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THE ROLE OF REPRODUCTION AND MORTALITY IN 
POPULATION FLUCTUATIONS OF PEROMYSCUS MANICULATUS 
AND MICROTUS OCHROGASTER ON NATIVE PRAIRIES 



by 



ROBERT G. ROLAN 



B. S., Kansas State University, 1959 



A MASTER'S THESIS 



submitted in partial fulfillment of the 



requirements for the degree 



MASTER OF SCIENCE 



Department of Zoology 



KANSAS STATE UNIVERSITY 
Manhattan, Kansas 



1961 



LD 

1901 

C X 



TABLE OF CONTENTS 

INTRODUCTION 1 

MATERIALS AND .METHODS 5 

RESULTS 8 

Preliminary Studies 8 

Population Fluctuations 23 

Population Structure 30 

Reproduction 32 

DISCUSSION 49 

SUMMARY 63 

ACKNOWLEDGMENT 65 

LITERATURE CITED 66 

APPENDIX OF TABLES OF SIGNIFICANT CHI-SQUARES 72 




INTRODUCTION 

Fluctuations in the numerical populations of small mammals have been 
the subject of intensive study during the past 30 years. Such changes in 
population density typically begin with a moderate growth rate, accelerate 
to an abnormally high rate and end with a "crash" after peak density has 
been reached. The fluctuations of some species of animals have been ob- 
served to be cyclic in character, with peaks and troughs occurring at 
regular intervals. Interest in these changes in density has been spurred 
by economic considerations. First, large populations of mice may cause 
extensive damage to crops due to their feeding habits. Second, such large 
populations may contribute to the increase of predators which may turn to 
livestock when the mice become less plentiful. Third, mice and/or their 
parasites may carry infectious diseases transmittable to man or livestock. 

It was recognized from the outset that these fluctuations are expressions 

of two major factors, natality and mortality; and subsequent studies have been 

primarily concerned with phenomena modifying the intensity of each. The 

earlier investigators studied the causes of population fluctuation so well 

summarized by Hamilton (1937). 

Increased population is fostered by three reproductive 
factors: 

1. An acceleration of the breeding rate; 

2. an increased number of young per litter, and 

3. the lengthening of the reproductive season, which 
allows for greater numbers of litters per year. 

Causes responsible for a decline in numbers of mice may be 
abiotic, such as climatic influences, or biotic, such as 
disease and predation. 

Jenkins (1948) listed factors inflicting death on Microtus . in what he con- 
sidered to be declining order of importance, as predation, climate, shortage 
of food and disease. In giving predation the primary role he was supported 



by Blair (19*8) who based his conclusions on observations of both Microtus 
and Peromvscus . 

Following Selye's (1946) original report on the general adaptation 
syndrome (GAS) and the diseases of adaptation, many workers began to study 
if and how stress might affect population dynamics. Christian (1956) 
studying the house mouse, and Louch (1956), the meadow vole, found decreased 
reproduction and increased mortality in their laboratory populations follow- 
ing the stress of overcrowding. Frank (1957) attributed the decimation of 
wild populations of Microtus arvalis to the exhaustion phase of the general 
adaptation syndrome. 

Chitty (1952), studying Microtus aqrestis . however, could not find that 
weather, disease, overcrowding, food deficiencies, predation, migration, 
change in age structure, season of birth, nor infestation with parasites 
were controlling factors in the mortality that he observed. They might 
better be termed contributing factors. He suggested that: "(l) strife 
during the breeding season resulted in (2) the early death of the young and 
physiological derangement among the adults. (3) The later progeny of these 
adults survived, but (4) were abnormal from birth and thus more susceptible 
to various mortality factors. (5) These constitutional defects, in a more 
severe form, were transmitted to the next generation." Experimental results 
in support of Chitty 1 s theory were found by Clarke (1955) with Microtus 
aqrestis , and by Christian and LeMunyan (1958) with Mus musculus . Christian 
and LeMunyan attributed the weakened physiological state of the progeny of 
crowded mice to insufficient quality or quantity of lactation. This weakened 
condition was persistent for two generations following removal of the stress. 
Such poor lactation, it should be noted, may be related to Louch' s finding 



3 



of aberrant maternal behavior in Microtus pennsvlvanicus under the same 
stress, overcrowding. Several recent field studies have yielded results 
more in accordance with Chitty's theory than with the general adaptation 
syndrome theory. The studies of Jameson (1953) on Peromvscus in California, 
and of Godfrey (1955) on Microtus are not inconsistent with the former. In 
another study, Hoffman (1958) noted that overwintering adult California and 
Montane voles from the peak population of a previous fall do not suffer the 
greatest mortality, as they might be expected to under the GAS theory, but 
it is rather their juvenile descendants that are the least viable. Chitty 
(1960) concluded that it is highly improbable that the action of physical 
factors is independent of population density. The effects of such "density 
independent" events as weather and climate become more severe as the number 
of animals increase and the physiological quality falls. "This hypothesis", 
he says, "overcomes two difficulties often met with in population studies: 
that there is no consistent evidence of (a) the mortality factors that are 
themselves influenced by population density in the manner required by one 
system of thought, or (b) the climatic catastrophes required by other 
systems." 

Numerous factors have been advanced as ones modifying reproductive 
success in small mammals. Asdell and Sperling (1941) studied the relation 
between size of the reproducing female and reproductive success; Baker and 
Ranson (1932) and Whitaker (1940) studied the effect of photoperiod on 
reproduction in Mjcrotyg and Peromvscus. Eskridge (1956) determined the 
effect of cold on male Peromvscus. Beer et al. (1957) and Helmreich (i960) 
were interested in the regulation of reproductive rate by prenatal mortality. 
Others have been concerned with techniques for measuring fecundity and 



reproductive success. Jameson (1947, 1950) found morphological criteria 
of fecundity in the testes of Microtus and Peromvscus . Deno (1937) dis- 
covered that placental scars indicate parousness in female mice. Davis 
and Emlen (1943) and Conaway (1955) investigated the possible use of 
placental scars for estimating litter size. Lauckhart (1957) and Hoffman 
(1958) have suggested that the quality of the available food is one of the 
biotic factors especially conduciva to high reproduction. Most interesting, 
however, in the light of recent emphasis on the general adaptation syndrome 
and intraspscific competition, are the relationships found between popula- 
tion density and reproduction. Hamilton (1937, 1940, 1941) maintained 
that, in the meadow vole, high levels of density were favorable to a high 
rate of reproduction; further increasing the rate of pregnancy, litter size, 
and length of the breeding season. Christian (1956) and Louch (1956), how- 
ever, both demonstrated experimentally that reproductive success declined 
as the population increased. Hoffman (1958) showed this to be true of wild 
populations of Microtus californicus and Microtus montanus . as have Martin 
(1956) and Fitch (1957) with Microtus ochrooaster . The majority of reports 
have upheld this conclusion, but conflicting reports have also been pub- 
lished. Jameson (1953) could find no consistent correlation between popula- 
tion density and reproduction, and Davis (1956) found reproductive rates to 
vary from time to time, but the wild population of Peromvscus observed 
remained essentially unchanged in numbers during the study period. It is 
the intent of this study to determine some of the factors affecting reproduc- 
tion in Peromyscus maniculatus and Microtus ochrooaster in Kansas, and how 
these changes are reflected in the population. 



The term "cycle" has become so closely identified with population work 
that it is often used incorrectly as a synonym for fluctuation. Cole, Rowan, 
Errington, and others (1954) have discussed the cyclic nature of certain popu- 
lation fluctuations from several viewpoints. Cole proposed that such cycles 
could be merely random variations, and gave certain empirical evidence to 
support his claim. The consensus, however, holds the view that cycles in- 
deed occur in some species, but there is no agreement on the controlling 
mechanism. In this paper the use of the term "cycle" will be restricted 
to regularly recurring peaks and troughs of population density, except 
in certain quoted material. 

MATERIALS AND METHODS 

Data for this study was collected by the Kansas Small Mammal Census. 
The K.S.M.C. was organized by Dr. H. T. Gier of Kansas State University in 
1949, independently of, but in general agreement with the principles used 
in the North American Small Mammal Census. Mammals were taken by the trap- 
line method, with two standard, snap mouse traps and one museum special 
placed in a cluster, but located in positions most likely to attract speci- 
mens, with 30 clusters spaced 40 to 50 feet apart. All traps were baited 
with a peanut butter-rolled oats mixture to which DDT was occasionally 
added when insects were likely to remove the bait. Cooperators throughout 
the state (Table 1) selected typical native grass prairie locations for the 
trap-lines. The lines were run twice a year, spring and fall; the spring 
trapping occurring during March and April, and the fall trapping during 
November and December, commencing after the first freeze. Each run was 
maintained for three consecutive nights, and the traps checked, emptied, 
and rebaited each morning. Specimens were frozen and sent to Dr. Gier for 









6 


Table 1. 


Trapping localities, cooperators, and dates of operation, 


1 


County 


: Cooperator 

• 


Institution 

• 
• 


s Dates 
I 


Anderson 


Bill R. Brecheisen 


«■ 


1959-60 


Atchison 


Eugene W. Dehner 


St. Benedict's College 


1953 


Cloud 


Harry C. Duncan 


Concordia High School 


1951-52 


Cowley 


William K. Bunyan 


Kansas State University 


1958-59 


Crawford 


Claude Leist 


Kansas State College, 
Pittsburg 


1951-52 




Ted S perry 




1953-55 




Horace Hays 




1956-60 




H. T. Gier 


Kansas State University 


1956-57 


Decatur 


Dolf Jennings 


Oberlin High School 


1954-56 




Paul Frederick 


Kansas State University 


1953 


Ellis 


Edwin P. Martin 


Ft. Hays Kansas State 
College 


1952-1960 


Finney 


Mickey Penny 


Garden City High School 


1951-56 




Roscoe C. Waldorf 


Garden City Jr. College 


1958-60 


Johnson 


Virgil E. Boatwright 


Shawnee Mission North H.S. 


1953-56 




Dwight L. Spencer 


•• H It II II 


1957-59 


Linn 


Marvin D. Schwilling 


Kansas Forestry, Fish and 
Game Commission 


1956-58 


Lyon 


Ted Andrews and 
Delta Kappa Chapter, 
Beta Beta Beta 


Kansas State Teachers 
College 


1951-60 


McPherson 


Eugene Krehbiel 


Kansas State University 


1958 




Robert G. Bellah 


Bethany College 


1959-60 


Osage 


Dolf Jennings 


Quenemo High School 


1951-52 



Table 1. (concl.) 



County 


1 Cooperator 


Institution 

• 
• 


' Dates 
i 


Republic 


Donald Gier 


Soil Conservation Service 


1958-59 


Riley 


H. T. Gier 


Kansas State University 


1951-60 


Saline 


Ronald R. Clothier 


Kansas Wesleyan University 


1953-54 




William V. Houston 


Salina Jr. High School 


1955-60 


Sedgwick 


Harold D. Swanson 


Friends University 


1951-53 


Shawnee 


Donald W. Janes 


Washburn University 


1958-60 


Smith 


Virgil E. Boatwright 


Smith Center High School 


1952-53 


Sumner 


James K. Maupin 


Wellington High School 


1953-55 



further analysis after weights and measurements were recorded. Data taken 
include weight, total length, tail length, hind foot length, and ear length. 
Female animals were examined for pregnancy and number of embryos, and for 
presence and number of placental scars. A limited number of male animals 
were examined for position, size, and condition of testes by macroscopical 
observation. In all, 48,898 trap-nights were set during the ten-year period 
from 1951 to 1961, resulting in a catch of 1,761 Peromvscus maniculatus 
b£ixdii and P. ipanjculatus nebrascensis. the prairie deer mice, and 467 
fj ' ic r°tMs P c taQqa?ter ophrogastar, the prairie vole. In addition, these 
species: Peromyscus ;eucopus,, Mcrotus pjnetorum . Sjqmodon hisoidus . 
UUk&amm f^loUs , UUkmiUtmm montanus . Peroanathns hisoidus . 
Onychpmys leucpgaster , Dipidpmys ordii, Synaotornvs coooeri . Spermophilus 
tridecimlineat.us , Blarina brevicauda. and Crvptotis parva were trapped in 
lesser numbers. 



The census data from all localities were pooled in order to increase 
the reliability of statistical procedures, and each locality was regarded 
as a random sample of the type of habitat specified. Statistical methods 
as outlined in Snedecor (1956) were used throughout this paper. Specimens 
were identified from descriptions in Hall (1955), and representative study 
skins were prepared and stored in the Museum of Zoology, Kansas State Univer- 
sity. All vernacular names used in this paper are those recommended by the 
American Society of Mammalogists Committee on Nomenclature (Hall, 1957). 

To count placental scars the uterus of each mouse was removed in. toto 
and placed on an index card. When held up to a light the scars were plainly 
visible along the mesometrial border of the uterus. In multiparous females 
two criteria were used to distinguish between sets of scars. Since the 
embryos tend to implant evenly spaced within a uterine horn, spacing of the 
scars was used to differentiate between sets. The intensity of pigmentation 
was used to determine the most recent set. Deno (1937) noted that this pig- 
ment, hemosiderin, gradually fades with age. A small colony of white mice 
was maintained in order to compare estimates of litter size made from 
placental scars with observed litters. 

RESULTS 

Preliminary Studies 

One of the major problems to be solved before an analysis of the 
reproductive data could be made was what constituted a sexually mature 
animal. For the most part, data were incomplete or unavailable on pelage 
patterns and tooth wear condition. A preliminary study was undertaken to 
determine which measurements to use as a substitute for known ages in the 



prairie deer mouse and prairie vole. Plates I and II show the extreme 
variability of tail length compared to body length in both species. It 
was concluded, therefore, that total length would include this additional 
variability if used. Next, weight and body length were compared in a 
similar manner as shown in Plates III and IV. Fitch's (1957) method of 
determining pregnancy by weight change suggested that pregnant females 
might introduce 6 certain amount of extra variability when age was esti- 
mated by weight. The weights of pregnant females are given as small circles 
in Plates III and IV. A great number of these animals tend toward the 
upper extremes of each body length coordinate. Fitch noted that other 
factors may also influence weight, one of the most important being the 
availability of moisture. Hence, body length was the dimension used for 
this study, reducing variability as much as possible. 

Female deer mice and voles were recorded accumulatively on graphs 
according to body length if they showed some indication of sexual maturity. 
The criteria used were the presence of corpora lutea, placental scars; and 
pregnancy. Such arrangement of these data resulted in the normal distribu- 
tions shown in Plate V. One of the characteristics of the normal distri- 
bution is the fact that approximately 95 per cent of the population showing 
the measured characteristic are expected to fall between two standard devia- 
tions on either side of the mean. The means and standard deviations were 
calculated for both species. The mean body length of sexually mature deer 
mice was 88.5 millimeters, and that of the voles 116.5 millimeters. The 
standard deviations were 7 and 9.5 respectively. Two standard deviations 
below the mean of the female deer mice was 75 millimeters. The lower limit 
for the voles was 97 millimeters. These values were taken to be unbiased 





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20 



estimates of the mean body length of females of the two species of rodents 
at attainment of sexual maturity, with a 95 per cent level of confidence. 

Sexual maturity in the males was determined on the basis developed by 
Jameson (1950) of testis length and epididymal condition. He gives the 
minimum length of the sexually active deer mouse testis as 8 mm. Jameson's 
previous report (1947) gives this minimum length of functional testis in the 
prairie vole as 7 mm. Only a small number of males were examined for testis 
condition, and all of these with testis measuring the minimum length or over 
fell well within the range of mature female body length, hence no size dif- 
ference between the sexes could be discovered. The work of Martin (1956) 
and Fitch (1957) suggests that male voles grow at a slightly more rapid rate 
and mature at a slightly greater weight and length. This difference is 
probably not significant for the purposes of this study. 

One of the difficulties inherent in all trapping studies is that 
nestling mice cannot be caught, at least not consistently. An attempt was 
made to estimate how soon young mice become ambulatory and susceptible to 
trapping after their birth. Two litters of deer mice were raised in the 
laboratory and weighed daily. The daily mean weights for these two litters 
are presented in Table 2. The second litter was obtained some days after 
birth and the age was estimated from the date the eyes opened, this being 
11 days post partum. The daily rate of gain varied a great deal between 
the two litters, but both weaned at approximately the same weight, 10 and 
10,6 grams, and at the same age, 19 days. It was concluded that young deer 
mice of three weeks of age or slightly younger are susceptible to trapping. 
Reference to Plate III will show that deer mice of 10 gram body weight are 
frequently caught. 

















21 


Table 2. 


Average weight 


s of two litters c 


>f £. 


maniculatus 


foi 


• the 




first 21 


days 


of life. 










Age (days) 




i Averaae weiqht in i 


srams 


: Litter 1 








Litter 2 


At birth 






1.62 










1 






2.06 










2 






2.74 










3 






3.00 










4 






3.60 








3.29 


5 






4.16 








3.87 


6 






4.46 








4.43 


7 






4.64 








4.95 


8 






4.98 








5.28 


9 






5.50 








5.91 


10 






5.94 








6.23 


11 






6.30 eyes 


open 






6.62 eyes open 


12 






6.64 








7.04 


13 






6.82 








7.43 


14 






7.10 








7.80 


15 






7.42 








7.96 


16 






8.19 








8.40 


17 






8.95 








8.90 


18 






9.13 








9.68 


19 






10.64 weaned 






10.00 weaned 


20 






12.13 








10.65 


21 






14.50 








11.44 


After entering 


1 the ] 


x>pulation, young 


deer 


mice remain 


sexually immature 


for 10 to 


40 days. 


Clark (1958) gives the 


age 


at maturity 


of 


deer mice fe- 


males as 


49 days, a 


nd that of the males as 


59 days. This < 


igrees well with 


Jameson's 


(1953) finding 


of 60 days for ma 


las, 


but not for 


the 


females 


which he 


gives as 35 days 


>. Further compli 


eating this matter, 1 


Blair (1940) 


gave this 


age as 63 


days. 


On the basis of 


the methods used 


1 to 


determine 


sexual ma 


turity it 


was decided to use Jameson's 


estimate of 


35 


days for the 


females* 


It is probable, 


, therefore, that 


femal 


e deer mice 


are 


available as 


juveniles 


for only 


about 


half the time that their male litter mates are. 



22 



Fitch (1957) gave the mean weight of newborn voles as 2.9 grams. 
Young voles began to appear in our trapping samples at 15 grams, and accord- 
ing to Fitch this weight is reached around 20 days of age. Probable age at 
sexual maturity of female prairie voles was estimated by Fitch as being 40 
days. There seems to be great variability associated with age at sexual 
maturity in voles. Fitch estimated the age at mating of some pregnant 
prairie voles as one month. Hamilton (1941) estimated the minimum age of 
mating of Microtus pennsylvanicus females as 25 days, and Greenwalt (1956) 
made an estimate of 14 days for M. californicus . The total length of prairie 
vole females was given as about 130 millimeters which corresponds roughly with 
the body length range between Age Classes I and II as presented in Table 3. 

For the purpose of detecting reproductive differences between animals 
of varying ages the mice were arbitrarily placed in six groups by body length. 
These groups were based on the standard deviations calculated for the dis- 
tributions represented by Plates III and IV. Age Classes II, III, IV, and V 
represent ranges of one standard deviation each. Age Classes I and VI are 
the extremes beyond two standard deviations from either side of the mean. 
These classes and their corresponding ranges, in millimeters, for each 
species are presented in Table 3. 

Table 3. Age classes of Perornvscns maniculatus and Microtus ochmaaster . 



: 



Class I : II : III : 

5 : t 



Peromvscus 74 or less 75-81 82-88 89-95 96-102 103 and up 
Mic r°t" s 96 or less 97-106 107-116 117-126 127-136 137 and up 



23 



Population Fluctuations 

The number of trap nights run in each month of the Kansas Small Mammal 
Census were not consistent from month to month, therefore the absolute 
number of animals caught could not be taken as a direct indication of the 
population density. Moreover, Stickel (1946, 1948) raised some objections 
to trap line censusing for estimating actual numerical densities. Fitch 
(1954) pointed out that trapping results may vary from month to month de- 
pending on the food supply of the animal in question. These trapping re- 
sults are, therefore, presented in the form of a relative index determined 
by dividing the number of animals caught by the number of trap nights set. 
All statistical comparisons of population density are based on the mean 
index of the same month. 

These fluctuations in population density between successive trapping 
dates are presented in Plate VI for Peromyscus and Plate VII for Microtus. 
A general similarity of the fluctuations in these species is obvious. Both 
started in 1951 with peak populations, both declined in April, both recovered 
to again peak in November. Vole numbers declined in December, but the deer 
mice remained at a peak. By March of 1952 the deer mice had declined, but 
voles had reached a low, but significant peak. The voles were unable to 
remain at high densities and by April had dropped quite low, a decline from 
which they were not to recover for four years. Deer mice had, however, 
reached their maximum density of the decade in that April before declining 
to the moderate densities maintained throughout 1953, 1954, and 1955. Deer 
mouse numbers fluctuated sharply through 1956 and the spring of 1957, reach- 
ing very low densities in March and November of 1956, and in March and April 
of 1957. By November of 1957, deer mouse density began to increase again. 





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28 


Voles seem to have 


begun to recover from 


the depression 


of their numbers in 


1956, and 


by April 


1957 they peaked. They subsequently 


declined for 


the 


rest of the year and ensuing winter. In 


1958 both deer 


mice and vol 


es 


reached s: 


Imiiltanamis peak densities in ADril. Microtus 


were able to 


main- 


tain thei: 


r numbers 


throuqh the summer, but Peromyscus went into an immediate 


decline. 


Voles remained at moderate den: 


sities throughoi 


ut 1959, but 


deer mice 


were rather scarce 


in March and November 


of that year. 


Both species 


were at 


quite low 


densities 


in April 1960, but recovered somewhat by November. 


Table 4. 


Population indices of Peromyscus maniculatus 


and Microtus 






ochroqaster cauqht 1951-1960. 








t 
t 


P. 


maniculatus : 


M. ochroqaster ; 


Trap- 




: i 


t 


: 


Year : 


Index 


: Numbers : 


Index : Numbers : 


nights 


March 












1951 


.0711 


33 


.0150 


17 


450 


1952 


.0320 


46 


.0185 


43 


1437 


1953 


.0305 


53 


.0006 


1 


1737 


1954 


.0353 


20 


- 


- 


567 


1955 


.0346 


31 


.0078 


7 


897 


1956 


.0108 


9 


.0144 


11 


837 


1957 


.0119 


10 


.0036 


3 


837 


1958 


.0428 


37 


.0023 


2 


864 


1959 


.0120 


17 


.0142 


20 


1413 


1960 


• 


- 


- 


• 


- 


April 












1951 


.0333 


36 


.0056 


6 


1080 


1952 


.0848 


84 


.0020 


2 


990 


1953 


.0254 


49 


.0031 


6 


1926 


1954 


.0240 


33 


- 


- 


1377 


1955 


.0330 


65 


.0010 


2 


1971 


1956 


.0372 


55 


.0047 


8 


1477 


1957 


.0107 


7 


.0441 


29 


657 


1958 


.0804 


170 


.0201 


40 


1989 


1959 


.0302 


77 


.0086 


22 


2544 


1960 


.0055 


13 


.0038 


9 


2373 













29 


Table 4. 


(concl. ) 












: P. maniculatus 


M. ochre 


qiastar 


: 


: 


: : 


: 




» Trap- 


Year 


t Index 


t Numbers : 


Index : 


Numbers 


: nights 


November 












1951 


.0656 


50 


.0709 


53 


747 


1952 


.0415 


71 


.0023 


4 


1710 


1953 


.0485 


134 


.0011 


3 


2761 


1954 


.0256 


29 


.0009 


1 


1134 


1955 


.0219 


88 


.0047 


18 


4015 


1956 


.0140 


16 


.0044 


5 


1143 


1957 


.0506 


71 


.0085 


12 


1404 


1958 


.0286 


66 


.0217 


50 


2304 


1959 


.0144 


29 


.0124 


25 


2019 


1960 


.0366 


69 


.0095 


18 


1890 


December 












1951 


.0747 


60 


.0049 


4 


810 


1952 


.0207 


18 


.0000 





870 


1953 


.0481 


13 


.0000 





270 


1954 


.0296 


8 


.0037 


1 


270 


1955 


.0514 


37 


.0014 


1 


720 


1956 


.0444 


36 


.0111 


9 


810 


1957 


.0406 


23 


.0018 


1 


567 


1953 


.0348 


108 


.0110 


34 


3099 


1959 


mm 


- 


_ 


_ 


m» 


1960 


* 


m 


- 


m 


mm 


Table 5. 


Peromyscus 


maniculatus. Population index. 


ten year 


monthly 




and seasonal averages. 












t Population 


t Animals 


1 


■ ' 






: index 


t caught 


: 


Trap-nights 


March 




.0283 


256 




9039 


April 




.0366 


589 




16084 


Spring 




.0336 


845 




25423 


November 




.0321 


554 




17238 


December 




.0403 


303 




7416 


Fall 




.0347 


857 




24654 





30 



Table 6. Microtus ochrogaster . Population index, ten year monthly 





and 


seasonal 


averages. 
















t 


Populsti 
index 


on 


• 

• 
« 


Animals 
caught 


i 

s 


Trap-nights 


March 

April 

Spring 

November 

December 

Fall 






.0125 
.0076 
.0090 
.0099 
.0067 
.0090 






104 
124 
228 
171 
50 
221 




9039 
16384 
25423 
17238 

7416 
24654 



Population Structure 



Sex Ratio . There were no significant variations of the total sex ratio 
of either species from the expected It 1 ratio. Peromyscus consistently 
averaged more males than females, and the March totals are significantly in 
favor of the males at the .05 level. Microtus . on the other hand, exhibited 
a significant ratio in favor of the females in April. These mean sex ratios 
are shown in Tables 7 and 8. The possible meaning of these findings will be 
discussed later. 

Age Structure . The ten year mean percentage of the six age classes 
were calculated for each month. These data are presented in Tables 9 and 
10. Monthly age structures are shown by Plates VIII and IX. It should be 
noted that these very seldom formed the pyramidal structure theoretically 
expected. The general picture presented by the ten year mean age distri- 
butions, for both species, is that of a relatively old population in March 
becoming increasingly younger, as a group, through the breeding season; then 
aging again in December. The ten year mean percentages of juvenile deer mice 
in April and November are high and about equal, but by far the greatest pro- 
portion of juvenile voles are present in November. This would indicate a 















31 


Table 7. 


Peromvscus maniculatus. 


Sex ratios 


. Ten year monthly and 




seasonal averages. 












1 




1 






* 
• 




t 


Sex ratio 


I 




Males 


: Females 




1 




: 






: 


March 




61 






130 


84 


April 




56 






262 


206 


Spring 




58 






392 


290 


November 




53 






235 


205 


December 




58 






151 


109 


Fall 




55 






386 


314 


Table 8. 


Microtus o 


chroqaster. S 


lex ratios. 


Ten year 


monthly and 




seasonal a 


verages. 












• 




t 






t 




t 


Sex ratio 


j 




Males 


: Females 




: 




| 








March 




56 






52 


41 


April 




36 






39 


68 


Spring 




46 






91 


109 


November 




53 






76 


67 


December 




36 






15 


27 


Fall 




49 






91 


94 


Table 9. 


Peromyscus 


maniculatus. 


Age structure. Ten 


year monthly 




average distribution in 


per cent. 








| 


: : 


■ 




: i 






« I 


: II t 


III j 


IV 


: V t 


VI 




• 
• 


• * 
• 


i 




: : 




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12 


18 


30 


30 


8 


2 


April 


20 


23 


38 


17 


2 


1 


November 


17 


22 


35 


21 


5 


less than 1 


December 


7 


30 


29 


23 


10 


1 





32 



Table 10. Microtus ochroqaster . Age structure. Ten year monthly 
average distribution in per cent. 



s : : t 

II '. III : IV t V : VI 
: t : l 



March 7 22 31 19 18 3 

April 9 23 38 18 11 1 

November 27 7 36 28 2 

December 8 28 18 41 5 



somewhat later peak of breeding for Microtus . 

No correlation could be established for either deer mice or voles be- 
tween the make up of the age structure and the density of the population. 
For example, in March of 1951 deer mouse population was high and so was the 
percentage of juveniles or Class I mice. In November of the same year the 
population density was great, but the percentage of juveniles was signifi- 
cantly low. 

Frequently there is great similarity between the age structures of the 
deer mice and voles. For instance, the above cited example applies also to 
voles. April 1955, November 1958, and April 1959 compare for low percentages 
of juveniles. November 1958 and March and April of 1959 are notable for their 
comparison of large percentages of old adult animals. This would imply that 
similar forces are working on the two species and they are responding in the 
same way; however, there are equally notable divergences such as December 
1956 when juvenile voles were plentiful, but juvenile deer mice were not. 

Reproduction 

Two aspects of the reproductive history of these rodents could be 
determined by dissection, the percentage of the population pregnant or 





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37 



recently pregnant and the litter size. Two others, prenatal mortality and 
litter succession could not be so determined for this study. 

The monthly percentages of deer mice and voles pregnant or having 
placental scars are given in Plates X and XI. These were statistically 
compared with their ten year monthly means that are to be found in Tables 
11 and 12. The greatest percentages of pregnancy were found in the spring 
months in both deer mice and voles. The percentages of females with scars 
increased steadily, on the average through the year in both species. No 
correlation could be established between the intensity of reproduction and 
the litter size. Similarly, no correlation could be found between reproduc- 
tion and resulting population changes. For example, deer mouse reproduction 
was slightly below average in March 1951. The April population declined 
and was extremely low in juvenile content. In March 1952 reproduction 
was even lower, but the population increased to peak density with a moderate 
proportion of juveniles. 

Litter size was determined by two different methods, embryo counts 
and placental scar counts. Neither of these are counts of the actual num- 
bers of young born, and prenatal mortality may alter the accuracy of these 
estimates somewhat. Like other population parameters, prenatal mortality 
undoubtedly varies in intensity with external factors. This has been demon- 
strated experimentally in the deer mouse by Helmreich (i960). Hamilton (1937) 
maintained that: 

Embryo counts of necessity do not give an exact criteria 
of the young produced. Resorbtion of embryos frequently occurs, 
especially in the period approaching the peak of a cycle, or at 
its culmination. Young are occasionally stillborn in captivity, 
and this undoubtedly occurs also in the wild state. Yet suf- 
ficient embryo counts made over a period of years do indicate 
whether the number of young per litter is increasing or decreas- 
ing. 













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42 



Table 11. Peromyscus maniculatus . Ten year monthly averages of per 
cent pregnant and per cent with placental scars. 



% 

pregnant 



March 
April 
November 
December 



33 
34 

4 




Number 
pregnant 



22 

56 

5 





with scars 



: Number t Sample 
i with scars : size 



21 
32 

34 
34 



14 
53 
39 
20 



66 
164 
116 

59 



Table 12. Microtus ochroaaster . Ten year monthly averages of per 
cent pregnant and per cent with placental scars. 



% 

pregnant 



March 
April 
November 
December 



40 

58 

5 

4 



: Number t % t Number : Sample 

: pregnant : with scars : with scars ; size 



14 

34 

4 

1 



3 
10 
16 

30 



1 

6 

12 

8 



35 
59 
77 
27 



As previously stated, no consistent relation was observed between de- 
crease in reproduction, as indicated by decrease in litter size, and popula- 
tion decline. Embryo counts are, however, subject to the same random error 
throughout, and are hence good relative estimates of actual litter size at 
any given time. 

Some objections have also been raised against the use of placental scars 
in estimating litter size, notably by Davis and Emlen (1948), who observed 
that placental scars accumulate and remain in the uterus with each succeed- 
ing litter. They apparently made no effort to differentiate between sets of 
scars. For the purposes of the present study only the most recent scars 
were counted, determined by the intensity of pigmentation and the spacing 
within the uterine horn. This is not difficult to do with a little experience. 



43 



Plate XII shows the method of counting placental scars. The litter size 
estimates made by this method compared very closely with those made by 
the embryo count method, as is shown in Tables 13 and 14. The composite 
mean embryo count for £. maniculatus was 4.29 and the mean placental scar 
count was 4.38. The mode was 4 with extremes of 1 to 7. Deer mouse litter 
size has variously been estimated at 3.05 (Svihla, 1932), 5.38 (Coventry, 
1937), 4.60 (Jameson, 1953), and 4.51 (Beer et al., 1957). M.. ochroaaster 
embryo counts gave a mean litter size estimate of 4.19, and scar counts 
one of 4.17. The modal number was again 4 with extremes of 1 to 9. The 
upper extremes for both species are based only on embryo counts. Whether 
or not these many young can be weaned undoubtedly depends on mammary 
function. Jameson (1947) estimated prairie vole litter size at 3.4, 
Martin (1956) at 3.18, and Fitch (1957) at 3.37. These counts were made 
from live-trapping data and are, therefore, probably somewhat lower due to 
mortality acting in the time span between the two methods. 

Tables 13 and 14 also illustrate the principle first shown by Asdell 
and Sperling (1941) and subsequently confirmed by other workers, that mean 
litter size tends to increase with the age of the mother. However, there 
seems also to be a slight decrease in mean litter size in the oldest mice. 
There is no mention of this apparent decrease in the literature. Also in 
agreement with the findings of Asdell and Sperling, the percentage of female 
deer mice and voles that are pregnant or have placental scars generally in- 
creases with age. These data are shown in Table 15. 

An experiment was undertaken to check the validity of placental scar 
counts. Ten female white laboratory mice with litters of known size were 
examined at various lengths of time after the birth of their young, up to 
45 days, for placental scars. The results are shown in Table 16. Some of 



EXPLANATION OF PLATE XII 



Fig. 1. Ventral view of the reproductive tract of a female 
Peromvscus maniculatus. in situ. Intestines, 
stomach, liver, and some other abdominal organs 
have been removed. Approximately 4X. 

R-Kid - right kidney 

OV - ovary 

C - colon 

UT - left uterine horn 

VAG - vagina 

UB - urinary bladder 



Fig. 2. The same reproductive tract removed from the mouse 
to show placental scars. This mouse was approxi- 
mately three weeks post-partum, having had an 
observed litter of five young. Five placental 
scars are present. Approximately 4X. 

OV - ovary 

UT - uterine horn 

PS - placental scar 

VAG - vagina 

MM - mesometrium 



US 




VAG 



UB 



Fig 1 



OV 







1-PS 




PS 



UT 




\S-PS 






*v 



PS . 



MM 



T 



Fig 1 



46 



Table 13. Peromyscus 


i maniculatus. 


Ten year average 


litter sizes. 




• 
• 


Average 


: Number 


: Number 


Age class 


i 


litter size 


: of litters 


t of embryos 


Embrvo counts 










I 




3.0 


2 


6 


II 




4.3 


7 


30 


III 




4.3 


22 


94 


IV 




4.4 


33 


144 


V 




5.3 


4 


21 


VI 




1.0 


1 


1 


Scar counts 








Number of scars 


I 




2.5 


2 


5 


II 




4.4 


12 


53 


III 




4.5 


36 


163 


IV 




4.5 


49 


220 


V 




4.1 


4 


21 


VI 




3.3 


3 


10 


Combined embryo 


and 






Number of embryos 


scar counts 




2.8 


4 


and scars 


I 




%rf * a** *».' \f %j ^ ,j 

11 


II 




4.4 


19 


83 


III 




4.4 


58 


257 


IV 




4.4 


82 


364 


V 




4.3 


19 


82 


VI 




2.3 


4 


11 













47 


Table 14. Microtus 


ochroqsster. 


Ten year average litter 


sizes. 


I 


Average 




1 


Number 


t Number 


Age class t 


litter size 




i 


of litters 


: of embryos 


Enjbryo counts 












I 















II 


3.8 






8 


30 


III 


3.9 






15 


58 


IV 


4.9 






10 


49 


V 


3.9 






7 


27 


VI 


4.0 






1 


4 


Scar, counts 










Number of scars 


I 















II 


3.0 






1 


3 


III 


4.1 






7 


29 


IV 


4.7 






12 


56 


V 


3.0 






4 


12 


VI 















Combined embryo and 










Number of embryos 


scar counts 










and scars 


I 















II 


3.7 






9 


33 


III 


4.0 






22 


87 


IV 


4.8 






22 


105 


V 


3.5 






11 


39 


VI 


4.0 






1 


4 


Table 15. Per cent 


animals pregnant 


or having placental 


scars in each 


age class 


> • 










Aqe Class 


I 


: II 


• 


III 


: IV : 


V : VI 


Peromyscus 8 


23 




41 


75 


64 80 


Microtus 


21 




42 


53 


70 50 





48 



Table 16. Comparison of placental scar counts to litter counts in 

iau& musculus. 

























• 








Mouse No. 








• 




* 1 


2 


3 


4 


5 6 7 


8 


9 


in 


t Total 


Scars 
Litter 


11 
11 


2 

1 


9 
11 


11 

10 


6 2 12 
8 1 12 


7 
8 


11 
11 


5 
4 


76 
77 



these females contained scars of previous pregnancies, but these were dis- 
tinguished by pigmentation and spacing, and were not used in estimating the 
size of the litter. Several explanations may be offered for the discrepancies 
that appear between the two counts. Where litter size is less than the scar 
count, as is the case in Mouse 2, 4, 6, and 10, the discrepancy may be due 
to the mother eating one or more of her young before the litter was counted, 
or it may be due to intrauterine loss after implantation. Conaway (1955) 
has shown that placental scars are not formed in rats prior to the seventh 
day of pregnancy, but after the ninth day scars of embryos dying before 
term are indistinguishable from those of survivors. More difficult to ex- 
plain are deviations in the other direction. Liu (1954) has occasionally 
found fusion of two placentae in deer mice and such placentae would, of 
course, leave but one scar. It seems unlikely, however, that this occurs 
frequently enough to explain the occurrence of more young than scars in 
mice 3, 5, and 8. There remains, of course, the possibility of error in 
counting. Whatever the cause of these deviations may be, they apparently 
cancel one another out in a series of counts. That is, they are random 
errors. This, and the emperical proof provided by the similarity in embryo 
and scar counts in the wild populations, should establish the validity of 
placental scar counts. 

No significant seasonal differences in mean litter size could be found 
in either the prairie deer mouse or prairie vole. Data presented by Jameson 



49 



(1947) and Fitch (1957) suggested thst such might be the case. However, the 
major seasonal variations shown in those papers were in months other than 
those sampled by the Kansas Small Mammal Census. Deviations from expected 
litter size are shown in Plate XIII for each month. These data were found 
by subtracting from the actual mean litter size a hypothetical mean litter 
size of a sample of the same age composition with individual litter sizes 
based on the means shown in Tables 13 and 14; 

Mean precipitation for each month, 1950-60 was taken from the U. S. 
Department of Commerce Climatological Data for Kansas. These data are 
shown as quarterly distribution, and accumulatively, for each year of the 
study in Plate XIV. They indicate a five-year drought extending from 1952 
through 1956. Comparison of the annual precipitation to the population 
fluctuations of deer mice and voles shows a general positive correlation. 

DISCUSSION 

The most striking correlation between population density and environ- 
ment is seen in the parallel of population fluctuations and amount of pre- 
cipitation. This correlation has been noted in Microtus ochroaaster by 
Martin (1956), and in Microtus and Peromvscus by Bradshaw (1956). The 
effect of rainfall is apparently through the plant growth that it initiates, 
for Martin found that changes in vole density follow the curve for the rate 
of growth of grasses. The emphasis here should be on the rapid growth of 
plants, for the preferred food of voles is the tender, growing stems. 
Martin, therefore, stated that mixed vegetation constitutes a more desirable 
habitat for voles, due to the variety of growing seasons represented, than 
do more homogenious vegetation. In this connection, it should be noted that 







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EXPLANATION OF PLATE XIV 

Mean quarterly distribution of precipitation in Kansas recorded 
accumulatively by years. 



PLATE XIV 



53 



X ' 

i 






■ i 
■ 

■ i 



■ ■ 






x> 



! 



■■•■ 



I 



t* — "f 

1954 1955 



>■ \ 

■si 



I 



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I ■ 
■ 

J 



■: 



i 






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•r — *r 

1956 1957 



I 

. . 

x 

1 






i ■ 

r ■ 



■ ■ M. 






QC^DEC 

JULY-SEPT 

APR-JUNE 

JAN-MAR 



v ' v* — *r 

1958 1959 I960 



r — t 

1952 1953 




54 



Johnson (1926) maintained that the entire biotic association, rather than 
any single agent, determines the distribution of Microtus ochroaaster . 
Interestingly, Hoffman (1958) observed that the height of the reproductive 
season for California and Montane voles corresponded with the peak growing 
seasons of grasses in their respective environments. He thought there 
might be some relation between grass protein content and reproduction. 

p erorc)Y§cy? roaniculatus differs from the vole in food habits in the 
relative amounts of leafy vegetation and seeds eaten, deer mice preferring 
the fruiting parts (Jameson, 1952; Williams, 1959). Following unfavorable 
conditions, the availability of preferred food occurs some months later for 
deer mice than for voles, the time it takes for the seed crop to mature. One 
would expect, therefore, that any response of deer mice to precipitation 
would be delayed. This expectation was met following the drought of the 
fifties. Microtus responded almost immediately to the increased precipitation 
in 1957 while the response of deer mice did not reach full expression until 
a year later. 

Another difference observed throughout the ten year period is the 
difference in relative density between deer mice and voles. Voles remained 
at consistently lower densities than deer mice, and during certain parts of 
the drought approached or reached local extinction. Apparently the high 
prairie habitat is not as favorable to voles as it is to deer mice. Dice 
(1922), Black (1937), Goodpastor and Hoffmeister (1952), and Martin (1956) 
all note that the prairie vole needs a relatively moist environment, although 
not as humid as that required by the meadow vole, M. Pennsylvania.!* . The 
vole needs fairly dense vegetation in which to construct runways that will 
provide effective protection against raptorial birds and other large predators. 
Another moisture need of voles is mentioned by Jameson (1947). Voles must, 



55 



apparently, dig their underground tunnels and chambers when the soil is 
moist. Numbers of voles may well be limited, in part, during dry periods 
by shortage of burrows and their inability to construct more in dry soil. 
Prairie voles are much more plentiful in riparian situations (Jameson, 
1947; Martin, 1956; Fitch, 1957), and remain available in such places dur- 
ing drought as long as water is also available. These areas undoutv 
serve as reservoirs for the repopulation of prairie grassl 
prolonged dry weather. Such repopulation is accomplished 
successive generations of juvenile voles (Martin, 1956) until the available 
habitat is filled. 

Precipitation may not be of an entirely beneficial nature. Martin 
(1956) suggested that the heavy rainfall in the spring of 1951 destroyed 
the infant voles. This would not account for the sudden drop in population 
density observed in both Microtus and Peromvscus in that spring, for such 
young animals do not contribute to the trapping data. Analysis of the 
percentage of juveniles in April, 1951, indicates that young voles might 
very well have been killed by the excessive rainfall, but deer mice cer- 
tainly were not greatly affected. This observed population drop can be 
attributed only to mortality among the adults. 

Snow-cover during the winter may also have an effect on the population. 
Both MiprotVg (Martin, 1956) and Peromvscus (Dunmire, 1960) store food for 
the winter. Snow-cover, therefore, would not be particularly harmful, at 
least when it does not remain too long. Dunmire (i960) measured the tem- 
perature of deer mouse nests beneath snow-cover, and found that the tem- 
perature seldom fell below freezing, even when the air temperature dropped 
to well below 0° Fahrenheit. The experimental work of Howard (1951), 



56 



Sealander (1951, 1952), Eskridge and Udall (1955), and Eskridge (1956) 
indicated that deer mice are quite capable of withstanding prolonged 
periods of freezing temperatures so long as food and nesting material were 
available. Huddling together of two or more mice also reduced loss of body 
heat. Inferentially, voles are equally capable of withstanding low temper- 
atures; theoretically even more capable, due to their greater size and 
smaller extremities. Eventually, however, food reserves are bound to be 
depleted if snow-cover remains long enough. Tunneling through the snow 
may provide a little more food, but certainly with hardship for the mice. 
The reduction of deer mouse and vole populations over the winter of 1959-60 
may have been due to prolonged snow-cover with resulting cold starvation of 
the mice. 

The apparent sex ratios observed in deer mice and voles present another 
interesting contrast. Consistently more male than female deer mice were 
trapped in each sample month during the decade. In March this difference 
is statistically significant. One must conclude that there actually were 
more males present than females, or some non-random factor influenced the 
trapping. There is no evidence to indicate that there was differential 
survival of the sexes during the winter; thus the first possibility must 
be discarded. Male deer mice may have larger home ranges than females. 
Blair (1943) found this to be so in P. maniculatus gracilis , as did 
Williams (1955) with P. maniculatus rufinus . Clayton (1952) was unable 
to detect any such difference, but since his study was done during the 
middle of winter a different situation may exist then than that of the 
breeding season. Quite possibly then, larger home ranges exist during 
the sample months, accounting for the apparent difference in sex ratios. 



57 



The especially large difference found in March might be explained as in- 
creased activity of males within their home ranges, or invasion of the home 
ranges of other mice in the search for mates at the beginning of the breeding 
season. Voles, on the other hand, did not exhibit an apparent sex ratio in 
favor of the males in March. This does not necessarily mean that male voles 
are not highly active during the breeding season. The peculiar habit of 
building runway systems, and the sharing of them by neighboring animals 
would tend to randomize the trapping results. A large group of voles may 
use a single runway system, and an individual vole will use several adjacent 
systems (Martin, 1956; Fitch, 1957). The especially low proportion of male 
voles in April may be explained by Frank's (1957) "condensation potential". 
He observed high mortality of male Microtus arvalis and resultant com- 
munities of females and shrinkage of home ranges at peak population densities. 
Such mortality was due, directly and indirectly, to aggressive behavior of 
the females, usually resulting in the males being driven away. These males 
were then more susceptible to predation. Frank thought this phenomenon 
delayed for a time the inevitable decline of the population, and termed it 
"Verdichtungspotential". The two April peaks observed in the Kansas Small 
Mammal Census, 1957 and 1953, were quite low in males, thus influencing the 
ten year average. Add to this the normal high antagonism of nursing female 
voles toward males observed by Fitch (1957) and another cause of low sex 
ratio during the breeding season can be perceived. Apparently, then, dif- 
ferent forces are influencing the observed sex ratios of deer mice and voles; 
the former being due to the larger home ranges of the males, and the latter 
to antagonistic behavior on the part of the females. 



58 



The annual reproduction of a species must be analyzed for three main 
parts, as noted by Hamilton (1937); breeding rate, litter size, and length 
of breeding season. The latter was more accurately termed "litter succession" 
by Frank (1957). In the early years of rodent population study it was 
thought that multiplication of the three terms would give a good estimate 
of natality. Several authors have actually presented formulae for this pur- 
pose. It is now known that it is not quite as simple as this for it is quite 
difficult to define the three terms. Two factors enter to complicate the 
picture; the age and the physiological state of the population as a whole. 
It is now well known that the age of a mouse bears a direct relation to the 
frequency with which it becomes pregnant, and the size of its litter, both 
of these increasing with age (Asdell and Sperling, 1941; Beer, et al., 1957; 
Jameson, 1947; Fitch, 1957). The Kansas Small Mammal Census data indicate 
that there may be a slight decline in the reproductive potential of mice 
in the oldest group. Obviously, a shift in the age composition of a popula- 
tion may introduce a large error into any calculations; such shifts are 
constantly occurring and are pronounced from season to season. 

The physiological state of the mice may also introduce error. Christian 
(1956), Christian and LeMunyan (1958) with house house, Louch (1956) with 
meadow voles, and Helmreich (i960) with the prairie deer mouse have shown 
experimentally that stress, in these cases overcrowding, reduces the natality 
by increasing intrauterine mortality. When complete loss of litters occurs, 
the effective breeding rate is reduced, and when part of a litter is lost, 
of course, litter size is reduced. 

Litter succession also presents several problems. First of these 
is the length of the breeding season. This may be partly under the control 
of photoperiod, which is relatively constant from year to year in a given 



59 



locality (Baker and Ranson, 1932a, b; Whitaker, 1940), but certain other 
factors must influence breeding season too. There is clear evidence that 
the breeding season of Peromyscus may be prolonged in certain years (Brown, 
1945; Beidleman, 1954). Beidleman found such late breeding to be correlated 
with above normal temperatures. Voles may even breed throughout the entire 
winter in some years, although at a greatly reduced rate (Martin, 1956). 
In both deer mice and voles, the factors which suppress breeding appear to 
act mainly on the females. Jameson (1947) found this to be so for Micrptus . 
as did Howard (1950) for JLeiaajt&CJis. The Cauda epididymi of mature males 
remained swollen and filled with motile sperm throughout the winter even 
though the testes may have been in the abdominal position. Eskridge (1956) 
has shown experimentally that freezing temperatures are not sufficient to 
inhibit the fecundity of male deer mice provided they have adequate food 
and nesting material* 

Secondly, mice do not reproduce at a maximum or constant rate through- 
out the breeding season. Female mice may become immediately pregnant after 
the birth of a litter due to a short post-partum estrus (Jameson, 1953; 
Fitch, 1957). The gestation period of such a pregnant, lactating mouse 
is several days longer than usual, apparently due to a delay in implantation 
while the embryos are temporarily arrested at the blastula state and float 
free in the lumen of the uterus (Kirkham, 1916). Following implantation 
development proceeds at a normal rate. No measurement of the frequency of 
post-paruous breeding has ever been made for any species. Hamilton (1937) 
believed that post-parous breeding was more frequent during high populations. 
Moreover, one must also allow for the summer oestival drop-off of breeding 
which occurs in deer mice (Blair, 1940; Jameson, 1953) and prairie voles 



60 



(Fitch, 1957). Thus the breeding season usually has two peak parts, vernal 
and autumnal, with lesser gradations of breeding intensity in between. 
Obviously, no constant can be found here for use in estimating annual 
natality. Estimates of natality can be made for specified times, and trends 
may be indicated when the population is sampled at frequent intervals, but 
these can be expressed only relatively, not in absolute numbers of young 
produced. It is clear from the Kansas Small Mammal Census data that natality 
is not necessarily reflected in corresponding changes in population density 
of either deer mice or voles. 

Various conditions have been advocated as mortality factors causing 
decline in rodent populations, these generally being infectious disease, 
predation, food supply, weather, and pathological hormone imbalances. Pre- 
viously, the hypothesis most consistent with the facts available was that 
overcrowded rodent populations were decimated by epidemics, due to the 
increased probability of infection. Investigation by Chitty (1954) showed 
that murine tuberculosis was not necessarily associated with population 
decline in voles, and may be epidemic in thriving populations. Bradshaw 
(1956) could find no evidence of tularemia or parathyphoids in Kansas 
rodents during a population low by serological techniques. 

Predation also has been credited with a controlling part in population 
processes. Situations occur where the density curves of a predator and 
its primary prey fluctuate in sequence with one another. The key question, 
of course, is does the predator destroy its food supply or is the decline 
of the prey due to some other reason. The observations of Chitty (1955), 
Godfrey (1955), and Lockie (1955) indicate that recurrent declines in vole 
numbers occur whether the pressure of predation is heavy or light. 



61 



The nature of rodent plagues such as that occurring in Oregon in 
1957, has suggested the possibility that mice may consume their whole food 
supply and be faced with subsequent starvation. Summerhayes (1941) could 
find no evidence that Microtus aorestis destroyed its food supply to the 
extent that it would seriously limit their numbers. This does not mean 
that voles cannot do serious agricultural damage. The other possibility 
is that the food supply is ruined by poor weather, and thus the mice de- 
cline. Evidence presented by the Kansas Small Mammal Census seems to 
implicate this latter possibility. It is suggested that the latter would 
be the usual pattern inasmuch as extremely high populations are relatively 
rare. Certainly there has been no plague of deer mice or voles in Kansas 
during the past ten years. It is likely that voles would be much more 
seriously limited by such failure of vegetation than deer mice, since the 
former are less adaptable to a wide range of environmental conditions 
(Jameson, 1955). The extreme low numbers of voles during the drought of 
the fifties, while the deer mice remained at moderate numbers, suggests that 
this is true. 

It is well documented that hormonal imbalances occur in many species 
of rodents under adverse environmental conditions. The main difficulty in 
the GAS theory is that mortality due to hypoglycemic shock has not been 
satisfactorily demonstrated in the field, or in caged populations of mice 
and voles for that matter. Christian (1950), Louch (1956), and Helmreich 
(1960) have demonstrated that a reduction in natality results from the stress 
of overcrowding. Louch, however, could not produce a numerical decline of 
adults in his experimental population of voles, although intraspecific compe- 
tition and symptoms of adrenal hyperactivity were high. Young voles in this 



62 



experiment were frequently killed or neglected by their mothers, suggesting 
that prolactin output may have been affected. Such a view would certainly 
agree with Christian and LeMunyan's (1958) hypothesis on the effect of 
milk quality or quantity on the progeny. Both of these views concur with 
that of Chitty (i960). Also of interest, Lindeborg (1950) has observed 
that pregnant female deer mice require better than twice as much water as 
others. Here is a factor that quite obviously might have a retarding 
effect on lactation during droughts such as the one observed in Kansas in 
the fifties. The idea that responses of rodent populations to mortality 
factors is modified by prenatal or neo-natal influences fits the data 
better than any other hypothesis. The varying responses of Kansas deer 
mice and voles cannot be explained by the GAS theory alone. The decline in 
vole numbers during the 1957 breeding season, and the decline of deer mice 
during the 1958 breeding season are not due to adverse climate, nor GAS, 
as severe mortality would be expected at the onset of winter or some other 
"catastrophic" event. Reproduction at these times was sufficient, and 
adult mortality did not seem to be especially severe; "senile" mice con- 
stituted s fair proportion of the population. The decrease in numbers must 
have been due to the failure of younger mice to survive in sufficient numbers 
to replace normal loss. The possibility that intraspecific strife between 
the parents of these young served as a teratogenic agent may have some 
credence. 

Thus the factors that modify death rate are also the ones modifying 
birth rate, by either decreasing ovulation or increasing intrauterine 
mortality. The fact that the same mortality factor does not always have 
the same effect, or any effect on the population density at all, is due to 



63 



the physiological state of the individual mice. No matter how many possible 
deaths a mouse escapes, one must eventually be the last. The previous ex- 
periences of that individual certainly play a major role in determining 
its susceptibility to the next crisis. Therefore, Errington's (1946, 1956) 
principle of population intercompensations is not strictly applicable. The 
death of one animal does not necessarily contribute materially to the sur- 
vival of the remaining in physiologically deranged populations, for the 
effect of that animal lives on for a time. 

The Kansas Small Mammal Census does not indicate that either Peromyscus 
or Microtus undergo cyclic population changes in this state. The accumula- 
tion of data over several more decades will be needed to clear this point. 

SUMMARY 

A statistical analysis of population characteristics and reproduction 
of Peromyscus maniculatus , the prairie deer mouse, and Microtus ochrogaster . 
the prairie vole, from random samples of the high prairie habitat in Kansas 
from 1951 to 1960 was made. Certain methods used were critically examined. 
In the absence of known age, some available measurement had to be used to 
estimate age. Total length and weight were found to be highly variable, 
hence body length was the measurement used. Six age classes were established 
on the basis of body length, with the first denoting immature animals. The 
use of placental scars in estimating litter size was investigated by comparing 
observed litters with scar counts in laboratory mice, and emperically by 
comparing embryo counts and scar counts of deer mice and voles. Such counts 
were found to be unbiased estimates of litter size. 



64 



Population structure, sex ratio, and age composition constantly changed. 
Deer mice males were found to average above the 1:1 expected ratio in March. 
This apparent high sex ratio was explained as being due to increased activity 
of the males at the start of the breeding season. Conversely, Microtus males 
averaged quite low in April. Since several April populations were very dense 
in numbers, this sex ratio was explained on the basis of the condensation 
potential of voles. Age structure, too, was fluid. On the average, popula- 
tions of both species were rather old in March, became progressively younger 
during the breeding season, and aged again by December. 

No consistent correlation could be found between reproductive rate and 
population changes. Similarly, no consistent relation between the age 
composition, or population changes, and the reproductive rate was discovered. 
Deer mouse and vole fluctuations from 1951 to 1958 paralleled the annual 
precipitation. It was concluded that mortality due to the drought was the 
major factor limiting population growth, possibly obscuring other factors 
operative during years of more normal precipitation. The inconsistencies 
of the various statistics at different times suggests that susceptibility 
to mortality factors varies from time to time, dependent on the physiologi- 
cal state of the animals. 



65 



ACKNOWLEDGMENT 

The writer wishes to express appreciation to his major professor, 
Dr. H. T. Gier, for access to the Kansas Small Mammal Census data, and 
for thought-provoking guidance and criticism of this study. Thanks are 
also due to all of the cooperators in the Census and their assistants. 
He wishes to thank his father-in-law, Dr. Maynard S. Johnson, and his 
father, Lt. Col. G. T. Rolan, for the financial aid that has permitted 
the completion of this paper. Finally, appreciation is due his wife, 
Caroline, for invaluable hours of help in arranging the data, and 
numerous other secretarial chores. 



66 



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Baker, J. R., and R. M. Ranson. 

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1958. 



67 



Chitty, Denis. 

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1952. 



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Influence of numbers on reproduction and survival in two experimental 
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Cole, LaMont C. 

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Conaway, C. H. 

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Davis, D. E. 

A comparison of natality in white-footed mice for four years. Jour. 
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Davis, D. E., and J. T. Emlen. 

The placental scar as a measure of fertility in rats. Jour. Wildl. 
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68 



Deno, R. A. 

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Some factors affecting the distribution of the prairie vole, forest 
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Dunmire, William W. 

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Errington, P. L. 

Predation and vertebrate populations. Quart. Rev. Biol. 21:144-177. 
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On the hazards of overemphasizing numerical fluctuations in studies 
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Factors limiting higher vertebrate populations. Science. 124:304-307. 
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Relationships between environmental stress as represented by freezing 
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Fitch, H. S. 

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1954. 



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The causality of Microtine cycles in Germany. Jour. Wildl. Mgmt. 
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Godfrey, G. K. 

Observations on the decline in numbers of two Microtus populations. 
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69 



Goodpastor, W. W. t and D. F. Hoffmeister. t^62-V71. 

Notes on the mammals of eastern Tennessee. Jour. Mamm. 33:362 371. 



1952. 



Greenwald, G. S. 



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Vernacular names of North American mammals north of Mexico. Univ. 

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Life and 'habits of field mice. Sci. Monthly. 1:425-434. 1940. 

Reproduction of the field mouse MJjcrotVS penns , ylvar4cus (Ord.) 

Cornell Univ. Agric. Exp. Stat. Memo 237:3-22. 1941. 

^Tegulation'of reproductive rate by intrauterine mortality in the deer 
mouse. Science 132:417-413. 1960. 

H ° ff The'rolfof reproduction and mortality in population fluctuations of 
voles (Microtus). Ecol. Monogr. 28(1): 79-109. 1958. 

H ° Wa Jint!r fecundity of caged male white-footed mice in Michigan. Jour. 
Mamm. 31(3): 319-321. 1950. 



Relation' between low temperature and available food to survival of 

small rodents. Jour. Mamm. 32:300-313. 1951. 

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Determining fecundity in male small mammals. Jour. Mamm. 31 (4): 
433-436. 1950. 



70 



Food of deer mice, Perotnyscus maniculatus and Peromyscus boylei in 
the northern Sierra Nevada, California. Jour. Mamm. 33:50-60. 1952. 



Some factors affecting the fluctuation of Microtus and peromvscus . 
Jour. Mamm. 36:206-209. 1955. 

Jenkins, H. 0. 

A population study of meadow mice ( Microtus ) in three Sierra Nevada 
meadows. Proc. Calif. Acad. Sci. 26:43-67. 1948. 

Johnson, Maynard S. 

Activity and distribution of certain wild mice in relation to biotic 
communities. Jour. Mamm. 7:245-277. 1926. 

Kirkham, W. B. 

The prolonged gestation period in suckling mice. Anat. Record. 
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Lauckhart, J. Burton. 

Animals cycles and food. Jour. Wildl. Mgmt. 21(2): 230-234, 1957. 

Lindeborg, Robert G. 

An adaptation of breeding Peromyscus maniculatus bairdii females to 
available water, and observations on changes in body weight. J. Mamm. 
31(1): 74-73. 1950. 

Liu, T. T. 

Monozygotic twinning in Peromvscus maniculatus . Jour. Mamm. 35:449-450. 
1954. 

Lockie, J. D. 

The breeding habits and food of short-eared owls after a vole plague. 
Bird Study 2:53-69. 1955. 

Louch, C. D. 

Adrenocortical activity in relation to the density and dynamics of three 
confined populations of Microtus pennsvlvanlcufi . Ecology 37: 701-713. 
1956. 

Martin, E. P. 

A population study of the prairie vole ( Microtus ochrogaster ) in 
northeastern Kansas. Univ. Kans. Mus. Nat. Hist. Pub. 8(6): 361-415. 
1956. 

Rowan, William. 

Reflections on the biology of animal cycles. Jour. Wildl. Mgmt. 
13(1): 52-60. 1954. 



71 



Sealander, J. A* 

Survival of Peromvscus in relation to environmental temperatures and 
acclimation at high and low temperatures. Amer. Midi. Nat. 46:257-311. 
1951. 



Food consumption in Peromvscus in relation to air temperature and 
previous thermal experience. Jour. Mamm. 33: 206-218. 1952. 

Selye, Hans. 

The general adaptation syndrome and the diseases of adaptation. 
Jour. Clin. Endocrinology. 6t 117-231. 1946. 

Snedecor, George W. 

Statistical methods applied to experiments in agriculture and 
biology. Iowa State College Press, Ames, Iowa. 1956. p. 523. 

Stickel, L. F. 

Experimental analysis of methods for measuring small mammal populations. 
Jour. Wildl. Mgmt. 10:150-159. 1946. 



The trap line as a measure of small mammal populations. Jour. Wildl. 
Mgmt. 12:153-161. 1948. 

Summerhayes, V. S. 

The effect of voles ( Microtus aqrestls ) on vegetation. Jour. Ecology 
29:14-48. 1941. 

Svihla, Arthur. 

A comparative life history study of the mice of the genus Peromvscus . 
Univ. Mich. Mus. Zool., Misc. Pub. 24. 1932. 

Whitaker, W. L. 

Illumination and reproduction in mice. Jour. Exp. Zool. 83:33-60. 
1940. 

Williams, Olwen. 

Home range of Peromvscus maniculatus rufinus in a Ponderosa pine 
community. Jour. Mamm. 36:42-45. 1955. 



Food habits of the deer mouse. Jour. Mamm. 40:415. 1959. 



72 



APPENDIX 
OF TABLES OF SIGNIFICANT CHI-SQUARES 



Table 


17. 


Peromvscus maniculatus. Population index. Siqnificant 
chi-squares. a^Q 5 


73 


Year 


• 


• • 

• « 

Sorinq : Fall : Soecial Monthlv Tests 




1951 
1952 
1953 
1954 
1955 
1956 
1957 
1958 
1959 
1960 




March 5.49 November 5.49 December 
April 12.37 

5.49 April 8.59 


8.59 


Table 


18. 


Microtus ochroaaster. PoDulation index. Siqnificant 
chi-squares. a « 5 




Year 




t t 

Spring . Fall • Special Monthly Tests 




1951 
1952 
1953 
1954 
1955 
1956 
1957 
1958 
1959 
1960 




16.16 March 16.16 November 49.49 
4.04 March 9.09 

4.04 April 20.45 

April 4.04 November 4.04 




■ 















74 


Table 


19. 


Peromyscus maniculatus. Sex 


: ratio. 


Significant 








chi-squares. 


a .05 












■ 


■ 
i 


t 


: 




Year 




: March 


t April 


: 


November : 


December 


1951 




6.76 






4.84 


29.16 


1952 












4.84 


1953 




14.44 






14.44 




1954 




25.00 


4.00 




11.44 


17.64 


1955 














1956 






4.84 




6.76 




1957 




4.00 


6.76 








1958 




5.76 










1959 




5.76 


6.76 








1960 






11.44 








Table 


20. 


Microtias ochroaaster. Sex 


ratio. 


Significant 








chi-squares. 


a .05 








Year 




• 

\ March 

• 


April 


t 
t 


• 
• 

November 

* 


December 


1951 












11.44 


1952 










11.56 




1953 






43.56 








1954 














1955 




11.44 






7.84 




1956 






4.00 






11.44 


1957 




11.44 


25.00 




25.00 




1958 












7.84 


1959 






5.76 




4.00 




1960 














- 















75 


Table 21. 


Peromyscus i 


naniculatus. 


Age structure. 


Signify 


sant 






monthly chi' 


-squares, a 


,05 














Aqe Class 








j I 


II 


III 


IV 


V 


VI 


March 














1951 


37.88 


8.20 




29.70 






1952 


6.06 




57.47 


6.90 


19.56 




1953 




4.34 




4.39 






1954 










8.69 




1955 


13.64 




3.86 




8.69 




1956 


13.64 


21.95 


65.19 




8.69 




1957 








4.76 


8.69 




1958 


9.47 












1959 










30.58 




1960 














April 














1951 


52.56 


76.34 


52.00 


20.48 






1952 






5.13 








1953 








7.09 






1954 


6.25 


24.90 


16.98 


7.09 






1955 


25.00 




16.98 


7.09 






1956 


25.00 






20.48 






1957 




29.87 


20.55 








1958 








5.75 






1959 


9.00 


25.28 




18.14 


225.00 


16.16 


1960 






6.11 


20.48 


12.76 




November 














1951 


17.42 


5.69 


8.62 




100.00 




1952 


5.74 


16.42 




11.81 






1953 


4.53 


6.88 


5.32 








1954 














1955 






5.32 


6.03 






1956 


11.98 


8.19 


27.49 


50.67 






1957 














1953 


5.74 


6.88 






73.47 




1959 










25.00 




1960 














December 














1951 


3.84 




17.53 


9.54 






1952 














1953 


7.53 




3.84 




11.11 




1954 


7.53 


19.04 


40.85 




25.00 




1955 


15.37 




7.00 








1956 


7.53 












1957 


3.84 




7.00 








1958 




9.04 










1959 














1960 





























76 


Table 22. 


Microtus ochroqaster. Age 


structure. 


Significant 


; monthly 






chi-squares. a 


.05 












: 




has Claj 


n 






i I 


II 


III 


IV 


V 


VI 


March 














1951 


39.32 


36.42 


11.97 




21.95 




1952 




9.35 




16.63 


8.20 




1953 


7.45 


28.21 


44.93 


23.46 


445.56 




1954 














1955 


7.53 


28.21 


74.80 


6.49 


21.95 




1956 




28.21 




18.78 






1957 


7.53 


7.05 




12.74 


21.95 




1958 


7.53 


45.69 


16.88 




21.95 




1959 








5.26 




99.31 


1960 














April 














1951 


9.89 


26.43 






12.36 




1952 


9.89 


29.87 




21.95 


155.36 




1953 


9.89 




33.28 


21.95 






1954 














1955 


9.89 




6.11 


8.20 


12.36 




1956 


175.26 


5.65 




21.95 






1957 




5.65 


13.76 




12.36 




1958 


23.78 








6.54 




1959 


9.89 


19.17 




13.28 




16.16 


1960 






10.37 




12.36 




November 














1951 


18.32 






12.70 






1952 




7.53 


6.25 








1953 




7.53 










1954 














1955 


15.66 


7.53 






4.59 




1956 




25.96 










1957 


58.65 




27.36 


13.39 






1958 


36.99 


3.84 




7.14 


100.00 




1959 














1960 














December 














1951 




38.89 






5.26 




1952 














1953 














1954 














1955 




38.89 


21.95 


908.02 


5.26 




1956 


84.51 


17.90 


17.34 


37.20 


7.58 




1957 




257. 14 


21.95 


69.49 


5.26 




1958 






6.78 


21.87 


10.32 




1959 














1960 

















THE ROLE OF REPRODUCTION AND MORTALITY IN 

POPULATION FLUCTUATIONS OF PEROMYSCUS MANICULATUS 

AND MICROTUS OCHROGASTER ON NATIVE PRAIRIES 



by 



ROBERT G. ROLAN 



B. S., Kansas State University, 1959 



AN ABSTRACT OF A MASTER^ THESIS 



submitted in partial fulfillment of the 



requirements for the degree 



MASTER OF SCIENCE 



Department of Zoology 



KANSAS STATE UN I VERS ITY 
Manhattan, Kansas 



1961 



The purpose of this study was to determine some of the factors 
affecting reproduction in Peromyscus maniculatus and Microtus ochroqaster 
in Kansas, and how these changes are reflected in the population. The 
small mammals were snap-trapped at various locations throughout the state 
in areas representing the high prairie habitat. From 1951 to the present 
this trapping program has been known as the Kansas Small Mammal Census, 
and has been coordinated by Dr. H. T. Gier of Kansas State University. The 
mice caught were measured, weighed, and analyzed for reproductive condition; 
pregnancy or presence of placental scars. To facilitate statistical analysis 
the census data were pooled without regard for locality, each of these being 
viewed as a random sample of the high prairie habitat of Kansas. 

Certain preliminary studies were made in order to examine critically 
some assumptions requisite to analysis of the data. In the absence of known 
ages of the mice, age had to be estimated by some measurement available. 
Total length and weight were found to be highly variable measurements. Thus, 
body length was the measurement used. The body lengths of parous females 
were found to be normally distributed. Six age classes were established on 
the basis of the probabilities of normal distribution, each class consisting 
of one standard deviation from the mean. Age Class I consisted of juvenile 
and sub-adult females, and the remaining five classes were all adults of 
varying ages. An attempt was made to establish a similar distribution of 
sexually mature males. Such data were quite limited and no difference 
could be found when compared to the distribution of parous female body 
lengths. Males were, therefore, assigned to the age classes previously 
established, by body length. 



Neonatal mice were not subject to snap-trapping. Observations on the 
growth of two litters of deer mice indicate that young mice are attracted 
to traps shortly after weaning. Numerous mice estimated to be recently, 
weaned appear in the census data. 

Fluctuations in numberical populations were examined. Both deer mice 
and voles attained peaks of population density in 1951 and early 1952, but 
soon declined. Peromvscus maintained moderate numbers to 1953, but Microtus 
practically disappeared from the high prairie. Voles attained peak densities 
again in 1957, a full year ahead of the deer mice. Both species stayed at 
moderate levels until the winter of 1959-60. In the spring of 1960 mice 
were quite scarce. From 1951 to 1958 the population levels were positively 
correlated with annual precipitation. The influence of precipitation is 
thought to be through its effect on plant growth. The earlier peak of 
Microtus following the drought of the fifties is attributed to differences 
in feeding habits. Voles tend to eat vegetation while deer mice prefer 
seeds. 

Sex ratios of deer mice averaged significantly high in males during 
March. It was concluded that this sex ratio was only apparent, being due 
to increased activity and range of male deer mice at the beginning of the 
breeding season, thus exposing them to a greater probability of being 
trapped. In Microtus . male voles averaged quite low in April. It was 
reasoned that the same factors are not operative in vole populations, in 
this regard, as in deer mice, due to the use of runway systems by numerous 
voles. The low ratio of male voles in April was considered real, and attri- 
buted to the phenomenon of condensation potential since two April populations 
were extremely dense. 



Age structure of the populations changed constantly. The general 
pattern was for the population to be proportionally high in older in- 
dividuals in March, become progressively younger during the breeding 
season, and to age again in December. No correlation could be estab- 
lished, however, between age composition of a population and its numerical 
density. Comparison between age structure of a population and its 
density through succeeding months was used to estimate survival or mor- 
tality in each age class. 

In general, the greatest percentage of the population found to be 
pregnant occurred in the spring. As the breeding season progressed, fewer 
females were pregnant at any given time, but the number with placental 
scars increased. Litter size was estimated for deer mice and voles by two 
methods; embryo and placental scar counts. Placental scar counts were 
validated with a control colony of laboratory mice, and by the emperical 
evidence of the great similarity of mean embryo and mean scar counts in 
Peromyscus and Mfrcrotus . Mean litter size was found to increase with age 
of the females except in the oldest group (Age Class VI) which averaged a 
somewhat smaller litter size. Frequency of breeding was found to increase 
with age. 

No consistent relation could be shown between high population density, 
age composition, climate, and mortality. Frequently, the heaviest mortality 
after adverse conditions occurred among the juvenile mice, but just as often 
it was among the adults. The discrepancy of the mortality patterns forces 
one to conclude that the mice responded differently at various times due to 
their own peculiar physiology. Thus a given mortality factor might be fatal 
to physiologically deranged population, but not to healthy ones. Any adverse 



condition may either reduce natality or contribute to the death of an 
animal; probably both. The major role in the dynamics of deer mice and 
vole population is, therefore, assigned to mortality. It has not been 
possible to demonstrate high natality overcoming the effects of adverse 
conditions in these species. 

If there are cyclic fluctuations independent of climatic conditions 
of Peromyscus and Microtus in Kansas, this study has not been of sufficient 
duration to show them. 

The division of the population into statistical age classes has been 
essential to this study. Splitting of sexually mature females into five 
groups, by body length, has permitted the demonstration of increased re- 
production with increased age. A decreased reproductive potential was 
found in the oldest females, a point not noted in the literature. Most 
important, however, has been the demonstration of differential survival of 
the age classes at various times. Conclusions cannot be properly drawn 
about the dynamics of wild mice from reproductive and density data alone; 
some measure of the relative survival of the young and adults must be con- 
sidered. During some declines of Kansas deer mice and voles the major 
mortality has occurred among the adults, as one would expect from the 
general adaptation syndrome theoiy. Other declines, however, have been 
due to heavy mortality of the juvenile mice with only normal attrition of 
the adults. This latter type closely fits Chitty's theory of population 
mortality.