COMPARATIVE POST-NAM. GROWTH
IN COTTON RATS (GENUS SJGMODONJ

Thesis for the Degree of Ph. D.
MICHIGAN STATE UNIVERSITY
JORGE V. JIMENEZ JIMENEZ
1969

 

 

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ABSTRACT
COMPARATIVE POST-NATAL GROWTH IN COTTON RATS
(GENUS SIGMODON)
By

Jorge V. Jimenez Jimenez

A comparative study of five species of cotton rats was
made to determine similarities and differences in external
and cranial morphology. These animals (a total of 610
individuals) were descendants of wild caught cotton rats
from different localities of Mexico.

The growth rate of the body dimensions can be divided
into three periods: From one to 40 days when there is a
maximum increase in length for all the body dimensions;
from 41 to 100 days when there is a moderate increase; and
from 101 to adulthood (200-400 days) when there is practi-
cally no growth.

The skull of the cotton rat presents two well-marked
patterns of growth (in the age periods of l-lOO days and
101 days-adulthood). At birth, the cotton rat skull is
egg-shaped. During the first 20 days of post-natal life
the skull expands rapidly in breadth and height. After
20 days, the skull begins to lengthen more rapidly changing
the shape from a rounded and short structure at birth to
an elongated structure characteristic of the adult cotton

rat.

Jorge V. Jimenez Jimenez

An aging formula was develOped using nine (of 21)
measurements taken. This formula may help, associated with
other aging methods, to age individual wild-taken cotton
rats.

Significant morphological differences were found when
progeny of intersubspecific crosses were compared with
their parent subspecies.

In comparing the growth rates of the population samples
of the five species of cotton rats, the species can be
divided into three groups. Siggodon hispidus was repre-
sented by a p0pulation sample having large dimensions and
was the largest cotton rat studied. Sigmodon alleni and
§.fulviventer were smaller and showed similar growth rates.
Sigmodon leucotis and §,ochrognathus were smallest and also
showed a similar pattern of growth. The differences found
indicates that §,1eucotis and §.ochrognathus are perhaps
most remote from the supposed parent stock (presumably
resembling §,hispidus). Sigmodon alleni and §.fulviventer,
on the other hand, appear close to §.hispidus in morpholog—
ical development and thereby may be more closely related to
§.hispidus.

When comparing morphological characteristics with the
preferred habitats of the species, it is found that the

smaller and shorter-tailed species (§,leucotis and

Jorge V. Jimehez Jimenez

§,ochrognathus) live usually in shallow, rocky soils on
sparsely-vegeted slopes whereas the other three species pre-
fer deep soils and abundant grass or shrub cover. Possible

reasons for these differences are discussed.

COMPARATIVE POST-NATAL GROWTH IN COTTON RATS

(GENUS SIGMODON)

BY

Jorge V. Jimehez Jimehez

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of
DOCTOR OF PHILOSOPHY
Department of Zoology

1969

ACKNOWLEDGMENTS

I am especially indebted to Professor Rollin H. Baker,
my advisor, professor and director of the Michigan State
University Museum, for his excellent criticism and
guidance, as well as for the use of the preserved skull
and skin materials in his charge. I am also indebted to
the members of my committee, Professors J. A. Holman,

R. Seltin, and R. A. Pennell for their guidance during
the study and for their careful reading of the manuscript.
I wish to thank Professor W. E. Cooper for his suggestions
about statistics. Fellow graduate students, especially
Larry Bowdre, provided stimulating discussions and advice
about my research. My wife Maria and.my children gave me
the greatest possible encouragement, support and spiritual

aid in my studies.

ii

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . .
MATERIAL AND METHODS . . . . .
Measurements . . . . . .

Procedures . . . . . . .

COMPARATIVE GROWTH OF COTTON RATS

Sexual Dimorphism . . . .

0

Comparative Growth of External Dimensions

Body Length . . . . .
Tail Length 0 O O O 0

Comparative Growth of Skull

Formula for Aging Cotton Rats . .

Growth Rates in Subspecific

Sigmgdon fulviventer .
DISCUSSION . . . . . . . . . .
SUMMARY . . . . . . . . . . .
LITERATURE CITED . . . . . . .

iii

CIOSBGB

Page

10
12
12
19
20
23
23
26
50

S4
61
68
72

Table

LIST OF TABLES

Page

Number of laboratory-raised cotton rats
used in study. 7

External dimensions and skull measurements
showing significant differences between
males and females in the five species of

Sigmodon. 18

An analysis of growth-rates in cotton rats
as shown by measurements of the length of
the body, tail, and hind-foot of different
age-classes of animals. 22

Development with age of the rostrum and
the braincase (expressed in percentages)
in relation to the skull lengthening

(eXpressed in terms of the condylo-basal

length) in Sigmodon hispidus. 31

Development with age of the rostrum and the
braincase (eXpressed in percentages) in
relation to the skull lengthening

(eXpressed in terms of the condylo-basal

length) in Sigmodon alleni. 32

Development with age of the rostrum and the
braincase (expressed in percentages) in
relation to the skull lengthening (expressed

in terms of the condylo-basal length) in
Sigmodon fulviventer. 33

Development with age of the rostrum and the
braincase (expressed in percentages) in
relation to the skull lengthening (expressed

in terms of the condylo-basal length) in
Sigmodon leucotis. 34

iv

Table

10

11

12

13
14
15

16

17

18

19

20

DevelOpment with age of the rostrum and
the braincase (eXpressed in percentages)
in relation to the skull lengthening

(expressed in terms of the condylo-basal

length) in Siggodon ochrognathus.

Comparison of growth of height and length
of braincase.

Comparative lengthening (eXpressed in
percentages of condylo-basal length) of the
rostrum and braincase.

Comparative broadening (eXpressed as ratios
of the width of braincase) of the inter-
orbital area and interparietal space.

Increase of length (in mm.) of the alveolar
space for the maxillary tooth-row.

Increase of length (in mm.) of the diastema.
Increase in palatal length.

Increase (expressed in percent) in the
length of three dimensions of the palatal
region, from 1-40 days (accelerating phase)
and from 41—100 days (decelerating phase).

Measurements of the zygomatic breadth and
breadth of the braincase for Sigmodon.

Ratios between length of the incisive
foramen and the diastema.

Constant values (b values) for each of the
nine measurements for the five species of

Sigmodon.

Specific numbers associated with each age-
groups of cotton rats, when the aging
formula is applied.

Subspecific crosses in Sigmodon
fulviventer.

Page

35

38

39

40

42
43
44

45

46

48

52

53

55

Table

mummy

Appendix A. Average and Extreme
Measurements of the Five Species of
Sigmodon in Eight Age-Classes.

Sigmodon hispidus major.

Sigmodon alleni.

Sigmgdon fulviventer.

Sigmodon leucotis.

Sigmodon ochrognathus.

Appendix B. Average and Extreme
Measurements of the Progeny of Crosses and
Their Parent Subspecies.

Si odon fulviventer minimus (Gallego,
Cfiiguahua) x S.fuIviventer melanotis
(La Barca, JaIisco).

Si modon fulviventer minimus (Gallego,
Cfiihuahua) x §.§.fu1viventer (Coyotes,
Durango).

Si odon fulviventer minimus (Boquilla,
Durango) x §.§.fuIviventer (Coyotes,
Durango).

Si odon fulviventer minimus (Gallego,
CHiEuEfiua).

Si don fulviventer minimus (Boquilla,
Durango).

Si odon fulviventer fulviventer (Coyotes,
Durango)

Si odon fulviventer melanotis (La Barca,
Ja1$scol.

vi

Page

75
77
79
81
83

85

86

87

88

89

90

91

Figure

1

10

LIST OF FIGURES

Map showing the approximate geographical
location of the species of Sigmodon.

Growth in dimensions of body, tail and
hind-foot in males and females of Sigmodon

hispidus.

Growth in dimensions of body, tail and _
hind-foot in males and females of Sigmodon
alleni.

Growth in dimensions of body, tail and
hind-foot in males and females of Sigmodon
fulviventer.

Growth in dimensions of body, tail and
hind-foot in males and females of Sigmodon
leucotis°

Growth in dimensions of body, tail and
hind-foot in males and females of Sigmodon

ochrognathus.

Graph of the curves of the body length for
all the five species studied of Sigmodon.

Graph of the curves of tail length for all
the five species studied of Sigmgdon.

Graph of the curves of hind-foot for all the
five species studied of Sigmgdon.

Graph of the curves of four cranial measure-
ments in relation with the condylo-basal

length.

vii

Page

13

14

15

16

17

21

24

25

27

Figure

11

12

13

14

15

16

Histogram of the relationships of growth
between the rostral region, the length of
the braincase, the height of the brain-
case, the postpalatal length with the
condylo-basal length.

Graph of the curves of the ratio: length
of the incisive foreman/length of the
diastema for all the species of cotton
rats.

Growth in dimensions of body, tail and
hind-foot of cotton rats of §.£.fulvi-
venter (Coyotes, Durango), §.§,mInImus
(Boquilla, Durango) and cross No. I
(§.£.fulviventer x §.£,minimus).

 

Growth in dimensions of body, tail and
hind-foot of cotton rats of §.£.fulvi-
venter (Coyotes, Durango), §,§3mIHIHEs
(GaIIego, Chihuahua), and cross No. 2
(§.£.fulviventer x §.£.minimus).

 

Growth in dimensions of body, tail and
hind-foot of cotton rats of S.f,minimus
(Gallego, Chihuahua), §.§,meIanotis (La
Barca, Jalisco) and cross No.
(§.£,minimus x §.£,melanotis).

Growth in dimensions of condylo-basal
length and height of skull of the progeny
of the crosses Nos. 1, 2 and 3, and their
parent subSpecies°

viii

Page

29

49

56

57

58

59

INTRODUCTION

The study of the comparative ontogenesis of rodents at
the interspecific level has been largely neglected, although
a few works exist on age and growth phenomena in single
species. One such work (Chipman, 1965) details age deter-
mination in Sigmodon hispidus, but this form is not
compared with other species in the genus. Thus, the
variability of five species of Sigmodon as laboratory-
raised animals of known ages has provided a unique
Opportunity to: (1) use comparative growth studies as a
measure of phylogenetic relationships in rodents; (2) develOp
an aging formula which makes it possible to determine
approximate ages of cotton rats of all five Species; and
(3) to study growth rates of the progeny of subspecific
crosses.

Rodents of the genus Sigmodon are of medium size
(225 to 330 mm. in total length) and have short tails (less
than one-half total length). The animals are robust with
short, round ears. The pelage is coarse and grizzled,
light brown to dark grey. The tips of the plumbeous-based

hairs of the underparts vary from white to fulvous to grey.

The skull has a heavy rostrum and prominent supraorbital
ridges continuing posterolaterad as temporal ridges. The
molar teeth are high-crowned with long, narrow S-shaped
enamel lOOps on the occlusal surfaces. Cotton rats are the
dominant grass-eating and runway-making rodents that are
found from sea level to more than 3,200 meters in south
temperate and tropical North America, through Central
America, and, that reach the southernmost limit of their
distribution in Perfi.

The species of Sigmodon are classified into two natural
groups, the §.hispidus group (one species) and the g.
fulviventer group (four species).

Cotton rats of the §.hisEidus group have a semi-naked
tail, with coarse annulations that are only partly obscured
by a short scattered hairs. Animals of this group mainly
inhabit south temperate and trapical zones. In these
regions, they are widely distributed and have an almost
continuous range. Sigmodon hispidus is the only species of
group although several subspecies are recognized.

Cotton rats of the §.fulviventer group have a tail
with dense hair that conceals the annulations. They inhabit
the trapical, temperate and boreal regions in southwestern
United States and western México (see Baker, 1969). The
group includes the four species: §.alleni,‘§.fulviventer,

§g$§2§2§i§ and §.ochrognathus.

The cotton rats studied in this work were all caught
in México, (see Figure l) in the following localities:
§,hispidus--TuxPén (Nayarit); §.alleni--Jachatengo (Oaxaca);

§.fulviventer--La Barca (Jalisco), Coyotes, Boquilla,

 

Canatlén (Durango), Gallego (Chihuahua), Ibarra

(Guanajuato); §.leucotis--Coyotes (Durango); §.ochrognathus--

 

Boquilla (Durango).

Most of our knowledge of the development, growth and
life history of the cotton rats is limited to §.hispidus.
Little is known concerning the other species, §.alleni,

§.fulviventer, §.leucotis and §.ochrognathus. The wide

 

 

distribution of §.hispidus in the United States has made
this Species more available for biologists to study. But
investigations of §.hispidus have been limited largely to
studies of basic life history in the laboratory (Svihla,
1929; Meyer, 1942; Meyer and Marsh, 1943) and under natural
conditions (Odum, 1955; Sealander and Walker, 1955; Goertz,
1965).

Ovulation in §.hispidus occurs during the late period
of the vaginal estrous. The gestation period is 27 days.
Six and one-half to 12 hours after partum ovulation may
again take place, with c0pulation following within 3 to 6
hours. The reproductive rate in the hispid cotton rat is
approximately one litter every 59 days (Sealander and Walker,

1955). Feral females in the United produce offspring from

 

 

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Figure 1. Map showing the approximate geographical
locations of the species of Sigmodon. Numbers
on the map refer to the following ocalities:
l. TuXpan, Nayarit; 2. Juchatengo, Oaxaca;

3. La Barca, Jalisco; 4. Canatlan, Durango;
5. Coyotes, Durango; 6. Boquilla, Durango;
7. Gallego, Chihuahua; 8. Ibarra, Guanajuato.

early spring until late autumn. The number of young per
litter vary from one to twelve and differ relative to the
species studied and whether the litter is born in nature
or in the laboratory (Brimley, 1923; Svihla, 1929; Meyer
and Meyer, 1944; Odum, 1955; Sealander and Walker, 1955;
Hoffmeister, 1963; Goertz, 1965).

At birth, the young is completely covered with a fine
coat of light colored and adpressed hair, thickest about
the head region. By the second day hairs begin to stand
up. The dorsal region of head, body and tail is generally
slate-grey, while the skin of the undersurfaces of these
regions is pink. The newly born cotton rats weight from
4.5 grams to 8 grams (Svihla, 1929; Hoffmeister, 1963).
Growth is rapid, the young gain as much as 2 grams per day
(Meyer, 1942). The time at which cotton rats reach sexual
maturity is variable. Size is not a good criterion,
because cotton rats may breed at about 40 days, at which
time they are still undergoing rapid growth (Asdell, 1946).
Haines (1961) believes that cotton rats are sexually mature
when their tail is at least 100 mm. long and the hind-foot
is at least 32 mm. long. Baker (1969) concludes that a
cotton rat less than 75 days old is a juvenile, between 75
and 200 days is a young adult, between 200 and 300 days old

is an adult, and more than 300 days old is an old adult.

MATERIAL AND METHODS

The Michigan State University Museum has a large series
of known-age skulls and skins of Sigmodon. The ages of
these species range from young of one day old to very old
adults (as much as 1129 days old). These animals are
descendants of cotton rats wild-caught in México and kept
under laboratory conditions. A total of 610 specimens were

studied and were grouped as shown in Table 1.

Measurements

A total of 21 measurements were taken on each speci-
men. The external measurements (length of the head and
body, tail and hind-foot) were taken from the specimen labels.
Measurements of the skull mainly follow Hall (1946). All
measurements were taken by the authors using a vernier
caliper to an accuracy of 0.1 mm.

Brief descriptions of the measurements taken are as
follows:

Length of head and body--the total length of the rat
from tip of the nose to the end of the tail (not including

the hairs) minus the length of the tail.

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Length of the tail--the distance from the upper base

of the tail to the tip, not including the hairs.
Length of the hind-foot--1ength of the left hind-foot

from the heel to the end of the middle claw.
Greatest length of the skull--the length from the tip

of the nasals to the posterior bulge of the braincase.
Height of skull--perpendicular distance between the

greatest convexity of the parietal bones at the median, and

a junction-line formed by a glass slide situated horizontal

to the basioccipital and basisphenoid bones (on the ventral

surface) minus the width of the glass slide.
Condylo-basal 1ength--distance along the midline of the

skull from a line connecting the anteriormost portions of

the premaxillae to a line connecting the posteriormost

margin of the occipital condyles.
Basal length--distance on skull from the anteriormost

inferior border of the foramen magnum to a line connecting

the anteriormost parts of the premaxillary bones.

Basilar length-~distance on skull from the anterior—
most inferior border of the foramen magnum to a line

connecting the posteriormost margin of the alveoli of the

first upper incisors.
Palatal length-~distance from the anterior parts of the

premaxillary bones to the anteriormost point on the posterior

border of the palantine bones.

Palatilar length--distance from the posteriormost
margins of the alveoli of the first upper incisors to the
anteriormost point of the posterior border of the palatine
bones.

Postpalatal length--least distance from the indentation
at the posterior end of the palatine bones to the anterior-
most border of the foramen magnum.

Length of the rostrum--distance connecting the posterior
margin of the anterorbital bridge of the maxillary bones
with the tip of the nasal bone on the same side.

Width of rostrum--greatest distance connecting the
most prominent lateral projections of the rostrum at the
anterior border of the infraorbital foramina.

Least interorbital breadth--the width of the most con-
stricted part of the interorbital space.
Interparietal width--the greatest width across the
interparietal bones.
Breadth of braincase--breadth of the skull across the
mastoid processes.
Zygomatic breadth--the greatest distance between the
lateral borders of the zygomata.
Alveolar length of the maxillary tooth-row--the length
of the alveoli of the three upper molariform teeth.
Length of the diastema--the distance from the posterior

margin of the alveoli of the upper incisor to the anterior

10

margin of the alveolus of the first cheek tooth.
Length of the incisive (anterior palatine) foramina--

the greatest length of this foramina.
Length of nasals--the greatest length of the nasal

bones.
Body weight was not taken into consideration because

under laboratory conditions this value is particularly
changeable and depends largely on feeding (Gebczynska,
1964), and because body weight is not a uniform variable

of age and therefore should not be used for analysis of

age structure in small mammals papulations (Dunaway and
Kaye, 1964).

Procedures
The external and skull measurements obtained were then

used to:
1. Determine the similarities and differences between males

and females in the five species of Sigmodon. The

Wilcoxon Matched-Paired Signed-Ranks Test, at 0.05 level,

was used.
Analyze comparatively the graphed growth-curves of the

body length, tail length and hind-foot length in all

species of Sigmodon to determine the actual increase
(and percent increase) in body dimensions daily and

during the first 200 days of life.

11

Determine the comparative rate of increase (and percent
increase) through time of the three principal regions of
the skull of the five species of cotton rats.

Analyze the growth of the palatal region by comparing

the changes through time of the alveolar length of the
maxillary tooth-row, the diastema length and the palatal
length.

Determine the comparative increase in the rostral length
using the ratio: length of the incisive foramen/length
of the diastema.

Determine the comparative increase in the skull breadth
in the five species of cotton rats by relating the growth
of the zygomatic breadth and the breadth of the braincase.
Analyze comparatively the interparietal space and the
interorbital area, expressed as ratios of breadth of the
braincase.

Derive an aging-formula by the Multiple Linear Regression
Method (Abbreviated Doolittle or Gaus-Doolittle Method)
to help to determine approximately the age of a cotton
rat on the basis of external size and skull dimensions.
Study statistically (using the Wilcoxon Rank-Sum Test)

the crosses between conspecific parents from geograph-

ically-separated papulations.

COMPARATIVE GROWTH OF COTTON RATS

Cotton rats of each species, born and reared in
captivity in the Michigan State University Museum Live
Animal Colony, were killed at intervals and prepared as
study specimens. The specimens of each species were first
segregated as to sex and then each sex-group was divided
into eight age-groups: one day, ten days, twenty days,
thirty days, forty days, fifty days, one hundred days,
and adults (200-400 days).

The averaged means for the various measurements taken
were obtained for each age-group and used in all the

statistical calculations.

Sexual Dimorphism

To determine if sexual dimorphism occurs (at least in
the papulation samples used herein) growth curves of the
three external dimensions were plotted in Figures 2-6. The
Wilcoxon Matched-Paired Signed-Ranks Test was applied (see

Table 2) to determine significant differences between the

sex-groups even though sample sizes were small in some age-

groups. Sigmgdon hispidus showed significant sexual
differences at the 0.05 percent level in body length

12

13

Female .——____
10.— Male -—---- ..” 1
1. Body length
2. Tail length
3. Hind-foot length I

)

  
 

l

 

 

 

 

 

10 26 36 50 so 100 adult
Age (days)

Fi ure 2. Growth in dimensions of body, tail, and
g hind-foot in males and females of Sigmodon

hispidus.

Female
10" Male
1. Body
2. Tail

 

 

14

 

length
length

3. Hind-foot length

 

 

 

16 2b 3b 45 56 100 adult

Figure 3.

Age (days)

' ° ' d hind-foot
Growth in dimensions of body, tail, an
in males and females of Sigmodon alleni.

15

10_. Female .———___

J Male ------

1. Body length

2. Tail length

3. Hind-foot length "-1

 

.2

 

 

16 26 36 46 so 100 adult
Age (days)

Fi ure 4. Growth in dimensions of the body, tail, and
g hind-foot in males and females of Sigmodon
fulviventer.

16

Female -———___
10" Male .......
. 1. Body length
.'2. Tail length
3. Hind-foot length

 

 

 

 

 

 

10 20 30 40 50 100 adult
Age (days)
Figure 5. Growth in dimensions of the body, tail, and

hind-foot in males and females of Sigmodon
leucotis.

 

 

17

Female

Male --------

' 1. Body length

2. Tail length

3. Hind-foot length

r I I I 17 1
10 20 30 40 50 100 adult
Age (days)

Figure 6. Growth in dimensions of the body, tail, and
hind-foot in males and females of Sigmodon

ochrognathus.

18

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19

(Chipman, 1965 and McIntire, 1944) and in length of the
hind-foot, and §,alleni showed significant sexual differences
at 0.05 percent level for greatest length of skull, palatal
length and length of the rostrum. Because the sample size
is small, all measurements for the following graphs and

tables are based on combined sexes.

Comparative Growth of External Dimensions

It was found that in the ontogeny of the cotton rat
that the rate of growth may be divided in all species into
three distinct age periods: 1-40 days, 41-100 days, 101
days-adulthood. In the first period growth is rapid, in
the second growth is reduced, and in the third growth almost
ceases.

Average and extreme measurements of these dimensions
for each age-group of each species are listed in Appendix
A. Some irregularities in these measurements reflect actual
differences in size between animals of the same age
(especially in §,leucotis), the small number of specimens
in some age-groups, the possible shrinkage after death of
some individuals, and the human error in making the measure-
ments. However, in Figures 7-9 these irregularities are
”smoothed-out" in an effort to present what is suspected to

be normal growth patterns.

20
Body Length. These curves (Figure 7) rise steeply from
the first day to near 100 days of age. There is rapid growth
to about 40 days, but between 40 to 100 days, the rate of
most species is slower. From 100 days to the adult stage,
the increase in body length is practically insignificant in
all species, with exception of §,hispidus, whose growth rate
seems to increase steadily throughout life. Sigmgdon
hispidus has the largest body length of all the cotton rats
studied, although §.fulviventer equals it in growth during
the first 40 days. Body growth in §,alleni is similar in
the first 10 days to that of §.leucotis and §.ochrqgnathus.
But after ten days, the rate of growth of §.alleni increases
and equals that of §.fu1viventer. Growth in §.leucotis and
smaller §.ochrognathus are similar.
Sigmgdon hispidus (Table 3) at 40 days old has reached

70.6 percent and at 100 days 86.8 percent of the adult body
size. Siggodon alleni and §.fulviventer at 40 days reach
respectively, 41.5 and 50.6 percent of their full growth;

at 100 days they reach 95.1 and 95.9 percent. This indi-
cates that §.fulviventer grows more rapidly than §.alleni

up to 40 days, but from 40 to 100 days the reverse is true
with §,alleni surpassing §.fulviventer in growth rate (19.5
and 13.0 percent, respectively). A similar reversal is noted
between the smaller §.leucotis and §.ochrognathus. Mostly

daily growth was achieved in the 1-40 days period (1.35 mm.

21

BODY LENGTH

.alleni

1. S 2
2. SLHIS idus ’
1 3. S. eucotis
4.Ԥ
5.§

.ochrognathus ’

.ffilviventer ’

 

 

 

' r

10 20 3O 4O 50 100 adult
Age (days)

I

Figure 7. Graph of the curves of the body length for all
the five studied species of Siqmgdon.

22

 

 

 

 

mo.o m.m o.oo1 a.om Po.o m.P o.oo_ m.mo_ __.o m.a o.oo_ o.Om_ arse.
No.0 m.m m.sm o.a~ 10.0 a.o a.ma m.~op a_.o a.e m.~a m.mm_ a co,
mm.o m.me N._m o.m~ ma.~ a.oe m.ma o.~o1 _a.1 a.om e.mm s.m~_ a as memwmmm.I
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No.o a.m ~.ma a.~m k_.o a.a 0.0m m.eo1 FN.o o.m_ a.ma o.mmp a oo_
ms.o s.am m.sa a._m am.~ a.am m.ew o.ea mo.~ e.om m.Nm a.ama a oa taa=a>.I
I I a.am m.m_ I I e.om e.mm I I m.~m m.mm sac _ IP>_at.m
oo.o 0.0 o.oo m.~m No.o m._ o.oop e.a1_ mo.o m.a o.oop N.Fo_ a_=ua
so.o _.e o.oop m.~m _m.o G.m1 ~.mm m.k_1 mm.o m.m_ F.mm m.mmp a cop
me.o ~.em m.~a o.om do._ m.mm e.~m w.ma ae._ m.ea e.ma m.NN_ a as I.
I I a.mm m.~l I I m.e~ o.~m I I F.am ~.mm ass 1 i=a_1a.m
mo.o P.a o.oop m.os _o.o o._ o.oo_ m.om_ am.o N.m_ o.oop a.mo~ p_=ua
ao.o a.o a.ma N.am NN.o m.m o.ma o.mm_ mm.o N.e_ m.em m.ea_ a cop III.
sm.o m.mm o.mm a.em mm.P m.oe m.mm e.e_e o~.~ m.ms e.oe m.ma_ u oa as .I
I I a.mm m.as I - e.m~ m.am I I m.k~. m.mm saw _ It mea.m
e\as a to aamaa_ space. u\55 a to asa=a_ aaa=a_ e\as a to apuad_ space, masses
cw tocH LucH uI: a uI: x c? Lucfi Loam ”was a ~weu,x a? LocH Loam Soon a zoos x Imm< mmwqum
.mpmewcm to mommepo one “coroeewu mo uooer:_s one .p_mu .xvon
mzu $0 cumcmp 9.3 we mucmexammmE .3 czocm mm mum.» copuou E. mmumgtcuzonm mo mwm>._.m:m :< .m 2.an

23

per day in §,leucotis to 2.2 mm. per day in §.hispidus);
least was in the age period between 100 and 200-400 days.

Tail Length. The tail (Figure 8) exhibits much of the
same type of growth in cotton rats as does body length.

The tail increases rapidly in length in the younger animals
(up to 40 days), but after this age, the rate of growth
decreases abruptly.

The longest tail at all ages is shown by §,hispidus
with §.a11eni second. However, the tail of the latter has
a similar growth pattern to that of §.fulviventer. Growth
in tail length of §,ochrognathus is intermediate between
§.fulviventer and §.leucotis. The latter species has the
shortest tail.

The tail lengths of the five species of cotton rats
reach, at the age of 100 days, from 85.1 percent (§,leucotis)
to 99.0 percent (§,hispidus) of the adult tail length. The
period of time from one day to 40 days shows the largest
percentage of growth. The daily growth-rate was greatest
in the age period between one to 40 days (1.98 mm. per day
in fi.hisgidus).

Hind-foot Length. The hind-foot (Figure 9) also grows
rapidly during early life. At about 40 days of age, there
is no significative growth.

Sigmodon leucotis has the smallest hind-foot at all

ages, except in the one day age-group, and §3hispidus has

10-I

H
SLJ

I J

 

24
TAIL LENGTH

alleni
His idus
Ieucotis

.ocHrognathus

 

mwaH
00000

Iwmwmm

.fuIviventer

 

 

A
‘ v— fi I

10 20 30 46 5'0 130 adult
Age (days)

Figure 8. Graphs of the curves of tail length for all
five species studied of Sigmodon.

25

HIND-FOOT LENGTH

 

l‘L
.
j 1. §.alleni
I2. .Eis idus
I. 3. _§_.Ieucoti's
J 4. §.ocfirognathus
5. §.fuIvIVenter

 

 

 

 

 

r [1 I T j I F fl

1 10 20 30 40 50 100 adult

Figure 9. Graphs of the curves of hind-foot length for all
the five studied species of Sigmodon.

26

the largest. Hind-foot growth in §.fulviventer is similar
to that in §.alleni while those of §.ochrognathus and
§.leucotis are least. The hind-foot length reaches adult
size by the time the cotton rats are 100 days old (94.5
percent in §.ochrognathus and 100 percent in §.alleni).
The daily growth rate in this age period varied between
species from 0.28 mm. to 0.54 mm. per day and was practi-

cally none in older animals.

Comparative Growth of Skull

The growth patterns of the cotton rats are the result
of: (1) early rapid expansion in the dorsal and lateral
aSpect of the braincase, and (2) slower growth of longer
duration resulting in the elongation of the base of the
braincase and the forward projection of the rostrum.

During the first 20 days of life, rapid growth of the
dorsal part of the braincase is the major factor determining
cranial form. After this early expansion, however, the
slower but steadily lengthening of the base of the braincase
and rostrum result in a reduction in the rounded dorsal
profile, correlated with the flattening and diminution in
the relative height of the cranium and with the posterior
elevation of the occiput and fronto-nasal area (Beer, 1954).

The growth relationships between different parts of the
skull are shown in Figures 10-11. The rostrum is represented

by the length of the nasal bones. The dimensions of the

27

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Amends use

 

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28

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.HH magmas

 

 

 

 

 

 

 

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30

braincase are obtained by measuring the length of the brain-
case (greatest length of the skull minus the length of the
nasals), the height of the braincase, and the basal length
of the braincase (postpalatal length).

The skull (as shown by the condylo-basal length)
increases in length throughout the life of the cotton rats.
This is further demonstrated by taking measurements of the
length of the rostrum and three aspects of the braincase
(see Tables 4-8 and Figure 11).

The lengthening of the rostrum with age was found to
be directly correlated with the increase in condylo-basal
length; however, other cranial dimensions grow progressively
slower as the age of the cotton rat increases (illustrated
in Figure 11). The rostral length increases in §.hispidus
only 9.9 percent of the condylo-basal length in the first
200 days of life, in other cotton rats to a maximum of
13.2 percent in §.alleni.

At one day old, §.alleni has prOportionally the longest
braincase (90.0 percent of the condylo-basal length) in
relation to skull length; §.hispidus has the smallest
(75.8 percent). However, at 200-400 days, §.leucotis has
the longest (67.1 percent) while §,alleni has the shortest
(62.4 percent). The braincase length grows slowly from
13.0 percent of the condylo-basal length in §.hispidus to

27.6 percent in §,a11eni during the 1-200 day period.

31

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33

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36

The basal part of the braincase (as shown by the post-
palatal length) is even slower in lengthening in the first
200 days of life, 2.9 percent of the condylo-basal length
in §.ochrognathus to 10.4 percent in §.alleni. Likewise,
the dorso-ventral expansion (height) of the braincase is
prOportionately slower-growing in relation to the lengthening
of the skull in the first 200 days after birth. In this
period the least growth is in §31eucotis (13.8 percent of
the condylo-basal length), the most in §.alleni (18.2
percent).

The changes in the cotton rat skull from a short,
rounded structure at one day of age to an elongated,
flattened condition at 200 days can be demonstrated by the
histograms in Figure 11. The height of the braincase in
one day old cotton rats is large in prOportion to the other
skull dimensions (i.e., rostral and postpalatal lengths),
but the subsequent rate of growth to elongate the skull far
exceeds that to heighten it. In fact, the measurements of
postpalatal length becomes greater than the height of the
skull by the time cotton rats are between 20 to 30 days old.
The rostral length in the 200 day-old cotton rats exceeds
the cranial height in all species of Sigmgdon but
§.§glviventer and §.1eucotis.

Comparison of the growth of the height of the braincase

and the length of the braincase using the ratios length of

37

braincase/height of the braincase and postpalatal length/
height of the braincase (Table 9) show that the antero-
posterior dimension of the braincase and the postpalatal
length grow progressively more rapidly than does the height
of the braincase.

The length of the skull can be expressed in terms of
the condylo-basal length, which may be divided into the
palatal and postpalatal lengths. The ratio of the post-
palatal length to the condylo-basal length and the ratio
of the palatal length to the condylo-basal length are
inversely related in all species of Sigmodon and for all
age-groups studied. When the skull lengthens the rostrum
maintains the same pattern of growth while the braincase
shows a progressive reduction in relative size (see Table
10).

The ratios of the least interorbital breadth and the
interparietal breadth to the breadth of the braincase in
the different species of Sigmodon are shown in Table 11.
The measurements of the breadth of the braincase at one day
old varies between 64.5 percent to 73.9 percent of the total
adult stage in the five species of cotton rats studied. At
the same age, the dimensions of the least interorbital
breadth are from 75.0 percent to 88.4 percent of the
condition of the adults. The interparietal breadth of the

38

o_.P No._
mo.~ mm._
op.~ Fm.P
mm.o mm.P

mm.o em.F
km.o em.P
mm.o n¢._

maguecmoizoonm

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.mmeucwmrn mo cumcwp use unmwo; to narcim to comwimasoo .m anm»

39

Table l0. Comparative lengthening (expressed-in percentage of
condylo-basal length) of the rostrum and braincase.

 

One day old Adult stage
Palatal Postpalatal Palatal Postpalatal
§3hisgidus - 52.2% 47.8% 57.5% 42.5%
_S_.a_l_l£n_1'_ 45.3 54.7 55.7 44.3
§,fulviventer 53.7 46.3 57.0 43.0
§,leucotis 48.5 51.5 56.7 43.3

§,ochrognathus 52.9 47.l 55.8 44.2

40

Table ll. Comparative broadening (expressed as ratios of the width of
the bra1ncase) of the interorbital area and interparietal

 

 

space.
Species Age Breadth of Least inter- Ratio Inter- Ratio
braincase orbital é%%_ parietal é¥%_
(l) breadth (2) breadth (3)
§,hisgidus 1 d 10.6 4.6 0.43 8.0 0.75
10 d 12.5 4.6 0.36 9.4 0.75
20 d 13.4 4.6 0.34 10.4 0.77
30 d 13.7 4.8 0.35 10.3 0.75
40 d 14.6 4.8 0.32 10.4 0.71
50 d 14.8 4.8 0.32 10.9 0.73
100 d 15.0 5.0 0.33 11.3 0.75
adult 16.4 5.6 0.34 12.1 0.73
1 d 10.5 4.4 0.41 8.5 0.80
10.0 12.1 4.5 0.37 9.9 0.81
20 d 12.4 4.6 0.37 10.1 0.81
30 d 12.5 4.6 0.36 10.6 0.84
40 d 12.9 4.8 0.37 10.3 0.79
50 d 13.0 4.9 0.37 10.9 0.83
100 d 13.2 5.1 0.38 11.2 0.84
adult 14.2 5.4 0.38 11.3 0.79
s. 1 d 10.2 3.9 0.38 8.5 0.83
" 10 d 11.4 4.4 0.38 10.0 0.87
20 d 13.1 4.4 0.33 10.5 0.80
30 d 13.7 4.6 0.33 11.0 0.80
40 d 14.1 4.6 0.32 11.2 0.79
50 d 14.2 4.6 0.32 11.5 0.80
100 d 14.5 4.8 0.33 11.5 0.79
adult 15.7 5.2 0.33 12.7 0.80
S.leucotis l d 10.5 4.5 0.42 8.3 0.79
""""“' 10 d 12.3 4.5 0.36 9.3 0.75
20 d 13.0 4.6 0.35 10.8 0.83
30 d - - - - - - - - - -
40 d 14.0 4.6 0.32 10.5 0.75
50.d - - - - - - - - - - .
100 d l4.6 5.0 0.34 11.8 0.80
adult 15.3 5.1 0.33 12.2 0.79
S.oChro- 1 d 9.8 4.2 0.42 7.7 0.78
"""77 12.0 4.4 0.36 9.7 0.80
M 283 12.6 4.3 0.34 9.6 0.76
30 d - - - - - - - - - -
13.4 4.7 0.35 10.4 0.77
53 g 13.5 4.6 0.34 10.7 0.79
100 d 13.9 4.9 0.35 10.2 0.73
adult 14.8 5.0 0.33 11.7 0-79

41

one-day old cotton rat is 65.8 percent to 75.2 percent as
broad as the same region in the adult stage.

It is shown in Table 11 that the interorbital area and
the interparietal space decrease slowly in breadth from
birth to adulthood, (except in §.leucotis and §.ochrggnathus)
in relation to the broadening of the braincase.

Three measurements of the palatal region of Sigmodon
were used to determine growth in this area during the first
one hundred day of life (see Tables 12-15).

The palatal length in §,hispidus and §.alleni
according to Tables 14-15 more than doubles (102 and 104
percent respectively) in the first 40 days of life, while
the other three species this increase is less, between 65
and 86 percent. The species seem in better accord when
comparing growth in alveolar length (Table 12) with an
increase between 50 and 67 percent in the first 40 days and
almost no growth thereafter. However, the increase in
length of the diastema (Table 13) shows marked variations
between species, from 71 to 116 percent growth in the first
40 days. As shown in all three of these measurements, the
palatal region of the skull has virtually completed its
growth by the time the cotton rats are 40 days old.

The growth rate of the lateral expansion of the skull

was determined by examining measurements of the breadth of

the braincase and the zygomata (see Table 16).

42

mama oo_

m.o m.m k.o m.o F.~ pa guacaP
mace oo_

P.o 0.0 N.o P.o N.o “8 mmmatucm
mane oe

m.~ F.~ N.~ ¢.~ N.~ pm mmmmtucw
~.m N.¢ m.¢ w.m ~.¢ sun _

 

 

 

 

mchmcmogsuoaw mwuousmpnw . empem>w>fismnw chPFm.

mautmmwgnw

UH

.3oguzuoou xgmppwxms mgp tom woman Lopom>Fm ago to A.EE cwv spasm, to mammtucH .m_ mpame

43

~.m
m.m

 

mscumumogzooam

m.m

o.o

o.e
~.<

 

mwuooamp.

w.

 

 

meme oo_

N.m o.w m.m pm cameo,

meme co.

N.F m._ ~.o um mmemtucw

mama oe

m.e m.~ _.¢ um mmmmgucw

~.m m.m o.¢ sac F
tas=m>t>P=tam P=m__8aw m=c1m8_;aw

.mewummvu on» mo A.EE cwv cpmcmp cm mmmmtocH .mp mpnme

44

m.m~

w.~

0.5
p.m

 

manumcmorzuonw

m.o—

m._

~.m
h.m

 

mSouamFfl

m.mp

m.m

~.m
n.m

 

me=o>w>pataw

 

o.a_ o.ml
e.m m.~
_.n s.m
m.m ¢.m

ical_8am maewmm.;aw

.gumcmF Pmum_mn cw mmmmgucH

msau oo_
pa gum=m_

mama cop
an omowgocw

mama oe
an mmmmtocw

use p

.ep opnmh

Table l5. Increase (expressed in percent) in the length of three

45

and from 4l-100 days (decelerating

dimensions of the palatal region, from l-40 days

bccelerating phase

phase).

Palatal length

 

l-40
§,hispidus l02%
§,gllggj_ l04
§,fulviventer 65
§,leucotis 70

§,ochrognathus 86

41-100

0.02%
0.03
0.03
0.01
0.01

Alveolar length
of maxillary

tooth-row
l-40 4l-l00
64% 0.03%
63 0.02
51 0.03
50 0.00
67 0.02

Length of diastema

1-40
102%
71
116
109
81

41-100
0.01%
0.03
0.02
0.00
0.02

46

w.¢~
m.mp
m.m~
¢.m—

m.~—
o.mp
w.m

macaw: ocsuoum

e.m~
m.mp
w.mp
c.5—

m.e_

m.o~

a.o
V

tom ANV ammocwmto on“ we cavemen was AFV spummtn ovumsomzn ms» to mucwEmtammoz

m.m~ w.mp
o.¢p m.m~
o.¢~ m.m—
o.m~ m.¢~
m.Np o.m~
m.op m.o_
.hwvununhrv
mwvousmp.m

 

~.mp n.P~
m.¢— m.mp
N.¢F m.mp
P.vP N.m~
n.m— n.n~
F.mF m.m~
¢.P_ m.m~
~.o_ ¢.op

hwy .hpyn
twpcm>w>F=w.m

N.e_ o.o~
N.ml P.m_
o.mF m.m_
m.~l ~.m_
m.~F s.ml
¢.~_ n.¢_
_.NP a.~_
m.o_ N.o
.hwpluunrv
lemFP8.m

¢.m~ ~.m~ mama oo¢-oo~
o.ml m.o~ mean oo_
o.¢~ m.mp axon om
m.e_ _.w. msau oe
~.mp ~.~_ made on
«.m_ ¢.m. msmc om
m.~_ m.m— mam—o 3
o.op o.o_ sou P
.mmwwmmwmww am<
Assam

.oF «Fame

47

At one day old, the breadth of the braincase is smaller

than the zygomatic breadth in all species except §.hispidus

and §.alleni. However, in all species the zygomata broadens

faster than the braincase. From one day to adulthood the

zygomatic breadth increases mest in §,hispidus (13.7 mm.)

and least in §,leucotis (9.2 mm.). Most growth of the

breadth of the braincase is in §,hispidus (5.8 mm.) and the

least in §,alleni (3.7 mm.).

The incisive foramen varies considerably in shape

(both individually and specifically) between the species

The shape of this elongate foramen may be oval

of Sigmgdon.
or sharply pointed.

incisive foramen in adult vary as to position in relation to

Furthermore, the posterior ends of the

the maxillary tooth-row. The posterior ends of the incisive

foramina in §,ochrggnathus and §,alleni do not reach a line

drawn between the anterior margins of the alveoli of this
tooth row; in §31eucotis the incisive foramina reaches but
does not extend beyond this line; and in §3hispidus and

§.fulviventer, the incisive foramina extend posteriorly

 

beYond this line. These findings relate to the relationships

between the length of the incisive foramen and the diastema,

as shown in Table 17 and Figure 12. Those species with the

formmina extending anteriorly in between tooth rows have the

largest ratios.

48

mm.o m~.o Fm.o wn.o

 

 

 

 

om.o when com
Pm.o me.c mm.o ma.o Fm.o axes cop

mm.o - - ew.o w~.o em.o when om

_m.o om.o ew.o om.o mm.o mama ca

- - - - mm.o aa.o mm.o mass on

we.o on.o oo.o om.o am.o mane o~

Fm.o an.o ew.o ow.o em.o memo op

mm.o mo.o No.0 we.o om.o sue P
msfimcmotcuofl 3w twang .E Scrum. .22 an. mat Em .E . m mm<

.memummwu asp ecu cognac; m>wmwocw mg» mo :pm:m_ :mwzpma owpmm .NF mpnme

Ratio: length incisive foramen/length of diastema

UllhuNH
so...

 

49

alleni
His idus
leucotis

ochrognathus

 

Imumonmun

 

fulviventer

 

 

--- *”*~x
C’f-‘ -~‘~-—-~_ Sx‘x-—*
~~“ fix ‘
~~“ 3“,“
.80 -———>"’ 5 ‘ -.. , \_:.f'
' , 1
. ..3
.70
.60
.501
1 10 20 30 40 50 100 .861.
Age (days)
Figure 12. Graph of the curves of the ratio: length of

the incisive foramen/length of the diastema
for all the species of cotton rats.

50

Interspecific relationships are demonstrated using the
ratio between the length of the incisive foramen and the
diastema (Figure 12) for all the species of cotton rats of
the genus Sigmpdon. The curves indicate that the incisive
foramen lengthens rapidly up to 20 days of age, although
maximum length is not obtained in §,leucotis until 40 days
of age and §,ochrognathus until 50 days. Following 20
days of age, the growth of the incisive foramen is greatly
reduced. The ratio as shown in Figure 12 also declines
because of the rapid increase of the lengthening of the

diastema after 20 days.

Formula for Aging_¢otton Rats

The availability of known-age specimens of the five
species of cotton rats makes possible the development of an
aging formula. To do this, the Multiple Linear Regression
Method (Abbreviated Doolittle or Sans-Doolittle Method) was
developed with 9 variables representing a selection of
nine (of the 21) measurements for each of the seven age-
groups (l—lOO days) of each species.

Assuming linear relationships between Y (age) and x
(selected parameter), the following formula was applied:
X

Y a a + b + b2 X2 + ...... + b9 X9

1 l
with

a : constant (intercept of the line) =0

51

constant value obtained from Table 18, specific for
apprOpriate measurements in each species.

length of the body.

length of the tail.

length of the hind-foot.
greatest length of the skull.
height of the braincase.
condylo-basal length.
palatal length.

length of the rostrum.
length of the nasal bone.

age of the specimen measured as determined, after

calculations, by consulting Table 19.

The aging-formula was tested with four known-aged

cotton rats belonging to each of the five species studied.

In 90 percent of the specimens the formula gave the correct

age.

Consequently, this aging-formula seems useful as

another method of aging to add to those now used, including

size, wearing of molar teeth, development of skull ridges

and condition of pelage.

52

00.m 1
ew.0 1

0¢.~F
n0.F1

00.? 1

NN.01
0N.N
F0.P1
00.p
m¢.F1

we

0~.0e
00.0
p0.0

No.0 1
NN.m_1

ne.w
P0.01
mF.F
00.01
0F.m

Fe.m 1
m0.0 1

05.0,

mN.0 1
0P.0 1

00.0

0N.0 1
00.PP1
m~.0 1
0F.0 1

00.0
00.01
No.0
em.01
cm.”

00.0
00.01
0~.01
mN.0

000.01

 

 

 

“0.01 .mm:pmcmogguonw
£5- 8.58360...”
8.5 $26333.m
8.? 26:;
Ed 3.0.3.50
Fa mmwumam
.8835

to mmwuwam m>wm ago not mpcmEossmmms 0 web to some not Amwapm> av mmzym> accumcou

.0_ open»

53

Table l9. Specific numbers associated

rats, when the aging formula

Species

§_.hispidus
_S_.alleni

§,fulviventer

§,leucotis

§,ochrognathus

1 day

18.80
—20.55
1.61
-11.51
286.97

10 d

28.54
-25.83
2.02
-15.67
403.39

20 d

37.42
-31.42
2.10
-19.52
434.42

with each age-group of cotton
is applied.

100 d

62.06
-40.41
4.55
-26.60
584.34

54

Growth Rates in Subspecific Crosses in Sigmodon fulviventer

Cotton rats of the species §.fulviventer (see Baker,
1969) range in size (both externally and cranially) from
small in northern México (Chihuahua) to large in southcentral
Mexico (Jalisco). Since known-age specimens of crosses
between parent of §.fulviventer from different localities
within this geographic area (see Figure l) were available
for study, measurements were taken to assess the growth
rates of the crosses as compared with parent stocks (Table
20).

Three body measurements (lengths of body, tail and
hind-foot) and two cranial dimensions (condylo-basal and
height of the braincase) were used (see Figures 13-16). The
Wilcoxon Rank-Sum Test at 0.05 level was used to determine
differences between progeny and parents. No significant
differences at 95 percent level were found between the
animals studied.

Cotton rats of the cross No. l grew more slowly than
their parents during the first 20 days of post-natal life
in the three body measurements studied (see Figure 13).

The growth of the animals of the cross No. 2 is intermediate

(Figure 14) in relation to the growth shown by their parents

in all the body dimensions used. The cotton rats of cross

No. 3 have a growth in the three body dimensions comparable

to those of their larger parent (§,£,melanoti§) at least

 

55

Table 20. Subspecific crosses in Sigmodon fulviventer.

 

 

 

 

 

 

 

 

 

 

 

Cross Subspecies involved Sample Localities of
No. * size parental stocks
1. S.f.minimus x _S_.j.fulviventer l3
§.j.minimus 57 vic.Boquilla,
Durango
S.f.fulviventer 60 vic.Coyotes,
“' Durango
2. S.f.minimus x _S_._1:.fulviventer 26
S.f.minimus 53 vic.Gallego,
"'" ‘ Chihuahua
S.f.fulviventer 6O vic.Coyotes,
_ ‘ ‘— Durango
3. S.f.minimus x _S_._.melanotis 79
S.f.minimus 53 vic.Gallego,
"" Chihuahua
S.f.melanotis 68 vic.LaBarca,
-‘ Jalisco

*Number of cross will be referred to in text.

56

. §.£.fulviventer -----

  
    

nf.minimus .0000

:0)

 

160- §.f. fu lviventer x
S. _.minimus

 

(A)

 

 

 

v *I’

10 20 30 40 50 100 adult
Age (days)

Figure 13. Growth in dimensions of body (A), tail (B),
and hind-foot (C) of cotton rats of
S. f. fulviventer (Coyotes, Durango), S. f. minimus
TBoquiIIa, Durango), and cross No. 1—
(S.f .fulviventer x S. f. minimus).

57

  
   

£0 £0 fUIViventer -----

It!)

 

.fdmnimus so...
.f. ulviventer x (A)

160‘ fu
8. _.min us

It!)

 

 

140‘

 

 

 

120'
100‘
(B)
80‘
60‘
40‘
/ . O . .
”' (C)
20'1/
V’
10 20 30 40 50 100 adult
Age (dayS)

Figure 14. Growth in dimensions of body (A), tail (B),
and hind-foot (C) in cotton rats of
S. f. fulviventer (Coyotes, Durango),
S. flminimus (Gallego, Chihuahua), and cross

No. 2 (S. f. fulviventer x S. f. minimus).

1

160%

100'

20‘

 

58

    

- — ‘~

   
  

_,melanotiss ""

f
.£,minimus x _.f.m _glanotis- '

 

 

 

/
_______ /'
0— z/
// (A)
z’
//
/
/
/
z’
/
(B)
'. ——— "’ " (C)
10 20 30 4O 50 100 adult
Age (dayS)

Figure 15. Growth in dimensions of body (A), tail (B),

and hind-foot (C) in cotton rats of
S. f. minimus (Gallego, Chihuahua), S. f. melanotis

(La Barca, Jalisco), and cross No. "3

Size (in cm)

[ID
Il'h

 

59

 

.melanotis °°°"

(a)

 

(b)

 

 

 

 

1 S.f.minimus x S.f.fulviventer

‘ §,£,minimus "'°'

 

 

 

 

 

 

 

_S_ofi-fUIViventer ----. . .....—-' _,____. . - '__:_— '
20 V
l‘
-2--- ___-_____:—=—g.-__fi___=—gg-—*=——'-’ (b)
" «pi-Fi—‘——‘—— —- —— — .
10 20 30 40 50 100 adult
Age (days)
Figure 16.

Growth in dimensions of condylo-basal length
(a) and height of the skull (b) of the progeny
of the crosses Nos. 1, 2, and 3 and their
parent subspecies.

60

until 50 days of age (see Figure 15). Older individuals
show that growth in the crosses are somewhat intermediate
between the growth of both parents.

The graph curves of the condylo—basal and height of
the braincase (see Figure 16) show that there are a somewhat
similar growth between the cotton rats of the different
crosses and their respective parent subspecies in all the

age-groups studied.

DISCUSSION

Massler and Schour (1951) have pointed out that the
skull of the albino rat shows two different types of growth.
Cotton rats also exhibit two patterns of growth. From one
day to 40 days most species of Sigmgdgn present an
accelerated and rapid growth in all the body and cranial
dimensions. During this accelerated and generalized phase
of growth the increase in body dimensions is almost double
(more than 80 percent). The second phase of growth is
present during the period of life from 40 days to the adult
stage. This period of life may be subdivided into two
different growth types. The first one, comprising the
period from 40 days to 100 days, results in a more
accelerated increase in all the dimensions studied and in
most species of cotton rats the adult size is reached in
this period of life. The second growth type is considered
to be from 100 days to the adult stage. This period is
characterized by a gradual cessation of growth. Hoffmeister
(1963) demonstrated that there is a little or no further
increase in the total length and tail length in

§,ochrognathus after 100 days old. Layne (1966) showed

61

62

that Peromyscus floridanus reaches 92 percent of the adult
size by the eighth week. The same is true for §3hispidus
(Meyer and Meyer, 1944) as the growth of individual cotton
rats is fairly constant through the first 50 days of life:
decreasing somewhat between 50 to 100 days and decreasing
markedly after 100 days of age. Dice and Bradley (1942)

studied Peromyscus maniculatus and determined that the deer-

 

mice grow rapidly in all body dimensions until they are 6
weeks old. The same authors (1942) have noted that the
slowing of growth rate after 4-6 weeks of age is correlated
with approaching sexual maturity. Meyer (1942) has demon-
strated that female cotton rats have their first estrum
between 20 and 30 days of age and that male testes descend
to the scrotum at the same time. These ages of attaining
sexual maturity correSpond well to the age at which the
young cotton rats change from a higher rate of growth

(1-40 days old) to a much lower growth rate (40-100 days of
age). The lepes of the growth curves demonstrated that
not all species of Sigmpdon grow equally rapidly. The two
smallest species §,ochrognathus and §,leucotis (see
Figures 7-9) begin to slow their rate of growth at an earlier
age (20 days of age) than the largest species, §,hispidus
(40-50 days of age). Those species that are intermediate

in size, §,alleni and §.fulviventer, begin to reduce their

63

rate of growth at an age which is somewhat intermediate
(30-40 days of life).

Dice and Bradley (1942) stated "... races of deer-mice
that are relatively larger or relatively smaller in any
dimensions at birth tend to retain these size relationships
as they grow to adult size". But cotton rats show a
different pattern of growth. The smaller Species, g.
ochrognathus and §,leucotis, are the largest at one day
old. Subsequently with the increase of age, they are
exceeded in growth by the other three species of cotton
rats (Figures 7-9).

Clark (1936) mentions that ratios between body
measurements indicate that the Species of the genus
Peromyscus differ greatly in body prOportions. Likewise,
the five species of cotton rats also show marked differ-
ences in the comparative growth of different body
prOportions. The sample of §.hispidus used in this study
(§,hispidus 2312; from near Tuxpan, Nayarit), is one of the
largest species (Bailey, 1902). The species of the
S.fulviventer present interesting patterns of growth. The

samples of S.fulviventer show a decline in overall size

 

from north to south (from Chihuahua to Jalisco). The
average means for all measurements and the growth rates of
§.fu1viventer compared favorably with those of §3alleni.

The smaller §.leucotis and §,ochrognathus show a somewhat

64

similar pattern of growth. These relationships between

§.alleni and §3fulviventer on one hand and §.leucotis and

 

§.ochrognathus on the other are related in a general sense
to environmental adaptations. Sigmodon alleni and
S.fulviventer live on deep soils covered with grass and

brush. Sigmodon leucotis and §.ochrgganthus live on shallow

 

soils on rocky slapes covered with bunch grass and some
shrubs.
Fox (1948) in studies with subspecies of Peromyscus

maniculatus has demonstrated a direct relationship between

 

tail length and environmental conditions; animals with longer
tails live in forested areas while those with shorter tails
live in Open areas. The hind-feet do not change. S'gmodon
hispidus, §.alleni and S.fulviventer have the longest tails
and hind—feet of all the Species studied and also live on
deep soils with grass—shrub cover. Sigmodon leucotis and
§.ochrognathus with the shortest tails and hind—feet inhabit
shallow soils often covered with sparse bunch grass.
Possible, the short tail as well as the small size are of
survival value to cotton rats living in sparse overhead
cover, especially for avoiding avian predators (hawks and
owls) which locate their prey from above by keen eyesight.
The cotton rat skull, with the increase in age, becomes
steadily more slender in its prOportions. The braincase of

Sigmodon becomes progressively deeper but not wider giving

65

the appearance of increased arching in cross section
(Findley and Jones, 1960). The skull of the cotton rat has
a pattern of growth different from that shown by external
measurements. Its growth is divisible into two well—defined
phases: from birth to 100 days and from 100 days to adult
stage. Most skull dimensions reach at least minimum adult
stage during the first phase of the cranial growth. Changes
in the skull length are due to increase in nasal length, or
in the length of the braincase, or in both dimensions. This
is also emphasized by Hall (1926) who pointed out that most
postnatal develOpment of the skull is concerned with muscular
develOpment since much of the formation of the brain (and
its case) occurred in prenatal time. As is shown in Table 4,
the length of the nasals and the condyloebasal length
progressively increase during the first 100 days of age, but
the breadth of the braincase grows faster than either during
the first 20 days of life at which time its size closely
approximates that of the adult. The rostral region is small
in the newborn, while the braincase in relatively large.
With the increase in age, the extension of the nasal bones
give the skull of the cotton rats its typical long-snouted
appearance. The larger the skull, the narrower it is in
relative sense (Findley and Jones, 1960), with the greatest

percent of increase in the breadth of the skull being in the

66

mastoid region which is associated with the increase in
growth of the zygomata (see Table 16).

The flattened appearance of old skull of Sigmodon is
increased by the developments of crest or ridges (Chipman,
1965). At 20 days to 30 days of life, the frontal bones
first show ridging in all species but §.ochrognathus where
the ridges do not appear until 30 to 40 days of age.

As was mentioned previously, the one day old rat has
small rostrum, a short postpalatal length and a conspicuous,
dorsally-expanded braincase (see also Hall, 1926). In
subsequent growth, the height of the braincase grows
relatively little compared to the lengthening of the rostrum
and the postpalatal area. In other words, the basal part
of the braincase elongates more rapidly than does the
dorsal part increase in size. The dimension of the post-
palatal length exceeds that of the height of the braincase
in the adults of all species (see Figure 11). But in the
growth of the rostrum, its adult dimensions exceeds that
of the height of the braincase only in §3hispidus, §.alleni
and §,ochrognathus. The growth of the rostrum seems also
to be correlated with time of weaning in cotton rats. This
event takes place between 10-25 days (Hoffmeister, 1963;
Meyer, 1942; Svihla, 1929). As shown in Figure 11 the
elongation of the rostrum (as also explained by Hall, 1926)

accelerates between 20-100 days of age. This rapid increase

67

in size appears to be directly correlated with the whole—
food-taking function of the rostrum.

Comparison of the growth data obtained from §.hispidus
from Tuxpan, Nayarit, with the growth data given by other
authors that studied the same species but from different
localities, shows that the Tuxpan sample is one of the
largest of the species. It is also.much larger than any
of the samples of other species of cotton rats studied
herein.

In viewing the growth data of the S.fulviventer group
it may be possible to separate the four species composing
this group into two categories: (1) §.alleni and S.fulyig
venter and (2) §,1eucotis and §,ochrognathus. The former
group show the larger dimensions at the adult stage and
the latter group the smaller. As Baker (1969) has pointed
out, §.1eucotis and §.ochrognathus may have separated
earlier from the ancestral stock of §3hispidus than
§.alleni and §,fulviventer. This earlier separation from
the §,hispidus stock may account for the great amount of
distinctive variation in body and cranial features common
to both species. The distinction showed by §.alleni and

S.fulviventer, however, show their closer relationship to

§,hispidus.

SUMMARY

Laboratory—raised cotton rats of the five recognized
species in the genus Sigmodon were killed at predetermined
intervals (1, 10, 20, 30, 40, 50, 100, and 200-400 days
after birth) and prepared as museum study specimens. These
known-age animals were then used to study differences and
similarities between the population samples of these rodents.
Postpalatal growth was determined by making a series of 21
external and cranial measurements of these known-age
samples (a total of 610 specimens). Some sexual dimorphism
was found in §,hispidus and §,alleni but was discounted
because of the small sample being used.

In external features, body length increased rapidly
from one day to 40 days. From 40 days to 100 days this
growth rate was slower and from 100 days to adulthood (200-
400 days)growth practically ceased. The population sample
of §,hispidus was the largest while those of §,alleni and
S.fulviventer showed an intermediate size and those of

S.leucotis and §,ochrognathus were smallest. The tail and

 

hind-foot in all species lengthened to adult size in the
first 40 days with §3hispidus having the longest appendages

and S.leucotis the shortest.

68

69

At birth the skull is oval in shape as the result of
the large expansion of the well-developed braincase and the
lack of growth of the rostrum. In all species, the brain-
case continues to expand in the first 20 days of post-natal
life to near the adult size. Subsequently, the skull
begins to lengthen rapidly with the deveIOpment of the
rostral area and the extension of the bones at the base of
the braincase. This growth transforms the rounded form
into the long-snouted condition typical of the adult cotton
rat. The adult size is attained in the first 100 days.

Skull measurements made show the correlation between
the initial rapid expansion of the braincase and the subse-
quent extension of the rostrum and basal part of the
braincase. Measurements which reflect the development of
the braincase are taken of the breadth and the height of
this feature. Those which reflect the elongation of the
skull are taken of the length of the rostrum and the post-
palatal length.

Normally the height of the braincase has a greater
dimension than the postpalatal length in the first days
after birth. However, at the 20-30 days period, the growth
of the postpalatal area increases to surpass in size the
former feature. In three species, §3hispidus, §,alleni and

§.ochrggnathus, the rostral length exceeds in size the height

70

of the braincase only after the animals are older than 100
days.

By the time the animals are 40 days old the palatal
length and the length of the maxillary tooth-row are near
the adult size, while the diastema is 50-67 percent
(depending on species) of the adult size. At birth the
zygomatic arches are small but expand rapidly laterally
compared with the slower-growing braincase. This develop-
ment along with the lengthening in the rostral region have
to do with the time of the weaning and develOpment of the
masticatory and facial muscles. Also, the change of the
growth from an accelerated phase (1-40 days) to a much
slower rate (40-100 days) seems correlated with the time
of sexual maturity in these species.

The smallest species, §,1eucotis and §,ochrognathus,
are actually the largest at birth; however, the other
species exceed them in size shortly thereafter. The
pOpulation sample representing the wide-spread §3hispidus
reaches the largest dimensions, with §,alleni and §r£ElXET
venter in slightly smaller position. Ecologically, these
three larger species are usually associated with deep soils
covered with heavy grass and brush. The two smaller
species seemingly prefer shallow soils on rocky lepes
covered with scattered bunch grass and some shrubs. The

short tails and small body size of §31eucotis and

71

§,ochrognathus may be of survival value to them on rocky
hillsides having sparse overhead cover.

Sigmodon leucotis and §.ochrognathus show patterns of
growth sufficiently different to set them apart from the
other species. If all modern species of cotton rats were
descended from a single hispid-like ancestoral stock, these
morphological findings indicate that S.leucotis and
§,ochrognathus may have evolved at a different time, per-
haps more remotely, than §.alleni and §.fu1viventer, which

have developmental characteristics more closely resembling

§,hispidus.

LITERATURE CITED

LITERATURE CITED

Asdell, S. A. (1946). Patterns of Mammalian Reproduction.
Comstock Publ. Co. New York.

Baer, M. J. (1954). Patterns of growth of the skull as
revealed by vital staining. Human Biol. 26:8-126.

Bailey, V. (1902). Synopsis of the North American species
of Sigmodon. Proc. Biol. Soc. Wash. 15:101-116.

Baker, R. H. (1969). Cotton rats of the Si don
fulviventer group (Rodentia: Muridae). Univ. Kansas

RubI}, Mus. Nat. Hist. Misc. Publ. (in press).

 

Brimley, C. S. (1923). Breeding dates of small mammals
at Raleigh, N. Carolina. Jour. Mamm. 4:263-264.

Chipman, R. K. (1965). Age determination of the cotton
rat (Sigmodon hispidus). Tulane Study in 2001. 12:
19-380

Clark, F. H. (1936). The estrous cycle of the deer-mice,
Peromyscus maniculatus. Contr. Lab. Vert. Biol. #1.

Dice, L. R. 8 R. M. Bradley (1942). Growth in the deer-
mice Peromyscus maniculatus. Jour. Mamm. 23:416-427.

Dunaway, P. B. a S. V. Kaye (1964). Weights of cotton rats
in relation to season, breeding and enV1ronmental
radioactive contamination. Amer. Midl. Nat. 71:141-

155.

Findley, J. S. 8 C. Jones (1960). Geographic variation in
the yellow-nosed cotton rat (Sigmgdon ochrognathus).

Fox, W. (1948). Variation in the deer-mouse, Peromyscus
maniculatus, along the Lower Columbia River. Amer.

M131. fiat. 40:420-452.

72

73

Gebczynska, z. (1964). Morphological changes occurring in
laboratory Microtus agrestis with age. Acta
Theriologica 9:67-79.

Goertz, J. W. (1965). Sex, age and weight variation in
cotton rats. Jour. Mamm. 46:471-477.

Haines, H. (1961). Seasonal changes in the reproductive
organs of cotton rat Sigmodon hispidus. Texas Jour.
Sci. 13:219-230.

Hall, H. R. (1926). Changes during growth in the skull of
the rodent Otos ermo hilus rammurus beecheyi. Univ.
Calif. Publ. 2001. 21:355-4 4.

Hall, E. R. (1946). Mammals of Nevada. Univ. Calif. Press,
Berkeley, L. A.

 

Hoffmeister, D. F. (1963). The yellow-nosed cotton rat
(Si odon ochrognathus) in Arizona. Amer. Midl. Nat.

7 : - 1.

Layne, J. N. (1966). Post-natal deve10pment and growth of
Peromyscus floridanus. Growth 30:23-45.

Massler, M. a I. Schour (1951). The growth pattern of the
cranial vault in the albino rat as measured by Vital
staining with alizarine red "S”. Anat. Rec. 110:

83-101.

McIntire, J. M. et a1. (1944). The nutrition of the cotton
rat (SigmodEfi'Erhispidus). Jour. Nutrition 27:1-9.

Meyer, B. J. (1942). The study of growth and reproduction
in the cotton rat. Ph.D. Thesis Univ. of Wisconsin.

Meyer, B. J. 8 R. K. Meyer (1944). The growth and repro-

duction of the cotton rat Sigmodon h.his idus under
laboratory condition. Jour. Mamm. 75:107-129.

DevelOpment and management

. h 1943).
Meyer, D. B. 8 M Mars ( er. Jour. Publ. Health 33:

of a cotton rat colony. Am
697-700.

Odum, E. P. (1955). An eleven year history of a Sigmodon
pOpulation. Jour. Mamm. 36:368-378.

74

Sealander, J. A. & B. Q. Walker (1955). A study of the
cotton rat in N. E. Arkansas. Proc. Arkansas Acad.
Sci. 8:153-162.

Svihla, A. (1929). Life history notes on Sigmodon h.
hispidus. Jour. Mamm. 10:352-353.

APPENDICES

APPENDIX A

Average and Extreme Measurements of the Five
Species of Sigmodon in Eight Age Classes

75

N.FN-o.ml m.a,-m.ml N.el-mp 8P-~.ml m_-a.mp N.m_-o._l m.o_-a.m m.a-m.a
m.o~ F.aP «.mt F.mp 3.4? a.~_ m.op ¢.a enscoP tsewsmpma
a.¢N-an_N m.NN-m.e_ ..m_-a_ ~.w_-e.mp a.o_-~.mp o.¢.-a.~_ ~.~P-op e.m-m
m mm e.a_ a.~_ 0.x, o.ep o.s_ m.op s.m comes, _apmlaa
«.mm-muom m.mm-enmu a.a~-m.m~ F.a~-o.mm s.w~-¢.e~ o.--m.o~ N.m_-s.~_ ml-m_
m mm a mu m.e~ a.m~ a.e~ a._m «.mp s.mF csmcm_ Laeamam
m.oe-eupm N.em-muem a.a~-a.a~ _.a~-m.mm ~.N~-a.mm m.m~-e._~ N.om-w.m_ el-¢_
x mm _ Nm w.a~ m.a~ s.e~ a.- m.m_ a.e_ eases, _mmem
m.Ns-PHmm m.mm.¢.m~ m.~m-m.om mm-mm e.a~-m.am N.e~-m.mm m.--a.o~ m.e_-a.m_
a o4 m.sm m._m om s.m~ l.mN a._N _.e_ .scaa _amua-o_xe=oo
m.ap-mus_ m.s_-m._l _.mp-a.~l mp-~.~_ m.mp-a.__ N.~_-FF m.o_-m.o_ e.m-_.m
m m_ o.mp m.ml o.NF _.N_ m.__ m.o_ m.m mmmacaara 18 pasta:
m.es-mnos m.os-~._m sm-~m s.mm-a.a~ m.lm-m.mw m.w~-m.m~ mm-~.m~ N.a_-a.mp
N we o.em a.~m m.lm m.om m.a~ o.s~ 1.x, __=xm .scmp emanamtw
menus Neumm mm-om am-em cm-em _m-am m~-mN m_-m_
s cc N am o.am 4.8m m.em m.a~ m.m~ m.¢p pooe-u=ee to assess
mm_uoop Ominoo_ am_-o_F mml-mol om_-ooa am-ma me-me m¢-om
m one 0 mm_ o.-F e.e__ w.~__ 8.0w m.me m.em lamp to assess
a_~nmal Nomumep Fo_-os. am_-mm_ mmp-mm_ m__-mm _a-mm Fe-me
a mom m on? m.om_ m.me_ m.s~P o.o_P a.mw e.mm seen to sesame
N~-u_=u< s-oo_ m-om m-os 4-0m o-o~ e-o_ a-e
mcwewumam to amassz new Amxmu cwv mommaP01mm< :mxmp mnemEmizmemz

 

towns wauwmww; cocoEmwm .P apps» 1 < xwucmaa<

76

0—10.0F 0.0P1N.PP 0.0F1~.0~ N.N_10.0F

0.0_ 0.0?
0.0F10.0 0.010.0
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F.0_1_.FF 0.Fp1m.m
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0.0p 0.0p
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p.~— 0.F—

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0.0— 0.0p
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F.0 ~.n

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VIEW F.N

7-1-

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0.x10.
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5.0

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5.0

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0.x10.0 n10.0
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77

F.0F1F.0P 0.0P10.0P 0.0P10.0_ ~.0_10.0— 0P10.0F 0.0F1P.__ 0.00-0.0 0.010.0
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0 F0 0 00 P.00 0.00 0.00 0.00 0.0_ 0.0_ cpmcm— F0000
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0 00 0.P0 0.00 0.00 0.00 0.00 _._0 0.00 .mcmp memn1o~>0:00
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00100 00100 00100 00100 00100 00100 00100 0010p
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00—100 —0_10F_ NFF100 00F100 00F1_0 00100 00100 00100
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APPENDIX B

Average and Extreme Measurements of the Progeny
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