, The
animal ls presumed To have been on The ploT aT T + I and is added To
ThaT ToTaI. A number of auThors (Newson and ChiTTy, I962; Krebs, I964;
Krebs I966; Krebs einal., I969) have recognized ThaT Their survival
raTes reflecT noT only morTaliTy facTors buT also The possible unequal
effecTs of immigraTion and emigraTion from The sTudy area. As Krebs
(I966) sTaTed, The daTa are expressed as minimum survival raTes beTween
Trapping periods (I4 days in This sTudy). The True survival raTe should
never be less Thaanhis raTe.

Minimum survival raTes (Table I9) for A: azarae, sexes and ages
pooled, suggesT ThaT There was a noTiceable change beTween reproducTive
and non—reproducTive seasons. This is True especially in PloT I where
each reproducTive season averaged 72—73 percenT, and The non-reproducTive
season, 83 percenT. On Hill PloT, The winTer survival raTe averaged 80
percenT, buT only sligthy lower, 79.5 percenT for The firsT reproduc-
Tive season, and much lower, 59 percenT, for The second reproducTive

season .

On The same Table, £5 [USVWIXEE (ploTs pooled) show a much lower

..I-‘li'..l|{‘."‘lll'llll liilllil'tiuil‘ {fa-Ill. ll III-I'llll..dllll

63

Table I9. Minimum survival raTes per I4 days for Akodon azarae, Oryzomys
nigripes and Oxyggcferis ruTilans. ToTal caTch released is in
parenTheses, followed by The TeTaI number of The same animals
capTured in subsequenT Trappinggperiods.

 

 

 

 

 

 

 

AKODON AZARAE ORYZOMX§_ OXYMYCTERIS
TRAP N IeIII PES RbT I LANS
PERIOD _PIoT J Hill PloT _P oTs Pooled PloTs Pooled
I Dec. '68
2 " " .72( 43) 3| .50( 2) I
3 Jan. '69 .69( 64) 44 .00( I) 0 l.00( 3) 3
4 " " .55(l07) 59 .OO( 4) 0 .67( 3) 2
5 " " .69( 80) 55 .75( 4) 3
6 Feb. " .77( 70) 54 .60( 5) 3
7 " " .84(I20)IOI I.OO( I) | .50( 6) 3
8 Mar. " .83(II5) 95 .84( 5|) 43 .80( 5) 4 .75( 4) 3
9 " " .72(I39)IOO .75( 89) 67 .54(l3) 7 I.OO( 6) 6
I0 Apr. " .76(|28) 97 .83( 83) 69 .43(23)I3 .83( 6) 5
II " " .62(I45) 9O .76(|O3) 78 .89(l8)|6 I.OO( 6) 6
l2 May " .90(l26)II3 .82(IO3) 84 .75(32)24 .88( 8) 7
I3 " " .85(III) 94 .8|(II7) 95 .76(38)29 I.OO( 6) 6
l4 June " {.89( 83) 74 .89(I00) 89 .88(43)30 l.00( 7) 7
l5 " " .78(Il0) 86 .88(Il4)IOO .76(2I)I6 I.00( 6) 6
l6 July " .82(I98) 80 .82(l27)IO4 .59(39)23 .9I(Il)l0
l7 " " .89(l83) 74 .83(I2|)IOO .52(44)23 I.00( 6) 6
l8 " " .85(l92) 78 .87(II2) 97 .2l(53)ll I.OO( 8) 8
I9 Aug. " .90(IO9) 98 .86( 78) 67 .33(I5) 5 .92(|2)Il
20 " " .73(I06) 77 .73( 98) 72 .29(I7) 5 .87(I5)I3
2| SepT. " .4I(IOO) 4| .72( 79) 57 .l7( 6) l .82(Il) 9
22 " " .84( 6|) 5| .83( 63) 52 .33( 3) I .85(l3)l|
23 OcT. " .94( 7|) 62 .86( 70) 6O .33( 3) I .92(|3)l2
24 " " .90( 79) 7| .78( 73) 57 .50( 4) 2 .86(I4)l2
25 Nov. " .87(.75) 65 .49( 57) 28 .33( 3) I .80(I5)|2
26 " " |.77( 78) 6O .69( 36) 25 .OO( 2) O .83( 6) 5
27 Dec. " .78( 58) 45 .53( 30) I6 .00( I) 0 .57( 7) 4
28 " " .6I( 57) 35 .54( I3) 7 .86( 7) 6
29 Jan. '70 .65( 40) 26 .47( I5) 7 .60(|5) 9
30 " " .79( 34) 27 .80( I0) 8 .69(I3) 9
3i " " .73( 30) 22 .53( I7) 9 .50( 8) 4
32 Feb. " .80( 40) 32 .58( 24) I4 .80(l5)l2
33 " " .74( 35) 26 .64( 28) l8 .OO( 3) 0 .68(l9)|3
34 Mar. " .82( 57) 47 .75( 57) 43 .OO( 2) 0 .69(I3) 9
35 " " .66( 87) 57 .54( 7|) 38 .83(I2)l0
36 Apr. " .73( 66) 48 .40( 82) 33 .00( I) 0 .44(I6) 7

 

 

 

 

 

 

64

survival raTe, abouT 58 percenT for The periods 9—20, The Time when
The mice were mosT abundanT. The Q. ruTilans populaTion (ploTs pooled)
had a survival raTe of abouT 70—76 percenT during The summers, buT in
The winTer iT rose To 92 percenT.

True survival raTes

 

In a series of field experimenTs wiTh MicroTus pennsylvanicus

 

and M. ochrogasTer, Krebs eT l. (I969) showed ThaT "survival raTes"

 

of These Species in large enclosures (where emigraTion could noT Take
place) were much higher (ofTen over 30 percenT higher) Than neighboring
unenciosed populaTions. This evidence could indicaTe ThaT a major
porTion of The survival raTe is lowered simply because of emigraTory
and immigraTory paTTerns. If iT is assumed ThaT These Two movemenTs
are approximaTely equal wiThin any given Time period, and if one
examines a populaTion during a non-reproducTive period, The decrease
in populaTion from one Time period To The nexT can be a reliable
measuremenT of survivalbiliTy. Because microTlnes may be in a non-
reproduchve sTaTe for only several monThs aT a Time or may breed
conTinuously ThroughouT The year (Krebs, I966; Krebs ei_§l:, I969),

This Type of analysis for esTimaTing a True survival raTe is

difficulT. Akodon azarae presenT The opporTuniTy To examine True survival

 

raTes since There is a 5-6 monTh period of non—reproducTion (Table I3)
during The winTer. This esTimaTe is calculaTed by dividing The minimum
number known alive aT one Trapping period by The number known alive aT
The nexT Trapping period. The daTa, presenTed in Table 20, show ThaT

The average esTimaTed True survival raTe is beTween 93-95 percenT per

I4 days. This Is 30-40 percenT higher Than The minimum survival raTe
calculaTed earlier, and as such, is much closer To The values deTermined

for enclosed populaTions of MicroTus.

65

Table 20. True survival raTes for Akodon azarae during The non—

reproducTive (winTer) season.

 

 

AKODON AZARAE

 

 

 

 

PloT I Hill PloT
Trap Survival Min. No. Survival Min. No.
Period Rafe Alive RaTe Alive
IO Apr. l969 2I4 I45
Il " " .87 l87 |.00 I45
l2 May " .99 l85 I.O6 l53
I3 " " .97 l80 .93 I43
l4 June " .99 I79 .99 I42
I5 " " .9I I62 I.I3 I60
I6 July " .93 I50 I.OO I60
l7 " " .97 I46 .88 I4I
l8 " " .95 l39 .9l l28
I9 Aug. " l.03 I43 l.0| I29
20 " " .89 I27 .82 l06
2| SepT. " .65 83 .85 90
22 " " I.I3 94 .92 83
23 OCT. " l.05 99 .96 80
24 " " .98 97 .93 74
.25 Nov. " .96 93 .57 42
Average True é I 427 (I396
(Survival RaTe Nél5 NéIS

X=95.I% X=93.l%

 

66

MovemenTs
The movemenT paTTerns of populaTions are parameTers which are
affecTed by densiTy, season, age and sex classes, and inTerspecific
compeTiTien. This secTion examines The movemenT paTTerns of The Three

aforemenTioned rodenT species in an efforT To undersTand how They are
affecTed by The above menTioned variables.

RecapTures per Trapping period

The percenTage of A: azarae which were capTured Two or Three

 

Times per Three day Trapping period is given in Table 2i. When sexes
and age classes were pooled, an average 42-44 percenT (Hill PloT and

(PloT l, respecTively) of The animals were recapTured. The subaduIT
individuals averaged for females, 25 and 37 percenT, and males, 40 and

I5 percenT (Hill PloT and PloT I, respecTively). The adulTs averaged more
Than This, wiTh males aT 43 and 46 percenT, and females, 48 and 48 percenT
(Hill PloT and PloT I, respecTively). If The adulTs are segregaTed by
.sex, season, and sTudy ploT (Table 23), The percenTage of 23.933533
ITecapTured show no clear Trend from season To season.

Oryzomys nigripes and O. ruTilans recapTure daTa are much less
CxompleTe per Trap period for parTs of The sTudy (Table 22). These Two
Swoecies, sexes pooled, have The Table 22 daTa grouped according To
Season and sTudy ploT, (Table 24). The daTa are limiTed for 0. nigripes,
SSHKxaThey seemed To lead an ephemeral exisTence in The sTudy siTes. The
"eacapfure values are raTher high when one considers ThaT A: §5§£§e_had
1‘he firsT epporTuany To enTer available Traps. DaTa for 9, ruTilans in
‘r1ne above Table suggesfs ThaT when A: EZE£§§_POPUIPTIO“S are high, as
‘ r1 PloT I, The former species are capTured less frequenle Than when

:fiL- azarae populaTions are down (e.g., PloT I second summer, and Hill

67

Table 2|. PercenTage of Akodon azarae which were capTured more Than once
during each Trapping period, arranged according To sex, age
class, and sTudy_ploT.

 

 

 

 

 

 

 

 

 

Subadulf SubadulT AdulT AdulT Pooled Sexes
Males Females Males Females and Ages
TRAP PloT Hill PloT Hill PloT Hill PloT Hill PloT Hill
PERIOD I PloT I PloT I PloT I PloT I PloT
lDec. '68 I7 00 I3 24 I7
2 " " 50 38 45 34 39
3Jan. '69 24 I9 52 55 42
4 " " 36 33 44 57 45
5 " " 30 20 2| 59 36
6Feb. " 29 IO 46 68 4|
7 v " 20 35 83 25 4| 42 40 60 36 38
8Mar. " 20 38 22 40 40 5| 36 73 34 48
9 " " 25 38 25 20 33 48 30 23 30 39
IOApr. " 29 40 25 07 38 4| 33 4| 34 36
II n n 20 33 29 38 49 48 43 56 45 49
|2May " 00 IO 57 36 36 50 33 5| 35 46
I3 " H 00 33 00 67 26 47 22 46 23 48
|4June " I00 50 00 38 60 40 68 39 63
|5 " n 67 33 00 42 38 43 46 42 39
I6July " 00 50 27 38 23 44 25 40
I7 " " 00 50 33 40 40 62 28 6| 35 60
l8 " " mo 39 47 52 52 46 49
'59Aug. " IOO 50 50 36 68 44 56 40 63
20 " " mo 50 00 36 43 58 43 47 43
2|SepT. " 00 42 4| 52 33 46 37
22 " " 50 00 66 84 52 69 6| 76
230c+. " 75 I00 76 76 53 73 67 76
24 " " mo 65 64 63 59 64 62
25Nov. " 53 70 45 69 49 69
2:65 " " 00 65 56 63 69 63 6|
2 7 Dec. " 00 52 33 58 38 65 36
2263 " " 00 |00 00 72 33 70 43 73 33
:599Jan. '70 20 50 00 53 00 40 I7 43 08
.:s() " " 20 IOO 25 78 00 50 77 50 I2
:5 I " " 25 20 60 57 33 82 23 65 25
.EsiiFfb' " 00 00 00 33 22 33 43 3| 29
:5 " 00 I7 I4 09 55 26 44 40 4| 22
:35’1Mar. " 00 I7 I3 00 42 20 55 35 38 2|
_>,55 " " 20 06 29 06 39 23 57 2| 43 I6
~+>€3Apr. " 00 08 I4 l9 6| 30 78 24 53 23
\

 

 

68

Table 22. PercenTage of Oryzomys nigripes and OxymycTeris ruTilans,
which were capTured more Then once duFTng each Trapang
period, and arranged wiTh sex and age classes pooled for
each sTudygploT.

 

 

 

 

 

 

 

Oryzomys nigripes OxymycTeris ruTilans

Trap Period PIOT I Hill PJoT PloT I LHill.PloT

I Dec '68 00

2 II I! 00 00

3 Jan '69 33 33

4 II II 80

5 II II 20

6 Feb " oo 33

7 " " 00 25 00 33
8 Mar " 00 I7 00 80
9 " " 07 09 50 ,
I0 Apr " 00 00 33
ll " " 2| 29 00 7|
I2 fay " I9 I7 00 00
I3 " " 22 57 00 67
I4 Jun " 38 43 83
I5 " " 2| I7 00 67
I6 July " 33 46 33
I7 " " 35 6O 00 80
I8 " n 43 , 00 20 29
'9 Aug " 54 25 29 38
20 " " 40 |00 67 75
2| SepT " 33 50 60 II
22 " " |00 50 00 56
23 OCT " |00 00 40 44
24 " " 50 |00 29 63
25 Nov " |00 00 80
26 II II 00 38
27 Dec " 00 50
28 " " 6o 40
29 Jan '70 38
30 I! II 33 00
3| N I! '0 00
32 Feb " 00 50 II
33 " " 00 I7 25
34 Mar " 40 38
35 " " 00 20 33
36 Apr " 00 60 00

 

 

 

 

 

69

Table 23. The proporTion of recapTured adulT Akodon azarae segregaTed
by sex, plof,and season. Preporlion is lhe average of
apprOpriafe daTa shown in Table 2|. Numbers in pareniheses
show average number of individuals capTured each period.

 

 

 

 

 

 

AdulT Males AdulT Females
PloT Hill PloT Hill
I PloT l PloT
Firsf Summer .37 .46 .44 .49
Periods l-lO
(37) (38) (40) (IB)
Winler
Periods ll—24 .44 .55 .43 .54
(50) (52) (40) (36)
Second Summer
Periods 25-34 .56 .36 .54 .45
(24) (ll) (22) (ll)

 

 

 

 

 

 

Table 24. The proporTion of recapTured Oryzomys nigripes and Oxymycieris

 

 

ruTilans, sexes and ages pooled;afid segregaTed by ploTTand

season. ProporTion is ihe average of appropriaTe daTa shown
in Table 22. Numbers in parenTheses show average number of

individuals capTured each period.

 

 

 

 

 

 

 

Oryzomys nigriges OxymycTeris ruTilans
PloT l Hill PloT PloT l Hill PloT

Firs? Summer .2l .49
(Periods l-IO) - - (3) (5)
Winfer .44 .42 .20 .SI
(Periods ll-24) (l8) (ll) (4) (7)
Second Summer - - .36 .3I
(Periods 25-34) (7) (6)

 

 

 

 

 

 

7O

PloT firsT summer and winTer).

SpaTial disTribuTion—cenTers Si acTiviTy

 

IT Is The proporTion of animals Taken more Than once during each
Trapping period ThaT forms The basis for The movemenT analysis. The
cenTer of acTiviTy (Hayne, I949) was calculaTed for each animal during
each Trapping period. From This poinT, all disTances To poinTs of re—
capTure was measured, and average values were Then calculaTed for each
recapTured individual. In These insTances where circular home range
daTa are desired, The mean disTance serves as The radius.

The spaTial disTribuTion, uTilizing The cenTers of acTiviTy, was
deTermined for each species by dividing PloT I and Hill PloT inTo four
quadranTs (Figure 2, 9). The number of animals capTured each Trapping
period for individual quadranTs was Then averaged for each season
(Table 25).

The daTa for A: azarae show ThaT for PloT I, mouse densiTy remained

 

highesT Tive ouT of six Times in QuadranTs A and B. IT was in These Two
quadranTs where vegeTaTion cover and densiTy values were highesT (Figure
3 shows an area wiThin A-B, Figure 7 for C—D). On Hill PloT, The mosT
heTerogeneous of The Tour quadranTs, QuadranT A showed The highesT
densiTies aT all seasons. A somewhaT unexpecTed resulT was The raTher
high values in QuadranTs C-D, boTh of which were covered raTher homo-

geneously wiTh sTands of Paspalum guadrifarium (Figure 9), a planT of

 

quesTionable nuTriTive value (Caswell 52.21:! l973), buT possibly
offering excellenT concoalmenT advanTages. DenslTy figures for HllI PloT
quadranTs show lower values Than PloT | for The firsT summer, wlTh
winTer values more equal To Those of PloT l winTer (noTe ThaT Hill PloT

winTer densiTy ls higher Than summer densiTy, conTrary To ThaT found In

.Leae:c ommce>m 65+ mc.+m.:o_mo can; now: +0: mm; no.cma
+mc+ cmc+ .uo_cea mc.aamc+ o.+.oeam m mc.c:u +0.6 >ua+m m :0 6663+660 ecez mm.oeam 6 +0 m.m:n.>.oc. 0c +.*

 

 

 

 

7|

 

 

 

 

 

 

 

 

 

m.. ... e.. m.m N.. v.0 Q =
0.0 ..N m.. m.o m.o v.0 o :
«.0 n.. o.m 0.0 m.. o.N m :
m.m m.m ewp INwNII, m.o m.o < +cmcumno
mz<..e:m m.mmeo>z>xo
n.o o.m m.N m.o m.m 0.. o :
m.. ..N o.m I m.m o.N o :
0.. 0.0 m._ I n.m 0.. m :
~.o m.. m.. m.o m.v 0.0 < +cmcnmao
mmd.mw.z whaom>mo
N.n 0.0m m.om m.m. 0.5. N.vm o :
w.o ..om 0.0. >.o ...N m.mm o =
0.0 v.mm <._N ..N. m.nm ..mm m z
..o. N.mm m.mm ..n. 0.5N m.om < +cmcumgo
m<m<w< zomox<
.vmumm .cedv .va.. .Lmd. Ao.I> .Led. .vmimm .LOd. .va.. .Ledv .o.I. .Lmdv
Leeanm ncm Le+c.3 coagum +m. Leagum new Lo+c.3 Leeanm +m.
Fo.d ...: . F04d
*.+o.d ...: ncm . +o.d c. m+cmcumav .mflm.>wvc_ L0+ vo.cmmlmc.aamc+

 

 

comm necs+amo mcm_.+:c m.Lm+o>E>mo 6cm .mma.cm.c mxeoNxco .emLmNm.covox< we Lassa: ommcm>< .mN m.nmh

72

PloT I). Second summer values were much lower Than firsT summer values
In boTh ploTs.

SpaTial disTribuTion of Q, nigripe§_was highesT in QuadranT C, The
mosT open vegeTaTively of PloT I and also QuadranTs C, D, dominaTed by

E, gyadrifarium, of Hill PloT. DaTa for PloT l 9, ruTilans show Quad—

 

ranT B wiTh The highesT densiTy The firsT summer, buT by The second,

The main populaTion,like in A, azarae, was shifTed To OuadranTs A, D
where cover was mosT available. On Hill PloT, QuadranT B mainTained The
highesT densiTy during The firsT summer, OuadranT A dominaTing winTer

and The second summer. BoTh of These quadranTs were The mosT vegeTaTiver
heTerogeneous of The four and, menTioned previously, favored by A,

azarae as well.

f§5fl1_gj_acTiviTy

The average disTance moved from The cenTer of acTiviTy (radii of
acTiviTy) was calculaTed for A, azarae, by sex, age, and sTudy ploT for
each Trapping period (Table 26). The age (subadulT, adulT) of each
Individual was esTablished according To iTs weighT aT firsT capTure in
each season. Radii of acTiviTy for each animal during any one season
were averaged TogeTher To give one value in compiling The Table.

If The seasonal average disTance moved from The cenTer of acTiviTy
ls examined (Table 27) for The adulTs, by a Three~way analysis of
variance (Table 28), significanT differences aTTribuTable To season,
sex, and ploT were all very significanT (p<.0005). A season x sex inTer-
acTion was noT significanT (.IO>p>.05).

Observance of Hill PloT_A; 933123 seasonal radii of acTiviTy paT-
Terns sTroneg suggesT ThaT winTer movemenTs were sllgthy more Than

Those of summer. In PloT I These mice sTeadily Increased Their radii of

 

Hill
PloT

l5|7446070897407'6658604846838
984567080886707785823603633746

3853088420358392279933'l364645
ll2|ll3222|ll|l 22'

 

AdulT Females

PloT

36'6336899982'3250038407526336270224

lulu/4333354336757475557575678800498576

4|90636842048Il09'529327085'466M5I638

 

Hill
PloT

l.7.92|.92IIIIIIn2n2.I 7.7. |.7.7292|.I.I.7.I.l.7.l. I.IIIIII
|.03AvnquRwAw2333nw4.6v|.Av337.4.837.7.7.7. 03A.AvAuII7.

O ....... O O . O O C
7.RVAVRJRJQ,Agave/7.0,Avo,Red.olnunu7.nvavnv Ago/R39277q3
960323031300€37.66A.n63.o,nvA.I.I.23Aun67.7. 7.7.Rsnu7.|.
I.7.|.7.3372237.7.4.7.339.|.332392|.|. I.

 

AdulT Males

39773676984062l29l65756039|6i8656032

 

DaTa grouped by sex, age class,and

73

 

 

 

 

Average disTance,in meTers, from The cenTer of acTiviTy, wiTh

N, for Akodon azarae.
sTudy ploT for each Trapping period.

 

 

 

Table 26.

 

 

ITI o
o |.7.o,9.o,Aus,A.d.zsfisd.£07.Re7,3u835eAefivsvo3nu7.nvl.olavnv|.o,nv7.Refiv
P
29274833934llll293985948026387862632
ll 22'232'27—"2 Ill-125222" IIIIIZ
T 0403202 90 9 07 5 08
.I lo 0425800 69 l 74 8 06
.Isil l 2
dl 28ll544 42 5 3i I I3
Mm T 02280045000 00 3704 00 6040 0000
%R o 063340244H5 06 586% 64 $036 0630
P
35833l54324 ll l22| 23 2ll3 l22l
% T 637525005 8 03 00 0500
m lo L6H558063 3 02 06 2800
M HP
T N 78342lll2 2 II II 3lll
M T Y 02084375005 9 045 0
w 0 09886324398 6 208 6
b P
W N l344662322l 2 III I
8 9 0
D 6H6HHHNNHHHH"HHHHHHHHHHHHHHH7HHHHNHN
0 9 9 9
R .
E o o o o o e V! 0 IT 0 o o o o O o
DI C n b r r V: n I! g p .T V C n b r r
e" an..." e" a" p" a" U" UH" U" 9" C" 0" e" a" H e" a" p
W D J F M A M J J A S O N D J F M A
R l234567890l234567890l234567890|23456
T llllllllll 22222222223333333

 

74

 

Ahmv .

 

 

 

 

 

 

 

 

Ava Amvv Ammv “0.. Ao.v AN.V “my vnImN muo_cea
mm.m vm.n 00.0 .h.m No.0 Nh.h wo.m mm.m LeEE:m UcN
“mo. ANmV .00.. Aom_v AONV Ao.v Aw. AN. VNI__ mno_Lom
mm.w m©.m hm.m mh.o NN.m _©.© ov.h no.5 Le+c.3
Ao.v Ann. Romy “no. Aw.v Ame. ANN. Aomv o.I. muomcem
no.h Nv.m O©.e NN.© Nm.N mm.n mo.m mm.v LmEEJm +mc_m
+0.1 ...I +o_m +o_a ...I . +o.m +0.1 .. . +o.m +o.m ...I +o.m
mm.mEmm +.:n< mm.mz +.:o< mm.memm +.:Umn:m mm.mz +.aumn:m

 

 

.+o.a >63+m 6cm «mmm_o mmm ~xem >5

.mmcmwm conox< L0+ .2 n+.3 .>+.>.+om +0 Lo+ceo 05+ sec+ ne>oe .mLm+ms c. .oocm+m.u mmmcm>m .mcommem .hN 6.66»

75

Table 28. Three-way analysis of variance for Table 27 adulTs.

 

 

Degrees

Source of Sum of of Mean F RaTio

VariaTion Squares Freedom Square

A (season) 600.6200 2 300.3l00 l3.9037 p<.000l
8 (sex) 277.236I l 277.236l |2.8354 p<.0004
C (ploT) 472.6985 l 472.6985 2l.8849 p<.OOOI
A x B

(inTeracTion) l06.|2l6 2 53.0608 2.4566 p<.0865
WiThin cell 730

 

Table 29. Seasonal average disTance, in meTers, moved from The cenTer
of acTiviTy, wiTh N,for Oryzomys nigripes and OxymycTeris
ruTilafls, by sex and sTudy ploT.

 

 

Oryzomys nigripes

 

OxymycTeris :gTilans

 

 

 

 

 

iales Females Males Females

PloT Hill PloT Hill PloT Hill PloT Hill

I PloT l PloT l PloT l PloT
FirsT Summer l6.67 20.l5 - - 36.07 20.77 35.65 25.24
(Perlods l-IO) (2) (4) (3) (3) (3) (4)
WinTer ll.22 l4.93 ll.35 l5.98 l5.66 l9.93 IO.68 l5.79
(Periods lI-24) (36) (I7) (l9) (6) (3) (9) (3) (l3)
Second Summer 30.00 6.00 - - l7.46 l6.24 l3.64 l3.29
(Periods 25-34) (I) (l) (5) (4) (9) (8)

 

 

 

 

 

 

 

 

 

 

76

acTiviTy as The sTudy progressed. This is noT principally a funcTion of
seasonal changes per s9, buT due To The sTeadily declining habiTaT
qualify. The guinea pigs (Eagle) are believed responsible for This
degradaTion of habiTaT, and a furTher discussion of This follows laTer.
Differences according To sex indicaTe ThaT males generally have sligthy
longer radii of acTiviTy Than females. When The inTensiTy of breeding
is high, as in PloT I and To a lesser exTenT Hill PloT, The firsT
summer females moved lesser disTances Than in The second summer charac-
Terized by low breeding acTiviTy. PloT effecTs, menTioned parTially in
reference To The influence of Eagle, indicaTe ThaT PloT l animals
probably had smaller radii of acTiviTy Than Those from Hill PloT. How—
ever, by The second summer, PloT l A, aggrae_had nearly equal or larger
radii of acTiviTy.

Oryzomys nigripes and 9, ruTilans showed ThaT The average disTance

 

moved from The cenTer of acTiviTy was 2-4 Times more Than A, azarae

 

(Table 29). The sparse daTa suggesTed liTTIe else in comparisons made
beTween seasons, sexes, and ploTs.

Dispersal of Akodon azarae

 

Dispersal, ThaT is, These movemenTs away from The main Trapping
area, can be esTimaTed for The pepulaTion in several ways in order To
beTTer undersTand a species' dispersal paTTern. Here, The dispersal
paTTern was deTermined by Trapping oTher sTudy areas in close associa-
Tlon To The main Trapping ploT. Removal PloT l and II are uTilized in
exacTiy This manner To invesTigaTe dispersal paTTerns in A, azarae,
However, due To The Toe clipping scheme used and The facT ThaT Removal
PloT l and II have noT been analyzed as yeT, Q, nigripes and O, ruTilans

daTa remain unTabulaTed.

77

One paramefer of dispersal which may be examined is sex raTio;
ThaT ls, does a cerTain age class or sex disperse during a given Time
(winTer, summer) aT a raTe much higher or lower Than expecTed? ln facT,
This Type of examinaTion has already been compleTed for all Three
Species earlier, when The sTudy area populaTions were divided inTo
residenT and recruiT segmenTs (Table II, l2). The recruiTs, referring
To These capTured during The non—breeding monThs, when recruiTmenT
Through reproducTion no longer is a facTor, are dispersers inTo a
given area, and can be examined in This IighT, even Though we have no
Idea of The disTance They have moved in order To enTer The sTudy ploTs.
An examinaTion of The above Tables and The discussion connecTed wiTh
Them should suffice for The momenT.

The sex raTios of A, azagae from PloT l and oThers unmarked from
The non~ploT areas of CroveTTo PasTure, and which dispersed inTo
Removal PloT I and II were also calculaTed (Table 30). PloT I adulTs
infrequenle enTered The compleTed Removal PloT | exclosure during The
firsT several monThs of Trapping because of Their relaTively low densiTy
and The effecTiveness of The exclosure. During Their increase The
guinea pigs (Efyia) likewise became increasingly more numerous, and in
addiTion, desTrucTive on The exclosure. AT firsT The few marked mice
capTured in Removal PloT l were placed on The nearesT PloT l side of
The exclosure wiTh The assumpTion ThaT These mice were including parTs
of boTh ploTs in Their home range. However, IT soon became apparenT ThaT
some of These marked mice were dispersing onTo The removal ploTs and
esTablishlng a home range. The decision To keep placing marked indivi-
duals in PloT l was based on The premise ThaT If They generally resided

in a nearby porTlon of The PloT l, They would Tend To remain; If noT,

.m_moo.>.no. mm..+ >:5.

 

mmo .AoAmo .

moo .X.
88
.Nm.

8.4.8..

mooébo. 89459 3.38..

 

8.4.
a.
oom.

.ko. .o.. .Nm. ,mcommmm
o.e. «.m. kmo. om_ooa
moo .Vo om .4. o. .9
.6. .m. .o.. enimm ..oo
85.. awe. ovo. Lassom

 

78

moovo
:N.
oam.

mmofiokmo. mmo .onmo. mmo .AaAmo.

o. .4. 8945? o. .3

 

0++0>0Lo

m+.:e<
. +0.a

0++m>0co

..m. .0.. .0.. va_. .Lea
mmv. mom. m.o. Lm+c.3
m+.:u< m+.:e< (m
0++e>0co . +0.m .>:n

 

 

omqooa kagm

HOJQ 4<>ozmm

 

 

_ H04m 4<>02mm

 

 

.c0>.m|0m_m 0cm m0.+mc xmm _n. *0

memec+06>c ..ac L0+ m0.+...nmnoca use .Amemmc+cmcma c.. m0~.m e.QEmm 0.0E0m .0L:+mmm 0++0>0Lo

 

oc30+ mmmcm mc.oomc+ ncm~mcommmm .wmmmm_o 0mm m:0.cm> +0 emLmNm c000x< m.mEm+ +0 c0.+c0&0cm .on 0.60%

79

Then Their subsequenT movemenTs would give an indicaTion of Their
homing abiliTy. Those which repeaTedly enTered each removal PloT were
counTed only once in ThaT ploT To arrive aT The sex raTio caiculaTions.
Unmarked adulT individuals which enTered The removal ploTs from oTher
parTs of The CroveTTo PasTure were removed and The daTa recorded under
"CroveTTo AdulTs".

The sex raTio (Table 30) resulTs for The juvenile and subadulT
group indicaTe ThaT females, like in Table II, Tended To enTer The
removal ploTs in sligthy higher proporTions Than males. Toe-clipped
Juveniles and subadulTs from PloT l were noT capTured (one excepTion)
in eiTher of The removal ploTs. The pooled winTer resulTs for "PloT l
AdulTs" suggesT ThaT significanle more males Than females were enTer-
lng The removal ploTs which is in agreemenT wiTh The winTer PloT l
residenT daTa. IT would be expecTed ThaT winTer "PloT l AdulTs", like
The PloT | recruiTs are in effecT recruiTs of The removal ploTs, and
should noT show any significanT differences. This was True for winTer
"CroveTTo AdulTs", where The resulTs show no significanT difference
from The expecTed l:I sex raTio, which agrees wiTh PloT l recruiT and
earlier snap-Trap daTa (Table II). WheTher The "PloT l AdulTs" differ-
ence is relaTed T0 The smaller sample size, The facT ThaT females are
sligthy smaller Than males, so become parT of The predominaTely female
subadulT group, or some oTher facTor is noT known. The pooled summer
resulTs for "PloT l AdulTs" mean liTTIe due To The small sample size,
while The "CroveTTo AdulTs" show slgnificanle fewer females Than ex-
pecTed, a resulT which agrees wiTh The breeding season (summer) PloT i
recruiT daTa (Table II).

During The period in which one or boTh of The removal ploTs were

80

 

m.. 0.m n._ m.. III 5.. m.o III III 0.m 0.m 0.n N.o. ..m w.© m.© 0.mN o.m_ 5<HOH 00 m

 

.0.. A... .0.. .0.. “coo ...o .NoN .ooo “coo A... A... .oom .mve ..om ...m .mom ...0. onm 05<2.z<
21 JC 2 Z Z Z I. II II II II II 6 l. C0 C. II
6 I. 9 .V C. ..l 6 8 9 .V Z O 0 Z .V 9 8 O
9 8 O z .V 9 8 0 Z V 9 8 _ _ _ _ _ _
. _ . . _ _ _ _ _ _ _ _ II 6 IL 9 C. I.
C. Z Z Z Z Z Z .I II I II II 0 0 Z .V 9 8
II 6 I. 03 .V C. .I 6 8 9 .V Z 8
.V 9 8 0 Z .V 9 8 0 Z .V 9

_ 5050 Z. m0350<o Hm<5 00 52.00 05 mmmhmz z. woz<km.o Hmm0<mz

 

.c0>.m 00.0 0. x50m0+00 0050+0.0 5000 c. 0.0 50.53
.0+0+ 05+ +0 +c0oc00 05+ .00.050+ .00005+c0.00 5. “00.05 50+ 0005+ 0.0 00005+50500 5. +05 0.05532
... 050 _ 0+0.0 .0>0500 5.5+.3 0.3+000 +0 +5.00 +0..+ 05+ 0+ . +0.0 5. 053+000 +00. +0 +5.00

05+ 505+ .0.0+05 c. .0050+0.0 +00500c 05+ 00 00+0.:0.00 .. +0.0 505+ 00.0N0 50005< +0 .00.000.Q .mm 0.505

 

 

 

©.m m.N III III ©.m 0.. 0.. 0.. III w.N N.0 0.. m.N N.0 .... .... m.m. N.mN 5(505 00 m
...m A... III III ANVN .0.. A..o A.Vo III AOVN A_VN .0.. A... A.VN .mvm ANvo .mvm AmvN. m5<x.z<
o.N mm. 5m. 0b. we. om. mm. om. 0.. No. 00 mm 00 0m N0 on m. o

 

55(3 m03m05oXm OH wmmhmz z. moz<5m.o Hmm0<mz

 

, .50>.m 00.0 0. >50m0+00 0050+0.0 5000 c.
0.0 50.53 .0+0+ 05+ +0 +c0oc00 05+ .00.050+ .00005+c0.00 c. “00.05 50+ 0005+ 0.0 00005+c0c00
5. +05 0.05532 .. +0.0 500+ >030 053+000 +0c.+ +0 05.+ 05+ +0 ...03 0.:00.0X0 +0.0 .0>0500

I _ +0.0 0+ 0.0+05 c. 0oc0+0.0 +00L00c 05+ 00 00+0.:0.00 .. +0.0 505+ 0050N0 50005< +0 .005000.o ..m 0.505

 

8i

In OperaTion, approximaTer 469 A, §z§:ae_were presenT of one Time or
anoTher In PloT l. AbouT 72 of Them, 0r l5.4 percenT, evenTually
appeared in The removal ploTs. The dispersal of These individuals from
PloT I, calculaTed as The nearesT disTance, in meTers, from The PloT l-
Removal PloT exclosure wall To The firsT poinT of capTure in eiTher of
The removal ploTs ls given In Table 3i. Following ThaT, Table 3i gives
dispersal disTances from The poinT of lasT capTure in PloT I To The
firsT poinT of capTure wiThin eiTher of The removal ploTs. In eiTher
case, nearly 45 percenT of The dispersers moved relaTively liTTIe, i,e,,
only 6—l8 m in Table 30, or 0-36 m as calculaTed in Table 32.

These same individuals, if summed aT The Trapping period aT which
They were firsT capTured in The removal ploTs, Then divided by The PloT
I minimum number known alive for The Time during which The animals dis-»
persed (Table 33) show no definiTe dISpersal raTe paTTerns. The May raTe,
calculaTed for Removal PloT I individuals only, is raTher high and
suggesTs a causal relaTionship wiTh a subsTanTial rainfall ThaT lefT
several inches of sTanding waTer on The CroveTTo PasTure for nearly a
week. July and especially AugusT raTes likewise mighT reflecT laTe
winTer subopTimal condiTions for A, 232533, New vegeTaTional growTh

began To slowly appear in SepTember.

Homing 01_Akodon azarae

 

Homing abiliTy for A, azarae very quickly became an area of
InTeresT during The removal sTudies because of The large number of
PloT l mice Trapped and removed, only To have Them reTurn The nexT day
or Two T0 or near The same Trap of original capTure. Homing was defined
as The reTurn of an animal during The same four day removal ploT Trap-

ping perlod To The same Trap where IT had been capTured or aT anoTher

82

 

mN..

0.

0.05

0

N.

 

00..

0.00

m

0.

00.0

0.0N

m

 

 

.m.mm

00..

m

 

00..

0.0.

5

 

00..

0.0N

m

 

mm..

0.05

m

0

 

50..

0.50

m

m

 

00..

0.

 

.0.n0

00..

5

 

0.

05..

0.00.

0

 

0.

50..

0.

0.00

0

 

0.

0N..

m.mm

5

N.

 

nn.N

n.5N

m

 

50.. 00..

0. 0.
0.N0 N.00

0 N.

 

0. 0N

 

00..

nN

50..

0N

m.00...00

m.

 

NM

0.

0m

.0
o 50 0.555 .0.
omzox
005.5 .oz .5.
owxox
50.33 m .0.
omzox

10.13 .02 Am.

 

m5<z.z< .oz A<.

 

0.N 00.

00.

05.

N0.

00.

mm.

0N.

0..

N0.

00

05

00

00

N0 on

m.

0

moz<5m.0 05.20:
.50.+0..0500 0.5+ +500050 0+ 000.50ml..0 50>0 005530
505+ .00.500 05.0005+ 5000 50+ 00+0.350+ 0503 0+00 .0.030.>.05. .0. 50+ 000.0 500+ 05.505 005.+ +0
505535 05+ .0. 050 .0505 0.5 >..03+00 50.53 505535 05+ .0. .0505 0+ >+.53+50000 050 +000. +0 005
50.53 0.05.50 +0 505535 05+ 00.>.m .<. "5+.3 005000 50505< 50+ 00050+0.0 030.50> +0 0000030 05.501 .0m 0.505

m0w5w2 z.

 

 

\

 

 

._ +O_Q _m>OE®m 0003.05. >.Cu*

mm 50 mm 05 5m 0m 5m. mm. N0. 05. mm. 0.N . 5050 ..05 .2.2

N . 0 m 0 _ 0 m. o. m N. 0 0mm0000.0 .oz

0.0 ..N 5.5 0.0 N.m ... _.n 0.0. N.0 0.0 *m.0 *m._ mmmmmm0m.o 0
mm on mm 0m 0m mm .N m. 5. m. n. ..
.1. Al 3 N m0 8 S V P. .l N V
a o. e O . a 8 n n n 0. d
O. U 3 A |.. d d 5 I U 1A J
. . . o . n... ..I.. . IA 9 .

00.0m0 02.00<05 oz< I529;

 

, I . ..005000.0 +0 5+505
305 5000 +0 05.55.005 05+ +0 . +0.0 5. 0>..0 53055 505535 535.5.5 05+ 050 ... 050 _ +0.0 .0>0500 0+
05.05000.0 505535 05+ 50 00005 .005000 50005< _ +0.0 50+ .+500500 5. ..005000.0 +0 0+0. 5.5+505 055 .00 0.505

 

83
Trap sTaTlon adjacenT To IT. This allowed The mouse To be capTured
again In a possible nine Traps, none more Than I7 m from The original.
If an Akodon was recapTured, This sTaTion Then served as a new cenTer
for possible successive capTures. The fourTh day was only examined To
deTermine If earlier animals had homed.

This meThod should give minimum values for homing abiliTy for
Three reasons. FirsT, animals for The firsT Three days of each Trapping
period were TabuIaTed. By so doing, animals capTured The second and
Third days did noT have equal Time To home as The firsT day. Second,
some animals were noT perceived To "home" unTil subsequenT Three day
Trapping periods. Third, mice could be recapTured adjacenT To The
homing area, and alThough iT was suspecTed from Their movemenTs, pasT
and presenT, ThaT They were in Their home range, These mice could noT
be considered. Since sexual differences do noT seem T0 exisT for The
shorT disTances involved here (Fisler, I962; Furrer, I973; Robinson and
Falls, I965), sexes were pooled.

Homing success aT various disTances (Table 34) for A, azarae for
The four day Trapping periods indicaTe ThaT an average 42.0 percenT of
The Individuals home, regardless of disTance. In addiTion, The raTio of
The number of Times an individual homes over The number of individuals
shows liTTIe difference (perhaps a smaller raTio) wiTh disTance.

HabiTaT QuallTy

Influence gj_pavia gfl_Ak0don populaTions

 

PloT llI (Figure 2) was esTablished in laTe spring (December, I969)

when IT was clear ThaT The Cavia aperea populaTion had reached subsTan-

 

Tlal numbers in The CroveTTo Field, and was in some way depressing

Akodon and Oryzomys numbers, and probably The oTher rodenT species as

84

well.

IT was hypoThesized ThaT Eagle, Through Their devasTaTion of
vegeTaTional cover, were negaTively affecTing Afgggn_densiTy. AlThough
1535321 is The principle myomorph grass—eaTer, food ls really no prob-
lem since Efllli graze on higher parTs of The vegeTaTion. However, 93113
do consume The inflorescences, so whaTever parT seeds conTribuTe To
The Akgggn dieT was presumeably IosT. AAodgn_and Qavia_were found To—
geTher in Sherman Traps aT leasT a dozen Times in The course of The
sTudy and aT no Time did The guinea pig show any sign of predaTIon upon
5539922 nor are There any recorded insTances of Them so doing.

ExcepT for December when The nine Sherman Traps were seT in PloT
III for five days, The monThs January—April each had four-day Trapping
periods. Five larger box Traps were examined daily for Qavi§_which may
have enTered The ploT during The monThs of December To 25 March I970.
By The end of March, Eavia_were enTering PloT III in large numbers.
From 25 March - I9 April I970, one dozen VicTor—Oneida No. 0 sTeeI Traps
were also placed wiThin The ploT To reduce The number 0f.§2¥l§‘ During
This period, Traps were checked aT leasT once a day and ofTen Twice.

AlThough Trapping in This manner is going To bias upwards any
aTTempT To give True values of QEXLE densiTies ouTside PloT lll, some
figures are necessary. From This 36 m x 36 m (.l3 hecTares) ploT, 45
guinea pigs were removed from I December — 25 March, a period of Il5
days. AfTer ThaT To I9 April (25 days), 65 Qavia_were capTured. On I5

March and I5 April, Two LuTreolina, one each day, were live—Trapped

 

I o o o O
and released where capTured. One huron, OaIIcTIS CUJa, likew|se was

 

Taken on 3| March. Of 22 adulT Cavia females examined during The

period from I-I9 April, 39 young were found In 2i animals, an average

Figure l4. Removal PloT l adjacenT To PloT lll — Removal PloT l exclosure.
NoTe The cane on The ground, The lack of inflorescences, and
The generally grazed appearance of each grass (Paspalum

elongaTum) clump.

Figure l5. Same posiTion and daTe as ThaT for Figure l4, buT wiTh 0
Turn of l80°, and now looking inTo PloT llI. There is some-
whaT less cane on The ground, The inflorescences are abundanT,

and The grass clumps are much denser and less dlsTurbed.

86

 

Figure I4

 

Figure I5

87

of l.9 (mode of one) per female. Rood (I972) found a mean liTTer size
of 2.l for This species.

The Akodon populaTion during This five monTh period was as follows:
4, 3, 7, I4, and I6, The laTTer figure nearly equal T0 The densesT
quadranT populaTion of A, azarae found in PloT l during The correspond-
ing Time, QuadranT A (25 Traps, .36 hecTares, versus 9 Traps, .l3
hecTares for PloT Ill).

One Akodon female was recorded pregnanT in December and Two oThers
conTribuTed a ToTal of 7—8 young in February and also in March. A
populaTion of I4—l6 may have been near The upper limiT for PloT lll
since ll of l4 or nearly 80% of The March Akodon were noT presenT in
April. Their disappearance could also be due in parT by predaTion as
Iell as emigraTion, The laTTer faciliTaTed by exclosure disTurbance as
22113 aTTempTed To junp over or dig under The meTaI sheeTing during

repeaTed aTTemst To enTer or exiT.

DISCUSSION

The firsT objecTive of This sTudy was To compare and conTrasT The
various demographic and ecological changes which occur in a yearly cycle
of grassland—inhabiTing SouTh American (ArgenTine) myomorph rodenT popu-
laTions, namely The grass—eaTer, A, azarae, The peromyscine-Iike, seed-
eaTing rice raT, O, nigripes, and Iasle, The insecT-eaTing Q, ruTilans.
This objecTive was only parle realized in The laTTer Two species because
for The sake of larger sample sizes, Their Two field populaTions were
ofTen pooled. Whenever possible, however, The Two field populaTions were
compared and where perTinenT, These resulTs are summarized. The main
objecTive, however, was To compare and conTrasT The basic demographic
and relaTed ecological and behavioral feaTures of The SouTh American
pampan species wiTh The NorTh American TemperaTe grassland myomorphs.
Before This discussion commences a brief commenTary concerning The origin
and evquTion of The aforemenTioned species follows.

PhylogeneTic HisTory of Akodon, Oryzomys, and OxymycTeris_

 

 

The Tribe SigmodonTini (subfamily CriceTinae, family Muridae), To

which Akodon, Oryzomys, OxymycTeris and a number of oTher genera belong,

 

 

is The mosT primiTive and diversified of New World murids (HershkoviTz,
I972). The adapTIve radiaTIon of The NeoTropical SigmodonTIni led To a
group consisTing of some 40 genera and ISO—200 species (HershkoviTz,
I972), which presenle occupy arboreal, pasToral, palusTrine, and aquaTic
habiTaTs and have equally variable food habiTs.

The origin of Sigmodonflni Is quesTIonabIe. HershkoviTz (l969) and

88

89

PaTTerson and Pascual (I972) supposed ThaT They originaTed in boreal
America, Then spread inTo Eurasia and SouTh America. However, HershkoviTz
(I972) poinTed ouT ThaT There is no evidence ThaT SouTh American
sigmodonTines exisTed in boreal America prior To Their invasion To The
former in The laTe Pliocene. He felT There is no likelihood ThaT They
Vgave rise T0, or were descended from Eurasian criceTines. Whereas earlier,
HershkoviTz (I962) aTTribuTed similariTies beTween sigmodonTines and
African (and Malagasy) criceTines as producTs of parallelism, he now
specuIaTes (HershkoviTz, I972) ThaT iT is likely ThaT ancesTraI
sigmodonTine sTock were rafTed from Africa To SouTh America during The
early TerTiary.

CyTogeneTic work by Bianchi e:_§l, (I97l) suggesTs To Them ThaT The
akodonTs, a progressive offshooT of The SigmodonTini, musT have radiaTed
from The cenTral Andean region bounded by souTheasTern Peru, norThern
Chile, Bolivia, and norThwesTern ArgenTina. They give no daTe for These
evenTs buT Reig and Linares (I969) lisT an already specialized pasToral
Akodon from ArgenTine as one of The oldesT (laTe Pliocene) known
sigmodonTines. OxymycTerines are described by HershkoviTz (I972) as
being long-clawed, shrew—like, semi-palusTrine, insecTivorous mice wiTh
akodonT anTecedenTs. In The oryzomine sTock, The small mice of The

scansorial subgenus Oligoryzomys, of which 9, nigripes is a member, did

 

evenTualIy invade grasslands, buT according To HershkoviTz, noT unTil
The PleisTocene.

The NorTh and CenTral American dlsTribuTed Peromyscinl, including
such lnsecT— and seed—eaTing genera as Baiomys, Peromyscus, and

RelThrodonTomys, is regarded by HershkoviTz (l972) as derived from

 

lsolaTed offshooTs of Eurasian criceTines; They are known from The

90

Oligocene. The Typically boreal_grass—eaTing MicroTInI, which wiTh The
peromycines are conTinuously referred To in This manuscripT, represenT
a laTe—appearing criceTid Tribe known firsT from The laTe Miocene (The
period of vasT grassland deveIOpmenT) of NorTh America, and The Pliocene
of Eurasia (Dawson, I967).

PopulaTion Changes

Seasonal densiTy changes

 

The A, azarae populaTion of PloT l liTerally exploded during The
summer preceding The Cavia invasion and remained high (greaTIy so from
abouT February To June, l969) ThroughouT The duraTion of The sTudy. By

The second summer A. azarae reached only one-Third T0 one-half of iTs

 

firsT summer's densiTy. This reducTion appeared due To The devasTaTion
of cover by Cavia, The Hill PloT A, azarae, on The oTher hand, showed
approximaTely The same populaTion densiTies for Those parTs of boTh
summers on which daTa were collecTed, Though somewhaT lower The second
breeding season. In conTrasT To PloT I, hardly any guinea pig acTiviTy
was noTed on The Hill PloT during This sTudy.

The PloT I Q, nigripes similarly had a high populaTion during The
firsT porTion of The l969 winTer, probably in response To a good seed

crop (a resulT of ample rain?) from Paspalum elongafum ThaT firsT

 

summer. None was ever recorded from PloT I offer period 25. Very likely
The 993 a were responsible for This drasTic decline of Q, nigripes

as nearly all infloresences were consumed before Their maTuriTy The
second summer. Only 9, ruTilans, possibly because of ITs mainly
InsecTivorous habITs appeared To escape The drop in densiTy experienced

by The former Two species.

9i

IrrupTIons and cyclic flucuaTIons

 

The high densiTy populaTions observed for A, azarae In PloT I and

 

Hlll PloT during The firsT summer of sTudy may be an example of an
lrrupTive evenT for The species. Crespo (I966) discusses oTher irrupTive

peaks which were documenTed in Buenos Aires Province for A. azarae and

 

several oTher species in l833, I872~73, I944, I963 and I964. Crespo (I944)
found ThaT irrupTIons, aT leasT during some years, were correIaTed wiTh
periods of abnormally high summer rainfall. If, afTer more inTensive
invesTigaTions This proves To be The case, Then The high densiTy evidenced
The summer—fall of l969—70 could be aTTribuTed To ThaT year's increased
precipiTaTion (I60 mm over The normal). lrrupTions of oTher grass-eaTing
criceTines, such as The NorTh American species of Sigmodon and Oryzgmys,
appear To be irregular in period and mainly influenced by climaTic

facTors (Erickson, I949; FleharTy eT al., I972; GoerTz, I964; Negus

eT al., I96l). In agreemenT wiTh Pearson (I967), There is no evidence

of rhyThmical cyclic flucTuaTIons in A59d92_similar in naTure To These
found for many NorThern Hemisphere microTines (ChiTTy I960, I967; Krebs,
I964, I966, I972; Krebs eT al., l969).

DensiTy esTimaTes and seasonal variaTions

 

DensiTy esTimaTes for many grass—eaTing microTines aT various
IocalITies and populaTion levels were summarized by Aumann (I965). More
recenT densiTy esTimaTes are given by Krebs (I964, I966), and Krebs
T al. (l969). EsTimaTes for Sigmodon (GoerTz, I964; Howard, I955;

a...

PeTersen, I973) and Oryzomys palusTris (Negus oi al., I96l) are wiThin

 

The same order of magniTude. For seed-eaTers, Terman (I966, I968) gives
compiled daTa for a number of differenT rodenTs, Including species of

Peromyscus. The resulTs suggesT ThaT Peromyscus populaTion levels are

 

 

Typically low. In addiTion, These populaTions exhibIT, on The average,

92

smaller seasonal mean variaTions Than The grass—eaTing microTines.

Terman's limiTed daTa for The seed-eaTing ReIThrodOnTomys megaloiis

 

mean variaTions appear To fall closesT To ThaT of The microTines.
InsecTivorous mice of The genus Onychomys, based on Their relaTive
densiTy To oTher species capTured The same Time (Egoscue, I960) inhabiT
areas aT even lower densiTies Than The seed-eaTers.

In summarizaTion, evidence is given To indicaTe ThaT A, azarae
reaches densiTies usually reporTed for microTines and oTher grass-eaTers,
and ThaT Q, nigripes is similar in pepulaTion levels and variaTion T0

NorTh American seed-eaTers. OxymycTeris ruTilans is somewhaT similar To

 

The insecT—eaTing Onychomys. lnformaTion provided by Egoscue (I960)
suggesTs ThaT This is True. Terman's (I966, I968) compiIaTion of NACSM

informaTion on The shrew Blarina brevicauda also is in agreemenT wiTh

 

This paTTern.

TrappabiliTy

 

Krebs of al. (I969) found ThaT MicroTus ochrogasTer was very

 

Trappable (90%) while M. pennsylvanicus was less so (SO—80%). AddiTionaIly,

 

AA pennsylvanicus was less Trappable during The summer Than aT oTher

 

seasons of The year. Akodon azarae was mosT similar To M. pennsylvanicus

 

 

in overall TrappabiliTy as well as summer Trapping success. However,
Krebs 21 al. achieved Their success in only Two consecuTive nighTs of
Trapping whereas here iT was Three.

TrappabiliTy of O, nig£ipe§_and O, ruTilans varied considerably
wiThin and beTween ploTs. In general, Their TrappabiliTy was less Than
ThaT found for A, 359523, The only excepTion was HIII PloT 9, ruTilans,
where The resulTs are comparable To These for A. azarae. TrappabiliTy

P"

daTa for NorTh American seed- and insecT-eaTing rodenTs are sadly lacking.

93

TrappabiliTy may differ beTween sex and age classes wiThin a

populaTion. SmITh (l968) found slighT differences In Mus musculus and

 

major differences in Peromyscus polionoTus TrappabiliTy, according To

 

sex. Females pregnanT and/or lacTaTing were noT as likely To be capTured.
Krebs §I_al, (l969) found liTTIe or no difference in Their Two species

of MicroTus, irrespecTive of breeding season. WiTh regard To age (weighT)
classes, Davis and Emlen (I956) found ThaT large (older) Norway raTs are
Trapped sooner Than smaller (younger) individuals. InformaTion concerning
TrappabiliTy wiThin a Three day Trapping period and segregaTed by species,
age, sex, and season will be discussed laTer under The heading "Move-
menT PaTTerns".

AdequaTe sample sizes of all species, plus species specific acTiviTy
paTTerns make TrappabiiiTy comparisons difficulT To evaluaTe. Since each
species does noT have an equal liklihood of enTering a Trap (perhaps
because individuals of anoTher species wiTh an earlier period of acTiviTy
have filled The Traps), The use of differenT Trapping schedules or
muITiple Trap sTaTions should be considered. This is assuming ThaT
behavioral differences are noT greaT enough To seriously affecT
TrappabiliTy beTween species.

RecruiTmenT

 

There appears To be liTTIe or no daTa reporTed on This demographic
parameTer for oTher rodenTs. RecruiTmenT inTo a populaTion may come from
new animals ImmigraTing To The sTudy area or by recording for The firsT
Time animals which were born wiThin The area and have aTTained Trappable
age. During The breeding season, These Two componenTs have yeT To be
saTisfacTorIly separafed.

For A, azarae, The daTa presenTed earlier show ThaT recruiTmenT Is

94

highesT during The summer (breeding) seasons and lowesT during The
winTer. Concerning 9, 212532222 reproducTive acTiviTy, as measured by
TesTIs and vaginal condiTion, and embryo palpaTion, seemed To be Too

low To accounT for a major porTion of recruiTmenT. lnsTead, if is
believed ThaT recruiTmenT raTes remained high because of ImmigraTion,
parTicularly in The CroveTTo PasTure (PloT l) where The highesT densiTies
formed. In facT, a breakdown of recruiTmenT by sTudy ploT showed ThaT The
9, Alg:lpes_recruifmenT raTe in PloT l was approximaTely double ThaT
found for Hill PloT.

OxymycTeris ruTilans presenTs The besT example of a change In

 

recruiTmenT paTTern due To season. ReproducTion occurred aT approxi-
maTely The same raTe year-round. Therefore, any greaT change in re—
cruiTmenT is likely due To a change in emigraTory—immigraTory paTTerns.
In 9, ruTilans, iT appears ThaT There was less vagiliTy in The winTer
season. DaTa presenTed earlier show ThaT The animals Tended To resTricT
Themselves To a smaller area during The winTer monThs.

An examinaTion of Q, ruTilans recruiTmenT paTTerns show ThaT The
drop in winTer raTe was The resulT of fewer adulTs immigraTing inTo The
sTudy ploTs. RecruiTmenT weighTs show approximaTely The same percenTage
(70-75 percenT) of adulTs in The firsT summer as in The winTer. During
The second summer, The same number of adulTs conTinued To be recruiTed
as during The firsT. However, a greaTer number of young animals were
also added To The ToTal recruiTmenT lisT. The resulT was ThaT only abouT
30-40 percenT of The recruiTs consisTed of adulTs. The reason for The
upsurge in The number of young recruiTs Is unknown. IT Is known ThaT
The number of young recruiTs amounfed To abouT 20~25 percenT of The

number of embryos counTed by palpaTlon during The firsT summer and

winTer. The second summer young recruiTs accounTed for nearly 90
percenT of The number of embryos counTed ThaT season. Assuming ThaT
juvenile morTaliTy is mosT likely To occur in The season The embryos
were counTed, and ThaT emigraTion-immigraTion raTes remained approxi-
maTely equal, The 90 percenT reading lndicaTes a high survival of young
compared To The former Two seasons. IT is inTeresTing To speculaTe
wheTher earlier high densiTies of A, azarae during The firsT suumer and
winTer had affecTed O, ruTilans juvenile and subadulT survivalbiliTy.

If The paTTern observed for Q, ruTilans is applicable for A, azarae,
which similarly showed resTricTed movemenTs during The winTer, Then high
summer recruiTmenT is a funcTion of higher adulT recruiTmenT raTes (aT
leasT higher Than winTer adulT recruiTmenT), as well as a funcTion of
The number of young animals reaching Trappable age.

Indeed, when The recruiTmenT segmenT of The populaTion Is divided
inTo adulT and young animals by weighT in The same manner as ThaT for
Q, ruTilans, iT was found ThaT adulT recruiTmenT in A, azarae for The
winTer season was only abouT 7—8 percenT of The ToTal number of animals
capTured each Trapping period. When The summer season was examined, The
number of adulT recruiTs each period increased 2-4 fold over The winTer
season percenTages.

Since adulT recruiTs may be viewed as dispersers (disperse inTo The
sTudy ploTs) The winTer—summer Trends jusT noTed should also be evidenT
In The dispersal raTe daTa deTermined In The PloT I To Removal PloT I
and II A, aza£33_movemenTs. This Is noT The case. If anyThing, The winTer
(May—AugusT) dispersal raTe (which incidenle agrees favorably wiTh The
adulT winTer recruiTmenT raTe) was sligthy higher Than The summer raTe.

There seems To be one likely InTerpreTaTIon of The low summer dispersal

96

raTes. Because of The EEXlB devasTaTion, Removal PloT I and II were

in very poor condiTion by The second summer. The area on which PloT l

was locaTed was The mosT mesic parT of The enTire CroveTTo PasTure,

excepT for porTions around The Arroyo Panfanoso. As a resulT, iT fared
besT during The summer and offered more cover and food Than The resT

of The field. By so doing, dispersal away from PloT l mighT have been

reduced, while recruiTmenT (dispersal info The ploT) would conTinue.

Sex RaTios 1;

NeonaTe A, azarae, as described in Appendix A, did noT show any
significanT deparTure (l:I.2) from The expecTed I:l sex raTio. The sex
raTios observed in The field daTa demonsTraTe ThaT females generally
form The smallesT proporTion of each period's caTch during The winTer
(non-breeding season). However, during The breeding season, The females
consTiTuTe a much higher proporTion of The caTch, alThough noT neces-
sarily over .500. Crespo (I966) did noT find This Trend in his snap—
Trapping sTudy. He reporTed an excess of males, and unlike here The
preponderance was aT iTs greaTesT during The peak of The reproducTive
season. These resulTs musT be an effecT of snap-Trapping, since similar
snap-Trapping resulTs were found in This sTudy (Table II).

Differences in sex raTios were noTed in This sTudy beTween residenT
and recruiT segmenTs of The A, azarae populaTion. This is evidenT in The
"ALL AGES" grouping where residenT females make up a significanle
smaller preporTion In The non—breeding season of boTh sTudy siTes,
whereas There is no significanT difference wiThin The recruiTs. In The
summer, residenT females consTiTuTed a greaTer proporTion Than They did
during The winTer season.

One explanafion for The recruiT—residenT sex raTio disparITy could

97

be ThaT The residenT mice do noT enTer The Traps as ofTen during The
non~breeding season. Because iT was mainly The residenT pepulaTion which
was doing The breeding, noT The recruiTs, high energy demands (Two or
Three fold increase of food inTake) during pregnancy and lacTaTion
(Kaczmarski, l966; Nelson and Evans, |96l), and possible increased
acTiviTy during periods of esTrus (aners, I954; Wang, l923) could
increase Their suscepTibiliTy To live—Trapping.

ln supporT of This explanaTion, reproducTion, which was highesT
during The high densiTy season, also shows The highesT proporTion of
females for The Two breeding seasons. UnforTunaTely, firsT breeding
season daTa are lacking for Hill PloT.

Krebs ei_§l, (l97l) found ThaT alThough laboraTory resulTs for

MicroTus pennsylvanicus and M. ochrogasTer documenTed l:I sex raTios in

 

 

neonaTes, field resulTs showed 53—55 percenT of The residenT Trappable
animals To be females. Secondly, and in conTrasT, The sex raTios of
newly caughT animals (recruiTs) showed a significanT pauciTy of females
(44-46 percenT). Their firsT finding of a high proporTion of residenT
females can perhaps be explained in lighT of my inTerpreTaTion of The

A, azarae sex raTio daTa. Since M. pennsylvanicus and M. ochrogasTer,

 

 

like many microTines, breed more or less conTinuously, Krebs e1_al,
could noT examine The effecT of breeding acTiviTy upon sex raTios.
OTher sTudies have demonsTraTed similar sex raTio resulTs. MicroTus

callfornicus appeared To follow This Trend aT Kreb's (I966) Tilden ConTrol

 

PloT, and GeTz (I960) found if pennsylvanicus in Michigan To do The same.

 

Crawley (l969) also presenTed The same Type of sex raTio resulTs for

CleThrioncmy§_glareolus and Apodemus sylvaTicus of England. However,

 

 

noT all fleld sTudies show The above Trend. GoerTz (l965a,b) sTaTed

98

ThaT when The Ejgmodon hjspidus populaTion densiTy was highesT buT

wiTh liTTIe breeding, There were sligthy more females Than males. During
Times of low densiTy buT more inTensive breeding, signifieanle more
males Than females were Trapped.

UnforTunaTely, iT is difficulT To deTermine if female A, azarae are
more suscepTible To live-Trapping during The breeding season Than during
The non—breeding season. One meThod of arriving aT an answer would be To
examine "TrappabiliTy" beTween The sexes, which has yeT To be done. One
reason for noT pursuing This analysis was based on The work of Krebs

T al. (i97l). Of several MicroTus pennsylvanicus and fl. ochrogasTer

 

 

populaTions sTudied, in only one pepulaTion (M, pennsylvanicus) was There

 

a significanT difference beTween The TrappabiliTy of The Two sexes. The
auThors concluded ThaT differenTial TrappabiliTy of males and females was
rare, and Therefore of lesser imporTance Than oTher facTors effecTing sex
raTios (3333' differenTial secondary sex raTios, recruiTmenT, growTh,
movemenTs, and survivalbiliTy).

AnoTher approach is To find wheTher The capTures per animal wiThin
each Three-day Trapping period vary by sex and season. If females are
capTured more Times per Trapping period during The reproducTive season,
Then inTuiTively There is a greaTer liklihood ThaT a larger percenTage
of The ToTal female populaTion will be capTured.

Akodon azarae recapTure daTa, analyzed wiThin each Three-day

 

Trapping period, supporT The above reasoning. Trappable aduiT females for
The high densiTy summer season of PloT l were capTured aT an average raTe
l0 percenT higher Than males for The firsT seven Trapping periods when
breeding levels were highesT (Table 2|). ResulTs for The firsT Ten Trapping

periods (Table 23), which includes The decline of breeding also shows This

:nm-\-—-I1

99

Trend for PloT l and Hill PloT, buT noT quiTe as sTrongiy. WinTer re-
capTures were nearly equal in boTh sexes for boTh ploTs. PloT l second
summer season resulTs, when breeding was less inTense, show overall
recapTures up (less densiTy, so less compeTiTion for Traps?) from The
firsT season, wiTh males sligthy more Than females. Hill PloT resulTs
for The second breeding season show females To be 9 percenT more suscep-
Tible To capTure Than males. This could be a reflecTion of The higher
inTensiTy of breeding (Table l3) compared To PloT |.

Only one sTudy was found ThaT examined capTures per animal. GenTry

(l968) sTudied pine mice, MicroTus pineTorum, in Two enclosed field ploTs.

 

His resulTs poinTed ouT ThaT females were capTured less ofTen Than males
during The summer buT noT during The winTer. This is jusT The conTrary
of The above resulTs, buT may be due To The fossorial habiTs of This
species.

in reference To The second finding by Krebs e:_al3 (l97i), ThaT
female recruiTs represenT a smaller proporTion of The sex raTio Than
The residenTs, The A, azarae_daTa are also in general agreemenT. As
discussed earlier, iT is The proporTion of residenT females during The
breeding seasons which are significanTiy higher. The recruiT females
(fewer reproducTive animals?) are noT significanTiy differenT from The
expecTed izi raTio. During The non—breeding season, however, The
residenT females drop significanTiy lower Than .500, and The recruiTs
remain relaTively unchanged. The difference in The Two segmenTs of The
female populaTion, parTicularly during The non—breeding season, musT be
a measure of The inTensiTy of selecTion on recruiT pepulaTions To arrive
aT residenT sex raTios. in summer (breeding) populaTions, behavioral

mechanisms Tied inTo The reproducTive cycle appear To be a driving force

1:: {ff-”'F—rll

lOO

in affecTing The difference in The preporTion of residenT females
capTured.

Oryzomys nig£i£e§_populaTions were found in This sTudy and by
Crespo (I966) To have an excess of males. InformaTion compiled by

Terman (i968) for The seed—eaTers of The genus Peromyscus indicaTes

 

ThaT This seems To be The case wiTh Them also. Dunaway (l968) found

jusT The opposiTe for ReiThrodonTomys humulis, alThough closer exami-

 

naTion of his daTa suggesT ThaT The preponderance of females were F_
normally capTured during The breeding season, and ThaT non-breeding
pepulaTions were nearer a l:I sex raTio, or even sligthy favored
males. Few of The Q, nigripes were discerned To be pregnanT, parTicularly
on PloT i, so This non—breeding populaTion would be more inclined To lean
Towards a higher proporTion of males.

The Q: ruTilans daTa, which heavily favor females, regardless of
Time of year, may in facT be aT leasT parle aTTribuTed To The conTinuous
reproducTive sTaTe of The females. These daTa Then follow The paTTern
which is apparenT in mosT oTher closely-sTudied rodenT populaTions; ThaT
is, The females are mosT prevalenT in breeding season Trapping records,
and less so during The non-breeding porTion of The year.

Fisler (l969, l97l), aTTempTed To correlaTe sex raTios wiTh
ecological and social organizaTional sysTems in Three species of

ReiThrodonTomys. He found males were favored during low densiTies and

 

breeding occurred year round. When breeding was aT iTs highesT, The male
sex raTio was even higher Than during periods of less inTensive breeding.
The reason for This preponderance of males during The breeding season is
unknown.

The consisTenle higher sex raTio of males To females irrespecTive

lOI

of Time of year, is ThoughT by Fisler To be an adapTive feaTure for
These low densiTy, small, secreTive mice where The number of Young—bearing
females are aT a premium. The reproducTive sTraTegy in such a case is To
have many reproducTively—acTive males moving abouT To assure inseminaTion
of any females ThaT come inTo esTrus, raTher Than assuming ThaT one male
may conTacT several females, as is probably The case in mosT high densiTy
species where social conTacT is noT a criTical problem.

ReproducTion

DeTails of The posT-naTaI developmenT and sexual maTuraTion of
young A, azarae, g. nigripes, and g. ruTilans, including a comparaTive
examinaTion of Them in relaTion To Their NorTh American counTerparTs, are
discussed in Appendix A.

The lengTh of The breeding season differs for each of The Three
ArgenTine species. The Uruguayan (330-3508) A, azarae bred from OcTober
Through May (Barlow, l969). in addiTion, Barlow menTioned The discovery
of one pregnanT individual collecTed in The souThern parT of The counTry
in SepTembeg i957. ln ArgenTina, Crespo ei._l, (I970) observed pregnanT
A; m in souThern Cordoba 330 50' — 340 35's.) from OcTober To May.
in The norThern parT of Buenos Aires Province (34O lS'S), Crespo (I966)
has Trapped pregnanT animals from OcTober To March. Pearson (l967), who
capTured A, aga£§e_near The ciTy of Buenos Aires (3508), found The
season of birThs To exTend from OcTober To April. FurTher To The souTh
(37O 45'5), The Balcarce populaTions were found To have a somewhaT
shorTer breeding season, exTending from The firsT week of November To
The middle of March or April, depending on The siTe.

-‘pm—

season during which lT is able To reach and malnTain hlgh densiTies

|02

(insTanTaneous raTes of pepulaTion growTh To r=.l3, very comparable To
Those of some microTines, according To Krebs el_al,, l973). During The
period of high densiTy growTh, A, azarae seemed To have a sligthy
exTended breeding season. WinTer breeding is a conmon occurrence in
microTines (Krebs, i964, I966; Krebs _T."l., I969), and Sigmodon_(60erTz,
I965) and Oryzomys (Negus 91 21,, l96l) in The souThernmosT parTs of
Their U.S. range.

ObservaTions by Crespo (l966), Pearson (l967) and from The presenT
sTudy show ThaT young of The year do parTicipaTe in reproducTion during
The same season in which They are born. My field daTa indicaTe The young
commence breeding when approximaTely Two monThs of age. However, iT
appears ThaT young do noT become reproducTively acTive if They have noT
reached Two monThs of age (abouT l8—20 g) by The lasT days of February
or firsT days of Maich.

In agreemenT wiTh The findings generally reporTed for microTines
(HamilTon, l937a; Keller and Krebs, l970; Krebs ei_al,, l973), A, azarae
aTTained sexual acTiviTy aT a lower age during The period of high densiTy
growTh (firsT summer, see Tables l6, l7). Secondly, liTTer sizes were The
same regardless of The populaTion densiTy. This lasT feaTure may or may
noT occur in microTine populaTions aT high densiTies (Schaffer and
Tamarin, l973). Concerning oTher grass—eaTing species, Odum (l955) found

ThaT Sigmodon hispidus liTTer sizes were greaTer in years of high densiTy

 

Than Those from years of low densiTy. Negus ej_al, (l96l) found The same
occurrence in pryzomys palusTris. Negus ei_ai, noTed ThaT younger animals
reached sexual maTuriTy aT This Time also. IT appears ThaT The feaTures

jusT menTioned are shared To one exTenT or anoTher by many rodenT species

and serve as mechanisms To increase producTivlTy during favorable

l03

condiTions.

ShifTing The discussion To 9, nigripes, Barlow (l969) believed ThaT
April and May were The monThs of peak reproducTion, and The earliesT
acTive female was found lacTaTing in January. Crespo (I966) mainTained
ThaT Q. nigripes reproduced during The same monThs as A, azarae; ThaT is,
from OcTober To April. The Balcarce animals were aT such low densiTies
during The early summer of boTh seasons ThaT iT is difficulT To seT a
daTe for The iniTiaTion of reproducTion. ScroTal animals were firsT
deTecTed in The lasT week of November. No embryos were palpaTed in The
few females presenT unTil January. Breeding conTinued inTo May, possibly
longer (AugusT?).

This sTudy, like Barlow's, suggesTs ThaT Q, nigripes probably breeds
laTer inTo The auTumn Than A, azarae. The period of peak reproducTion
could noT be discerned in This sTudy, alThough a good proporTion of The
populaTion was reproducTively acTive in February and March. No females
were capTured more Than once. This may be due in parT To Trap morTaliTy
and a higher diluTion raTe. Since several female 9, nigripes weighing
l6 9 (abouT Three monThs of age) in January were or shorle afTer became
pregnanT, iT appears ThaT young born early in The reproducTive season
do become sexually maTure The same season.

However, wheTher early young of The year do reproduce during The
same reproducTive season can be quesTioned. One of The young animals
referred To in The above paragraph, firsT capTured The firsT week of
January aT l6 9 Took nearly Two monThs To gain 5 g (iT was deTecTed To
be pregnanT aT iTs second capTure). If The laboraTory posTnaTal growTh
daTa approximafed growTh raTes under field condiTions, Then lT Takes

approximaTely Three monThs of age To reach lO—l2 g, and since growTh is

..A‘fl’
u

l04

already reduced by ThaT Time, iT would Take aT leasT anoTher Two monThs
To reach l6 9. This would make The animal abouT five monThs of age by
January, wiTh a birTh daTe around The firsT of AugusT or earlier if
winTer growTh is reTarded in This species. The laTTer possibiliTy may
be True since scroTal (=breeding) males were noT found afTer May, and
IS g animals were sTill capTured as laTe as SepTember. Crespo (l966)
also reporTed low weighTs for young mice in laTe winTer.

ReproducTive daTa for seed-eaTers, primarily Peromysgus, are scanTy

 

(Terman, l968) and noT readily comparable in many aspecTs wiTh The

similarly poor 9, nigripes informaTion. Howard (l949) found ThaT prairie

deermice (E, maniculaTus bairdi) formed a bimodal disTribuTion of birThs,

 

wiTh a peak in laTe spring produced by overwinTering adulTs, and a
higher peak in The fall produced by young born earlier in The year. The
same Trend appeared in The easTern harvesT mouse (Dunaway, l968). lT is
suspecTed To occur in Q, EiEElE223 There may also be a similar synchroni-
zaTion of breeding in A. azarae (Table l3). OverwinTering adulTs produce
a peak of young in The spring. A few old adulTs and Their firsT liTTer
conTinue for one or aT mosT, Two iiTTers more before Their young began
To breed jusT as fall arrives.

Year—round breeding of 9, ruTilans was suspecTed by Barlow (l969),

and documenTed here. His sample of The oTher Uruguayan insecTivorous

myomorph, ScapTeromys Tumidus, showed ThaT This animal also appeared To

 

breed year-round. Perhaps insecTivorous rodenTs, in a given seT of
climaTic condiTions, have a more sTable year-round food supply Than do
grass" and seed—eaTers.

in areas where food supplies dwindle during The much cooler winTer

(unlike The less harsh climes of Uruguay and Balcarce, ArgenTine),

 

l05

conTinuous breeding may noT be possible for insecT—eaTers. WinTer
breeding has noT been reporTed for The NorTh American Onychomys (Egoscue,
l960; Horner and Taylor, l968), presumeably because of The pauciTy of
preper foodsTuffs or perhaps oTher environmenTal facTors. The besT known

of The NorTh American shrews, Blarina brevicauda, is apparenle capable

 

of aT leasT low inTensiTy winTer breeding (Dapson, l968). WinTer food
habiTs of B, brevicauda indicaTe ThaT animal food conTinues To be of
major imporTance, alThough vegeTaTion inTake approximaTely doubles in i
volume (HamilTon, I930).
Survival RaTes

One of The mosT difficulT parameTers To calculaTe for mice is
survival. According To pasT researchers, The survival of a rodenT depends
on such facTors as age, movemenTs, sex class, season, dispersal, recruiT-
menT, predaTion and pepulaTion phase (GeTz, l960; Krebs, I966; Krebs
_:‘ai., l969, l973). Some of These parameTers have been or will be
covered in oTher secTions of The discussion. OThers are noT criTical To
The poinT which is To be sTressed here; ThaT is, are survival raTes of
_grass—eaTers and seed-eaTers differenT?

Unlike HolarcTic microTines, A, azarae does noT breed in The winTer,
so any populaTion decline aT This season can be mainly aTTribuTed To
morTaliTy. This assumes ThaT immigraTion and emigraTion approximaTely
compensaTe each oTher. True survival raTes for The Trappable populaTion,
Then, should be closer To 90-95 percenT for The winTer season Than The
minimum survival raTe of 80—83 percenT.

Can The difference beTween These Two values be explained in lighT of

The daTa analyzed Thus far? Dispersal raTes for PloT l equalled abouT

 

|06

5 percenT per monTh, regardl $5 of season, based on The dispersal daTa.
if The road side of PloT l is considered a formidable barrier To
dispersal and someThing near 5 percenT also disperses from The oTher
Two sides for which no daTa are available, a minimum of l5 percenT is
known To disperse each monTh, or 7.5 percenT beTween Trapping periods.
This laTTer figure approximaTes The winTer recruiTmenT (dispersal inTo)
raTe (9 percenT). Changes in movemenT (radii of acTiviTy) and adulT
recruiTmenT, (and presumeably dispersal Though noT shown here), and
The onseT of reproducTion make iT unreliable To obTain summer True
survival raTes.

The effecT of predaTion on A, az§:§e, as indicaTed by an analysis
of over 300 predaTory mammal scaTs and bird—of—prey pelleTs collecTed

from The CroveTTo PasTure during The duraTion of This sTudy suggesT

ThaT predaTors conTribuTed aT mosT only several percenT To The A, azarae

(and The oTher Two species also) morTaliTy raTe per monTh.
As sTaTed by Krebs ei__l, (l973), deaTh raTe measuremenTs are
available for a relaTively small number of rodenT pepulaTions. However,

iT appears ThaT A, azarae enjoys a raTe roughly comparable To ThaT for

microTines (Krebs, l964, l966; Krebs g: 21,, I969). OxymycTeris ruTilans

 

may fiT in The caTegory wiTh A, azarae and The microTines. Survival
values for Q, nigripes are somewhaT below The former Two ArgenTine
species. This is noT whaT mighT be expecTed since oTher seed—eaTers such

as Those sTudies ciTed by Terman (l968) for ReiThrodonTgmys humulis

 

sTrongly suggesT ThaT populaTion Turn—over Takes longer in These forms
as compared To grass-eaTers. Perhaps The disappearance of Q, nigripes
from The communiTy is due To emigraTion from The sTudy siTes To oTher

areas wiTh more food resources or is a resulT of Their grouping behavior

 

|O7

(Crespo, l966), discussed laTer.
MovemenTs

Much of The informaTion originally covered in The "RESULTS" secTion
concerning movemenTs and dispersal have been discussed under The headings
previous To This and will noT be Thoroughly covered here. Emphasis will
be given To as yeT undiscussed Tepics.

The spaTial disTribuTion, uTilizing The cenTers of acTiviTy, was
deTermined for each species by dividing boTh PloT l and Hill PloT inTo
four quadranTs. For PloT l animals, A, azarae and Q, ruTilans were
densesT in These areas where vegeTaTion cover was mosT dense. As
vegeTaTion characTerisTics changed under The influence of Eavia_so did
The densiTies of The above Two species shifT To Take advanTage of

remaining cover. Onyzomys nigripes was noT so resTricTed To vegeTaTive

 

cover during The firsT half of The sTudy. The qualiTy of vegeTaTive

cover was found by vegeTaTion analysis To be much less in PloT l by The
second summer. The Eagle exclosure experimenT documenTs The damage These
animals can do To a fallow pasTure planTed in selecTed grasses. The
effecTs of The large densiTy build—up of 92112 is reflecTed in The lower
densiTies (disappearance of Q, nigripes) and changes in The spaTial
disTribuTion of The small rodenT inhabiTanTs as They adjusTed To The

new carrying capaciTy of The pasTure. The Hill PloT species, parTicularly
A, azarae, reflecT The heTerogeneiTy of The cover in ThaT The difference
from one quadranT To anoTher was much less Than in PloT I. This was noT

expecTed because of The large percenTage of pure Paspalum quadrifarium

 

sTands. ApparenTiy This unpalaTable grass offers an abundanT seed supply
in season, as well as abundanT cover (including a Thick layer of ground

debris) for lnverTebraTes and nesTing acTiviTies.

 

l08

"RecapTure per Trapping period" daTa indicaTe The probabiliTy of
recapTuring a mouse more Than once during any Three-day Trapping period.
Plausible reasons why species differences in recapTure were discussed
earlier. WiThin species differences suggesTed ThaT residenT adulT

A. azarae females were recapTured more frequenle Than males.

w

 

MovemenT daTa (Table 27) for adulT females in The high—densiTy
season in PloT l, and To a cerTain degree Hill PloT, sTrongly suggesT
ThaT during seasons of high reproducTive inTensiTy, The disTances

Traveled is reduced. Krebs (I966) sTaTed ThaT MicroTus callfornicus

 

males averaged longer movemenTs beTween capTures Than females during
The breeding season, buT This was noT The case during The non—breeding
epoch. V

A more correcT inTerpreTaTion, parTicularly in reference To A,
azarae, should include The recogniTion ThaT female mice under high
inTensiTy breeding will show markedly decreased movemenTs (and become
recapTured more ofTen). ReproducTive acTiviTy of The females reduces
The lengTh of movemenTs beTween capTures more Than do differenT seasons
or densiTies. This reasoning is sensible when one realizes The differenT
energy demands and behavioral paTTerns associaTed wiTh The gesTaTion
and care of The young. In such periods of reproducTion, The females
resTricT Themselves more To foraging near The nesT. This behavior presenTs
no food—acquiring problems since food, be iT insecTs, seeds, or grasses,
is generally noT resource—limiTed during periods of peak reproducTion.
A sTudy by Blair (l953) suppchs my inTerpreTaTion of The movemenT daTa.
He found ThaT Two Species of kangaroo raTs in a souThern New Mexico
mesquiTe associaTion ranged less widely aT abouT The onseT of The re-

producTive period.

 

l09

By examining movemenT daTa wiThin a Trapping period, such as was
done in The radii of acTiviTy analysis, one is beTTer able To observe
changes in movemenT paTTerns associaTed wiTh breeding females Than wiTh
novemenf daTa calculaTed beTween Two or Three Trapping periods (2—4
weeks). ReproducTive Trends mighT be losT in These longer Time spans
where a female could be caring for young one period and The nexT Time
Trapped be non-pregnanT and non—lacTaTing (young weaned).

PloT differences were influenced To a large degree by The 93313.
in CroveTTo PasTure. However, iT was noTable To observe The smaller
radii of acTiviTy for A, azarae in PloT l compared To Those in Hill PloT
during The firsT summer. This is assuming ThaT densiTy was noT a criTical
facTor.

Sexual differences in movemenT lengThs and home ranges of small
rodenTs, regardless of species, is The general rule (Blair, l953; Brown,
l962; STickle, l968), wiTh males having somewhaT longer movemenTs Than
females. This is The case here wiTh A, azarae, buT no definiTe Trend
was clearly evidenT wiTh The oTher Two species.

Whereas mosT sTudies demonsTraTe ThaT rodenTs have smaller home
ranges in The winTer Than in The summer (Blair, l953; STickle, l968),
Hill PloT A, azarae_appeared To increase Their radii of acTiviTy during
The winTer season. By shifTing Their acTiviTy Towards The warmer daylighT
hours, cold TemperaTures are less of a criTical facTor, and furTher move—
menTs may resulT in response To a decreased supply of available lnverTe-
braTes, seeds, and oTher foodsTuffs. UnforTunaTely, liTTle movemenT daTa
are available on souThern UniTed STaTes grass-eaTers where TemperaTure
may be less of a limiTing facTor Than food. AbundanT food and/or cover

resulTs in smaller ranges Than when These resources are limiTed (Blair,

IiO

I953; STickle, l968).

Home ranges and movemenTs of mammals are affecTed by Their TrOphlc
sTaTus (McNab, i963; Sanderson, l966). WiThin The rodenTs, The insecT—
eaTing Onychomvs have The largesT home range and daily movemenTs,
followed by The seed—eaTing Percmyscus, (even The insecTivorous shorT-

Talled shrew, Blarina brevicauda according To Blair (I953), has a range

 

as large or larger Than EEKEEXEEE§.C8PTUFGd under similar condiTions)
and lasle, The grass-eaTing microTines. The ArgenTine species sTudied
here showed The same Trends. The insecTivorous Q: ruTilans were found
To have movemenTs longer or aT leasT similar To The seed—eaTing Q:
nigripes, and A, azarae, like The microTines, was acTive over much
shorTer disTances.
Dispersing and Homing

Dispersal of animals was suggesTed by Lidicker (l962) To offer
Three possible advanTages To The individual. These are: (I) The disperser
mighT come inTo conTacT wiTh more individuals (perhaps also in a more
habiTable locaTion) and breed more ofTen; (2) The geographic spreading
of geneTic maTerial increases The chances of new advanTageous recombi-
naTions of geneTic maTerial such ThaT offspring will have greaTer
heTerozygosiTy and increased fiTness; and (3) individuals which leave
high pepulaTion densiTies may in some cases have a higher survivalbiliTy
Than Those which remained during The populaTion crash.

As Myers and Krebs (l97l) demonsTraTed in field pepulaTions of

 

 

MicroTus_pennsyIvanicus and Ag ochrogasTer, There appears To be geneTic
differences beTween dispersing and residenT populaTions. This supporTs
Howard's (I960) hypeThesls ThaT a geneTic polymorphism influences The

Tendency To diSperse, and Erringion's (I956) and Lidicker's (l962) Theory

 

ThaT emigraTion Is a key componenT of populaTion regulaTion. ChiTTy‘s
(l960) hypoThesis ThaT a behavioral polymorphism regulaTes populaTion
densiTy was supporTed when dispersing and residenT mice displayed
differenT behavioral TraiTs; A, azarae was examined To approximaTe
losses, by sex and season, from PloT l aTTribuTabie To dispersal, and
noT parTicularly To esTimaTe iTs value in pepulaTion regulaTion.

Sex raTios of A, azarae dispersing inTo The removal ploTs show ThaT
CroveTTo PasTure adulTs were more likely To reTain a l:I sex raTio during
The winTer (non—breeding season). During The second summer (breeding
season), The number of females capTured dropped significanTiy. UnforTu-
naTely, The removal ploTs were noT in eperaTion The firsT summer and
The low sample sizes of PloT I dispersers discourage meaningful evalua-
Tion.

IT was noTed earlier in The discussion of recruiT-residenT sex
raTios ThaT adulT residenT female A, EEEEEE Typically made up a larger
proporTion of The sex raTio during The breeding season. IT is believed
ThaT The opposiTe Trend of fewer female dispersers aT This Time is due
in parT To many of The residenTs becoming esTablished in a seT area
during The reproducTive season. The small number of Juveniles is indica-
Tive of The facT ThaT A, azarae, like oTher mice, generally do noT
disperse unTil puberTy (Blair, i953; ChrisTian, I970, Myers and Krebs,
l97l).

Myers and Krebs (l97l) reporTed a general excess of dispersing male
MicroTus in Their sTudies, parTicularly as Their daTa suggesT, during a
period of raTher inTense breeding. lnsTead of menTioning The above
possibiliTy, ThaT is, The sedenTary naTure of The reproducTive females,

They believed ThaT The excess of males mosT likely reflecTs The facT ThaT

 

|l2

males move greaTer disTances, cerTainly a facTor buT perhaps noT a major
one.

ApproximaTely Three quarTers of The marked A, azarae dispersed no
farTher Than IOO m info The removal ploTs, based on The four—day monTth
Trapping Tallies. iT ls assumed ThaT mosT of The dispersers had become
esTablished on The removal ploTs, and ThaT furTher dispersal would noT
conTinue. The homing daTa collecTed for A, aga£ae_indicaTe ThaT This is
True a large proporTion of The Time. WhaT effecT The removal ploTs had
in creaTing a "sink" which mighT encourage dispersers To sTop moving is
a valid poinT. Dispersal disTances given by Blair (l953) for various
small mammals and by STickel (I968) for Peremyscus suggesT no greaT
deviaTion from ThaT observed. A more valid TesT would be To examine The
AcTiviTy PloT daTa and compare The number and disTances Traveled for
PloT l dispersers There wiTh Those in The removal ploTs.

The monThly raTe of dispersal appeared To flucTuaTe some according
To severe weaTher condiTions and seasonal (winTer) deTerioraTion of The
habiTaT. Concurring wiTh The sTudy by Myers and Krebs (l97l) on MicroTus,
The dispersal raTe of A, azarae was found To be raTher consTanT, irrespec-
Tive of densiTy. However, iT may be under seasonal (reproducTive?)
influences, based on The earlier discussion of dispersal inTo (recruiTmenT)
PloT I.

Homing abiliTy of A, azarae appears To be well developed. The
adapTIve significance of homing in mice was discussed by Furrer (I973)

in his sTudy of homing in Peromyscus maniculaTus. According To his work

 

and ThaT by Robinson and Falls (I965) for MicroTus pennsylvanicus, The

 

raTher censTanT success aT homing in A. azarae mighT be expecTed for

even greaTer disTances. Where Robinson and Falls found an almosT linear

m 1—”

ll}

relaTionship in Aflcroig§_beTween disTance and homing success, Furrer

reporTed a curvilinear funcTion which was formed if long disTances (500
m or more) were involved. WheTher differences were due To The differenT
social organizaTion and lower mobiliTy of AficroTus compared To The seed-

eaTing Peremyscus is an inTeresTing poinT considered by Furrer. If in

 

facT furTher sTudies subsTanTiaTe This difference, iT would be inTeresTing
To find where A, azarae fiTs lnTo The scheme. UnforTunaTely,The disTances
examined in This sTudy were raTher shorT.

AcTiviTy and Food HabiTs

Akodon azarae is acTive day and nighT, wiTh generally higher peaks

 

of acTiviTy during The TwilighT and The early evening hours. On The

oTher hand, The seed—eaTing Oryzomys nigripes appears To be sTrichy

 

nocTurnal and The insecTivorous OxymycTeris ruTilans, diurnal (pers.

 

daTa from AcTiviTy PloT). A furTher discussion of Their acTiviTy paTTerns
and Their significance follows.
mm

Fornes and Massoia (l965) found A, agagag_To be acTive day and
nighT during snap—Trapping sTudies in AugusT (winTer) aT Miramar, Buenos
Aires Province, ArgenTina. Crespo (I966), snap-Trapping during various
monThs in The ParTido of Rojas, Buenos Aires Province, sTaTed ThaT A,
aga:ae_was mosT acTive during nccTurnal hours, while a parTially sympaTric
species, A, EEEEEEEEJ was mainly diurnal. Barlow (l969) likewise discovered
The same resulTs wiTh his Uruguayan A, azarae_and A, obscurus snap-Trapped
during December—May (mainly summer).

In The presenT sTudy, A, azarae became inacTive during mosT of The
day and more acTive during The TwilighT and evening hours of summer. This

Trend reversed iTself during The winTer when evening achvlTy decreased

 

ll4

and diurnal acTiviTy increased. Presumeably, a shifT in acTiviTy paTTerns,
In order To avoid TemperaTure exTremes, would resulT in ThermoregulaTory
energy savings for The species. The behavioral flexibiliTy To change
acTiviTy paTTerns according To cerTain climaTic facTors no doubT accounTs
for some of The divergence in findings, as does The various meThods used
in deTermining The periods of acTiviTy.

Oryzomys nigripes were found by Fornes‘and Massoia (I965) To be

 

nocTurnal, alThough some were capTured during The hours of dusk. In

addiTion, They found all buT Two (ouT of fifTeen) OxymycTeris ruTilans

 

acTive beTween noon and 3:00 pm. Barlow (l969) gave no daTa for Oryzomys

nigripes, buT found ThaT OxymycTeris :ujgs_(= 9, ruTilans) was primarily

 

diurnal alThough specimens had been Trapped aT dusk and The early evening.

The sligthy larger swamp raT, ScapTeromys Tumidus, a less specialized

 

insecTivorous mouse (based on sTomach morphology) and lnhabiTing mesic
areas in Uruguay wiTh Q, ruTilans, was mainly nocTurnal, and Thus
compeTiTion was ThoughT To be minimal.

lT appears ThaT all NorTh American grass—eaTers (MicroTus, Sigmodon,

 

Oryzomys) are generally acTive boTh day and nighT wiTh possible crepuscular
peaks (Ambrose, I973; Baker, l969; Calhoun, I945a,b; HamilTon, I946;
HanieId, i940; HeidT, l97l; OsTerberg, I962; Pearson, l959, I960).
Oryzomv§_may be The mosT nocTurnal of The group (Harris, I953; Negus e:
31,, l96l), buT seemingly is The mosT insecTivorous (Sharp, I967). Seed—

eaTers (Peromyscus, ReiThrodonTemvs, and The heTeromyids) and InsecT—

 

eaTers (pnychomys) are mainly nocTurnal (Falls, l968; Jahoda, I970;
Pearson, l959, I960, Reynolds, I960; Tlnkle and Harmon, l970). WlTh The
excepTlon of The inSOCllVOrOUS and diurnal Q, ruTilans, The ArgenTine

species follow The above acTiviTy paTTerns.

|l5

AcTiviTy paTTerns and eaTing‘AabiTs

Seed— and insecTreaTers consume food which is higher in digesTible

 

proTein and, as CarleTon (I973) believed, carbohydrafes. Such foods
remain longer in The digesTive TracT. As KosTelecka—Myrcha and Myrcha
(I964) demonsTraTed, even microTines which are fed seeds show a 2.5 To
3 Times slower food passage Through The alimenTary TracT Than when fed
green planT parTs. STomach morphology and food habiTs of criceTines are
raTher difficulT To correlaTe compleTely (CarleTon, I973), mainly
because The varieTy of foods eaTen over various seasons is unknown.

STudies by Jameson (I952) and WhiTaker (I966) on Peromyscus and Mus,

 

and BaTzli and PiTelka (l97l) on MicroTus californicus showed The

 

diversiTy of foods which may be consumed by a "seed-eaTer" or a "grass-
eaTer". IT is imporTanT To poinT ouT, however, ThaT grass—eaTers eaT
more seeds Than seed~eaTers eaT of grass. Seed-eaTers in Turn eaT mere
inverTebraTe maTeriaI Than mosT grass-eaTers. In This regard, The True
sTaTus of A, azarae remains quesTIonabIe. lTs food habiTs (auThor,
cursory examinafion of collecTed alcoholics; Barlow, l969) show ThaT
animal maTerial may be an imporTanT componenT of iTs dieT aT leasT
during some seasons of The year. According To Ellerman (l94l), flESEEfl.
denTiTion, oTher Than being somewhaT hypsodonT, unlike oryzomine sTock,

shows no more modificaTion for grass—eaTing Than relaTed Calomys,

ZygodonTomyg, or even someTimes PhylloTis, possibly The mosT herbivorous

 

of The group (DorsT, l97l, l972).
As for 9, 31351333, Barlow (l969) found ThaT each of Ten sTomachs
examined conTained green planT maTerial, while five conTained some

inverTebraTe remains. OxvmyeTeris ruTilans sTomachs (Twelve) conTained

 

nearly all inverTebraTe maTerial. The sTomachs of The True lnsecT-eaTers

ll6

(Onychomys, pxymycTerjs, SgapTeremys) are modified for Their specialized

 

dieT (Horner, I962; lorner §:_al,, I964; Barlow, I969).

While The acTiviTy of grass-eaTers is more or less dicTaTed by
food requiremenTs ThaT necessiTaTe numerous feeding forays, granivorous
and insecTivorous species can conTain Their acTiviTy rhyThms wiThin a
favorable parT of The day. For Those in The warmer climes, evening
presenTs The Time when TemperaTures are IowesT and wafer, in The form

of dew, is mosT available. Even The grass-eaTing MicroTus californicus,

 

which swiTches To a mainsTay of grass seeds during The dry summer monThs
(BaTzli and PiTeIka, l97l), also assumes a predeminaTely nocTurnal acTiviTy

paTTern (Pearson, I960). Meserve (I97l) discovered ThaT M. ochrogasTer in

 

wesTern Kansas became acTive mainly during The TwilighT hours of sunmer
and spring, buT diurnal during winTer. Through analysis of Their droppings,
iT appeared ThaT The winTer mice relied on Their underground cached food,
and perhaps appeared above ground only when TemperaTures were less exTreme
(daylighT hours).

In addiTion To cooler TemperaTures and more favorable moisTure
condiTions, Two oTher facTors may be of imporTance in influencing nocTurnal
acTiviTy. The firsT, food availabiliTy, is of mosT imporTance To insecTiv-
orous xeric-inhabiTing species such as Onychomys where The prey are mosT
acTive (scorpions) or mosT easily capTured (grasshoppers) during The
evening (Horner e:_§l,, I964). To a mesic—inhabiTlng species (g,g,,g,
ruTilans) or even The shrews (OsTerberg, I962), nocTurnaliTy evidenle
is noT necessary.

The remaining faclor is predaTion. PasToral species of canlds, felids,
musTelids, The hawks, falcons and even some owls ThaT subslsT heavily on

rodenTs are commonly diurnal or crepuscular in hunTlng habiTs; These are

 

ll7

The hours when The largesT biomass of rodenTs, The grass—caTers, are
acTive. IT is hypoThesized ThaT The nocTurnaI seedeeaTers, because of
Their acTive food searching forays Through and over planTs, would be

besT proTecTed from predaTion during The evening. Even when discovered,
escape would seem more likely, due To lessened visibiliTy by The predaTor
(since iT is evening). These mice also have more of a Three-dimensional
escape ploy (verTically Through The habiTaT as well as horizonTally)
compared To The Two—dimensional escape paTTern (usually along a well 3
esTablished runway sysTem) uTilized by grass~eaTers. In addiTion, The
facT ThaT seed-eaTing mice are elusive and less common Than Their grass—
eaTing coinhabiTanTs would lead To hunTing seiecTion away from, noT
Towards, The Temporal and spaTial requiremenTs of These mice.

A number of pasToral rodenT predaTors are (or aT leasT unTil
recenle) inhabiTanTs of The Balcarce area: one fox, Two caTs, Two
musTelids, Three owls, Three hawks or falcons and several snakes. Their
poTenTial for effecTing selecTive pressures on The Temporal (and spaTiaI)
paTTerns of Their prey is probably subsTanTial. However, few if any
sTudies have been conducTed which aTTempT To correlaTe The Temporal
hunTing sTraTegy of These predaTors wiTh Their prey. Of inTeresT in This
vein is The survival value of mainTaining a diurnal acTiviTy paTTern as
found in boTh The insecTivorous Q, ruTilans (does iTs disTincTive smell
proTecT iT from predaTion?) and The similar—sized insecTivorous didelphid,

Monodelphis dimidiaTa (pers. daTa; Fornes and Massoia, I965).

 

Several oTher sTudies have aTTempTed To compare The dynamics of
grass—eaTers wiTh seed—eaTers, Hansson (I97I) presenTed a comparison of

The acTiviTy paTTerns, feeding habiTs and populaTion dynamics of

herbivorous and granlvorous mice in Scandinavia. He mainTained There was

ll8

a clear inverse relafion beTween seed and animal food occurrences in
. granivores which varied according To Their needs. The herbivores needed
more waTer Than The granivores, probably oTher or greaTer amounTs of
micro—nuTrienTs, and had a more even diel rhyThm of food inTake Than
The nocTurnal granivores. The need for seed food was very evidenT in
The granivorous species, indicaTing To Hansson by iTs effecTs on The
pepulaTion dynamics of These animals, ThaT There was a causal relaTion-

ship beTween frucTificaTion and dynamics. SmiTh (l97l), like Hansson,

 

found food To be a limiTing facTor in The densiTy of Peromyscus polionoTus,

 

a seed—eaTer. The primary consumers, like Augioigs, have failed To show
any populaTion increase when food was added under field condiTions (Krebs
and DeLong, I965). SmiTh (l97l) Then ciTed a paper by Slobodkin ei_al,
(I967) as supporTing evidence for The above findings. The laTTer conTended
ThaT animals which are planT producT consumers or omnivores (Peromyscus,
9, nigripes) are food-limiTed while Those which are vegeTaTion consumers
(microTines, probably A. azarae) are noT.

EaTing habiTs and meTabolic raTes

 

Grass-eaTing myomorphs, like hoofed animals, spend a large porTion
of Their Time eaTing. Grass, unlike seeds or insecTs, is high in cellulose
and low in proTein; ThaT is, The mice consume a relaTively bulky, low
energy food (EilioTT, I963). A raTher efficienT digesTive TracT has
developed (Golley, I960) To readily uTilize The energy from This Type
of foodsTuff. As a resulT of a fasT passage Time of low qualiTy food
Through The guT (Jameson, I947), These mice ofTen remain acTive aT various
levels over a 24 hour day.

CorrelaTed wiTh Their frequenT bouTs of acTiviTy, Hanield (I939)

deTermined The meTabolic raTe of flicrofus callfornicus To be subsTanTlally

 

ll9

 

higher Than ThaT for The seed—eaTing Peromyscus maniculaTus. He aTTrib-

f‘iv

uTed This To The greaTer feeding acTiviTy and likewise wiTh The Type of

food eaTen by MicroTus pennsylvanicus. Since Then, a number of oTher

 

peeple, including Packard (l968), have examined oTher species of
microTines and found The same resulTs. Packard believed ThaT a higher
raTe of meTabolism is correlaTed wiTh The maximal level of heaT produc—
Tion ThaT The TemperaTe—derived voles are able To aTTain during periods
of cold exposure.

The grass—eaTing criceTine Siemodon, CenTral American in origin,
does noT show The high meTabolic raTes of microTines. Bowers (l97l)
examined several species and geographical races of Sigmodon, and found
ThaT Their resTing meTabolism was in The range expecTed for animals of
Their size. ObservaTions by Cockrum (I948) and Dunaway and Kaye (l96l)
subsTanTiaTe The claim ThaT Sigmgdgn reaching Their norThern limiTs
(Kansas, Tennessee) of NorTh America are noT well-adapTed for These
norThern climes. They noTed a high morTaliTy during severe winTers, and
aT The leasT, coTTon raTs appeared To commonly suffer winTer weighT
losses (Chipman, I966; Dunaway and Kaye, I96I, I964; FleharTy gingi.,
I972; GoerTz, I965; Sealander and Walker, I955). IT is known ThaT
esTimaTed meTabolic needs for S, hispidus in Georgia during The winTer
are nearly 70 percenT higher Than summer raTes (Wagner, l970). Oryzomys
palusTris may also lose a significanT amounT of weighT from cold sTress
(Negus ei_al,, l96l).

WheTher A, agggae_will be found To be mosT relaTed in meTabolism To
ThaT Typified by The microTines or The NorTh American grass-eaTing

criceTines (Sigmodon, Oryzomys) remains To be answered. Some species of

 

Akodon are adapTed To The much cooler environs of Tierra del Fuego

 

l20

(Ellerman, l94l), and The aITipIano (Cabrera, l96l). DaTa presenTed for

A, azarae by Underhill (l973) on a proTein efficiency sTudy suggesT ThaT

 

aT leasT some AAggEfl_may have a high raTe of meTabolism. However, The
nervousness of A, a3§£ae_ThroughouT The experimenT may have played a
role in iTs slow growTh. ExperimenTs presenle underway suggesT ThaT
resTing meTabolic raTes for A, azarae are nearly comparable To Those of
microTines. In facT, following The cyTogeneTic work of Bianchi ei.al,
(l97l), The finding of high meTabolic raTes in This group would supporT
Their hypoThesis ThaT The akodonTs musT have radiaTed from The cenTral
Andean region. This region is TemperaTe in naTure and would allow Akodon,
Through iTs meTabolic adapTions, To become esTablished where possible

in similar areas ThroughouT non—Tropical SouTh America. Unlike Siqmodon

and Or/zom/s, preliminary examinaTion shows liTTIe change in weighT (in

 

facT, some gain) in winTer A, azarae. Similarly, microTine growTh daTa
(Krebs, I964; Krebs gi_al,, l969) agree wiTh This. In addiTion, There
was very liTTIe Trap morTaliTy during The colder monThs. This is in
conTrasT To The seed-eaTing 9, nigripes which frequenle suffered high
winTer morTaliTy despiTe safeguards (boards and cloThs over all Traps,
and coTTon or ToileT paper inside). This species appeared Torpid In The
Traps a number of Times, a phenomenon known To be relaTively common in

such NorTh American seed—eaTing criceTines as Baiom\s, PerognaThus,

 

Peromyscus, and ReiThrodonTomys (Hudson, I973). IT is also known ThaT

 

 

9, nigripes may pracTice winTer huddllng. Crespo (I966) excavaTed several
.groups consisTing of one or more females and young of The year. However,
winTer huddling, like nesT building, food caching, and oTher acTiviTies
are found spordically occurring in boTh grass— and seed—eaTers (Dunaway

and Kaye, l96l; Frank, l957; HarT, I97l; Jameson, I947), and are mechanisms

 

l2l

shorT of hibernaTion ThaT provide ways of conserving energy during
periods of lnacTiviTy and inclemenT weaTher. There was noT any morTaliTy
or TorpidiTy discerned In winTer live—Trapped O. ruTilans.

Runwav—makiro and burrowin acTiviTies
z a ._

 

Runway—making and burrowing acTiviTies, alThough noT limiTed To
grass—eaTing myomorphs (Baker, l969, l97l; HamilTon, I946; Harris, I953;
Jameson, I947; STark, I963; STickel and STickel, l949), are cerTainly of
imporTance in Their biology. Runway-making appears To be of Two Types.
The firsT is besT characTerized by mosT microTines. Their runways are
acTively mainTained by cuTTing grass and forbs which grow on iTs surface.
'These paThways may be shorT, lasTing only a meTer or Two from The nesTing
siTe, or They may exTend many meTers and inTerconnecT wiTh each oTher.
Sigmodon generally do noT seem To produce The complexiTy or numbers of

i
-.-.—.- ”1.. v—o—n—o.

runways seen in many of The microTines, and Oryzomys palusTris may

 

consTrucT even fewer.
The second Type of runway—making (made by A, azarae) is found in
many of The NorTh American grassland—inhabiTing sciurids, such as

Spermophilus TridccemlineaTus, S. franklinii, and S, beecheyi, The

 

guinea pigs, Eagle and EElEEJ and some Sigmodon. These animals make
runways, generally shorT (several meTers aT besT), and unless used very
frequenle, are raTher diffuse. PaThways are mainTained enTirer by
consTan Treading over The vegeTaTion. My observaTions and Those of
Barlow’s (I969) supporT The conclusion ThaT neiTher 9, nigripes nor
g} Ilgilans are runway—makers.

Burrowing ls anoTher feaTure well developed in mosT microTlnes.
These burrows may be exTended i5 cm or more inTo The soil and exTend
ssxeral meTers or more in lengTh, wiTh blind Tunnels, food sTorage and

lairine comparTmenTs, and nesTing siTes. Sigmodon seem To prefer burrcwlng

51‘?-

l22

inTo such places as banks, diTches, and under flaT rocks or oTher surface

debris. WhaTever The case, The burrows are noT complex. Oryzomys palusTris,

 

due To iTs generally palusTrine habiTaT, apparenle does liTTIe digging.

Akodon azarae burrows were only discovered, Then examined closely

 

several Times. Burrowing does noT seem To occupy much of Their Time,
since few were observed. In one case, a hole wenT sTraighT down for l2—
l5 cm and ended in a globular nesT. In anoTher case, a burrow wenT 5-IO
cm in depTh and more or less sTraighT for 30—4O cm in lengTh before iT
resurfaced. A Third burrow was more exTensive. One enTrance was near a
ThisTle (EiIEiHE) and The burrow exTended down To a depTh of 5—6 cm,
Then leveled off. AfTer a disTance of 5—IO cm, a blind Tunnel, perhaps
under consTrucTion, exTended 5—lO cm. The main burrow conTinued onward
for 30,40 cm where if ended in a lO—l2 cm globular nesT. All These burrows
were assumed To be Those of A, azarae. None, including nesTs, could be
aTTribuTed To 9, DiEIiBEE.Or.9° ruTilans. The laTTer, conTrary To iTs
common name of burrowing mouse, did noT appear To consTrucT many, if any,
burrows, since none was discovered in This sTudy. lnsTead iT is believed
ThaT Their long foreclaws and shrew—like poinTed nose funcTion in digging
and rooTing for shallow sub—surface inverTebraTes.

Since liTTIe is known of The feeding behavior of The ArgenTine
species, iT is perTinenT To end This secTion wiTh several commenTs on
9, nigripes. During The acTiviTy sTudy, one Individual was viewed by
flashlighT as iT climbed To a heighT of I.O—|.5 m To forage on The

inflorescences of Paspalum elongaTum. Balance was mainTained by The

 

relaTively large hind feeT and long Tail, leaving The forefeeT free for
manipulaTlve Tasks. On several oTher occasions, afTer Trap release during
The day, animals, driven presumably by hunger, would immediaTely climb

one of The nearesT P. elongnfnm clumps wiTh amazing agillTy and commence

l23

feeding as described.
To summarize This secTion, myomorph grass~eaTers, including A.
azarae, are generally acTive day and nighT. Species which consume

primarily seeds and/or animal maTerial (Peromyscus, ReiThrodonTomys,

 

Oryzomys nigripes) are mainly nocTurnal. When grass—eaTers, such as

 

MicroTus californicus or Oryzomys palusTris consume a large percenTage

 

 

of seed or animal maTerial, They Tend To be more nocTurnal. InsecTivorous F‘

myomorphs (ScapTeromys, Onychomys) and shrews (Blarina brevicauda) are

"-‘I

mainly acTive aT nighT or during The TwilighT hours. A sTriking excepTion
is The insecTivorous 9, ruTilans which is diurnal in iTs habiTs.

AlimenTary and general meTabolic adapTions for consuming a given
food Type (grass, seeds, insecTs) in TemperaTe grassland regions and
perhaps predaTion pressure influence To a large degree The basic acTiviTy
paTTern for each group. Food is believed To be a more imporTanT facTor
limiTing populaTion densiTy in Q, ruTilans and Q, nigripes Than in The
grass-eaTing A, azarae.

Runway— and burrow—making seem To be besT developed in TemperaTe
_ grass—eaTing species. Runways mainTained by The animals acTively cuTTing
grass and forbs (versus Those mainTained by Trampling) are The mosT
advanced Types in grass-eaTers. Those wiTh The more Tropical, palusTrine,
or nongrass—eaTing habiTs are less advanced burrowers.

Oryzomys nigripes was found To be highly adapTed for climbing grasses

 

and forbs in order To consume The sTill aTTached seeds. OxymycTeris

 

ruTilans, like 9, nigripes did noT appear To consTrucT burrows or run-
ways. lT is believed ThaT iTs The long foreclaws and shrew-like poinTed

nose of The former funcTion in rooTing for subsurface inverTebraTes.

SUMMARY AND CONCLUSIONS

A sevenTeen—monTh field sTudy of The comparaTive ecology and

pepulaTion dynamics of Three abundanT grassland myomorphs was conducTed

on several one-hecTare siTes in The viciniTy of Balcarce, Province of

Buenos Aires, ArgenTina. These Three species, namely, The herbivorous

Akodon azarae, The granivorous Oryzomys nigripes, and The insecTivorous

 

 

OxymycTeris ruTilans were compared wiTh Their "ecological equivalenTs",

 

inhabiTing TemperaTe grasslands in NorTh America. IT was found ThaT:

PopulaTions of_A. agagge during periods of high densiTy exceeded 200
animals/hecTare, comparable in densiTy and raTe of increase To ThaT
found in NorTh American grasseeaTers. By laTe winTer The densiTy was
reduced To approximaTely 50 animals/hecTare. AlThough sporadic
populaTion irrupTions like Those of NorTh American Sigmodon and
possibly Qrvzomys occur, cyclic populaTion flucTuaTions like Those
of microTines have noT been reporTed. Typical of granivores, S}
nigripes reached 50 animals/hecTare The firsT summer and fall when
The seed crop was abundanT. By The following winTer and early summer,
The densiTy was so reduced ThaT several Three-day Trapping periods

would go by before There were furTher capTures. OxymycTeris ruTilans

 

had The IowesT buT mosT sTable populaTions, wiTh densiTies flucTuaTing
beTween 3*l5 mice/hecTare.

Sex raTios of A, azarae suggesTed ThaT, like in many oTher grass-
eaTers, females make up a higher proporTion of The ToTal caTch Than
lnales during The breeding (summer) season Than in The non—breeding

sueason. This was parTicularly True in residenT versus recruiT animals.
l24

l25

Oryzomys nigripes consisTenle had a much smaller proporTion of
females,a feaTure common To The social organizaTion of oTher seed—
eaTers. The preporTion of female 9, ruTilans was higher, perhaps a

funcTion of iTs conTinual reproducTive acTiviTy. RaTios of A, azarae

 

coming inTo one of The sTudy ploTs indicaTed ThaT adulT females dis—
persed less ofTen Than males during The breeding season, and abouT
equally during The winTer (non—breeding) season.

ReproducTion of A. azarae, unlike oTher grass-eaTers, from similar
laTiTudes, was sTrongly seasonal, wiTh liTTers born from The monThs
of November To April. LiTTer size averaged 4.6, and gesTaTion lengTh,
22.7 days. Delayed implanTaTion may occur. Young were successfully
weaned aT l4-l5 days of age. The growTh raTe of The young did noT
aTTain The level found in The NorTh American grass—eaTing microTines.
Sexual maTuriTy occurred aT Two monThs of age. Young born laTe in

The year did noT become sexually maTure unTil The following breeding

season. Oryzomys nigripes were noTed To breed from January To May.

 

LiTTer size averaged 3.6, buT gesTaTion period was noT esTablished.
Young were successfully weaned aT l4—l5 days of age. Sexual maTuriTy

may noT be reached unTil The following reproducTive season. OxymycTeris

 

ruTilans bred in all seasons of The year. LiTTer size averaged 3.l,
buT gesTaTion period was noT esTablished. Young were successfully
weaned aT l4 days of age. Sexual maTuriTy occurred near Three monThs
of age. AlThough A, azarae is a seasonal breeder, The species realizes
a much higher reproducTive poTenTlal Than The seed—eaTing Q, nigripes
or The insecT~eaTlng Q, [Elilflfléx This agrees well wiTh our knowledge

C>f myomorph grass-, seed—, and inverTebraTe—eaTers in NorTh America.

4. The minimum survival raTes for A. azarae averaged sligthy higher
during The winTer (BO—83 percenT) Than The summer (59-79.5 percenT).
ATTrlTlon during The winTer season indicaTed ThaT The survival raTe

was closer To 93—95 percenT per fourTeen days. Oryzomys nigripes had

 

minimum survival raTes near 60 percenT for The Time when They were
mosT abundanT. 9, ruTilans was calculaTed To have a survival raTe of
70-76 percenT during The summers, buT in The winTers iT was closer
To 92 percenT. Dispersal, especially in summer, seemed To be a prime
facTor in lowering The minimum survival raTes. PredaTion appeared To
be insignificanT. These findings make iT difficulT To esTablish
wheTher A, azarae, Q, fligiiflféx and 9, ruTilans have survival raTes
comparable To Their NorTh American ecological equivalenTs.

5. he spaTial disTribuTion (cenTer of acTiviTy) of The mice on The sTudy
ploTs was influenced by vegeTaTive cover. A shifT in disTribuTion for

A. azarae and O. nigripes was noTed in one ploT when increasing

 

numbers of guinea pigs (Cavia aperea) caused a deTerioraTion in

 

habiTaT qualiTy. Oryzomys nigripes was capTured raTher homogeneously,

 

buT by The heighT of The habiTaT deTerioraTion had disappeared. On
anoTher ploT where The vegeTaTion consisTed of a greaTer number of
dominanT grasses, A, azarae and 9, ruTilans were generally capTured
in The mosT heTerogeneous habiTaT, whereas 9, nigripes was found in
The porTion where vegeTaTive cover was mosT dense and highesT.

6. dovemenTs (radii of acTiviTy) of A, azarae Typifled gramlnivorous
rodenTs, being The leasT of The Three species. WinTer and male move-
menTs were only sligthy longer Than summer and female movemenTs,
respecTively. Female movemenTs during a summer of high reproducTive

inTensiTy were greale resTricTed when compared To a summer when

 

|27

breeding was less inTense. The vegeTaTive desTrucTion by Egvia‘
seemed To be a facTor favoring The increase in The radius of acTiviTy
of A, aggr§g_as The sTudy progressed. LimiTed daTa for 9, Aigripes
and Q, rgillaflg_showed only ThaT Their movemenTs were 2~4 Times more
exTensive Than ThaT for A. azarae and in general agreemenT wiTh Their
NorTh American counTerparTs. Dispersal informaTion for A, azarae
indicaTed ThaT nearly 75 percenT of The mice moved IOO m or less
from Their previous cenTer of acTiviTy. This species homed readily
considering The shorT disTances TesTed (less Than 200 m).

7. The diel acTiviTy paTTerns of A, azarae_and 9, nigripes, buT noT The

insecTivorous O. ruTilans, agree wiTh whaT is known of ecologically—

...—...

 

similar Species in NorTh America. Specifically, The grass-eaTing

A. azarae_was acTive aT nearly all Times, wiTh peaks during The

 

crepuscular hours. The seed—eaTing O. nigripes was acTive exclusively

m-

 

aT nighT. Unlike The nocTurnal insecTivorous NorTh American grass-
hOpper mice (Onychomys) which inhabiT arid grasslands or The mesic-

inhabiTing shrew Blarina brevicauda, 9, ruTilans appeared To be

 

exclusively diurnal. Energy demands and The Type of food (grasses,
grains, or insecTs) play a major role in dicTaTing The acTiviTy
paTTern for each species. AlThough A, azarae is The primary myomorph
herbivore, iT appears To consume more inverTebraTes Than NorTh
American grass—eaTers. High meTabolic raTes for A, azarae are
comparable To Those of microTines. Burrowing and runway—making
behaviors were less developed in A, aga£§e_Than The microTines.
The above feaTures indicaTe ThaT The grass-eaTing A, azarae resembles
The NorTh American grass-eaTing myomorphs in many respecTs. Oryzomys

nigripes, in as much as The daTa will allow, Typifies a longeTailed

l28

scansorial seed—eaTer, such as NorTh American grassland forms of

Peromyscus or PeiThrodonTomys. Only The insecTivorous O. ruTilans is a

 

 

form noT Truly represenTed in NorTh America and appears To have an ecolog-
ical posiTion overlapping Those of boTh grasshOpper mice, Qflyghgmys, and
shrews such as Blarina. The number of grass—eaTing and seed—eaTing
myomorph species is comparable To ThaT found aT similar laTiTudes in Tall
grass areas of NorTh America. A ToTal of Two myomorph grass—eaTers (one

uncommon), five seedweaTers (includes Mus musculus) and one insecT—eaTer

 

inhabiTed The sTudy siTes. The number of principal myomorph grass—eaTing
species (one) To seed—eaTers (four To five) agrees wiTh The hypoThesis

suggesTing ThaT seed—eaTers divide Their consumer niche more efficienle
Than The grass—eaTers. The seed—eaTers of NorTh American grasslands, un—
like Those found in This sTudy, are augmenTed sligthy in species numbers
by The presence of The zapodids (jumping mice) and heTeromyids (kangaroo
raTs, pockeT mice). The insecTivore niche may also allow several species

To inhabiT one area. OxymycTeris ruTilans, and The marsupials, Monodelphis

 

 

dimidiaTa and possibly LuTreolina crassicaudaTa, appear To fill The

 

insecTivore niche normally filled by Onychomys and shrews in NorTh America.

"1.1%:
{nan . .1-

LIST OF REFERENCES

LIST OF REFERENCES

Ambrose, H. V. lll. l973. An experimenTal sTudy of some facTors affecT-
ing The spaTial and Temporal acTiviTy of MicroTus pengsylvanicus.
J. Mamm., 54: 79—ilO.

 

Anonynous. l965. Economic analysis and feasibiliTy sTudy of Balcarce
AgriculTural College. Michigan STaTe UniversiTy, Foreign
AgriculTural Programs. 247 pp.

Asdell, S. A. I964. ”PaTTerns of Mammalian ReproducTion". Cornell
UniversiTy Press, lThaca, N.Y. 670 pp.

Aumann, G. D. l965. MicroTine abundance and soil sodium levels. J. Mamm.,
46: 594,604.

Baker, R. H. l969. CoTTon raTs of The Sigmodon fulvivenTer group. Misc.
Publ. Mus. NaT. HisT., Univ. Kans., 5|:l77—232.

 

l97l. NuTriTional sTraTegies of myomorph rodenTs in NorTh

 

American grasslands. J. Mamm., 52: BOO-805.

Bailey, V. and C. Sperry. l929. Life hisTory and habiTs of The grass-
hopper mouse, genus Onychomys. USDA Tech. Bull., I48: l—l9.

Barlow, J. l969. ObservaTions on The biology of rodenTs in Uruguay. Life
Sci. ConTr., Roy. OnT. Mus., 75: l—59.

BaTzli, G. and F. PiTelka. l97l. CondiTion and dieT of cycling pepulaTions
of The California vole, MicroTus callfornicus. J. Mamm., 52: l4l-
l6}.

 

Bianchi, N. 0., O. A. Reig, O. J. Molina, and F. N. DulouT. l97l.
CyTogeneTics of The SouTh American akodonT rodenTs (Cricefidae).
l. A progress reporT of ArgenTinian and Venezuelan forms. Evol.,
25: 724—736.

Blair, W. F. l953. PopulaTion dynamics of rodenTs and oTher small
mammals. Advances in CeneTics, 5: l—4I.

Bowers, J. R. l97l. ResTing meTabolic raTe in The coTTon raT Sigmodon.
Physiol. Zeol., 44zl37—i48.

Brambell, F. W. R. and l. W. Rewiands. i936. ReproducTion of The bank
vole (fiyelgmy§_glarnolus, Schreber). l. The oesTrous cycle of The

female. Trans. Roy. Soc. Lond., Ser. B, 226: 7l—97.

l29

Brody, S. l945. "BioenergeTics and GrowTh”. Reinhold Publ. Corp., N.Y.
l023 pp.

Brown, L. E. l962. Home range in small mammal communiTies. in: 8. Glass
(ed.). Survey of Biological Progress, Academic Press, N.Y., 4: l3l-
l79.

Cabrera, A. I953. Esquema fiTogeografico de la Republica ArgenTina.
RevisTa del Museo do La PlaTa, Seccion BoTanica, 8: 87—l68.

l968. VegeTacion de la Provincia do Buenos Aires. in: A. L.
Cabrera (ed.). Flora de la Provincia de Buenos Aires. Talleres
Graficos l.S.A.G., Buenos Aires, l: l2l—l22.

 

Cabrera, A. l96l. CaTalogo de los mamiferos de America del Sur. Rev.
Arg. Cien. NaT. "Bernardino Rivadavia”, Cien. Zool., 4: 309-732.

and J. Yepes. I960. "damiferos Sud Americanos". EdiTores
Ediar S.Av Buenos Aires. 370 pp.

 

Calhoun, J. B. l945a. Diel acTiviTy rhyThms of The rodenTs fiicrofgs
ochrggasTer and Sigmodon hispidus hispidus. Ecol., 26: 25l—273.

 

 

l945b. TwenTy—four hour periodiciTies in The animal

 

kingdomT—ParT ll. The verTebraTes. J. Tenn. Acad. Sci., 20: 29l-
308.

CarleTon, M. D. I973. A survey of gross sTomach morphology in New World
CriceTinae (PodenTia, Muroidea), wiTh comnenTs on funcTional
inTerpreTaTions. Misc. Publ. Mus. Zool., Univ. Mich., l46: l-43.

Caswell, H., F. Reed, 8. N. STephenson, and P. A. Werner. I973. PhoTo-
synTheTic paThways and selecTive herbivory: a hypoThesis. Amer.
NaT., IO7: 465—480.

Chipman, R. K. l966. CoTTon raT age classes during a populaTion decline.
J. iamm., 47: l38-l4l.

ChiTTy, 0. I960. PopulaTion processes in The vole and Their relevance
To general Theory. Can. J. Zool., 38: 99-ll3.

I967. The naTural seiecTion of self—regulaTory behaviour
in animal populaTions. Proc. Ecol. Soc. AusT., 2: 5l-78.

 

ChrisTian, J. J. l970. Social subordinaTion, pepulaTion densiTy, and
mammalian evoluTion. Sci., l68: 84—90.

Cockrum, E. L. l948. The disTribuTion of The hispid coTTon raT in Kansas.
Trans. Kans. Acad. Sci., 5i: 306—3l2.

Conaway, C. H. l97l. Ecological adapTion and mammalian reproducTion.
Biol. Reprod., 4: 239-247. '

l3l

Cowan, I. McT. and M. G. ArsonaulT. l954. ReproducTion and growTh in
The creeping vole, MicroTus oregoni serpens Merriam. Can. J. Zool.,
') WWW—- W
3z: l98—208.

Crawley, M. C. l969. Some populaTion dynamics of The bank vole,
CleThrionemys glareolus and The wood mouse, Apodemus sylvaTicus
in mixed woodland. J. Zool., l60: 7l—89.

 

 

Crespo, J. A. l944. Relaciones enTre esTados climaTicos y la ecologia
de algunos roedores de campo (CriceTidae). Rev. Arg. Zoogeogra.
4: l57—l44.

l966. Ecologia de una comunidad de roedores silvesTres
en el parTido do Rojas, Provincia de Buenos Aires. Rev. Arg. Cien.
NaT. "Bernardino Rivadavia" e lnsT. Nac. lnvesT. Cien. NaT.,
Ecologia, l: 79—l44.

 

Crespo, J. A., M. A. SabaTTini, M. J. PianTanida, and G. de Villafane.
I970. EsTudios ecologicos sobre roedores silvesTres. Observaciones
sobre densidad, reproduccion y esTrucTura de comunidades de
roedores silvesTres en el sur de Cordoba. Comision Nacional de
EsTudio y Lucha ConTra la Fiebre Hemorragica ArgenTina, SecreTaria
de Salud Publica. 45 pp.

Daniel, J. F. l9l0. ObservaTions on The period of gesTaTion in whiTe
mice. J. ExpT. Zool., i: 865-870.

Dapson, R. W. l968. ReproducTion and age sTrucTure in a pepulaTion of
shorT-Tailed shrews, Blarina brevicauda. J. Mamm., 49: 205—2l4.

 

Davis, D. E. and J. T. Emlen. l956. DifferenTial TrappabiliTy of raTs
according To size and sex. J. Wildl. Mng., 20: 326—327.

Dawson, M. R. l967. Fossil hisTory of The families of RecenT mammals.
In: S. Anderson and J. K. Jones, (eds.). "RecenT Mammals of The
World". Ronald Press 00., N.Y. pp. l2-53.

Dunaway, P. B. l968. Life hisTory and populaTional aspecTs of The easTern
harvesT mouse. Amer. Midi. NaT., 79: 48-67.

Dunaway, P. B. and S. V. Kaye. l96l. CoTTon raT morTaliTy during severe
winTer. J. Namm., 42: 265-268.

l964. WeighTs of coTTon raTs in relaTien
To season, breeding, and environmenTal radioacTive conTaminaTion.
Amer. Midi. NaT., 7|: l4l—l55.

 

anors, J. T. I954. SponTaneous acTiviTy in The raT. BriT. J. Anim. Behav.,
2: 25-30.

Egoscue, H. J. l960. Laborafcry and field sTudies of The norThern grass-
hopper mouse. J. Namm., 4i: 99—IIO. -

 

 

|32

Eilerman, J. R. |94l. The families and genera of living rodenTs.
BriT. Mus. (HaT. HisT.), 2: i—690.

EllioTT, F. C. I963. The meadow vole (MicroTus pennsylvanicus) as a
bioassay TesT organism for individual iorage planTs. QuarT. Bull.,
Mich. Agr. ExpT. STa., 46: 58—72.

 

Erickson, A. B. i949. Summer populaTions and movemenTs of The coTTon
raT and oTher rodenTs on The Savannah River Refuge. J. Mamm.,
30: l33—l40.

Fails, J. B. l968. AcTiviTyuln: J. A. King (ed.). "Biology of Peromyscus".
Amer. Soc. Mamm. Spec. Publ., 2: 543-570.

 

Fisier, G. F. l969. Mammalian organizaTional sysTems. Los Angeles Co.
Mus., ConTr. Sci., I67: l—32.

l97l. Age sTrucTure and sex raTio in populaTions of

 

ReiThrodonTomys. J. Mamm., 52: 653-662.

FiTch, H. S. l957. AspecTs of reproducTion and developmenT in The
prairie vole (MicroTus ochrogasTer). Univ. Kans. Publ., Mus. NaT.
HisT., l0: l29—l6l.

 

FiTTkau, E. J. l969. The fauna of SouTh America. In: E. J. FiTTkau,
J. lilies, H. Klinge, G. H. Schwabe, H. Sioii (eds.). "Biogeography
and Ecology in SouTh America”. Dr. W. Junk N. V., Publishers, The
Hague, NeTherlands, 2: 624-658.

FleharTy, E. D., J. R. ChoaTe, and M. A. Mares. i972. FlucuaTions in
populaTion densiTy of The hispid coTTon raT: facTors influencing
a "crash". Bull. SouTh. Calif. Acad. Sci., 7l: i32-l38.

Fornes, A. and E. Massoia. i965. Micromamiferos (Marsupialia y. RodenTia)
recoiecTados en la iocalidad bonaerense de Miramar. Physis, 25:
99-i08.

FosTer, J. l96l. Life hisTory of The Phenacomys vole. J. Mamm., 42: l8!-
l98.

 

Frank, F. l957. The causaliTy of microTine cycles in Germany. J. Wildl.
Mng., 2|: |i3-l2i.

Furrer, R. K. i973. Homing of Poromyscus maniculaTus in The channelled
scablands of easT—cenTrai WashingTon. J. Mamm., 54: 466—482.

 

GenTry, J. B. l968. Dynamics of an enclosed populaTion of pine mice,
MicroTus_pjneTorum. Res. Popul. Ecol., IO: 2l-30.

 

GeTz, L. L. l960. A pOpulaTion sTudy of The vole, MicroTus pennsylvanicus.
Amer. Midi. NaT., 64: 392~405.

 

a. j l96l. Home ranges, TerriTorialiTy, and movemenT of The
meadow vole. J. Mamm., 42: 24~36.

 

Godfrey, G. K. l953. A Technique for finding MicroTus nesTs. J. Mamm.,
34: 503—505.

GoerTz, J. W. I964. The influence of habiTaT qualiTy upon densiTy of
coTTon raT populaTions. Ecol. Monogr., 34: 359—38l.

I965a. Sex, age and weighT variaTIon in coTTon raTs.
J. Mamm., 46: 47I—477.

 

l965b. ReproducTive variaTion in coTTon raTs. Amer. Midi.
NaT., 74: 329—340.

 

Goiley, F. B. I960. AnaTomy of The digesTive TracT of MicroTus. J. iamm.,
4i: 89-99.

Goldman, E. A. i9I8. The rice raTs of NorTh America (genus Oryzomys).
USDA Bur. Biol. Sur., NA Fauna No. 43. I00 pp.

HamilTon, W. J. Jr. i930. The food of The Soricidae. J. Mamm., ll: 26—
39.

I937a. The biology of microTine cycles. J. Agr. Res.,
54: 779-790.

 

|937b. GrowTh and life span of The field mouse. Amer.
NaT., 7i: SOO~507.

 

I94I. ReproducTion of The field mouse, MicroTus
pennsyivanicus (0rd). Cornell Univ. Agr. ExpT. STaTion, Memoir 237.

23 pp.

 

 

I946. HabiTs of The swamp rice raT, Oryzomys palusTris
palusTris (Harlan). Amer. Midi. NaT., 36: 730—736.

 

HamilTon, W. J. lli. I962. ReproducTive adapTions of The red Tree mouse.
J. Mamm., 43: 486-504.

Hanson, L. l97l. Small rodenT food, feeding and populaTion dynamics.
A comparison beTween granivorous and herbivorous species in
Scandinavia. Gikos, 22: I83-I98.

Harris, V. T. I953. Ecological reiaTionships of meadow voles and rice
raTs in Tidal marshes. J. Mamm., 34: 479—487.

HarT, J. S. I97I. RodenTs. In: G. C. WhiTTow, (ed.). "ComparaTive
Physiology of ThermoregulaTion. Mammals? 2: I—I49.

Hanield, D. M. I935. A naTurel hisTory sTudy of MicroTus callfornicus.
J. Mamm., IO: 26l—27I.

Hanield, D. M. i939. RaTe of meTabolism In MicroTus and Peromyscus.
MurreieT, 20: 54—56. r_v

 

I940. AcTiviTy and food consumpTion in MicroTus and
Peromyscus. J. Mamm., 2i: 29-36.

 

 

HaTT, R. T. I930. The biol0gy of The voles of New York. RooseveiT WiIdI.
Bull., 5: 5i3-623.

Hayne, D. W. I949. CaiculaTion of size of home range. J. Mamm., 30: I—I8.

HeidT, G. A. I97I. Daily summer acTiviTy of The meadow vole, MicroTus
pennsylvanicus. Mich. Academician, 3: 3I—39.

 

HershkoviTz, P. I959. NomenclaTure and Taxonomy of The neoTropicaI
mammals described by Oifers, l8i8. J. Mamm., 40: 337—353.

I969. The evoluTion of mammals on souThern conTinenTs. VI.
The recenT mammals of The NeoTropicaI Region: a zoogeographic and
ecological review. QuarT. Rev. Biol., 44: I-70.

 

I972. The RecenT mammals of The NeoTropicaI Region: a
zoogeographic and ecological review. In: A. KeasT, F. C. Erk and
B. Glass, (eds.). "EvoluTion, Mammals, and SouThern ConTinenTs".
SUNY Press, Albany, pp. 3II-43I.

 

HoffmeisTer, D. F. and L. L. GeTz. I968. GrowTh and age-classes in The
' prairie vole, MicroTus ochrogasTer. GrowTh, 32: 57-69.

 

Horner, B. E. I962. The gasTric fundus of The grasshopper mouse. Amer.
Zool., 2: 4I6.

i968. GesTaTion period and early developmenT in Onychomys
IoucogasTer brevicaudus. J. Mamm., 49: 5I3-5I5.

 

 

and J. M. Taylor. I968. GrowTh and reproducTive behavior
in The souThern grasshopper mouse. J. Marm., 49: 644—660.

 

, and H. A. Padykuia. I964. Food habiTs
and gasTric morphology of The grasshopper mouse. J. Mamm., 45: 5|3-
535.

 

Howard, W. E. I949. DiSpersaI, amounT of inbreeding and longeviTy in a
local populaTion of prairie deer mice on The George Reserve,
souThern Michigan. ConTr. Lab. VerT. Biol., Univ. Mich., 43: I-50.

fiw I960. InnaTe and environmenTaI dispersal of individual
verTebraTos. Amer. Midi. NaT., 63: I52~I6I.

 

Howell, A. H. I9I4. Revision of The American harvesT mice (genus
ReiThrodonTomys). USDA Bur. Biol. Sur., NA Fauna No. 36. 97 pp.

 

I35

Hudson, J. W. I973. TorpidiTy in mammals. In: G. C. WhiTTow, (ed.).
"ComparaTive Physiology of ThermoregulaTion. Special AspecTs of
ThermoreguIaTion", 3: 97—I65.

Huff, J. N. I973. The "disTress" cry of InfanT deermice, Peromyscus:
physical characTerisTics, specific differences, social funcTion.
Ph.D. Thesis, DepT. Zool., Michigan STaTe UniversiTy.

 

 

Jahoda, J. C. I970. The effecTs of rainfall on The acTiviTy of Onychomys
IeucogasTer breviauriTus. Amer. Zool., I0: 326.

 

James, P. E. I959. LaTin America. The Odyssey Press (3rd. ed.). New York,
N.Y. 942 pp.

Jameson, E. W., Jr. I947. NaTural hisTory of The prairie voie (mammalian
genus MicroTus). Univ. Kans. Publ., Mus. NaT. HisT., I: I25—I5I.

,I952. Food of The deer mice, Peremyscus manicuIaTus
and Peromyscus boylei, in The norThern Sierra Nevada, CaiifornTa.
J a i'iiJiiim. , 33: 50"600

 

 

 

Jimenez, J. V. I969. ComparaTive posTnaTal growTh in coTTon raTs (genus
Sigmoigg). Ph.D. Thesis, DepT. Zool., Michigan STaTe UniversiTy,

9| pp.

 

Kaczmarski, F. I966. BioenergeTics of pregnancy and IacTaTion in The
bank vole. AcTa Therioi., II: 409—4I7.

Kaye, S. V. I96I. Laborafory Iife hisTory of The easTern harvesT mouse.
Amer. Midi. NaT., 66: 439—45I.

Keller, B. L. and C. J. Krebs. I970. MicroTus populaTion biology III.
ReproducTion changes in fiucTuaTing populaTions of M. echrogasTer
and EL pennsylvanicus in souThern Indiana, I965-67.-Ecol. Monogr.,
40: 263:294.

 

 

King, H. D. I9I3. Some anomalies in The gesTaTion of The albino raT
(Mus norvegicus albinus). Biol. Bull., 24: 377-39I.

 

King, J. A. l958. MaTernai behavior and behavioral developmenT in Two
subspecies of Peromyscus maniculaTus. J. Mamm., 39: I77—l90.

 

, J. 0. Deshales, and R. WebsTer. I963. Age of weaning in
Two subspecies of deer mice. Sci., I39: 483-484.

 

KosTeiecka-Myrcha, A. and A. Myrcha. I964. The raTe of passage of food-
sTuffs Through The alimenTary TracTs of cerTain MicroTidae under
laboraTory condiTions. AcTa Therioi., 9: 37-53.

Krebs, C. J. I964. The lemming cycle aT Baker Lake, NorThwesT TerriTories,
during I959—62. Archc lnsT. N. A. Tech. Pap., No. I5. |O4 pp.

_I966. Demographic changes In qucTuaTIng populaTions of
MicroTus caiiiornicus. Ecol., 36: 239-273.

 

 

, M. S. Gaines, B. L. Keller, J. H. Myers, and R. H. Tamarin.
., I79: 35—4I.

 

I913:rPopulaTion cycles in small rodenTs. Sci

, B. L. Keller, and R. H. Tamarin. I969. MicroTus populaTion
biology: demographic changes In flucluaTing populaTions of M.
ochrcgasTer and M. pennsylvanicus in souThern Indiana. Ecol.

50; 587—607. ‘“

 

 

Kruckenberg, S. M., H. T. Gier and S. M. Dennis. I973. PosTnaTal develop—

menT of The prairie vole, MicroTus ochrogasTer. Lab. Anim. Sci.,
23: 53-55.

 

Landry, S. Jr. I970. The RodenTia as omnivores. QuarT. Rev. Biol.,
45: 35l—372.

Layne, J. N. I959. GrowTh and developmenT of The easTern harvesT mouse,
ReiThrodenTomys humulis. Bull. Florida STaTe Mus., Biol. Sci.,
4: 6l—82.

, l968. Onfegeny. In: J. A. King (ed.). "Biology of Peromyscus
(RedenTia)". Amer. Soc. Mamm. Spec. Publ., 2: I48-253.

 

 

Lidicker, W. Z. I962. EmigraTion as a possible mechanism permiTTing The
regulaTion of populaTion densiTy below carrying capaeiTy. Amer. NaT.,
96: 29— 33.

MarTin, E. P. I956. A populaTion sTudy of The prairie vole (MicroTus
ochrogasTer) in norTheasTern Kansas. Univ. Kans. Publ., Mus. NaT.
HisT., 8: 36l—4I6.

 

Masseia, E. and A. Fornes. I965. ConTribucion al conocimienTo de los
roedores miemorfos argenTinos vinculados con la fiebre hemorragica
ArgenTina. Comision Nacional Coordinadora para EsTudio y Lucha
ConTra Fiebre Hemorragica de ArgenTina, I‘.inisTerio AsisTencia
Social y Salud Publica de la Nacion. 29 pp.

I967. El esTado sisTemaTico, disTribucion geografica y
daTos eToecologicos de aigunos mamiferos neoTropicales (Marsupialia
Y RodenTia, con la descripcion de Ca breramys, genero nuevo
(CriceTidae). De AcTa Zoologica LiIIoana, 23: 407-430.

 

 

, R. Weinberg, T. de Fronza. I968. Nuevos aporTes ai
conocimienTo de Ias especies bonaerenses del genero Calomys
(RodenTia-CriceTidae). RevisTa de InvesTigaciones Agropecuarias,
INTA, Serie I, Biologia y Produccion Animal, 5: 65-92.

 

McNab, B. K. I963. BieenergeTIcs and The deTerminaTion of home range
size. Amer. NaT., 97: I33-I40.

Lbserve. P. L. l97l. PopuIaTion ecology of The prairie vole, MicroTus
ochr gasTer, in The wesTern mixed prairie of Nebraska. Amer. Midi.
NaT. , 86: 4l7—433.

Myers, J. H. and C. J. Krebs, I97I. Sex raTios In open and enclosed
vole populaTions: demographic implicaTions. Amer. NaT., l05:
325—344.

Morrison P. R., F. A. Ryser, and R. L. STrecker. I954. GrowTh and The
deveIOpmenT of TemperaTure regulaTIon in The Tundra redback vole.
J. Mamm., 35: 376—386.

Negus, N. C., Gould, E. and R. K. Chipman. |96I. Ecology of The rice
raT, Oryzomys palusTris (Harlan), on BreTon island, Gulf of
Mexico, wifhwa criTique of The social sTress Theory. Tulane
STud. Zool., 8: 93—i23.

 

Nelson, M. M. and H. M. Evans. I96l. DieTary requiremenTs for IacTaTion
In The raT and oTher laboraTory animals. in: Ken, S. K. and A. T.
Cowie (eds.). "Milk: The Mammary Gland and ITs SecreTion", 2: i37—
I9I.

Newson, J. and D. ChiTTy. I962. Haemoglobin levels, growTh and survival
in Two MicroTus populaTions. Ecol., 43: 733-738.

Odum, E. I955. An eleven year hisTory of a Sigmoden populaTion. J. 4amm.,
36: 368-378.

OsTerberg, D. M. I962. AcTiviTy of small manmals as recorded by a
phoTographic device. J. Mamm., 43: 2|9—229.

Packard, G. C. I968. Oxygen consumpTion of MicroTus monTanus in relaTion
To ambienT TemperaTure. J. Mamm., 49: 2l5—220.

 

Pearson, O. P. I959. A Traffic survey of MicroTus~ReiThrodonTomys runways.
J. Mamm., 40: I69—I80.

I960. HabiTs of MicroTus caiifornicus revealed by
auTomaTic phoTographic recorders. Ecol. Monogr., 30: 23I-249.

 

 

I960. HabiTs of harvesT mice revealed by auTomaTic
phoTographic recorders. J. Mamm., 4|: 58-74.

 

I967. La esTrucTura por edades y la dinamica reproducTiva
en una poblacion de raTones de campo Akodon azarae. Physis, 27: 53-
58.

 

 

PeTersen, M. K. I973. lnTeracTiens beTween The coTTon raTs, Sigmoden
fulvivenTer and S, hispidug, Amer. Midi. NaT., 90: 3|9-333.

 

PInTer, A. J. I970. ReproducTion and growTh for Two species of grass—
hopper mice (Onychgmys) in The laboraTory. J. Mamm., 5|: 236-243.

Ranson, R. M. l94l. Pre—naTaI and infanT morTaliTy In a laboraTory
populaTion of voles (MicroTus agresTis). Proc. Zool. Soc. Lond.,

 

I38
Reig, O. A. and O. S. Linares. I969. The occurrence of Akodon in The

upper Pliocene of ArgenTina. J. Mamm., 50; 643—6477

Reynolds, H. G. I960. Life hisTory noTes on Merriam's kangaroo raT in
souThern Arizona. J. Mamm., 4|: 48—58.

RingueleT, R. A. I955. Panorama zoogeografico de la Provincia de Buenos
Aires. NoTas Museo de La PIaTa; Zoologia, I8: I—i5.

l96l. Rasgos de la 200geografia de la ArgenTina. Physis,
22: l5|—|70.

 

Robinson, W. L. and J. B. Falls. I965. A sTudy of homing of meadow mice.
Amer. Midl. NaT., 73: l88~224.

Rood, J. P. I966. ObservaTions on The reproducTion of Peromyscus in
capTiviTy. Amer. Midi. NaT., 76: 496-503.

 

I972. Ecological and behavioural comparisons of Three genera
of ArgenTine cavies. Anim. Behav. MonOgr., 5: I—83.

 

Sanderson, G. C. I966. The sTudy of mammal movemenTs — a review. J. Wildl.
Mng., 30: 2l5—235.

Schaffer, W. M. and R. H. Tamarin. I973. Changing reproducTive raTes and
populaTion cycles in lemmings and voles. Evol., 27: III-l24.

Sealander, J. A., Jr. and B. Q. Walker. I955. A sTudy of The coTTon raT
in norThwesTern Arkansas. Proc. Ark. Acad. Sci., 8: l53-I62.

Seidel, D. R. and E. S. BooTh. I960. Biology and breeding habiTs of The
meadow mouse MicroTus monTanus, in easTern WashingTon. Walla Walla
Coll. Publ. DepT. Biol. Sci., 29: I-I2.

 

Selle, R. M. I928. MicroTus callfornicus in capTiviTy. J. Mamm., 9: 93-
98.

 

Sharp, H. F., Jr. l967. Food ecology of The rice raT, Oryzomys palusTris
(Harlan), in a Georgia salT marsh. J. Mamm., 48: 557-563.

Slobodkin, L. 8., F. E. SmiTh and N. G. HairsTon. l967. ReguIaTion in
TerresTrial ecosysTems, and The implied balance of naTure. Amer.
NaT., IOI: l09-l24.

SmiTh, C. F. I936. NoTes on The habiTs of The long—Tailed harvesT mouse.
J. Mamm., l7: 274—278.

SmiTh, M. H. I967. Sex raTios in laboraTory and field populaTions of The
old-field mouse, Peromyscus polionoTus. Res. Popul. Ecol., 9: I08-
ll2.

 

I968. A comparison of differenT meThods of capTurIng and
esTimaTing numbers of mice. J. Mamm., 49: 455n462.

 

I39

I97l. Food as a limiTing facTor in The populaTion

 

ecoloQY of Peromyscus polionoTus (Wagner). Ann. Zool. Fennici,
8: IO9—l|2.

 

STark, H. E. I963. NesTing habiTs of The California vole, MicroTus
californicus, and microclimaTic facTors affecTing iTs nesTs. Ecol.,
44: 663—669.

 

 

STickel, L. F. l968. Home range and Travels. In: J. A. King (ed.). "Biology
of Peromyscus (RodenTia)". Amer. Soc. Mamm. Spec. Publ., 2: 373-4ll.

 

, and W. H. STickel. I949. A Sigmodon and Baiomys_populaTion
in ungrazed and unburned Texas prairie. J. Mamm., 30: I4I-I50.

 

Svihia, A. I93I. Life hisTories of The Texas rice raT (Oryzomys palusTris
Texansis) J. Mamm., i2: 238—242.

 

I932. A comparaTive life hisTory sTudy of The mice of The
genus Peromyscus. Misc. Publ. Mus. Zool., Univ. Mich., 24: I-39.

 

 

Svihia, R. D. I93I. NoTes on deserT and dusky harvesT mice (ReiThrodonTomys
negaloijs megaiofis and R: E: nigrescens.) J. Mamm., I2: 363-365.

 

 

 

I936. Breeding and young of The grasshopper mouse (Onychomys
ieueogasTer fuscogriseus. J. Mamm., l7: |72—I73.

 

 

Terman, C. R. I966. PopulaTion flucTuaTIons of Peromyscus maniculgips and
' oTher small mammals as revealed by The NorTh American Census of
Small Mammals. Amer. Midi. NaT., 76: 4I9-426.

 

 

l968. PopulaTion dynamics. In: J. A. King (ed.) "Biology of
Peromyseus (RodenTia)”. Amer. Soc. flamm. Spec. Publ., 2: 4I2—450.

 

 

Tinkle, D. and W. Harmon, I970. Seasonal surface acTiviTy in PerognaThus
californicus. Amer. Midi. NaT., 84: 257-259.

 

 

Underhill, A. I973. Use of NeoTropIcal rodenTs in proTein efficiency
sTudies. Lab. Anim. Sci., 23: 499-503.

Wagner, C. K. I970. Oxygen consumpTion, ambienT TemperaTure and exereTion
of phosphorus—32 in coTTon raTs. Ecol., 5|: 3II-3I7.

Wang, C. H. I923. The relaTion beTween "sponTaneous acTiviTy" and oesTrous
cycle in The whiTe raT. Comp. Psychol. Monogr., 2: I—27.

WhiTaker, J. 0. I966. Food of Mus musculusIPeromyscus manicuiaTus, and
Peromyscus Ieucepus In Vigo CounTy, Indiana. J. Mamm., 47: 473-
486. ‘—

 

 

WhiTmoyer, T. F. I956. A laboraTory sTudy of growTh raTe in young Micngigs
pennsylvanicus. M. S. Thesis, DepT. Zool., Michigan STaTe UniversiTy,
62 pp.

 

 

Yang, K., Krebs, C. J. and B. L. Keller. I970. SequenTlal live—Trapping
and snap—Trapping sTudies of MicroTus populaTions J. Mamm., 5|:5l7-526.

APPENDIX A

APPENDIX A

COMPARATIVE POSTNATAL DEVELOPMENT OF AKODON AZARAE,

ORYZOMYS NIGRIPES, AND OXYMYCTERIS RUTILANS

 

INTRODUCTION

Purpose
This appendix presenTs The resulTs and discussion concerning The
laboraTory aspecTs of a comparaTive posTnaTal developmenT sTudy of The
Three mosT numerous grassland myomorphs encounTered in The field siTes,
namely, Akodon azarae, Oryzomys nigripes, and OxymycTeris ruTilans. These
daTa are Then compared wiTh Those presenle known abouT Their NorTh
American grassland counTerparTs in an efforT To deTermine wheTher similar

adapTional reproducTive responses have occurred Through The process of

convergenT evoluTion.

I40

REPRODUCTION ~ LABORATORY RESULTS

PosTnaTaI DevelopmenT of Akodon azarae

 

MeThogg

 

Females in advanced pregnancy were separaTod from The harem
(ld'To 3—539) and mainTained in separaTe cages. When The gesTaTion
lengTh was being esTablished, The male was also Transferred To Take
advanTage of The posTparTum esTrus. LiTTer checks were made aT leasT
once per day and ofTen Twice unTil The young were born. When found,

The liTTer size and sex were noTed and each neonaTe was measured, in
mm, for ToTal, Tail, hind feeT, and ear lengThs, and also weighed To
The nearesT O.| 9. These measuremenTs, Taken while The animal was
eTherized, were ThereafTer Taken aT weekly inTervaIs To The TweIfTh
week, by which Time The animals conTinued To gain only an impercepTible
amounT. During This period various oTher developmenTal characTerisTics,
such as hair growTh, incisor erupTion, hearing, eye opening, sexual
maTuraTion, and moITing paTTerns were noTed.

MesT long—Term growTh measuremenTs were Taken from l9 liTTers.
Sample sizes varied from one age To anoTher due To cannibalism (common
for cerTain females wiThin The firsT few days posTparTum), deaTh
(accidenfal or afTer new harems were esTablished), Tail loss, pregnancy,
or failure To measure. All animals were mainTained on a commercial
mouse chow and an d IIbITgM_waTer supply.

”— “...-“

GesTaTIon period

 

In mosT cases, male Akodon remained wiTh The female during pregnancy
1
"I

I42

and for some 24 hours afTer parTuiTion, if separaTed from her of all.
Based on vaginal smears for sperm, There is a pesTparTum esTrus of
undeTermined lengTh. However, The liTTIe available daTa indicaTed ThaT
if The female was noT inseminaTed wiThin 24 hours of parTuriTion, she
would noT be recepfive again for several days. From This procedure, an
esTimaTe of gesTaTion lengTh was made. Of eleven females on which The
Time of inseminaTion was known, gesTaTion lengTh ranged from 20-25 days,
wiTh a mean of 22.7 days. A female nursing six young during pregnancy
had The IongesT gesTaTion period, 25 days, buT anoTher female nursing
Three had The shorTesT, 20 days.

LTTTer size

 

Of 43 53 azarae liTTers raised in The laboraTory, There was a
mean of 4.6 young per liTTer, a mode of 4, and a range of 2-I0. This
average iTTer size was noT significanTiy differenT (p>.05) Than ThaT
found for PloT I and Hill PloT animals (4.3,mode 4, range l-8, N=|20)
palpaTed for young, alThough The variance was higher (p<.05) in The
lab colony. There were neiTher differences (p>.O5) in mean liTTer size

beTween The Two breeding seasons of PloT | nor beTween The second

breeding seasons (Hill PloT was esTablished afTer The firsT season) of
Hill PloT and PloT i.

AlThough females ouTnumbered males by a raTio of l.2:l.0 in The
laboraTory sTock, a goodness of fiT X2 TesT showed no significanT
difference aT The .05 level from The expecTed I:| sex raTio. A negaTive

cerrelaTion beTween liTTer size and neonaTe weighT was found (N=22,

If

r .60, p<.0l). Cafe in Table I—A show This correlaTlon aT boTh exTremes
of IITIer size, and also The range of variaTion in weighTs wiThin liTTers.

By examining field records, iT was possible To find The approximaTe weighT

I43

 

 

0.." N.w N.a n.o sea aces

N._ e.N 0.0. N.em _.w_ N.N m._ o.m a.aN n.mm N.a_ m.e_ a.Ne _.wn m.e _.mn _.Nm m.e +cmuLam
_ N e. me m_ a _ N ON aN m. m a m _ a N. _ .>_ac_
.02

m_ e. m. N. __ o. __ o. a a N m N a m N a m ><a

 

 

 

 

 

noceao >__:+ o>o oco +mme_ +<

02.2mmo m>m

ee>Lemoo +mc_m

oz_m<mz

ue+asco Lem_oc_ eco +mmo_ +<

mmOm_oz_ awed:

mmOmmoz_

mmKOJ

 

 

 

.omLon cooOx< c_ cocooaam +mc_+ mc_couo o>o new
.mc_cmoc .co_+azco Lem_oc_ +0 mo_+m_co+omcmzo _m+cero_o>eo 65+ can; emm +0 co_+:n_c+mmn secesoocm .NI< e_ame

 

 

 

 

o.NIm.N _e.N h N.NIm.N Nm.N m
o.NIN.N .N.N N o.NIo.w on.m m
v.NI_.N NN.N o m.MIo.N mo.m n
_.mIm.N om.m o m.NIo.N 0N.N N
v.NIm._ mN.N o N.mI_.m m_.n N
m.NI_.N oe.m o m.nIo.m mN.m N
mmcom +gmmo3 com: o- I Le++_a emcee +Lm_o3 comm mNWm La++mo
.omcmwm cooox< c_ mco++__ coea+mn 0+

nocmaeoo .c_£+_3 co_+m_ce> ecmmez one NEgo; cmoe 0+ oN_m Lo++__ +0 a_nmcom.m_mm ._I< owums

I44

of a female when she was known noT To be pregnanT, as deTermined by
palpaTion. An aTTempT was made To correlaTe This weighT wiTh The number
of palpable embryos found wiThin her one Trapping period earlier or
laTer. AT Times several periods In eiTher direcTion were examined if
The animal was maTure and her weighT appeared sTable. The resulTs, boTh
ploTs pooled, demonsTraTe ThaT There Is a posiTive correlaTion (N=IIO,
r=.2|, p<.05) beTween palpable liTTer size and female weighT.

Physical developmenT

 

WiThln The firsT 24 hours (Figure A—I) The neonaTe's dorsum,
including The head, Tail, Top of The feeT, and planTar Tubercles, were
siighTiy pigmenTed. Only The eyelids were darkly pigmenTed. LaTeraIly,
The dorsal pigmenTaTion blended info The venTer and The flesh-colored
remaining porTions of The body. Conspicious Through The venTer skin
were The incomplefely fused sTernal elemenTs and The superficial aspecTs
of The miik~filled sTomach, The inTesTine, and liver.

The eye lens were noT generally visible because of The very
pigmenfed eyelids. PorTions of The fronfal and parieTal cranial suTures
were evidenT Through The skin. The ears, Their peripheral edges sligthy
pigmenTed, were noT folded aT birTh. AlThough unpigmenTed, The claws
were wellmdevelopod, buT exTended only sligthy beyond The unguai phlanges.
A few whiTe hairs were generally scaTTered over The head and The dorsum
and appeared To be The fuTure guard hairs. These hairs, especially Those
on The head, were visible wiTh The naked eye, buT were besT observed wiTh
sTereo-binocularscope. The whiTe mysTacial vibrissae were well—developed,
The IongesT reaching approximaTer 3 mm In lengTh.

The neonaTe was generally noT vocal when handled. There was no

abiliTy To uprighT ITseIf. The fall was normally found Tucked beTween

I45

The legs for The firsT few days. Many young, aside from The Typical
uncoordinafed movemenTs, were capable of performing a springing acTion.
This movemenT, sfarTed from The normal, sligthy doubled posiTion of
The resTing neonaTe, resulTed in a very quick sTraighTening of The body
and a kicking ouTwards. AT Times, liTTer maTes would likewise respond,
resulTing in a major disTurbance. This Type of behaviorism has also

been noTed in ReiThrodonTomys humulis (Layne I959).

 

AT approximaTely 4 days of age, (Figure A—2) The neonaTe was able
To righT iTself, alThough any quick crawling movemenT was likely To
send The animal sprawling. The dorsal guard hairs, more plenTifui by
now, were approximaTely l mm in lengTh. The yellowish-Tipped underfur,
barely above The skin surface, was presenT aT This age or day 5 on The
head and possibly neck region, and in a few of The more advanced
neonaTes, The anTerior porTion of The back. The onseT of underfur growTh
gave The mouse a velveT—TexTured coaT during This sTage. There was
usually a subsTanTial amounT of epidermal scaling in a wide area in
fronT of The advancing underfur. The venTer was coaTed wiTh a sparse
covering of whiTe guard hairs abouT 0.5 mm in lengTh. The sTernal
elemenTs had fused and The visceral organs noTed earlier were sligthy
less visible because of a Thicker, more pigmenTed skin.

By The age of 7 days (Figure A—3) almosT all A. azarae had The
dorsal underfur presenT over The enTire dorsum while in The remaining,
The underfur was lagging very sligthy on The rump area. The guard
hairs were approaching 2.0 mm in lengTh, while The underfur was approxi-
maTely O.5—O.l mm. Underfur on The ears, legs, and feeT was well-developed.

Epidermal scaling conTinued laTerally and also occurred on The upper hind

limbs. Scaling began on The pecToral region, and in some, on The lower

l46

Figure A—l. NeonaTe Akodon azarae.

 

Figure A-2. Fourwday—old Akodon azarae.

 

 

I47

 

Figure A-l

 

Figure A—2

I48

Figure A~3. Seven—day-old‘Akodon'azarae.

 

Figure A-4. FourTeen~day—old Akodon azarae.

 

 

 

 

I49

 

Figure A-3

 

Figure A-4

I50

porTions of The venTer where underfur appeared. In mosT cases, The
underfur was less Than 0.5 mm in lengTh, The guard hairs, |.O mm. The
liver and sTomach were sTill fairly conspicious Through The venTer skin.
The nipple areas were jusT sligthy disfinguishable from The surrounding
skin. The longesT mysTacial vibrissae were 8—l0 mm. LiTTle behavioral
change had occurred since previously. The mice were more sTeady on Their
feeT Than aT four days, buT sTlll had problems mainTaining Their balance.
AT leasT some conTinued To kick abruple when disTurbed, buT oThers,
especially laTer, were beginning To crawl ouT of The nesT, scurrying
aimlessly. ‘

AT l4 days of age (Figure A-4),_A. azarae was well-developed (see
nexT paragraph also), and many could be weaned. The dorsal underfur was
approximaTely 5.0—7.0 mm, while The guard hairs were 8.0-9.0 mm in lengTh.
The venTral underfur was abouT 3.0—4.0 mm, and The guard hairs 5.0-6.0
mm in lengTh, Thus hiding all signs of The viscera and nipple areas
previously described. The longesT mysTacial vibrissae measured abouT
ll.O—l2.0 mm. The incisors, whiTe~Tipped for The firsT O.5—l.0 mm afTer
erupTion, sTill showed some whiTe, alThough This area would disappear
wiThin The nexT week or Two.

From The fifTh day and TerminaTing aT weaning, anoTher series of
raTher imporTanT developmenTal characTerisTics also appeared. Table 2-A
gives The days aT which incisor erupTion, hearing, and eye Opening were
flrsf observed in A, azarae, The young Akodon were TesTed for response
To sound by holding The mouse wiThin several inches of The mouTh, Then
producing a loud sound wiTh The Tongue and roof of The mouTh. Such a
sTimulus invariably eliciTed a sTarTle response from These animals which

had developed hearing.

AlThough weaning of animals under laboraTory condiTions was highly
arTiflcial, daTa for A, azarae lndicaTed ThaT under condiTions of
available food and waTer, The mice could be weaned wiThin l-3 days afTer
eye Opening; ThaT is, aT approximaTely Two weeks of age.

MolT

 

Barlow (l969) discussed molT paTTerns of Uruguayan A, azarae, based
on field—Trapped animals. Consequenle, no deTailed sTudy of molT paTTerns
was underTaken here. However, during weekly posTnaTal measuremenTs of
animals, cerTain feaTures were noTed. A posT-juvenile molT was observed
on The venTer of 35—day-old animals. Several animals sacrificed aT 2| days
showed no evidence of melanin deposiTion on The skin, while if was very
evidenT on The venTer of 28—day—old individuals. Apparenle This posT-
juvenile molT, difficulT To observe in live animals (only on venTer here)
because of The similariTies in hair color and TexTure beTween juveniles
and adulTs, lasfed for several weeks. Some animals firsT noTiced wiTh a
venTer molT paTTern aT 35 days conTinued To show pelage changes unTil 63

days of age.

 

 

GrowTh- weighT gain and maTuraTion

The mean weighT of l08 neonaTe A, azarae, pooled sexes, was approxi-
maTely 2.5 g, abouT l3 percenT of The adulT female weighT, based on The
average for 84wday—old laboraTory—raised individuals. If based on field
daTa, females averaging approximaTely 25 9 produced young. On This basis,
The neonaTe is abouT lO percenT of The female weighT. ln Terms of average
adulT weighT, The mean A. azarae neonaTe liTTer weighT, based on The mean
liTTer size of 4.6 as deTermined in The lab, was 60.5 percenT, or abouT
45 percenT lf weighTs of field daTa animals are considered.

The growTh by weighT of neonaTe Akodon To maTuriTy, wiTh The mean,

Figure A—5.

l52

Mean weighT, range, and i,2 SE for pooled sexes of Akodon

azarae. Numbers above The mean designaTe female (Top)

and male (boTTom) sample sizes. Numbers below The mean
wiThouT parenTheses give The preporTion of perforaTe
females To ToTal females; Those wiTh ParenTheses give The
proporTion of pregnanT females To ToTal non-nursing

females.

 

I53

 

 

 

 

 

 

 

 

 

 

 

 

 

 

m><o z_ .Iem_m emem< me<
me we on no em me me mm mm _N a. k
d u d - 4 ~ ~ - - - a -
\ LYN
A.
a mee
‘1 no a
e
..o
. 1 mm.
mm. 00.
Th:
om . ..I L c
A__.c .l s
am.c l. a . NM ..N_
. Amm.v l. oa
:osfim V i c. 3 L:
.l m_
v I...
_N
2. mm ..e_
.v G
e . ..m.
.f E
v i ON
om 1 ON
mm
lr- ll BI—v 1 NW
.s _ mm
if. .T VN _N L VN
m . om mm
on «.N J .mm
_N
mm
mm ale wene_c

.‘vl

-Lll‘D | El

if'l'x/Elt') N l

i 2 S.E., range, and sample sizes, is shown in Figure A—5. ProporTional
growTh, mean, and range of weighT measuremenTs are shown for each seven
day period from birTh To 84 days of age in Table A-3. GrowTh conTinued
beyond 84 days of age (l2 weeks) buT only aT a very slow raTe. Approxi-
maTe Time of one-half growTh was also calculaTed.

LaboraTory reared 84-day—old young appeared To weigh The same as
similar-sized individuals snap~Trapped in The field. To TesT This, head—
body lengThs of 84-day-old lab born mice were divided inTo 5.0 mm
incremenTs. Then The body lengThs, similarily apporTioned, of The same
approximaTe number of snap—Trapped mice were Taken from field daTa for
The summer period, January-February l969. The mean weighTs wiThin each
body lengTh incremenT did noT differ significanTiy (p>.05). Sexes were
pooled since There was no significanT difference beTween The means.
However, There was a sTrong Tendency in laTer weeks for adulT males To
weigh more,Than adulT females. This Tendency was significanT (p<.05)
in mosT of The winTer (non-reproducTive) populaTions from boTh field
ploTs where mean weighTs were compared.

Aging by weighT alone is noT accuraTe because each age group exhibiTs
considerable variaTion (Figure A—S). Even so, weighT quickly separaTes
live-Trapped animals inTo raTher crude age caTegories. Individual A, azarae
were considered To be juveniles if equal To or less Than l2 9 in weighT, aT
which Time They were approximafely 30 days of age. Since weaning occurred
when The mice were approximafely l4 days of age, They were a parT of The
Trappable populaTion for abouT Two weeks. SubadulTs were These animals
weighing more Than l2 9 buT less Than l8 9, a mean weighT They approached
aT abouT 60 days of age.

By This age, The firsT female Akodon showed signs of pregnancy by

palpaTlon (embryo deTecTion was noT possible unTil afTer The firsT 7-l0
days). From ThaT poinT in Time, pregnancies conTinued aT a moderaTe raTe
(Figure A-5). Male maTuraTion, based on scroTal developmenT and manual
phallic proTrusion, appeared in several animals (4.7 percenT) aT abouT
35 days of age. By The age of 60 days (male mean weighT, I7.} 9), all
The males were sexually well—developed alThough This was noT The case

in The field. For The purpose of This sTudy, adulTs, regardless of sex,
were considered To be These animals ThaT weighed l8 or more grams.

OuT of approximaTely a dozen harems where The males were as old or
older Than The females, The firsT liTTer was noT born unTil The males
were: 83, 9|, 98, 99, IOO days old. Since The gesTaTion period for
A, azarae_is approximaTely 23 days, The youngesT male did noT breed
unTil abouT 60 days of age. Perhaps experienced, maTure females maTed
To young males would give a beTTer indicaTion of male sexual maTuriTy
(buT experience indicaTed ThaT older animals do noT readily accepT
inTroducTions).

GeomeTric growTh raTe

 

GeomeTric growTh raTes provide a sensiTive basis of comparison of
growTh paTTerns. The mean insTanTaneous raTe of growTh was calculaTed
from The change over Time in mean weighT of a number of animals.

Brody (l945) gives The formula for insTanTaneous raTes of growTh
as K=loggW2~log WI, or more simply, k=log fl2_f, where k=insTanTaneous
relaTiveTEaTe of growTh for a given uniT ofwTime, flgarafio weighT aT

WI
T2 To weighT aT TI, and, T=Tlme inTervaI in days beTween observaTions.

 

Figure A-6 shows The mean weighT of Akodon, sexes combined, ploTTed
on a semi—log scale for The firsT 84 days (l2 weeks). The slope of The

line, represenTing The insTanTaneous raTe of growTh, approximaTes a

l56

Figure A-6. Weekly average raTe of growTh (weighT) for pooled sexes of

Akodon azarae. The sTraighT lines roughly indicaTe The

 

insTanTaneous raTe of growTh. Sample sizes are given above

each poinT.

 

 

 

 

 

 

 

m><g z_ .Iem_m mmee< mo<

am we on me on me we mm mm _N e_

a u d n J a _ d H u a
7
r)

mm
me
me em me we we

©I< mane-m

 

DOUGH \0 Ln

('3
(‘4

iHOlEM

SNVHO NI

 

|58

 

 

 

 

 

 

 

 

 

moo. amo. Nmo. mmo. _oo. oo_. Iii .o.m._
o.omio.__ m.o_io.o ..m_im.o o.m_|o.n o.m_iv.m m.wie.m N.mlo._ emcee
A_.m_v Am.q_v Ao.N_v Av.o_v AN.mo .Am.mv Am.mv cmmx
m.me _.ek o.mo o.am m.me o.km m.~_ 5+30Lm .aoLa
erm_m3
moo. woo. moo. o_o. moo. No_. iii .w.m._
m_iN_ m_im_ v_im_ m_io_ m_io mum mum mmcmm
Am.m_v Am.m_v Ao.m_v Ao.m_o An.o_v Ao.ov Ao.mv cam:
m.>m o.mo o.~o o.oo ..nn o.mv m._N 5+30cm .aocu
Ieozmo w<m
moo. moo. moo. __o. oeo. .mo. iii .o.m._
_N-m_ _N-m_ _N-e_ om-o_ m_-m_ m_-__ . mun mmcmm
Am.o_v A_.o_v an.o_v Ao.o_v Ah.o_v Ao.m_v An.nv cmm;
o.oo N.oo N.vo o.oo o.¢o o.mo m.om c+30co .aocd
Foam axwm
moo. moo. moo. o_o. omo. Neo. iii .owm._
Noion _ouem ooi_n monmn nmiom _oioe melon mocmm
Am.nmv An.mov Am.onv Am.nnv Ao.mov Ao.mmv Am._qv cam:
m.mm m.em n.2m N.om _.Ok n.5m o.Ne 5+;ocm .aoua
Ieozmo yoom
ooo. o_o. N_o. mmo. who. moo. iii .o.m._
mklkm oelmm melee amlme mmlkm mmlem k_l__ magma
A_.moo Ao.oov Am.omo Aw._mv A_.evv Av.omv Ao.m_v amen
e.mm o.mm m.mm 0.05 m.me _.mm m.om c+eoLm .aoLm
Thmzmo umch
moo. moo. moo. omo. meo. omo. is: .o.m._
eo~imm ooolmm_ <m_iom_ ov_n_N_ om_loo _oimn ooiov mmamm
Ah.om_ ho.ev_o Am.mm_o. Aw.om_v Ao.___v Ao._mv Am.mmv cmm:
o.mo o.no m.mo w.ow ..mo o.ov m.mm n+3obm .aocd
Iemzua J¢eow
me mm mm _N v. n L+L_m
we >mo
m><o z_ wo<

.m+cesocsmmee +_:om n_0i>mu:ao co ummmn mcom+m~ao_mo __< .mmcmmm am L0+

o_m3 ncm .mg+oce_ Lee ucm .+00+ nc_c .>v0n .__m+ ._m+o+ c0+ c+30um +_m£ioco +m mom e+me_x0ca¢m
.A.o.m._o c+30cm +0 m+mc msoocm+co+mc_ >mmmo .mmocmc ~mcmoe .n+ceocea cow n+zocm _mco_+L0uoLd .mi< e.nmh

l5?

L_'J

 

 

 

 

 

 

 

o_o. ooo. ooo. ooo. ooo. __o. .o.m._
k.mmio.e_ o.om-o.e_ ~.omle.m_ m.mm-o.m_ n.mm-m.~_ m._~-e.__ moomo
o.o_ Am.o_o xo.k_o Ao.o_o Am.e_o Am.e_o Am.o_o cam:
o oo_ o.mo , e.mo o.oo o.oo o.oo 5+;ouo .ooLa
e:o_m;
--- moo. _oo. _oo. _oo. ooo. .o.m..
m_-m_ m_-m_ m_-m_ m_-m_ m_-m_ e_-m_ mocma
m.w Am.m_o Ao.e_o Ao.m_o Ae.m_o “o.m_o Am.m_o . cams
o.oo_ N..o_ e.oo N.oo o.oo m.ko c+30Lo .ooLa
Ieozoo mew
--- _oo. ooo. _oo. _oo. _oo. .o.m._
_N-o_ _N-o_ _N-o_ _N-o_ _N-k_ _N-k_ emcee
N.m xo.o_o xo.o_o Ao.o_o Ao.o_o Ak.o_o xo.o_o cmme
o.oo_ o.oo e.oo k.oo o.oo o.oo o+3oLo .noLa
Poop ox_x
moo. _oo. moo. moo. moo. moo. .o.a._
mo_-oo eo_loo mo_-oo _o_-mo oo_-mo oo-_o moomm
m.m Ao.koo Ak.moo xo.eoo xo.moo Ak.moo Ao.ooo ammo
o oo_ o.oo o.eo o.oo m.mo e.mo nwsoLo .oouo
Ieozmo yoom
_oo. ooo. ooo. moo. moo. eoo. .owa.o
Nk-_o mklmo ek-mo ekl_o aniom mkiom magma
o.o Ao.koo Am.koo Am.koo AN.koo x_.ooo Ao.eoo can:
o oo_ o.oo o.oo e.oo o.ko _.oo o+30uo .ooea
xeosoo some
ooo. _oo. _oo. moo. moo. moo. .owo._
mk_-mm_ mk_-om_ ek_-oe_ Ne_-ke_ oe_-me_ ko_-oe_ mocmo
..k xe.eo_o xo.mo_o Am.mo_o xk._o_o “o.oo_o Ao.mm_o cam:
o.oo_ k.oo m.oo m.oo k.oo o.eo o+30po .noLo
Ieozmo oeeoe
Amsmoo eo km on mo om ow
c+30co N\_
mseo z. oo<

 

.Aoe:c_+coov nl< m_nmh

l60

sTraighT line for shorT disTances. The raTes of growTh based upon The
Trend of The average weighT growTh curves have been calculaTed for The
firsT week aT k=.l09, and for The second, k=.06l. For The growTh of
Afigdgfl from birTh To day 7, The value of k=.|09 signifies ThaT The
insTanTaneous percenTage raTe of growTh is approximaTely l0.9 percenT
per day during ThaT 7 days of growTh. Table A—3, in parT, gives
insTanTaneous raTes of growTh of weighT by 7 day Time incremenTs.

GrowTh — body measuremenTs

 

ProporTional growTh (percenTage of ToTal, based on ThaT of 84-day-
old adulTs), mean values, ranges, and insTanTaneous raTes of growTh for
A, agagae_are given in Table A-3 for measuremenTs of ToTal lengThs of
Tail, head and body, hind fooT, and heighT of ear from noTch for weeks
0—l2. The head—body and hind fooT lengThs of The neonaTe, when compared
To Those of The adulT, were disproporTionally longer Than oTher body
measuremenTs, while The Tail and ear were much shorTer. However, The
Tail and ear grew fasTer during The firsT l4 days, Thus reducing This
difference. The approximaTe age of one-half growTh, also shown in Table
A-3, is a reliable index of posTnaTal developmenT, and like proporTional
growTh or growTh raTe measuremenTs, is ofTen used when comparing differenT
pepulaTions or Taxa.

PosTnaTal DevelopmenT of Oryzomys nigripes

 

MeThods
The daTa presenTed here are limiTed; Two pair of adulTs gave birTh
To four liTTers and because of cannibalism, escape or sickness, only
Two animals survived pasT 84 days. WeighTs measuremenTs, and observaTions
were made as in A, EEELEQ unless noTed dlfferenle. Animals were mainTained

on a dieT of mixed grain (oaTs, wheaT, cracked corn, sorghum) and an

l6!

d liblTum waTer supply.

 

-.—

LiTTer size and gesTaTiofl_period.

 

Of Three liTTers observed shorle afTer birTh, all conTained four
young. Field daTa (Table I4) show The liTTer size To average 3.6. One
female gave birTh 44 days afTer her previous parTiTuaTion, alThough The
.gesTaTion period was cerTainly shorTer Than This. One field animal was
capTured in alTernaTing Trapping periods, and in each insTance was in
advanced pregnancy. This indicaTes a gesTaTion period nearer 28 days.

GrowTh and maTuraTioA,

 

AT birTh, The neonaTe weighed 2 9 (To The nearesT gram), abouT ll
percenT and The liTTer, approximaTely 44 percenT The weighT of a l8 g
adulT (field daTa). STandard measuremenTs and weighT of 9, nigripes are
given only for The firsT 2| days and again aT 84 days (Table A—4). The
mean insTanTaneous raTe of growTh, also included in The above Table was
calculaTed for The firsT 2| days; The percenTage of ToTal growTh, based
on The measuremenTs of The Two 84—day—old individuals are also shown.

Individual 9, nigripes were considered To be juveniles if equal To
or less Than 7 g in weighT, aT which Time They were abouT 30 days old.
Those more Than 7 g buT less Than lllg were classified as subadulTs, or
approximafely 30-90 days of age. Individuals weighing ll or more grams
were designaTed as adulTs. The adulT age class was esTablished somewhaT

more arblTrarily Than ThaT for Akodon azarae. None of The lab animals

 

showed any exTernal signs (e.g. perforaTe vagina, scroTal TesTes) of
sexual maTuriTy during The 84 day posTnaTal developmenT sTudy. However,
field daTa (Table l8) suggesT ThaT some animals near The 84 day weighT

were becoming sexually maTure.

 

 

l62

 

 

 

 

 

 

 

 

 

 

 

 

 

.. ooo. emo. oeo. --- .o.m._
__-o_ elm o.e elm o.N magma
m.o_ m.o_ o.o o.e m.m o.N cam;
o.oo_ _.em _.om m.mm o.o_ o+zoLo .ooLa
exo_m;
--- mmo. e... ooo. --- .o.m.
m_-m_ N_-o_ __lo o.e o.m mmcmm
_.o_ m.N_ k.o_ _.o o.o o.N cam:
o.oo_ o.om o.Nk o.Nm o.o_ o+30uo .ooLa
Ieozmo new
in- o_o. meo. omo. --- .o.m.”
o.mm oN-o_ o.-o_ e_-N_ o-» moomm
o.m o.om a.o_ m.e_ o.N_ m.e came
o.oo_ m.ao N.mk o.em m._m o+30co .ooLo
wooL o:_i
--- N_o. Nmo. kmo. --- .o.e. _
ok-kk molom _o-am oelme omlom magma
o._ m.ee o.oo o.om o.ee o.km cmoz
o.oo. o.me _.Nk m.km e.ee o+3oLo .ooLo
Eozm o mom
--- omo. ooo. oo_. --- .o. m._
o.oo melon _olom omlmm o_-o. mooam
o.__ o.oo k.me o.om _.em k.k_ cam:
o.oo_ o.ok o.om o.om e.o_ r+zoLo .ooz
xeozmo swan
--- kmo. meo. kmo. --- .o.m._
ek_-me_ oe_-om_ o._loo_ oolee omlmm moomm
..o m.mkw o.em_ _.N__ o._o e.am com o
o.oo_ o.ek o.eo _.eo m..m +3oco .ooL
mJMo aoeow
Amsmoo eo om e_ e L+Lom
c+30co m\. we >00
o>,o z. mo<
mmo _cc.c m>EoN>co
.m+coeecam
name +_3om o.0:>muuom :0 women mco_+m_:o_mo _.< .mcm.m+oc m_cm+o>e>xo ocm mmomummc|mxeoN>Lo L0+
+mem2 ncm .mz+mco_ me one .+00+ nc_c .>o0n .__m+ ._m+o+ L0+ n+3oLo +_mcueco +0 mom e+mE_x0coam
new A.o.m._o c+30co +0 e+mc mooocm+cm+mcm >__mo .meocmc .mcmoe .A+cmocmo c_v c+30co _mco_+L0o0ca .vs< o.omh

I63

 

 

 

 

 

 

 

 

.m_o_mmoo m+cmeecammms o+mco_+c0ooLo 0: 0m ..mE_cm o.0|>mnueo co m+e_osooc_ __mkw
in: moo. woo. woo. iii .o.m._
oe om o. __ olm emcee
o.o_ o.oe o.om o.o_ o.__ N.o cam:
o.oo_ N.am o.om o.NN m.m_ o+3ouo .ooLa
exoumg
iii o_o. NNo. 00.. iii .o.m._
o.o_ o.e_ o.m_ o.o miv mmcmm
m.m o.o_ o.o_ o.N_ o.o m.v cam:
o.oo_ m,no o.on m.om o.om c+30co .aoca
Ieozmo mmm
iii m_o. omo. ooo. iii .owa.o
o.om o.mN o._N o.>_ N_io mocmm
o.N o.om o.om o._m o.>. v.o_ cam:
o oo_ o.oo o.oo e.mo o.oo n+2oco .ooLa
Food oz“;
in: ooo. omo. _mo. iii .m.m._
o.om. o.oo_ o._o_ o.on nmi_m mmomd
>.N o.om. o.mo_ o._o_ o.on o.om cmez
o.oo_ e._o o.ok o.oo m.Ne 5+3oLo .ooLa
Ieozmo >00
In: iii woo. woo. iii .owm._
in: ii: o.mv o._m mmi>_ mocmu
in: ii: iii o.me o._m w.o_ cmm:
iii iii iii Iii *iil +30Lm .aoLu
efiozmg o.Kh
:1: iii moo. mmo. iii .o.m.~
iii iii oo_ o.oo_ oolom wound
is: it: is: oe_ o.oo_ N.an cmmx
III iii iii III *lii z+30cm .aoLd
Iewzm AKpmk
Am>moo om _N a. n c+c_m
£+30Lm N\_ we >mo
m><o z_ mo<

 

mcm__+nc m_cm+o>exxo
.Aomoc_+coov vu< o_nmk

I64

Physical deveIOpmeAj

 

The Q, nigripes neonaTe (Figure A—7) was pigmenTed llgthy above;
This pigmenTaTion faded laTerally inTo The flesh—colored venTer. The
eyelids were The darkesT pigmenTed parT of The individual alThough The
lens remained visible. The ankles and Tail dorsum were sligthy pigmenTed,
as could be The planTar Tubercles. The cranial suTures were visible as
were The rib cage, liver, and milk—filled sTomach. The ears were folded aT
birTh buT parTed wiThin The firsT 24 hours. The whiTe mysTacial vibrissae
were approximaTely 3 mm in lengTh. Hair, if presenT, was barely visible
over The dorsum and head regions. The neonaTes aT Times emiTTed high
piTched squeaks of several seconds duraTion when disTurbed. No oTher
conspicious behavioral feaTures were observed. '

AT four days of age (Figure A—8), The dorsal guard hairs were
approximaTely l mm in lengTh. Brownish underfur, visible for The firsT
Time, covered mosT of The head and dorsum, wiTh The excepTion of The
rump area. Conspicious aT iTs leading edges was The epidermal scaling,
mosT evidenT on The sides of The dorsum, around The ears, and The Upper
parTs of The limbs. Sparse guard hairs, approximaTely 0.5 mm in lengTh
were visible on The venTer for The firsT Time. The rib cage, liver, and
sTomach remained visible. The young were readily able To uprighT Them-
selves from a smooTh surface by This Time. Crawling Too was well-developed.

By 7 days of age (Figure A~9) The dorsal guard hairs were 2—3 mm in
lengTh, The underfur abouT l mm. VenTrally, The guard hairs were l-2 mm,
The underfur, l mm or less in lengTh. Some epidermal scaling was occurring
on The venTer. The liver and sTomach were sTlll visible Through The venTral
hair. The nipples of The female were easily dlsTinguishable. The longesT

mysTacial vibrissae approached 7 mm in lengTh. OUT of a liTTer of four

Figure A-7. Neonafe Oryzomys nigripes. NoTe folded ears.

 

Figure A-8. Four-day-old Oryzomys nigripes,

 

I66

 

Figure A-7

 

Figure A-8

I67

Figure A-9. Seven-day-old Oryzomys nigripes.

Figure A-l0. FourTeen-day-old Oryzomys nigripes.

 

 

I68

 

Figure A-9

 

Figure A—IO

 

I69

anlmals, all could hear for The firsT Time and had Their eyes open aT
leasT a sliT. The lower incisors were near erupTion, The uppers less
so. The young balanced exTremely well and remained moTionless or crawled
on one's finger wiThouT falling. The Tail was acTively uTilized for
sTabillzaTion.

AT fourTeen days (Figure A—IO) The young, probably weaned by now,
were well-furred, very acTive, buT sTill somewhaT clumsy in Their move-
menTs, a condiTion noT observed by 2| days.

PosTnaTal DevelopmenT of OxymycTeris ruTilans

 

MeThods

As wiTh 9, nieripes, The daTa here also are limiTed. More Than l2
pregnanT females live-Trapped in areas ouTside of The sTudy grids, Then
broughT info The lab, losT Their young Through aborTion, reabsorpTion,
or cannibalism. One female successfully raised one of four young (she
aTe one, and Two oThers were losT in a fosTering experimenT wiTh The

coney raT, ReiThrodon auriTus). ATTemst To breed OxymycTeris in The

 

 

lab failed. Animals were mainTained on a dieT of mixed grain, like ThaT
Agiven To 9, nigripes, and meaT (mice or Cavia flesh) several Times per
week, and an §d_libiTum waTer supply.

LiTTer size and gesTaTion period

 

Of The Three liTTers observed shorle afTer birTh, all conTained
four young. Field daTa (Table l5) show The liTTer size To average 3.l.
AT birTh, The neonaTe weighed 5-8 9 (average, 6.2 g), abouT l2.5 percenT,
and The liTTer, approximaTely 52 percenT of The weighT of a 48 g adulT.
lf based on field daTa, females averaging 86 g produced young. On This'
basis, The neonaTe ls abouT 7, and The liTTer, 22 percenT of The female

weighT. Several field anlmals were capTured ln alTernaTe Trapping perlods,

l70
and in a few insTances were in advanced sTages of pregnancy. This

suggesTs a gesTaTion period near 28 days.

GrowTh and maTuraTlon

 

STandard measuremenTs and weighT for 9, ruTilans are given only
for The firsT 2| days and again aT 84 days (Table A—4). The insTanTaneous
raTe of growTh, also included in The above Table was calculaTed for The
flrsT 2| days; The percenTage of ToTal growTh, is based on The measure—
menTs of The one 84-day-old individual.

individual 0, ruTilans were considered To be juveniles if equal To
or less Than 28 g in weighT, aT which Time They were abouT 30 days old.
Those more Than 28‘g buT less Than 48 g were classified as subadulTs,
or Those approximaTely l-3 monThs of age. Individuals weighing 48 g or

more were designaTed as adulTs. Like in Oryzomys nigripes, The adulT

 

age class was esTablished somewhaT arbiTrarily. None of The lab animals
showed any signs of sexual maTuriTy during The 84 day posT—naTal
deveIOpmenT sTudy. However, field daTa (Table I8) suggesT ThaT some
animals near The 84 day weighT. were becoming sexually maTure.

Physical deveIOpmenT

 

The Q, ruTilans neonaTe (Figure A-ll) was ligthy pigmenTed above
buT faded laTerally inTo The flesh-colored venTer. The eyelids were The
darkesT pigmenTed parT of The individual, alThough The lens remained
visible. The ankles, dorsal aspecTs of The feeT, Tail, dorsum, and The
planTar Tubercles were pigmenTed. The cranial suTures were visible as
was The rib cage, liver, and sTomach. The ears were noT folded aT birTh.
The whiTe mysTacial vibrissae were approximafely 5 mm in lengTh. The
dorsum and head were sparsely covered wiTh erupTing guard hairs.

Like The former Two species, The young were noT observed To remain

aTTached To The nipples of The female when she lefT The well-consTrucTed

 

l7l

nesT. The young aT Times emiTTed 2 or 3 squeaks of shorT duraTion when
handled. No oTher conspicious behaviorisms were observed.

By four days of age (Figure A-l2), The enTire head and dorsal areas
were covered wiTh a dense buT shorT (under I mm) coaT of lighT rusTy-
colored underfur; The guard hairs were approximaTely 2 mm in lengTh.
Epidermal scaling occurred aT The leading edges of The pelage, parTicu-
larly on The laTeral surfaces and on The hind legs. LlTTle developmenT
was observed on The venTer; The sparse guard hairs were approaching l mm
in lengTh, and The nipple areas were fainle visible. The whiTe-Tipped
lower incisors were barely erupTed; The uppers were presenT The following
day. The young mainTained Their balance well, buT crawling was less
developed.

AT 7 days of age (Figure A—l3), The dorsum was covered wiTh guard
hairs 4 mm and underfur 2 mm in lengTh. The venTeg wiTh The visceral
organs barely visible,was coaTed wiTh underfur l.0-l.5 mm, and guard
hairs 3 mm in lengTh. The epidermal scaling was essenTially compleTed.
The TeaTs were easily observed. The one remaining animal was responsive
To sound for The firsT Time, and had boTh eyes sligthy open. When
Touched or a sharp sound was made, The rodenT jerked violenle away.
AdulTs ofTen responded similarily when disTurbed.

By l4 days of age (Figure A-l4), The young were well-furred and
adulT-like, and probably acquired mosT of Their own food. Barlow (l969)
idenTified The raTher sTrong odor which emanaTes from This species as
reminiscenT of acrolein (acrylic aldehyde). This chemical is used
commercially To warn of Toxic odourless gas leaks from mechanical
sysTems. Barlow hypoTheslzed ThaT This sTrong, peneTraTlng scenT may

serve To discourage poTenTial predaTors.

l72

Figure A-Il. Neonafe OxymycTeris ruTilans. NoTe The darkened planTar

 

Tubercles.

Figure A-l2. Four-day-old OxymycTeris ruTilans.

 

 

Figure A-ll

 

Figure A-l2

Figure Aei3. Seven-day—old OxymycTeris ruTilans.

 

Figure A-l4. FourTeen—day-old OxymycTeris ruTilans.

 

 

Figure A—I3

 

Figure A—l4

 

DISCUSSION

The laboraTory posTnaTal deveIOpmenT daTa given for Akodon azarae,

 

Oryzomys nigripes, and OxymycTeris ruTilans were noT presenTed merely

 

 

for The reason ThaT liTTIe was known of Their posTparTum onTogeny. IT
was anTicipaTed ThaT sufficienT informaTion would resulT which could
lead To an undersTanding of whaT reproducTive aTTribuTes are shared by
These Three species wiTh Their ecological equivalenTs in The TemperaTe
grasslands of NorTh America. For a more Thorough discussion of The
evoluTion of SouTh American myomorphs and The role of naTural seiecTion
in effecTing changes dealing wiTh reproducTion and posTnaTal developmenT
for a given species wiThin a specific habiTaT and consumer level, one
is also advised To read The DISCUSSION found in The main TexT.
GesTaTion Period

GesTaTion period (Table A~5) is The firsT reproducTive parameTer
examined which shows some very definiTe Trends. AlmosT wiThouT excepTion,
The grassland inhabiTing and grass-eaTing microTines are characTerized
by a gesTaTion period of 20—22 days which is liTTIe affecTed in lengTh
(1,2,,no delayed implanTaTlon) by oTher energy demands on The female,
mainly The nursing of iTs currenT offspring. This is noT The case wiTh

a few microTines specialized for oTher niches (The Tree mouse, Phenacomys

 

 

Iongjcaudus, HamilTon, l962; The woodland lnhabiTing bank vole,

 

Clefhrionomys glareolus, Brambell and Rowlands, |936), aT leasT some

 

murids (Mus musculus, Daniel, l9l0; RaTTus norvegicus, King, l9l3), and

 

 

criceTines (mosT Peromyscus, Layne l968; Onychomys, Egoscue, l960;
l76

I77

 

 

.Ammo_v mc>m4 mom .msowx90Lma +0 mmmomam cmc+o coax

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ammm_v m_;_>m m.mn n.o_ m. mo.m o._ nwnmm *m:+m_:o_cwe muom>eoLme
A_mm_v m_c_>m “Aomm_v I.
c+_em “Ammm_v Eacxooo m.om n.__ m.N_ o.N m._ vmnmm m_+o_mome .m
Ammo_o mc>ms “A_mm_v
osmx “hwea_v >m3mcso m.oe o.e_ N.m N.m N._ em m__ses£ msgo+c0uocc+_m
>nz+m +commcu o.ov _.__ m_ o.m o.m mm“ mmamLm_c m>soN>Lo
>es+m +cmmmLa m.NN N.~ mm _.m N.o mm“ mcm__+:L maam+oxs>xo
Aonm_v Lm+c_a .I
unm©m_c Lo_>m» e LmCLOI o.me o.N_ om mum e.N hm msu_LLo+ .o
Aonm_v
Lm+c_e “Am©m_v Loccoz o.mm m.m om o.m m.N union Lm+mmmoo3m_ m>roro> ro
nkMth
m_c_>m nn_om_v ..m +o
msmoz “Aoem_s co+__gmx N.NN o. m mm o.m _.m mm m.e+ms mu mesonsmw
z28 nm_ em: m.m_ _. e me” m.e _.e --- Aoo_x9 V Lm+ mewsmse um
>co_oo no_ em: m.om m. m _om m.o m.m --- Aoo_xw ; mmgummcm .m
Avvm_v Lm;ow2 w L962 v.mm m. w om_ o.m m.o mm “.mov m:w_ 8m .5 couosu.m
.Ibrmw_v cm+mom n.mv m. m mm m.v e.m _Nm m>mops mexuunimmu
Awmm_v ._m +o com.LLo2 o.m_ N. o om o.m _.m NNI_N mn+mo_cnsc x>loemorn a
Aqmm_v ._m +o com_LL02 _.mq n. 0 mm n.© mm._ in: 922+:L mrgouo_pyimrm
Aomo_v c2+cm2 v.0m q. m we N.m m.m _N Lo+mmuul no Am
Aoom_v L+oom w _mnmom w.vm m. m ow m.v N.N o._mnm.om cwcm+gle mu
Ammm_v o._om o.om v. v No m.v n.N NN:_N m 9 n.._ano mm
“_qm_v co+_25m1 m._m o. 0 me m.m n.m _N m.. E: >_>mrcwa .2
Aqmm_v +_3mcomc< w cmzoo v.0m m. m mm o.m n._ emimm 2“ remain m +0.02:
>n3+m +cmmmca m.ooso.oe N.m_ o. o_ mm:m_ o.q m.m n.mm mmLmum co oxq
+£m2m3 +£m_m3 +£m203 mN_m +£m_m3 Amsmov
ooL30m +mzo¢ o+ +_3o< 0+ +_3o< Lo++24 o+mcooz vo_cmm
Lm++_4 m+mcooz mmmco>< mmmco>< mmmco>< co_+m+mow
+coocem +cmocmd
.mgmcoe02E mc_+moi+omm c_ ncm .Inmmm .immmcm
ocm+comc< ucm cmo_coe< L+Loz mc2+_nm;c_ ncmfimmmcm oEOm L0+ +_:vm mc+ +0 +mc+ 0+ +cm2m3 Lm++2_
ucm o+mcooc +0 +cmocma .mmpm Lo++__ ommLo>m .m+gm203 +_3um ucm o+mcomc mmmcm>m .no_cmg co_+m+mow .mu< o2nme

I78

Svihla, l936). Grass—eaTlng criceTines such as Oryzomys palusTris and

 

Sigmodon have very precocial young, whose dependence on Their moTher is
less, so delayed ImplanTaTlon as an energy conserver would be of
quesTionable value.

In conTrasT To The shorT gesTaTion period of microTines, The seed-

eaTers (ReiThrodonTomygg Peromyscus) have a period sligthy longer, wiTh

 

The insecTivorous species (Onychomys) The longesT. IT is presumed ThaT

The same selecTive forces are acTing on The ArgenTine species; A, azarae,
The main myomorph herbivore, has a reasonably shorT gesTaTion inTermediaTe
beTween ThaT of microTines and The criceTid seed—eaTers. UnforTunaTely,

we can only speculaTe aT This daTe ThaT Q, nigripes, The seed—eaTer, and
‘9..£giilans, The insecT—eaTer, would have correspondingly longer gesTa-
Tion periods according To Their respecTive consumer level. How True This
Trend will be for 9, ruTilans is quesTionable. lTs larger size and more
mesic affiniTies, when compared To Onychomys, may mask any clear-cuT
differences. Layne (I968), discussed The imporTance of These facTors in

Peromyscus.

 

PosTparTum EsTrus

According To Conaway (l97l), posTparTum esTrus seems To have
developed independenle many Times and iTs disTribuTion ls sporadic
ThroughouT mammals. Asdell (l964) noTed ThaT iT follows no phylogeneTic
paTTern. As an example, iT does occur in many buT noT all species of

Peromyscus (alThough This is a poinT of conTenTion; see Rcod, I966), The

 

gain or loss of posTparTum esTrus Is one of The mosT easily made of The
major reproducTive adjusTmenTs. lTs main purpose in small shorT life-
span mammals seems To be as a mechanism for increasing producTIvlTy.

WlTh excepTion of some Peromyscus llsTod by Asdell and Oryzomys nigripes

 

 

and OxymycTeris ruTilans on which IT was noT esTablished, all oTher

 

I79

species in Table A-5 are known To have a posTparTum esTrus.
LiTTer Size

Average liTTer sizes (Table A—5) show a very definiTe posiTlve
Trend Towards larger liTTer sizes wiTh grass—eaTers buT generally
smaller liTTers for seed- and InsecT-eaTers. The Three ArgenTine species
examined here also show This Trend.

ReproducTive Efficiency

Frank (I957) menTioned ThaT one of The facTors which affecTs The
reproducTive poTenTial of a species is iTs reproducTive efficiency. This
Is compuTed as The average liTTer size weighT divided by The average
weighT of The reproducing females, Times IOO percenT. WhaT This measure-
menT sTrongly suggesTs is ThaT one species is puTTing an equal, more, or
less of an "invesTmenT" inTo reproducTion aT a given Time compared To

anoTher species. Frank found ThaT MicroTus arvalis liTTer weighT amounTed

 

To 53.2 percenT of The moTher's weighT, while in mosT oTher small rodenTs,
20—30 percenT is The average (Table A~53 Layne, I968). NeonaTes of Akgggg
§;§£§e_are relaTively large when compared To adulT weighT, and wiTh The
respecTable liTTer size, boTh conTribuTe To The high reproducTive
efficiency found in This species. Several dissimilar species in Table A-5

show equally high reproducTive efficiencies (e.g., ReiThrodonTomys humulis,

 

Onychomys Torridus, CIeThrionomys ruTilus).

 

GrowTh - WeighT
Daily insTanTaneous raTes of growTh (Table A-6) for The firsT week
of growTh (where greaTesT differences In raTes are found) were calculaTed
for a number of differenT species To deTermine if differenT raTes could
be found which would parallel Their food habiTs. Layne (l968) calculaTed

growTh raTes for various species of Peromyscus and found a subsTanTial

 

I80

Table A-6. Daily insTanTaneous raTes of growTh for approximaTely The firsT
week for some grassland inhabiTing NorTh American and ArgenTine
grass-, seed-, and insecT—eaTing myomorphs.

 

 

 

 

 

 

 

 

Species Daygiafger k Source
Akodon azarae 0-7 .l09 PresenT sTudy
A, olivaceus 0-7 .l00 MSU lab colony
MicroTus oregoni 0-9 .l62 Cowan and ArsenaulT (I954)
‘M. pennsylvanicus 0-7 .I52 HamilTon (I941)
f} callfornicus 0-7 .l68 Hanield (I935)
EL E9!:929§. 0-7 .II4 Seidel and BooTh (I960)
.M. ochrogasTer 0-7 .I3l MarTin (I956)
CleThrienomys ruTilus 0~7 .l57 Morrison T al. (I954)

 

DicrosTonyx rubricaTus 0-7 .l49 Morrison T I. (I954)

 

 

 

 

Sigmodon hispidus O-IO .l08 Meyer and Meyer (I944)
(Louisiana)

S: fulvivenTer 0-7 .I29 MSU lab colony
(Durango)

Oryzomys palusTris 0-7 .ll4 HamilTon (I946)

OxymycTeris ruTilans 0—7 .082 PresenT sTudy

 

Onychomys leucogasTer 0-7 .090 Horner (l968)

 

Q. Torridus 0-7 .084 Homer and Taylor (l968)

Oryzomys nigripes 0-7 .079 PresenT sTudy

 

ReiThrodonTomys humulis 0—7 .087 Layne (I959)

 

Peromyscus maniculaTus* 0-7 .l29 Layne (l968)

 

 

*For oTher species of Peromyscus, see Layne (l968).

 

amounT of variablllTy aT The subspeciflc as well as specific levels. The
daTa presenTed here does indicaTe a generally higher growTh raTe in The
herbivorous microTines. However, The oTher species IisTed, including
Those from ArgenTina, regardless of food habiTs, have somewhaT lower
values.

Physical DevelopmenT

NoT all growTh ls devoTed To increasing The amounT of flesh, REE sg,
Much of The energy uTilized by The young goes info The developmenT of
homeoThermic (including pelage growTh), and IocomoTor conTrol. OTher
onTogeneTic changes, such as TooTh formaTion, eye opening, hearing and
oTher complex neuromuscular advances also musT Take place before The
young can be weaned. Table A—7 examines The daTa for The developmenT of
incisor erupTion, hearing, eye opening, and perhaps The mosT ill-defined
buT neverTheless mosT imporTanT developmenT of all, weaning.

Concerning The laTTer, King 9i al. (I963) aTTempTed To define
weaning in deer mice as The age aT which The young mice gain weighT (lab
chow, waTer, ad libiTum) upon separaTlon from The female and liTTer maTes.
WheTher This definiTion will wiThsTand The rigors of TesTing under field
condiTions remains To be answered. MosT resulTs in The llTeraTure show
weaning To occur from several days To a week afTer eye opening, depending
upon The species and The invesTigaTor.

AlThough There appears To be slighT species differences in develop-
menTal Times, Siamodon Is The mosT precocial (Table A-7) myomorph In
NorTh America and possibly The wesTern hemisphere. This genus has a
gesTaTion lengTh The same (e.g., Qflyghgmys) or only sligthy higher Than
oTher species IisTed on Table A-5. This genus is aT one exTreme where

neonaTe weighT is less Than whaT mighT be expecTed for a 27—day-old

Table A-7. Time of firsT observance of several developmenTal characTer-
. isTics in some grassland inhabiTing NorTh American and
ArgenTine grass—, seed-, and insecT—eaTing myomorphs. Time
expressed in days.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

lncisors Hearing Eye Opening Weaning
I-Akodon azarae 6-7 8 l2 l4-l5
Z-Microfus pennsylvanicus 6-7 7-8 8-9 l2-l4
Bjfl. callfornicus 4-6 7-8 9-I0 I4
4—M. monTanus 5~6 IO-IZ IO l2-l5
S-E. £31951 5 5 I0 lO-ll.5 I3
6—M. ochrogasTer I 8-9 lO-Il I6
7¥CleThrionomys ruTilus 2 - 9-l0 l7-I8
8—Sigmodon hispidus (Louisiana) l-2 Sev. days l-2 IO-20
9-8. fulvivenTer (Nexico) O-I 0-2 I-2 IO-ZO
IO-Cryzomys palusTris 5 - 8 II-I3
ll-Onychcmys leucogasTer IO-II I5 |8-20 23
I2—O. Torridus IO-Il l4 l6-20 20
l3-0xymycTeris ruTilans 5 7 7—8 l4
I4-Cryzomys nigripes 8-9 7? 7-8 I4
l5-PeiThrodonTonys humulis 6-7 8-9 7-8 2|
I64§TTEigfib+L§ 4-5 9 lI-l2 I9-2l
I7-Eengmyscus maniculaTus* 5.2 I2-l3 l2-l4 l8

 

 

*For oTher species of Peromyscus, see Layne (l968).

 

l-PresenT sTudy

2-HamilTon (l94l); Godfrey (I953); pers. daTa

3~Selle (I928); Hanield (l935); Jenkins FoundaTion for Research, Salinas,
California (pers. comm.)

4-Seidel and BooTh (I960)

5-Cowan and ArsenaulT (I954)

6-Kruckenberg ef al. (I973); MarTin (I956)

7-Morrison eT EIL—TT954)

8-0dum (19557; Svihla (l929); MSU lab colony

9-MSU lab colony

lO-HamilTon (I946)

ll-Horner (l968)

I2~Horner and Taylor (l968)

l3-PresenT sTudy

l4ePresenT sTudy

IS—Kaye (l96l); Layne (I959)

l6-SmiTh (I936); Svihla (|93l)

l7-Svihla (I932); King (I958); King 3I_§l: (I963); Huff (I973)

I83

(gesTaTion Time) Individual in relaTion To The adulT. ReproducTive
sTraTegy for These large herbivorous criceTines appears To have direcTed
energy noT inTo biomass buT insTead inTo feaTures which allow precocialiTy,
and hence independence (aT relaTively small size) from The moTher wiThin a

very shorT Time. Oryzomys palusTris is anoTher species which has a raTher

 

precocial deveIOpmenT.

Second To The grass—eaTlng Sigmodon and Oryzomys in physical
developmenT are mosT of The microTines. They noT only mainTain a high
growTh raTe buT in addiTion are capable of independence by Two weeks of

age. OTher NorTh American species (Qnychomys, ReiThrodonTomys, and

 

Peromyscus average anoTher week longer before weaning. The Three

 

ArgenTine species are noTable because each was successfully weaned by
l4 days of age.
Sexual MaTuriTy
in relaTively shorT-lived species such as ThaT found in mosT rodenTs
under naTural condiTions, sexual maTuriTy aT an early age would be a
obvious asseT, and Therefore an imporTanT aTTribuTe of reproducTive
poTenTiaI.

Under lab condiTions, a few female Akodon azarae have a perforaTe

 

vagina by four weeks of age, alThough as a group iT is noT unTil They
aTTain 6-8 weeks of age before perforaTion and The firsT pregnancies
are commonly noTed. Males do noT seem To sTarT breeding unTil They Too

reach Two monThs. Oryzomys nigripes young showed no change from The

 

non—perforaTe, non-scroTal condiTion during The l2 weeks in which They
were observed. WolghTs of field anlmals indicaTe ThaT l2 weeks Is The
very earliesT aT which boTh sexes even begin To show signs of sexual

maTuraTion. OxymycTeris ruTilans daTa also indicaTe ThaT animals (aT

 

l84

leasT females) are JusT sTarTing To reach sexual maTuriTy aT 84 days of
age (Twelve weeks). ReproducTive maTuriTy for These laTTer Two species

Then, is delayed Two To Three Times longer Than ThaT found for A, azarae.

 

The microTines (Table A~8) are characTerized by Their exTremely
early daTe of sexual maTuriTy. Frank (I957) in parTicular menTioned a
specific example where The European fl} arvalis, which suckle unTil
The sevenTeenTh day, have shown a perforaTed vagina by The elevenTh and
were maTed by old males by The ThirTeenTh. OTher grass-eaTers noTed

(i.e., Sigmodon hispidus and Oryzomys palusTris) are raTher precocial

 

 

in Their developmenT, considering Their adulT size. Seed—eaTers and
insecT—eafers on The oTher hand, generally maTure no fasTer Than and

usually slower Than These larger herbivores.

Table A-8. Age of sexual maTuriTy, in weeks, in some grassland inhabiTing
NorTh American and ArgenTine grass—, seed-, and insecT-eaTing

myomorphs.

 

Age of Sexual

Species

MaTuriTy

(in weeks)

Source

 

Akodon azarae

 

MicroTus sp.

Phenacomys ungava

 

Sigmodon_hispidus

 

Oryzomys palusTris

 

Oryzomys nigripes

 

 

Onychomys leucogasTer

 

9, Torridus

OxymycTeris ruTilans

 

ReiThrodonTomys_humulis

 

R. meealoTis

Peromyscus maniculafus*

 

6—8

4—6

|2+
8-l2
l3

5-7

PresenT sTudy

Frank (I957); HamilTon
(l94l); MarTin, (I956);
Cowan and ArsenaulT
(I954); Seidel and BooTh
(I960); Hanield (I935)
FosTer (l96l)

Meyer and Meyer (l944)
HamilTon (I946)

PresenT sTudy

Svihla (I936)

Horner and Taylor (l968)
PresenT sTudy

Layne (l959)

Svihla (I93l)

Layne (I968)

 

¥For oTher species of Peromyscus,

 

see Layne (l968).

S UWARY
The comparaTive posTnaTal developmenT of Three ArgenTine grassland

inhabiTing myomorphs was presenTed; namely, Akodon azarae, The predominanT

 

, grass—eaTer; Oryzomys nigripes, a peromyscine-like seed-eaTer; and The

 

large (86 g) InsecT—eaTing OxymycTeris ruTilans. ResulTs demonsTraTe ThaT

 

among The Three, A, azarae mosT likely has The shorTesT gesTaTion period,
The largesT neonaTe To adulT weighT raTio, The highesT reproducTive
efficiency, The fasTesT firsT week of growTh, by weighT, a nearly equal
or sligthy slower developmenT in incisor appearance, aTTainmenT of
hearing, eye opening, and weaning daTe, and Iasle, a much quicker Time

of sexual maTuriTy. Oryzomys nigripes, in relaTion To 9, ruTilans, has

 

a higher neonaTe To aduiT weighT raTio, a higher reproducTive efficiency,

a sligthy slower firsT week of growTh, a slower raTe of lncisor erupTion,
buT comparable developmenT in hearing, eye opening and day of weaning.
Sexual maTuriTy appears To Take a lengThy period of Time for developmenT

in boTh species; field daTa suggesTs ThaT iT may Take longer in 9: nigripes.
Concerning 9: ruTilans alone, The raTher precocial developmenT of This

large mouse in The areas of incisor erupTion, hearing, eye opening and

day of weaning is noTabIe.

In comparison To NorTh American grassland myomorphs, The same basic
paTTern exisTs. The grass-eaTers, parTicularly The microTines, have The
fasTesT posTnaTal developmenT. The microTines are alone in having a very
fasT growTh raTe. Even Those which have a moderaTe growTh raTe (a measure

of biomass only) have The available energy Transformed inTo precocial

I86

l87

developmenT of locomoTion, eye opening, and oTher physical~behavioral
facTors. ReproducTive efficiency does noT seem To be characTerisTic of
microTlnes. lnsTead, lT commonly appears in The smaller species of
many groups, and funcTions To mainTain liTTer sizes aT The level of Their
larger relaTives, all oTher variables being equal.

The seed- and insecT-eaTing species are more alike in many of Their
developmenTal feaTures. This may be aTTribuTed To Their food habiTs,
which probably overlap more Than do The grass-eaTers wiTh eiTher one.

Akodon azarae appears To be inTermediaTe in onTogeneTic developmenT To

 

The grass—eaTers and seed-eaTers. Oryzomys nigripes are equally or

 

sligthy more precocial Than A: azarae, buT produce smaller liTTers and

reach sexual maTuriTy aT a laTer daTe, as in seed-eaTers. OxymycTeris

 

ruTilans, oTher Than sharing a similar dieT and possibly a delayed sexual
maTuraTion, is unlike Onychomys, iTs only myomorph counTerparT in NorTh

America. OxymycTeris is more precocial in iTs early developmenT, is

 

nearly Triple The size of Onychomys aT adulThood, and reaches sexual
maTuriTy aT a raTher laTe age, buT Then commences To breed ThorughouT
The year. The facT ThaT Onychomys commonly inhabiTs xeric areas where
selechve forces would presumably differ may help in explaining The

divergence beTween The Two animals.

"IIIIII'IIIIIIIIIIIIIIIIIT