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Reproductive success and survival of the young in Peromyscus Britton, Mary Martha 1966

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REPRODUCTIVE SUCCESS AND SURVIVAL OF THE YOUNG IN PEROMYSCUS by MARY MARTHA BRITTON B.Sc., U n i v e r s i t y of B r i t i s h Columbia, 1963. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of . ZOOLOGY We accept t h i s t h e s i s as conforming t o the re q u i r e d standard ;THE UNIVERSITY OF BRITISH COLUMBIA MAY, I 9 6 6 . In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements fo r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study . I f u r t h e r agree that permiss ion f o r ex -t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n -c i a l gain s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Depa rtment i ABSTRACT The object of t h i s study was t o compare the r o l e of changes i n reproduction and m o r t a l i t y i n r e g u l a t i n g popula-t i o n d e n s i t y i n the deer mouse, Peromyscus maniculatus, and i f p o s s i b l e to discover some of the f a c t o r s a f f e c t i n g the r e p r o d u c t i v e r a t e . Observations were made on n a t u r a l and e x p e r i m e n t a l l y reduced populations on the U n i v e r s i t y Endow-ment Lands i n 196*+ and 1 9 6 5 . In both years numbers remained r e l a t i v e l y steady during the summer, increased i n the f a l l , when immature animals r e -placed the a d u l t s , and g r a d u a l l y d e c l i n e d over the w i n t e r . Animals were about e q u a l l y abundant at comparable times i n both years, f a l l d e n s i t i e s being about h,72 mice per acre. The s t a t i o n a r y s t a t e of these populations was a s s o c i a t e d w i t h a poor reproductive performance on the part of the females, whose breeding success v a r i e d between areas, and was greater i n 1 9 6 ^ than i n 1 9 6 5 . N o change i n l i t t e r s i z e or i n p r e n a t a l l o s s was observed during the p e r i o d of study. The males, i n c o n t r a s t , were s e x u a l l y a c t i v e from March t o September on a l l areas i n both years. The greatest l o s s of mice occurred between b i r t h and age at f i r s t capture, a f t e r which j u v e n i l e s survived at the same rate as the a d u l t s . S u r v i v a l was poorer during the breeding season than during the w i n t e r , and s u r v i v a l of males was poorer than that of females. i i P o pulations whose numbers had. been ex p e r i m e n t a l l y r e -duced and whose age s t r u c t u r e had been altered, were not s i g -n i f i c a n t l y d i f f e r e n t from the n a t u r a l populations i n mean monthly body weights, reproductive performance, or s u r v i v a l . Mean monthly body weights and reproductive performance were lower, and s u r v i v a l of the young from b i r t h to age at f i r s t capture was higher i n 1965 "than i n 196*+. The p r o p o r t i o n of subadults which became fecund was greatest on t h i s area i n The s t a t i o n a r y state of these populations was maintained by changes i n s u r v i v a l r a t h e r than by changes i n reproductive r a t e . The reproductive performance of the females was f a i r l y constant whereas the l o s s of young from b i r t h to age at f i r s t capture v a r i e d . Loss of the young i s a t t r i b u t e d to t h e i r death or emigration i n response to aggressive i n t e r a c t i o n s w i t h -i n the p o p u l a t i o n . i i i ACKNOWLEDGMENTS I wish t o thank Dr. Dennis C h i t t y f o r s u p e r v i s i n g t h i s t h e s i s . I a l s o wish to thank Dr. J . T. McFadden f o r advice on the s t a t i s t i c a l a n a l y s i s . I was supported by the N a t i o n a l Research C o u n c i l of Canada during 1963-1966 and by a Teaching A s s i s t a n t s h i p i n 196^-1965. TABLE OF CONTENTS Page Abstract i Acknowledgments i i i L i s t of f i g u r e s and t a b l e s .. i v I . I n t r o d u c t i o n I I I . Study areas 3 I I I . Methods 6 A. Dead samples 6 B. Mark-recapture s t u d i e s 7 1 . Acadia (experimental populations) , 7 2 . N a t u r a l populations 8 C. Laboratory colony 8 IV. R e s u l t s 10 A. P o p u l a t i o n s t r u c t u r e and changes '.. 10 1 . Density 10 2 . Age s t r u c t u r e 12 3 . Sex r a t i o 16 M a t u r i t y 17 a. Males 17 b. Females .. • 2 5 B. Reproduction 2 9 1 . Length of breeding season 29 2 . P r o p o r t i o n of pregnant females 2 9 3 . Number of l i t t e r s per season 30 h. Factors a f f e c t i n g l i t t e r s i z e 3 1 5 . Ovulation rate and l i t t e r s i z e 32 C. M o r t a l i t y and s u r v i v a l r a t e s 3*+ 1 . P r e n a t a l m o r t a l i t y . 3*+ a. Loss of ova p r i o r t o i m p l a n t a t i o n .... 38 b. Loss of embryos a f t e r i m p l a n t a t i o n ... 38 2 . P o s t n a t a l m o r t a l i t y 39 a. Loss of young from b i r t h t o age at f i r s t capture 39 b. Minimum s u r v i v a l r a t e s f o r a d u l t s .... ^3 3 . Disease, p a r a s i t i s m , and p r e d a t i o n ^5 2 Page V. D i s c u s s i o n *t8 A. P o p u l a t i o n changes ^8 B. Reproduction ^9 C. M o r t a l i t y 51 D. E x t r i n s i c f a c t o r s 52 E. Seasonal increase i n numbers 55 F. Relevance of present data t o e x i s t i n g t h e o r i e s 56 VI. Summary 6h B i b l i o g r a p h y 67 Appendices A. Pregnant u t e r i from dead samples, 196h . 73 B. Pregnant u t e r i from dead samples, 1965 • 7'+ C. Pregnant u t e r i of known age from colony. 75 i v LIST OF FIGURES AND TABLES Figures T i t l e Page 1 Map of part of U n i v e r s i t y Endowment Lands showing trapping areas and roads 5 Table T i t l e Page I Monthly changes i n numbers and d e n s i t y of marked populations 1 1 I I Weight group, pelage and m a t u r i t y of deer mice 1 2 I I I Aging of deer mice i n groups based on degree of tooth wear lk IV Age s t r u c t u r e and body weight d i s t r i b u -t i o n of deer mice i n n a t u r a l p o p u l a t i o n s , data f o r I 9 6 U - -5 combined 1 8 V Age s t r u c t u r e and body weight d i s t r i b u -t i o n s of deer mice i n experimental p o p u l a t i o n s , 196 Lt—5 1 9 VI Mean monthly t e s t e s weights, 196 Lt~5 . . . 2 0 VII D i s t r i b u t i o n of sperms i n cauda epididymis by t e s t e s weight 2 1 V I I I D i s t r i b u t i o n of weight of seminal v e s i c l e s i n r e l a t i o n t o weight of t e s t e s , data f o r 196U--5 combined . 2 3 IX D i s t r i b u t i o n of t e s t e s s i z e i n r e l a t i o n to t e s t e s weight, data f o r 1 9 6 * + - ! ? com-bined 2h X Comparison of reproductive data f o r dead samples, 196k~5 2 7 XI Comparison of reproductive data f o r marked po p u l a t i o n s , 196U--5 2 8 X I I M^ ean number of l i t t e r s per season 3 1 V Table T i t l e Page X I I I L i t t e r s i z e i n r e l a t i o n to maternal weight, data f o r 196 1 +-5 combined 3 5 XIV Monthly d i s t r i b u t i o n of l i t t e r s i z e , data f o r 196 1 +-5 combined 3& XV Number of ova and embryos i n w i l d f e -males and number of young born i n the l a b o r a t o r y 37 XVI P r e n a t a l m o r t a l i t y , 196*+-5 hi XVII P r o p o r t i o n of young s u r v i v i n g to k—6 weeks of age ^2 XVIII Minimum s u r v i v a l rates of j u v e n i l e s and a d u l t s *+3 XIX Minimum s u r v i v a l r a t e s f o r the n a t u r a l p o p u l a t i o n s , 196*+-5 ^6 XX Minimum s u r v i v a l r a t e s f o r experimental po p u l a t i o n s , 196 ) +-5 ^7 XXI P o t e n t i a l seasonal increase i n numbers 57 I . INTRODUCTION The numbers of small mammal populations are f r e q u e n t l y determined by changes i n both b i r t h r a t e s and death r a t e s . There i s some controversy i n the l i t e r a t u r e over the r e l a -t i v e importance of these changes i n r e g u l a t i n g p o p u l a t i o n numbers. Regulation i s sometimes thought to be achieved by v a r i a t i o n s i n one parameter while the other remains constant. One concept i s that reproductive rate i s r e l a t i v e l y constant and, t h e r e f o r e , any adjustment t o p o p u l a t i o n d e n s i t y i s achieved e s s e n t i a l l y through v a r i a t i o n s i n death r a t e (Lack, 195*+) • In b i r d s , f o r example, Southern ( 1 9 5 9 ) suggests that t h e i r b i r t h rate " i s not f l e x i b l e enough to r a i s e or lower the p o p u l a t i o n l e v e l f o r adjustment to normally observed devia-t i o n s . " Adjustments through changes In death r a t e r a t h e r than b i r t h rate are t y p i c a l of f l u c t u a t i n g populations such as snow-shoe hares (Green and Evans, 19 ! +0a, 1 9 L f 0 b , 19 ! +0c), lemmings (Krebs, 196'+), and v o l e s ( C h i t t y , 1 9 5 2 ) , though as reported by C h i t t y ( 1 9 5 2 ) and C h i t t y and C h i t t y ( 1 9 6 2 ), d e c l i n e s may be accompanied by reduced reproductive r a t e s . The c o n t r a s t i n g p o i n t of view maintains that the repro-ductive rate i s s u f f i c i e n t l y reduced t o account f o r a s t a t i o n -ary or d e c l i n i n g state of a p o p u l a t i o n . Indeed, according t o C h r i s t i a n ( 1 9 5 7 ) 5 such changes are mainly responsible f o r the r e g u l a t i o n of numbers. Wynne-Edwards ( 1 9 6 2 , p.20) contends that animal populations can l i m i t t h e i r numbers by decreasing 2 t h e i r reproductive r a t e s as po p u l a t i o n d e n s i t y i n c r e a s e s . The purpose of the present study was t o make an accurate d e s c r i p t i o n of reproductive r a t e s and f a c t o r s a f f e c t i n g them, and t o compare the r o l e of changes i n reproduction and m o r t a l -i t y i n determing population numbers i n the deer mouse, Peromyscus maniculatus austerus ( B a i r d ) . F i e l d work was c a r r i e d out from 20 February - 16 December, 196*+ and 26 Feb-ruary - 25 October, 1965 on the U n i v e r s i t y Endowment Lands, U n i v e r s i t y of B r i t i s h Columbia. 3 I I . STUDY AREAS The U n i v e r s i t y Endowment Lands are i n the Puget Sound Lowlands b i o t i c area (Cowan and Guiguet, 1965) . The f o r e s t i s dominated by Douglas f i r (Pseudotsuga t a x i f o l i a ) , western red cedar (Thu.ja p l i c a t a ) , and western hemlock (Tsuga hetero-p h y l l a ) w i t h mixtures of red a l d e r (Alnus rubra), vine maple (Acer circinatum) and broadleaf maple (Acer macrophyllum). Regrowth se c t i o n s are dominated by red a l d e r . The dominant p l a n t s of the undergrowth are salmon berry (Rubus s p e c t a b i l i s ) , red e l d e r (Sambucus c a l l i c a r p a ) , red huckleberry (Vaccinlum  p a r v i f o l l m ) , s a l a l ( G a u l t h e r i a s h a l l o n ) , sword f e r n (Poly-stichum munitum), and bracken (Pteridium aquilinum). The f o r e s t f l o o r i s covered with f a l l e n logs i n va r i o u s stages of decay and a t h i c k , spongy l a y e r of l e a f l i t t e r . Mice were trapped on s e v e r a l areas i n the Endowment Lands ( F i g . l ) during the two summers of f i e l d work. The areas used f o r mark-recapture s t u d i e s were as f o l l o w s : -Acadia (A) : A s e m i - i s o l a t e d area of approximately 8 . 8 acres, bounded by roadways on three sides and by a stand of young red. a l d e r on the f o u r t h , i n which very few deer mice were trapped. This area, was used f o r experimental populations i n 196^ and 1965. (37 trap p o s i t i o n s ) Golf Course (G): A s e m i - i s o l a t e d area of approxi-mately 2 .3 acres on the U n i v e r s i t y Golf Course used f o r a n a t u r a l p o p u l a t i o n i n (7 trap p o s i t i o n s ) Chancellor ( C , Lookout (L) , Marine Drive (M) , and Wreck Beach (W) : Areas i n the general f o r e s t of approximately 0 . 7 acre, 2.1 acres, 1.2 acres, and 0 . 8 acre, r e s p e c t i v e l y , used f o r n a t u r a l populations i n 1 9 6 5 . The s i z e s given f o r these areas do not include an e x t r a s t r i p to a l l o w f o r ranges extend-ing outside the area trapped. ( 5 , 8 , 10 and 5 trap p o s i t i o n s , r e s p e c t i v e l y ) . The areas used f o r c o l l e c t i n g m a t e r i a l f o r autopsy were as f o l l o w s : 196^ Spring Summer F a l l A, B, H H, M A, F, G, J H, M A, I , K, M C', D, E, F, H, I , N, 0 A, I , L, M Winter F i g . l . Map. of part of U n i v e r s i t y Endowment Lands showing trapping areas and roads. Dots i n d i c a t e f o r e s t e d areas. 6 I I I . METHODS Changes i n p o p u l a t i o n s t r u c t u r e , r e p r o d u c t i o n , and s u r v i v a l were s t u d i e d by a u t o p s y of dead specimens and by m a r k - r e c a p t u r e s t u d i e s . A l l deer mice were t r a p p e d w i t h Longworth l i v e t r a p s . A. DEAD SAMPLES Data on m a t u r i t y , organ w e i g h t , r e p r o d u c t i v e r a t e , and p r e n a t a l m o r t a l i t y were o b t a i n e d from dead samples. A u t o p s i e s were performed on the deer mice c o l l e c t e d from the f i e l d a r e a s l i s t e d above, c o r p s e s were saved, and the f o l l o w i n g d a t a were r e c o r d e d : A l l specimens: Date, l o c a t i o n , sex, body w e i g h t , p e l a g e , degree of t o o t h wear, and I n 196*+, s p l e e n weight and a d r e n a l w e i g h t . A l l mice were weighed t o 0 . 5 gm; a l l organs t o 0 . 1 mg. M a l e s : T e s t e s s i z e and w e i g h t , s e m i n a l v e s i c l e s i z e or w e i g h t , and i n 1965, v i s i b i l i t y of t u b u l e s and presence of sperms i n the cauda e p i d i d y m i s . Females: S t a t e of l a c t a t i o n , s i z e of n i p p l e s , s i z e of mammary g l a n d s , p e r f o r a t i o n of the v a g i n a l o r i f i c e , number of c o r p o r a l u t e a , number and approximate age of embryos, number of s c a r s , and weight of e n t i r e u t e r i n e t r a c t . Because p r e l i m i n a r y a n a l y s i s of changes i n weight of the s p l e e n and a d r e n a l g l a n d s i n d i c a t e s t h a t no new i n f o r m a t i o n was b e i n g added t o t h a t a l r e a d y a v a i l a b l e i n the l i t e r a t u r e , t h i s t h e s i s w i l l be concerned o n l y w i t h changes i n p o p u l a t i o n s t r u c t u r e , r e p r o d u c t i v e r a t e , and s u r v i v a l r a t e of the s t u d y p o p u l a t i o n s . 7 B. MARK-RECAPTURE STUDIES Data, on p o p u l a t i o n growth and s u r v i v a l were o b t a i n e d from m a r k - r e c a p t u r e s t u d i e s . Each s t u d y a r e a was t r a p p e d once each week, but f o r one n i g h t o n l y so t h a t l a c t a t i n g f e m a l e s . were kept away from t h e i r l i t t e r s as l i t t l e as p o s s i b l e . The deer mice were brought i n t o the l a b o r a t o r y , marked w i t h number-ed monel m e t a l e a r t a g s ( f i n g e r l i n g f i s h t a g s ) , examined, and r e t u r n e d t o the f i e l d the same morning. The f o l l o w i n g d a t a were r e c o r d e d : A l l specimens: Date, l o c a t i o n , t a g number, sex, body w e i g h t , and p e l a g e . A l l mice were weighed t o 0.5 gm. M a l e s : T e s t e s s i z e . F emales: S t a t e of l a c t a t i o n , s i z e of n i p p l e s , p e r -f o r a t i o n of the v a g i n a l o r i f i c e , and pregnancy. 1 . A c a d i a ( e x p e r i m e n t a l p o p u l a t i o n s ) . Many mice were removed from t h i s a r e a i n November 1 9 & 3 I n an e x t e n s i v e t r a p p i n g - o u t program ( C h i t t y , unpub. d a t a ) . I n F e b r u a r y 1 9 6 ^ the r e m a i n i n g p o p u l a t i o n of 2 1 males and 2 9 f e -males was reduced t o f i v e males and seven f e m a l e s i n an attempt t o c r e a t e a p o p u l a t i o n of low d e n s i t y t h a t might l e a d t o an i n -c r e a s e d p r o d u c t i o n and s u r v i v a l of young. B r e e d i n g began i n mid-March, but because r e c r u i t m e n t of the j u v e n i l e s was p o o r , the a d u l t s were removed at the end of May. F o u r young males and seven young f e m a l e s were l e f t on the a r e a , and, i n a d d i t i o n , one young male and two young f e m a l e s born i n the l a b o r a t o r y were i n t r o d u c e d . 8 In February 1 9 ° 5 the overwintered p o p u l a t i o n of 30 males and *+5 females was again reduced, l e a v i n g s i x p a i r s of re s i d e n t ' animals on the area. Because s e v e r a l of the deer mice had d i s -appeared, four females from the l a b o r a t o r y were released a month l a t e r , and f i v e more males and s i x females were introduced at the end of June. 2. N a t u r a l populations• Several populations were trapped t o obtain i n f o r m a t i o n about p o p u l a t i o n changes and reproductive changes under n a t u r a l c o n d i t i o n s and t o serve as c o n t r o l s f o r the experimental popula-t i o n s . G. LABORATORY COLONY A breeding colony, s t a r t e d i n the spring of 196*+, provided animals f o r f i e l d and l a b o r a t o r y experiments. The estrous c y c l e was studied from v a g i n a l smears, which were made by p i p e t -t i n g warm i s o t o n i c s a l i n e s o l u t i o n i n t o the vagina, withdrawing the l i q u i d and spreading i t on a microscope s l i d e to dry. The smear was stained with methylene blue. No r e g u l a r estrous c y c l e was observed. Timed matings were set up i n order to determine the age of <• embryos found i n the f i e l d samples. Males and females were kept i n separate cages during the day and put together at n i g h t . Vaginal smears were made the f o l l o w i n g morning. The presence of sperms i n d i c a t e d that mating had occurred. The females were autopsied between the seventh and eighteenth day of g e s t a t i o n and the f o l l o w i n g data were recorded2 number of corpora l u t e a , number of embryos, and the weight of the e n t i r e u t e r i n e t r a c t . 10 IV. RESULTS A. POPULATION STRUCTURE AND CHANGES 1. Density. P o p u l a t i o n d e n s i t y changes con t i n u o u s l y , more n o t i c e a b l y i n the f a l l than during the breeding season. In order to c a l -c u l a t e d e n s i t y , one must know the number of r e s i d e n t animals on a study p l o t of known area. The areas of the study p l o t s i n the general f o r e s t were correc t e d f o r ranges extending outside the area trapped by a d d i t i o n of a s t r i p , a r b i t r a r i l y 100 f e e t wide, to each side of the p l o t . Twice t h i s distance gives a value of s i m i l a r magnitude to the 199 and 258 f e e t given by S t i c k e l ( i960) as the average, distances between successive cap-tures f o r Peromyscus leucopus populations at high and low d e n s i -t i e s and the 172 f e e t given by Brant (1962) f o r P. mahiculatus. The 200 f e e t assumed f o r these populations may be t o o ; s m a l l as mice o c c a s i o n a l l y moved *+00 f e e t or more between successive t r a p -pings. The longest distance recorded was approximately 950 f e e t . D e n s i t i e s on a l l areas were of s i m i l a r order of magnitude (< 10) (Table I ) . By making comparisons w i t h i n an area, one can determine the d i r e c t i o n of change i n numbers, or d e n s i t y , i n a p o p u l a t i o n . On the Golf Course In 196*+, f o r example, the number of deer mice r e -mained r e l a t i v e l y steady during the summer breeding season, be-gan to increase i n September, and continued to r i s e through the end of October. With two exceptions, t h i s p a t t e r n of i n c r e a s i n g TABLE I Area Monthly changes i n numbers and density of marked populations. Acadia 1 9 6 ^ - 5 8 . 8 a Month N p_ Mar. 12 1 . 3 6 Apr. 15 1 . 7 1 May 2 0 2 . 2 8 June 17 1 . 9 3 J u l y 20 2 . 2 8 Aug. 32 3 . 6 2 Sept. 5 . 0 0 Oct. 50 5 . 6 8 Nov. 56 6 . 3 6 Dec. 51 5 . 8 0 Feb.. h5+ 5 . 1 1 Acadia 1965 8 . 8 a . N D 12 1 . 3 6 12 I . 3 6 13 l.*+8 lh 1 . 5 9 13 1 A 8 15 1 . 7 1 2 1 2 . 3 9 28+ 3 . 1 8 Golf Course 196!+ 2 . 3 a N D 7 3 . 0 ^ 7 3 . 0M-7 3.0'+ 6 2 . 6 1 5 2 . 1 8 7 3 - 0 ^ 9 3 . 9 1 17+ 7 . 3 9 Chancel-l o r 1965 3 A a * N D 19 5 . 5 9 28 8 . 2 3 2h 7 . 0 6 26 . 7 . 6 5 2 5 7 . 3 5 30 8 . 8 2 26 7 . 6 5 1^ 1+.12 Look-out 1 9 6 5 5 . 7 a * N D 7 1 . 2 3 11 1 . 9 3 13 2 . 2 8 12 2 . 1 1 10 1 . 7 5 13 2 . 2 8 10 1 . 7 5 17+ 2 . 9 8 Marine Drive 1965 >+.8a* N D 15 3 . 1 2 12 2 . 5 0 16 3 . 3 3 2 1 If. 38 19 3 . 9 6 30+ 6 . 2 5 Wreck Beach 1 9 6 5 3 . 8 a * N 19 = 22 19 1 5 19 2h 19 13 D 5 . 0 0 5 . 7 9 5 . 0 0 3 . 9 5 5 . 0 0 6 . 3 2 5 . 0 0 3 A 2 N -D -* _ + -no.of deer mice captured de n s i t y adjusted acreages of study areas i n the general f o r e s t , a c t u a l acreages are given i n t e x t , p.3 data from f i r s t trap-out period only 12 numbers In the f a l l was observed, on the other areas, the amount of increase from the beginning t o the end of the breeding season being two to three f o l d . On the Chancellor and Wreck Beach areas, however, numbers dropped i n the f a l l . 2. Age s t r u c t u r e . Age s t r u c t u r e i n f l u e n c e s both reproduction and m o r t a l i t y and, t h e r e f o r e , the d i r e c t i o n of change i n numbers of a popula-t i o n . The age groups used i n t h i s study were based on body weight and pelage. The a r b i t r a r y weight groups roughly c o r r e s -pond to age groups or phases i n the l i f e c y c l e . These groups are as f o l l o w s : • TABLE I I . Weight group, pelage, and m a t u r i t y of deer mice. Weight group Pelage P r o p o r t i o n mature* 6 . 0 - 1 0 . 5 gm. juvenal 0 . 0 0 0 ( 5 6 ) 1 1 . 0 - 1 3 . 5 gm. juvenal,subadult 0 . 0 6 8 ( 2 3 6 ) lh.0 - 1 6 . 5 gm. adult 0 . 2 6 8 ( 2 0 5 ) 1 7 . 0 - 1 9 . 5 gm. adult 0 . 7 1 6 ( 1 1 6 ) 2 0 . 0 - 2 2 . 5 gm. a d u l t 0 . 8 5 7 ( 3 5 ) 2 3 . 0 + adult 1 . 0 0 0 ( 5 ) * Data from autopsy specimens, number examined i n parentheses. J u v e n i l e s and a d u l t s were e a s i l y separated by the appearance of the pelage, but subadults were l e s s e a s i l y d i s t i n g u i s h e d from a d u l t s . The gray juvenal pelage i s replaced by the dark brown subadult pelage at about two months of age; the subadult pelage i s replaced over a p e r i o d of s e v e r a l months by the l i g h t e r brown adult pelage. McCabe and Blanchard ( 1 9 5 0 ) reported a s i m i l a r molt sequence. 13 Separation of mice of d i f f e r e n t age groups was checked i n -dependently by the degree of tooth wear. The c r i t e r i a f o r the f o l l o w i n g groups, based on the amount of wear of the enamel and cusps of the upper molars, were adapted from Sheppe ( 1 9 5 8 ) . The ages of deer mice i n d i c a t e d i n the groups were obtained from animals of known age, used i n mark-recapture s t u d i e s (Table I I I ) . I . M 3 ( t h i r d upper molar) not f u l l y eruped; mice approximately 2-6 weeks of age and i n juvenal pelage; I I . M 3 f u l l y erupted but showing l i t t l e or no wear; mice approximately 2-3+ months of age and i n subadult pelage; I I I . M 3 basined but cusps s t i l l apparent, M 1 and M 2 showing appreciable wear but cusps s t i l l prominent; mice approximately h-8 months of age and i n adult pelage; IV. A l l molars basined, cusps of M 3 worn away, cusps of M 1 and M 2 g r e a t l y worn but r e -entrant angles s t i l l apparent; mice 10-1*+ months of age; V. Cusps and re-entrant angles of M 1 and M 2 almost e n t i r e l y worn away, roots u s u a l l y p r otruding w e l l beyond the a l v e o l u s ; mice more than 1 5 months of age. Shorten (195*+), studying s q u i r r e l s from d i f f e r e n t areas, a l s o found that the amount of tooth wear progressed w i t h age, and that the rate of tooth wear may have been a f f e c t e d by varying food c o n d i t i o n s . V a r i a t i o n s i n food c o n d i t i o n s may have a f f e c t e d the rate of tooth wear i n mice from d i f f e r e n t p a r t s of the Endowment Lands. The age s t r u c t u r e and body weight d i s t r i b u t i o n s of the marked populations are given i n Tables IV and V. The weight Ik TABLE I I I . Aging of deer mice i n groups based on degree of tooth wear. Deer mice from mark-recapture s t u d i e s . Group N Mean body _ weight (gm.) I 11* 7.6k I I 27 1 2 . 5 0 I I I 12 1 5 . 9 6 IV 18 1 7 . 7 8 V 1 2 0 . 5 0 S.E. Estimated Age. ± 0 . 3 2 2 - 6 wks. ± 0 . 3 3 2 - 3 mos. - 0 . 5 9 k-Q mos. to. 63 10-1!+ mos. 1 5 + mos. N - No.of deer mice examined - Deer mice i n Group I from dead samples 15 used f o r each animal i s i t s average weight during each month's t r a p p i n g . Year c l a s s e s are separated by the dashed l i n e . A n a l y s i s of variance shows that the mean monthly body weights of the n a t u r a l populations tended t o decrease s l i g h t l y from 196'+ to 1965, but that the d i f f e r e n c e s are not s i g n i f i c a n t ( , 1 0 > P> . 0 5 ) . With the exception of the Wreck Beach area, on which the mice were h e a v i e r , the d i f f e r e n c e s between areas are not s i g n i f i c a n t ( P > . 0 5 ) . Examination of c r i t e r i a other than body weight i n d i c a t e s that a l l the n a t u r a l populations were very s i m i l a r ; t h e r e f o r e , data on age s t r u c t u r e were pooled. The approximate age s t r u c t u r e of the n a t u r a l p o p u l a t i o n s , shown i n Table I.V, i s seen to change from the beginning to the end of the breeding season. The mice g r a d u a l l y gained weight from the 11 .0-13.5 gm. and 1^.0-16.5 gm. weight groups i n March t o the 17 .0-19 .5 gm. weight group i n J u l y . In August animals i n the 11 .0-13.5 gm. and 17 .0-19 .5 gm. weight groups formed a bimodal d i s t r i b u t i o n . In October animals i n the 11 .0-13.5 gm. and l 1 + . 0 - l 6 . 5 gm. weight groups made up most of the po p u l a t i o n . By March the overwintered deer mice had begun t o come Into breeding c o n d i t i o n and a few had begun t o produce young. Even though the females were fecund, few pregnancies were recorded before May. Few young appeared before J u l y , and most not u n t i l September. By September the older animals had begun t o l o s e weight and to go out of breeding c o n d i t i o n , many had d i e d , and few overwintered. 16 S i m i l a r weight changes were observed, i n the experimental populations on Acadia i n the two years. In 196*+ the a d u l t s be-gan to breed at the end of March, and j u v e n i l e s began to appear at the end of A p r i l . The a d u l t s were removed at the end of May, l e a v i n g the j u v e n i l e s on the area. The j u v e n i l e s matured i n June and t h e i r young began to appear i n J u l y . A few of these young matured l a t e r i n the summer, but most d i d not. The a d u l t s went out of breeding c o n d i t i o n i n September. Toward f a l l un-marked a d u l t s appeared which may have been young that had grown up on the area or may have been immigrants. The f a l l and winter p o p u l a t i o n was composed of a few a d u l t s born i n the spring ( > 1 7 . 0 gm.) and younger animals born i n the summer and f a l l (<17.0 gm.). In 1965 the ad u l t s on Acadia d i d not come i n t o breeding con-d i t i o n u n t i l A p r i l . Young began to appear i n May but few were caught before August. The a d u l t s began to go out of breeding c o n d i t i o n In September and by October few remained. Immature mice i n the 11 .0-13.5 gm. and l 1 + . 0 - l 6 . 5 gm. weight groups made up the f a l l p o p u l a t i o n . The mean monthly body weights of the experimental popula-t i o n s decreased from 196*+ to 1965, as t e s t e d by a n a l y s i s of variance ( . 0 5 > P> . 0 1 ) , but were not s i g n i f i c a n t l y d i f f e r e n t from those of the n a t u r a l p o p u l a t i o n s . 3 . Sex r a t i o . The d i s t r i b u t i o n of sexes by body weight f o r the marked 17 populations i s shown i n Tables IV and V. The d i s t r i b u t i o n of sexes i n the n a t u r a l populations was n e a r l y equal i n March, June, and J u l y . In the other months the number of males was g r e a t e r than the number of females, perhaps r e f l e c t i n g greater movement of the males. There were more adu l t males than f e -males, e s p e c i a l l y i n the 1 7 . 0 - 1 9 . 5 gm. weight group. The sex r a t i o f o r the other weight groups was n e a r l y equal. The d i s t r i b u t i o n of sexes i n the experimental p o p u l a t i o n on Acadia i n 1 9 6 ^ was n e a r l y equal f o r each month u n t i l August, when the number of females became greater than the number of males, e s p e c i a l l y In the 1 1 . 0 - 1 3 . 5 gm. and 1 7 . 0 - 1 9 . 5 gm. weight groups. During the breeding season there were more females than males i n the 2 0 . 0 - 2 2 . 5 gm. and 2 3 . 0 + gm. weight groups. In 1 9 6 5 the sex r a t i o f o r months and f o r weight groups was n e a r l y equal. The data f o r both years c o n t r a s t w i t h those of the n a t u r a l popula-t i o n s , i n which there were more adult males than adult females. h. M a t u r i t y , a. Males. The mean monthly t e s t e s weights, given i n Table V I , were much lower i n 1 9 6 5 than i n 196^ ( . 0 1>P, t = 3 . 6 9 2 , 7 d . f . ) . In both years, however, the mean te s t e s weight increased from March u n t i l August and then r a p i d l y decreased u n t i l l a t e f a l l . This seasonal change i n mean t e s t e s weight r e f l e c t s changes i n both age s t r u c t u r e and f e c u n d i t y . A male i s fecund when there i s an abundance of m o t i l e sperms 18 TABLE IV. Age s t r u c t u r e and body weight d i s t r i b u t i o n of deer mice i n n a t u r a l p o p u l a t i o n s , data f o r 196^-5 combined,, Dashed l i n e separates year classes,, Body weight (gm„) Mar„ Apr. May June J u l y S ? S ? 9 9 S 9 6.0- - - N _ 1 1 1 - 1 . -11 .0- 3 7 k 12 2 10 X X X 1 X 2 5 2 m - .o- h 5 22 10 2h 16 10 25 3 5 17. ')- 7 1 9 1 22 2 20 6 25 23 2 0 . 0 - - - - - 3 1 1 2 3 3 23.0+ - - - - - - - 2 T o t a l 1*+ 13 35 2h 52 30 32 36 36 35 Grand Aug. Sept. Oct. T o t a l T o t a l & 9 9 «? 9 9 h 2 2 1 1 6 8 12 20 12 5 13 9 30 28 70 75 i*+5 6 7 \ v. 12 11 27 13 108 97 205 23 20 19 15 13 5 138 73 211 '+ • 7 1 2 1 2 13 17 30 - - - - - - 2 2 *+9 hi h7 38 72 59 337 276 613 19 TABLE V. Age s t r u c t u r e and body weight d i s t r i b u t i o n s of deer mice i n experimental p o p u l a t i o n s , 196^f-5« Dashed l i n e separates year c l a s s e s . Dotted l i n e separates second and t h i r d generations,, 196k Body weight (gm.) Mar, Apr. May June J u l y Aug 6 V L P 9 cT ¥ 6 . 0 - - - - - 1 1 l - -11 .0- - - - ~\ \ 1 k 1 - 3 - \ 2 2 l^-.O- 5 - - 2 5 k - - 1 1 1 7 . 0 - 5 l 5 k 3 1 3 3 5 k 1 2 0 . 0 - - - 1 2 ^ \ - 2 - l - k 23.0+ l - 2 - 1 \ - - l - 3 T o t a l 5 7 6 7 7 13 8 9 7 8 7 11 Body weight (gm.) 1965 Mar 0 Apr, May June J u l y Aug, s ¥ ? ¥ 9 ¥ ? 6 . 0 - - - - \ \ - - - - - 1 -11 .0- 2 - 2 \ \ ~ 1 - - 1 1 1 1 l L f . 0 - 2 1 - - 1 6 2 2 1 h 1 1 7 . 0 - k 2 3 - 3 - 1 • 1 3 3 1 2 2 0 . 0 - - — 3 2 1 h 1 1 1 2 2 23.0+ - - - - 1 T o t a l 6 5 6 2 6 9 7 7 7 6 9 6 Sept. Oct. Nov. 0* 9 9 9 1 - 1 1 1 5 2 3 if k 9 9 7 7 7 15 9 if 6 5 8 2 8 1 7 - 5 1 1 2 - 1 - -L9 25 15 26 23 28 Sept. Oct. T o t a l d" 9 ¥ 0" ? _ 2 2 3 2 8 6 13 13 23 26 2 2 6 If 19 20 1 5 1 6 17 19 1 - 1 12 7 - - - - 1 .1 lk 22 26 7k 75 Grand Dec. T o t a l T o t a l c? ? 9 1 2 5 6 11 ^ 10 23 31 5k 12 15 50 50 100 2 3 3^ * f l 75 - k 25 29 - - 11 11 19 30 116 I6if '280 Grand T o t a l 5 +9 39 36 19 1 l*f9 TABLE V I . Mean monthly t e s t e s weights, 1 9 6 ^ - 5 . 196*+ 1965 Month No.males examined Mean t e s t e s weight (mg.) + S.E. No.males examined Mean te s t e s weight (mg.) + S.E. Mar. 16 2 3 1 . 3 1 2 5 . 7 2 12 1 2 6 . 5 2 2 1 . 5 7 Apr. 9 2 M + . 8 8 2h.ll 13 8 9 . 5 3 2^.9*+ May 1 5 3 0 1 . 9 5 5 5 . 3 9 27 1 2 9 . *+8 1 3 . 8 3 June 3 302.M-7 7 . 7 1 28 1 2 9 . 7 2 1 1 . 2 2 J u l y 18 3 2 3 . 5 5 ^ 0 . 2 9 15 3 ^ 7 . 3 6 *+5.07 Aug. 36 1 ^ 5 . 9 2 3 0 . 6 3 19 *+6.21 1 2 . 2 2 Sept. 2M- 1 1 6 . 5 2 3 3 . 1 9 6 2 ^ . 6 0 3 . 7 3 Oct. 32 1 9 . 6 3 6 . 9 0 67 1 7 . 6 9 2 A l Nov. 11 7 . 9 6 l.hl - - -Dec. 7 1 1 . 9 3 1.7*+ _ 2 1 and a c e r t a i n amount of semen produced. Fecundity was deter-mined i n 1 9 6 5 from the presence of m o t i l e sperms i n the cauda epididymis (Perry, 19*+5) and from the enlargement of the tubules i n the cauda epididymis (Jameson, 1950) and from the development of the seminal v e s i c l e s . The d i s t r i b u t i o n of males w i t h and without m o t i l e sperms i s as f o l l o w s : -TABLE V I I . D i s t r i b u t i o n of sperms i n cauda epididymis by t e s t e s weight. Presence of Sperms Testes Weight No.without No.with (mg.) sperms sperms 0 - 1 0 0 102 16 1 0 0 - 2 0 0 5 32 2 0 0 - 3 0 0 0 lk-3 0 0 - >+00 0 11 h-00 + 0 7 Sperms were present i n animals with small t e s t e s , but many of these males, e s p e c i a l l y those with t e s t e s weighing l e s s than 1 0 0 mg., had only a few sperms; t h e r e f o r e , only males whose t e s t e s weighed more than 100 mg. were considered to be fecund. Table V I I I shows that growth of the seminal v e s i c l e s i n -i t i a l l y lagged behind that of the t e s t e s but continued a f t e r the t e s t e s had stopped i n c r e a s i n g i n weight. P r e l i m i n a r y a n a l y s i s of the data i n d i c a t e s that t h i s l a g i n the growth of the seminal v e s i c l e s may be seasonal. The t e s t e s of l i v e animals cannot be weighed; t h e r e f o r e , the r e l a t i v e s i z e of the t e s t e s , determined by s l i p p i n g them between the thumb and f o r e f i n g e r , was recorded as regressed or 22 small ( S ) , small-medium (S/M), medium (M), medium-large (M/L), or l a r g e (L) t o correspond w i t h t e s t e s weighing 0 - 1 0 0 mg., 1 0 0 - 2 0 0 mg., 2 0 0 - 3 0 0 mg., SOO-i+OO mg., or ^00+ mg. , r e s p e c t i v e -l y . In order to t e s t the accuracy of these s u b j e c t i v e measure-ments, t e s t e s of dead specimens were given a r e l a t i v e s i z e and then d i s s e c t e d out and weighed. A n a l y s i s of the data shows that the c o r r e l a t i o n between r e l a t i v e s i z e and a c t u a l weight i s h i g h l y s i g n i f i c a n t i n both years. The data f o r the two years i s combined i n Table IX. Since males w i t h t e s t e s weighing more than 1 0 0 mg. were con-s i d e r e d to be fecund and the c o r r e l a t i o n between t e s t e s s i z e and t e s t e s weight i s h i g h l y s i g n i f i c a n t , then males with t e s t e s scored as small-medium or l a r g e r should a l s o be fecund. These c r i t e r i a of f e c u n d i t y were used to estimate the p r o p o r t i o n of fecund animals i n the populations s t u d i e d . The p r o p o r t i o n of fecund males in. the autopsy samples and the marked populations i s shown i n Tables X and X I . In 196*+ the ad u l t males i n the dead samples were fecund from.March through October, though they began to go out of breeding con-d i t i o n i n August. In 19&5 'the males were fecund from March through August, the maximum being i n J u l y , f o l l o w e d by a sudden drop i n August. This r a p i d change i n breeding c o n d i t i o n may have been hastened by an outbreak of b o t f l y l arvae (Cuterebra sp.) which p a r a s i t i z e d the deer mice. The p r o p o r t i o n of fecund males was lower i n 1 9 6 5 than i n 196*+ (,01>P, X 2 = 1 +1.609 w i t h 1 d . f . ) . TABLE; V I I I . D i s t r i b u t i o n to weight of of weight of seminal v e s i c l e s i n r e l a t i o n t e s t e s , data f o r 196*4—5 combined. Testes weight (rag.) 0 . 0 - 5 0 . 0 -Wei 100.0-ght of seminal v e s i c l e s (mg.) 150.0- 2 0 0 . 0 - 250.0+' 300.0- 350.0+ 0.0- 175 - - _ -5 0 . 0 - 35 - - _ -1 0 0 . 0 - 30 1 - _ -150.0- 17 3 - _ -200.0 12 5 - _ -2 5 0 . 0 - 2 8 h 2 1 - - -3 0 0 . 0 - 2 6 6 1 - - - -3 5 0 . 0 - - 1 2 6 h 1 1 -^ o o . o - - - h 3 3 3 1 -*+5o.o+ _ 5 3 - 2 1 3 2h TABLE IX. D i s t r i b u t i o n of t e s t e s s i z e i n r e l a t i o n t o te s t e s weight,data f o r 196*+-5 combined. Testes-weight R e l a t i v e t e s t e s s i z e (mg.) S S/M M M/L L 0 . 0 - 137 5 0 . 0 - 32 1 0 0 . 0 - 7 22 1 5 0 . 0 - - 18 6 2 0 0 . 0 - •-- h 13 2 2 5 0 . 0 - - - 7 5 -3 0 0 . 0 - - 13 3 5 0 . 0 - .-. _ _ 7 i+ ^oo.o- - - - 10 ^-50.0+ - - - 1*+ 25 The seasonal changes i n the p r o p o r t i o n of fecund males was a l s o observed i n the marked p o p u l a t i o n s . In both the n a t u r a l and experimental populations the males were fecund from March through September, but the p r o p o r t i o n of fecund males was lower In the n a t u r a l populations ( . 0 5 > P> . 0 1 , X = 5.311 w i t h 1 d . f . ) . More j u v e n i l e s became fecund In the ex-perimental p o p u l a t i o n i n 196h (k/lQ) than i n e i t h e r 1965 ( 0 / l 6 ) or the n a t u r a l populations . b. Females. The sexual m a t u r i t y of females i n autopsy specimens may be determined by the presence of corpora l u t e a i n the ovaries ( L e s l i e , Venables, and Venables, 1952; Jameson, 1953). In both 196M- and 1965 there were females w i t h corpora l u t e a through-out the year (Table X) , and i n 196*+, f o u r of s i x females i n November and one of two females i n December had corpora l u t e a . The p r o p o r t i o n of females with corpora l u t e a Increased during the summer and then decreased i n the f a l l , as the ..females went out of breeding c o n d i t i o n . Although corpora l u t e a I n d i c a t e ovarian a c t i v i t y , the r e s t of the u t e r i n e t r a c t may be i n a c t i v e (Jameson, 1953) . In many of the females with corpora l u t e a , the v a g i n a l o r i f i c e was im-p e r f o r a t e and the females could not have mated. P e r f o r a t i o n of the v a g i n a l o r i f i c e , p a r t i c u l a r l y i n l i v e animals, may be used as an i n d i c a t i o n of sexual a c t i v i t y ( C l a r k , 1938). In young females the vagina opens at the f i r s t estrum; hence, p e r f o r a t i o n 26 of the v a g i n a l o r i f i c e may he used t o assess the sexual m a t u r i t y of young animals brought i n from the f i e l d . As an i n -d i c a t i o n of reproductive c o n d i t i o n , however, p e r f o r a t i o n of the v a g i n a l o r i f i c e i s not always r e l i a b l e . The vagina becomes open only during the breeding season, but i t may be closed f o r a short time during pregnancy. A female that had a p e r f o r a t e vagina, th a t was v i s i b l y pregnant, or that was l a c t a t i n g was considered to be fecund. The p r o p o r t i o n of fecund females i n the dead samples and marked populations i s given i n Tables X and X I . The seasonal aspects of the breeding season are more apparent than they were i n the males. P r e l i m i n a r y a n a l y s i s of the autopsy data i n d i c a t e d that females with corpora l u t e a were not n e c e s s a r i l y fecund. In 196Li- females were fecund from March through September, the height of the breeding season being from June through August; i n 1 9 6 5 the females were fecund from May through September, the peak being i n J u l y . The p r o p o r t i o n of fecund females was lower i n 1 9 6 5 than i n l$6h ( . 0 1 > P, X 2 =. 10.96^ w i t h 1 d . f . ) . The p e r i o d of f e c u n d i t y i n the marked populations was s i m i l a r to those reported f o r the dead samples. Females i n the n a t u r a l populations and on Acadia In 196k were fecund from March through September and, on Acadia i n I 9 6 5 from May through Septem-ber. More j u v e n i l e s became fecund on Acadia, i n 1961+ ( 3 / 2 3 ) than i n e i t h e r 1 9 6 5 ( 0 / 1 2 ) or the n a t u r a l populations (O / 2 3 ) . TABLE X. Comparison of reproductive data f o r dead samples, 196 1 +-5. Males Females Month No. No. No. No.with No. N a d u l t s fecund adults corpora l u t e a fecund preg: 1 9 6 ^ Mar. 16 16 . 13 k h 0 Apr. 9 9 7 3 3 0 May 1 5 15 10 10 6 1 June 3 3 k *f 1+ 2 J u l y 1*+ 1*+ • 11 11 11 6 Aug. 22 13 2 1 21 20 5 Sept. 8 7 7 6 6 3 Oct. 12 8 8 7 0 0 T o t a l 99 8 5 . 8 1 66 5fr 17 1 9 6 5 Mar. 12 7 k 3 0 0 Apr. 13 5 13 1+ 0 0 May 26 17 2 5 7 2 0 June 28 17 28 10 10 J u l y Ih lk 13 13 13 8 Aug. lh h 18 15 8 1 Sept. 5 0 11 11 1 0 Oct. 31 0 2 1 15 0 0 T o t a l 1>3 6h 133 78 3*+ 11 2 3 TABLE X I . Comparison of reproductive data f o r marked popul a t i o n s , 1964--5 . Males Females No. No. No. No. No. No. juvei Month Adults fecund a d u l t s fecund pregnant (both ; N a t u r a l p o p u l a t i o n s , data . f o r 1 9 6 4 — 5 combined. Mar. 14- 9 13 2 1 0 Apr. 35 23 2^ 4- 0 1 May 51 4 -3 29 12 2 3 June 31 2 9 33 30 7 1 J u l y 31 30 33 32 1 5 10 Aug. 33 21 3>+ 22 10 26 Sept. 32 6 28 4- 0 8 Oct. 4-1 0 2 5 0 0 12 T o t a l 268 l 6 l 218 106 35 6 1 Experimental p o p u l a t i o n , 1 9 6 4 - . Mar. 5 5 7 5 1 0 Apr. 5 5 7 7 ~J 1 May 6 6 5 9 7 5 6 June 6 Q 7 7 6 J u l y 3 ^ 7 9 6 6 Aug. 5 5 9 9 6 6 Sept. 14- 9 22 13 4- 10 Oct. 12 0 2 1 0 0 6 T o t a l 56 38 91 57 32 4-1 Experimental p o p u l a t i o n , 1 9 6 5 . Mar. 6 6 5 0 0 0 Apr. 6 6 2 0 0 - 0 May 6 6 8 8 1 1 June 7 7 7 6 4- 1 J u l y 6 5 5 5 «. 3 Aug. 7 7 5 4- 3 4-Sept. 3 2 7 1 0 12 Oct. 7 0 9 0 0 7 T o t a l 4-8 39 4-8 24- 11 28 2 9 B. REPRODUCTION Mature females breed from March to October, bearing l i t t e r s of 2 - 9 young ( A s d e l l , 1 9 6 L 0 . The females are poly-estrous and have a postpartum e s t r u s . These c h a r a c t e r i s t i c s , plus a short g e s t a t i o n p e r i o d of 2 2 - 2 7 days and the r a p i d maturation of young of both sexes i n 7 - 8 weeks (Clark., 1938) , endow the mice wi t h a high p o t e n t i a l rate of i n c r e a s e . 1. Length of the breeding season. The number of pregnant females caught i n 196h and 1 9 6 5 I s given i n Tables X and X I . In 196h the f i r s t pregnant female was caught on Acadia i n March, and the l a s t , a l s o caught on Acadia, i n September. The f i r s t pregnant female i n the n a t u r a l p o p u l a t i o n was not caught u n t i l May. Mice i n juvenal pelage were l a s t caught i n e a r l y November. In 1 9 6 5 the f i r s t pregnant female was caught on Chancellor i n March, and the l a s t on Acadia i n l a t e August. Breeding d i d not. begin-on Acadia u n t i l May, and on Lookout u n t i l J u l y . Mice i n j u v e nal pelage were l a s t caught.in l a t e October. 2 . P r o p o r t i o n of pregnant females. Females i n a l l stages of pregnancy or with recent p a r t u r i -t i o n s i t e s were used to c a l c u l a t e the p r o p o r t i o n of pregnant f e -males i n the dead samples. More females were pregnant i n 196*+ ( 0 . 2 ^ 0 ) than i n 1 9 6 5 ( 0 . 1 1 5 ) ( . 0 1>P, X 2 = 7.15*+ w i t h 1 d.f.) The greatest p r o p o r t i o n of pregnant females was i n J u l y . 30 The p r o p o r t i o n of pregnant females i n the marked popu-l a t i o n s was estimated from females which were more than 12-14-days pregnant or which were obviously l a c t a t i n g . Females l e s s than 12-14- days pregnant were missed because the embryos are not l a r g e enough to cause a marked increase i n body weight or d i s t e n s i o n of the abdomen. V a g i n a l smears were i m p r a c t i c a l , as a s e r i e s of smears i s needed to determine a female's r e -productive c o n d i t i o n . The p r o p o r t i o n of pregnant females v a r i e d i n the n a t u r a l populations from 0 .094- on Lookout t o 0 . 2 6 5 on Marine D r i v e , and i n the experimental populations on Acadia from 0 . 3 5 2 i n 1964- t o 0 . 2 1 6 i n 1 9 6 5 . The p r o p o r t i o n of pregnant females i n the experimental populations was not s t a t i s t i c a l l y g reater than those i n the n a t u r a l p o p u l a t i o n s , as t e s t e d by a n a l y s i s of variance (P > . 0 5 ) , but tended to be greater i n 1964-(.10> P > . 0 5 , X 2 = 2 . 8 0 7 with 1 d . f . ) . 3 . Number of l i t t e r s p'er season. Throughout the breeding season breeding appeared t o be random r a t h e r than synchronized. In 1965? however, many of the females became pregnant during the f i r s t two weeks of J u l y . This was the only period of synchronized breeding observed. With the exception of Acadia, 1964-, the number of l i t t e r s per season f o r females that were present i n the marked popula-t i o n s f o r the d u r a t i o n of the breeding season i s ta b u l a t e d be-low. In 1964- the overwintered females on Acadia were removed 31 at the end of May and the j u v e n i l e s l e f t on the area began breeding i n June. Had the overwintered females not been r e -moved, they would probably have borne two t o three more l i t t e r s during the summer. TABLE X I I . Mean number of l i t t e r s per season. P o p u l a t i o n No.of No.of Mean no.of females l i t t e r s l i t t e r s + p e r female - S.E. N a t u r a l Mar.-Sept. ,196^+ 2 3 1 . 5 0 ± . 5 0 Mar.-Sept. , 1 9 6 5 20 28 l.ifO - . 1 1 Experimental Mar.-May, 196*+ 7 9 1 . 2 9 J . 1 8 June-Sept. ,196*+ 5 1*+ 2 . 8 0 ' J . 3 7 Mar.-Sept. , 1 9 6 5 7 lk 2 . 0 0 - . 3 1 The average number of l i t t e r s per season appeared t o be great-er i n 196'+ from June to September and i n 1 9 6 5 on Acadia than i n the n a t u r a l populations i n the r e s p e c t i v e years, but the d i f f e r -ences are not s t a t i s t i c a l l y s i g n i f i c a n t , as t e s t e d by Student's t - t e s t . h. F a c t o r s a f f e c t i n g l i t t e r s i z e . L i t t e r s i z e may vary w i t h maternal age and weight, p a r i t y , and season of b i r t h ( S n e l l , 19*+1; Beer et a l , 1 9 5 7 ; Newson, 196'+; C a l d w e l l and Gentry, 1 9 6 5 ). L i t t e r s i z e was recorded f o r the deer mice i n the l a b o r a t o r y colony and f o r the autopsy specimens. I n s p e c t i o n of the l a b o r a t o r y data showed no c o n s i s t e n t r e -l a t i o n between l i t t e r s i z e and maternal age or between l i t t e r s i z e and p a r i t y . P r e l i m i n a r y a n a l y s i s of the f i e l d data, how-ever, i n d i c a t e s that there may be some r e l a t i o n s h i p between the 32 number of v i s i b l e embryos per l i t t e r and maternal weight and between the number of embryos per l i t t e r and season of b i r t h . The data f o r the two years were combined.' The tendency f o r l i t t e r s i z e t o increase i n heavier (or older) females i s shown i n Table X I I I . The number of l i t t e r s was greater In the heavier females. The seasonal changes i n l i t t e r s i z e are shown i n Table XIV. The number of embryos per l i t t e r increased from May t o a peak i n August and then decreased. The number of l i t t e r s was great-est i n J u l y , 5. Ovulation rate and l i t t e r s i z e . Information on o v u l a t i o n r a t e , l i t t e r s i z e , and p r e n a t a l m o r t a l i t y was obtained from f i e l d and l a b o r a t o r y animals. The reproductive t r a c t s were preserved i n a mixture of a c e t i c a c i d , e t h a n o l , and f o r m a l i n , c l e a r e d through benzene i n t o benzyl ben-zoate and viewed w i t h oblique l i g h t ( O r s i n i , 1 9 6 2 a , 1 9 6 2 b , 1 9 6 3 ) . The number of corpora l u t e a , i m p l a n t a t i o n s i t e s , embryos, recent p a r t u r i t i o n s i t e s , and u t e r i n e scars was counted. Only those data p e r t a i n i n g t o pregnant u t e r i were used because I was unable to separate d i f f e r e n t sets of corpora l u t e a or d i f f e r e n t sets of scars i n s e v e r a l cases. The approximate stage of development of pregnant u t e r i i n the f i e l d m a t e r i a l was estimated by comparing t h e i r development wit h known stages from timed pregnancies set up i n the l a b o r a t o r y . The sta t e of development used to separate i m p l a n t a t i o n 33 s i t e s and embryos was p l a c e n t a t i o n , or, approximately, day nine. Further i n f o r m a t i o n on pregnant u t e r i i s given i n Appendices A, B and C. Table XV shows the average number of corpora l u t e a , im-p l a n t a t i o n s i t e s , embryos, and l i v e young born per l i t t e r i n 1964- and I 9 6 5 . None of the d i f f e r e n c e s between means i n each category i n the two years i s s t a t i s t i c a l l y s i g n i f i c a n t , as t e s t e d by the Student's t - t e s t . The d i f f e r e n c e between the number of young per l i t t e r and the number of corpora lutea. per l i t t e r i s not s i g n i f i c a n t (P> . 0 5 ) . The mean l i t t e r s i z e , c a l -c u l a t e d from the pooled data and used i n f u r t h e r computations, was 4-.82 young per l i t t e r . The number of corpora l u t e a formed was assumed to be equal to the number of ova shed even though some evidence of p o l y o v u l y was found. Two pregnant u t e r i i n which the number of corpora l u t e a was one l e s s than the number of embryos were found i n the f i e l d m a t e r i a l . In a t h i r d i n s t a n c e , a female from the colony had no corpora l u t e a i n the r i g h t ovary and f i v e i n . t h e l e f t , and of nine embryos, the four i n the horn having no corpora l u t e a were dead and the f i v e i n the l e f t horn were a l i v e . I t seems u n l i k e l y t o me that four embryos i n one pregnancy should have a r i s e n from m u l t i p l e o v u l a t i o n s . The incidence of m u l t i p l e o v u l a t i o n i n Peromyscus manlculatus reported by Beer et a l (1957) i s very low, but can be greater i n some i n d i v i d u a l s (Hartman, .1926) . I f polyovuly d i d not occur, then the corpora l u t e a may 3*+ have atrophied. The l i t e r a t u r e , however, i n d i c a t e s that the corpora l u t e a are present f o r the d u r a t i o n of pregnancy i n t h i s and other species (Beer et a l , 1 9 5 7 ; S n e l l , 1 9 Li-l; Long and Evans, 1 9 2 2 ; Harper, 196 Lh; Newson, 196*+). The average number of corpora l u t e a was 5 « l 8 - 0 . 1 6 . The average number of im p l a n t a t i o n s i t e s per l i t t e r was 5 . 0 0 - 0 . 2 0 and the average number of embryos per l i t t e r was '+.95 - 0 . 2 3 . The range i n number of embryos per l i t t e r was three t o seven and the mode f i v e . The average number of young per l i t t e r born i n c a p t i v i t y was H - . 6 0 - O.lh. The range was one t o eight and the mode f i v e . C. MORTALITY AND SURVIVAL RATES Death may occur at any stage of development from the time of o v u l a t i o n on. Both p r e n a t a l and p o s t n a t a l m o r t a l i t y a f f e c t the s i z e , s t r u c t u r e , and reproductive c a p a c i t y of a popul a t i o n . 1 . P r e n a t a l m o r t a l i t y . P r e n a t a l m o r t a l i t y i n c l u d e s l o s s e s before and a f t e r im-p l a n t a t i o n (Davis and G o l l e y , 1 9 & 3 ; Newson, 196*+). Loss of un-f e r t i l i z e d and nonviable ova i s expected p r i o r to i m p l a n t a t i o n (Harper, 196*+) and l o s s of embryos, p o s s i b l y through l a c k of s u f f i c i e n t blood to the pl a c e n t a , i s expected during mid-preg-nancy (Reynolds, 1 9 5 5 ) . Owing to the small number of pregnant u t e r i i n various stages of development, the only d i v i s i o n s used were i m p l a n t a t i o n and p l a c e n t a t i o n . P l a c e n t a t i o n was used t o separate implantations from embryos. The method used to e s t i -35 TABLE X I I I . L i t t e r s i z e i n r e l a t i o n t o maternal weight, data f o r 1964—5 combined. N o . v i s i b l e embryos per l i t t e r 3 4-5 6 7 Maternal weight (gm.) 14-. 0 - 1 7 . 0 - 2 0 . 0 -2 1 2 4-1 6 4-1 T o t a l No. l i t t e r s 3 5 10 l T o t a l 11 12 23 Avg.no.embryos per l i t t e r 4-.00 5 . 0 9 5 . 3 3 5.oo 36 TABLE XIV. Monthly d i s t r i b u t i o n of l i t t e r s i z e , data f o r 196*+-!? combined. N o . v i s i b l e embryos per l i t t e r 3 >+ 5 7 May 1 Month June J u l y 1 1 2 3 6 5 Aug, 1 1 3 1 Sept. 1 1 1 T o t a l No. l i t t e r s 3 5 10 9 1 T o t a l 1*+ 28 Avg.no.embryos per l i t t e r (3.00) h.2$ 5.1*+ 5.30 b.,67 5.00 TABLE XV". Number of ova and embryos i n w i l d females and number of young born i n the l a b o r a t o r y . Stage of development Corpora l u t e a (ova) Implantation s i t e s Embryos Live young at birth"* 196H-N X ± s . E 21 5.29 ^ . 2 1 21 5.1M- ±0.29 13 5.23 ^0.28 10 t o . 1+0 N X ±S.E. 12 5.00 io .25 12 i f . 7 5 ±0.28 7 h.k3 - 0 . 3 7 79 *+.6i io .15 Years combined N X ±S.E. 33 5.18 ±0.16 33 5.oo ±0.20 20 i+.9? i o . 2 3 89 *+.6o -o.lh N - No.of l i t t e r s . - l i t t e r s born i n colony 38 mate the approximate stage of development of pregnant u t e r i was given on page 3 3 . a. Loss of ova p r i o r to. i m p l a n t a t i o n . Loss of ova p r i o r to i m p l a n t a t i o n was estimated from the d i f f e r e n c e between the number of corpora l u t e a and the number of i m p l a n t a t i o n s i t e s . No attempt was made to recover f r e e ova or b l a s t o c y s t s from the u t e r i ; hence, only u t e r i i n which some of the ova survived to im p l a n t a t i o n were used. The p r o p o r t i o n of l i t t e r s showing l o s s was about 0 . 3 0 , and the p r o p o r t i o n of ova l o s t p r i o r to i m p l a n t a t i o n was about 0 . 1 0 (Table XVI A). There was no i n d i c a t i o n that l o s s e s were d i s p r o p o r t i o n a t e l y greater where the number of ova shed was gr e a t e r . T o t a l l o s s of ova p r i o r to i m p l a n t a t i o n was not estimated. b. Loss of embryos a f t e r i m p l a n t a t i o n . Loss of embryos a f t e r i m p l a n t a t i o n was estimated from the d i f f e r e n c e between the number of i m p l a n t a t i o n s i t e s and the number of l i v e embryos. Although the amount of l o s s may vary wi t h the stage of pregnancy (Perry, 1 9 5 5 , Harper, 196 Lt-), the small number of pregnant u t e r i made f u r t h e r s u b d i v i s i o n im-p r a c t i c a l . The p r o p o r t i o n of l i t t e r s showing l o s s was 0 . 2 0 - 0 . 2 5 , and the p r o p o r t i o n of embryos l o s t a f t e r i m p l a n t a t i o n was about 0 . 0 5 (Table XVI B). T o t a l l i t t e r l o s s was not estimated. In Table XVI C pr e i m p l a n t a t i o n l o s s i s shown as l o s s of ova and p o s t i m p l a n t a t i o n l o s s , as l o s s of embryos. T o t a l l o s s of ova from o v u l a t i o n i n l i t t e r s 1 0 - 2 3 days old was about 0 . 0 8 , 39 w i t h l o s s e s d i s t r i b u t e d n e a r l y e q u a l l y before and a f t e r im-p l a n t a t i o n . The d i f f e r e n c e s i n l o s s of ova between the two years are not s i g n i f i c a n t (P> . 0 5 ) . 2. P o s t n a t a l m o r t a l i t y . a« Loss of young from b i r t h to f i r s t capture. The p r o p o r t i o n of young l o s t at b i r t h i n the f i e l d i s un-known, but i f the estimate of the number of young born per l i t -t e r i n the l a b o r a t o r y i s r e p r e s e n t a t i v e of the f i e l d s i t u a t i o n , then the drop from 4-.95 embryos per pregnancy to 4-.60 young per l i t t e r at b i r t h i s a l o s s of 7 . 1 per cent. Complete l o s s of l i t t e r s i s probably quite low, as 8 7 . 1 per cent ( 6 1 / 7 0 ) of the pregnant females were l a c t a t i n g when subsequently caught. The p r o p o r t i o n of pregnant females which suckled l i t t e r s was not s i g n i f i c a n t l y greater i n the experimental p o p u l a t i o n (34-/38) than i n the n a t u r a l populations ( 2 7 / 3 2 ) . Nothing i s known about the l o s s of young from 'birth to about 4—6 weeks of age, when they would have been captured f o r the f i r s t time. In f u r t h e r d i s c u s s i o n t h i s w i l l be r e f e r r e d t o as age at f i r s t capture. Loss of n e s t l i n g s may be due to i n -herent weakness, disease, d e s e r t i o n by the mother, p r e d a t i o n , and other accidents such as f l o o d i n g . The estimate of l o s s must cover t h i s e n t i r e p e r i o d from b i r t h to age at f i r s t capture, and enables one to compare the p o t e n t i a l number of young born on an area w i t h the number of j u v e n i l e s caught. Loss of young, however, may be greater than 1+0 such an.' estimate would i n d i c a t e . There are various p o s s i b l e estimates of l i t t e r s i z e and, t h e r e f o r e , of numbers born. L i t t e r s i z e may be estimated from the number of embryos per l i t t e r or the number born per l i t t e r or from the combined data on l i t t e r s i z e . The p o t e n t i a l number born i s then c a l c u l a t e d by m u l t i p l y i n g the number of young per l i t t e r by the number of pregnancies ( l i t t e r s ) . I t I s probable, however, that not a l l pregnancies were r e -corded. Several females which had had l i t t e r s during the breed-ing season, but which had not been caught when pregnant, were caught and autopsied i n the f a l l . Females, which showed signs of having l a c t a t e d when caught f o r the f i r s t time, may or may not have had t h e i r l i t t e r s on the same area. J u v e n i l e s , i n d i s p e r s i n g , may have migrated onto or away from the study areas. A few j u v e n i l e s appeared on some of the study areas i n 1965 before any pregnancies had been recorded f o r those areas. Some of the unmarked immature animals, caught i n the f a l l , may have been present e a r l i e r i n the breeding sea-son, but not caught, or they may have been immigrants onto the study areas. This I n f l u x of young i n the f a l l was e s p e c i a l l y n o t i c e a b l e on Lookout and Acadia, i n 1965. The l o s s , or s u r v i v a l , of young f o r each month may be e s t i -mated by comparing the number of pregnancies w i t h the number of j u v e n i l e s caught i n the f o l l o w i n g months (Table X I ) . The sur-v i v a l of young was poor during the breeding season but tended 4-1 TABLE,.XVI. P r e n a t a l m o r t a l i t y , 1964--5 . A. P r e i m p l a n t a t i o n m o r t a l i t y i n l i t t e r s 4— 9 days o l d . No. ova l o s t . L i t t e r s '  No.with P r o p o r t i o n Year 0 1 2 3 T o t a l l o s s w i t h l o s s 1964- 7 2 0 1 10 3 .300 1 9 6 5 5 1 1 0 7 2 . 2 8 6 T o t a l 12 3 1 l 17 5 .294-B. P o s t i m p l a n t a t i o n m o r t a l i t y i n l i t t e r s 1 0 - 2 3 days o l d . No.embryos reso r b i n g L i t t e r s  No.with P r o p o r t i o n Year 0 1 T o t a l l o s s w i t h l o s s 1964- 6 2 0 8 2 . 2 5 0 1 9 6 5 4- 1 0 5 1 . 2 0 0 T o t a l 10 3 0 13 3 . 2 3 1 C o T o t a l p r o p o r t i o n of ova l o s t i n l i t t e r s 1 0 - 2 3 days o l d . No. No.corpora No. No. P r o p o r t i o n Year l i t t e r s l u t e a s i t e s embryos ova. l o s t 1964- 13 73 7 0 68 .04-1 1 9 6 5 7 34- 33 31 . 0 2 9 T o t a l 2 0 107 1 0 3 99 0O37 Ova No.corpora No. P r o p o r t i o n l u t e a s i t e s ova l o s t 50 4-5 . 1^0 36 33 »083 86 78 . 0 9 3 Embryos No. s i t e s 4-2 2 5 No. embryos 4-0 24-P r o p o r t i o n embryos l o s t .04-8 . 04-0 67 64- .04-5 P r o p o r t i o n embryos l o s t . 0 2 9 . 0 6 1 Combined l o s s of ova and embryos . 0 6 8 . 0 8 8 . 0 3 8 . 0 7 5 4-2 to improve toward f a l l . The o v e r a l l s u r v i v a l of young from b i r t h to age at f i r s t capture was estimated by comparing the number of young that could have been produced by pregnant females on each area w i t h the number of j u v e n i l e s a c t u a l l y caught. Numbers born were es-timated from the number of pregnancies observed and from the number of young per l i t t e r (4- .82) , c a l c u l a t e d from the combined data. TABLE XVII-. P r o p o r t i o n of young s u r v i v i n g to 4—6 weeks of age. P o p u l a t i o n N a t u r a l Golf Course, 1964-C h a n c e l l o r , 1 9 6 5 Lookout , 1 9 6 5 Marine Dr. , 1 9 6 5 Wreck Beach,1965 Experimental Acadia, 1964-Acadia , 1 9 6 5 N - no.of pregnancies B - estimated no.of young born C - no.of j u v e n i l e s captured P - estimated p r o p o r t i o n of young s u r v i v i n g to 4—6 weeks of age. The d i f f e r e n c e s between areas are not s i g n i f i c a n t , as t e s t e d by a n a l y s i s of variance ( P > . 0 5 ) ; t h e r e f o r e , the data, f o r a l l areas were combined to give 78 pregnancies and 130 j u v e n i l e s . Of the 376 young expected from these pregnancies, the p r o p o r t i o n of young caught was 0 . 3 4 - 6 . Thus, even on a conservative b a s i s , only about one t h i r d of the young born were marked. Although the d i f f e r e n c e s between the experimental and H B C 4- 1 9 . 2 8 7 O.36.3 11 5 3 . 0 2 16 0 . 3 0 2 3 14-. 4-6 7 0.4-84-9 4-3.33 19 0.4-38 8 3 8 . 5 6 12 0 . 3 1 2 32 154-.24- 4-1 0 . 2 6 6 11 5 3 . 0 2 28 0 . 5 2 8 *+3 n a t u r a l populations i n the p r o p o r t i o n of young s u r v i v i n g t o h-6 weeks of age are not s t a t i s t i c a l l y s i g n i f i c a n t , the d i f -ferences i n s u r v i v a l on Acadia In 196*4- and i n 1965 are h i g h l y s i g n i f i c a n t ( . 0 1>P, X:2 = 12.100 w i t h 1 d . f . ) . b. Minimum s u r v i v a l r a t e s of .juveniles and a d u l t s . Deer mice that were no longer caught on an area may have d i e d , emigrated, or have been missed f o r some other reason. I n d i v i d u a l s l i v e d an unknown time a f t e r t h e i r l a s t capture and, t h e r e f o r e , one knows nothing about the m o r t a l i t y r a t e . One can c a l c u l a t e the minimum s u r v i v a l r a t e , the p r o p o r t i o n of animals re l e a s e d at time t , caught at time t + 1. J u v e n i l e and adult s u r v i v a l f o r each four-week trapping p e r i o d were compared before c a l c u l a t i n g the minimum s u r v i v a l r a t e s f o r the study p o p u l a t i o n s . P r e l i m i n a r y a n a l y s i s of the data shown below i n d i c a t e s that the rat e s of disappearance f o r the j u v e n i l e s are f a i r l y uniform throughout the breeding season. TABLE X V I I I . Minimum s u r v i v a l r a t e s of j u v e n i l e s and a d u l t s . J u v e n i l e s Adults Males Females Males Females Month R MSR H MSR R MSR R MSR Apr. 3 0.667 3 1.000 55 0.7*+5 hi 0.976 May 7 0.71*+ 9 0.778 50 0.780 *+5 O.Shh June 9 O.667 7 0.71*+ 1+1 0.756 *+3 0.7*+*+ J u l y 16 0.688 10 0.700 37 .0.757 hi 0.732 Aug. 25 O.680 17 0.706 50 0.580 5h 0.685 Sept. 13 O.769 16 0.812 19 o.6S*+ 20 0.850 Oct. 8 0.875 12 O.667 6 1.000 15 0.800 R - No.of mice released MSR - Minimum s u r v i v a l r a t e s A n a l y s i s of variance shows no s i g n i f i c a n t d i f f e r e n c e s between e i t h e r age or sex, but does show d i f f e r e n c e s between trapping periods ( , 0 5 > P > . 0 1 ) ; t h e r e f o r e , the data f o r age and sex were pooled. In other words, once a j u v e n i l e had been marked, i t s chances of s u r v i v a l were as good as an a d u l t ' s . The minimum s u r v i v a l r a t e s f o r each four-week trapping p e r i o d are given i n Tables XIX and XX. Because the numbers are so s m a l l , minimum s u r v i v a l r a t e s vary rather widely. A n a l y s i s of variance r e v e a l s no s i g n i f i c a n t d i f f e r e n c e s i n s u r v i v a l bet-ween areas, even though s u r v i v a l began t o decrease on Chancellor i n J u l y 1 9 6 5 and on Wreck Beach i n August 1 9 6 5 . The reason f o r the d e c l i n e i n numbers and i n s u r v i v a l on these two areas i s not known. The s u r v i v a l r a t e s f o r males were s i g n i f i c a n t l y lower than those f o r females, averaging 0 . 7 5 8 and 0 . 8 1 6 , r e s p e c t i v e l y ( . 0 5 ^ P > . 0 1 ) . These r a t e s were c a l c u l a t e d by d i v i d i n g the t o t a l number of males recaptured by the t o t a l number of males r e l e a s e d , and s i m i l a r l y f o r the females. Clear seasonal changes i n s u r v i v a l were observed ( , 0 1 > P ) . S u r v i v a l was lowest during the breeding season from June through August ( 0 . 7 1 1 ) and highest during the winter ( O . 8 3 2 ) . S u r v i v a l was a l i t t l e poorer i n 1 9 6 5 than i n 196*f ( , 0 5 > P > . 0 1 ) . The seasonal and y e a r l y r a t e s were c a l c u l a t e d as f o r the males. The o v e r a l l s u r v i v a l per month was 0 . 7 9 0 . k5 3. Disease, p a r a s i t i s m , and p r e d a t i o n . With the exception of two females caught on the Wreck Beach area, none of the deer mice from the f i e l d appeared diseased. The two mice had damp, clumped f u r even though the c o t t o n i n the ne.st boxes was dry. One female remained on the area f o r three months, the other four months. The deer mice were p a r a s i t i z e d by t i c k s , l i c e , and f l e a s , and during J u l y and August, by l a r v a e of the b o t f l y , Cuterebra sp. Of kO mice p a r a s i t i z e d w i t h b o t f l y l a r v a e , a l l but nine badly p a r a s i t i z e d ones were recaptured i n apparently good h e a l t h . S a d l e i r ( 1 9 6 5 ) a l s o reported p a r a s i t i s m by b o t f l y l a r v a e . The traps were d i s t u r b e d s p o r a d i c a l l y by raccoons (Procyon  l o t o r ) but only once was^there evidence of a k i l l . Douglas s q u i r r e l s (Tamiasciurus douglasi) may a l s o have d i s t u r b e d the t r a p s . Four s h o r t - t a i l e d weasels (Mustela erminea) were caught and removed from the areas during the two years. Owls and house cats were a l s o heard or seen. 4-6 TABLE XIX. Minimum s u r v i v a l r a t e s f o r the n a t u r a l p o p u l a t i o n s , 1964--5. P e r i o d 26 Feb. ,1964-26 Mar. ,1964-23 Apr., 1964-21 May,1964-18 June, 1964-16 J u l y , 196'+ 13 Aug., 1964-10 Sept. ,196»+ 26 Feb. ,1965 26 Mar. ,1965 23 Apr.,1965 21 May,1965 18 June,1965 16 July,1 9 6 5 13 Aug.,1965 10 Sept. , 1965 Males R C MSR 4- 4- 1.000 3 3 1.000 4- 3 0.750 3 2 0.667 3 3 1.000 2 0.667 7 5 0.714-4- -11 9 0.818 25 21 0..84-0 '+3 30 0..698 36 27 0.750 32 23 0.719 33 25 0.758 4-5 25 0.556 27 - -Females R C MSR 3 3 1.000 3 3 1.000 2 2 1.000 3 2 O.667 2 2 1.000 2 2 1.000 3 1 0.333 1 - -11 11 1.000 21 19 0.905 26 25 0.962 31 25 0.806 30 24- 0.800 34- 24- 0.706 >+2 26 0.619 28 _ _ R -C -MSR -no.of deer mice released at time t . no.of deer mice recaptured at time t + 1. minimum s u r v i v a l r a t e . M-7 TABLE XX. Minimum s u r v i v a l r a t e s f o r experimental p o p u l a t i o n s , 1 9 6 M - - 5 . Males P e r i o d R C 26 Feb. ,196'+ h M-26 Mar. , 1 9 6M- 5 5 23 Apr. , 1 9 6M- 6 5 2 1 May, 196M- 6 5 18 June , 1 9 6M- 5 M-16 J u l y , 1 9 6M- 8 7 13 Aug. , 1 9 6M- 13 10 10 Sept . , 1 9 6 M - 18 16 8 Oct. , 1 9 6M- 1 M - 13 5 Nov. , 1 9 6M- 23 2 0 3 Dec. , 1 9 6M- 19 1 1 1 26 Feb. , 1 9 6 5 6 5 26 Mar.. , 1 9 6 5 6 1+ 23 Apr. 1 9 6 5 5 5 2 1 May,1965 7 6 l o June , 1 9 6 5 9 7 16 J u l y , 1 9 6 5 9 5 13 Aug.. 1 9 6 5 9 6 10 Sept. , 1 9 6 5 12 -MSR 1 . 0 0 0 1 . 0 0 0 0 . 8 3 3 0 . 8 3 3 0 . 8 0 0 0 . 8 7 5 0 . 7 6 9 0 . 8 8 9 0 . 9 2 9 o . 8 7 0 o 0 . 8 3 3 2 0 . 8 3 3 0 . 6 6 7 1 . 0 0 0 0 . 8 5 7 0 . 7 7 8 0 . 5 5 6 0 . 6 6 7 R 7 7 9 9 6 10 Female s C 7-7 9 9 7 18 15 20 20 27 20 33 26 32 2 M - 1 M-7 7 8 10 5 8 13 2 >+ 7 7 6 M-7 MSR 1 . 0 0 0 1 . 0 0 0 1 . 0 0 0 1 . 0 0 0 0 . 6 6 7 0 . 7 0 0 0 . 8 3 3 1 . 0 0 0 0.7M-1 0 . 7 8 8 o 0 . 9 0 9 2 0 . 5 0 0 0 . 5 7 1 1 . 0 0 0 0 . 8 7 5 0 . 6 0 0 0 . 8 0 0 0 . 3 7 5 R - no.of deer mice rel e a s e d at time t . C - no.of deer mice recaptured at time t + 1 . MSR - minimum s u r v i v a l r a t e . 1 - no.of deer mice recaptured 26 F e b . , 1 9 6 5 . 2 - average MSR f o r three trapping p e r i o d s . 4-8 V. DISCUSSION In t h i s s e c t i o n the data w i l l be i n t e g r a t e d w i t h the r e s u l t s of other workers. F o l l o w i n g t h i s w i l l be a d i s c u s s i o n of the relevance of these observations on deer mice to e x i s t i n g t h e o r i e s on the r e g u l a t i o n of numbers. A. POPULATION CHANGES Each year since 1962, when S a d l e i r (1965) began trapping deer mouse populations on the U n i v e r s i t y Endowment Lands, numbers have remained r e l a t i v e l y steady during the summer, i n -creased suddenly to a peak i n the f a l l , and then g r a d u a l l y de-c l i n e d over the win t e r . With one exception, no marked decrease i n numbers p r i o r to.the onset of the breeding season has been observed. S a d l e i r (1965) observed a drop i n numbers i n one of h i s study populations i n June 1963, the pop u l a t i o n began breed-ing i n J u l y . There has been l i t t l e f l u c t u a t i o n i n numbers from year to year. The changes i n pop u l a t i o n s t r u c t u r e have been that most a d u l t s of the summer breeding p o p u l a t i o n die by f a l l and are r e -placed by s e x u a l l y immature animals which overwinter and mature i n the f o l l o w i n g spring t o form a new breeding p o p u l a t i o n . Few j u v e n i l e s appear during the summer breeding season. Thus, there i s an annual turnover of i n d i v i d u a l s i n the p o p u l a t i o n . S i m i l a r changes i n population numbers and s t r u c t u r e have been reported by many workers (Howard, 194-9; Snyder, 1956; Hoffmann, 1958; Newson 1964-; Krebs, 1964-). 4-9 B. REPRODUCTION The l e n g t h of the breeding season has been quite v a r i a b l e . Breeding had begun when S a d l e i r began h i s studies i n May 1962 and extended to September. In 1963 the breeding season extend-ed from June to September, i n 1964- from March to September, and i n 1 9 6 5 from May to August. No e s t i v a l pause has been observed. Breeding seasons of s i m i l a r d u r a t i o n have been reported by Sheppe ( 1 9 5 8 ) f o r t h i s subspecies (P. m. a u s t e r u s ) , by Beer et a l ( 1 9 5 7 ) f o r P. m. b a i r d i i and P. m. g r a c i l i s , and by McCabe and Blanchard ( 1 9 5 0 ) f o r P. m. gam b e l i i . Winter breeding has been reported f o r P. m. gambelii i n C a l i f o r n i a (Jameson, 1 9 5 3 ) . Few females, however, s t a r t e d breeding i n the n a t u r a l po-p u l a t i o n s u n t i l May i n 1964- and i n a l l populations u n t i l May i n 1965? even though, as a l s o noted by S a d l e i r ( 1 9 6 5 ), the females were p e r f o r a t e e a r l i e r i n the s p r i n g , A p o s s i b l e reason f o r t h i s delay i n breeding i s that the f i r s t estrous c y c l e s may have been s t e r i l e , as has been observed i n Clethrionomys (Brambell and Rowlands, 1 9 3 6 ) , Peromyscus leucopus ( B e n d e l l , 1 9 5 9 ) , and i n P. maniculatus (Jameson, 1 9 5 3 ) . The p r o p o r t i o n of adult or overwintered females that be-came pregnant was low, O . I 6 3 i n 1 9 6 3 , 0 . l 6 l i n the n a t u r a l po-p u l a t i o n s In 1964- and 1 9 6 5 , and 0 . 3 5 2 and 0 . 2 1 6 i n the e x p e r i -mental populations In 1964- and 1 9 6 5 , r e s p e c t i v e l y . The f r e -quency of pregnancy i n females a l i v e throughout the breeding season i n the n a t u r a l populations i n 1964- and 1 9 6 5 was 1 . 5 50 l i t t e r s per breeding female, and i n the experimental popula-t i o n s , 1.3 before June 196M- and 2.8 a f t e r , and 2 .0 i n 1965. The incidence of postpartum mating- i n the marked populations was very low, (6/27 = 0 . 2 2 2 ) , most females only becoming pregnant again two to three weeks a f t e r p a r t u r i t i o n . This gap between pregnancies suggests a p e r i o d of l a c t a t i o n anestrus. McCabe and Blanchard (1950) reported an average of four l i t t e r s per breed-ing female per season. The number of embryos per l i t t e r observed i n the f i e l d was h.95 and the number of young per l i t t e r i n the l a b o r a t o r y was h.6Q. Sheppe (1958) reported 5.6 embryos per l i t t e r f o r s e v e r a l subspecies of P. maniculatus i n B r i t i s h Columbia. The number per l i t t e r reported f o r other subspecies i s h.28 young f o r P. m. b a l r d i i i n Michigan (Howard, 19*+9) , 5.06 embryos f o r P. m. gam-b e l i i near Berkeley (McCabe and Blanchard, 1950) and H-.60 embryos i n the S i e r r a Nevada i n northern C a l i f o r n i a (Jameson, 1953)5 and 5.38 embryos f o r P. m. g r a c i l i s i n Ontario (Coventry, 1937). The average l i t t e r s i z e i n P. m. austerus was a f f e c t e d by maternal weight, or age. Larger females c a r r i e d l a r g e r numbers of embryos. This phenomenon was a l s o observed by Beer et a l (1957) f o r P. m. b a i r d i i and P. m. g r a c i l i s . A l a r g e r number of young per l i t t e r has been reported at higher a l t i t u d e s , but a longer breeding season and more l i t t e r s at lower e l e v a t i o n s ( A s d e l l , 196H-). 51 C. MORTALITY The decrease i n l i t t e r s i z e during pregnancy was from 5.18 ova shed (corpora l u t e a ) to 5.00 i m p l a n t a t i o n s i t e s t o 4-.95 embryos. The p r o p o r t i o n of ova l o s t was approximately 0 . 1 0 . This decrease i s comparable to that observed by Beer et a l (1957) f o r P. m. b a l r d i i . 5.77 ova to 5.25 s i t e s t o 5-08 embryos, and a p r o p o r t i o n of 0.12 ova l o s t ; and f o r P. m. g r a c i -l i s , 5.86 ova t o 5.4-5 s i t e s to 5.32 embryos, and a p r o p o r t i o n of 0.10 ova l o s t . Complete l o s s of l i t t e r s i s probably quite low, as shown by the p r o p o r t i o n of pregnant females which, when subsequently caught, were l a c t a t i n g (61/70 = . 8 7 1 ) . Neither l o s s at b i r t h nor n e s t l i n g m o r t a l i t y was observed. Howard (194-9) reported that s e v e r a l l i t t e r s born i n the spring were dead at b i r t h or died s h o r t l y a f t e r . He observed a decrease i n l i t t e r s i z e of n e s t l i n g s i n P. m. b a i r d i i from 4-.28 young per l i t t e r at 0-2 days, to 4-. 17 at 3-7 days to 3.39 at 8-21 days and 3.28 from 21 days t o d i s p e r s a l . He a l s o observed that the l a r g e s t l o s s of young occurred at about three weeks a f t e r b i r t h , or j u s t a f t e r weaning. The p r o p o r t i o n of young l o s t from b i r t h to age at f i r s t capture was 0 .65 i n P. m. austerus compared wi t h 0.82 i n P. m. gambelii (McCabe and Blanchard, 1950). I found that the sur-v i v a l of the young was poor during the breeding season and im-proved toward f a l l , but that once the j u v e n i l e s had been marked, 52 they survived as w e l l as the a d u l t s . S a d l e i r (1965), however, observed that j u v e n i l e s e n tering the summer breeding popula-t i o n survived worse than those e n t e r i n g the f a l l p o p u l a t i o n . My observations on n a t u r a l populations were made over a short-er p e r i o d of time and could not be used t o make t h i s comparison. B e n d e l l (1959) found s i m i l a r s u r v i v a l rates f o r j u v e n i l e s and a d u l t s i n P.. leucopus p o p u l a t i o n s , and that no matter what time of year the young entered the p o p u l a t i o n , they survived as w e l l as the a d u l t s . The s u r v i v a l of ad u l t s was good during the winter (0.332) but dropped during the summer breeding season ( 0 . 7 1 1 ) . The observed average s u r v i v a l per month of 0 .80 was a l s o reported by S a d l e i r (1965) f o r . t h i s subspecies, and by B e n d e l l (1959) f o r P. leucopus. D. EXTRINSIC FACTORS Disease may be an important m o r t a l i t y f a c t o r i n nature, but C h i t t y (195*+) has shown that the presence of an e p i z o o t i c i s not n e c e s s a r i l y c o r r e l a t e d - w i t h high m o r t a l i t y i n n a t u r a l p o p u l a t i o n s . Except f o r two mice caught on the Wreck Beach area, none of the animals from the f i e l d appeared diseased. Hence, there was no evidence of an endemic e p i z o o t i c disease k i l l i n g animals i n l a r g e r numbers i n the spring than i n the f a l l . 53 Predators have been assumed to be r e s p o n s i b l e f o r keeping numbers i n P. maniculatus populations i n check (Howard, 194-9) ? and even, to cause p o p u l a t i o n d e c l i n e s (Brant, 1962). Although predators were present on the U n i v e r s i t y Endowment Lands, there i s no i n d i c a t i o n that they took more mice at the beginning of the season than at the end. V a r i a t i o n s i n weather could e x p l a i n part of the d i f f e r e n c e s i n l e n g t h of the breeding season, and, perhaps, the amount of r e p r o d u c t i o n between the two years. The m i l d weather i n 1964-was probably conducive t o reproduction and the c o l d spring and hot, dry summer i n 1965 were probably i n i m i c a l , but In both years the amount of breeding was below the p o t e n t i a l c a p a c i t y . In each year breeding began at d i f f e r e n t times i n the marked popu-l a t i o n s . Thus, weather by i t s e l f d i d not i n h i b i t breeding. Weather may a l s o have a f f e c t e d s u r v i v a l i n the two years although d i f f e r e n c e s i n the s u r v i v a l r a t e s between populations were not s t a t i s t i c a l l y s i g n i f i c a n t . More s e n s i t i v e a n a l y s i s d i d point up d i f f e r e n c e s between years, s u r v i v a l being poorer i n 1965 than i n 1964-. There i s no d i r e c t evidence, however, that these d i f f e r e n c e s were due to adverse weather c o n d i t i o n s . The food supply was assumed to be s u f f i c i e n t , and no e x t r a food was put i n the f i e l d . Food, however, could be scarce i n the spring and become more p l e n t i f u l during the summer and enough food could be stored i n the f a l l to c a r r y the p o p u l a t i o n through the winter.- Food was probably not scarce on Acadia i n the spring 5h of 1965 since the number of deer mice was exp e r i m e n t a l l y r e -duced from 75 to 12; hence, n e i t h e r the onset of breeding i n the sprin g nor production of l i t t e r s l a t e r i n the summer could be d i r e c t l y c o n t r o l l e d by the food, supply. The mice caught i n the s p r i n g , many of which were unmarked, may have been l i v i n g on the area but not p r e v i o u s l y caught or may have immigrated onto the area to f i l l the vacuum created by trapping i t out. Po p u l a t i o n numbers were very s t a b l e from year t o year, es-p e c i a l l y on the Golf Course, while the food supply probably v a r i e d ; t h e r e f o r e , food supply by i t s e l f was not r e g u l a t i n g the popu l a t i o n s . Bendell (1959), working on P. leucopus, and Krebs and DeLong (1965), working on Mi c r o t u s , have reported that populations do not increase i n d e f i n i t e l y when given an u n l i m i t -ed supply of food, and the problems of pop u l a t i o n r e g u l a t i o n s t i l l remain. Trapping may have caused an increase i n l o s s of l i t t e r s be-ing suckled. L a c t a t i n g females which were caught i n the traps were kept away from t h e i r l i t t e r s overnight, l e a v i n g the young without food or p r o t e c t i o n f o r a maximum of about 12 hours. T o t a l l o s s of l i t t e r s , however, was f a i r l y low, as only 9 oxxt of 70 pregnant females showed no signs of having l a c t a t e d when subsequently caught. The observed seasonal change i n s u r v i v a l of young i s not due to t r a p p i n g , as females were subjected to the same trapping procedure i n the f a l l as i n the s p r i n g . 55 E. SEASONAL INCREASE IN NUMBERS The seasonal increase i n numbers observed i n the marked po-p u l a t i o n s was only 1.04-. This f i g u r e was c a l c u l a t e d by compar-ing the number of ad u l t s present i n A p r i l (77) wi t h the number s t i l l a l i v e In October (10) plus the number of j u v e n i l e s s u r v i v -ing t o October ( 7 0 ). Some of the unmarked animals caught i n the f a l l may have grown up on the area but were not included i n t h i s c a l c u l a t i o n , as they may e q u a l l y w e l l have been immigrants. The t o t a l observed three f o l d increase i n numbers was probably due to immigration i n t o the pop u l a t i o n s . The p o t e n t i a l seasonal increase i n numbers can be estimat-ed from the f o l l o w i n g assumptions about reproduction and s u r v i v -a l . Reproduction: Number of young per l i t t e r i s 5.00. Sex r a t i o at b i r t h i s u n i t y . Adults bear one l i t t e r per month between May and September. J u v e n i l e s mature at 8 weeks of age. S u r v i v a l : Adult s u r v i v a l rate per 4- weeks i s 0 . 8 0 . J u v e n i l e s u r v i v a l rate from b i r t h to h weeks i s 0.35. By applying these assumptions t o a cohort of 10 females and 10 males a l i v e In A p r i l (week 0 ) , one f i n d s that 61.86 animals would be present In October (week 28) (Table X X I ) . This represents a three f o l d p o t e n t i a l increase In numbers In co n t r a s t to v i r t u a l -l y no increase due to reproduction i n the marked po p u l a t i o n s . Loss of mice over the winter at the rate of 0.17 per month would reduce numbers to the previous spring's l e v e l . A po p u l a t i o n of 56 100 mice i n the s p r i n g , f o r example, would grow to 300 mice by f a l l and dwindle to 98 mice by the next s p r i n g . The model allows f o r exact replacement of the p o p u l a t i o n , but other patterns of po p u l a t i o n change are a l s o p o s s i b l e . r Under c e r t a i n c o n d i t i o n s such as a clement w i n t e r , s u r v i v a l would probably be b e t t e r and the spring population could i n -crease. On the other hand adverse weather c o n d i t i o n s would r e -duce population numbers. F. REVELANCE OF PRESENT DATA TO EXISTING THEORIES Andrewartha and B i r c h (195*+) have suggested that the num-ber of animals i n a n a t u r a l p o p u l a t i o n i s l i m i t e d by e x t r i n s i c environmental f a c t o r s , p a r t i c u l a r l y by shortage of time due t o adverse weather c o n d i t i o n s when the r a t e of increase r i s p o s i -t i v e . In a previous s e c t i o n on e x t r i n s i c f a c t o r s i t was decided that v a r i a t i o n s i n weather were not of primary importance i n po-p u l a t i o n r e g u l a t i o n since there were no s i g n i f i c a n t changes In breeding success between the two years and since p o p u l a t i o n num-bers were very stable from year to year. Although m o r t a l i t y i s the main f a c t o r , changes i n i t are not due to c l i m a t i c condi-t i o n s alone but, perhaps, to s o c i a l i n t e r a c t i o n s w i t h i n the po-p u l a t i o n s . Wynne-Edwards (196.2, 1961+) has advanced the idea that animal populations can adjust t h e i r reproductive r a t e s to avoid over-e x p l o i t a t i o n of the renewable resources of the environment. He says that a population receives i n f o r m a t i o n about i t s numbers TABLE XXI. P o t e n t i a l seasonal increase i n numbers. Time (weeks) Generation P a r e n t a l Males Females F i r s t F 4 F i r s t Fa Second F Thir d F2 Second F 4 F i r s t F2 Second F2 T h i r d F A F i r s t F2 Fourth E. F i f t h S i x t h F 1 S u r v i v a l of p a r e n t a l cohort of 10 females and 10 males and t h e i r offspring:-. 0 4- 8 12 16 20 24- 28 10.00 • .-8. oo 6.4-0 5.12 4-. 10 3.28 2.62 2.10 10.00 8 .00 6.4-0 5.12 4-. 10 3.28 2.62 2.10 — 4-0.00 14-. 00 11.20 3.96 7.13 5.74- 4-. 59 — — — — 22.4-0 7.84- 6.28 5.02 - - - - - 17.94- 6.28 5.02 — — - - - - 14-. 35 5.02 - - 32.00 11.20 8.96 7.18 5.74- 4-. 59 - - - - - 17.94- 6.28 5.02 — — — — - - 14-. 35 5.02 - - - 25.60 8.96 7.18 5.74- 4-. 59 - - - - - - 14-.35 5.02 — — - — 20.25 7.18 5.74- 4-. 59 - — - - - 16.4-0 5.74- 4-. 59 - - - - - - 13.10 4 .^59 vn T o t a l 61.86 Assumptions: Reproduction: Number of young per l i t t e r i s 5 . 0 0 . Sex r a t i o at b i r t h i s u n i t y . Adults bear one l i t t e r per month between weeks 4- and 28, J u v e n i l e s mature at 8 weeks of age. M o r t a l i t y : Adult s u r v i v a l r a t e per 4- weeks i s 0 . 8 0 . J u v e n i l e s u r v i v a l r a t e from b i r t h to 4- weeks i s O .35. 58 through s o c i a l i n t e r a c t i o n s and t e r r i t o r i a l behavior and that i these i n t e r a c t i o n s between members of the p o p u l a t i o n can prevent some of them from breeding and so, i n e f f e c t , reduce the b i r t h r a t e . According to these i d e a s , the high aggressive l e v e l ob-served at the beginning of the breeding season by S a d l e i r ( 1 9 6 5 ) and Healey ( 1 9 6 6 ) would be i n t e r p r e t e d as an adaptation which favo r s s u r v i v a l of the population by l i m i t i n g the number of breed-ing animals i n an area, and the shortened breeding seasons ob-served i n some populations and the low p r o p o r t i o n of pregnancies as f u r t h e r adaptations f o r preventing over-population. That i s , he b e l i e v e s that populations that don't c o n t r o l t h e i r reproduc-t i v e r a t e s w i l l destroy themselves and be replaced by popula-t i o n s that do c o n t r o l t h e i r reproductive r a t e s . Various a u t h o r i t i e s , however, have objected to t h i s view on the grounds that i t i s c o n t r a r y to the present view on n a t u r a l s e l e c t i o n of the i n d i v i d u a l (Braestrup, 1 9 6 3 ; Lack., 196'+; P e r r i n s , 1 9 6 Lf; Smith, I96H-). Smith (1964-) discusses behavior which "leads an i n d i v i d u a l not to breed i n circumstances i n which other mem-bers of the species are breeding s u c c e s s f u l l y " and p o i n t s out that " I f the d i f f e r e n c e between breeders and non-breeders i s gene-t i c a l l y determined, then i t i s the breeders whose genotype i s per-petuated. 1! He a l s o p o i n t s out that " i t i s u n l i k e l y that species are o f t e n d i v i d e d i n t o a l a r g e number of small and completely i s o l a t e d groups,", which would a l l o w a l t r u i s t i c behavior to evolve and be perpetuated, e s p e c i a l l y i f mating i s random. 59 C h r i s t i a n ( 1 9 5 7 ) has suggested that a behavioral-physio-l o g i c a l mechanism, operating through the p i t u i t a r y - a d r e n o c o r t i c a l a x i s , a f f e c t s reproduction and m o r t a l i t y ( a l s o see C h r i s t i a n and Davis, 1 9 6 5 ; C h r i s t i a n , L l o y d , and Davis, 1 9 6 5 ) . He says that "there i s decreased f e r t i l i t y and f e c u n d i t y , increased i n t r a u t e r i n e and Infant m o r t a l i t y w i t h increased p o p u l a t i o n den-s i t y and that the increased i n f a n t m o r t a l i t y i s at l e a s t p a r t l y due to p a r t i a l f a i l u r e of l a c t a t i o n ^ " According to t h i s view, the low reproductive r a t e would be i n t e r p r e t e d as a necessary consequence of r e l a t i v e l y high numbers or high s o c i a l pressure, and the hypertrophy of the adrenal glands i n the summer, found i n t h i s study and i n those reported by C h i t t y , 1 9 6 1 ; C h i t t y and C l a r k e , I 9 6 3 ; C h r i s t i a n , 1 9 6 2; and Krebs, 1964- as evidence f o r endocrine feed-back mechanisms, a c t i n g t o reduce reproduction and increase p r e n a t a l m o r t a l i t y . Negus e_t a l ( 1 9 6 1 ) discussed evidence which i m p l i e s that d e n s i t y and s o c i a l contact are not " n e c e s s a r i l y important f a c t o r s i n f l u e n c i n g adrenal weight changes i n n a t u r a l populations'.".. In t h i s study no s i g n i f i c a n t d i f f e r e n c e s i n p r e n a t a l mort-a l i t y were found. The p r o p o r t i o n of pregnant females which were l a c t a t i n g when subsequently caught was not s i g n i f i c a n t l y greater i n the experimental populations (3*+/38) than i n the n a t u r a l po-p u l a t i o n s ( 2 7 / 3 2 ). The a c t u a l d e n s i t y on the study areas was quite low i f one considers .the number of mice the areas could have supported, as i n d i c a t e d by the number of mice removed from 6o Acadia i n February 1965. The amount of reproduction on Acadia tended to be greater i n 1964- than i n 1965 and may have been i n -v e r s e l y r e l a t e d to the number of mice on the area immediately p r i o r t o the breeding season. These observations could e q u a l l y w e l l be explained by v a r i a t i o n s i n the weather i n the two years or, perhaps, by changes i n behavior. Crowcroft and Rowe (1953, 1963) have reported that f e c u n d i -t y and reproduction decreased i n house mice caged together i n l a r g e numbers, and that breeding was resumed when the d e n s i t y was reduced by al l o w i n g the populations to disperse i n t o l a r g e r areas. New dominance-subordination r e l a t i o n s h i p s were formed i n the a d d i t i o n a l area. Petrusewicz (1957, 1963) induced p o p u l a t i o n growth by changing a pop u l a t i o n to a d i f f e r e n t k i n d of cage, by adding or removing mice. He suggested that h i s r e s u l t s were caused by a l t e r e d s o c i a l r e l a t i o n s i n the p o p u l a t i o n . Increased reproduction i n f i e l d populations has a l s o been a t t r i b u t e d to d i s r u p t i o n of s o c i a l s t r u c t u r e . L l o y d (1963) found that when the number of r a b b i t s i n a pop u l a t i o n was reduced immediately p r i o r to the onset of spring breeding, the p o p u l a t i o n increased more than i f no r a b b i t s had been removed. Sheppe (1965) obtained midsummer breeding i n populations of P. leucopus, which normally have an e s t i v a l pause, by in t r o d u c i n g mice i n very la r g e numbers onto i s l a n d s i n Ontario. S o c i a l contacts have been shown to increase p r e n a t a l mor-t a l i t y . The presence or odor of strange males f o r s e v e r a l hours 61 w i t h i n a few days a f t e r insemination may cause r e s o r p t i o n of l i t t e r s i n Mus musculus (Parkes and Bruce, 1961) and i n P. m. b a i r d i i (Bronson and E l e f t h e r i o u , 1 9 6 3 ). Strange males, how-ever, seldom remained i n an area f o r any l e n g t h of time, as 27 per cent (4-5/165) of the males i n the marked populations were caught only once. The only prolonged exposure to strange odors would be i n the traps four n i g h t s per month. Thus the r e s o r p t i o n of l i t t e r s due to pregnancy block would not be expect-ed to be important i n reducing the b i r t h r a t e . The e f f e c t s of odors need not be d e l e t e r i o u s . Odors and scent markings leave i n f o r m a t i o n about animals and t h e i r a c t i v i t i e s , as i n the hamster ( E i b l - E i b e s f e l d t , 1953) and the dog (Lorenz, 195'+). C h i t t y ( i 9 6 0 ) has proposed that "under appropriate circum-stances, i n d e f i n i t e increase i n p o p u l a t i o n d e n s i t y i s prevented through a d e t e r i o r a t i o n i n the q u a l i t y of the p o p u l a t i o n [andl that the e f f e c t s of independent events such as v/eather, become more severe as numbers r i s e and q u a l i t y f a l l s : : n . He has r e l a t e d changes i n s u r v i v a l , reproductive success, and body weight t o phases i n the population c y c l e i n v o l e s ( C h i t t y , 1 9 5 2 , C h i t t y and C h i t t y , 1962) . Krebs (196li-) , working on lemmings, reported a shortened breeding season, f a i l u r e of j u v e n i l e s to mature, and increased body weights of a d u l t s when numbers were g r e a t e s t . He suggested that the changes i n numbers could be r e l a t e d to changes i n aggressive behavior. S a d l e i r ( 1 9 6 5 ) observed that deer mice were more aggressive during the breeding season than i n the f a l l , and that the l e v e l of aggressiveness i n f i e l d populations was greater at times of decreased s u r v i v a l . Healey (1966) found that j u v e n i l e s u r v i v a l was i n v e r s e l y r e l a t e d to the l e v e l of aggressiveness of the a d u l t s . McCabe and Blanchard (1950, p. 97) suggested that "the key to s u r v i v a l [of j u v e n i l e s ] l a y i n s u c c e s s f u l establishment elsewhere, not i n i n t o l e r a n c e or com-p e t i t i o n near the nests, "but once a j u v e n i l e had l e f t i t s nest i t would be d r i v e n away from any refuge already occupied. According to these ideas, the poor reproductive performance observed i n t h i s study would be i n t e r p r e t e d as a change i n the q u a l i t y of the p o p u l a t i o n s , perhaps r e f l e c t i n g the l e v e l of aggressiveness, and as a symptom of l i v i n g i n an unfavorable en-vironment. The greater p r o d u c t i v i t y and maturation of the young observed i n 196'+ could be a s s o c i a t e d with higher mean monthly body weights. Lack (1954-) has suggested that changes i n s u r v i v a l rather than changes i n b i r t h r a t e regulate p o p u l a t i o n numbers. The reasons given f o r t h i s point of view are t h a t , i n b i r d s , b i r t h r a t e i s not f l e x i b l e enough to a f f e c t p o p u l a t i o n numbers and that fewer young are fledged from c l u t c h e s of l a r g e r than aver-age s i z e . The number of embryos per l i t t e r , however, was quite v a r i a b l e (3-7) but no evidence was observed f o r more embryos dying i n l a r g e r l i t t e r s than i n smaller ones, or, i n the l a b o r a -t o r y , f o r fewer young to be weaned i n l a r g e r l i t t e r s . No s i g n i -f i c a n t d i f f e r e n c e s between years were found i n the average num-6 3 ber of embryos per l i t t e r . That changes i n p o p u l a t i o n numbers are adequately ac-counted f o r by changes i n s u r v i v a l i s shown by the d i f f e r e n c e s i n reproductive r a t e and s u r v i v a l of the young i n the two years on the experimental area. Numbers at the end of each season were approximately the same.- In 1964- reproductive per-formance was comparatively good but s u r v i v a l of the young was poor; i n 1 9 6 5 , the reverse was observed. -Differences i n r e -productive performance were not s t a t i s t i c a l l y s i g n i f i c a n t ; d i f f e r e n c e s i n s u r v i v a l were s t a t i s t i c a l l y s i g n i f i c a n t . Deer mouse populations are s e l f - r e g u l a t o r y , changes i n death rate r a t h e r than changes i n reproductive rate being more important i n maintaining the s t a t i o n a r y s t a t e of these popula-t i o n s . There i s some evidence that a g o n i s t i c behavior i s the mechanism tending to space the animals out and e l i m i n a t i n g the excess animals from the p o p u l a t i o n s . The suggestion was made that changes i n reproductive performance may be a s s o c i a t e d with changes i n the aggressive l e v e l of the p o p u l a t i o n . 64-VI. SUMMARY 1. The r o l e of changes In reproductive rate and death r a t e i n determining p o p u l a t i o n numbers i n the deer mouse, Peromys-cus maniculatus. was compared by studying n a t u r a l and e x p e r i -mental populations on the U n i v e r s i t y Endowment Lands i n 1964-and 1965. 2. Weekly l i v e trapping of the populations showed that num-bers remained r e l a t i v e l y steady over the summer, increased i n the f a l l , and g r a d u a l l y d e c l i n e d over the w i n t e r . In two po-p u l a t i o n s numbers d i d not increase i n the f a l l of 1965. By f a l l immature animals which overwintered to form the next year's breeding p o p u l a t i o n had replaced most of the a d u l t s from the summer breeding p o p u l a t i o n , r e s u l t i n g i n an annual overturn of the p o p u l a t i o n s . 3 . Males were fecund from March to September i n both years. Females bore l i t t e r s from March to September i n 1964- and from March to August i n 1965. Few l i t t e r s were born before May i n e i t h e r year. Seasonal v a r i a t i o n s i n breeding c o n d i t i o n were not so marked, i n the males as they were i n the females. 4-. L i t t e r s i z e was s i m i l a r i n both years and seemed to be i n -dependent of p a r i t y , but to be greater i n heavier females and to change, seasonally. 5. The p r o p o r t i o n of ova l o s t was 0.10 and was r e l a t i v e l y con-stant during the p e r i o d of study. The r a t e of l o s s of young 6 5 from b i r t h to age at f i r s t capture averaged 0 . 6 5 and. was great-est during the breeding season, decreasing toward f a l l . The s u r v i v a l r a t e s of the j u v e n i l e s and the a d u l t s were not s t a t i s -t i c a l l y d i f f e r e n t . S u r v i v a l was s i g n i f i c a n t l y lower during the breeding season ( 0 . 7 1 1 ) than during the winter ( 0 . 8 3 2 ) , and s i g n i f i c a n t l y lower i n the males ( 0 . 7 5 8 ) than i n the females ( 0 . 8 l 6 ) . . The o v e r a l l s u r v i v a l rate of the a d u l t s was 0 . 7 9 0 per month. 6 . Reduction of overwintered populations p r i o r t o the breed-ing season i n attempts to induce the populations to have higher reproductive r a t e s were not e n t i r e l y s u c c e s s f u l . Although there were no s i g n i f i c a n t d i f f e r e n c e s between the n a t u r a l and e x p e r i -mental populations i n mean monthly body weights, reproductive performance, or s u r v i v a l , there were d i f f e r e n c e s between the ex-perimental populations i n 1964- and 1 9 6 5. Body weights and r e -productive performance were lower, and s u r v i v a l of the young from b i r t h t o age at f i r s t capture was higher i n 1 9 6 5 than i n 1964-. The p r o p o r t i o n of subadults that became fecund was great-est i n t h i s .area i n 1964-. 7 . The e f f e c t s of e x t r i n s i c environmental f a c t o r s on popula-t i o n numbers were examined. The e f f e c t s of disease, p a r a s i t e s , and predators d i d not seem to be severe enough to account f o r the r e g u l a t i o n of numbers. Clement weather was a s s o c i a t e d w i t h the longer breeding season observed i n 1964- but v a r i a t i o n s i n weather d i d not seem to a f f e c t m o r t a l i t y . The food supply was assumed to 66 be adequate although I t was not s t u d i e d ; evidence from other s t u d i e s i n d i c a t e s that an u n l i m i t e d food supply does not pre-vent p o p u l a t i o n numbers from d e c l i n i n g . 8. Changes i n death-rate r a t h e r than In b i r t h - r a t e were more important i n r e g u l a t i n g p o p u l a t i o n numbers. Wynne-Edwards'!s concept of group s e l e c t i o n and s e l f - l i m i t a t i o n of numbers by co n v e n i e n t l y decreasing the reproductive rate was r e j e c t e d on t h e o r e t i c a l grounds. C h r i s t i a n ' s idea that changes i n repro-d u c t i v e rate are mainly r e s p o n s i b l e f o r r e g u l a t i o n was not supported. That changes i n po p u l a t i o n numbers are adequately accounted f o r by changes i n m o r t a l i t y was supported by the data. The suggestion was made that changes In reproductive p e r f o r -mance may be a s s o c i a t e d w i t h changes i n the aggressiveness of the p o p u l a t i o n . 67 BIBLIOGRAPHY Andrewartha, H.G. and B i r c h , L.C. 1954-. The d i s t r i b u t i o n and abundance of animals. U n i v e r s i t y of Chicago P r e s s , Chicago. A s d e l l , S.A. 1964-. P a t t e r n s of mammalian reproduction. 2 n d Ed. Constable, London. 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I n v e s t i g a t i c n pf e x p e r i m e n t a l l y i n d u c e d p p p u l a t i p n grpwth.Ek.61.'Pplska,Se'ir:-i.'A, £ : 2 8 1 - 3 0 9 . 71 P e t r u s e w i c z , K. 1 9 6 3 . P o p u l a t i o n growth i n d u c e d by d i s t u r b a n c e i n t he e c o l o g i c a l s t r u c t u r e of the p o p u l a t i o n . E k o l . P o l s k a , Ser.A, 1 1 : 8 7 - 1 2 5 . R e y n o l d s , S.R.M. 1 9 5 5 . G e s t a t i o n mechanisms. R e p r o d u c t i o n and i n f e r t i l i t y , Symposium, M i c h . S t a t e U n i v . 7 1 - 7 8 , S a d l e i r , R.M.F.S. 1 9 6 5 . The r e l a t i o n s h i p between a g o n i s t i c be-h a v i o u r and p o p u l a t i o n changes i n the deermouse, Peromyscus  m a n i c u l a t u s (Wagner). J . A n i m . E c o l . 3]+: 331-35-2 . Sheppe, W.A. 1 9 5 8 . S y s t e m a t i c and e c o l o g i c a l r e l a t i o n s of Peromyscus oreas and P. m a n i c u l a t u s . Unpub.Ph.D. T h e s i s , U n i v . B r i t i s h Columbia. Sheppe, W.A. 1 9 6 5 . U n s e a s o n a l b r e e d i n g i n a r t i f i c i a l c o l o n i e s °f Peromyscus l e u c o p u s . J . Mammal. |+6: 64-1-64-6. S h o r t e n , M. 1954-. S q u i r r e l s . C o l l i n s , , London. S m i t h , J.M. 1964-. Group s e l e c t i o n and k i n s e l e c t i o n . N a t u r e , 2 0 1 : 114 - 5 - U 4 - 7 . Snedecor, G.W. 1 9 5 6 . S t a t i s t i c a l methods a p p l i e d t o ex p e r i m e n t s i n a g r i c u l t u r e and b i o l o g y . 5 t h Ed. Iowa S t a t e C o l l e g e P r e s s , Ames. S n e l l , G.D. 194-1. B i o l o g y of the l a b o r a t o r y mouse. B l a k i s t o n Company, P h i l a d e l p h i a . S nyder, D.P. 1 9 5 6 . S u r v i v a l r a t e s , l o n g e v i t y , and p o p u l a t i o n f l u c t u a t i o n s i n the w h i t e - f o o t e d mouse, Peromyscus l e u c o p u s , i n s o u t h e a s t e r n M i c h i g a n . M i s c . P u b l . M u s . Z o o l . , U n i v . M i c h . No.9 5 . 33 P. S o u t h e r n , H.N. 1 9 5 9 . M o r t a l i t y and p o p u l a t i o n c o n t r o l . I b i s , 1 0 1 : 4-29-4-36. S t e e l , R.G-.D. and T o r r i e , J.H. i 9 6 0 . P r i n c i p l e s and p r o c e d u r e s of s t a t i s t i c s . M c G r a w - H i l l Book Company, I n c . , New Yo r k . S t i c k e l , L.F. i 9 6 0 . Peromyscus ranges at h i g h and low p o p u l a t i o n d e n s i t i e s . J . Mammal. 4-1: 4-33-4-4-1. Wecker, S.C. 1 9 6 2 . The e f f e c t s of bot f l y p a r a s i t i s m on a l o c a l p o p u l a t i o n of the w h i t e - f o o t e d mouse. E c o l o g y , 4^ .: 5 6 1 - 5 6 5 . Wynne-Edwards, V.C. 1 9 6 2 . A n i m a l d i s p e r s i o n i n r e l a t i o n t o s o c i a l b e h a v i o u r . O l i v e and Boyd L t d . , E d i n b u r g h and London. 72 Wynne-Edwards, V.C. 1964. Population control i n animals. Sci.Amer. 211: 68-74-. s APPENDIX A. Pregnant u t e r i from dead samples, 196M-. Date female caught 9 May, 1964- . 17 June, 1964-2 9 June, 1964-2 J u l y , 1964-2 J u l y , 1964-18 J u l y , I 9 6 H -18 J u l y , 1964-18 J u l y , 1964-19 J u l y , 1964-19 J u l y , 1964-23 J u l y , 1964-2 Aug., 196'+ 3 Aug., 19.6M-3 Aug., 1964-8 Aug. ,196M-14- Aug., 196M-16 Aug., 1964-16 Aug. ,196M-11 Sept.,196^ 11 S e p t . , 1964-12 S e p t . , 1964-M a t e r n a l N o . c o r p o r a No.embryos U t e r i n e Avg. wt. Approximate weight l u t e a weight per embryo day of (gm.) R L R L (mg.) (mg.) pregnancy 14-. 0 2 2 1* 3 708. 4 236.I 13 19.0 4- 2 M- 1 4-893.5 - Recent p a r t u r i t i o n 1 5 . 0 2 2 2 2 1223.4- 21 19.0 1 h 1 1+ 1657.5 3 3 L 5 1*+ 2 1 . 5 3 4 2 4- - - Recent p a r t u r i t i o n 2 0 . 0 3 3 3 180.9 30.2 5 2 1 . 5 1 h 1 1250.5 250.1 13 2 2 . 0 2 2 2 3 6956.9 1391.4- 22 2 2 . 0 3 2 3 2 187.8 37.6 7 2 0 . 5 1 3 1 3 9 8 A 24-.6 KM-17.0 3 1 3 1 .83.M- 20.8 k 2 0 . 0 3 3 3 3 - -' Recent p a r t u r i t i o n 17.0 4- 2 4- 1 107.1 21.4- 4-21.0 1 5 1 5 - - Recent p a r t u r i t i o n 18.0 3 3 3 3 5897.8 983.0 19 19.0 3 2 0 2 66.4- 33.2 k 21.0 3 3 3 4 H 6 5 2 . 4 - 1665.3 2h 2 0 . 5 2 4 2+ 4- 9528.8 1905.8 25 17.0 2 4- 2 M- 1»M-3.!+ 23.9 5 1 5 . 0 ? 2 2 1 1 0 5 . 0 3 5 . 0 7 1 5 . 5 4- 2 1 4 - - Recent p a r t u r i t i o n R - r i g h t , L - l e f t . * - a l l embryos dead. + - i n c l u d e s one dead embryo. APENDIX B. Pregnant u t e r i from dead samples, 1 9 6 5 . Date M a t e r n a l N o . c o r p o r a No.embryos U t e r i n e Av.g. wt. Approximate female weight l u t e a weight per embryo day of caught (gm.) R. L R. L (mg.) pregnancy 2 5 June, 1 9 6 5 1 5 . 5 1 h 1 4- 1 1 5 6 . 5 2 3 1 . 3 13 2 5 June, 1 9 6 5 2 2 . 5 1 3 1* 3 . - - Recent p a r t u r i t i o n 11 J u l y , 1 9 6 5 1 7 . 0 1 4-. 1 4- 7 2 9 6 . 0 14-59.2 23 15 J u l y , 1 9 6 5 1 8 . 0 1 5 1 5 2 8 7 . 0 '+7.8 8 19 J u l y , 1 9 6 5 1 7 . 5 2 2 2 2 3 5 8 6 . 6 8 9 6 . 6 20 J u l y , 1 9 6 5 1 9 . 0 >+ 2 5+ 0 2 4 9 5 . 8 624-.0 16 20 J u l y , 1 9 6 5 1 9 . 0 3 1 3 1 1 1 8 . 9 2 9 . 7 6 2 1 J u l y , 1 9 6 5 1 7 . 0 2 4 2 4 14-7.0 2 4 . 5 6 26 J u l y , 1 9 6 5 21.0 0 5 0 5 1 1 6 . 0 2 3 . 2 5 27 J u l y , 1 9 6 5 1 2 . 0 3 2 —» 2 '+0.2 13.4- k 30 J u l y , 1 9 6 5 1 7 . 5 6 0 6 0 5 9 7 . 1 9 9 . 5 11 4 Aug. , 1 9 6 5 1 8 . 0 2 2 2 2 - - Recent p a r t u r i t i o n R - r i g h t , L - l e f t . * - a l l embryos dead. + - i n c l u d e s one. dead embryo. APPENDIX C. Pregnant u t e r i of known age from c o l o n y . Date female a u t o p s i e d M a t e r n a l weight (gm.) No. c o r p o r a l u t e a R L No.embryos R L U t e r i n e weight (mg.) Avg. wt. per embryo (mg.) • : Known day of pregnancy 5 Aug. , 1 9 6 5 2 1 . 0 0 5 4-* 5 2 6 6 1 . 6 5 3 2 . 3 1 5 18 Aug.,1965 2 2 . 0 4- 2 4- 2 7 7 2 . 7 1 2 3 . 8 12 19 Aug. , 1 9 6 5 1 9 . 5 3 4- 1 3+ 6 3 6 . 8 2 1 2 . 3 13 25 Aug. , 1 9 6 5 1 6 . 5 4- 2 4- 2+ 4-57 .0 91.4- 11 26 Aug. , 1 9 6 5 1 9 . 0 5 2 5 2 504-9.8 721.4- 17 29 Aug. , 1 9 6 5 2 1 . 0 2 5 1 4- 2 3 1 . 0 4-6.2 8 21 Feb. , 1 9 6 6 1 8 . 0 2 3 2 3 364-. 6 7 2 . 9 10 9 Mar. , 1 9 6 6 1 9 . 0 2 3 2 3 214-.1 4-2.8 7 21 Mar. , 1 9 6 6 1 9 . 0 1 4- 1 4- 2 5 5 . 5 5 1 . 1 9 14- A p r . , 1 9 6 6 1 9 . 0 3 2 •3 2 4-801.3 9 6 0 . 3 18 R - r i g h t , L - l e f t . * - a l l embryos dead. + - i n c l u d e s one dead embryo. 

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