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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 a c c e p t t h i s t h e s i s as conforming t o the r e q u i r e d standard  ;THE UNIVERSITY OF BRITISH COLUMBIA MAY,  I966.  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  requirements f o r an advanced degree at the U n i v e r s i t y of Columbia, for  I agree t h a t the L i b r a r y  r e f e r e n c e and s t u d y .  I further  s h a l l make i t  freely  the  British available  agree that p e r m i s s i o n f o r  ex-  t e n s i v e c o p y i n g 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 g r a n t e d by the Head o f my Department o r by h i s  representatives.  understood t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r cial  g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n  Depa rtment The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada  It  is  finan-  permission.  i ABSTRACT The  o b j e c t of t h i s study was  t o compare the r o l e  of  changes i n r e p r o d u c t i o n and m o r t a l i t y i n r e g u l a t i n g p o p u l a t i o n d e n s i t y i n the deer mouse, Peromyscus m a n i c u l a t u s , i f p o s s i b l e t o d i s c o v e r some of the f a c t o r s a f f e c t i n g reproductive rate.  Observations  and the  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 p o p u l a t i o n s on the U n i v e r s i t y Endowment Lands i n 196*+ and  1965.  I n both y e a r s numbers remained r e l a t i v e l y steady  during  the summer, i n c r e a s e d i n the f a l l , when immature a n i m a l s p l a c e d 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  winter.  A n i m a l s were about e q u a l l y abundant at comparable times b o t h y e a r s , f a l l d e n s i t i e s being about h,72 The  mice per  s t a t i o n a r y s t a t e of these p o p u l a t i o n s was  196^  associated  was  success v a r i e d between a r e a s , and was  than i n 1965.  N o  change i n l i t t e r  observed d u r i n g the p e r i o d of The  in  acre.  w i t h a poor r e p r o d u c t i v e performance on the p a r t of the whose b r e e d i n g  re-  females,  greater i n  s i z e or i n p r e n a t a l l o s s  study.  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 y e a r s . The  g r e a t e s t l o s s of mice o c c u r r e d between b i r t h and age  f i r s t c a p t u r e , a f t e r which j u v e n i l e s s u r v i v e d at the same r a t e as the a d u l t s .  S u r v i v a l was  t h a n d u r i n g the w i n t e r , and t h a t of  females.  poorer  d u r i n g the b r e e d i n g  s u r v i v a l of males was  poorer  season than  at  ii P o p u l a t i o n s whose numbers had. been e x 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 a l t e r e d , were n o t 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 p o p u l a t i o n s i n mean monthly body w e i g h t s , r e p r o d u c t i v e performance, or s u r v i v a l . Mean monthly body weights  and r e p r o d u c t i v e performance were  l o w e r , and s u r v i v a l of the young from b i r t h t o age a t f i r s t c a p t u r e was h i g h e r i n  1965  "than i n  196*+.  The p r o p o r t i o n of  s u b a d u l t s which became fecund was g r e a t e s t on t h i s a r e a i n  The s t a t i o n a r y s t a t e of these p o p u l a t i o n s 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 r e p r o d u c t i v e rate.  The r e p r o d u c t i v e performance of the females was  fairly  c o n s t a n t whereas the l o s s of young from b i r t h t o age a t 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 t o t h e i r  death or e m i g r a t i o n i n response t o a g g r e s s i v e 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 .  iii  ACKNOWLEDGMENTS  I w i s h 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 this thesis. for  I a l s o w i s h t o thank D r . J . T. McFadden  a d v i c e on t h e s t a t i s t i c a l a n a l y s i s .  by t h e N a t i o n a l Research C o u n c i l  I was s u p p o r t e d  of Canada d u r i n g  and by a Teaching A s s i s t a n t s h i p i n 196^-1965.  1963-1966  TABLE OF CONTENTS  Page Abstract  i  Acknowledgments L i s t of f i g u r e s and  i i i t a b l e s ..  iv  I.  Introduction  I  II.  Study a r e a s  3  Methods A. Dead samples B. M a r k - r e c a p t u r e s t u d i e s 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 ) 2. Natural populations C. Laboratory colony  6 6 7 7 8 8  III.  IV.  ,  Results A. P o p u l a t i o n s t r u c t u r e and changes '.. 1. D e n s i t y 2 . Age s t r u c t u r e 3 . Sex r a t i o Maturity a. Males b. Females .. • B. R e p r o d u c t i o n 1. Length of b r e e d i n g season 2 . P r o p o r t i o n of pregnant females 3 . Number of l i t t e r s per season 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 5 . O v u l a t i o n r a t e and l i t t e r s i z e C. M o r t a l i t y and s u r v i v a l r a t e s 1. P r e n a t a l m o r t a l i t y . a. Loss of ova p r i o r t o i m p l a n t a t i o n .... b. Loss of embryos a f t e r i m p l a n t a t i o n ... 2. Postnatal m o r t a l i t y a. Loss of young from b i r t h t o age at f i r s t capture 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 . D i s e a s e , p a r a s i t i s m , and p r e d a t i o n  10 10 10 12 16 17 17 25 29 29 29 30 31 32 3*+ 3*+ 38 38 39 39 ^3 ^5  2  Page V.  VI.  Discussion A. P o p u l a t i o n changes B. R e p r o d u c t i o n C. M o r t a l i t y D. E x t r i n s i c f a c t o r s E. S e a s o n a l i n c r e a s e i n numbers F. Relevance of p r e s e n t d a t a t o e x i s t i n g theories  *t8 ^8 ^9 51 52 55  Summary  6h  Bibliography Appendices  A. Pregnant u t e r i from dead samples, 196h . B. Pregnant u t e r i from dead samples, 1965 • C. Pregnant u t e r i of known age from c o l o n y .  56  67 73 7'+ 75  iv LIST OF FIGURES AND  TABLES  Figures  Title  1  Map of p a r t of U n i v e r s i t y Endowment Lands showing t r a p p i n g a r e a s and roads  Table I II III IV  V  Page  Title  Page  M o n t h l y changes i n numbers and d e n s i t y of marked p o p u l a t i o n s  11  Weight group, p e l a g e and m a t u r i t y of deer mice  12  Aging of deer mice i n groups based on degree of t o o t h wear  lk  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 , d a t a f o r I 9 6 U - - 5 combined  18  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 e x p e r i m e n t a l p o p u l a t i o n s , 196 t—5  19  L  VI VII VIII  IX  5  Mean monthly t e s t e s w e i g h t s ,  196 t~5 . . . L  D i s t r i b u t i o n of sperms i n cauda e p i d i d y m i s by t e s t e s weight  20 21  D i s t r i b u t i o n of weight of s e m i n a l 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 . 23 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 t e s t e s w e i g h t , d a t a f o r 1 9 6 * + - ! ? combined  2h  X  Comparison of r e p r o d u c t i v e d a t a f o r dead samples, 196k~5  27  XI  Comparison of r e p r o d u c t i v e d a t a f o r marked p o p u l a t i o n s , 196U--5  28  M^ean number of l i t t e r s per season  31  XII  V  Table XIII  Title  Page  L i t t e r s i z e i n r e l a t i o n t o maternal w e i g h t , d a t a f o r 196 +-5 combined  35  M o n t h l y d i s t r i b u t i o n of l i t t e r d a t a f o r 196 +-5 combined  3&  1  XIV  size,  1  XV  XVI  XVII  Number of ova and embryos i n w i l d f e males and number of young born i n the laboratory  37  Prenatal mortality,  hi  196*+-5  P r o p o r t i o n of young s u r v i v i n g t o k—6 weeks of age  XVIII XIX XX  ^2  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  *+3  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  Minimum s u r v i v a l r a t e s f o r e x p e r i m e n t a l p o p u l a t i o n s , 196 +-5  ^7  P o t e n t i a l s e a s o n a l i n c r e a s e i n numbers  57  )  XXI  I.  INTRODUCTION  The numbers of s m a l l mammal p o p u l a t i o n s are f r e q u e n t l y d e t e r m i n e d by changes i n b o t h b i r t h r a t e s and d e a t h r a t e s . There i s some c o n t r o v e r s y 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 t h e s e changes i n r e g u l a t i n g p o p u l a t i o n numbers.  R e g u l a t i o n i s sometimes thought t o be a c h i e v e d by  v a r i a t i o n s i n one parameter w h i l e the o t h e r remains c o n s t a n t . One  concept i s t h a t r e p r o d u c t i v e r a t e i s r e l a t i v e l y  c o n s t a n t 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*+) •  I n b i r d s , f o r example,  Southern (1959)  suggests  t h a t t h e i r b i r t h r a t e " i s not f l e x i b l e enough t o r a i s e or l o w e r the p o p u l a t i o n l e v e l f o r adjustment t o n o r m a l l y observed d e v i a tions."  Adjustments t h r o u g h changes I n d e a t h r a t e r a t h e r t h a n  b i r t h r a t e are t y p i c a l of f l u c t u a t i n g p o p u l a t i o n s such as snowshoe h a r e s (Green and Evans, 19 +0a, 1 9 f 0 b , !  L  1 9 + 0 c ) , lemmings !  ( K r e b s , 196'+), and v o l e s ( C h i t t y , 1 9 5 2 ) , though as r e p o r t e d 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 ) , accompanied  d e c l i n e s may  be  by reduced r e p r o d u c t i v e r a t e s .  The c o n t r a s t i n g p o i n t of v i e w m a i n t a i n s t h a t the r e p r o d u c t i v e r a t e 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 a r y or d e c l i n i n g s t a t e of a p o p u l a t i o n . Christian (1957)  5  Indeed, a c c o r d i n g t o  such changes are m a i n l y r e s p o n s i b l e 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  t h a t a n i m a l p o p u l a t i o n s can l i m i t t h e i r numbers by d e c r e a s i n g  2 t h e i r r e p r o d u c t i v e r a t e s as p o 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 p r e s e n t  study was t o make an a c c u r a t e  d e s c r i p t i o n of r e p r o d u c t i v e 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 r e p r o d u c t i o n and m o r t a l -  i t y i n determing p o p u l a t i o n numbers i n the deer mouse, Peromyscus m a n i c u l a t u s  austerus  (Baird).  F i e l d work was  c a r r i e d out from 20 F e b r u a r y - 16 December, 196*+ and 26 Febr u a r y - 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  II.  STUDY AREAS  The U n i v e r s i t y Endowment Lands a r e i n the Puget Sound Lowlands b i o t i c a r e a (Cowan and G u i g u e t ,  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 ) , w e s t e r n r e d cedar  (Thu.ja p l i c a t a ) , and w e s t e r n hemlock (Tsuga h e t e r o -  phylla) with mixtures  of r e d a l d e r (Alnus r u b r a ) , v i n e maple  (Acer c i r c i n a t u m ) and b r o a d l e a f maple (Acer m a c r o p h y l l u m ) . Regrowth s e c t i o n s a r e dominated by r e d a l d e r .  The dominant  p l a n t s o f the undergrowth a r e salmon b e r r y (Rubus s p e c t a b i l i s ) , r e d e l d e r (Sambucus c a l l i c a r p a ) , r e d h u c k l e b e r r y  (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 ( P o l y s t i c h u m munitum), and bracken ( P t e r i d i u m a q u i l i n u m ) .  The  f o r e s t f l o o r i s covered w i t h f a l l e n l o g s i n v a r i o u s stages o f decay and a t h i c k , spongy l a y e r of l e a f Mice were trapped  litter.  on s e v e r a l areas i n the Endowment Lands  ( F i g . l ) d u r i n g the two summers of f i e l d work.  The areas used  f o r m a r k - r e c a p t u r e s t u d i e s were as f o l l o w s : A c a d i a (A) : A s e m i - i s o l a t e d a r e a of a p p r o x i m a t e l y 8 . 8 a c r e s , bounded by roadways on t h r e e s i d e s and by a stand of young red. a l d e r on the f o u r t h , i n which v e r y few deer mice were t r a p p e d . This area, was used f o r e x p e r i m e n t a l p o p u l a t i o n s i n 196^ and 1965. (37 t r a p p o s i t i o n s ) G o l f Course ( G ) : A s e m i - i s o l a t e d a r e a of a p p r o x i m a t e l y 2 . 3 a c r e s on the U n i v e r s i t y G o l f 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 t r a p positions) C h a n c e l l o r ( C , Lookout (L) , Marine D r i v e (M) , and Wreck Beach (W) : Areas i n the g e n e r a l f o r e s t of a p p r o x i m a t e l y 0 . 7 a c r e , 2.1 a c r e s , 1.2 a c r e s , and 0 . 8 a c r e , r e s p e c t i v e l y , used f o r n a t u r a l p o p u l a t i o n s  i n 1965. The s i z e s g i v e n f o r these a r e a s do not i n c l u d e an e x t r a s t r i p t o a l l o w f o r ranges e x t e n d i n g o u t s i d e the a r e a t r a p p e d . ( 5 , 8, 10 and 5 t r a p positions, respectively). The a r e a s 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 follows: 196^ Spring Summer Fall Winter  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  Fig.l.  Map. of p a r t of U n i v e r s i t y Endowment Lands showing t r a p p i n g a r e a s and r o a d s . Dots i n d i c a t e f o r e s t e d a r e a s .  6 III.  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 b y a u t o p s y o f d e a d s p e c i m e n s a n d b y mark-recapture studies. Longworth l i v e A.  A l l d e e r m i c e were t r a p p e d  with  traps.  DEAD SAMPLES D a t a on m a t u r i t y ,  organ weight, r e p r o d u c t i v e  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  r a t e , and  from dead samples.  Autopsies  w e r e p e r f o r m e d on t h e d e e r m i c e c o l l e c t e d f r o m t h e f i e l d areas l i s t e d were  above, corpses  were s a v e d , a n d t h e f o l l o w i n g d a t a  recorded: All  s p e c i m e n s : D a t e , l o c a t i o n , s e x , body w e i g h t , p e l a g e , d e g r e e o f t o o t h w e a r , a n d 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 m i c e were w e i g h e d t o 0 . 5 gm; a l l o r g a n s 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 o r w e i g h t , a n d i n 1965, v i s i b i l i t y o f t u b u l e s a n d p r e s e n c e o f sperms i n t h e cauda e p i d i d y m i s . Females: State of l a c t a t i o n , size of n i p p l e s , s i z e o f mammary g l a n d s , p e r f o r a t i o n o f t h e v a g i n a l o r i f i c e , number o f c o r p o r a l u t e a , number a n d a p p r o x i m a t e age o f e m b r y o s , number o f s c a r s , a n d 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 t h e s p l e e n and a d r e n a l glands was b e i n g  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  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 t h e 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 c o n c e r n e d o n l y w i t h c h a n g e s i n p o p u l a t i o n structure, reproductive populations.  r a t e , and s u r v i v a l r a t e of t h e s t u d y  7 B.  MARK-RECAPTURE STUDIES Data, on p o p u l a t i o n g r o w t h and  from mark-recapture s t u d i e s . e a c h week, b u t f o r one n i g h t  Each  s t u d y a r e a was  as l i t t l e  females  as p o s s i b l e .  .  The  i n t o t h e l a b o r a t o r y , m a r k e d 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 r e t u r n e d t o the f i e l d  t r a p p e d once  o n l y so t h a t l a c t a t i n g  were k e p t away f r o m t h e i r l i t t e r s d e e r m i c e were b r o u g h t  s u r v i v a l were o b t a i n e d  fish  t h e same m o r n i n g .  t a g s ) , examined, The  following  and  d a t a were  recorded: All  specimens: D a t e , l o c a t i o n , t a g number, s e x , body w e i g h t , and p e l a g e . A l l m i c e were w e i g h e d t o 0.5 gm.  Males:  Testes  size.  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 , p e r f o r a t i o n o f t h e v a g i n a l o r i f i c e , and p r e g n a n c y . 1.  Acadia (experimental populations). Many m i c e were removed f r o m t h i s a r e a i n November 1 9 & 3  an e x t e n s i v e t r a p p i n g - o u t program February 196^ m a l e s was to  ( C h i t t y , unpub. d a t a ) .  the r e m a i n i n g p o p u l a t i o n of 2 1  reduced t o f i v e males  and  males  survival  mid-March,  r e c r u i t m e n t o f t h e j u v e n i l e s was  the  a d u l t s were removed a t t h e end  and  s e v e n young f e m a l e s were l e f t  o f May.  attempt  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 but because  of young.  In  and 2 9 f e -  s e v e n f e m a l e s i n an  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  In  B r e e d i n g began i n  F o u r young  on t h e a r e a , a n d ,  poor, males  i n addition,  one young m a l e and two young f e m a l e s b o r n i n t h e l a b o r a t o r y were introduced.  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 a g a i n reduced, l e a v i n g s i x p a i r s of r e s i d e n t ' animals  on t h e a r e a .  Because s e v e r a l of the deer mice had d i s -  appeared, f o u r females from t h e l a b o r a t o r y were r e l e a s e d a month l a t e r , and f i v e more males and s i x females were i n t r o d u c e d a t t h e end of June. 2.  Natural populations• S e v e r a l p o p u l a t i o n s were t r a p p e d t o o b t a i n i n f o r m a t i o n  about p o p u l a t i o n changes and r e p r o d u c t i v e 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 e x p e r i m e n t a l  popula-  tions. G.  LABORATORY COLONY A breeding  animals  c o l o n y , s t a r t e d i n t h e s p r i n g of 196*+,  f o r f i e l d and l a b o r a t o r y e x p e r i m e n t s .  provided  The e s t r o u s  c y c l e was s t u d i e d 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 v a g i n a , the l i q u i d and s p r e a d i n g  withdrawing  i t on a m i c r o s c o p e s l i d e t o d r y .  smear was s t a i n e d w i t h methylene b l u e .  No r e g u l a r  The  estrous  c y c l e was observed. Timed matings were s e t up i n order t o determine t h e age of <•  embryos found i n the f i e l d i n separate  samples.  Males and f e m a l e s were kept  cages d u r i n g t h e day and put t o g e t h e r a t n i g h t .  V a g i n a l smears were made the f o l l o w i n g morning. sperms i n d i c a t e d t h a t mating had o c c u r r e d . a u t o p s i e d between t h e seventh  The presence o f  The f e m a l e s were  and e i g h t e e n t h day of g e s t a t i o n  and t h e f o l l o w i n g d a t a were r e c o r d e d 2  number of c o r p o r a 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. A. POPULATION STRUCTURE AND 1.  RESULTS CHANGES  Density. P o p u l a t i o n d e n s i t y changes c o n 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 d u r i n g the b r e e d i n g  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 on a study p l o t of known a r e a .  The  animals  areas of the study p l o t s i n  the g e n e r a l f o r e s t were c o r r e c t e d f o r ranges e x t e n d i n g  outside  the a r e a 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 wide, t o each s i d e of the p l o t .  feet  Twice t h i s d i s t a n c e g i v e s a  v a l u e of s i m i l a r magnitude t o the 199  and 258  feet given  by  S t i c k e l ( i 9 6 0 ) as the average, d i s t a n c e s between s u c c e s s i v e capt u r e s f o r Peromyscus leucopus t i e s and the 172 The  p o p u l a t i o n s at h i g h and low d e n s i -  f e e t g i v e n by Brant  (1962) f o r P.  200 f e e t assumed f o r these p o p u l a t i o n s may  mahiculatus.  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 s u c c e s s i v e t r a p pings.  The  l o n g e s t d i s t a n c e r e c o r d e d was  approximately  950  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 (<  feet. 10)  (Table I ) . By making comparisons w i t h i n an a r e a , 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 G o l f 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 d u r i n g the summer b r e e d i n g  season, be-  gan t o i n c r e a s e i n September, and c o n t i n u e d t o r i s e through end  of October.  the  W i t h two e x c e p t i o n s , 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  M o n t h l y changes i n numbers and d e n s i t y of marked p o p u l a t i o n s . Acadia  196^-5 8.8a  Acadia  1965 8.8a.  196!+ 2.3a  Chancellor  Wreck Beach  N  >+.8a* N D  N  1965 5.7a*  1965 3.8a*  N  Mar.  12  1.36  12  1.36  7  3.0^  19  5.59  7  1.23  19 =  5.00  Apr.  15  1.71  12  I.36  7  3 . 0M-  28  8.23  11  1.93  22  5.79  May  20  2.28  13  l.*+8  7  3.0'+  2h  7.06  13  2.28  15  3.12  19  5.00  June  17  1.93  lh  1.59  6  2.61  26 . 7 . 6 5  12  2.11  12  2.50  15  3.95  July  20  2.28  13  1A8  5  2.18  25  7.35  10  1.75  16  3.33  19  5.00  Aug.  32  3.62  15  1.71  7  3-0^  30  8.82  13  2.28  21  If.  38  2h  6.32  5.00  21  2.39  9  3.91  26  7.65  10  1.75  19  3.96  19  5.00  28+ 3 . 1 8  17+  7.39  1^  1+.12  17+ 2 . 9 8  30+ 6 . 2 5  13  3A2  Oct.  50  5.68  Nov.  56  6.36  Dec.  51  5.80  Feb..  h5+ 5 . 1 1  D  1965  p_  D  D  Marine Drive  N  N  N  1965 3Aa*  Lookout  Month  Sept.  D  Golf Course  N - no.of deer mice c a p t u r e d D - density * _ a d j u s t e d acreages of study areas i n the g e n e r a l f o r e s t , a c t u a l acreages are g i v e n i n t e x t , p.3 + - d a t a from f i r s t t r a p - o u t p e r i o d o n l y  D  12 numbers I n the f a l l was of  observed, on the o t h e r a r e a s , the amount  i n c r e a s e from the b e g i n n i n g t o the end of the b r e e d i n g  b e i n g two t o t h r e e f o l d .  season  On the C h a n c e l l o r and Wreck Beach  a r e a s , however, numbers dropped i n the f a l l . 2.  Age  structure.  Age  s t r u c t u r e i n f l u e n c e s both r e p r o d u c t i o n and  mortality  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 p o p u l a tion.  The age groups used i n t h i s study were based on body  weight and p e l a g e .  The a r b i t r a r y weight groups r o u g h l y c o r r e s -  pond t o 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, p e l a g e , and m a t u r i t y of deer m i c e . Weight group 6.0 11.0 lh.0 17.0 20.0 23.0  +  Pelage  1 0 . 5 gm. 1 3 . 5 gm. 1 6 . 5 gm. 1 9 . 5 gm. 2 2 . 5 gm.  Proportion mature*  juvenal juvenal,subadult adult adult adult adult  * Data from autopsy i n parentheses.  0.000 0.068 0.268 0.716 0.857 1.000  (56) (236) (205) (116) (35) (5)  specimens, number examined  J u v e n i l e s and a d u l t s were e a s i l y s e p a r a t e d by the appearance of t h e p e l a g e , but s u b a d u l t s were l e s s e a s i l y d i s t i n g u i s h e d adults.  from  The gray j u v e n a l pelage i s r e p l a c e d by the dark brown  s u b a d u l t pelage at about two months of age; the s u b a d u l t  pelage  i s r e p l a c e d 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. molt  sequence.  McCabe and B l a n c h a r d ( 1 9 5 0 ) r e p o r t e d a s i m i l a r  13 S e p a r a t i o n of mice of d i f f e r e n t age groups was checked i n d e p e n d e n t l y by the degree of t o o t h 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 m o l a r s , 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 o b t a i n e d from a n i m a l s of known age, used i n m a r k - r e c a p t u r e s t u d i e s (Table I I I ) . I.  II.  III.  IV.  V.  M 3 ( t h i r d upper molar) not f u l l y eruped; mice a p p r o x i m a t e l y 2-6 weeks of age and i n juvenal pelage; M 3 f u l l y e r u p t e d but showing l i t t l e or no wear; mice a p p r o x i m a t e l y 2-3+ months of age and i n s u b a d u l t p e l a g e ; M 3 b a s i n e d but cusps s t i l l a p p a r e n t , M 1 and M 2 showing a p p r e c i a b l e wear but cusps s t i l l prominent; mice a p p r o x i m a t e l y h-8 months of age and i n a d u l t p e l a g e ; A l l molars b a s i n e d , cusps of M 3 worn away, cusps of M 1 and M 2 g r e a t l y worn but r e e n t r a n t a n g l e s s t i l l a p p a r e n t ; mice 10-1*+ months of age; Cusps and r e - e n t r a n t a n g l e s of M 1 and M 2 almost e n t i r e l y worn away, r o o t s u s u a l l y p r o t r u d i n g w e l l beyond the a l v e o l u s ; mice more than 1 5 months of age.  S h o r t e n (195*+), s t u d y i n g s q u i r r e l s from d i f f e r e n t a r e a s , a l s o found t h a t the amount of t o o t h wear p r o g r e s s e d w i t h age, and t h a t the r a t e of t o o t h wear may have been a f f e c t e d by v a r y i n g food c o n d i t i o n s .  V a r i a t i o n s i n f o o d c o n d i t i o n s may  have  a f f e c t e d the r a t e of t o o t h 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 p o p u l a t i o n s are g i v e n i n T a b l e s IV and V.  The weight  Ik  TABLE I I I .  Group I  Aging of deer mice i n groups based on degree of t o o t h wear. Deer mice from m a r k - r e c a p t u r e s t u d i e s .  N _  11*  Mean body weight (gm.)  S.E.  Estimated Age.  7.6k  ±0.32  2 - 6 wks.  II  27  12.50  ±0.33  2 - 3 mos.  III  12  15.96  -0.59  k-Q  IV  18  17.78  to. 63  V  1  20.50  mos.  10-1!+ mos. 15  +  mos.  N - No.of deer mice examined - Deer mice i n Group I from dead samples  15 used f o r each a n i m a l i s i t s average weight d u r i n g each month's trapping.  Year c l a s s e s a r e s e p a r a t e d by the dashed  line.  A n a l y s i s of v a r i a n c e shows t h a t t h e mean monthly body w e i g h t s of the n a t u r a l p o p u l a t i o n s tended t o decrease  slightly  from 196'+ t o 1965, but t h a t t h e d i f f e r e n c e s a r e n o t s i g n i f i c a n t (,10>P>.05).  W i t h t h e e x c e p t i o n of the Wreck Beach a r e a , on  which t h e mice were h e a v i e r , t h e d i f f e r e n c e s between a r e a s a r e not s i g n i f i c a n t  (P>.05).  E x a m i n a t i o n of c r i t e r i a o t h e r t h a n  body weight i n d i c a t e s t h a t a l l the n a t u r a l p o p u l a t i o n s were v e r y s i m i l a r ; t h e r e f o r e , d a t a on age s t r u c t u r e were p o o l e d . The approximate age s t r u c t u r e o f t h e 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 t o change from the b e g i n n i n g t o t h e  end o f t h e b r e e d i n g season.  The mice g r a d u a l l y g a i n e d weight  from t h e 1 1 . 0 - 1 3 . 5 gm. and 1^.0-16.5 gm. weight groups i n March t o t h e 1 7 . 0 - 1 9 . 5 gm. weight group i n J u l y .  I n August a n i m a l s  i n t h e 1 1 . 0 - 1 3 . 5 gm. and 1 7 . 0 - 1 9 . 5 gm. weight groups formed a bimodal d i s t r i b u t i o n .  I n October a n i m a l s i n t h e 1 1 . 0 - 1 3 . 5 gm.  and l + . 0 - l 6 . 5 gm. weight groups made up most of t h e p o p u l a t i o n . 1  By March t h e o v e r w i n t e r e d deer mice had begun t o come I n t o b r e e d i n g c o n d i t i o n and a few had begun t o produce young.  Even  though t h e f e m a l e s were f e c u n d , few p r e g n a n c i e s were r e c o r d e d b e f o r e May. Few young appeared b e f o r e J u l y , and most n o t u n t i l September.  By September t h e o l d e r a n i m a l s had begun t o l o s e  weight and t o go out of b r e e d i n g c o n d i t i o n , many had d i e d , and few o v e r w i n t e r e d .  16 S i m i l a r weight changes were observed, i n the e x p e r i m e n t a l p o p u l a t i o n s on A c a d i a i n the two y e a r s .  I n 196*+ t h e a d u l t s be-  gan t o breed a t the end of March, and j u v e n i l e s began t o appear at  t h e end of A p r i l .  The a d u l t s were removed a t t h e end of May,  l e a v i n g t h e j u v e n i l e s on t h e a r e a .  The j u v e n i l e s matured i n  June and t h e i r young began t o appear i n J u l y .  A few of t h e s e  young matured l a t e r i n t h e summer, but most d i d n o t . went out of b r e e d i n g c o n d i t i o n i n September.  The a d u l t s  Toward f a l l un-  marked a d u l t s appeared w h i c h may have been young t h a t had grown up on the a r e a or may have been i m m i g r a n t s .  The f a l l and w i n t e r  p o p u l a t i o n was composed of a few a d u l t s born i n t h e s p r i n g ( > 1 7 . 0 gm.) and younger a n i m a l s born i n t h e summer and f a l l  (<17.0 gm.). In  1965 the a d u l t s on A c a d i a d i d n o t come i n t o b r e e d i n g con-  dition until April.  Young began t o appear i n May but few were  caught b e f o r e August.  The a d u l t s began t o go out of b r e e d i n g  c o n d i t i o n I n September and by October few remained.  Immature  mice i n t h e 1 1 . 0 - 1 3 . 5 gm. and l + . 0 - l 6 . 5 gm. weight groups made 1  up t h e f a l l  population.  The mean monthly body w e i g h t s of t h e e x p e r i m e n t a l p o p u l a t i o n s d e c r e a s e d from 196*+ t o 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  different  from those o f t h e 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 p o p u l a t i o n s i s shown i n T a b l e s 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 p o p u l a t i o n s was n e a r l y e q u a l i n March, June, and J u l y .  I n the o t h e r months the number of males  was  g r e a t e r than the number of f e m a l e s , perhaps r e f l e c t i n g g r e a t e r movement of the males.  There were more a d u l t males t h a n 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 o t h e r weight groups was n e a r l y e q u a l . The d i s t r i b u t i o n of sexes i n the e x p e r i m e n t a l p o p u l a t i o n on A c a d i a i n 1 9 6 ^ was n e a r l y e q u a l f o r each month u n t i l  August,  when the number of females became g r e a t e r than the number of males, e s p e c i a l l y I n the 1 1 . 0 - 1 3 . 5 gm.  and 1 7 . 0 - 1 9 . 5 gm.  weight  groups.  D u r i n g the b r e e d i n g season t h e r e were more females than males i n the 2 0 . 0 - 2 2 . 5 gm.  and 2 3 . 0 + gm. weight groups.  I n 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 e q u a l .  The  d a t a f o r both y e a r s c o n t r a s t w i t h those of the n a t u r a l p o p u l a t i o n s , i n which t h e r e were more a d u l t males than a d u l t f e m a l e s . h.  Maturity, a. M a l e s . The mean monthly t e s t e s w e i g h t s , g i v e n 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 y e a r s , however, the mean t e s t e s weight i n c r e a s e d 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  s e a s o n a l 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  fecundity.  A male i s fecund when t h e r e 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 w e i g h t 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 , d a t a f o r 196^-5 combined,, Dashed l i n e s e p a r a t e s year classes,,  Body weight (gm„)  6.011.0m-.o-  Mar„ S  ?  -  -  3 h  17. ')-  7  20.023.0+  -  Total  1*+  7  5  Apr. N  S  ?  _  1  k  22  12 10  1  9  1  -  -  -  -  -  13  35  2h  May  June  9  1 2 2h  1 10 16  X  X  X  X  July  9  S  -  1  . -  1  2  5  10  25  3  9  Aug.  Sept.  9  9  &  Oct. «?  Total  9  Grand Total  9  h  2  2  1  1  6  8  12  20  2  12  5  13  9  30  28  70  75  i*+5  5  6  7  12  11  27  13  108  97  205  23  20  19  15  13  5  138  73  211  \  v.  22  2  20  6  25  23  3  1  1  2  3  3  '+ • 7  1  2  1  2  17  30  -  -  13  -  2  -  -  -  -  -  -  2  2  52  30  32  35  *+9  h7  38  72  59  337  276  613  36  36  hi  19 TABLE V.  Body weight (gm.)  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 e x p e r i m e n t a l p o p u l a t i o n s , 196^f-5« Dashed l i n e s e p a r a t e s year c l a s s e s . D o t t e d l i n e s e p a r a t e s second and t h i r d generations,, 196k  Mar,  -  6.011.0-  17.0-  5  20.0-  -  23.0+ Total  5  P  VL  -  May  -  -  1  1  k  1  2  5  -  -  l  5  k  3  -  1  2  ^ \  l  -  2  -  1  7  6  7  7  13  -  \  Mar  ¥  9  9  ¥  -  -  \  \ \ \  2  3  if  k  9  ^  10  23  31  5k  1  9  7  7  7  15  9  12  15  50  50  100  k  1  if  6  5  8  2  8  2  3  3^  *fl  75  l  -  k  1  7  k  25  29  -  l  -  -  11  11  7  8  I6if  '280  2  2  5  k  -  -  1  1  3  3  5  -  2  -  7  3 11  L9  5  1  1  2  -  1  -  -  -  25  15  26  23  28  19  June  July  Aug,  9  ¥  ?  d"  -  _  ¥  0"  ?  2  2  3  2  5  -  -  1  1  -  -  1  1  1  1  8  6  13  13  23  26  +9  2  1  h  1  2  2  6  If  19  20  39  1  5  1  6  17  19  36  1  -  1  12  7  19  -  -  1  1  22  26  7k  75  l*f9  -  -  1  6  2  -  3  -  1  •1  3  3  1  2  h  1  1  1  2  2  2  1  17.0-  k  2  3  20.0-  -  —  3  2  1  23.0+  -  -  -  -  1  Total  6  5  6  9  9  -  2  6  Oct.  -  -  2  Grand Total  Sept.  Total  7 7  7 6  9 6  .1  lk  ? 2  5  6  30  -  l f.0-  9  5  - \  2  ~  Grand Total  1  3  ¥  ?  Total  1  -  9  Dec.  1  -  8  9  Nov.  -  -  -  0*  Oct.  1  l  \  Sept.  c? 1  1  May  Apr,  0  -  11.0L  Aug  cT  9 ~\  July  June  1965  s 6.0-  6 -  l^-.O-  Body weight (gm.)  Apr.  116  11  TABLE V I .  Mean m o n t h l y t e s t e s w e i g h t s , 1 9 6 ^ - 5 .  196*+  1965  Mean t e s t e s + S.E. weight (mg.)  Month  No.males examined  No.males examined  Mar.  16  231.31  25.72  12  126.52  21.57  Apr.  9  2M+.88  2h.ll  13  89.53  2^.9*+  May  15  301.95  55.39  27  1 2 9 . *+8  13.83  June  3  302.M-7  7.71  28  129.72  11.22  July  18  323.55  ^0.29  15  3^7.36  *+5.07  Aug.  36  1^5.92  30.63  19  *+6.21  12.22  Sept.  2M-  116.52  33.19  6  2^.60  3.73  Oct.  32  19.63  6.90  67  17.69  2Al  Nov.  11  7.96  l.hl  -  -  -  Dec.  7  11.93  1.7*+  Mean t e s t e s + S.E. weight (mg.)  _  21 and a c e r t a i n amount of semen produced. mined i n 1 9 6 5  from the presence  F e c u n d i t y was d e t e r -  of m o t i l e sperms i n the cauda  e p i d i d y m i s ( P e r r y , 19*+5) and from the enlargement t u b u l e s i n the cauda e p i d i d y m i s (Jameson, 1950) development of the s e m i n a l v e s i c l e s .  of the  and from the  The d i s t r i b u t i o n of males  w i t h and w i t h o u t 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 e p i d i d y m i s by t e s t e s w e i g h t .  T e s t e s Weight (mg.)  0 100 200 300 h-00  +  Presence No.without sperms  100 200 300 >+00  102 5 0 0  of Sperms No.with sperms  16 32 lk-  0  11 7  Sperms were p r e s e n t i n a n i m a l s w i t h s m a l l t e s t e s , but many of these males, e s p e c i a l l y those w i t h t e s t e s w e i g h i n g l e s s 100 mg.,  than  had o n l y a few sperms; t h e r e f o r e , o n l y males whose  t e s t e s weighed more than 100 mg.  were c o n s i d e r e d t o be f e c u n d .  Table V I I I shows t h a t growth of the s e m i n a l v e s i c l e s i n i t i a l l y l a g g e d behind t h a t of the t e s t e s but c o n t i n u e d 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 w e i g h t .  Preliminary analysis  of the d a t a i n d i c a t e s t h a t t h i s l a g i n the growth of the s e m i n a l v e s i c l e s may  be s e a s o n a l .  The t e s t e s of l i v e a n i m a l s 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 r e c o r d e d as r e g r e s s e d o r  22 s m a l l ( S ) , small-medium (S/M), medium (M), medium-large or l a r g e (L) t o c o r r e s p o n d w i t h t e s t e s w e i g h i n g 0 - 1 0 0 1 0 0 - 2 0 0 mg., ly.  2 0 0 - 3 0 0 mg.,  SOO-i+OO mg.,  (M/L), mg.,  or ^00+ mg. , r e s p e c t i v e -  I n o r d e r t o t e s t the a c c u r a c y of these s u b j e c t i v e measure-  ments, t e s t e s of dead specimens t h e n d i s s e c t e d out and  were g i v e n a r e l a t i v e s i z e  and  weighed.  A n a l y s i s of the d a t a shows t h a t 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 d a t a f o r the two y e a r s i s combined i n Table IX.  S i n c e males w i t h t e s t e s w e i g h i n g more t h a n 100 mg.  were con-  s i d e r e d t o be f e c u n d and the c o r r e l a t i o n between t e s t e s  size  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 w i t h t e s t e s s c o r e d as small-medium or l a r g e r s h o u l d a l s o be f e c u n d .  These  c r i t e r i a of f e c u n d i t y were used t o e s t i m a t e the p r o p o r t i o n of f e c u n d a n i m a l s i n the p o p u l a t i o n s s t u d i e d . The p r o p o r t i o n of f e c u n d males in. the autopsy samples and the marked p o p u l a t i o n s i s shown i n T a b l e s X and X I .  I n 196*+  the a d u l t males i n the dead samples were fecund from.March t h r o u g h October, though t h e y began t o go out of b r e e d i n g cond i t i o n i n August.  I n 19&5 'the males were f e c u n d 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.  T h i s r a p i d change i n b r e e d i n g c o n d i t i o n  may  have been hastened by an outbreak of b o t f l y l a r v a e ( C u t e r e b r a sp.) which p a r a s i t i z e d the deer mice. males was lower i n 1 9 6 5 1  d.f.).  The p r o p o r t i o n of f e c u n d  than i n 196*+ (,01>P, X  2  = +1.609 w i t h 1  TABLE; V I I I .  Testes weight (rag.)  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 , d a t a f o r 196*4—5 combined.  0.0-  50.0-  0.0-  175  50.0-  35  -  100.0-  30  1  150.0-  17  3  200.0  12  250.0-  Wei ght of s e m i n a l v e s i c l e s (mg.) 150.0200.0250.0+' 300.0100.0-  350.0+  _  5  -  _  -  _  -  2  8  h  2  1  -  -  -  300.0-  2  6  6  1  -  -  -  -  350.0-  -  1  2  6  h  1  1  -  ^oo.o-  -  -  h  3  3  3  1  -  *+5o.o+  _  5  3  _ _  -  2  1  3  2h  TABLE I X . 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 t e s t e s w e i g h t , d a t a f o r 196*+-5 combined. Testesweight (mg.)  S  0.0-  137  50.0-  32  100.0-  7  22  150.0-  -  18  6  200.0-  •--  h  13  Relative testes S/M  250.0-  -  300.0-  -  350.0-  .-.  ^oo.o-  -  ^-50.0+  -  size M  -  M/L  L  2 7  5  -  13 _  -  _  7  i+  -  -  10  -  1*+  25 The  s e a s o n a l changes i n the p r o p o r t i o n of f e c u n d males  was a l s o observed i n t h e marked p o p u l a t i o n s . I n both t h e n a t u r a l and e x p e r i m e n t a l p o p u l a t i o n s the males were f e c u n d from March through September, but t h e p r o p o r t i o n of f e c u n d males was lower I n t h e n a t u r a l p o p u l a t i o n s ( . 0 5 > P > . 0 1 , X 5.311  with 1 d.f.).  More j u v e n i l e s became f e c u n d I n the ex-  p e r i m e n t a l p o p u l a t i o n i n 196h (k/lQ) or t h e n a t u r a l p o p u l a t i o n s b.  =  than i n e i t h e r 1965 ( 0 / l 6 )  .  Females. The  s e x u a l m a t u r i t y of females i n autopsy specimens may  be determined by the presence of c o r p o r a l u t e a i n t h e o v a r i e s ( L e s l i e , V e n a b l e s , and V e n a b l e s , 1952; Jameson, 1 9 5 3 ) .  In  b o t h 196M- and 1965 t h e r e were females w i t h c o r p o r a l u t e a t h r o u g h 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 c o r p o r a l u t e a . The p r o p o r t i o n of females w i t h c o r p o r a l u t e a I n c r e a s e d d u r i n g the summer and then decreased i n t h e f a l l , as t h e ..females went out of b r e e d i n g c o n d i t i o n . A l t h o u g h c o r p o r a l u t e a I n d i c a t e o v a r i a n a c t i v i t y , the r e s t of t h e u t e r i n e t r a c t may be i n a c t i v e  (Jameson, 1 9 5 3 ) .  I n many  of t h e females w i t h c o r p o r a l u t e a , t h e v a g i n a l o r i f i c e was imp e r f o r a t e and the females c o u l d n o t 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 a n i m a l s , may be used as an i n d i c a t i o n of s e x u a l a c t i v i t y ( C l a r k , 1938).  I n young  f e m a l e s the v a g i n a opens a t t h e f i r s t estrum; hence, p e r f o r a t i o n  26 of t h e v a g i n a l o r i f i c e may he used t o a s s e s s t h e s e x u a l m a t u r i t y of young a n i m a l s brought i n from the f i e l d .  As an i n -  d i c a t i o n of r e p r o d u c t i v e c o n d i t i o n , however, p e r f o r a t i o n o f t h e v a g i n a l o r i f i c e i s n o t always r e l i a b l e .  The v a g i n a becomes open  o n l y d u r i n g the b r e e d i n g season, but i t may be c l o s e d f o r a s h o r t time d u r i n g pregnancy.  A female t h a t had a p e r f o r a t e v a g i n a ,  t h a t was v i s i b l y p r e g n a n t , or t h a t was l a c t a t i n g was c o n s i d e r e d t o be f e c u n d . The p r o p o r t i o n of fecund females i n the dead samples and marked p o p u l a t i o n s i s g i v e n i n T a b l e s X and X I .  The s e a s o n a l  a s p e c t s of the b r e e d i n g season a r e more apparent than t h e y were i n t h e males. P r e l i m i n a r y a n a l y s i s of t h e autopsy d a t a i n d i c a t e d t h a t f e m a l e s w i t h c o r p o r a l u t e a were n o t n e c e s s a r i l y f e c u n d .  In  196 i- f e m a l e s were fecund from March through September, the L  h e i g h t o f t h e b r e e d i n g season being from June t h r o u g h August; i n 1 9 6 5 the females were fecund from May t h r o u g h September, t h e peak being i n J u l y .  The p r o p o r t i o n of f e c u n d females was l o w e r  i n 1 9 6 5 t h a n 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 o f f e c u n d i t y i n the marked p o p u l a t i o n s was s i m i l a r t o those r e p o r t e d f o r t h e dead samples.  Females i n the  n a t u r a l p o p u l a t i o n s and on A c a d i a I n 196k were f e c u n d from March t h r o u g h September and, on A c a d i a i n I 9 6 5 from May t h r o u g h September.  More j u v e n i l e s became fecund on Acadia, i n 196 + ( 3 / 2 3 )  i n e i t h e r 1965 (0/12)  1  or t h e n a t u r a l p o p u l a t i o n s ( O / 2 3 ) .  than  TABLE X. Month  196^  Comparison of r e p r o d u c t i v e d a t a f o r dead samples, 196 +-5. 1  Males No. No. adults fecund  No. adults  Mar. Apr. May June July Aug. Sept. Oct.  16 9 15 3 1*+ 22 8 12  16 9 15 3 1*+ 13 7 8  Total  99  85  Mar. Apr. May June July Aug. Sept. Oct.  12 13 26 28  7 5 17 17  13 25 28  5 31  0 0  18 11 21  Total  1>3  6h  133  1965  Ih lh  lk h  .  No. fecund  N preg:  h  • 11 21 7 8  10 *f 11 21 6 7  3 6 1+ 11 20 6 0  0 0 1 2 6 5 3 0  81  66  5fr  17  k  3 1+ 7 10 13 15 11 15  0 0 2 10 13 8 1 0  0 0 0  78  3*+  11  13 7 10  k  .  Females No.with corpora l u t e a  13  k 3  8  1 0 0  23  TABLE X I .  Month  Comparison of r e p r o d u c t i v e p o p u l a t i o n s , 1964--5.  Males No. No. Adults fecund  No. adults  d a t a f o r marked  Females No. No. fecund pregnant  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. 14Mar. 9 13 2 1 4Apr. 35 23 2^ 0 May 51 29 12 2 4-3 June 31 29 33 30 7 July 31 30 33 32 15 Aug. 33 21 3>+ 22 10 Sept. 432 6 28 0 Oct. 4-1 0 25 0 0 Total  268  l6l  Experimental population, Mar. 5 5 Apr. 5 5 6 May 5 6 June 6 July ^ 3 Aug. 5 5 14Sept. 9 Oct. 12 0 Total  56  38  218  4-8  39  0  1 3  1 10 26 8 12  106  35  61  1  9 22 21  5 7 7 7 9 9 13 0  0  0 1 6 6 6 6 10 6  91  57  32  4-1  0 0 8 6 5  0  0 0 1 1 3  1964-.  7 7 9 Q  7  Experimental population, 1965. Mar. 6 6 5 Apr. 6 6 2 May 6 6 8 June 7 7 7 July 6 5 5 Aug. 7 7 5 Sept. 3 2 7 Oct. 7 0 9 Total  No. juvei (both ;  4-8  ~J  5 7 6 6  4-  0-  1  4«.  1 0  3 0 0  12 7  24-  11  28  4-  4-  29 B.  REPRODUCTION Mature females breed from March t o October, b e a r i n g  l i t t e r s of 2 - 9 young ( A s d e l l , 1 9 6 0 . L  The females are p o l y -  e s t r o u s 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 ,  p l u s a s h o r t 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 m a t u r a t i o n of young of b o t h sexes i n 7 - 8 weeks (Clark., 1938) , endow the mice w i t h a h i g h p o t e n t i a l r a t e of i n c r e a s e . 1.  Length of the b r e e d i n g season. The number of pregnant females caught i n 196h and 1 9 6 5 I  g i v e n i n T a b l e s X and X I .  s  I n 196h the f i r s t pregnant female  was caught on A c a d i a i n March, and the l a s t , a l s o caught on A c a d i a , 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 j u v e n a l p e l a g e 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 C h a n c e l l o r i n March, and the l a s t on A c a d i a i n l a t e August. not. begin-on A c a d i a u n t i l May,  Breeding d i d  and on Lookout u n t i l J u l y .  Mice  i n j u v e n a l p e l a g e were l a s t c a u g h t . i n l a t e October. 2.  P r o p o r t i o n of pregnant f e m a l e s . Females i n a l l s t a g e s of pregnancy or w i t h r e c e n t p a r t u r i -  t i o n s i t e s were used t o 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. ( 0 . 2 ^ 0 ) than i n 1965  (0.115)  More f e m a l e s were pregnant i n 196*+ (.01>P, X  2  = 7.15*+ w i t h 1 d.f.)  The g r e a t e s t 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 popul a t i o n s was e s t i m a t e d from f e m a l e s which were more t h a n 12-14days pregnant or which were o b v i o u s l y l a c t a t i n g .  Females l e s s  than 12-14- days pregnant were m i s s e d because the embryos a r e n o t l a r g e enough t o cause a marked i n c r e a s e 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 t o determine a f e m a l e ' s r e productive condition. 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 p o p u l a t i o n s from 0.094- on Lookout t o 0 . 2 6 5 on Marine D r i v e , and i n the e x p e r i m e n t a l p o p u l a t i o n s on A c a d i a from 0 . 3 5 2 i n 1964- t o 0 . 2 1 6 i n 1 9 6 5 . the  The p r o p o r t i o n of pregnant f e m a l e s i n  e x p e r i m e n t a l p o p u l a t i o n s was not s t a t i s t i c a l l y  greater  t h a n 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 v a r i a n c e (P > . 0 5 ) , but tended t o be g r e a t e r i n 1964(.10> P > . 0 5 , X 3.  2  = 2.807 with 1 d.f.).  Number of l i t t e r s p'er season. Throughout  the b r e e d i n g season b r e e d i n g appeared t o be  random r a t h e r than s y n c h r o n i z e d . I n 1965? however, many of the females became pregnant d u r i n g the f i r s t two weeks of J u l y . T h i s was the o n l y p e r i o d of s y n c h r o n i z e d b r e e d i n g observed. W i t h the e x c e p t i o n of A c a d i a , 1964-, the number of l i t t e r s per  season f o r f e m a l e s t h a t were p r e s e n t i n the marked p o p u l a -  t i o n s f o r the d u r a t i o n of the b r e e d i n g season i s t a b u l a t e d below.  I n 1964- the o v e r w i n t e r e d females on A c a d i a 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 a r e a began  b r e e d i n g i n June.  Had the o v e r w i n t e r e d females not been r e -  moved, t h e y would p r o b a b l y have borne two t o t h r e e more l i t t e r s d u r i n g the summer. TABLE X I I .  Mean number of l i t t e r s per  Population  season.  No.of females  No.of litters  Mean no.of litters per female - S.E.  Natural Mar.-Sept. ,196^+ Mar.-Sept. , 1 9 6 5  2 20  3 28  1.50 l.ifO  ± -  .50 .11  Experimental Mar.-May, 196*+ June-Sept. ,196*+ Mar.-Sept.,1965  7 5 7  9 1*+ lk  1.29 J 2 . 8 0 'J 2.00 -  .18 .37 .31  +  The average number of l i t t e r s per season appeared er  i n 196'+ from June t o September and i n 1965  t o be g r e a t -  on A c a d i a t h a n i n  the n a t u r a l p o p u l a t i o n s i n the r e s p e c t i v e y e a r s , 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-test. h.  Factors affecting l i t t e r L i t t e r s i z e may  size.  v a r y w i t h m a t e r n a l age and w e i g h t , 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 ; C a l d w e l l and G e n t r y , 1 9 6 5 ) .  Newson, 196'+;  L i t t e r s i z e was r e c o r d e d f o r the  deer mice i n the l a b o r a t o r y c o l o n y 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 d a t a 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 p a r i t y .  s i z e and m a t e r n a l age or between l i t t e r  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 d a t a , how-  e v e r , i n d i c a t e s t h a t t h e r e 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 m a t e r n a l weight  and  between the number of embryos p e r l i t t e r and season of b i r t h . The d a t a f o r the two y e a r s were combined.' The tendency f o r l i t t e r  size t o increase i n heavier (or  o l d e r ) females i s shown i n Table X I I I .  The number of l i t t e r s  was g r e a t e r I n the h e a v i e r f e m a l e s . The  s e a s o n a l changes i n l i t t e r  The number of embryos per l i t t e r i n August and then d e c r e a s e d . est  i n July,  5.  O v u l a t i o n r a t e and l i t t e r  s i z e are shown i n Table XIV.  i n c r e a s e d from May  The number of l i t t e r s was g r e a t -  size.  I n f o r m a t i o n on o v u l a t i o n r a t e , l i t t e r m o r t a l i t y was  t o a peak  s i z e , and p r e n a t a l  o b t a i n e d from f i e l d and l a b o r a t o r y a n i m a l s .  r e p r o d u c t i v e t r a c t s were p r e s e r v e d i n a m i x t u r e of a c e t i c  The acid,  e t h a n o l , and f o r m a l i n , c l e a r e d t h r o u g h benzene i n t o b e n z y l benzoate and viewed w i t h o b l i q u e l i g h t  (Orsini, 1962a, 1962b,  1963).  The number of c o r p o r a l u t e a , i m p l a n t a t i o n s i t e s , embryos, r e c e n t p a r t u r i t i o n s i t e s , and u t e r i n e s c a r s was  counted.  Only those  d a t a p e r t a i n i n g t o pregnant u t e r i were used because I was to  unable  s e p a r a t e d i f f e r e n t s e t s of c o r p o r a l u t e a or d i f f e r e n t s e t s of  scars i n several 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 e s t i m a t e d by comparing  t h e i r development  w i t h known stages from timed p r e g n a n c i e s set up i n the l a b o r a t o r y . The  s t a t e of development used t o s e p a r a t e 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 , o r , a p p r o x i m a t e l y , nine.  F u r t h e r i n f o r m a t i o n on pregnant  Appendices A, B and  day  u t e r i i s given i n  C.  Table XV shows the average number of c o r p o r a 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  c a t e g o r y i n the two y e a r s 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 , t e s t e d by the S t u d e n t ' s t - t e s t .  The  as  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 c o r p o r a lutea. per litter  (P> . 0 5 ) .  i s not s i g n i f i c a n t  The mean l i t t e r  c u l a t e d from the p o o l e d d a t a and used i n f u r t h e r was 4-.82  young per  computations,  assumed t o be  t o the number of ova shed even though some evidence found.  l u t e a was  Two  cal-  litter.  The number of c o r p o r a l u t e a formed was  was  size,  pregnant  equal  of p o l y o v u l y  u t e r i i n which the number of c o r p o r a  one l e s s than the number of embryos were found i n the  f i e l d material.  I n a t h i r d i n s t a n c e , a female from the c o l o n y  had no c o r p o r a 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 n i n e embryos, the f o u r i n the horn h a v i n g 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 . seems u n l i k e l y t o me  It  t h a t f o u r embryos i n one pregnancy s h o u l d  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  i n c i d e n c e of m u l t i p l e  o v u l a t i o n i n Peromyscus m a n l c u l a t u s r e p o r t e d by Beer et a l (1957) i s v e r y low, but can be g r e a t e r i n some i n d i v i d u a l s .1926) .  (Hartman,  I f p o l y o v u l y d i d not o c c u r , then the c o r p o r a l u t e a  may  3*+ have a t r o p h i e d .  The l i t e r a t u r e , however, i n d i c a t e s t h a t t h e  corpora l u t e a are present  f o r t h e d u r a t i o n of pregnancy i n t h i s  and o t h e r s p e c i e s (Beer e t a l , 1 9 5 7 ; S n e l l , 19 i-l; Long and L  Evans, 1 9 2 2 ; H a r p e r , 196 h; Newson, 196*+). L  The average number o f c o r p o r a l u t e a was 5 « l 8 - 0 . 1 6 . The average number of i m p l a n t a t i o n s i t e s p e r l i t t e r was 5.00  - 0 . 2 0 and the average number of embryos p e r l i t t e r was  '+.95 - 0 . 2 3 . The range i n number o f embryos p e r l i t t e r was t h r e e t o seven and t h e mode f i v e . The average number of young p e r l i t t e r born i n c a p t i v i t y was H - . 6 0 - O.lh. C.  The range was one t o e i g h t and t h e mode f i v e .  MORTALITY AND SURVIVAL RATES Death may occur a t 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 r e p r o d u c t i v e c a p a c i t y of a p o p u l a t i o n . 1.  Prenatal mortality. 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*+). f e r t i l i z e d and n o n v i a b l e (Harper,  Loss of un-  ova i s expected p r i o r t o i m p l a n t a t i o n  196*+) and l o s s of embryos, p o s s i b l y t h r o u g h l a c k of  s u f f i c i e n t blood t o the p l a c e n t a , i s expected d u r i n g nancy (Reynolds,  1955).  mid-preg-  Owing t o t h e s m a l l number of pregnant  u t e r i i n v a r i o u s stages of development, the o n l y 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 . separate  i m p l a n t a t i o n s from embryos.  P l a c e n t a t i o n was used t o The method used t o e s t i -  35 TABLE X I I I .  No.visible embryos per l i t t e r  L i t t e r size i n r e l a t i o n t o maternal weight, d a t a f o r 1964—5 combined.  M a t e r n a l weight 14-. 0 17.0-  3  2  4-  1  5  2  (gm.)  20.01  T o t a l No. litters  3 5  4-  6  10  6  4-  7  1  l  Total  11  12  23  Avg.no.embryos p e r litter 4-.00  5.09  5.33  5.oo  36 TABLE XIV.  No.visible embryos per l i t t e r 3  M o n t h l y d i s t r i b u t i o n of l i t t e r d a t a f o r 196*+-!? combined.  May 1  June  Month July  Aug,  1  size,  Sept. 1  T o t a l No. litters 3  >+  1  3  1  5  2  6  1  1  10  5  3  1  9  1  7  1  1*+  Total Avg.no.embryos per l i t t e r (3.00)  5  h.2$  5.1*+  28  5.30  b.,67  5.00  TABLE XV".  Number o f ova and embryos i n w i l d females and number of young born i n t h e l a b o r a t o r y . Years combined  196H-  Stage of development  N  X  ±s.E  N  X  ±S.E.  N  X  ±S.E.  Corpora l u t e a (ova)  21 5.29 ^ . 2 1  12 5 . 0 0 io.25  33 5.18  ±0.16  Implantation s i t e s  21 5.1M- ±0.29  12 i f . 7 5  ±0.28  33 5.oo  ±0.20  Embryos  13 5.23 ^0.28  7 h.k3 - 0 . 3 7  20 i+.9?  io.23  L i v e young a t birth"*  10  t o . 1+0  79 *+.6i  N - No.of l i t t e r s . - l i t t e r s born i n c o l o n y  io.15  89 *+.6o -o.lh  38 mate t h e approximate stage of development  of pregnant u t e r i  was g i v e n 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 t o i m p l a n t a t i o n was e s t i m a t e d from t h e  d i f f e r e n c e between the number of c o r p o r a l u t e a and t h e number of i m p l a n t a t i o n s i t e s .  No attempt was made t o r e c o v e r 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, o n l y u t e r i i n which some of t h e ova s u r v i v e d t o i m 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 t h e p r o p o r t i o n of ova l o s t p r i o r t o 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 t h a t 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 g r e a t e r where the number of ova shed was g r e a t e r .  Total loss  of ova p r i o r t o i m p l a n t a t i o n was n o t e s t i m a t e d . 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 e s t i m a t e d from t h e  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 t h e number of l i v e embryos.  A l t h o u g h t h e amount of l o s s may v a r y  w i t h t h e stage of pregnancy ( P e r r y , 1 9 5 5 , H a r p e r , 196 t-), t h e L  s m a l l 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 impractical.  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 0.05  (Table XVI B ) .  T o t a l l i t t e r l o s s was not e s t i m a t e d .  In Table XVI C p r 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 o l d 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 b e f o r e and a f t e r implantation.  The d i f f e r e n c e s i n l o s s of ova between the two  y e a r s are not s i g n i f i c a n t 2.  (P> . 0 5 ) .  Postnatal mortality. a  «  Loss of young from b i r t h t o f i r s t c a p t u r e . 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 e s t i m a t e of the number of young born per ter  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  lit-  situation,  t h e n the drop from 4-.95 embryos per pregnancy t o 4-.60 young per l i t t e r a t b i r t h i s a l o s s of 7 . 1  per c e n t .  l i t t e r s i s p r o b a b l y q u i t e low, as 8 7 . 1  Complete l o s s of  per cent ( 6 1 / 7 0 )  of the  pregnant females were l a c t a t i n g when s u b s e q u e n t l y caught.  The  p r o p o r t i o n of pregnant f e m a l e s which s u c k l e d l i t t e r s was not s i g n i f i c a n t l y g r e a t e r i n the e x p e r i m e n t a l p o p u l a t i o n  (34-/38)  t h a n i n the n a t u r a l p o p u l a t i o n s ( 2 7 / 3 2 ) . N o t h i n g i s known about the l o s s of young from ' b i r t h t o about 4—6 weeks of age, when t h e y would have been c a p t u r e d f o r the  f i r s t time.  I n 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 c a p t u r e .  Loss of n e s t l i n g s may  be due t o i n -  h e r e n t weakness, d i s e a s e , d e s e r t i o n by the mother,  predation,  and o t h e r a c c i d e n t s such as f l o o d i n g . The e s t i m a t e of l o s s must c o v e r t h i s e n t i r e p e r i o d from b i r t h t o age a t f i r s t c a p t u r e , and e n a b l e s one t o compare the p o t e n t i a l number of young born on an a r e a w i t h the number of j u v e n i l e s caught.  Loss of young, however, may  be g r e a t e r t h a n  1+0  such an.' e s t i m a t e would i n d i c a t e . There a r e v a r i o u s p o s s i b l e e s t i m a t e s of l i t t e r t h e r e f o r e , of numbers born.  L i t t e r s i z e may be e s t i m a t e d from  the number of embryos per l i t t e r or  s i z e and,  or the number born p e r l i t t e r  from the combined data on l i t t e r  size.  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 p r e g n a n c i e s  (litters).  I t I s p r o b a b l e , however, t h a t not a l l p r e g n a n c i e s were r e corded. ing  S e v e r a l females which had had l i t t e r s  d u r i n g the breed-  season, but which had not been caught when p r e g n a n t ,  caught and a u t o p s i e d i n the f a l l .  Females, which showed s i g n s  of h a v i n g l a c t a t e d when caught f o r the f i r s t not have had t h e i r l i t t e r s  were  t i m e , may or may  on the same a r e a .  J u v e n i l e s , i n d i s p e r s i n g , may have m i g r a t e d onto or away from the study a r e a s . s t u d y a r e a s i n 1965 for  those a r e a s .  A few j u v e n i l e s appeared on some of the  b e f o r e any p r e g n a n c i e s had been r e c o r d e d Some of the unmarked immature a n i m a l s , caught  i n the f a l l , may have been p r e s e n t e a r l i e r i n the b r e e d i n g son, but not caught, or t h e y may have been immigrants study areas.  T h i s I n f l u x of young i n the f a l l was  n o t i c e a b l e on Lookout and Acadia, i n  sea-  onto the  especially  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 p r e g n a n c i e s 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 s u r -  v i v a l of young was poor d u r i n g the b r e e d i n g 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  .  Litters ' No.with Proportion loss with loss  Ova No. sites  No.corpora lutea  Proportion ova l o s t  Year  0  1  2  3  1964-  7  2  0  1  10  3  .300  50  4-5  .1^0  1965  5  1  1  0  7  2  .286  36  33  »083  12  3  1  l  17  5  .294-  86  78  .093  Total  Total  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 resorbing  Litters No.with loss wPirtohp o lr ot si so n  No. sites  Embryos No. embryos  Proportion embryos l o s t  Year  0  1  1964-  6  2  0  8  2  .250  4-2  4-0  .04-8  1965  4-  1  0  5  1  .200  25  24-  . 04-0  10  3  0  13  3  .231  67  64-  .04-5  Total Co  Total  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. embryos  Proportion ova. l o s t  Combined l o s s of ova and embryos  Year  No.corpora lutea  1964-  13  73  70  68  .04-1  .029  .068  1965  7  34-  33  31  .029  .061  .088  20  107  103  99  0O37  .038  .075  Total  No. sites  Proportion embryos l o s t  No. litters  4-2 to  improve toward  fall.  The o v e r a l l s u r v i v a l of young from b i r t h t o age at f i r s t c a p t u r e was e s t i m a t e d by comparing the number of young t h a t c o u l d have been produced by pregnant females on each a r e a 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 e s -  t i m a t e d from the number of p r e g n a n c i e s 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 t o 4—6 weeks of age.  Population Natural G o l f Course, 1964Chancellor,1965 Lookout,1965 Marine D r . , 1 9 6 5 Wreck Beach,1965 Experimental A c a d i a , 1964Acadia,1965 N B C P -  C  H  B  411 3 9 8  19.28 53.02 14-. 4-6 4-3.33 38.56  7 16 7 19 12  O.36.3 0.302 0.4-840.4-38 0.312  32 11  154-.2453.02  4-1 28  0.266 0.528  no.of p r e g n a n c i e s e s t i m a t e d no.of young born no.of j u v e n i l e s c a p t u r e d e s t i m a t e d 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 a r e a s 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 v a r i a n c e ( P > . 0 5 ) ; were combined  t h e r e f o r e , the data, f o r a l l a r e a s  t o g i v e 78 p r e g n a n c i e s and 130 j u v e n i l e s .  Of the  376 young e x p e c t e d from these p r e g n a n c i e s , the p r o p o r t i o n of young caught was 0 . 3 4 - 6 .  Thus, even on a c o n s e r v a t i v e b a s i s ,  o n l y about one t h i r d of the young born were marked. A l t h o u g h the d i f f e r e n c e s between the e x p e r i m e n t a l and  *+3 natural populations  i n t h e 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 a r e n o t 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 A c a d i a I n 196*4- and i n 1965 a r e h i g h l y  s i g n i f i c a n t ( . 0 1 > P , X:  2  b.  = 12.100 w i t h 1 d . f . ) .  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 . Deer mice t h a t were no l o n g e r caught on an a r e a may have  d i e d , e m i g r a t e d , or have been missed f o r some o t h e r  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 c a p t u r e and, t h e r e f o r e , one knows n o t h i n g  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 t h e minimum s u r v i v a l r a t e , t h e p r o p o r t i o n of a n i m a l s r e l e a s e d a t time t , caught a t time t + 1. J u v e n i l e and a d u l t s u r v i v a l f o r each four-week t r a p p i n g 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 t h e  d a t a shown below i n d i c a t e s t h a t t h e r a t e s of d i s a p p e a r a n c e f o r the j u v e n i l e s a r e f a i r l y u n i f o r m TABLE X V I I I .  Month Apr. May June July Aug. Sept. Oct.  season.  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 adults.  Juveniles Males Females R MSR MSR H 3 7 9 16 25 13 8  throughout t h e breeding  0.667 0.71*+ O.667 0.688 O.680 O.769 0.875  3 9 7 10 17 16 12  1.000 0.778 0.71*+ 0.700 0.706 0.812 O.667  Adults Males Females R MSR MSR R 55 50 1+1 37 50 19 6  R - No.of mice r e l e a s e d MSR - Minimum s u r v i v a l r a t e s  0.7*+5 0.780 0.756 .0.757 0.580 o.6S*+ 1.000  hi  *+5 *+3  hi 5h 20 15  0.976 O.Shh 0.7*+*+ 0.732 0.685 0.850 0.800  A n a l y s i s of v a r i a n c e 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 t r a p p i n g p e r i o d s ( , 0 5 > P > . 0 1 ) ; t h e r e f o r e , the d a t a f o r age and sex were pooled.  I n o t h e r 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 t r a p p i n g p e r i o d are g i v e n 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 v a r y r a t h e r w i d e l y .  Analysis  of v a r i a n c e 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 b e t ween a r e a s , even though s u r v i v a l began t o decrease i n J u l y 1 9 6 5 and on Wreck Beach i n August 1 9 6 5 .  on C h a n c e l l o r  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 f e m a l e s , a v e r a g i n g 0 . 7 5 8 and 0 . 8 1 6 , r e s p e c t i v e l y (.05^ P >.01).  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 r e c a p t u r e d 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 f e m a l e s . C l e a r s e a s o n a l changes i n s u r v i v a l were observed S u r v i v a l was l o w e s t d u r i n g the b r e e d i n g  (,01>P).  season from June  August ( 0 . 7 1 1 ) and h i g h e s t d u r i n g the w i n t e r ( O . 8 3 2 ) . was a l i t t l e p o o r e r i n 1 9 6 5 than i n 196*f ( , 0 5 > P > . 0 1 ) .  through  Survival The  s e a s o n a l 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. o v e r a l l s u r v i v a l p e r month was  0.790.  The  k5 3.  D i s e a s e , p a r a s i t i s m , and  predation.  W i t h the e x c e p t i o n of two females caught on the Wreck Beach a r e a , none of the deer mice from the f i e l d appeared d i s e a s e d . 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 a r e a f o r t h r e e  months, the o t h e r f o u r 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  and d u r i n g J u l y and August, by l a r v a e of the b o t f l y , sp.  fleas,  Cuterebra  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  b a d l y p a r a s i t i z e d ones were r e c a p t u r e d i n a p p a r e n t l y good h e a l t h . S a d l e i r ( 1 9 6 5 ) a l s o r e p o r t e d p a r a s i t i s m by b o t f l y l a r v a e . The  t r a p s were d i s t u r b e d s p o r a d i c a l l y by r a c c o o n s  l o t o r ) but o n l y once was^there evidence s q u i r r e l s (Tamiasciurus traps.  d o u g l a s i ) may  of a k i l l .  (Procyon  Douglas  a l s o have d i s t u r b e d the  Four s h o r t - t a i l e d weasels ( M u s t e l a erminea) were caught  and removed from the areas d u r i n g the two y e a r s . c a t s were a l s o heard  or seen.  Owls and house  4-6 TABLE X I X . 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.  Period  R  26 26 23 21 18 16 13 10  43 43 3  26 26 23 21 18 16 13 10  Feb. ,1964Mar. ,1964Apr., 1964May,1964June, 1964J u l y , 196'+ Aug., 1964Sept. ,196»+ Feb.,1965 Mar.,1965 Apr.,1965 May,1965 June,1965 July,1965 Aug.,1965 Sept.,1965  7  411 25 '+3 36 32 33 4-5 27  Males C MSR 43 3 2 3 2 5 9 21 30 27 23 25 25  -  1.000 1.000 0.750 0.667 1.000 0.667 0.714-  -  0.818 0..84-0 0..698 0.750 0.719 0.758 0.556  -  R  Females C MSR  3 3 2 3 2 2 3 1  3 3 2 2 2 2 1  1.000 1.000 1.000 O.667 1.000 1.000 0.333  11 21 26 31 30 34>+2 28  11 19 25 25 242426  1.000 0.905 0.962 0.806 0.800 0.706 0.619  -  _  -  _  R - no.of deer mice r e l e a s e d a t time t . C - no.of deer mice r e c a p t u r e d a t time t + 1. MSR - 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 e x p e r i m e n t a l populations, 196M--5.  Period  R  26 26 23 21 18 16 13 10 8 5 3  Feb. ,196'+ Mar. , 1 9 6 M Apr. , 1 9 6 M May, 196MJune , 1 9 6 M J u l y ,196MAug. , 1 9 6 M Sept . , 1 9 6 M Oct. , 1 9 6 M Nov. , 1 9 6 M Dec. ,196M-  23 19  26 26 23 21 lo 16 13 10  Feb. , 1 9 6 5 Mar.., 1 9 6 5 Apr. 1965 May,1965 June , 1 9 6 5 J u l y ,1965 Aug.. 1 9 6 5 Sept. , 1 9 6 5  6 6 5 7 9 9 9 12  R C MSR 1 2  h 5 6 6 5 8 13 18 1M-  -  Males C M-  5 5 5 M-  7 10 16 13 20 11  1  5 1+ 5 6 7 5 6  -  Female s R MSR C  MSR 1.000 1.000 0.833 0.833 0.800 0.875 0.769 0.889 0.929 o.870 0.833 0.833 0.667 1.000 0.857 0.778 0.556 0.667  o 2  7 7 9 9 6 10 18 20 27 33 32 M-  7 7 8 10 5 8  13  77 9 9 7 15 20 20 26  2M-  2 >+ 7 7 6 M-  7  1  1.000 1.000 1.000 1.000 0.667 0.700 0.833 1.000 0.7M-1 0.788 0.909  o 2  0.500 0.571 1.000 0.875 0.600 0.800 0.375  no.of deer mice r e l e a s e d a t time t . no.of deer mice r e c a p t u r e d a t time t + 1 . minimum s u r v i v a l r a t e . no.of deer mice r e c a p t u r e d 26 F e b . , 1 9 6 5 . average MSR f o r t h r e e t r a p p i n g p e r i o d s .  4-8  V. In  DISCUSSION  t h i s s e c t i o n the d a t a w i l l be i n t e g r a t e d w i t h t h e  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 t h e r e l e v a n c e of these  o b s e r v a t i o n s on deer mice t o 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 s i n c e 1962, when S a d l e i r (1965) began t r a p p i n g  deer mouse p o p u l a t i o n s 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 d u r i n g t h e summer, i n creased  suddenly  t o 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 t h e w i n t e r .  W i t h one e x c e p t i o n , no marked decrease  i n numbers p r i o r t o . t h e onset of t h e b r e e d i n g observed.  S a d l e i r (1965) observed  season has been  a drop i n numbers i n one of  his  study p o p u l a t i o n s i n June 1963, the p o p u l a t i o n began breed-  ing  i n July.  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 t o y e a r . The  changes i n p o p u l a t i o n s t r u c t u r e have been t h a t most  a d u l t s o f the summer b r e e d i n g p o p u l a t i o n d i e by f a l l and a r e r e p l a c e d by s e x u a l l y immature animals which o v e r w i n t e r and mature i n the f o l l o w i n g s p r i n g t o form a new b r e e d i n g p o p u l a t i o n . j u v e n i l e s appear d u r i n g t h e summer b r e e d i n g  season.  Few  Thus, t h e r e  i s an annual t u r n o v e r 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 p o p u l a t i o n numbers and s t r u c t u r e have been r e p o r t e d by many workers (Howard, 194-9; Snyder, 1956; Hoffmann, 1958; Newson 1964-; K r e b s ,  1964-).  4-9 B.  REPRODUCTION The l e n g t h of the b r e e d i n g season has been q u i t e v a r i a b l e .  B r e e d i n g had begun when S a d l e i r began h i s s t u d i e s i n May 1962 and extended t o September.  I n 1 9 6 3 the b r e e d i n g season e x t e n d -  ed from June t o September, i n 1964- from March t o September, and i n 1 9 6 5 from May t o August.  No e s t i v a l pause has been observed.  B r e e d i n g seasons of s i m i l a r d u r a t i o n have been r e p o r t e d by Sheppe ( 1 9 5 8 ) f o r t h i s s u b s p e c i e s (P. m. a u s t e r u s ) , by Beer e t 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 B l a n c h a r d ( 1 9 5 0 ) f o r P. m. g a m b e l i i .  W i n t e r b r e e d i n g has  been r e p o r t e d f o r P. m. g a m b e l i i i n C a l i f o r n i a (Jameson, 1 9 5 3 ) . Few f e m a l e s , however, s t a r t e d b r e e d i n g i n the n a t u r a l pop u l a t i o n s u n t i l May i n 1964- and i n a l l p o p u l a t i o n s u n t i l May i n 1 9 6 5 ? 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 d e l a y i n b r e e d i n g i s t h a t the f i r s t e s t r o u s c y c l e s may have been s t e r i l e , as has been observed i n C l e t h r i o n o m y s ( B r a m b e l l and Rowlands, 1 9 3 6 ) , Peromyscus l e u c o p u s ( B e n d e l l ,  1 9 5 9 ) ,  and  i n P. m a n i c u l a t u s (Jameson, 1 9 5 3 ) . The p r o p o r t i o n of a d u l t or o v e r w i n t e r e d females t h a t became 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 I n 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 m e n t a l p o p u l a t i o n s I n 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 b r e e d i n g season i n the n a t u r a l p o p u l a t i o n s i n 1964- and 1 9 6 5 was 1 . 5  50  litters  p e r b r e e d i n g f e m a l e , and i n t h e e x p e r i m e n t a l  t i o n s , 1.3  b e f o r e June 196M- and 2.8 a f t e r , and 2 . 0 i n 1965.  The i n c i d e n c e of postpartum  mating- i n t h e marked p o p u l a t i o n s was  v e r y l o w , (6/27 = 0 . 2 2 2 ) , most females  o n l y becoming pregnant  a g a i n two t o t h r e e weeks a f t e r p a r t u r i t i o n . pregnancies Blanchard ing  T h i s gap between  suggests a p e r i o d of l a c t a t i o n a n e s t r u s .  (1950) r e p o r t e d an average of f o u r l i t t e r s  McCabe and p e r breed-  female p e r season. The number of embryos p e r l i t t e r observed  h.95  popula-  i n the f i e l d  was  and the number of young p e r l i t t e r i n the l a b o r a t o r y was  h.6Q.  Sheppe (1958) r e p o r t e d 5.6 embryos p e r l i t t e r f o r s e v e r a l  s u b s p e c i e s of P. m a n i c u l a t u s i n B r i t i s h Columbia. per l i t t e r r e p o r t e d f o r o t h e r s u b s p e c i e s i s h.28  The number young f o r P. m.  b a l r d i i i n M i c h i g a n (Howard, 19*+9) , 5.06 embryos f o r P. m. gamb e l i i near B e r k e l e y (McCabe and B l a n c h a r d , 1950) and H-.60 embryos i n t h e S i e r r a Nevada i n n o r t h e r n C a l i f o r n i a 5.38  (Jameson, 1953)5 and  embryos f o r P. m. g r a c i l i s i n O n t a r i o ( C o v e n t r y , The average l i t t e r  1937).  s i z e i n P. m. a u s t e r u s was a f f e c t e d by  m a t e r n a l w e i g h t , or age. L a r g e r females c a r r i e d l a r g e r numbers of embryos. (1957)  T h i s phenomenon was a l s o observed by Beer e t a l  f o r P. m. b a i r d i i and P. m. g r a c i l i s .  A larger  number  of young p e r l i t t e r has been r e p o r t e d a t h i g h e r a l t i t u d e s , but a l o n g e r b r e e d i n g season and more l i t t e r s ( A s d e l l , 196H-).  at lower e l e v a t i o n s  51 C.  MORTALITY The decrease i n l i t t e r  5.18  ova shed ( c o r p o r a l u t e a ) t o 5.00 i m p l a n t a t i o n s i t e s t o  4-.95 embryos. 0.10. al  s i z e d u r i n g pregnancy was from  The p r o p o r t i o n of ova l o s t was a p p r o x i m a t e l y  T h i s decrease i s comparable  (1957) f o r P. m. b a l r d i i . 5.77  t o t h a t observed by Beer e t ova t o 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 t o 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 p r o b a b l y q u i t e l o w , as shown  by t h e p r o p o r t i o n of pregnant females w h i c h , when s u b s e q u e n t l y c a u g h t , were l a c t a t i n g  (61/70 = . 8 7 1 ) .  n o r n e s t l i n g m o r t a l i t y was observed.  Neither loss at b i r t h Howard (194-9) r e p o r t e d  that several l i t t e r s  born i n t h e s p r i n g were dead a t b i r t h or  died shortly 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 p e r l i t t e r a t 0-2 days, t o 4-. 17 a t 3-7 days t o 3.39 a t 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 t h a t the l a r g e s t l o s s of  young o c c u r r e d a t about t h r e e weeks a f t e r b i r t h , or j u s t  after  weaning. The p r o p o r t i o n of young l o s t from b i r t h t o age a t f i r s t c a p t u r e was 0 . 6 5 i n P. m. a u s t e r u s compared w i t h 0.82 i n P. m. g a m b e l i i (McCabe and B l a n c h a r d , 1 9 5 0 ) .  I found t h a t t h e s u r -  v i v a l of the young was poor d u r i n g t h e b r e e d i n g season and improved toward f a l l , but t h a t once t h e j u v e n i l e s had been marked,  52  t h e y s u r v i v e d as w e l l as t h e a d u l t s .  S a d l e i r (1965), however,  observed t h a t j u v e n i l e s e n t e r i n g t h e summer b r e e d i n g t i o n s u r v i v e d worse t h a n those e n t e r i n g the f a l l My o b s e r v a t i o n s  popula-  population.  on n a t u r a l p o p u l a t i o n s were made over a s h o r t -  e r p e r i o d of time and c o u l d n o t 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 r a t e s f o r j u v e n i l e s and a d u l t s i n P.. l e u c o p u s p o p u l a t i o n s , and t h a t no m a t t e r what time of year t h e young e n t e r e d  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 a d u l t s was good d u r i n g the w i n t e r  but dropped d u r i n g t h e summer breeding  season ( 0 . 7 1 1 ) .  observed average s u r v i v a l p e r month of 0 . 8 0 was a l s o by S a d l e i r (1965) f o r . t h i s s u b s p e c i e s , for  P.  D.  EXTRINSIC FACTORS  and by B e n d e l l  (0.332) The  reported (1959)  leucopus.  D i s e a s e 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 t h a t 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 h i g h m o r t a l i t y i n n a t u r a l populations.  E x c e p t f o r two mice caught on the Wreck Beach  a r e a , none of the a n i m a l s from t h e f i e l d appeared  diseased.  Hence, t h e r e was no evidence of an endemic e p i z o o t i c d i s e a s e killing fall.  a n i m a l s i n l a r g e r numbers i n the s p r i n g t h a n i n the  53 P r e d a t o r s have been assumed t o be r e s p o n s i b l e f o r keeping numbers i n P. m a n i c u l a t u s p o p u l a t i o n s i n check (Howard, 194-9) ? and even, t o cause p o p u l a t i o n d e c l i n e s ( B r a n t , 1962).  Although  p r e d a t o r s were p r e s e n t on the U n i v e r s i t y Endowment Lands, t h e r e i s no i n d i c a t i o n t h a t they took more mice at the b e g i n n i n g of t h e season than a t the  end.  V a r i a t i o n s i n weather c o u l d e x p l a i n p a r t of the d i f f e r e n c e s i n l e n g t h of the b r e e d i n g season, and, perhaps, r e p r o d u c t i o n between the two y e a r s .  the amount of  The m i l d weather i n  1964-  was p r o b a b l y conducive t o r e p r o d u c t i o n and the c o l d s p r i n g and h o t , d r y summer i n 1965  were p r o b a b l y i n i m i c a l , but I n both y e a r s  the amount of b r e e d i n g was  below the p o t e n t i a l c a p a c i t y .  In  each year b r e e d i n g began at d i f f e r e n t times i n the marked populations.  Thus, weather by i t s e l f d i d not i n h i b i t  Weather may  breeding.  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  a l t h o u g h 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 p o p u l a t i o n s 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  p o i n t up d i f f e r e n c e s between y e a r s , s u r v i v a l being p o o r e r i n t h a n i n 1964-.  1965  There i s no d i r e c t e v i d e n c e , however, t h a t these  d i f f e r e n c e s were due t o adverse weather c o n d i t i o n s . The f o o d s u p p l y was  assumed t o be s u f f i c i e n t , and no e x t r a  f o o d was put i n the f i e l d .  Food, however, c o u l d be s c a r c e i n  the s p r i n g and become more p l e n t i f u l d u r i n g the summer and enough f o o d c o u l d be s t o r e d i n the f a l l t o c a r r y the p o p u l a t i o n through the w i n t e r . - Food was p r o b a b l y not s c a r c e on A c a d i a i n the s p r i n g  5h  of 1965 s i n c e t h e number of deer mice was e x p e r i m e n t a l l y r e duced from 75 t o 12; hence, n e i t h e r t h e onset of b r e e d i n g  i n the  s p r i n g n o r p r o d u c t i o n of l i t t e r s l a t e r i n t h e summer c o u l d be d i r e c t l y c o n t r o l l e d by t h e food, s u p p l y .  The mice caught i n t h e  s p r i n g , many of which were unmarked, may have been l i v i n g on the a r e a but not p r e v i o u s l y caught or may have immigrated onto the a r e a t o f i l l  the vacuum c r e a t e d by t r a p p i n g i t o u t .  P o p u l a t i o n numbers were v e r y s t a b l e from year t o y e a r , e s p e c i a l l y on the G o l f Course, w h i l e t h e f o o d s u p p l y  probably  v a r i e d ; t h e r e f o r e , food s u p p l y by i t s e l f was not r e g u l a t i n g the populations.  B e n d e l l (1959), working on P. l e u c o p u s ,  and DeLong (1965), working on M i c r o t u s , have r e p o r t e d  and Krebs that  p o p u l a t i o n s do not i n c r e a s e i n d e f i n i t e l y when g i v e n an u n l i m i t ed s u p p l y of f o o d , and the problems of p o p u l a t i o n r e g u l a t i o n still  remain. Trapping  ing  suckled.  may have caused an i n c r e a s e i n l o s s of l i t t e r s beL a c t a t i n g f e m a l e s which were caught i n the t r a p s  were kept away from t h e i r l i t t e r s without  o v e r n i g h t , l e a v i n g the young  food or p r o t e c t i o n f o r a maximum o f about 12 h o u r s .  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 o n l y 9 oxxt of 70 pregnant females showed no s i g n s of having subsequently  caught.  l a c t a t e d when  The observed s e a s o n a l change i n s u r v i v a l  of young i s not due t o t r a p p i n g , as females were s u b j e c t e d t o the same t r a p p i n g procedure i n the f a l l as i n t h e s p r i n g .  55 E.  SEASONAL INCREASE IN NUMBERS The s e a s o n a l i n c r e a s e i n numbers observed i n t h e marked po-  p u l a t i o n s was o n l y 1.04-. ing  T h i s f i g u r e was c a l c u l a t e d by compar-  t h e number of a d u l t s p r e s e n t  i n A p r i l (77) w i t h t h e number  s t i l l a l i v e I n October (10) p l u s t h e number o f 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 a r e a but were n o t i n c l u d e d i n t h i s c a l c u l a t i o n , as t h e y may e q u a l l y w e l l have been i m m i g r a n t s .  The  t o t a l observed t h r e e f o l d i n c r e a s e i n numbers was p r o b a b l y due to  immigration  i n t o the p o p u l a t i o n s .  The p o t e n t i a l s e a s o n a l i n c r e a s e i n numbers can be e s t i m a t ed from the f o l l o w i n g assumptions about r e p r o d u c t i o n and s u r v i v al. Reproduction: Number of young p e r l i t t e r i s 5.00. Sex r a t i o a t b i r t h i s u n i t y . A d u l t s bear one l i t t e r p e r month between May and September. J u v e n i l e s mature a t 8 weeks of age. Survival: A d u l t s u r v i v a l r a t e p e r 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 t o h weeks i s 0.35. By a p p l y i n g these assumptions t o a c o h o r t  of 10 f e m a l e s and 10  males a l i v e I n A p r i l (week 0 ) , one f i n d s t h a t 61.86 animals be p r e s e n t  I n October (week 28) (Table X X I ) .  would  This represents a  t h r e e f o l d p o t e n t i a l i n c r e a s e I n numbers I n c o n t r a s t t o v i r t u a l l y no i n c r e a s e due t o r e p r o d u c t i o n i n t h e marked p o p u l a t i o n s . Loss of mice over t h e w i n t e r a t the r a t e of 0.17 p e r month would reduce numbers t o the p r e v i o u s s p r i n g ' s l e v e l .  A p o 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 t o 300 mice by f a l l and dwindle t o 98 mice by t h e next s p r i n g . The model a l l o w s f o r exact replacement of the p o p u l a t i o n , but o t h e r p a t t e r n s of p o p u l a t i o n change a r e 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 ,  survival  would p r o b a b l y be b e t t e r and t h e s p r i n g p o p u l a t i o n c o u l d i n crease.  On the o t h e r hand adverse weather c o n d i t i o n s would r e -  duce p o p u l a t i o n numbers. F.  REVELANCE OF PRESENT DATA TO EXISTING THEORIES Andrewartha and B i r c h (195*+) have suggested t h a t the num-  ber of animals environmental  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 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 t h e r a t e of i n c r e a s e r i s p o s i tive.  I n 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 d e c i d e d  t h a t v a r i a t i o n s i n weather were n o t of p r i m a r y  importance i n po-  p u l a t i o n r e g u l a t i o n s i n c e t h e r e were no s i g n i f i c a n t changes I n breeding  success  between the two y e a r s and s i n c e p o p u l a t i o n num-  bers were v e r y s t a b l e from year t o y e a r .  Although  mortalityi s  the main f a c t o r , changes i n i t a r e not due t o c l i m a t i c  condi-  t i o n s a l o n e but, p e r h a p s , t o 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 populations . Wynne-Edwards (196.2, 196 +) has advanced t h e i d e a t h a t 1  p o p u l a t i o n s can a d j u s t t h e i r r e p r o d u c t i v e r a t e s t o a v o i d e x p l o i t a t i o n of the renewable r e s o u r c e s  animal  over-  of the environment.  says t h a t a p o p u l a t i o n r e c e i v e s i n f o r m a t i o n about i t s numbers  He  TABLE X X I .  P o t e n t i a l seasonal increase  i n numbers.  S u r v i v a l of p a r e n t a l c o h o r t of 10 females and 10 males and t h e i r o f f s p r i n g : - . Time (weeks) Generation Parental Males Females First F First F Second F Third F Second F First F Second F Third F First F F o u r t h E. Fifth Sixth F 4  a  2  4  2  2  A  2  1  0  4-  8  12  10.00 10.00  • .-8. oo 8.00  6.4-0 6.4-0  —  4-0.00  14-. 0 0  5.12 5.12 11.20  —  -  —  -  —  —  -  -  —  —  —  -  -  -  — —  -  —  —  32.00 -  11.20  —  -  —  25.60  —  -  -  16 4-. 10 4-. 10 3.96 22.4-0  -  20  3.28 3.28  7.13  7.8417.94-  -  -  7.18 17.94-  8.96  7.18  8.96  20.25  -  -  -  -  7.18 16.4-0  -  Assumptions: Reproduction: Number of young p e r l i t t e r i s 5 . 0 0 . Sex r a t i o a t b i r t h i s u n i t y . A d u l t s bear one l i t t e r p e r month between weeks 4- and 28, J u v e n i l e s mature a t 8 weeks of age. Mortality: A d u l t s u r v i v a l r a t e p e r 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 t o 4- weeks i s O.35.  24-  28  2.62 2.62 5.746.28 6.28 14-. 35 5.746.28 14-. 35 5.7414-.35 5.745.7413.10  2.10 2.10 4-. 59 5.02 5.02 5.02 4-. 59 5.02 5.02 4-. 59 5.02 4-. 59 4-. 59 4^.59  Total  61.86  vn  58  through  s o c i a l i n t e r a c t i o n s and i  t e r r i t o r i a l b e h a v i o r and  that  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 b r e e d i n g and  birth  rate.  According  so, i n e f f e c t , reduce the  t o these i d e a s , the h i g h a g g r e s s i v e l e v e l  s e r v e d at the b e g i n n i n g  ob-  season by S a d l e i r ( 1 9 6 5 )  of the b r e e d i n g  and H e a l e y ( 1 9 6 6 ) would be i n t e r p r e t e d as an a d a p t a t i o n w h i c h f a v o r s s u r v i v a l of the p o p u l a t i o n by l i m i t i n g the number of ing  animals  i n an a r e a , and the s h o r t e n e d  s e r v e d i n some p o p u l a t i o n s and  breeding  seasons  the low p r o p o r t i o n of  as f u r t h e r a d a p t a t i o n s f o r p r e v e n t i n g o v e r - p o p u l a t i o n .  breedob-  pregnancies That i s ,  he b e l i e v e s t h a t p o p u l a t i o n s t h a t 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 d e s t r o y themselves and  popula-  be r e p l a c e d by  t i o n s t h a t do c o n t r o l t h e i r r e p r o d u c t i v e r a t e s . V a r i o u s a u t h o r i t i e s , however, have o b j e c t e d t o t h i s v i e w on the grounds t h a t i t i s c o n t r a r y t o the p r e s e n t v i e w 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 ( B r a e s t r u p , 1 9 6 3 ; 1 9 6 f ; S m i t h , I96H-). L  Smith (1964-) d i s c u s s e s b e h a v i o r w h i c h " l e a d s  an i n d i v i d u a l not t o breed i n c i r c u m s t a n c e s b e r s of the s p e c i e s are b r e e d i n g  petuated. ! 1  i n which o t h e r mem-  s u c c e s s f u l l y " and p o i n t s out  t h a t " I f the d i f f e r e n c e between b r e e d e r s t i c a l l y determined,  Lack., 196'+; P e r r i n s ,  and non-breeders i s gene-  then i t i s the b r e e d e r s  whose genotype i s p e r -  He a l s o p o i n t s out t h a t " i t i s u n l i k e l y t h a t s p e c i e s  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 s m a l l 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 b e h a v i o r t o e v o l v e and  be p e r p e t u a t e d ,  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 t h a t a b e h a v i o r a l - p h y s i o -  l o g i c a l mechanism, o p e r a t i n g through the  pituitary-adrenocortical  a x i s , a f f e c t s r e p r o d u c t i o n 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 D a v i s , 1 9 6 5 ;  C h r i s t i a n , L l o y d , and D a v i s , 1 9 6 5 ) .  He  says  t h a t " t h e r e i s decreased f e r t i l i t y and f e c u n d i t y , i n c r e a s e d i n t r a u t e r i n e and  I n f a n t m o r t a l i t y w i t h i n c r e a s e d p o p u l a t i o n den-  s i t y and t h a t the i n c r e a s e d i n f a n t m o r t a l i t y i s at l e a s t due  t o 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 r e p r o d u c t i v e r a t e would be i n t e r p r e t e d as a consequence of r e l a t i v e l y h i g h numbers or h i g h s o c i a l and the h y p e r t r o p h y i n t h i s study and  of the a d r e n a l glands i n those  endocrine and  pressure,  C h i t t y and  and K r e b s , 1964- as e v i d e n c e f o r  feed-back mechanisms, a c t i n g t o reduce  increase prenatal m o r t a l i t y .  reproduction  Negus e_t a l ( 1 9 6 1 )  e v i d e n c e which i m p l i e s t h a t d e n s i t y and " n e c e s s a r i l y important  necessary  i n the summer, found  r e p o r t e d by C h i t t y , 1 9 6 1 ;  C l a r k e , I963; C h r i s t i a n , 1962;  partly  discussed  s o c i a l c o n t a c t are  not  f a c t o r s i n f l u e n c i n g a d r e n a l weight changes  i n n a t u r a l populations'.".. I n 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 morta 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 s u b s e q u e n t l y  caught was  i n the e x p e r i m e n t a l  populations  pulations (27/32).  The  q u i t e low i f one have supported,  not s i g n i f i c a n t l y g r e a t e r  (3*+/38) than i n the n a t u r a l po-  a c t u a l d e n s i t y on the study a r e a s  was  c o n s i d e r s .the number of mice the areas c o u l d 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 r e p r o d u c t i o n on A c a d i a  tended t o be g r e a t e r 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 t o the number of mice on the a r e a p r i o r t o the b r e e d i n g  season.  immediately  These o b s e r v a t i o n s c o u l d e q u a l l y  w e l l be e x p l a i n e d by v a r i a t i o n s i n the weather i n the two or,  years  p e r h a p s , by changes i n b e h a v i o r . Crowcroft  and Rowe  (1953, 1963)  t y and r e p r o d u c t i o n decreased  have r e p o r t e d t h a t f e c u n d i -  i n house mice caged t o g e t h e r i n  l a r g e numbers, and t h a t b r e e d i n g was resumed when the d e n s i t y was reduced by a l l o w i n g the p o p u l a t i o n s t o d i s p e r s e i n t o l a r g e r a r e a s . New  dominance-subordination  a d d i t i o n a l area.  r e l a t i o n s h i p s were formed i n the  Petrusewicz  (1957, 1963)  induced  population  growth by changing a p o p u l a t i o n t o a d i f f e r e n t k i n d of cage, by adding  or removing mice.  He suggested  t h a t 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 . I n c r e a s e d r e p r o d u c t i o n i n f i e l d p o p u l a t i o n s has a l s o been a t t r i b u t e d t o 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 .  Lloyd  (1963)  found t h a t when the number of r a b b i t s i n a p o p u l a t i o n was immediately  p r i o r t o the onset  of s p r i n g b r e e d i n g , the p o p u l a t i o n  i n c r e a s e d more than i f no r a b b i t s had been removed. o b t a i n e d midsummer b r e e d i n g  reduced  Sheppe  i n p o p u l a t i o n s of P. l e u c o p u s ,  (1965) which  n o r m a l l y have an e s t i v a l pause, by i n t r o d u c i n g mice i n v e r y l a r g e numbers onto i s l a n d s i n O n t a r i o . S o c i a l c o n t a c t s have been shown t o i n c r e a s e p r e n a t a l mortality.  The presence o r 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 i n s e m i n a t i o n 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) (Bronson and E l e f t h e r i o u , 1 9 6 3 ) .  bairdii  and i n P.  m.  Strange males, how-  e v e r , seldom remained i n an a r e a f o r any l e n g t h of t i m e , as 27 per cent (4-5/165) of the males i n the marked p o p u l a t i o n s were caught o n l y once.  The  only prolonged  exposure t o strange  odors would be i n the t r a p s f o u r 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 t o pregnancy b l o c k would not be ed t o be important  i n r e d u c i n g the b i r t h r a t e .  odors need not be d e l e t e r i o u s .  The  expect-  e f f e c t s of  Odors and scent markings l e a v e  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) Chitty (i960)  and the dog  ( L o r e n z , 195'+).  has proposed t h a t "under a p p r o p r i a t e  circum-  s t a n c e s , i n d e f i n i t e i n c r e a s e 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  t h a t 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  related  changes i n s u r v i v a l , r e p r o d u c t i v e s u c c e s s , and body weight t o phases i n the p o p u l a t i o n c y c l e i n v o l e s ( C h i t t y , 1952, and C h i t t y , 1962) . a shortened b r e e d i n g  Krebs (196 i-) , working l  on lemmings, r e p o r t e d  season, f a i l u r e of j u v e n i l e s t o mature, and  i n c r e a s e d body weights suggested  Chitty  of a d u l t s when numbers were g r e a t e s t .  He  t h a t the changes i n numbers c o u l d be r e l a t e d t o changes  i n aggressive behavior.  S a d l e i r ( 1 9 6 5 ) observed  were more a g g r e s s i v e d u r i n g the b r e e d i n g  t h a t deer mice  season than i n the  fall,  and  t h a t the l e v e l of a g g r e s s i v e n e s s i n f i e l d p o p u l a t i o n s  g r e a t e r at times of d e c r e a s e d s u r v i v a l . t h a t j u v e n i l e s u r v i v a l was  H e a l e y (1966) found  i n v e r s e l y r e l a t e d t o the l e v e l  a g g r e s s i v e n e s s of the a d u l t s . 97)  was  of  McCabe and B l a n c h a r d (1950,  p.  suggested t h a t "the key t o 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  e l s e w h e r e , not  p e t i t i o n near the n e s t s , "but  i n i n t o l e r a n c e or com-  once a j u v e n i l e had  i t would be d r i v e n away from any r e f u g e a l r e a d y According  left  i t s nest  occupied.  t o these i d e a s , the poor r e p r o d u c t i v e  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  vironment.  g r e a t e r p r o d u c t i v i t y and m a t u r a t i o n of the young  The  observed i n 196'+  as a symptom of l i v i n g i n an u n f a v o r a b l e  en-  c o u l d be a s s o c i a t e d w i t h h i g h e r mean monthly  body w e i g h t s . Lack (1954-) has  suggested t h a t changes i n s u r v i v a l r a t h e r  t h a n changes i n b i r t h r a t e r e g u l a t e p o p u l a t i o n numbers.  The  reasons g i v e n f o r t h i s p o i n t of v i e w 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 t o a f f e c t p o p u l a t i o n numbers and t h a t fewer young are f l e d g e d from c l u t c h e s of l a r g e r than  aver-  age  quite  size.  variable dying  The (3-7)  number of embryos per l i t t e r , however, was but no evidence was  observed f o r more embryos  i n l a r g e r l i t t e r s than i n s m a l l e r ones, o r , i n the  t o r y , f o r fewer young t o be weaned i n l a r g e r l i t t e r s .  labora-  No  f i c a n t d i f f e r e n c e s between y e a r s were found i n the average  signinum-  63  ber of embryos per  litter.  That changes i n p o p u l a t i o n numbers are a d e q u a t e l y  ac-  c o u n t e d 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 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 of the young i n the y e a r s on the e x p e r i m e n t a l a r e a . season were a p p r o x i m a t e l y formance was  Numbers at the end of each  the same.- I n 1964-  reproductive per-  c o m p a r a t i v e l y good but s u r v i v a l of the young  i n 1965,  poor;  two  the r e v e r s e was  observed.  was  -Differences i n re-  p r o d u c t i v e performance were not s t a t i s t i c a l l y 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  significant;  significant.  Deer mouse p o p u l a t i o n s are s e l f - r e g u l a t o r y , changes i n death r a t e r a t h e r than changes i n r e p r o d u c t i v e r a t e being more important tions.  i n m a i n t a i n i n g the s t a t i o n a r y s t a t e of these  There i s some evidence  popula-  t h a t a g o n i s t i c b e h a v i o r i s the  mechanism t e n d i n g t o space the a n i m a l s e x c e s s a n i m a l s from the p o p u l a t i o n s .  out and e l i m i n a t i n g the The  s u g g e s t i o n was made  t h a t changes i n r e p r o d u c t i v e performance may  be a s s o c i a t e d w i t h  changes i n the a g g r e s s i v e l e v e l of the p o p u l a t i o n .  64VI. 1.  The  SUMMARY  r o l e of changes I n r e p r o d u c t i v e r a t e 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 m a n i c u l a t u s .  was  compared by s t u d y i n g n a t u r a l and e x p e r i -  m e n t a l p o p u l a t i o n s on the U n i v e r s i t y Endowment Lands i n 1964and  1965.  2.  Weekly l i v e t r a p p i n g of the p o p u l a t i o n s showed t h a t num-  b e r s remained r e l a t i v e l y steady over the summer, i n c r e a s e d 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 . p u l a t i o n s numbers d i d not i n c r e a s e i n the f a l l fall  immature animals  year's breeding  which o v e r w i n t e r e d  3.  of 1965.  t o form the next  p o p u l a t i o n , r e s u l t i n g i n an a n n u a l  from March t o September i n b o t h y e a r s .  Females bore l i t t e r s from March t o September i n 1964- and March t o August i n 1965.  not 4-.  Few  Seasonal v a r i a t i o n s i n breeding  s i m i l a r i n both y e a r s and  5.  in  c o n d i t i o n were  so marked, i n the males as they were i n the L i t t e r s i z e was  from  l i t t e r s were born b e f o r e May  females. seemed t o be i n -  dependent of p a r i t y , but t o be g r e a t e r i n h e a v i e r females to  By  populations.  Males were fecund  e i t h e r year.  po-  p o p u l a t i o n had r e p l a c e d most of the a d u l t s  from the summer b r e e d i n g o v e r t u r n of the  I n two  and  change, s e a s o n a l l y . The p r o p o r t i o n of ova l o s t was  s t a n t d u r i n g the p e r i o d of s t u d y .  0.10 The  and was  r e l a t i v e l y con-  r a t e of l o s s of young  65 from b i r t h t o age at f i r s t c a p t u r e averaged 0 . 6 5 and. was  great-  est  The  d u r i n g the b r e e d i n g  season, d e c r e a s i n g toward f a l l .  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 t i c a l l y different. breeding  S u r v i v a l was  season ( 0 . 7 1 1 )  s i g n i f i c a n t l y lower d u r i n g the  than d u r i n g the w i n t e r ( 0 . 8 3 2 ) ,  s i g n i f i c a n t l y lower i n the males ( 0 . 7 5 8 ) (0.8l6)..  The  statis-  than i n the  o v e r a l l s u r v i v a l r a t e of the a d u l t s was  and  females 0 . 7 9 0 per  month. 6. ing  Reduction  of o v e r w i n t e r e d p o p u l a t i o n s p r i o r t o the  season i n attempts  breed-  t o induce the p o p u l a t i o n s t o have h i g h e r  r e p r o d u c t i v e 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 m e n t a l p o p u l a t i o n s i n mean monthly body w e i g h t s ,  reproductive  p e r f o r m a n c e , or s u r v i v a l , t h e r e were d i f f e r e n c e s between the p e r i m e n t a l p o p u l a t i o n s i n 1964- and 1965.  Body w e i g h t s  ex-  and r e -  p r o d u c t i v e performance were l o w e r , and s u r v i v a l of the young from b i r t h t o age a t f i r s t c a p t u r e was h i g h e r i n 1965 1964-. est 7.  than i n  The p r o p o r t i o n of s u b a d u l t s t h a t became fecund was  great-  i n t h i s .area i n 1964-. The  e f f e c t s of e x t r i n s i c e n v i r o n m e n t a l  t i o n numbers were examined.  The  f a c t o r s on  popula-  e f f e c t s of d i s e a s e , p a r a s i t e s ,  and p r e d a t o r s d i d not seem t o be severe enough t o account f o r the r e g u l a t i o n of numbers.  Clement weather was  l o n g e r b r e e d i n g season observed did  a s s o c i a t e d w i t h the  i n 1964- but v a r i a t i o n s i n weather  not seem t o a f f e c t m o r t a l i t y .  The  f o o d s u p p l y was  assumed t o  66  be adequate a l t h o u g h  I t was not s t u d i e d ; e v i d e n c e from o t h e r  s t u d i e s i n d i c a t e s t h a t an u n l i m i t e d f o o d s u p p l y does not p r e v e n t 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 d e a t h - r a t e  important  r a t h e r t h a n I n b i r t h - r a t e were more  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 conveniently decreasing t h e o r e t i c a l grounds.  of numbers  by  the r e p r o d u c t i v e r a t e was r e j e c t e d on  C h r i s t i a n ' s i d e a t h a t changes i n r e p r o -  d u c t i v e r a t e are m a i n l y 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 n o t supported.  That changes i n p o p u l a t i o n numbers are  accounted f o r by changes i n m o r t a l i t y was supported  adequately by the d a t a .  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J a m e s o n , E.W.,Jr. 1950. D e t e r m i n i n g mammals. J.Mammal. 3 i 4-33-4-36.  f e c u n d i t y i n male  small  :  /L  Jameson,E.W., J r . 1953. R e p r o d u c t i o n o f d e e r m i c e ( P e r o m y s c u s m a n i c u l a t u s a n d P. b o y l e i ) i n t h e S i e r r a N e v a d a , C a l i f o r n i a . J . Mammal. 4-4~-58. K r e b s , C . J . 1964-. The l e m m i n g c y c l e a t B a k e r L a k e , N o r t h w e s t T e r r i t o r i e s , d u r i n g 1959-1962. A r c t i c I n s t . N.Amer.Tech.Pap. No.15. 104- p p . K r e b s , C . J . a n d D e L o n g , K.T. 1965. A M i c r o t u s s u p p l e m e n t a l f o o d . J . Mammal. 4^5: 56l>^~573^  population  L a c k , D. 1954-. The n a t u r a l r e g u l a t i o n o f a n i m a l U n i v e r s i t y P r e s s , London. L a c k , D. 1965. E v o l u t i o n a r y e c o l o g y .  numbers.  with Oxford  J . A n i m . E c o l . 3^+: 2 2 3 - 2 3 1 .  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The e f f e c t s o f b o t 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 o f t h e 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 . W y n n e - E d w a r d s , 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. P o p u l a t i o n c o n t r o l i n animals. Sci.Amer. 211: 68-74-.  s  APPENDIX A. Date female caught  9 May, 1964- . 17 J u n e , 19642 9 J u n e , 19642 J u l y , 19642 J u l y , 196418 J u l y , I 9 6 H 18 J u l y , 196418 J u l y , 196419 J u l y , 19641 9 J u l y , 196423 J u l y , 19642 Aug., 196'+ 3 Aug., 19.6M3 Aug., 19648 Aug. ,196M14- Aug., 196M16 Aug., 196416  A u g . ,196M-  11 Sept.,196^ 11 S e p t . , 196412 S e p t . , 1964-  P r e g n a n t u t e r i f r o m d e a d s a m p l e s , 196M-. Maternal weight (gm.) 14-. 0 19.0 15.0 19.0 21.5 20.0 21.5 22.0 22.0 20.5 17.0 20.0 17.0 21.0 18.0 19.0 21.0 20.5 17.0 15.0 15.5  No.corpora lutea R L 2 42 1 3 2 3 1 3 3 41 3 3 3 2 2 ?  4-  2 2 2 h 4 3 1 2 2 3 1 3 2 5 3 2 3 4 4-  2 2  No.embryos R  L  1*  3 1 2  M-  2 1 2 3 h 2 3 1 3 3 41 3 0 3 2+ 2 2 1  Uterine weight (mg.)  1+  43 1 3 2 3 1 3 1 5 3 2 4 4-  M1 4  R - right, L - left. * - a l l embryos dead. + - i n c l u d e s one dead embryo.  Avg. wt. p e r embryo (mg.)  708. 4  236.I  4-893.5 1657.5  1223.433L5  180.9 1250.5 6956.9 187.8  30.2 250.1 1391.437.6 24-.6 20.8  -  98A  .83.M-  107.1 5897.8  66.4H652.49528.8 1»M-3.+ 105.0 !  -  -  -'  21.4-  983.0 33.2 1665.3 1905.8 23.9 35.0 -  Approximate day o f pregnancy 13 Recent p a r t u r i t i o n 21 1*+ Recent p a r t u r i t i o n 5 13 22 7 KMk Recent p a r t u r i t i o n 4Recent p a r t u r i t i o n 19 k 2h  25 5 7 Recent p a r t u r i t i o n  APENDIX B.  Pregnant  Date female caught  u t e r i from dead samples,  Maternal weight (gm.)  1965.  No.corpora lutea R. L  No.embryos R.  L  Uterine weight (mg.) 1156.5  Av.g. wt. p e r embryo  Approximate day of pregnancy  25  June,1965  15.5  1  h  1  4-  25  June,1965  22.5  1  3  1*  3  11  July,1965  17.0  1  4-.  1  4-  7296.0  14-59.2  23  15  July,1965  18.0  1  5  1  5  287.0  '+7.8  8  19  July,1965  17.5  2  2  2  2  3586.6  896.6  20  July,1965  19.0  >+  2  5+  0  2495.8  624-.0  20  July,1965  19.0  3  1  3  1  118.9  29.7  6  21  July,1965  17.0  2  4  2  4  14-7.0  24.5  6  26  July,1965  21.0  0  5  0  5  116.0  27  July,1965  12.0  3  2  —»  2  '+0.2  13.4-  k  30  July,1965  17.5  6  0  6  0  597.1  99.5  11  18.0  2  2  2  2  -  -  4 Aug.,1965  -  .  R - right, L - left. * - a l l embryos dead. + - i n c l u d e s one. dead embryo.  231.3  -  23.2  13 Recent  parturition  16  5  Recent  parturition  APPENDIX C. Date female autopsied 5  Aug.,1965  Pregnant  u t e r i of  known age  from c o l o n y .  No.corpora lutea R L  No.embryos R  21.0  0  5  Maternal weight (gm.)  L  Uterine weight (mg.)  Avg. wt. p e r embryo (mg.)  4-*  5  2661.6  532.3  15  • : Known day o f pregnancy  18  Aug.,1965  22.0  4-  2  4-  2  772.7  123.8  12  19  Aug.,1965  19.5  3  4-  1  3+  636.8  212.3  13  25  Aug.,1965  16.5  4-  2  4-  2+  4-57.0  91.4-  11  26  Aug.,1965  19.0  5  2  5  2  504-9.8  721.4-  17  29  Aug.,1965  21.0  2  5  1  4-  231.0  4-6.2  8  21  Feb.,1966  18.0  2  3  2  3  364-. 6  72.9  10  9 Mar.,1966  19.0  2  3  2  3  214-.1  4-2.8  7  21  Mar.,1966  19.0  1  4-  1  4-  255.5  51.1  9  14- A p r . , 1 9 6 6  19.0  3  2  •3  2  4-801.3  960.3  18  R - right, L - left. * - a l l embryos dead. + - i n c l u d e s one d e a d embryo.  

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