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MECROTUS PENNSYLVAMCUS T0
PREDATIQN BY DOMESUC CATS

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VULNERABILITY 0F MEADOW VOLES
MICROTUS PENNSYLVANICUS T0

PREDATION BY DOMESTIC CATS

BY

Donald Pith-hristian

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of
MASTER OF SCIENCE

Department of Zoology

1973

ACKNOWLEDGEMENTS

I wish to thank members of my guidance committee: Walter H.
Conley, James H. Asher, and especially Rollin H. Baker, for their
guidance and assistance.

Also acknowledged are landowners Max Furney and the Hyram
Kitchen family for their cooperation; Richard W. Hill for his sugges-
tions on this manuscript; and the graduate students of the Michigan
State University Museum for their criticisms and suggestions. The
assistance of Richard J. McCleod and Ulreh V. Mostoskey is acknowledged.

Finally, I would like to thank my wife, Sandy, for her support

and encouragement during this research and manuscript preparation.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS . . . . . . . . .
TABLE OF CONTENTS . . . .
LIST OF TABLES
LIST OF FIGURES
ABSTRACT
INTRODUCTION
The Problem . . . . . .
The Prey

Hypotheses . . . . . . . . . . .
Basis of the Method for Testing Hypotheses

The Predator . . . .
THE STUDY AREA . . . . . . . .
METHODS .
RESULTS

SMALL RODENT POPULATION
Numbers and Age Classes .
Sex Ratios . . . . . .
Reproduction
Mortality
ANALYSIS OF PREDATION .
DISCUSSION .
SUGGESTIONS . . . . . . . . .
LITERATURE CITED .

APPENDIX: A MODEL OF PREDATOR FEEDING BEHAVIOR

.20
.20
.23

\O NO‘O‘W—i

12

15

. 30
. 36
. 38
. 39
. 42

LIST OF TABLES

TABLE PAGE
I. Sexes and ages of tagged Microtus recovered from
cat fecal matter during the entire study. . . . . 31
2. Sexes and ages of tagged Microtus recovered from
cat fecal matter within Groups I and II. . . . . . 33
3. Comparison between Group I and Group II tags in
the amount of predation suffered by each sex and

age segment. . . . . . . . . . . . . . . 34

LIST OF FIGURES

FIGURE PAGE
1. Map of the study area. . . . . . . . . . . . II
2. Minimum numbers of Microtus alive . . . . . . . 17
3. Minimum numbers of Microtus alive . . . . . . . 18
A. Proportion of the minimum number of Microtus alive

represented by each age class. . . . . . . . . 19
5. Proportion of male voles in the minimum number alive

during each trap period. . . . . . . . . . . 21
6. Changes in female vole reproductive measures . . . . 22
7. Proportion of male voles with scrotal testes . . . . 24
8. Survivorship of all Microtus, trap periods 1-13 . . . 26
9. Survivorship by sex, trap periods 1-13. . . . . . 27
10. Survivorship by age at first capture, trap periods 1-13.28
11. Survivorship of voles alive during trap periods 1-10

and periods 11-13. . . . . . . . . . . . . 29

12. A schematic model of predator feeding behavior. . . . 43

ABSTRACT

To investigate differential vulnerability to predation among
sex and age segments of a population of Microtus pennsylvanicus, and
between Microtus and other small rodents, the small mammal population
of a 0.8 hectare old field in southern Michigan was live-trapped,
ear-tagged, and released during the summer and fall, 1972. ‘Fecal
matter of domestic cats (Felis catus) was collected and analyzed for
the presence of ear tags. It was found that among the rodents preyed
upon, the observed numbers of species, sexes, and ages taken were not
significantly different than those expected on the basis of abundance.
A significant seasonal change occurred in the numbers of subadult and
adUlt voles taken by cats, and corresponded to changes in the age
structure of the Microtus population. The data indicate that differ-
ent sex and age segments of meadow voles are taken in proportion to
their abundance in the population.

 

INTRODUCTION

The Problem

It has generally been recognized that predation is, at least in
part, a density-dependent phenomenon, and many studies have dealt with
its quantitative aspects. For example, Holling (1965, 1966) investi-
gated the reSponses of predators to changes in prey density; Pearson
(1964, 1966, 1971) was concerned with the numbers of California voles

(Microtus californicus) taken by carnivores at different points in the

 

population cycle. These and other studies have illustrated that numbers
of predators and prey are important determinants of any predatory out-
come. Holling (1959) notes several other factors which are essential

to an assessment of predation, among which he included the character-
istics of the prey.

The kinds of prey variables which might influence vulnerability
to predation cover a wide range of morphological and behavioral topics.
Anatomical characteristics such as body size (see Ashmole, 1968; Brooks,
1968; Estes and Goddard, 1967) and coloration (Dice, 19h7) may certainly
have an effect on predation. Baker (1971) has suggested possible differ-
ences in risk between grass-eating and seed-eating myomorph rodents --
the former may take less risk by remaining under cover when gathering
food, whereas seed—eaters must cover greater areas and may expose them-
selves more in search of seed-bearing plants. Seed-eaters, however, are

largely nocturnal, and thus may be less conspicuous than the generally

diurnal or crepuscular grass-eating rodents.

The responses of a prey animal to a predator should also be
considered. Mech (1966) found that wolves on Isle Royale “tested” the
reactions of moose, and rarely killed prime moose that stood their
ground. Errington (1967) relates similar reSponses of mink to adult
muskrats that did not attempt escaping after being detected. Schoener
(1971) discusses this potential risk of injury to a predator as a
frequently ignored handling cost.

While these prey characteristics may remain fairly constant for
any one species, or the differences among Species in a multi-species
system may do likewise, the social and demographic attributes of a
population of prey animals may vary through time. Paul Errington (1943),
in his analysis of predation by mink upon muskrats, was perhaps the
first to consider the operation of these factors in populations of
mammalian prey. He found a relationship between changes in p0pulation
density, with their concurrent changes in intraspecific friction, and
the vulnerability of certain segments of a muskrat population to pre-
dation. A similar situation might exist in other microtine rodents:
recent research into the population processes of voles of the genus
Microtus has pointed out that demographic and socio-behavioral attri-
butes may vary among segments of a population and may change dramatically
with the population cycle (Krebs, 1966; Myers and Krebs, 1971). The
question is raised as to whether or not these or other differences
might affect the vulnerability of different sex or age segments of a
Microtus population to predation by a carnivore. The research reported

herein was designed to test an hypothesis about differential vulner-

ability to predation among segments of a population of meadow voles
(Microtus pennsylvanicus). An hypothesis of secondary interest was
that of differential vulnerability to predation between Microtus

pennsylvanicus and other small rodents, notably deermice (Peromyscus

 

maniculatus), jumping mice (Zapus hudsonius), and house mice (Mus

 

musculus). This was intriguing because of the preponderance of Microtus
over these other rodents in carnivore diets as reported by several

food habits studies (Bradt, 1949; Korschgen, 1957).

The Prey

The prey species of primary interest in this study, Microtus
pennsylvanicus (Family Cricetidae, Subfamily Microtinae), is widely
distributed over North America (Hall and Kelson, 1959). The subSpecies
represented in Michigan, M-.B- pennsylvanicus 0rd, measures 120-188 mm
in total length, and weighs between 20 and 69 grams (Burt, 1946). These
voles are uniformly dark brown above with paler sides and silver-tipped
ventral hair. Their food consists primarily of grasses and sedges, and
they are generally found in moist low areas and in old field situations.
They are primarily crepuscular, although they are active throughout the
day and night (Burt, 1946),

Meadow voles, like many other microtine rodents, are subject to
periodic fluctuations in numbers, and it is through the study of the
demographic changes accompanying these fluctuations that they have
received considerable attention in recent years. These studies have
furthered the I'interpretation of animal populations as a composite of

qualitatively different individuals” (Myers and Krebs, 1971), and it

is upon this interpretation that the present study is based. Data
supporting the existence of differences in quality among individuals
of a Microtus pennsylvanicus population have been reported in several
studies.

Several papers have discussed differences in aggressive behav-
ior between sexes of meadow voles. Significant antagonism between
females, but not among males or between males and females has been
reported by Getz (1972). Some of this behavior may be associated with
reproductive activity -- Clarke (1956) noted high aggression by female

Microtus agrestis during the last stages of pregnancy and with subse-

 

quent nursing of litters. Changes in aggressive behavior seem also to
accompany the population cycle in both Microtus pennsylvanicus and_M.

ochrogaster (Krebs, 1970).

 

Research into the role of dispersal in the regulation of vole
populations has yielded considerable evidence of qualitative differences
between dispersing and resident animals. Dispersing males are generally
more aggressive than residents during peak population density (Myers
and KrebS, 1971). These I'intolerant'l animals tend to move into less
densely populated areas as population density increases. Dispersing
males show less exploratory behavior in a maze than do males from con-
trol populations.

These authors did not find, as is often thought, that young
males are more apt to disperse than are adult males or young females.
Body weights of dispersing males were not different from controls.

They did, however, find a greater tendency for juvenile females (S 22 g)

to diSperse than juvenile males. Differences in reproductive condition

were found between dispersing animals and controls. The proportion of
subadult males with testes in a scrotal position, and of female sub-
adults with perforate vaginae (each indicating attainment of breeding
condition) were greater in diSpersing animals than in residents.

Differences between male and female meadow voles in size of and
restriction to a home range have been observed. Getz (1961) found male
voles to have larger home ranges than females in two southern Michigan
habitats. He suggests that this may be due to one or two factors: the
restriction of female activity while caring for young, and/or the wide
ranging search by males for mates. Getz also observed that a greater
percentage of females than males had exclusive home ranges, and that
females with established home ranges outnumbered established males
during most of his study.

That some of these sex- or age-related behaviors might influence
mortality is suggested by the rates of survival reported by Krebs-gt
.31. (1969). They found female survival rates to be high and relatively
constant (until the population they were studying declined), while
survival rates of males were highly variable. Male and female rates
of survival were not highly correlated, and they concluded that I'the
loss process must be sex selective.” Age differences in survival were
also noted. Subadult males and females exhibited lower survival rates
than did adults, eSpecially during the summer breeding season when
densities were high. It should be noted that in live-trapping studies
of the type conducted by Krebs t al. (1969), survival refers to dis-

appearance from the grid due to any factor (e._g., death, dispersal).

Hypotheses

Which, if any, of these sex- or age-related behaviors might
affect vulnerability to predation? Several hypotheses would appear
justified. Adult males, because of their greater tendency to roam,
might expose themselves more to predators. Subadult males and females
in breeding condition, through an inclination to disperse, could be
more liable to predation than other age segments. Neonates may also
be highly vulnerable. Due to the heterogeneity of existing evidence,
hypotheses of no difference due to sex or age were postulated. The
only hypothesized differences would be due to abundance. The null
hypothesis for Microtus sex and age segments was phrased as follows:

Each sex and age segment of a Microtus population will be cap-

tured and consumed by a carnivore in proportion to that seg-

ment's abundance in the environment.
The hypothesis of differential vulnerability between Microtus pennsyl-
vanicus and other rodent species parallels that for vole sexes and ages,
.1..§., the only differences among species in vulnerability would be

due to differences in abundance.
Basis of the Method for Testing the Hypotheses

In order to test these hypotheses, it would be necessary to
1) monitor the changes in abundance of Microtus sex and age classes,
and of the other rodent Species; and, 2) identify animals eaten by
carnivores. The first could be accomplished by live-trapping and
marking the rodent population, as was done in the previously-mentioned
demographic studies. In order to determine the identity of animals

preyed upon, some marker was needed. Several studies (_._g., Pearson,

1966) have analyzed fecal matter for mandibles to identify rodent
species eaten by a carnivore. This technique cannot, however, be used
to identify the sex and age of small rodent prey. Thus, it was neces-
sary to devise some marker, to be placed on a prey animal, that would
be ingested and passed by a carnivore consuming that prey. The small,
numbered monel fingerling gill tags which have been used to identify
rodents in demographic studies, seemed well-suited to the purpose.
These tags, which are attached to the ear and measure 2 mm by 8 mm
when applied, would allow rapid identification of animals both in the
field during live-trapping and in carnivore droppings. It was first

necessary to test whether a carnivore would ingest a tagged rodent.

 

Several Microtus and Peromyscus were fed to each of 2 pet domestic
cats, this species having been selected as the carnivore for this
study. All rodents were eaten without hesitation by the cats, which
showed no reaction to the tags. Thus, it was considered feasible to
tag live-trapped animals in the field, and to collect carnivore drop-

pings and analyze them for tags.
The Predator

Domestic cats (Felis catus) were selected as predators for this

 

study for several reasons:
1. Microtus form a major part of the diet of field-roaming
domestic cats (Bradt, 1949; Hubbs, 1951).
2. Domestic cats are abundant and reasonably easy to study.
3. There is growing concern among wildlife biologists, pest

control operators, and others about the increasing numbers

of field-roaming and free-living domestic cats and their effect
on populations of small birds and mammals (Kuroda, 1968; McKee,
1967; Troy, 1951). The ecology of the domestic cat is not

well understood.

Domestic cat food habits, studied by Eberhard (1954), Hubbs
(1951), McMurray and Sperry (1941), and others, have shown that these
cats generally prey upon whatever small mammal or bird is most avail-
able or abundant. They occasionally take such larger animals as rab-
bits, pheasants, and ducks. Bradt (1949) found that one cat killed
1600 rodents in an 18 month period, of which about 75% were meadow
voles. Insects occupy a considerable part of the diet during Spring
and summer.

While several studies (Kuo, 1930; Leyhausen, 1960) have dealt
with the development of the domestic cat's reSponse to prey animals
and with its killing technique, little work on field-roaming domestic
cats, other than the above mentioned food habits studies, has been
done. In the only published work on field behavior found by this
author, Leyhausen (1965) discussed social organization and found that
domestic cats frequently shared common hunting grounds, but usually did

not hunt together, generally avoiding close contact in the field.

THE STUDY AREA

This study was carried out in farmland near Williamston, Mich-
igan (section 17, T 3N, R 1E, Ingham County). The small rodent pop-
ulation of an approximately 0.8 hectare (2 acre) old field was sampled
by live-trapping. An adjacent farm, directly across a paved county
road from the trapping grid, was occupied by approximately 10 domestic
cats, the number varying with the movements of wandering toms. The
resident cats were fed dry food and table scraps, but supplemented their
diet by hunting mice, primarily on the trapping grid. The grid had
been fallow for about 5 years, having last been sown in llSudex“ (DeKalb
trade name for an annual hybrid forage plant). Three general vegeta-
tional types were found on the area (see Figure 1): a central area,
consisting primarily of blue-grass LE22.§B‘); a brushy strip along the
south and east consisting of burdock (Arcticum 33.), stinging nettles
(Urtica 33.), and scrub willows (Sallxngp.); and an area of brome-
grass (Bromus 23°) and mixed herbaceous plants along the north and west
borders. The grid was bordered on the north by the county road, on the
west by an alfalfa field, and on the east and south by a small creek.
Beyond the creek was a pasture to the east and a fallow field to the
south.

A wide variety of vertebrates and invertebrates existed on the
study area. Several predators in addition to domestic cats were pres-

ent. One weasel (Mustela frenata) was captured. Although not observed,

 

10

the presence of raccoons (Procyon lotor) and domestic dogs was evidenced
by their droppings. A red-tailed hawk (Buteo jamaicensis) was observed
once in the adjacent alfalfa field, but was not seen on or over the
trapping grid.

Potential prey animals, in addition to the small rodents of
particular interest in this study, included pheasant (Phasianus col-
chicus), which were abundant on the trapping grid. Crickets (Gryllidae)
and grasshoppers (Acrididae and Tettigoniidae), which are common prey
of domestic cats during the summer months (Hubbs, 1951; McMurray and
Sperry, 1941), were present in large numbers. Sparrows (Fringillidae)
and frogs (Rana pipiens and R. sylvatica) were occasionally captured
in traps. Shrews (Blarina brevicauda and Sorex cinereus) were common
(on the study area. These are often captured and killed by cats, but

infrequently eaten (Nader and Martin, 1962).

Other mammalian species on the study area included muskrat

(0ndatra zibethicus), whitetail deer (Odocoileus virginianus), and

opossum (Didelphis marsupialis).

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Map of the study area.

Figure 1.

METHODS

Data on the independent variable, i._§., the relative abundances
of sex and age segments of Microtus and of the several other small
mammals, were collected by an intensive live-trapping and marking
program which took place between 19 July and 1 November, 1972. Box
traps with a trip mechanism of the type described by Fitch (1950) were
used on a permanent grid of 8 meter spacing, 118 traps being used to
cover the area. Traps were placed 4 meters from the north and west
borders; border traps on the other sides fell between 0 and 6 meters
from the boundaries.

The traps were placed in 9 rows, numbered from north to south,
each containing the following number of traps: rows 1 through 4, 18
traps each; row 5, 16; row 6, 13; row 7, 8 traps; row 8, 6; and row
9, 3 traps. Each trap location was assigned a number based on its row
and location within a row. Traps were baited with rolled oats, small
amounts of which were placed both at the mouth of the trap and behind
the trip.

Each of the 16 trap periods was three nights long. Traps were
baited in late afternoon or early evening, inSpected and closed the
following morning, and baited and re-set that evening. Each trap
period ended when the traps were closed on the third morning. The traps

were left set during the day when temperatures were low enough to pre-

sent little danger of overexposure to animals in traps. The first day

12

13

of each trap period was spaced one week from the others with the
following exceptions: trap periods 1 through 3 were on 9 successive
days, and periods 4 and 5 occurred 1 week later on 6 successive days.
These trap periods were compressed to allow rapid tagging of as many
animals as possible at the beginning of the study. Trap periods 9 and
10 were spaced 2 weeks apart.

Nine additional traps were placed in areas of highest Microtus
density during trap periods 8 through 12. These were then removed when
they failed to increase capture success.

Individual mice and voles were identified by attaching the
previously mentioned fingerling tags (Salt Lake Stamp Co.) on the ear
at first capture. Voles were occasionally captured with torn ears,
indicating that they had likely lost previously-applied tags. This
loss, however, was minimal: only 15 individuals were captured in this
condition.

Several kinds of data were recorded upon trapping an animal:
species, location on the grid, sex, weight, tag number (if recaptured),
and, in the case of Microtus, reproductive condition. Weights were
obtained to the nearest gram using a Hanson diet scale. Reproductive
condition was assessed in the manner described by Krebs (1966). For
males, the position of the testes, whether scrotal or abdominal, was
noted. Several reproductive measures were used for females. The con-
dition of the vagina, whether perforate or not, was used as a general
indicator of breeding condition. However, as mentioned by Myers and
Krebs (1971), this indicator seems to be most reliable for subadults,

since its use with adults is complicated by the occurrence of preg-

14

nancy. The size of the nipples was used as an indicator of lactation.
The condition of the pubic symphysis, whether closed, slightly open,
or open, gives some idea of past parturitions (see Hall and Newton,
1946). In addition, females with bulging abdomens were noted as being
”likely pregnant“.

Age classes were established on the basis of body weight, as
described by Chitty (1952). Voles weighing 22 grams or less were
classified as juveniles; those greater than 22 9 but less than or
equal to 32 g as subadults; and those weighing more than 32 grams were
classified as adults.

Data on the dependent variable, the number of animals of each
sex and age segment of Microtus and of each species that was preyed
upon by cats, were obtained by collecting and analyzing cat droppings
for ear tags. These scats were collected in various locations, primar-
ily on the trapping grid and in flower beds in the farmhouse yard.
Droppings were analyzed by x-ray on sheets of Plexiglas. The scats
containing ear tags were then easily segregated from the others and
the tags removed.

The use of this method is based on Several assumptions. First,
it is assumed that tagging will have no effect, or that any effect will
be systematic among groups, on survivorship of mice and voles, on vul-
nerability to predation, or on ingestion of mice by carnivores. It is
assumed that no bias will be introduced by locations of scats, j, 2,,
that individual carnivores do not select prey differently than others,
and that each cat does not use a specific scatorium. The assumption is
made that trappability is random and does not change with time among

the marked segments of the population.

RESULTS

The results will be presented in 2 sections, one dealing with
the characteristics of the small rodent population during the course
of the study; the other, with the analysis of predation upon these
animals. Since the numbers of Peromyscus, Zapus, and M33 captured
were small, no attempt was made to evaluate changes in numbers and

other demographic parameters for these Species.
SMALL RODENT POPULATION

A total of 5640 trap-nights were logged, during which 576 total
captures were made on 261 Microtus, 18 Peromyscus maniculatus, IO.ME§
musculus, and 1 ggpgs_hudsonius. Capture success was consistently low
throughout the study, varying between 5 and 20% between trap periods.
Trap mortality was minimal: 3 Microtus were found dead in traps. One
of these was an adult female which was known to be alive on the grid
for 4 weeks previously. The other 2 animals dying were 1 male and 1
female subadult that apparently drowned or chilled during a heavy rain-

storm.

Numbers and Age Classes

Numbers of Microtus alive during the study were estimated by
the direct enunwration technique described by Krebs (1966). In this

technique, the minimum number of mice alive at time t is obtained by

15

16

summing two counts: 1) the number caught at time t; and 2) the number
of previously tagged animals caught after time t, but not at that time.
Use of this method avoids making the assumption inherent in capture-
recapture methods of random sampling between marked and unmarked seg-

ments of the population.

\
1..

The minimum number of Microtus alive fluctuated widely between
trap periods, so minimum numbers were calculated for 2 trap period
intervals (except that trap periods 1-3 were combined). These data,
illustrated in Figure 2, page 17, show a decrease in total numbers
during trap periods 6 and 7, perhaps due to decreased trappability,
accounted for in part by the very hot humid weather at these times.
This dip was followed by an increase during trap periods 8 and 9,
another decrease through trap period 13, and a slight increase through
period 15. When minimum numbers are calculated for longer periods of
time, as shown in Figure 3, page 18, deviations from a smooth curve
are lessened, and the general trend of decreasing numbers of Microtus
emerges.

Figures 2 and 3 also illustrate minimum numbers of Microtus
alive in each age class throughout the study. The minimum number of
juveniles alive‘decreased slightly throughout the study, while the
number of subadults, after a decrease during trap periods 6 and 7,
increased. The number of adult voles decreased steadily.

Since the relative abundances of the different age classes were
also of interest, the proportions of the total minimum number alive in
each age class were calculated, and are shown in Figure 4, page 19.

Juveniles represented between 12 and 20 % of the total population

MINIMUM N0. ALIVE

7O

60

50

40

30

20

10

I7

  
 

 

 

r.
total
f subadult
adult
juvenile
TRAP
1-3 4-5 6-7 8-9 10-1 12-13 14-15 PERIOD
JUL AUG SEPT OCT
MONTH

Figure 2. Minimum numbers of Microtus alive.

MINIMUM N0. ALIVE

TRAP
PERIOD

120

100

80

60

40

20

 

   

18

  

 

 

Figure 3.

L.—
total
h
..—0 subadult
\ /
\\\ ///
.\Vfi adult
\ juvenile
I-ST 6-9 10-13 14-16
19 JUL- 15 AUG- 20 SEP- 16 OCT-
9 AUG 5 SEP 11 OCT 1 NOV
DATES

Minimum numbers of Microtus alive.

PROPORTION

I.

19

  

° subadult

\\\ //’ adult

‘/ \
juvenile

 

 

1 l l
_19 JUL- 15 AUG- 20 SEP- 16 OCT-
9 AUG 5 SEP 11 OCT 1 NOV

DATES

Figure 4. Proportion of the minimum number of Microtus
alive represented by each age class.

20

throughout. A Chi-square goodness-of-fit test was used to test for
significant changes in the proportion represented by each age segment.
The changes in the juvenile proportion were not significant (each
X%(1.45, P>.20). The decrease in the subadult proportion during the
second time interval (15 August to 5 September) was not significant
(X2=2.58, P>.10), but the increases in proportion during the following
2 time intervals were significant (X2=21.33, P<.001; X2=5.26, P<.05).
The increase in the adult proportion from the first to the second time
interval was not significant (X2=.97, P>.30); a Significant decrease
(X2=5.OO, P<.05) occurred between the second and third intervals. The
decrease observed in the fourth interval adult proportion was not

significant (X2=1.96, P>.10).

Sex Ratios
Sex ratios (proportion of males of the minimum number of Microtus
alive are shown in Figure 5, page 21. None of these ratios is signif-

icantly different from.1:1 (P>.20).

Reproduction

As stated in the Methods section, reproductive activity was
assessed by examining the external sexual characteristics of voles.
While these measures are not as accurate as autopsies, they provide a
crude measure of changes in reproduction (Krebs _£_§l,, 1969).

Probably the best measure of a population's breeding activity
is the percentage of females with enlarged nipples, j._§., lactating
(Krebs t 1., 1969). These percentages are illustrated in Figure 6,

page 22. The percentage of adult females that were lactating increased

21

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PROPORTION

1.

22

    

 

 

 

perforate
F adults
' o\
\
\Q
- \ perforate
subadults
P
lactating
adults
19 JUL- 15 AUG- 20 SEP- 16 OCT-
9 AUG 5 SEP 11 OCT 1 NOV
DATES

Figure 6. Changes in female vole reproductive measures.

23

through the first part of October, and then dropped to zero during the
last 2 weeks of the study, indicating a decline in breeding. This
interpretation is made with no little reservation, since samples were
small (9. 3., a minimum number of only 5 adult females were alive
during the closing 2 weeks of the study).

The proportions of subadult and adult females with perforate
vaginae are also shown in Figure 6. This percentage for adult females
increased throughout the study, but, as previously stated, the use of
this measure for mature females may be invalid. The proportion of
subadult females in breeding condition decreased throughout the study.

The proportion of adult females judged pregnant on the basis
of bulging abdomen remained between .46 and .60, and showed no increas-
ing or decreasing trend during the study. These proportions are con-
siderably higher than those reported by Krebs, Keller, and Tamarin
(1969), and seem to contradict other measures of female reproduction.
These factors have led the author to seriously question his lack of
bias in using this technique, and for this reason, these data on
pregnancy have not been used in assessing reproductive activity.

Breeding condition of males was judged by the position of the
testes. The proportion of males with testes in a scrotal position is
shown in Figure 7, page 24, for adults, subadults, and both ages

combined.
Mortality

Mortality in a live-trapping study is defined as disappearance

from the grid, and thus includes dispersal off of the trapping area.

PROPORTION

 

24

 

a\\.
“x .
‘\\.q\\ A adult
5

\9 combined
subadult

19 JUL- 15 AUG- 20 SEP- 16 OCT-

9 AUG 5 SEP 11 OCT 1 NOV

DATES
Figure 7. Proportion of male voles (subadult, adult,

and combined) with testes in a scrotal
position.

25

Survivorship curves, such as that shown in Figure 8, page 26, were
calculated in the following manner. The week in which each vole was
first captured was designated week zero. Thus, at week zero, 100 %
of the animals were alive on the grid. The proportion remaining at
each following week is lower than the previous week by the proportion
that was captured for the last time during that week. These curves
were constructed for all voles captured for the first time in trap
periods 1 through 13. Trap period 13 was the last in which animals
first captured were not still being re-captured at the end of the
study.

These curves were linearized by plotting on semi-logarithmic
graph paper to facilitate comparisons between sexes and ages. The
points conformed closely to a straight line, with the exception of
the period from week zero to week one, which was excluded from the
semi-log plots. Figure 9, page 27, shows survival for males and
females. After the first week, both sexes survived at approximately
the same rate.

Survivorship of the different ages is illustrated in Figure
10, page 28. After differential survival in the first week, all ages
survived at approximately the same rate.

Since it was of interest to examine changes in survivorship
over time, the survivorship of voles alive during trap periods 1-10
(n=182) was compared with that of those alive during periods 11-13
(n=60). These data, shown in Figure 11, page 29, illustrate that
survival was lower in the latter time period; a t-test of mean sur-

vivorship showed this difference not to be significant.

1

26

 

 

 

a: \o 4- (V c:
UNINIVNHU NOIlUOdOUd
Figure 8. Survivorship of all Microtus, trap periods 1-13,

WEEKS

PROPORTION REMAINING

.06

.04

.02

.01

27

 

 

 

 

 

 

 

 

 

 

.
Q
‘ Females (n=72)
.
Males (n=96)
.
a
O
I n l l
1 2 3 4 5
WEEKS

Figure 9. Survivorship by sex, trap periods 1-13.

 

PROPORTION REMAINING

.00

.60

.40

.20

.10

.06

.04

.02

.01

28

 

 

 

 

 

 

 

 

 

 

 

Q
jk\\\ Subadults (n=64)
1a
l I l 1 l
0 I 2 3 4 5
WEEKS
Figure 10. Survivorship by age at first capture, trap

periods 1-13.

 

PROPORTION REMAINING

.00

.60

.40

.20

.10

.06

.04

.02

.01

29

 

 

 

 

1-10 (n=182)

 

 

11-13 (n=60)

 

 

 

 

L A l 1 l
1 2 3 4 5
WEEKS

Figure 11.

Survivorship of voles alive during trap
periods 1-10 and periods 11-13.

 

30

No data are available on survival of those very young animals
which have not yet reached trappable age, about 4-6 weeks, at which
time they weigh 20-25 grams. The index relating recruitment of young
into the population to lactating females employed by Krebs (1966)
could not be used because of the small number of lactating females

captured in this study.
ANALYSIS OF PREDATION

A total of 40 ear tags was recovered from approximately 160
scats. All of the tags were from Microtus. The sexes and ages of
voles whose tags appeared in scats, along with expected numbers
(based on 261 total voles of which 133 were males, 128 females, 45
juveniles, 9O subadults, and 126 adults), X2, and P values, are shown
in Table 1, page 31. The X2 tests show no differences between observed
and expected numbers for sex or age classes or for ages within sex.

A conspicuous, although not significant, difference is seen between
males and females -- the former being taken less often than expected,
the latter more often. These data indicate that sex and age segments of
voles are preyed upon in proportion to their abundance.

Since the relative abundances of adults and subadults changed
throughout the study, it was of interest to compare changes in the
amount of predation suffered by each age class at different times. To
accomplish this, the first 20 tags (Group I) recovered were compared
with the second 20 (Group II). The age structure of the population
during the times tagged animals in each group were known to be available

to predators (Group I -- 19 July to 22 September; Group II -- 25 Sep-

Table 1. Sexes and ages of tagged Microtus recovered from cat fecal

matter during the entire study.

 

 

 

 

 

CLASS OBSERVED EXPECTED* X P
Male Juvenile 4 2
Subadult 4 5 1.26 .704 .80
Adult 8 9
Female Juvenile 4 5
Subadult 10 10 0.31 >.95
Adult 10 9
Males 16 20
Females 21+ 20 1.60 .30<.50
Juveniles 8 7
Subadults 14 15 0.26 J>.95
Adults 18 19
n=40

* Based on the proportion of total captured in each class over the

entire study.

32

tember to 1 November) was computed to derive expected values on the
basis of abundance. The expected, observed, X2, and P values illus-
trated in Table 2, page 33, show that, within each of these time
periods, each age and sex class was preyed upon in proportion to its
abundance.

A X2 test was used to test for differences between Group I
and Group II tags. The proportion of Group I tags represented by each
class was used to establish expected values for that class in Group II
tags. The observed and expected numbers, along with X2 and P values,
are shown in Table 3, page 34. The significant difference between
Group I and Group II tags in the amount of predation suffered by each
age class may be attributed almost entirely to the deviations between
observed and expected values for subadults and adults. These differ-
ences correspond to the changes in relative abundances of each of these
age classes, and further indicate that age classes of meadow voles in
this study were preyed upon in proportion to their abundance.

The minimum number of voles alive during the time when Group 1
animals were in the population was 169, while 110 were alive when
Group II animals were available to predators. Thus, the kggwfl per-
centages of predation were 11 % (20/169) for Group I animals and 18 %
(20/110) for Group II animals. This represents a significant increase
over time in the proportion of animals alive that were caught by
predators (X2=5.33, .02<P<.05). The cause(s) of this increase are not
known, but several conjectures may be made. Since no change in vul-
nerability, beyond changes in abundance, was observed for any one sex

or age class, it would appear that, whatever the cause, it affected

33

Table 2. Sexes and ages of tagged Microtus recovered from cat fecal

matter within Groups I and II.

 

 

 

 

 

Group I
CLASS OBSERVED EXPECTED X2 P
Males 8 ll
Females 12 9 1.82 .30<.50
Juveniles 4 3
Subadults 4 5 0.28 .>.95
Adults 12 12

n=20

Group II
CLASS OBSERVED EXPECTED X2 P
Males 7 10
Females 13 10 1.80 .30<.50
Juveniles 5 3
Subadul ts 9 9 1.25 .70 < .80
Adults 6 8

 

34

Table 3. Comparison between Group I and Group II tags in the amount

of predation suffered by each sex and age segment.

 

 

 

 

 

Group I
% 0F

CLASS OBSERVED TOTAL
Males 8 40
Females 12 6O
Juveniles 4 20
Subadults 4 20
Adults 12 60

Group II
CLASS OBSERVED EXPECTED* X2 P
Males 8 8
Females 12 12 0 00 >°99
Juveniles 5 4
Subadults 9 4 8.12 .02<.05
Adults 6 12

 

*Based on % of total represented by each class in Group 1 tags.

35

each age and sex segment equally. No data is available on the number
of predators hunting on the area, but a larger number might account
for an increase in percent predation. Vegetative cover began deter-
iorating with the first frost in late September, which may have
influenced vulnerability.

It was of interest to examine the effects upon vulnerability of
the attainment of breeding condition and behaviors associated with
reproduction. Seventy percent of all subadult and adult females had
perforate vaginae (from the summation of capture data for all trap
periods), compared with 85 %.of those which were known to be preyed
upon. This does not represent a significant difference (X2=0.64,
P>.30). Likewise, no association was found between pregnancy or
lactation and the liability to predation.

For males, 69 % of all subadults and adults had testes in a
scrotal position (again, from the summation of capture data for all
trap periods), compared with 75 % of those males recovered in scats.
This represents no difference. Thus, these data Indicate that
reproductive activity has no effect on vulnerability to predation.

As previously stated, no tagged Mus, Zapus, or Peromyscus were
recovered in scats. This difference is striking, but not significant.
Individuals of these species represented 10 % (29/290) of all rodents
captured. The number expected in scats would thus have been 4, which
Is not significantly different from the observed zero (X2-3.06, PJ>.05).
Thus, these data indicate that each rodent species on the area was

taken In proportion to its abundance.

DISCUSSION

The data gathered in this study indicate that domestic cats
captured small mammal prey in proportion to abundance. Different
segments of the Microtus population were preyed upon in proportion to
their abundance overall, and changes in abundance of age classes were
directly reflected in changes in the amount of predation suffered by
each class. The various rodent Species on the trapping grid were
apparently also captured in relation to their relative abundance.

This apparent lack of differential predation among segments
of the Microtus population may be explained in at least two ways:

1) all voles, regardless of sex or age, are equally liable to predation,
or 2) individual voles may differ in vulnerability, but the distribution
of vulnerability is the same within each segment of the population.

That is, there may be some highly vulnerable voles, but these animals
comprise an equal proportion of all segments. Given the existence of
behavioral differences within a vole population, this latter hypothesis
would seem more tenable.

The lack of differential predation among sexes or ages may be
in part a function of the hunting strategy of domestic cats. Hornocker
(1970) has hypothesized that canids (g. 3., wolves, gaflL§.lgEg§), which
run their prey down, are more apt to take sick, weak, or aged animals,

while felids, such as the mountain lions (Fejis concolor) he was study-

 

ing, which either approach slowly to within attacking distance or wait

36

37

in ambush, are apt to take any animal, regardless of condition, that
is in a vulnerable position. House cats often employ a sit-and-wait
strategy, and thus may take any animal that approaches within pouncing
distance. It would seem that if differences existed among prey animals
in experience, activity, range, and so on, these differences would
affect liability to predation by domestic cats. However, unless these
differences were distributed unequally among segments of a population,
results such as those obtained in this study might be expected.

The decreasing numbers and low survival (see Krebs _£__L., 1969)
would indicate that this vole population was in a decline phase; a
different distribution of vulnerability among sexes and ages might be
expected during another phase in the microtine cycle. Thus, the con-
clusions of this study should be limited to this population at this
point in time.

In summary, it appears that, at least for this study, no differ-
ential vulnerability exists among segments of a vole population to
predation by domestic cats. Also, there appears to be no difference

in vulnerability to predation between Microtus and the other rodent

species studied.

38

SUGGESTIONS

The author would like to suggest solutions to some of the
problems encountered in this study. One of the limitations on the
interpretation of these data stems from the lack of knowledge about
the number of cats hunting on the area. Cats were occasionally observed
during morning and afternoon, but never more than 2 at a time. The
number hunting on the area at night was not determined. The use of
an enclosure which would allow maintaining a constant and known number
of predators would have been helpful. But since dispersal has been
found to be an important part of microtine rodent population biology,
it would be necessary for such an enclosure to prevent movement of
cats and yet allow free dispersal of voles into and out of the area.
An enclosure would also facilitate scat collection, which was one of
the major problems in this study,

Trap predation was somewhat of a problem, so it would be
advantageous to devise some method of securing traps that would not

be a serious hindrance to checking, baiting, and setting.

LITERATURE CITED

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Zoology, 17(3): 292-304.

Baker, R. H. 1971. Nutritional strategies of myomorph rodents in
North American grasslands. J. Mamm., 52(4): 800-805.

Bradt, G. W. 1949. Farm cat as predator. Mich. Cons., July-Aug., 1949.

Brooks, J. L. 1968. The effects of prey size selection by lake plank-
tivores. Systematic Zoology, 17(3): 273-291.

Burt, W. H. 1946. The Mammals of Michigan. Univ. Mich. Press, Ann
Arbor, xv + 288 pp.

Chitty, D. 1952. Mortality among voles (Microtus agrestis) at Lake
Vyrnwy, Montgomeryshire in 1936-9. Phil. Trans. Royal Soc. London,
Ser. B, 236: 505-552.

Clarke, J. R. 1956. The aggressive behavior of the vole. Behavior,

9: 1-23.

Dice, L. R. 1947. Effectiveness of selection by owls of deermice
Peromyscus maniculatus which contrast in color with their back-
ground. Contrib. Lab. Vert. Biol. Univ. Mich., no. 34: 1-20.

 

Eberhard, T. 1954. Food habits of Pennsylvania house cats. J. Wildl.
Mgmt., 18(2): 284-286.

Errington, P. L. 1943. An analysis of mink predation upon muskrats in
north-central United States. Iowa State Coll., Res. Bull. 320:
797-924.

. 1967. Of Predation and Life. Iowa State Univ. Press.
Ames. xii + 277 pp.

 

Estes, R. 0., and J. Goddard. 1967. Prey selection and hunting behavior
of the African wild dog. J. Wildl. Mgmt., 31(1): 52-70.

Fitch, H. S. 1950. A new style live-trap for small mammals. J. Mamm.,

31(3): 364-365.

Getz, L. L. 1961. Home ranges, territoriality, and movement of the
meadow vole. J. Mamm., 42(1): 24-36.

39

4O

Getz, L. L. 1972. Social structure and aggressive behavior in a pop-
ulation of Microtus pennsylvanicus. J. Mamm., 53(2): 310-317.

Hall, E. R., and K. R. Kelson. 1959. The Mammals of North America.
Vols. I and II. Ronald Press. New York.

Hall, K., and W. H. Newton. 1946. The normal course of the separation
of the pubes in pregnant mice. J. Physiol., 104: 346-352.

”011109, C. S. 1959. The components of predation as revealed by a study
of small mammal predation of the EurOpean sawfly. Can. Ent.,

91: 293-320.

, 1965. The functional response of predators to prey
density and its role in mimicry and population regulation. Mem.
Ent. Soc. Can., 45: 5-60.

 

1966. The functional response of invertebrate preda-
tors to prey density. Mem. Ent. Soc. Can., no. 48. 86 pp.

 

Hornocker, M. G. 1970. An analysis of mountain lion predation upon
mule deer and elk in the Idaho Primitive Area. Wildl. Monogr.,
no. 21, 39 pp. The Wildlife Society.

Hubbs, E. L. 1951. Food habits of feral house cats in the Sacramento
Valley. Cal. Fish and Game, 37(2): 177-189.

Korschgen, L. J. 1957. Food habits of coyotes, foxes, house cats, and
bobcats in Missouri. Mo. Cons. Comm., P-R Report, no. 15, pp. 1-64.

Krebs, C. J. 1966. Demographic changes in fluctuating populations of
Microtus californicus. Ecol. Monogr., 36(3): 239-273.

1970. Microtus population biology: Behavioral changes

 

associated with the population cycle in M. ochrogaster and-M.
pennsylvanicus. Ecology, 51(1): 34-52.

Krebs, C. J., B. L. Keller, and R. H. Tamarin. 1969. Microtus pop-
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of M; ochrogaster and M. pennsylvanicus in southern Indiana.
Ecology, 50: 587-607.

 

 

Kuo, Z. Y. 1930. The genesis of the cat's response to the rat. J.
Comp. Psychol., 11(1): 1-35.

Kuroda, N. 1968. A bird census in the Imperial Palace (Japan) for
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Tierpsychologie, Beiheft 2, 1960 (Eng. sum.).

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Leyhausen, P. 1965. The communal organization of solitary mammals.
Zool. Soc. London, Symposia, no. 14: 249-263.

McKee, R. 1967. Catbirds and/or bird cats. Mo. Cons., July, 1967:
13-15.

McMurray, R. B., and C. S. Sperry. 1941. Food of feral house cats in
Oklahoma, a progress report. J. Mamm., 22(2): 185-190.

Mech, L. D. 1966. The wolves of Isle Royale. Fauna Natl. Parks, U. 5.
Fauna Series, no. 7, 210 pp.

Myers, J. H., and C. J. Krebs. 1971. Genetic, behavioral, and repro-
ductive attributes of dispersing field voles Microtus pennsylvan-
icus and Microtus ochrogaster. Ecol. Monogr., 41(1): 53-78.

 

Nader, I. A., and R. L. Martin. 1962. The shrew as prey of the domes-
tic cat. J. Mamm., 43(3): 417.

Pearson, O. P. 1964. Carnivore-mouse predation: an example of its
intensity and bioenergetics. J. Mamm., 45: 177-188.

. 1966. The prey of carnivores during one cycle of mouse
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. 1971. Additional measurements of the impact of carni-
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41-49.

 

Schoener, T. W. 1971. Theory of feeding strategies. .Lfl Richard F.
Johnston (Ed.), Annual Review of Ecology and Systematics, Vol. 2,
1971. Annual Reviews, Inc.

Troy, W. N. 1951. Are cats the PCO's biggest competitor? Pest Control,
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APPENDIX: A MODEL OF

PREDATOR FEEDING BEHAVIOR

Several authors (Holling 1965; Schoener, 1971) have derived
mathematical models describing predation, but these equations often
assume control of predator and prey variables. Thus it was difficult
to classify the types of prey variables of interest in this study. A
schematic model for the behavior of a feeding predator, shown in Figure
12, page 43, was devised for this organizational purpose. This model
includes not only prey characteristics, but also predator and environ-
mental variables.

The feeding behavior of a predator has been segmented into
several steps. These are depicted in the model as ”decisions” with
”yes'-' or ”no” alternatives. A ”yes” alternative to a step indicates
that the predator has completed that ”decision” and thus moves on to
the next. For example, a ”yes” alternative to ”search for prey”
signifies that the predator is searching for prey, and the next step
in the sequence is to detect prey. The model begins with the assump-
tion of an active predator, and proceeds through the several steps,
each of which will be explained in turn. A ”no” alternative to ”search”
indicates that the predator will undertake some other activity. After
a ”no“ alternative to any other step, the predator returns to searching.

When the predator has begun searching, its next task is to

detect prey, which is dependent on his capabilities and on various

42

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

43

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ACT'VE 17 lPREY BEHAVIOR]
SEARCH ' II.________I IHABITAT C
FOR PREY L CLIMATE
OTHER N0 5 Er—J—
ACT'V'" Rm
'0 CT A‘W
LEXPERIENCEI——- Zfl'////. PREY ‘3
I
SOCIAL 8 DEMO-
5' 5 4‘ GRAPHIC CHAR
V L T
PREF R N E 2 Y,
E. A’ ’47
D I} I (BODY SIZE—8 '
APPROACH PALATABILITY
EVASIVE
ICAPABILITYff. BEHAVIOR
LNO YES
0 F A R
CAPTURE
CAPABILITY 0F TN-
DAL YES JURING PREDATOR
HANDLE
[EAL VEs
FOOD STOMACH
INTAKE CONTENT
4_ L

Figure 12. A schematic model for predator feeding behavior. Prey
variables on right, predator variables on left.

 

 

44

prey and environmental characteristics collectively termed ”vulnera-
bility”. It is upon this task of detecting prey that the prey variables
of primary interest in this study are thought to exert their influence.

After detecting a potential prey animal, the predator must next
recognize that animal as an acceptable prey, which depends in part upon
body size and palatability of the prey, and also upon the predator's
preference, which may change with experience. Increased hunger may
cause a predator to relax its restrictions on what prey are acceptable.

When a prey animal has been detected and determined to be
acceptable, it must next be approached and Captured. The success in
these tasks may be a function of the predator's experience and capabil-
ity, and of the experience and behavior of the prey in evading and
defending itself against the predator. Capability of the prey to in-
jure the predator must also be given consideration at this step.

Once a predator has made contact with the prey, it must next
handle it, which has been defined in this model as killing, preparing
(plucking, skinning, etc.), and ingesting the prey. Once the prey is
eaten, a summation index labeled ”stomach contentll is increased, and
the predator assesses its hunger level and whether or not it will con-
tinue to search for prey.

In summary, this model provides a different view of feeding
behavior than that suggested by mathematical models. It allows depic-
tion of the effects of various prey, predator, and environmental

variables upon specific behaviors of a feeding predator.

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