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The lemming cycle at Baker Lake, N.W.T., during 1959-61 Krebs, Charles J. 1962

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THE LEMMING CYCLE AT BAKER LAKE, N.W.T., DURING 1959-61  by  CHARLES J . KREBS B.S.,  U n i v e r s i t y o f Minnesota, 1 ° 5 7  M.A., U n i v e r s i t y o f B r i t i s h Columbia,  195?  A T h e s i s Submitted i n P a r t i a l F u l f i l m e n t o f The Requirements f o r t h e Degree o f Doctor o f Philosophy In t h e Department of Zoology  We a c c e p t t h i s t h e s i s a s conforming t o t h e required  THE  standard  UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1962  In p r e s e n t i n g the  t h i s thesis i n p a r t i a l fulfilment of  requirements f o r an advanced degree a t t h e  British  Columbia, I agree t h a t the  a v a i l a b l e f o r reference  and  study.  University  of  L i b r a r y s h a l l make i t f r e e l y I f u r t h e r agree t h a t  permission  f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may g r a n t e d by the  Head o f my  It i s understood t h a t f i n a n c i a l gain  April  be a l l o w e d w i t h o u t my  Zoology  The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada. Date  1.  representatives.  copying or p u b l i c a t i o n of t h i s t h e s i s f o r  s h a l l not  Department o f  Department o r by h i s  be  1962  Columbia,  written  permission.  F A C U L T Y O F G R A D U A T E STUDIES  mi PROGRAMME OF T H E  FINAL ORAL E X A M I N A T I O N FOR T H E DEGREE OF  DOCTOR OF PHILOSOPHY  of CHARLES JOSEPH KREBS B.S., University of Minnesota, 1957 M.A., University of British Columbia, 1959 PUBLICATIONS Krebs, C. J. 1961 Population dynamics of the Mackenzie Delta reindeer herd, 1938-1958. Arctic 14 (2): 91-100.  IN ROOM 3332, BIOLOGICAL SCIENCES BUILDING THURSDAY, APRIL 19th, 1962 A T 3:00 P.M.  COMMITTEE  IN  CHARGE  Chairman: F. H . SOWARD J. F. BENDELL D. H . CHITTY I. McT. C O W A N H. B. H A W T H O R N W. S. HOAR  Z. A. MELZAK W . B. SCHOFIELD T. M . C. TAYLOR N . J. WILIMOVSKY A. J. WOOD  External Examiner: D. G. C H A P M A N University of Washington  T H E LEMMING C Y C L E A T BAKER L A K E , N.W.T., DURING 1959-61 A three year study covering one cycle in numbers of the brown lemming (Lemmus trimucrotiatus) and the varying lemming (Dicrostonyx groenlandicus) has been carried out at Baker Lake, Keewatin, N.W.T. An attempt was made to describe the events of the cycle in detail by snap trapping and live trapping techniques and by detailed autopsies on about 3 >00 animals, and from this to determine what explanations would fit the observations. Increase began from very low numbers in the summer of 1959 with tremendous population growth occurring over the winter of 1959-60. Little further increase occurred in the peak summer of I960. A great decline occurred over the winter of 1960-61, and this decline continued through the summer of 1961 in the Main Study Area. Two changes in reproduction were associated with this cycle— changes in the length of the breeding season and in the weight at sexual maturity. Winter breeding occurred only in the period of increase, and a shortened summer breeding season occurred in the peak and to some degree in the decline. Young male Lemmus matured in the summer of increase but not in the peak or in the decline; young females matured in the increase and decline summers but not in the peak. The major change in mortality was a very high juvenile mortality in the summer of the decline. Changes in the quality of the animals were manifested not only by these reproductive and mortality changes but also by a 20-30% increase in mean body weights of the adults in the peak summer compared to the increase or decline summers. The role of the extrinsic factors is considered. There was no widespread destruction of the habitat, or any evidence of qualitative or quantitative food shortage in the animals of the decline. Neither predators, disease, nor parasites seemed to be the cause of the observed changes in mortality. The role of the instrinsic factors is also considered. Summer adrenal and spleen weights showed no clear relationship to the cycle. The amount of wounding on skins showed strong seasonal and yearly changes and was not a simple function of density.  It was concluded from these observations that the lemming cycle could not be adequately explained by the conventional extrinsic factors such as food supply, but rather that it is essentially a self; regulatory phenomenon. The stress hypothesis proposed by Christian was also rejected as an adequate explanation of these events. The idea of Chitty that populations change in quality during changes in density was fully supported by these results. Behavioral changes in the population may constitute the crux of the lemming cycle, and Chitty's suggestion that these fluctuations may represent a genetic polymorphism involving aggressive behavior is rot refuted by these data.  G R A D U A T E STUDIES Field of Study: Zoology Population Dynamics  P. A. Larkin  Wildlife Management  L McT. Cowan  Economic Entomology Comparative Vertebrate Embryology Quantitative Methods in Zoology  K. Graham P. Ford P. A. Larkin  Other Studies: Animal Growth and Nutrition Plant Ecology Synoptic Oceanography Dynamic Oceanography Biogeography  A. J. Wood and J. Biely V. J. Krajina G. L. Pickard G. L. Pickard R. W. Pillsbury  ABSTRACT A t h r e e y e a r study c o v e r i n g one c y c l e i n numbers o f t h e brown and v a r y i n g lemmings has been c a r r i e d out a t Baker Lake, Keewatin, N.W.T. An attempt was made t o d e s c r i b e t h e events o f t h e c y c l e i n d e t a i l b y snap t r a p p i n g and l i v e t r a p p i n g t e c h n i q u e s  3U00  animals  and b y d e t a i l e d a u t o p s i e s on about  and f r o m t h i s t o determine what e x p l a n a t i o n s would f i t t h e  observations. Increase began f r o m v e r y low numbers i n t h e summer o f 1°59 tremendous p o p u l a t i o n growth o c c u r r i n g over the w i n t e r o f f u r t h e r i n c r e a s e o c c u r r e d i n the peak summer o f over t h e w i n t e r o f of  l°6l  l°60-6l,  i960.  1959-60.  with Little  A great d e c l i n e occurred  and t h i s d e c l i n e c o n t i n u e d through t h e summer  on t h e Main Study A r e a . Two changes i n r e p r o d u c t i o n were a s s o c i a t e d w i t h t h i s c y c l e -  changes i n t h e l e n g t h o f t h e b r e e d i n g maturity. shortened  season and i n t h e weight a t s e x u a l  W i n t e r b r e e d i n g o c c u r r e d o n l y i n t h e p e r i o d o f i n c r e a s e , and a summer b r e e d i n g  the d e c l i n e .  season o c c u r r e d i n t h e peak and t o some degree i n  Young male Lemmus matured i n t h e summer o f i n c r e a s e b u t n o t i n  the peak o r i n t h e d e c l i n e j young f e m a l e s matured i n t h e i n c r e a s e and d e c l i n e summers b u t n o t i n t h e peak. The  major change i n m o r t a l i t y was a v e r y h i g h j u v e n i l e m o r t a l i t y  i n t h e summer o f t h e d e c l i n e . Changes i n t h e q u a l i t y o f t h e animals were m a n i f e s t e d  not o n l y  by these r e p r o d u c t i v e and m o r t a l i t y changes but a l s o by a 20-30% i n c r e a s e i n mean body w e i g h t s o f t h e a d u l t s i n t h e peak summer compared t o t h e i n c r e a s e o r d e c l i n e summers. The  r o l e o f the e x t r i n s i c f a c t o r s i s considered.  w i d e s p r e a d d e s t r u c t i o n o f t h e h a b i t a t , o r any evidence q u a l i t a t i v e f o o d shortage  i n t h e animals  There was no  of quantitative or  of the d e c l i n e .  Neither predators,  d i s e a s e , n o r p a r a s i t e s seemed t o be t h e cause o f t h e observed mortality.  changes i n  ii The  r o l e of the i n t r i n s i c f a c t o r s i s a l s o considered.  a d r e n a l and s p l e e n w e i g h t s showed no c l e a r r e l a t i o n s h i p t o t h e The  Summer cycle.  amount o f wounding on s k i n s showed s t r o n g s e a s o n a l and y e a r l y changes  and was  not a simple f u n c t i o n o f d e n s i t y . I t was  c o n c l u d e d from t h e s e o b s e r v a t i o n s t h a t t h e lemming c y c l e  c o u l d not be a d e q u a t e l y e x p l a i n e d by t h e c o n v e n t i o n a l e x t r i n s i c f a c t o r s  such  as f o o d s u p p l y , but r a t h e r t h a t i t i s e s s e n t i a l l y a s e l f - r e g u l a t o r y phenomenon. The  s t r e s s h y p o t h e s i s proposed by C h r i s t i a n was  e x p l a n a t i o n of these events.  The  a l s o r e j e c t e d as an adequate  i d e a of C h i t t y t h a t p o p u l a t i o n s change i n  q u a l i t y d u r i n g changes i n d e n s i t y was  f u l l y supported by t h e s e  B e h a v i o r a l changes i n the p o p u l a t i o n may  results.  c o n s t i t u t e the c r u x o f the lemming  c y c l e , and C h i t t y ' s s u g g e s t i o n t h a t t h e s e f l u c t u a t i o n s may  represent a genetic  polymorphism i n v o l v i n g a g g r e s s i v e b e h a v i o r i s not r e f u t e d b y t h e s e  data.  iii TABLE OF CONTENTS  Page 1  INTRODUCTION DESCRIPTION  OF AREA AND CLIMATE  3  VEGETATION AND HABITATS  8  POPULATION DENSITY . . , „ * . . , . . . ,  Ik  Methods  ill  Live Trapping  ll+  Snap T r a p p i n g  17  Other Census Methods  19  Results  20  Live Trapping  20  Snap T r a p p i n g  . *  2i|  Other Census Methods  30  Summary and C o n c l u s i o n s  30  REPRODUCTION  33  Methods  33  Results  36  Length  o f B r e e d i n g Season  L i t t e r Size  . . . . . . . .  Embryo Rates Summary and C o n c l u s i o n s MORTALITY  h3 $1  Pregnancy Rates Age a t Reproductive  36  Maturity  57 6I4. 68 69  Methods  69  Results  69  iv Page Prenatal Mortality  69  Post-natal Mortality  73  Adults  73  Juveniles  83  Summary and C o n c l u s i o n s  . •  89 90  MOVEMENTS AND MIGRATIONS Methods  90,  Results  90  L o c a l Movements  90  Migrations  91  Summary and C o n c l u s i o n s  98  CHANGES IN EXTRINSIC FACTORS  100  Weather  100  Predators  101  Disease  and P a r a s i t e s  103  IOI4.  Food Summary and C o n c l u s i o n s  .  CHANGES IN INTRINSIC FACTORS Methods  UO I l l I l l  Age D e t e r m i n a t i o n  I l l  Mean Body Weights  112  Organ Weights  lilt  Results  116  Body Weight D i s t r i b u t i o n s  116  Mean Body Weights  125  Organ Weights  129  F a t Changes  137  V Page Social Relationships  138  Summary and C o n c l u s i o n s BISCUSSION  li|3 1U5  . . .  H i s t o r i c a l Approaches and Background Reproduction  Hi5 •  II48 152  Mortality Migrations  iSk  Weather and Synchrony  158  Predators  161  Disease  . . . . . . . .  and P a r a s i t e s  l6l  Body Weight Changes Three C u r r e n t Hypotheses  162 . . . . . . . . . . .  165  F o o d Supply h y p o t h e s i s  l6£  C h r i s t i a n ' s S t r e s s Hypothesis  167  C h i t t y ' s Hypothesis  170  Conclusions . * SUMMARY LITERATURE CITED  . . . . .  17U 175 177  vi FIGURES Page F i g u r e 1.  F i g u r e 2.  Map showing t h e l o c a t i o n o f Baker Lake, t h e Main Study A r e a , and t h e p e r i p h e r a l sampling areas  k  Location of the l i v e t r a p p i n g quadrats o f the Main Study A r e a  15  F i g u r e 3. • Summer h a b i t a t d i s t r i b u t i o n i n D i c r o s t o n y x  F i g u r e U.  F i g u r e 5«  and Le minus on t h e Main Study A r e a  29  G e n e r a l i z e d d e n s i t y changes 1959-61. Numbers i n d i c a t e r e l a t i v e changes i n numbers f o r each s p e c i e s based on l i v e trapping  31  Components o f r e p r o d u c t i o n i n p o l y e s t r o u s mammals  F i g u r e 6.  Figure  7.  G e n e r a l i z e d annual c h r o n o l o g y o f g e n e r a t i o n s and l i t t e r s f o r Lemmus and D i c r o s t o n y x  37  113  Body weight d i s t r i b u t i o n s f o r Lemmus males, J u l y  16-31, I960 and 1961.. .  . . . «B  F i g u r e 8.  P i t e l k a ' s food supply hypothesis  166  Figure  C h r i s t i a n ' s s t r e s s hypothesis  169  9»  F i g u r e 10. C h i t t y s h y p o t h e s i s 1  . 172:  vii  TABLES Pa Table 1.  Temperature and p r e c i p i t a t i o n data during l°5>9-6l r e l a t i v e to the mean values f o r 1950-60 as recorded by the Baker Lake Meteorological Station . . . . . . . . . .  5  Table 2.  Depth of snow on ground during e a r l y winter . . .  7  Table 3.  Area covered by the p r i n c i p a l habitat types on the Main Study Area  . 10  Table h»  Dominant plant species i n the habitats of the Main Study Area, New Lake, and Prince M v e r . . . 11  Table 5«  Dominant plant species i n the habitats of the Aberdeen Lake area  Table 6.  Table 7» Table 8.  Dominant plant species i n the moss habitats on the islands of Baker Lake and the south bank of the The Ion mouth  12  .13  Numbers of Lemmus on Quadrat # 1 during 1959-61  21  Numbers of Dicrostonyx on Quadrat # -3  i n 1960-61  22  Table 9.  lemmus snap trapping indices, 1959-61  25  Table 10.  Dicrostonyx snap trapping indices, 1959-61  Table 11.  Timing of summer breeding periods i n Lemmus females, 1959-61 Timing of summer breeding periods i n Dicrostonyx females, 1959-61  Table 12.  . . . 27  38 39  Table 13.  Length of the summer breeding seasons of Lemmus and Dicrostonyx, Main Study Area, 1959-61. UO  Table lli*  Winter and spring breeding of Lemmus and Dicrostonyx, 1958-61  Table 15.- Number of corpora lutea i n Lemmus females, Main Study Area, summers 1959-61 Table 16. Table 17.  Number of corpora lutea i n Dicrostonyx females, Main Study Area, summers 1959-61 . . . Number of embryos i n Lammus females, Main Study Area, summers 1959-61  \x2 h$ U6 i;7  viii Page T a b l e 18. Number o f embryos i n D i c r o s t o n y x Main Study A r e a , summers  females,  U8  1959-61  Table 19. Number o f c o r p o r a l u t e a and embryos i n Lemmus females a t Aberdeen Lake, summers Table 20.  Number o f c o r p o r a l u t e a and embryos i n D i c r o s t o n y x females a t Aberdeen Lake, summers  T a b l e 21.  g, Lemmus, summers  50  1959-61  52  Crude pregnancy r a t e s p e r 15 days p e r female  >30.5 T a b l e 23.  1960-61 . . . .  Crude pregnancy r a t e s p e r 16 days p e r female  >20.5 T a b l e 22.  1960-61 I4.9  g, D i c r o s t o n y x , summers  1959-61 . . . . . .  5U  Weight a t r e p r o d u c t i v e m a t u r i t y i n Lemmus males, summers  1959-61  58  Table 21;. Weight a t r e p r o d u c t i v e m a t u r i t y i n Lemmus females, summers T a b l e 25.  1959-61  . 59  Weight a t r e p r o d u c t i v e m a t u r i t y i n D i c r o s t o n y x males, summers  1959-61  60  T a b l e 26. Weight a t r e p r o d u c t i v e " m a t u r i t y i n D i c r o s t o n y x females,  summers  1959-61  61  T a b l e 27. Median body weights a t m a t u r i t y f o r Lemmus and D i c r o s t o n y x males and f e m a l e s ,  1959-61 . . . . . . 63  Table 28. Crude embryo r a t e s p e r 16 days p e r female  >20.5 T a b l e 29.  g, Lemmus,  1959-61  65  Crude embryo r a t e s p e r 15 days p e r female  66  >30.5 g, D i c r o s t o n y x , 1959-61 Table 30.  P a r t i a l p r e n a t a l m o r t a l i t y d a t a f o r Lemmus females,  T a b l e 31.  summers  Main Study A r e a . . . .  72 75  Minimum s u r v i v a l r a t e e s t i m a t e s f o r Lemmus Quadrat #1  .  .  .  .  76  Minimum s u r v i v a l r a t e e s t i m a t e s f o r M c r o s t o n y x , summer  T a b l e 35.  1959-61,  i960  summer 1961, T a b l e 3k*  71  Minimum s u r v i v a l r a t e e s t i m a t e s f o r Lammus summer  T a b l e 33.  Main Study A r e a . . . .  P a r t i a l p r e n a t a l m o r t a l i t y data f o r Dicrostonyx f e m a l e s , summers  T a b l e 32.  1959-61,  i960,  Quadrat #  2. . . .  77  Minimum s u r v i v a l r a t e e s t i m a t e s f o r D i c r o s t o n y x , summer  I96I,  Quadrat  #3  78  ix Page T a b l e 36.  Mnimum  s u r v i v a l r a t e s f o r lemmus c o n v e r t e d t o a  28 day base T a b l e 37.  79  Minimum s u r v i v a l r a t e s f o r D i c r o s t o n y x c o n v e r t e d t o 80  a 28 day base T a b l e 38.  S u r v i v a l e s t i m a t e s f o r j u v e n i l e Lemmus on Quadrat # summer  Table 39.  1,  I960  85  S u r v i v a l e s t i m a t e s f o r j u v e n i l e Lemmus on Quadrat #  1,  summer 1961 Table UO.  S u r v i v a l e s t i m a t e s f o r j u v e n i l e D i c r o s t o n y x on Quadrat  T a b l e I4.I.  #2,  i960  87  summer 1961  88  Length o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s of  T a b l e U3.  summer  S u r v i v a l e s t i m a t e s f o r j u v e n i l e D i c r o s t o n y x on Quadrat # 3 ,  T a b l e Jj.2.  86  l i v e t r a p p i n g f o r Lemmus, summer  i960,  I4J4..  T a b l e U5»  T a b l e 1+6.  # 2 . 91  Length o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e t r a p p i n g f o r Lemmus, summer 1961,  Table  Quadrat  Quadrat # 1 .  .  92  Length o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e t r a p p i n g f o r B i c r o s t o n y x , summer I960, Quadrats # 2 and # 3  93  Length o f every movement r e c o r d e d w i t h i n p e r i o d s o f L i v e t r a p p i n g f o r D i c r o s t o n y x , summer 1961, Quadrat # 3 and v i c i n i t y S t a n d i n g f o r a g e i n grams p e r 0.5  9k  s q . meter d r y weight 105  at t h e end o f summer Table lj.7«  Estimate o f percentage forage u t i l i z a t i o n  i n the spring  of t h e d e c l i n e , June 1961 T a b l e 1+8.  Body weight d i s t r i b u t i o n s f o r Lemmus males on t h e Main Study A r e a ,  T a b l e 1+9 •  1959-61  1960-61  121  1960-61  122  Body weight d i s t r i b u t i o n s f o r Lemmus males on the o u t l y i n g a r e a s , summer 1961  T a b l e 53.  118  Body weight d i s t r i b u t i o n s f o r D i c t o s t o n y x males a t Aberdeen Lake,  T a b l e 52.  1959-61  Body weight d i s t r i b u t i o n s f o r Lemmus males a t Aberdeen Lake,  T a b l e 51.  117  Body weight d i s t r i b u t i o n s f o r D i c r o s t o n y x males on t h e Main Study A r e a ,  Table 50.  108  . . . .  123  Mean body w e i g h t s f o r Lemmus males o f the w i n t e r and s p r i n g g e n e r a t i o n s , summers  1959-61 . ,  126  X Page T a b l e 5U»  Mean body weights  f o r D i c r o s t o n y x males of the  1959-61  w i n t e r and s p r i n g g e n e r a t i o n s j summers Table 55*  S t a n d a r d i z e d mean organ weights i n d e x f o r Lemmus males,  T a b l e 56. T a b l e 57.  .  131  S t a n d a r d i z e d mean organ w e i g h t s ( m i l l i g r a m s ) and f a t index f o r Lammus f e m a l e s , 1959-61 S t a n d a r d i z e d mean organ w e i g h t s  index f o r Dicrostonyx females,  . . . . .  . . . .  133  ( m i l l i g r a m s ) and f a t  1959-61  T a b l e 59•  Amount o f wounding shown on s k i n s of Lemmus males  T a b l e 60.  from the Main Study A r e a , 1959-61 Amount of wounding shown on s k i n s of D i c r o s t o n y x males from the Main Study A r e a ,  . 132  ( m i l l i g r a m s ) and f a t  1959-61  S t a n d a r d i z e d mean organ weights  128  ( m i l l i g r a m s ) and f a t  1959-61  i n d e x f o r D i c r o s t o n y x males, T a b l e 58.  . . . . .  1959-61  13k  U4O  Ihl  xi  AC MOWLEDGMENT S  I w i s h t o e x p r e s s my g r a t i t u d e t o S r . Dennis C h i t t y and D r . I a n MoT.  Cowan f o r t h e i r s t i m u l a t i o n and h e l p t h r o u g h o u t t h i s study.  I am  a l s o i n d e b t e d t o Mr. Andrew H. Macpherson and the Canadian W i l d l i f e S e r v i c e f o r i n f o r m a t i o n and a s s i s t a n c e i n the f i e l d work.  Support f o r  t h e f i e l d work was p r o v i d e d b y the A r c t i c I n s t i t u t e o f N o r t h America. I was supported and d u r i n g  during  1961-62  1959-61 b y t h e  N a t i o n a l Research C o u n c i l o f Canada,  b y the N a t i o n a l Science Foundation  a g r e a t presumption t o p u t ray name t o t h i s manuscript  o f the U.S.  It is  because many o f t h e  i d e a s were c o l l e c t e d f r o m d i s c u s s i o n s w i t h f r i e n d s and a s s o c i a t e s both a t the Bureau o f A n i m a l P o p u l a t i o n , O x f o r d , U.B.C. two  and a t t h e Department o f Zoology,  Thanks are a l s o due t o my w i f e who s e r v e d as a f i e l d  summers.  assistant f o r  F i n a l l y , I want t o express my thanks t o t h e people  Lake whose c o o p e r a t i o n has made t h i s study most p l e a s a n t .  o f Baker  INTRODUCTION  A l l a n i m a l p o p u l a t i o n s f l u c t u a t e i n numbers. f l u c t u a t i o n s are s m a l l , i n o t h e r s l a r g e .  In some t h e s e  I n some a g a i n t h e s e  a r e i r r e g u l a r , i n o t h e r s t h e y t e n d t o be r e g u l a r .  fluctuations  Some s m a l l mammals i n  p a r t i c u l a r show f l u c t u a t i o n s w h i c h are l a r g e i n magnitude and  relatively  r e g u l a r i n o c c u r r e n c e , and t h e s e f l u c t u a t i o n s are r e f e r r e d t o as  "cycles"•  Vie are concerned here w i t h a well-known example o f t h e s e f l u c t u a t i o n s  —  t h e lemming c y c l e o f the t u n d r a . Two  s p e c i e s o f lemmings i n h a b i t the c e n t r a l Canadian  arctic,  the brown lemming (Lemmus t r i m u c r o n a t u s ) and the v a r y i n g lemming (Dicrostonyx groenlandicus).  Both a r e s m a l l f u r r y r o d e n t s w i t h v e r y  s h o r t t a i l s and e a r s , and u s u a l l y weight  60-100  grams when f u l l y grown.  The brown lemming remains brown a l l y e a r round, but the v a r y i n g lemming i s w h i t e i n w i n t e r and g r e y i n summer. t h e y e a r , burrowing  Both s p e c i e s are a c t i v e  throughout  under t h e snow i n t h e w i n t e r and o c c u p y i n g burrows  dug i n the ground d u r i n g t h e summer.  T h e i r f o o d c o n s i s t s o f green p l a n t s  i n summer and dormant buds and r o o t s i n w i n t e r .  I n summer the v a r y i n g  lemming t e n d s t o occupy the d r i e r h a b i t a t s and t h e brown lemming t h e wetter s i t e s .  There  i s an a n n u a l o v e r t u r n o f p o p u l a t i o n , no  l i v i n g more t h a n one y e a r . and young a n i m a l s may  B r e e d i n g may  individuals  o c c u r a t any time o f t h e y e a r  mature a t 3-i| weeks o f age i n b o t h s p e c i e s .  g e s t a t i o n p e r i o d i s 19-21 between t h r e e and n i n e .  The  days and the l i t t e r s i z e v a r i e s s e a s o n a l l y Thus these s p e c i e s have a tremendous p o t e n t i a l  rate of i n c r e a s e . The  o b j e c t i v e o f t h i s r e s e a r c h program was  t o s t u d y the  p o p u l a t i o n dynamics o f the brown lemming and t h e v a r y i n g lemming over a f u l l c y c l e i n numbers a t Baker Lake, Keewatin,  N.W.T. i n t h e  Canadian  2 B a r r e n Grounds. events was  The  f i r s t purpose o f t h i s study was  o f t h e lemming c y c l e o f the B a r r e n Grounds.  to describe The  t o e x p l a i n t h e s e e v e n t s i n a comprehensive t h e o r y .  d i r e c t i n g my  approach i s t h i s :  c o n d i t i o n s f o r the occurrence ( d e s c r i p t i o n ) has now  what are t h e n e c e s s a r y o f a lemming c y c l e .  been r e a l i z e d f o r one  The  cycle.  The  the  second purpose The and  basic question sufficient  first  purpose  second purpose  ( e x p l a n a t i o n ) i s not y e t r e a l i z e d , but the r e s u l t s suggest which of t h e c u r r e n t e x p l a n a t i o n s are i n a d e q u a t e . not i n c o n s i s t e n t w i t h t h e observed  A h y p o t h e s i s i s c o n s i d e r e d which i s  events  and the i n f o r m a t i o n needed from  f u t u r e work i s n o t e d . The p l a n o f t h i s p a p e r i s as f o l l o w s . of  After a brief description  t h e study a r e a , the h a b i t a t s o c c u p i e d b y the two  s p e c i e s of lemmings  w i l l be d e s c r i b e d and t h e n p o p u l a t i o n d e n s i t y changes w i l l be  considered.  Then r e p r o d u c t i o n , m o r t a l i t y , and movements w i l l be a s s e s s e d i n r e l a t i o n to  t h e d e n s i t y changes.  Changes i n e x t r i n s i c and  a f f e c t the p o p u l a t i o n w i l l t h e n be c o n s i d e r e d .  intrinsic  f a c t o r s which  F i n a l l y , a d i s c u s s i o n of  t h e e n t i r e work w i l l be g i v e n w i t h an attempt t o i n t e g r a t e t h e s e w i t h contemporary i d e a s .  results  DESCRIPTION OF AREA AND CLIMATE  The  Baker Lake s e t t l e m e n t i s on t h e northwest  c o r n e r o f Baker  Lake near the mouth o f t h e T h e l o n R i v e r i n t h e e a s t c e n t r a l B a r r e n Grounds. The whole a r e a l i e s i n t h e Canadian  Shield.  The t e r r a i n i n g e n e r a l i s  f l a t t o g e n t l y r o l l i n g , m o s t l y c o v e r e d w i t h g l a c i a l d r i f t w i t h few o u t c r o p s o f bedrock 30$  showing.  of the t o t a l area.  Lakes d o t t h e l a n d s c a p e , occupying  Drainage  i s p o o r and even l i g h t  perhaps  summer r a i n s c a n  cause c o n s i d e r a b l e l o c a l f l o o d i n g . T h i s s t u d y was c a r r i e d o u t i n t h e a r e a shown i n F i g u r e 1. The Main Study A r e a o c c u p i e s about 3 s q . m i l e s j u s t n o r t h o f t h e Baker Lake s e t t l e m e n t .  A l l i n t e n s i v e work was c a r r i e d out on t h i s a r e a .  Other  a r e a s marked on t h e map are o u t l y i n g a r e a s sampled once o r t w i c e d u r i n g each summer.  I n a d d i t i o n some sampling was c a r r i e d out a t t h e Canadian  W i l d l i f e S e r v i c e camp on Aberdeen Lake (6U° 3 7 ' N, 99°  Uk  %  W),  about  113) m i l e s west o f Baker Lake. The weather f o r Baker Lake i s summarized i n T a b l e 1 f o r and t h e mean v a l u e s f o r t e n y e a r s ' r e c o r d s are g i v e n f o r  1959-61,  comparison.  Summer weather v a r i e d c o n s i d e r a b l y between t h e d i f f e r e n t y e a r s . The  summer o f 195°  rather dry. The  was c o l d and wet, w h i l e t h e summer o f I960 was warm and  The summer o f 1961 was i n t e r m e d i a t e , c o o l t o warm and a g a i n d r y . s p r i n g p h e n o l o g i e s o f the t h r e e y e a r s were q u i t e d i f f e r e n t .  S p r i n g I960 was t h e t a r l i e s t .  The s p r i n g e v e n t s o f 1959  began about  8-20  days b e h i n d those o f s p r i n g I960, and t h e s p r i n g events o f 1961 were 2-6 days b e h i n d those o f I960.  T h i s i s r e f l e c t e d i n t h e d a t e s a t which Baker Lake  was e n t i r e l y f r e e o f i c e s  31 J u l y 1959,  12 J u l y I960, and 17 J u l y 1961.  There were c o r r e s p o n d i n g d i f f e r e n c e s i n t h e time o f onset o f summer b r e e d i n g in  lemmings.  4  FIGURE 1.  Map  showing t h e l o c a t i o n o f Baker Lake, the Main Study A r e a ,  and t h e p e r i p h e r a l  sampling a r e a s .  TABLE  1.  Temperature and p r e c i p i t a t i o n d a t a d u r i n g  1959-61  and the mean v a l u e s f o r  1950-60  as r e c o r d e d b y t h e Baker Lake M e t e o r o l o g i c a l S t a t i o n . JAN.  FEB.  MAR.  APR.  MAY  JUNE  JULY  AUG.  SEPT.  OCT.  NOV.  DEC.  -18  -2 +1 +2 +2  15  35 46 Ul 39  50 5u 53 52  46 52 46 50  38 38  14  19  -4 -7  -6  38  18  -4  1.78 1.28 0.2U 1.55  2.71 1.11 1.98 1.72  1.07 1.76  1.60  Tr  Tr Tr  1.16  0,20  Tr  -  1.0 0.8  8.7 7.3  2.1 1.5  7.6 1.7  0.9  3.8  3.6  2.9  "WHOLE  YR  Mean Monthly Temperature (°F.)  -22  1959  I960 1961 Mean  1950-60  -26 -31 -28  -28  -25 -21 -27  -22 -25 -15  -  -  0.8 1.0 4.4 1.8  l.U 2.3 3.1 2.3  28  16 22  4.14  +9.8 +12.0  -19 +10.7  T o t a l R a i n f a l l (in.)  1959 I960 1961 Mean  1950-60  Total  -  Tr  0.89 0.I4 0.37 0.22 0.73 Tr* Tr Tr  6.45 5.89 5.58  Snowfall (in.)  1959 I960 1961 Mean  -  1950-60  # Tr » trace  3.2 1.6 0.6 1.8  1.5 4.9 4.2 3.6  6.1 0.8 1.0 1.8  Tr _  1.0 0.6  -  -  Tr Tr  30.0 5925.6 ;5o23.1  6 E v e n t s o v e r the autumn f r e e z e - u p a r e a l s o o f c o n s i d e r a b l e importance f o r lemmings.  The 1959  f r e e z e - u p was c h a r a c t e r i z e d b y a  l a c k o f f r e e z i n g r a i n and a q u i c k b u i l d u p o f a p r o t e c t i v e snow c o v e r , t h u s n n n i m i z i n g ground i c i n g and burrow f l o o d i n g .  By c o n t r a s t t h e  I960  f r e e z e - u p was accompanied by v e r y wet c o n d i t i o n s , o s c i l l a t i n g  freezing-  thawing, and a l a c k o f a good snow cover u n t i l mid-December.  This contrast  between  1959 and i960 To  i s brought out i n T a b l e  2.  sum up, t h e summer o f 1959 was c o l d and wet b u t was f o l l o w e d  b y a q u i c k , d r y f r e e z e - u p and an i d e a l w i n t e r .  The summer o f  i960 was  warm and d r y b u t was f o l l o w e d b y a slow, wet f r e e z e - u p and a bad e a r l y winter.  The summer o f 1961 was warm and d r y a l s o .  7  TABLE 2 . Depth o f snow on ground d u r i n g e a r l y w i n t e r . Depth o f Snow Oct. 1 9 5 9 - 6 0  1  0  Oct. 1 2  1 9 6 0 -  61  Tr#  2  Mean  1 9 5 0 - 6 0  0  1  *  Trace  Nov.  5  3  (inches) 1  Nov.  1 5  Dec.  9  9  1 1  *  2:  3»  5  7  1  TEGETATION AND  HABITATS  Not much i s known about p l a n t a s s o c i a t i o n s i n the Arctic. in  Nevertheless,  t h i s study.  Canadian  some scheme o f h a b i t a t c l a s s i f i c a t i o n was  What f o l l o w s i s an attempt t o d i v i d e the v e g e t a t i o n  the Baker Lake a r e a i n t o h a b i t a t s which i n p r i n c i p l e might be the e n t i r e Barren One  of  applied to  Grounds.  o f t h e most s t r i k i n g c h a r a c t e r i s t i c s o f a r c t i c  i s i t s extreme v a r i a b i l i t y from one  small area t o the next.  vegetation T h i s produces  a c o r r e s p o n d i n g l y g r e a t i n t e r s p e r s i o n o f h a b i t a t s and g i v e s t h e o f one  needed  impression  g r e a t continuum o f v e g e t a t i o n r a t h e r t h a n d i s t i n c t h a b i t a t s .  Nevertheless,  t h e r e are d i s t i n c t h a b i t a t s which can be r e c o g n i z e d  even  though t r a n s i t i o n s are v e r y common. Three c r i t e r i a were used t o d i s t i n g u i s h h a b i t a t s . approximation, can r e c o g n i z e  As a  first  t h e d o m i n a t i n g i n f l u e n c e seems t o be water, and thus a v e g e t a t i o n continuum from d r y t o wet.  we  From t h i s p e r s p e c t i v e  the l i c h e n s o c c u r i n d r y a r e a s , the heath ( E r i c a c e a e ) p l a n t s i n m o d e r a t e l y d r y a r e a s , and the sedges and mosses i n wet  areas.  I have i n t r o d u c e d  f u r t h e r f a c t o r i n t o the h a b i t a t c l a s s i f i c a t i o n , t h a t of relief.  and l e s s t h a n one areas.  ( t h i c k clumps o f Eriophorum, u s u a l l y about 10"  foot i n diameter).  This microtopographical The type  Both t h e s e  recorded.  and microtopography.  a l s o made f o r most s t a t i o n s .  All  done s u b j e c t i v e l y by l o o k i n g at A subjective appraisal of  o r t h r e e dominant s p e c i e s w i t h i n a f i v e f o o t r a d i u s of t h e  s t a t i o n was  tall  f o r lemmings.  o f h a b i t a t a t each t r a p p i n g s t a t i o n was  the vegetation, drainage,  in  s t r u c t u r e s occur i n wetter  r e l i e f i s important  h a b i t a t c l a s s i f i c a t i o n i n t h i s study was  the two  microtopographical  T h i s i n v o l v e s hummocks (low rounded mounds,' 10-18" t a l l , 1-2'  d i a m e t e r ) and t u s s o c k s  a  There was  trapping  not enough time t o  do  9 a n y t h i n g more o b j e c t i v e . The p r i n c i p a l h a b i t a t s a r e l i s t e d i n T a b l e 3 which a l s o g i v e s t h e a r e a c o v e r e d b y each on t h e Main Study A r e a . obtained from a combination The  These f i g u r e s were  a e r i a l photo and f o o t s u r v e y o f t h e a r e a .  h a b i t a t s found on t h e Main Study A r e a are s i m i l a r t o those  a t New Lake, P r i n c e R i v e r , and Aberdeen Lake.  The dominant p l a n t s p e c i e s  found i n t h e s e h a b i t a t s are s i m i l a r on t h e f i r s t t h r e e o f t h e s e f o u r a r e a s , and T a b l e k g i v e s t h e s e d a t a . d i f f e r e n t s p e c i e s dominating i n T a b l e 5. and  The Aberdeen Lake a r e a shows somewhat t h e same h a b i t a t s , and t h e s e d a t a a r e g i v e n  The c h i e f d i f f e r e n c e i s t h a t t h e r e a r e no D r y a s i n t e g r i f o l i a  Cassiope t e t r a g o n a a t Aberdeen Lake, b u t Ledum groenlandicum  and  E r i o p h o r u m a r e more abundant t h e r e . A s l i g h t l y d i f f e r e n t s e r i e s o f h a b i t a t s o c c u r s on t h e i s l a n d s a t the west end o f Baker Lake and on t h e s a n d p l a i n a l o n g t h e south bank o f the T h e l o n R i v e r mouth.  These h a b i t a t s a r e c h a r a c t e r i z e d b y a dominance o f  mosses, as shown i n T a b l e 6. I n summary, because t h e r e i s no s a t i s f a c t o r y b o t a n i c a l c l a s s i f i c a t i o n o f t u n d r a p l a n t a s s o c i a t i o n s 11 h a b i t a t t y p e s have been e s t a b l i s h e d t o d e s c r i b e t h e v e g e t a t i o n o f the Baker Lake a r e a .  The c r i t e r i a used t o  d i s t i n g u i s h h a b i t a t s were d r a i n a g e , p l a n t s and m i c r o t o p o g r a p h i c a l  relief.  The p l a n t s p e c i e s found i n these h a b i t a t s v a r y s l i g h t l y from a r e a t o a r e a .  10  TABLE 3 .  A r e a c o v e r e d b y t h e p r i n c i p a l h a b i t a t t y p e s on the Main  Study  Area*  HABITAT TYPE  NUMBER OF ACRES  ROCK and ROCK-LICHEN LICHEN  29k  15.0  53  2.7  7U2  LICHEN HEATH  5  HEATH and HEATH HUMMOCK HEATH SEDGE and HEATH SEDGE HUMMOCK %  % OF TOTAL LAND SURFACE  38.0  0.2  506  25.9  SEDGE TUSSOCK  3  0.1  SEDGE HUMMOCK  290  Hi.?  67  3.3  SEDGE MARSH  T o t a l area cover typed (exclusive o f lakes)  1958  100.1  (3.05 s q . m i l e s )  * Heath-sedge hummock c o n t r i b u t e s about 30% o f t h e s e f i g u r e s , sedge t h e r e m a i n i n g %10  m  heath  11 TABLE I*.  Dominant p l a n t s p e c i e s i n t h e h a b i t a t s o f t h e Main Study A r e a ,  New Lake, and P r i n c e R i v e r .  Figures i n the table r e f e r t o the frequency  w i t h w h i c h t h e p l a n t s p e c i e s was r e c o r d e d as dominant a t t r a p p i n g s t a t i o n s i n the given h a b i t a t .  HEATH  HEATH HUM.  HEATH SEDGE  HEATH SEDGE RTJMM.  18.2$  31.5$  2iul#  9.1  143.1  32.5  81.8  26.2  U8.0  100.0  8.5  26.8  3.8  -  -  0.7  1.6  2.6  2.1  SPECIES  LICHEN HEATH  Lichens  91.5% *  Dryas  integrifolia  28.8  -  glandulosa  28.8  66.7  26.3  72.2  23.8  27.8  U5.5  Ledum groenlandicum  21.3  5.6  9.1  Empetrum nigrum  25.0  38.9  27.3  Arctostaphylos rubra  18.8  Betula  Vaccinium Cassiope  uliginosum tetragone  Rhododendron lapponicum Rubus chamaemorus S a l i x spp. Eriophorum spp. Carex s p p . Juncus s p p . Mosses T o t a l Number o f Stations  -  5.6  -  mm  mm  -  -  2.5  5.6  9.1  2.5  -  mm  -  23.8  80  61.1  18  11  0.8 2.3  -  SEDGE SEDGE TUSSOCK HUM.  7.9$ <  2.6  kl.k  7.9  1.6  mm  -  2.6  0.8  -  2.6  -  -  8.5  U.9  7.0$  20.U 2U.6 6.3  1.U  -  2.6  SEDGE MARSH  mm  1.0  -  -  mm  -  -  5.3  U.2  1.9  U.6  18.7  100.0  U3.0  61.0  86.9  77.2  26.3  2M  9U.3  3U.6  15. h  28.2  50.5  U7.7  U8.8  38.7  30.5  330  123  -  U7.U  38  Hi2  # The two most common dominants f o r each h a b i t a t are u n d e r l i n e d .  105  TABIE 5 * area.  Dominant p l a n t s p e c i e s i n t h e h a b i t a t s o f t h e Aberdeen Lake  F i g u r e s i n t h e t a b l e r e f e r t o t h e f r e q u e n c y w i t h which t h e p l a n t  s p e c i e s was r e c o r d e d  a s dominant a t t r a p p i n g  SPECIES  LICHEN HEATH  Lichens  96.9% 1 0 0 . 0  Ledum groenlandicum Betula  Vaccinium v i t i s - i d e a  I|3.8  Empetrum nigrum  3 . 1 6 . 3  Rubus charaaemorus  3 . 1  E r i o p h o r u m spp. Carex spp.  6 . 3  Mosses Grasses T o t a l Number o f Stations  HE. TH-SEDGE and SEDGE TUSSOCK H. S. HUMMOCK *  2$.7%  11.855  91.it  58.8  5.7  glandolusa  1  2  3  .  2  5  s t a t i o n s i n the given  28.6  -  SEDGE HUMMOCK  9.1$ 72.7  U.6  habitat.  SEDGE MARSH  mm  mn  -  -  9.1  -  mm  1U.3  17.6  U5.7  9U.1  68.2  75.0  31.U  Ul.2  59.1  95.8  UO.O  U7.1  63.6  70.8  ll.U 35  mm  17  9.1 22  12.5 2k  * The two most common dominants f o r each h a b i t a t t y p e a r e u n d e r l i n e d .  15 TABLE 6.  Bominant p l a n t s p e c i e s i n t h e moss h a b i t a t s on t h e i s l a n d s o f  Baker Lake and t h e south bank o f t h e T h e l o n mouth.  Figures i n the table  r e f e r t o t h e f r e q u e n c y w i t h which t h e p l a n t s p e c i e s was r e c o r d e d a s dominant a t t r a p p i n g s t a t i o n s i n t h e g i v e n h a b i t a t .  LICHEN HEATH  SPECIES  integrifolia  Vaccinium  uliginosum  9-5 9.5  58.8 5.9  Vaccinium v i t i s - i d e a  52.U  -  Betula  23.8  11.8  . 19.0  26.5  U2.9  5.9  glandulosa  Empetrum nigrum Ledum groenlandicum S a l x x spp. Car ex spp. Mosses Grasses Other s p e c i e s T o t a l Number o f S t a t i o n s  -  Hi.3  9.5  -  21  MOSS  -  100.0% #  Lichens Dryas  MOSS HEATH  27.3  U.5 U.5 27.3 U.5  MOSS SEDGE  -  2.0  -  11.8  -  -  2.0  Hi.7  59.1  6U.7  2.9  31.8  82 .U  22i2  100.0  96.1  8.8  13.6  29.U  2.9 3h  U.5 22  2.0 51  # The two most common dominants f o r each h a b i t a t t y p e a r e u n d e r l i n e d .  POPULATION DENSITY  The accurate  first  requirement o f a l l p o p u l a t i o n work must be  i n f o r m a t i o n about changes i n d e n s i t y .  p o p u l a t i o n d e n s i t y I have used one  moderately  To determine t r e n d s i n  census method, l i v e t r a p p i n g ,  and  f o u r o t h e r methods, snap t r a p p i n g , v i s u a l e s t i m a t e s , t r a c e i n d i c e s f o r f e c e s , and d r o p p i n g  boards.  Of t h e s e f i v e methods o n l y l i v e  p r o v i d e s a d i r e c t census of numbers.  The  trapping  o t h e r f o u r merely g i v e an  index  more o r l e s s p r o p o r t i o n a l t o a c t u a l d e n s i t y . METHODS Live  Trapping Live trapping i s the best technique  because i t g i v e s a d i r e c t count.  f o r estimating  density  A c o n s i d e r a b l e amount o f e f f o r t  expended i n a l i v e t r a p p i n g program i n t h i s study, but v a r i o u s plagued the r e s u l t s .  The  not  s o l v e d u n t i l l ? 6 l and  was  not f i x e d u n t i l 1961  less  was  difficulties  most s e r i o u s p r o b l e m was  trap-deathsj t h i s  even t h e n n o t e n t i r e l y .  The b a s i c  and  consequently  t h e d a t a f o r 1959  was  technique and I960 are  complete. Three q u a d r a t s f o r l i v e t r a p p i n g were e s t a b l i s h e d d u r i n g  course  o f t h i s study  ( F i g u r e 2).  Area (acres) Quadrat #1  the  T h e i r s p e c i f i c a t i o n s are as f o l l o w s :  Length o f one side ( f e e t )  Trap s p a c i n g (feet)  No.of t r a p s  1.9  288  18  289  Quadrat # 2  31.3  700  50  225  Quadrat # 3  3.5  350  $0  6k  Quadrat # 1 was  s e t up i n 1959.  Quadrat # 2 was  e s t a b l i s h e d i n I960 when  i t became apparent t h a t movements and home ranges were f a r l a r g e r t h a n c o u l d be measured by Quadrat #  1.  Quadrat #  3  was  a l s o s e t up i n  i960  15  FIGURE 2 .  L o c a t i o n o f t h e l i v e t r a p p i n g q u a d r a t s o f t h e Main Study A r e a .  A b b r e v i a t i o n s r e f e r t o h a b i t a t t y p e s ( r » r o c k , lb. » l i c h e n h e a t h , hh =» h e a t h hummock, h s =» h e a t h sedge and h e a t h sedge hummock, sh «* sedge hummock and sm = sedge  marsh).  17 weight.  The c o h o r t o f a d u l t animals p r e s e n t a t the b e g i n n i n g o f t h e  summer b r e e d i n g are r e f e r r e d t o as the w i n t e r g e n e r a t i o n . breeding tends t o occur synchronously  throughout  young appear i n waves d u r i n g t h e summer. r e f e r r e d t o as f o l l o w s :  summer  t h e p o p u l a t i o n , summer  These summer l i t t e r s are  Y^ summer young = f i r s t  young = second summer l i t t e r j  Since  summer L i t t e r j Y^  and Y-j* summer young = t h i r d summer  1  summer  litter.  These groups o f young are r e a d i l y s e p a r a t e d b y body weights u n t i l l a t e • summer when growth r a t e s d e c l i n e . is  A f u l l d i s c u s s i o n o f a g i n g problems  g i v e n i n a l a t e r s e c t i o n (see Body W e i g h t s ) .  Snap  Trapping Snap t r a p p i n g was done both s y s t e m a t i c a l l y and n o n - s y s t e m a t i c a l l y .  The  s y s t e m a t i c l i n e s were s e t up as Type B l i n e s o f t h e North  Census o f S m a l l Mammals (Calhoun,  19U8),  American  i . e . 20 s t a t i o n s spaced  50*  at  in  a s t r a i g h t l i n e with three traps p e r s t a t i o n w i t h i n a f i v e f o o t r a d i u s  of  the stake.  100'  W i t h a few e x c e p t i o n s t h e s e l i n e s were s e t up i n p a i r s  a p a r t (Calhoun,  19U8,  recommends ij.00 ) and p a r a l l e l . 1  l i n e s were s e t o u t on t h e Main Study A r e a i n 1959*  spaced  Eight p a i r s of  No new l i n e s were  added t h e r e i n I960, b u t f o u r a d d i t i o n a l p a i r s were s e t out i n 1961.  On  the o u t l y i n g a r e a s two p a i r s o f l i n e s were s e t o u t i n 1959, two a d d i t i o n a l p a i r s i n I960, and f i v e and o n e - h a l f a d d i t i o n a l p a i r s i n 1961.  In general  on the Main Study A r e a each l i n e was t r a p p e d t w i c e d u r i n g t h e summer and A u g u s t ) .  These l i n e s a r e r e f e r r e d t o as standard l i n e s because  are r e t r a p p e d each y e a r .  (June they  Each s i n g l e s t a n d a r d l i n e r u n f o r one p e r i o d  ( t h r e e days) r e p r e s e n t s 180 s t a n d a r d t r a p n i g h t s and a l l snap t r a p p i n g i n d i c e s i n t h i s paper are expressed  i n numbers o f lemmings p e r 100 s t a n d a r d  trap nights. Non-systematic snap t r a p p i n g was done p r i m a r i l y t o o b t a i n specimens f o r autopsy.  S t a t i o n s were p l a c e d a t i r r e g u l a r i n t e r v a l s wherever  18 t h e r e seemed t o be any chance o f c a t c h i n g a lemming. p l a c e d around each s t a t i o n .  Two t r a p s were  The t r a p s were removed a f t e r t h r e e days  and t h e same p l a c e was never t r a p p e d t w i c e .  These l i n e s caught two t o  t h r e e t i m e s as many lemmings p e r t r a p n i g h t as t h e - s t a n d a r d Museum S p e c i a l t r a p s were used throughout r a i s i n s were used a s b a i t . was  Although  used on a l l snap t r a p s throughout  lines.  t h i s study and  i t was p r o b a b l y unnecessary,  bait  t h i s s t u d y u n t i l t h e matter c o u l d  be i n v e s t i g a t e d . Some d i f f e r e n c e between t h e two s p e c i e s f o r standard sampling  s h o u l d be p o i n t e d o u t .  Lemmus i s a c r e a t u r e o f the wet h a b i t a t s  and uses w e l l d e f i n e d runways i n moving about. difficult  snap t r a p  I t s occupied holes are  t o f i n d because t h e r e a r e so many p o s s i b l e s i t e s .  Dicrostonyx,  on t h e o t h e r hand, i s more o f t e n a c r e a t u r e o f the d r i e r h a b i t a t s and does not move a l o n g w e l l d e f i n e d runways.  I t s o c c u p i e d h o l e s are o f t e n e a s y  t o f i n d and are marked b y a mound o f f r e s h l y dug sand o r p e a t .  Thus runway  t r a p p i n g i s most e f f e c t i v e f o r c a t c h i n g Lemmus, and burrow o r den t r a p p i n g i s most e f f e c t i v e  f o r catching Dicrostonyx.  The d i f f e r e n t h a b i t a t s o f t h e  two  species introduce a f u r t h e r complication.  wet  (Lemmus) h a b i t a t s a r e r e a s o n a b l y e x t e n s i v e and t h e d i s t r i b u t i o n o f  Lemmus i s f a i r l y u n i f o r m over t h e s e .  I n t h e Baker Lake a r e a t h e  But, a l t h o u g h t h e D i c r o s t o n y x  h a b i t a t s a r e e q u a l l y o r even more e x t e n s i v e , a r e a s s u i t a b l e f o r d i g g i n g burrows are r e s t r i c t e d and t h i s r e s u l t s i n a c o n t a g i o u s type o f d i s t r i b u t i o n f o r Dicrostonyx.  T h i s c o m p l i c a t e s sampling  t r a p l i n e s may c o m p l e t e l y miss t h e s e of the a c t u a l d e n s i t y .  c o n s i d e r a b l y , because  standard  " c o l o n i e s " and thus g i v e a b i a s e d i d e a  The r e s u l t i s t h a t Lemmus d e n s i t i e s a r e e s t i m a t e d  b e t t e r by s t a n d a r d snap t r a p p i n g l i n e s t h a n are D i c r o s t o n y x d e n s i t i e s i n the Baker Lake a r e a . Two q u e s t i o n s about t h e snap t r a p p i n g t e c h n i q u e  s h o u l d be c o n s i d e r e d  19 at t h i s p o i n t . density?  F i r s t , does snap t r a p p i n g p r o v i d e a good i n d e x of p o p u l a t i o n  The number o f animals caught  i n a t r a p l i n e depends not o n l y on  p o p u l a t i o n d e n s i t y but a l s o on t h e weather, h a b i t a t , amount o f home range s i z e , and p r o p o r t i o n of young a n i m a l s .  activity,  For t h i s reason  i n d i c e s from snap t r a p p i n g must be t r e a t e d w i t h c a u t i o n .  numerical  Trends i n d e n s i t y  are shown q u i t e w e l l , but the a c t u a l n u m e r i c a l v a l u e o f t h e s e t r e n d s must remain vague.  In p a r t i c u l a r ,  are meaningless  s t a t i s t i c a l confidence l i m i t s f o r these  indices  u n l e s s the e f f e c t s o f a l l the secondary v a r i a b l e s mentioned  above can be n e g l e c t e d .  I n t h i s study t h e r e are independent  sources o f  d e n s i t y e s t i m a t e s , such as l i v e t r a p p i n g , and these can be compared t o the snap t r a p p i n g i n d i c e s t o see how The  w e l l these d i f f e r e n t estimates  agree.  second q u e s t i o n i s whether r e p e a t e d t r a p p i n g of t h e same  l i n e s b o t h w i t h i n and between y e a r s has i n t e r f e r e d w i t h the c y c l i c we  are a t t e m p t i n g t o o b s e r v e .  s i n c e new  changes  T h i s q u e s t i o n can be answered i n d i r e c t l y ,  s t a n d a r d l i n e s were s e t out e v e r y y e a r .  any c a t c h d i f f e r e n c e s o c c u r r e d between new  We  can e n q u i r e whether  l i n e s i n y e a r x and o l d l i n e s  i n the same y e a r , t a k i n g i n t o account h a b i t a t d i f f e r e n c e s between l i n e s . Comparing t h e numbers caught i n new  and o l d l i n e s , we  i n c a t c h e i t h e r i n I960 o r i n 1 9 6 l , and  Other i n d i r e c t evidence s u p p o r t s t h i s .  o f f w i t h o u t a c a t c h i s almost  differences  so I conclude t h a t snap t r a p p i n g  t h e same l i n e s does not a f f e c t t h e c y c l i c changes we observe.  found no  The  are a t t e m p t i n g  to  number o f t r a p s s e t  always e q u a l t o o r g r e a t e r than the number o f  lemmings caught, w h i c h suggests t h a t a f a i r number o f lemmings escape b e i n g t r a p p e d even i n the immediate v i c i n i t y o f the t r a p l i n e . we  In addition, i f  c o n s i d e r the s i z e o f the a r e a o v e r which t h e t r a p l i n e s are spread  and  t h e m o b i l i t y o f the lemmings, i t i s c l e a r t h a t o n l y a minute f r a c t i o n o f the p o p u l a t i o n i s b e i n g removed by t r a p p i n g . Other Census Methods  20 V i s u a l e s t i m a t e s o f d e n s i t y changes were o b t a i n e d by c o u n t i n g the number o f lemmings seen p e r . h o u r o f w a l k i n g on t h e t u n d r a .  This i s  o b v i o u s l y a crude i n d e x o f d e n s i t y b u t i t does p r o v i d e v a l u a b l e supplementary i n f o r m a t i o n f o r a r e a s where no l i v e t r a p p i n g was done. Trace i n d i c e s o f f r e s h f e c e s were made i n 1°5°  and 1°60 b y  d o i n g l i n e t r a n s e c t s through h a b i t a t t y p e s , d r o p p i n g a 3' by l e v e r y t e n f e e t , and r e c o r d i n g presence A g a i n t h i s i s a crude  1  rectangle  o r absence o f f r e s h green  droppings.  i n d e x but i t has the advantage o f b e i n g v e r y q u i c k l y  done. F i n a l l y , d r o p p i n g boards were used as suggested b y Emlen e t a l .  (1957).  T h i s t e c h n i q u e was used i n 1959 and  I960  but discontinued i n  1961 because i t i n v o l v e d a c o n s i d e r a b l e amount o f work and merely d u p l i c a t e d o t h e r census i n f o r m a t i o n . RESULTS Live  Trapping Table 7 g i v e s t h e numbers o f Lemmus on Quadrat # 1 (1.9  i n 1959-61, and Table  (3.5  8  acres)  g i v e s t h e numbers o f D i c r o s t o n y x on Quadrat #  3  a c r e s ) i n 1959-61. W h i l e t h e r e were few o r no D i c r o s t o n y x on Quadrat  # 1 i n any o f t h e y e a r s , Quadrat # 3 had a lemmus p o p u l a t i o n o f 20 i n August  I960,  b u t none i n e i t h e r  animals  1959 o r 1961.  The f i r s t p o i n t t h a t emerges from these t a b l e s i s t h e g r e a t i n c r e a s e i n numbers from  1959  to  i960  and subsequent d e c l i n e i n  We c a n e s t i m a t e these changes q u a n t i t a t i v e l y . August  1959  t o June  i960  I n Lemmus the i n c r e a s e from  i s 28-fold, and i f we a c c e p t t h e argument from  snap t r a p p i n g g i v e n below, t h a t t h e p o p u l a t i o n b e f o r e the was  approximately  1961.  e q u a l t o t h e August  58-fold i n c r e a s e over the w i n t e r o f  i960  melt-off  p o p u l a t i o n , we have an e s t i m a t e d  1959-60.  a crude e s t i m a t e o f the a c t u a l i n c r e a s e .  I960  T h i s must be c o n s i d e r e d as o n l y  There was p r o b a b l y a minimal  21  Numbers o f Lemmus on Quadrat # 1 d u r i n g 1959-61."  TABLE 7.  DATE OF SAMPLING  1959  WINTER GENERATION  SUMMER GENERATION*  V  Y '»  1  5-10  August  TOTAL ANIMALS  0  1  August 11-23 I960 28'  June 18-20 July  6-8  J u l y 28-30  25-27  August  26  22'  8  30  12*  25  ia  8  16  1  2  20  J  1U  J  2  1961 June 12-18 June  58  3  3  19-25  5  June 2 6 - J u l y 2  5  J u l y 3-9  3  July  10-16  2  J u l y 17-23  2  2  J u l y 2U-30  2  2  1  3  1  h  J u l y 31-Aug. 6  -  August 7-13  -  August ll|-20  -  August 21-27  -  0  -  1  1  0  -  August 28-Sept. 1  -  1  Superscripts i n the table give trap m o r t a l i t i e s . 2 Y-j_ = f i r s t summer l i t t e r , Y-^' second summer l i t t e r , m  Y^  B  = third  summer  litter.  0  1  22  TABLE 8,  Numbers o f D i c r o s t o n y x on Quadrat # 3 i n 1960-61.  1959 July  2U-Aug. 1  August I960  6-10  25-27  August  .2 SUMMER GENERATION  WINTER GENERATION  DATE OF SAMPLING  V  y n 1  TOTAL ANIMALS  (3)  (3)  (6)  (1)  (2)  (3)  10  13  1  38  11  1961 June  5-11  June  12-18  June  19-25  1  1  9  9  11  June 2 6 - J u l y 2  $  *  3-9  1  1  8  8  July July  10-16  5  J u l y 17-23  5  J u l y 2U-30  k  J u l y 31-Aug. 6  1  h  August  2  1  7-13  3 1  August lU-20 August  2  21-27  0  August 28-Sept. 1 Superscripts ^Y^ = f i r s t  summer l i t t e r , Y^  Y]_" = t h i r d The 1959  i n the table give trap  = second summer l i t t e r ,  summer l i t t e r .  figures r e f e r t o l i v e trapping  v i c i n i t y o f where Quadrat # not  1  mortalities.  3 was  s t r i c t l y comparably t o t h e  done a l o n g a l i n e i n t h e  s e t out i n  i960.  1960-61 f i g u r e s .  They a r e t h u s  23 25-fold i n c r e a s e i n Lemmus over t h e w i n t e r o f  1959-60  and t h i s i n c r e a s e  may have been as much as 50-fold. The  1959-60 w i n t e r  i n c r e a s e o f D i c r o s t o n y x cannot be e s t i m a t e d  i n the same way, s i n c e Quadrat # 3 was n o t s e t up u n t i l I960. t h a t the May  I960 p o p u l a t i o n  e q u a l l e d t h a t i n August  were about 3-6 animals on the a r e a i n August 1959 e s t i m a t e d i n c r e a s e i s 5-10 f o l d over t h e w i n t e r  I960,  I f we assume  and t h a t t h e r e  (see Table 8), t h e  1959-60.  These  crude  e s t i m a t e s suggest t h a t D i c r o s t o n y x p r o b a b l y d i d n o t i n c r e a s e a s much as Lemmus over t h e w i n t e r o f The of  I960.  1959-60  on t h e Main Study A r e a .  Lemmus p o p u l a t i o n a t l e a s t  doubled  i t s numbers i n t h e summer  From t h e amount o f t r a p m o r t a l i t y i n v o l v e d i n t h i s e s t i m a t e and  a d d i t i o n a l d a t a from Quadrat # 2, i n c r e a s e i s 2-3' f o l d between  a reasonable estimate of t h i s  15 June  and 31 August  i960  summer  i n Lemmus.  No  e s t i m a t e can be made f o r D i c r o s t o n y x f r o m l i v e t r a p p i n g d a t a . F i n a l l y , we c a n estimate t h e d e c l i n e o v e r the w i n t e r o f  1960-61.  The Lemmus p o p u l a t i o n on Quadrat # 1 d e c l i n e d f r o m 58 t o 5 between August I960 t o June 1961, a 90-95% d e c r e a s e .  The D i c r o s t o n y x p o p u l a t i o n on  Quadrat # 3 d e c l i n e d f r o m 38 t o 11 o v e r the same p e r i o d , a 70-80% d e c r e a s e . These crude f i g u r e s a l l o w us t o conclude t h a t D i c r o s t o n y x p r o b a b l y d i d n o t decrease as much as Lemmus over the w i n t e r o f 1960-61 on t h e Main Study A r e a . The  d e c l i n e c o n t i n u e d i n b o t h s p e c i e s t h r o u g h the summer o f 1961  on t h e l i v e t r a p p i n g a r e a .  There were v e r y few summer young i n t h e 1961  samples, an important p o i n t t o which we s h a l l r e t u r n l a t e r , and t h e r e was no r e c o v e r y o f numbers. Although i t i s p o s s i b l e t o e s t i m a t e the number o f lemmings p e r a c r e , I have n o t done so because t h e d a t a a r e t o o i m p r e c i s e .  Actual  d e n s i t i e s are v e r y much a l o c a l phenomenon and do n o t h e l p us t o understand the  cycle.  2k Snap T r a p p i n g T a b l e s 9 and 10  give;.-the?snap t r a p p i n g i n d i c e s f o r Lemmus and  D i c r o s t o n y x and show i n a g e n e r a l way  1959  t o abundance i n  I960  the g r e a t changes from s c a r c i t y i n  and the subsequent  1961.  decline i n  These changes  i n abundance o c c u r r e d i n b o t h s p e c i e s and on a l l the areas t r a p p e d . These d a t a suggest t h a t t h e Lemmus c y c l e was t h a n the D i c r o s t o n y x c y c l e .  The  more pronounced  i n d i c e s f o r the Main Study A r e a change  p r o p o r t i o n a l l y as f o l l o w s :  1959  However, we  I960  1961  Lemmus  1  50  10  Dicrostonyx  5  hO  15  must beware o f comparing Lemmus i n d i c e s w i t h D i c r o s t o n y x i n d i c e s  because the d i f f e r e n c e s i n b i o l o g y between t h e s e s p e c i e s must a f f e c t absolute value of these i n d i c e s .  So a l l t h a t we  the  can say i s t h a t the d a t a  suggest t h a t Lemmus f l u c t u a t e d more s t r o n g l y than D i c r o s t o n y x . The variable  snap t r a p p i n g i n d i c e s i n t h e summer o f 1961  (compare, f o r example, Lemmus on Nine M i l e I s l a n d a t 12.22  Lemmus on t h e Main Study A r e a i n August a t due p a r t l y t o the f a c t t h a t two 1961.  are p a r t i c u l a r l y  On some areas t h e r e was  0.62).  with  This v a r i a b i l i t y i s  d i f f e r e n t types of d e c l i n e s occurred i n moderate abundance i n s p r i n g and a s t e a d y  decrease t h r o u g h the summer w i t h no r e c o v e r y (Main Study A r e a , P r i n c e R i v e r , Thelon R i v e r ) .  On o t h e r areas t h e r e was  moderate abundance i n s p r i n g w i t h  some r e c o v e r y o f numbers t h r o u g h the summer (Aberdeen  Lake, New  M i l e I s l a n d , Nine M i l e I s l a n d , Long I s l a n d , Second I s l a n d ) . of these d i f f e r e n t types of d e c l i n e s t o C h i t t y , 1955) One  The  Lake, Ten significance  ( r e s p e c t i v e l y t y p e s G and H a c c o r d i n g  and t h e i r a s s o c i a t e d c h a r a c t e r i s t i c s w i l l be d i s c u s s e d l a t e r .  f u r t h e r d e t a i l o f d e n s i t y changes d u r i n g the c y c l e was  by snap t r a p p i n g r e s u l t s .  There was  a sharp drop i n d e n s i t y i n the  shown spring  25 TABLE  9.  Lemmus snap t r a p p i n g i n d i c e s ,  LOCATION AND TIME PERIOD  DRY HABITATS N  MEDIUM HABITATS  • LEMMUS  2  1959-61.  3  N  LEMMUS  WET HABITATS N  LEMMUS  Main Study A r e a  1959 June July August September  50U 50U 153  0.0 0.0 0.0 0.0  504 378 68U  0.79 0.00 5.56  576  711  1-10  I960 June ."^July August  1961 June July August  1332 1377 1377 810  3377 180  0.15 0.0 0.0 0.0  4.87  1377  6.11 15.5U  0.0  16U7  0.2U  0.0  -  0.0  -  -  1017 1260  0.0  2268 990 990 477  0.09 0.10 0.20 0.21  999 6.91 198 6.57 999 21.42 1449  1.10  1773  0.30  0.11  1764 1935  126  0.79 0.0  2.1)3  -  576 216 387 1620  1.11  226  3.54  218  18.35  54  33.30  306  27.12  96 2U  23.95  12.50  391 216 387  17.65 18.06 27.13  1017  1.42  0.62  Other Areas 1959 August 12-17 Prince River Ten M i l e I s . TheIon R i v e r August 26-Sept. 5 New Lake  18  213  -  2k  -  0.0  1.85 0.78  I960 J u l y 13-18 Aberdeen Lake J u l y 20-23 New Lake August 15-18 Prince River Ten M i l e I s . TheIon R i v e r 1961 J u l y 1-13 New Lake  J u l y 26-29 Aberdeen Lake J u l y 17-20 Long I s l a n d  201  0.0  -  -  15  6.67  213  17.37  -  -  mm  -  171  0.0  540  I.48  306  0.0  279  2.15  486  8.44  126  0.0  207  2.90  207  7.73  -  (continued)  26 TABLE 9  (continued)  DRY HABITATS-  LOCATION AND TIME PERIOD  1  LEMMUis 3  N  1 9 6 1 (Cont'd) J u l y 2l;-27 Second I s l a n d August lU-19 Prince River Ten M i l e I s . Nine M i l e I s . Thelon River September 1 - 1 0 New Lake  3 6 9 2 . 9 8 18 0 . 0 126 2 . 3 8 8 1 12,3U 1 5 3 0 . 0  MEDIUM HABITJTS N  2 7  LEMMUS  WET HABITATS N  0.0  6 . 2 5  2 7  3 . 7 0  Ii32 180 90 360  2 9 7  0 . 6 7  6 9 6  9 0  0 . 0  -  3 . 7 0  B r y H a b i t a t s = l i c h e n heath, h e a t h , h e a t h hummock and moss h e a t h . Medium H a b i t a t s « h e a t h sedge and h e a t h sedge hummocked. Wet H a b i t a t s = sedge hummock, sedge t u s s o c k ,  sedge marsh, moss, and  moss sedge. 2  N = number o f s t a n d a r d  trap nights.  ^LEMMUS - number o f Lemmus caught p e r 1 0 0  LEMMUS  standard t r a p n i g h t s .  3.01 7.78 12.22 3.33 3.02  27 TABLE 10. D i c r o s t o n y x snap t r a p p i n g i n d i c e s , 1959-61  LOCATION AND TIME PERIOD  DRY HABITATS  1  MEDIUM HABITATS  MET HABITATS  o  N  2  DICRO.  J  N  DICRO.  N  DICRO.  Main Study A r e a  1959 June July August September 1-10  711  501; 5ol+ 153  0.1+2 0.20 0.20  0.0  I960 June July August  5ol+  2.98  378 681+  10.05  1961 June July August  576 1017 1260  Other A r e a s 1959 August 12-17 TheIon R i v e r I960 J u l y 13-18 Aberdeen Lake August 15-18 TheIon R i v e r 1961 J u l y 26-29 Aberdeen Lake August 11+-19 Thelon R i v e r  7.31  1.01+ 2.56 0.79  1332 1377 1377 810  0.68 0.51 0.29  2268 990 990  0.26 0.20  0.12  U77  0.63  999  0.30 0.51  1.31  1377 180 1377  2.22  1.09  999  161+7 1017 1773  1.1+0 0.88 0.85  11+1+9  -  213  0.0  201  1U.U3  213  U.69  -  306  5.56  279  153  0.0  226  27  198  0.0  o.l+o  0.1+8  1761+ 1935  0.62 0.56  -  387  0.52  6.61+  218  2.29  -  387  0.26  6.1+5  1+86  2.06  0.0  360  0.0  ^ D r y H a b i t a t s = l i c h e n h e a t h , h e a t h , h e a t h hummock, and moss h e a t h . Medium H a b i t a t s •> h e a t h sedge and h e a t h sedge hummock. Wet H a b i t a t s • sedge hummock, sedge t u s s o c k , sedge marsh, moss and moss sedge.  2  N • number o f s t a n d a r d t r a p n i g h t s . 3 -DICRO. = number o f D i c r o s t o n y x caught p e r 100 s t a n d a r d t r a p n i g h t s .  28 of  I960  i n Lemmus j u s t as the snow was  T h i s drop was  m e l t i n g and summer b r e e d i n g began.  r e g i s t e r e d i n the s t a n d a r d t r a p l i n e e s t i m a t e s as f o l l o w s : equal  l i n e s E, F  June 1-h (melt-off)  2/3  ^August I-I4.  equal  decrease  •  June U-Ijl 2 to 3 f o l d increase ( a f t e r the m e l t o f f )  s i x other l i n e s  G i v e n t h i s s e t of r e l a t i o n s h i p s , we  1 r  ^August  k-l$  e s t i m a t e a 67% d e c l i n e i n d e n s i t y o f  Lemmus over the m e l t - o f f , but t h i s i s p r o b a b l y an o v e r e s t i m a t e  because  o f i n c r e a s e d movements o f animals d u r i n g t h i s p e r i o d ( t h u s i n c r e a s i n g t r a p line catches).  Perhaps a 30% m o r t a l i t y e s t i m a t e i s c l o s e r t o the t r u t h .  T h i s s p r i n g d e c l i n e o c c u r r e d i n s p i t e o f the absence o f b i r d p r e d a t o r s o n l y sparse p o p u l a t i o n s o f mammalian p r e d a t o r s .  Whether t h i s s p r i n g  d e c l i n e a l s o o c c u r r e d i n D i c r o s t o n y x c o u l d not be The  and  determined.  s p r i n g t r a p p i n g d a t a r e f l e c t a change i n h a b i t a t d i s t r i b u t i o n  between the two  s p e c i e s over the c y c l e .  d r y , medium, and wet  I f we  (as i n T a b l e s 9 and 10)  d i v i d e the h a b i t a t s i n t o  and p l o t the percentage  t h e t o t a l numbers caught i n each type o f h a b i t a t , we shown i n F i g u r e 3.  o b t a i n the  of  results  There i s an i n v e r s e r e l a t i o n s h i p between D i c r o s t o n y x  and Lemmus such t h a t the s p e c i e s which i s most abundant o c c u p i e s the g r e a t e s t range o f h a b i t a t s . the peak summer o f  Thus Lemmus g r e a t l y expanded i t s h a b i t a t spectrum i n  i960,  while Dicrostonyx contracted i t s habitat  a l t h o u g h i t a l s o i n c r e a s e d c o n s i d e r a b l y i n numbers.  spectrum  These changes c o m p l i c a t e  somewhat the i n t e r p r e t a t i o n o f d e n s i t y changes observed i n a s i n g l e h a b i t a t , because a g i v e n number o f animals  spread over many h a b i t a t s w i l l o b v i o u s l y  be l e s s dense t h a n the same number i n one h a b i t a t o n l y .  The e x p l a n a t i o n of  these changes i n h a b i t a t s e g r e g a t i o n p r o b a b l y l i e s i n some form o f i n t e r s p e c i f i c  29  FIGURE 3»  Summer h a b i t a t d i s t r i b u t i o n  Main Study A r e a .  i n D i c r o s t o n y x and lemmus on t h e  O r d i n a t e i s t h e p e r c e n t a g e o f t o t a l numbers caught i n  each t y p e o f h a b i t a t .  LEMMUS  30 i n t e r f e r e n c e , but we  have no d i r e c t e v i d e n c e t h a t t h i s i s the  case.  Other Census Methods V i s u a l e s t i m a t e s were o b t a i n e d f o r Lemmus as f o l l o w s : 1959  -  0.U3  Lemmus seen p e r 100  1960 - 85.0  11  «  "  11  1961 - 0.51  n  »  "  11  hours w a l k i n g  » 0  These e s t i m a t e s a p p l y o n l y t o the summer. t h e f a l l f r e e z e - u p lemmings may The  e x t e n t o f the  (based on 1;65  »  (  »  »  ( »  9  hours)  316  )  n  •» 393  "  )  D u r i n g the s p r i n g m e l t - o f f and  become much more n o t i c e a b l e .  i960  c y c l i c h i g h may  r e p o r t s of lemming abundance as f o l l o w s :  May  —  be i n d i c a t e d from  visual  Chesterfield Inlet,  Rankin  I n l e t , C o r a l Harbor, Eskimo P o i n t ; J u l y — G a r r y Lake, B e v e r l y Lakej August —  C h a n t r y I n l e t j and September ~  f r o m these r e p o r t s t h a t the m i l e s by 1+00  i960  Repulse  Bay,  Ferguson  Lake.  I t i s apparent  500  h i g h o c c u r r e d o v e r a t l e a s t an a r e a  m i l e s o f the c e n t r a l a r c t i c , thus showing t h a t the c y c l e a t  Baker Lake was  not m e r e l y a l o c a l  effect.  D a t a o b t a i n e d from t r a c e i n d i c e s and d r o p p i n g b o a r d s w i l l be p r e s e n t e d here because t h e y add n o t h i n g new SUMMARY AND  not  t o the o b s e r v a t i o n s above.  CONCLUSIONS  F i g u r e k summarizes t h e d e n s i t y changes i n Lemmus and D i c r o s t o n y x over  1959-61. 1959  Summer:  T h i s was  a summer o f v e r y low numbers o f b o t h  s p e c i e s , w i t h D i c r o s t o n y x somewhat more abundant t h a n Lemmus.  It  was  e v i d e n t b y September t h a t some i n c r e a s e had o c c u r r e d but numbers were s t i l l v e r y low.  1959-60 W i n t e r :  1  Tremendous p o p u l a t i o n growth o c c u r r e d  t h i s w i n t e r i n b o t h s p e c i e s , t h e crude  25-50 f o l d  May I960.  i n Lemmus and  5-10  over  estimates of t h i s increase being  f o l d i n D i c r o s t o n y x f r o m September  1959  to  51  FIGURE  k»  G e n e r a l i z e d d e n s i t y changes,  1959-61.  Numbers i n d i c a t e r e l a t i v e  changes i n numbers f o r each s p e c i e s b a s e d on l i v e t r a p p i n g . details.  See t e x t f o r  1/5  cr  TYPE  H  TYPE  G  UJ CD  o  _J  SUMMER  '  1959 LEMMUS  <l  DICROSTONYX  7  WINTER  '  SUMMER  1  WINTER  '  SUMMER  1  1961  I960 1  28  58  5  4?  20?  38  1 1  1 2  32 1960  Summer:  The s p r i n g p o p u l a t i o n o f Lemmus d e c l i n e d c o n s i d e r a b l y  when t h e snow m e l t e d and summer b r e e d i n g began. between 67% and 30%.  T h i s m o r t a l i t y was p r o b a b l y  By August t h e Lemmus p o p u l a t i o n had r i s e n 2-3  f r o m i t s lowest p o i n t i n June and was then  fold  s l i g h t l y above t h e s p r i n g d e n s i t y .  The D i c r o s t o n y x p o p u l a t i o n a l s o i n c r e a s e d d u r i n g t h i s summer, b u t i t i s n o t known whether t h e y showed the same drop i n numbers a t t h e m e l t - o f f .  Densities  were h i g h e s t i n t h i s c y c l e d u r i n g August I960.  1960-61 W i n t e r :  A severe  over t h i s w i n t e r , e s t i m a t e d from August  i960  1961 decline.  t o June  Summer:  at  decrease  90-95%  i n population d e n s i t y occurred  i n Lemmus and  70-80% i n Dicrostonsoc  196l. There were two p a t t e r n s found i n t h i s summer o f  On t h e Main Study A r e a and two o u t l y i n g a r e a s t h e d e c l i n e  i n both  s p e c i e s through the summer w i t h no r e c o v e r y  1955)•  On f i v e  continued  (Type G d e c l i n e j C h i t t y ,  o t h e r o u t l y i n g areas p a r t i a l r e c o v e r y o c c u r r e d t h r o u g h t h e  summer (Type H d e c l i n e ) . d e n s i t i e s i n both  By t h e end o f t h i s summer on the Main Study A r e a  s p e c i e s were about e q u a l t o those  a t the s t a r t o f the  study. R e p o r t s were r e c e i v e d t h a t lemming numbers were a l s o h i g h w i t h i n a l a r g e area o f the c e n t r a l a r c t i c i n  i960.  REPRODUCTION  P o p u l a t i o n d e n s i t y changes because o f r e p r o d u c t i o n , m o r t a l i t y , or migration.  I n t h i s s e c t i o n we s h a l l d e a l w i t h the f i r s t  o f these  primary population f a c t o r s . METHODS Reproductive  d a t a c a n o n l y be o b t a i n e d f r o m dead a n i m a l s , and  s i n c e most o f t h e s e were o b t a i n e d b y snap t r a p p i n g we must hope t h a t snap t r a p p i n g samples t h e p o p u l a t i o n randomly. assumption are p a r t l y a v o i d e d  i n t h e a n a l y s i s which f o l l o w s b y t r e a t i n g  s e p a r a t e l y each g e n e r a t i o n , t h e d i f f e r e n t time p e r i o d s .  The d i f f i c u l t i e s o f t h i s  summer l i t t e r s ,  and t h e v a r i o u s  F o r example, t o lump o l d a d u l t and summer young females  t o g e t h e r f o r an a n a l y s i s would t a x t h e assumption t h a t t h i s group i s sampled randomly, whereas i f we t r e a t o l d and young females s e p a r a t e l y t h e assumption t h a t sampling  i s random w i t h i n each group i s p r o b a b l y  Complete a u t o p s i e s were performed on almost a l l a n i m a l s s k i n s and s k u l l s were saved and t h e f o l l o w i n g d a t a were All  specimens:  date,  valid.  trapped^  recorded:  s p e c i e s , sex, xraight, t o t a l l e n g t h , h i n d f o o t  l e n g t h , f a t i n d e x , a d r e n a l weight, s p l e e n weight, l e n s w e i g h t , stomach weight, l o c a t i o n and h a b i t a t where caught. Males o n l y :  Females o n l y :  t e s t e s p o s i t i o n and weight, e p i d i d y m i s t u b u l e s v i s i b l e o r not, s i z e o f seminal v e s i c l e s . Whether l a c t a t i n g o r n o t , v a g i n a p e r f o r a t e o r not, s i z e of u t e r u s , number o f p l a c e n t a l s c a r s , number, s i z e and age o f embryos, number o f c o r p o r a l u t e a and c o r p o r a a l b i c a n s i n each ovary, combined weight o f u t e r u s and embryos.  Males were judged as f e c u n d o r non-fecund b y whether o r n o t t h e e p i d i d y m i s t u b u l e s were v i s i b l e t o t h e naked eye (Jameson, was almost no ambiguity  i n determining  1°5>0).  There  t h i s , b u t i n the few d o u b t f u l  a c c e s s o r y d a t a on t h e s i z e o f t h e s e m i n a l v e s i c l e s and t h e weight and p o s i t i o n o f the t e s t e s were u t i l i z e d .  cases  3U Females were c l a s s e d as mature o r immature b y the presence o r absence o f c o r p o r a l u t e a i n the o v a r i e s .  T h i s c r i t e r i o n i s more r e f i n e d  t h a n the c r i t e r i o n o f p e r f o r a t e o r n o n - p e r f o r a t e v a g i n a ( L e s l i e , and Venables,  Venables,  1952).  Females were c l a s s e d as pregnant macroscopically v i s i b l e  swellings.  i f t h e u t e r u s showed  The g e s t a t i o n p e r i o d o f Lemmus has Thompson (1°55  been measured i n o n l y a few c a s e s .  a) g i v e s 20 and 20|  days f o r two i n d i v i d u a l s , and i n t h e p r e s e n t study two p r e g n a n c i e s were t i m e d a t 21 and 21^ days. i n d i c a t e d f o r Lemmus. two  Thus an approximate g e s t a t i o n o f 21 days i s  F o r D i c r o s t o n y x Manning (195k)  c a s e s , and Quay and Quay  observations.  (1956)  give  21  days f o r  days as a maximum f o r f i v e  Thus an approximate g e s t a t i o n o f 19-21  f o r Dicrostonyx.  g i v e s 19-21  days i s i n d i c a t e d  Assuming t h a t b o t h s p e c i e s o f lemmings f o l l o w , i n g e n e r a l ,  the type o f development shown b y l a b o r a t o r y r a t s and mice, we may e s t i m a t e t h a t pregnancy becomes m a c r o s c o p i c a l l y v i s i b l e  on t h e s i x t h day a f t e r  impregnation. Embryos were aged i n t h e f o l l o w i n g way i n o r d e r t o c a l c u l a t e back t o t h e date o f i n s e m i n a t i o n . 3.3  B i r t h weights were determined  t o average  grams i n Lemmus (Thompson, 1955  a j t h i s study) and about 3.0 grams i n  1956).  L a b o r a t o r y mouse embryo growth c u r v e s  D i c r o s t o n y x (Quay and Quay,  f o r weight and crown-rump l e n g t h (Enzmann, 1935)  were c o n v e r t e d t o t h e  g e s t a t i o n p e r i o d and b i r t h weight o f each s p e c i e s o f lemming, and t a b l e s o f expected weight and crown-rump l e n g t h f o r each day o f g e s t a t i o n were constructed.  As a f u r t h e r check a n a t o m i c a l changes a s s o c i a t e d w i t h development  i n t h e r a t (Henneberg, 1937)  were adapted  i n the same way t o t h e lemmings.  The use t o w h i c h these a g i n g d a t a a r e p u t i s such t h a t a c c u r a c y o n l y w i t h i n + 2 days i s n e c e s s a r y , and t h u s t h e assumptions made here c r i t i c a l f o r t h e r e s u l t s which f o l l o w .  are not r e a l l y  35 P l a c e n t a l s c a r s are formed embryos and  a t the i m p l a n t a t i o n s i t e s o f  show up as a r e a s o f b l a c k p i g m e n t a t i o n on the  s i d e o f the u t e r u s (Conaway,  1955)*  the o n l y use made o f t h e s e d a t a was  mesometrial  A l t h o u g h these s c a r s were  counted,  i n t h e c l a s s i f i c a t i o n o f females as  n u l l i p a r o u s (no embryos o r p l a c e n t a l s c a r s ) , P r i m i p a r o u s (embryos o r one set  o f p l a c e n t a l s c a r s p r e s e n t , ) o r m u l t i p a r o u s (embryos and  s c a r s p r e s e n t , o r two  o r more s e t s o f s c a r s ) .  placental  These s c a r s t e n d t o fade  w i t h age, but t h i s causes few problems i n animals o f s h o r t l i f e lemmings.  span  C o r p o r a a l b i c a n t i a (degenerate c o r p o r a l u t e a ) were a l s o  like  counted;  b u t , as w i t h t h e p l a c e n t a l s c a r s , t h e o n l y use made o f these' d a t a was  to  c l a s s i f y f e m a l e s as n u l l i p a r o u s , p r i m i p a r o u s , o r m u l t i p a r o u s . Corpora l u t e a were counted i n the o v a r i e s o f pregnant w i t h the a i d o f a b i n o c u l a r d i s s e c t i n g m i c r o s c o p e .  These  females  structures  show up v e r y c l e a r l y i n the s m a l l o v a r y o f a lemming, p a r t i c u l a r l y i n animals f r e s h l y dead. to  O v a r i e s p r e s e r v e d i n f o r m a l i n are much more  count w i t h o u t d e t a i l e d h i s t o l o g i c a l work.  r a t e we  In order t o assess ovulation  must assume t h a t each corpus luteum r e p r e s e n t s one o v u l a t e d egg  and thus t h a t t h e r e are no p o l y o v u l a r f o l l i c l e s formed. for  difficult  or accessory corpora l u t e a  There i s almost no e x p e r i m e n t a l o r h i s t o l o g i c a l evidence on lemmings  these p o i n t s .  Quay (I960) f o u n d v e r y few  trinuclear primordial f o l l i c l e s polyovular f o l l i c l e s  (about 0,1%)  binuclear  and  i n D i c r o s t o n y x , and t h i s s u g g e s t s t h a t  are n o t i m p o r t a n t i n t h i s s p e c i e s .  In general corpora  l u t e a counts agree w i t h embryo counts f o r b o t h s p e c i e s j o n l y v e r y r a r e l y are  t h e r e fewer c o r p o r a l u t e a t h a n embryos, and r a r e l y more t h a n one t o  t h r e e more c o r p o r a l u t e a than embryos.  U n t i l f u r t h e r s t u d i e s are made,  the a n a l y s i s which f o l l o w s must r e s t on the unproven assumption l u t e a counts a c c u r a t e l y and c o n s i s t e n t l y measure o v u l a t i o n r a t e . no r e a s o n y e t t o doubt t h i s  assumption.  that corpora There i s  36 R e s o r b i n g embryos were r e c o g n i z e d because t h e y were s m a l l e r t h a n normal embryos.  O b v i o u s l y these  s i z e d i f f e r e n c e s are e a s i e r t o detect i n  l a r g e r embryos l a t e i n pregnancy, and t h i s i n t r o d u c e s some u n c e r t a i n t y i n a s s e s s i n g one a s p e c t o f p r e n a t a l m o r t a l i t y . embryo r a t e s o n l y l i v e  embryos were counted.  In calculating l i t t e r  s i z e and  Prenatal mortality i s discussed  i n t h e s e c t i o n on m o r t a l i t y . RESULTS Reproduction s e v e r a l components.  i s a complex v a r i a b l e which may be broken down i n t o  F i g u r e 5 g i v e s a schematic  a n a l y s i s o f the components  of r e p r o d u c t i o n i n p o l y e s t r o u s mammals, and i n t h e remainder s e c t i o n we w i l l attempt  t o a s s e s s some o f t h e s e p a r t i c u l a r  of this  components.  Length o f B r e e d i n g Season Summer b r e e d i n g i n lemmings b e g i n s when the snow m e l t s i n s p r i n g and t h i s t e n d s t o s y n c h r o n i z e b r e e d i n g p e r i o d s f o r the r e s t o f the summer. Almost a l l mature females a 5-10 (the  (the w i n t e r g e n e r a t i o n ) are impregnated  d a y p e r i o d a t t h e m e l t - o f f j 20-21  Y-^ summer young).  Post-partum  late  i s dropped (the Y-^' summer young).  (Y^** young) and a f o u r t h l i t t e r may be produced, b u t by  summer t h e o r i g i n a l synchrony  tendency  days l a t e r t h i s l i t t e r i s dropped  b r e e d i n g i s v e r y common i n both s p e c i e s ,  and t h u s 3 weeks l a t e r a second l i t t e r A third litter  within  b r e a k s down.  T h i s synchronous b r e e d i n g  makes i t p o s s i b l e t o t r e a t summer r e p r o d u c t i o n i n terms o f b i o l o g i c a l  p e r i o d s r a t h e r t h a n c h r o n o l o g i c a l ones.  T a b l e s 11 and 12 g i v e t h e t i m i n g o f  summer b r e e d i n g p e r i o d s i n Lemmus and D i c r o s t o n y x . The  l e n g t h o f t h e summer b r e e d i n g seasons  of  1959-61  and D i c r o s t o n y x on the Main Study Area a r e g i v e n i n T a b l e 13.  i n Lemmus The b e g i n n i n g  of b r e e d i n g i n e v e r y case c o i n c i d e s w i t h the m e l t i n g o f the snow, and i t i s v a r i a t i o n s i n t h e end o f t h e summer b r e e d i n g season t h a t must be for  here.  I n 1959  accounted  t h e r e was no evidence t h a t b r e e d i n g ceased i n t h e f a l l  57 FIGURE 5»  Components o f r e p r o d u c t i o n i n p o l y e s t r o u s  TOTAL YEARLY REPRODUCTION  MONTHLY EMBRYO RATES  LITTER SIZE  LENGTH OF BREEDING SEASON  PREGNANCY RATE (during breeding  season)  mammals.  58 TABLE 11.  Timing o f summer b r e e d i n g p e r i o d s i n Lemmus f e m a l e s ,  1959-61.  Dates g i v e n a r e i n s e m i n a t i o n d a t e s ; t o o b t a i n p e r i o d s o f b i r t h add 21 days.  PERIOD  YEAR  ii  I Winter  n i  if  Generation  1959  June 12-20  1960  May  1961  June 5 - l U  29-June  10  J u l y 3-11  J u l y 2U-Aug. 6  June 16-30  J u l y 8-19  NO BREEDING  June 2 6 - J u l y 6  J u l y 18-28  August 6-?  ?  Y^ Summer Young 1 9 5 9  J u l y 2I+-Aug. 6  ?  I960  J u l y 8-19  NO BREEDING  1961  J u l y 18-28  August 7-?  Y j • Summer Young 1 9 5 9  August 19-31  I960  NO BREEDING  1961  August 8-?  59 TABLE 12. T i m i n g o f summer b r e e d i n g p e r i o d s i n D i c r o s t o n y x  1959-61.  females,  Dates given are insemination dates; t o obtain p e r i o d s o f  b i r t h add 20 d a y s .  PERIOD  TEAR  IV  III  II  WINTER GENERA! ION 1959  June 18-28  J u l y 10-18  August 1-10  1960  May 31-June 8  June 22-28  July  16-2U  NO BREEDING  1961  June 2-LU  June 2 8 - J u l y 10  J u l y 21- ?  NO BREEDING  Y  x  ?  SUMMER YOUNG  1959  NO BREEDING ?  I960  NO BREEDING  NO BREEDING  1961  J u l y 17-21  NO BREEDING  Y  x  « SUMMER YOUNG  1959  -  -  1960  -  -  1961  .  -  NO BREEDING ? -  NO BREEDING NO BREEDING  4o TABLE 13.  l e n g t h o f t h e summer b r e e d i n g seasons o f Lemmus and D i c r o s t o n y x ,  Main Study A r e a , 1959-61.  TEAR  LENGTH IN DAYS  1  TIME PERIOD  2  IEMMITS  1959  80  June 12. - September 15 +  1960  70  May 29 - August 9  1961  8U  June 5 - August 28  DICROSTONYX  1959  73  June 18 - August 30 + ?  1960  7k  May 3 1 - August 13  1961  69  June 2 - August 10  O n l y June - August counted i n t h i s  figure.  F i r s t i n s e m i n a t i o n date t o l a s t b i r t h d a t e .  Ul i n Lemmus, as pregnant  specimens were s t i l l b e i n g o b t a i n e d i n the  h a l f o f September when I l e f t . way  Whether D i c r o s t o n y x behaved i n the same  i s not known because o n l y t h r e e s m a l l young and one l a c t a t i n g  female were caught  a f t e r the end o f August.  I n 1961  a f t e r J u l y 20 o r  b r e e d i n g seemed t o have stopped  mid-August i n D i c r o s t o n y x and by t h e end o f August i n lemmus. v e r y few mature females l e f t males were caught  by  There were  i n e i t h e r s p e c i e s by August and no mature  on the Main Study A r e a a f t e r August 1 i n Lemmus or  August 3 i n D i c r o s t o n y x . to  adult  I n I960 summer b r e e d i n g  stopped a t the end o f J u l y , no Lemmus b e i n g impregnated D i c r o s t o n y x a f t e r J u l y 25.  first  Under such c i r c u m s t a n c e s  p i n p o i n t the end o f summer b r e e d i n g i n 1961,  e x e r c i s e d i n i n t e r p r e t i n g these  i t i s rather  difficult  and some care must be  figures.  A t Aberdeen Lake changes i n the l e n g t h of the summer b r e e d i n g season o f D i c r o s t o n y x seemed even more s t r i k i n g than the changes on the Main Study A r e a .  I n I960 widespread  s t o p p i n g by J u l y 15,  was  e v i d e n c e was  and i n 1961 b y J u l y 25.  obtained that breeding The p r e c i s e end of t h e  b r e e d i n g season a t Aberdeen Lake cannot be g i v e n f o r e i t h e r y e a r because of  no August d a t a .  Thus b r e e d i n g seemed t o be c u r t a i l e d i n both summers  but s l i g h t l y e a r l i e r i n I960 t h a n i n 1961. l i k e those on the Main Study  Lemmus a t Aberdeen Lake behaved  Area.  The e x t e n t o f w i n t e r and s p r i n g b r e e d i n g ( i . e . b r e e d i n g under the snow) i n both s p e c i e s i s g i v e n i n Table lU. based  on body weight  d i s t r i b u t i o n s o f June 1959»  The d a t a f o r 1958-59 a r e A few young D i c r o s t o n y x  were f o u n d which must have been b o r n d u r i n g the s p r i n g , but no young Lemmus. Both  s p e c i e s b r e d e x t e n s i v e l y i n the w i n t e r of 1959-60.  Lemmus were o b t a i n e d i n A p r i l and May, F e b r u a r y , and A p r i l . 18, J a n u a r y 17,  Pregnant  and March 2U,  Pregnant  female  and b r e e d i n g males i n December,  female D i c r o s t o n y x were o b t a i n e d on November and b r e e d i n g males i n November,  January,  42  TABLE H i . W i n t e r and s p r i n g b r e e d i n g o f Lemmus and D i c r o s t o n y x ,  WINTER BREEDING"'  SPRING BREEDING  1958- 59  none ?  none ?  1959- 60  extensive  YEAR  LEMMUS;  1960- 6 1  extensive  none  some  1958- 59  none 1  some  1959- 60  extensive  some  DICROSTONYX  1960- 6 1  1  none  W i n t e r - September t o A p r i l 1 5  o S p r i n g - A p r i l 16 t o May 3 1  some  1958-61,  1*3 F e b r u a r y , March, A p r i l , and May. o b t a i n e d from t h i s w i n t e r only a qualitative  (57  S i n c e o n l y a few w i n t e r specimens were  D i c r o s t o n y x and 21 Lemmus), these d a t a g i v e  i d e a of w i n t e r b r e e d i n g .  the o t h e r hand, there was 65 D i c r o s t o n y x and 21+5  no b r e e d i n g d e t e c t e d i n e i t h e r s p e c i e s (based  Lemmus c o l l e c t e d throughout  b r e e d i n g d i d o c c u r i n 1961, on A p r i l 16 and May  3.  In t h e w i n t e r o f 1960-61, on  and pregnant  the w i n t e r ) .  on  Spring  female D i c r o s t o n y x were o b t a i n e d  Although no pregnant  female  Lemmus were o b t a i n e d ,  females w i t h f r e s h p l a c e n t a l s c a r s and young animals were caught  i n late  May. In  summary, the major changes i n the l e n g t h o f the b r e e d i n g  season over the lemming c y c l e were:  ( l ) extensive winter breeding only i n  t h e phase o f i n c r e a s e (1959-60); and  (2) s h o r t e n i n g of the summer b r e e d i n g  season b o t h of the peak y e a r  (I960) and o f the d e c l i n e (1961).  These e f f e c t s  occurred i n both s p e c i e s . L i t t e r Size L i t t e r s i z e a t b i r t h i s a f u n c t i o n o f the o v u l a t i o n r a t e and the p r e n a t a l m o r t a l i t y r a t e s . by c o u n t i n g embryos i n pregnant  An a p p r o x i m a t i o n females.  We  to l i t t e r  size i s obtained  need t o f i n d out whether t h e r e  are any changes i n number o f c o r p o r a l u t e a o r number o f embryos p e r female  over the lemming c y c l e . There are a t l e a s t e i g h t i n t e r r e l a t e d v a r i a b l e s t h a t may  litter  pregnant  size:  season, f o o d s u p p l y , body weight,  age, p a r i t y ,  affect  lactation,  p o p u l a t i o n d e n s i t y and s o c i a l s t r u c t u r e , and p h y s i o l o g i c a l and g e n e t i c changes in  constitution.  year  Thus t o say t h a t l i t t e r  s i z e d i f f e r s between y e a r x  o r group a and group b, i s t o say v e r y l i t t l e .  and  I t i s necessary to  c o r r e c t f o r as many of these v a r i a b l e s as p o s s i b l e i n an assessment o f s i z e changes and t o compare o n l y groups of s i m i l a r c o m p o s i t i o n .  litter  These f a c t s  have not always been a p p r e c i a t e d b y workers a s s e s s i n g r e p r o d u c t i o n and much  c o n f u s i o n has t h u s  resulted.  A p r e l i m i n a r y " a n a l y s i s o f t h e d a t a i n d i c a t e d t h a t body weight p e r se (indeptendent o f p a r i t y and season) had no e f f e c t on c o r p o r a l u t e a or  embryo counts, and t h i s v a r i a b l e was d e l e t e d from t h e f i n a l  analysis.  T a b l e s 1$ and 16 g i v e t h e number o f c o r p o r a l u t e a o f pregnant Lemmus and D i c r o s t o n y x f e m a l e s from t h e Main Study A r e a , and from d a t a we are l e d t o t h e f o l l o w i n g c o n c l u s i o n s :  these  (1) O v u l a t i o n r a t e i n b o t h  s p e c i e s changes s e a s o n a l l y , d e c l i n i n g f r o m h i g h e r v a l u e s a t t h e s t a r t o f the summer t o lower v a l u e s i n t h e l a t e summer.  (2) P r i m i p a r o u s  females  t e n d t o have lower o v u l a t i o n r a t e s t h a n m u l t i p a r o u s females i n b o t h s p e c i e s b u t t h e d i f f e r e n c e s a r e s l i g h t , i n so f a r as can be g e n e r a l i z e d f r o m t h e few  samples which c o n t a i n b o t h groups.  (3) P r i m i p a r o u s summer young have  s i g n i f i c a n t l y lower o v u l a t i o n r a t e s t h a n m u l t i p a r o u s w i n t e r g e n e r a t i o n a d u l t s b r e e d i n g a t t h e same t i m e .  (U) F i n a l l y , and most important f o r o u r purposes,  t h e r e are 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 o v u l a t i o n r a t e s o f e i t h e r s p e c i e s between t h e y e a r s , when we compare s i m i l a r groups o f a n i m a l s . T a b l e s 17 and 18 g i v e t h e number o f embryos o f pregnant and D i c r o s t o n y x females from t h e Main Study A r e a .  Lemmus  P r e c i s e l y the same f o u r  c o n c l u s i o n s drawn from t h e c o r p o r a l u t e a d a t a c a n be a p p l i e d t o t h e s e embryo d a t a . S i n c e a l l these d a t a p e r t a i n o n l y t o t h e Main Study A r e a , i t i s r e a s o n a b l e t o enquire whether t h e s e r e s u l t s a r e l o c a l o r g e n e r a l .  Fortunately  d a t a a r e a v a i l a b l e from Aberdeen Lake, 115 m i l e s west o f Baker Lake, f o r I960 and 1961.  T a b l e 19 g i v e s t h e c o r p o r a l u t e a and embryo counts f o r  Lemmus a t Aberdeen Lake i n Dicrostonyx.  i960  and  1961,  and T a b l e  20 t h e  These d a t a show t h e s e a s o n a l change observed  r a t e and l i t t e r  size.  same d a t a f o r above i n o v u l a t i o n  The Lemmus do not show any d i f f e r e n c e between  ovulation rate or l i t t e r  s i z e i n summer young and w i n t e r g e n e r a t i o n a d u l t s ,  c o n t r a r y t o what was o b s e r v e d above.  F i n a l l y , t h e r e are no s i g n i f i c a n t  4 TABIE 15.  5  Number o f C o r p o r a L u t e a i n Lemmus f e m a l e s , Main Study-  A r e a , Summers  GROUP  1969-61.  I PERIOD N  I I PERIOD  MEAN  SE  6.92  ±.36  N  MEAN  I I I PERIOD SE;  N  MEAN  SE.  3  7.33  +.67  13 6.62  ±.33  I V PERIOD N  MEAN  SE  WINTER GENERATION  1959 Primiparous  12  Multiparous  -  -  I960 Primiparous Multiparous  1$ 10  7.80 8.10  ±.39 ±.28  8  7.25 -  ±.31  18 7.72  ±.21*  NOT BREEDING  1961 Primiparous Multiparous  Y  ±  5 8.20  +.37  8 6.75  ±31  1  7.00  SUMMER YOUNG  1959 Primiparous Multiparous  -  -  7 5.57  +.37  -  -  10 5.20 -  +.36  -  -  13 5.31 -  ±.29  -  -  -  -  -  -  —  -  I960 Primiparous Multiparous  NOT BREEDING  1961 Primiparous Multiparous  Y  1  1 SUMMER YOUNG  1959 Primiparous Multiparous  5  3.80 -  i960 Primiparous Multiparous  NOT BREEDING  -  1961 Primiparous Multiparous  -  1  U.00 -  +.37  46  TABLE 16.  Number o f c o r p o r a l u t e a i n D i c r o s t o n y x f e m a l e s , Main S t u d y  A r e a , summers  1959-1961.  GROUP  I PERIOD  I I PERIOD  N  MEAN  SE  9  7.00 ±0.50  I I I PERIOD  N  MEAN  SE  2  6,50 ±0.50  N  MEAN  I  5.00  SE  TOTTER GENERATION  1959 Primiparous Multiparous  -  I960 Primiparous  9  7.00 +0.U7 1  Multiparous  2  9.50 +1.50 9  1961  Primiparous M uSUMMER l t i p a r YOUNG ous  6.00 7.33 +0.80  6  5.67 +0.8U mm  23 1  6.96 ±0.35 7.00 7  8.1(3 ±0.72  1  9.00  1959 Primiparous  -  -  -  I960 Primiparous  -  1961 Primiparous  -  2  U.50 ±0.50  4  TABLE  17,  7  Number o f embryos i n Lemmus f e m a l e s , Main Study A r e a , summers  GROUP  I PERIOD N  MEAN  I I PERIOD SE.  N  MEAN  I I I PERIOD SE  1959-61.  I ? PERIOD  N  MEAN  SE  N  3  6.67  ±.67  ±.28 13  6.23  ±.28  ±.U9  8  6.75  ±.31 1  7  5.U3  ±.U8  MEAN  SE  WINTER GENERATION  1959 Primiparous Multiparous  I960 Primiparous Multiparous  12  6.33 -  ±.36  -  15 10  7.27 7.50  ±.37 ±.3U  18 7.11  8  7.00 -  *.38  5 7.80  -  NOT BREEDING  1961 Primiparous Multiparous Y  x  6.00  SUMMER YOUNG  1959 Primiparous Multiparous  -  —  I960 Primiparous Multiparous  10 5»00  -  1961 Primiparous Multiparous Y  x  13  U.92  -  -  -  -  —  -  -  -  -  -  -  -  ±.30  NOT BREEDING  +.33 -  « SUMMER YOUNG  1959 Primiparous Multiparous  5  3.80  I960 Primiparous Multiparous  NOT BREEDING  1961 Primiparous Multiparous  1  U.00  +.37  48 TABLE 18. summers  Number o f embryos i n D i c r o s t o n y x f e m a l e s , Main Study A r e a ,  1959-61.  GROUP  I PERIOD N  MEAN  9  6.11  I I PERIOD SE  I I I PERIOD  N  MEAN  SE  N  MEAN  2  6.00  ±1*00  1  SE  WINTER GENERATION  1959 Primiparous  Multiparous  ±0.U6  -  1.00  I960 Primiparous  9  6.11  ±0.51 1  6.00  Multiparous  2  6.00  ±2.00 9  5.00  ±0.58  6  U.67  1961 Primiparous Multiparous  23 1  5.61 1.00  ±0.ljl  5.29  ±0.61+  1  8.00  Y  ±  7  ±0.i;9  SUMMER I0UNG  1959 Primiparous  -  I960 Primiparous  -  -  -  -  -  1961 Primiparous  2.  2.50  ±1.50  4 TABLE 19.  9  Number o f c o r p o r a l u t e a and embryos i n lemmus females a t  Aberdeen Lake, summers  1960-61.  GROUP  I PERIOD N  I I PERIOD  1  MEAN  SE'  9.00  tO.Ul  N  MEAN  5  7.20  I I I PERIOD SE  N  MEAN  SE  NUMBER OF CORPORA LUTEA Winter  Generation  I960 Primiparous Multiparous  1961 Primiparous Multiparous  k  1  +1.07  h  5.50 ±0.91  3  6.00  3  6.00 ±0.1+7  1  6.00  9.00 ±0.00  Y^ Summer Young  I960 Primiparous  1961 Primiparous  NUMBER OF EMBRYOS Winter  Generation  I960 Primiparous Multiparous  1961 Primiparous Multiparous  U  1  8.75 +0.63  5  7.00 ±1.00  9.00  k  5.50 ±0.91  3  6.00  3  6 . 0 0 ±0.U7  ±0.00  Y-|_ Summer Young I960  Primiparous  -  1961 Primiparous  -  Insemination dates r I I P e r i o d June  -  I P e r i o d - June k-93  21+-27, 1960j  1  6.00  I960; June 10-12:, 1961;  I I I P e r i o d - J u l y 6-10, I96O5 J u l y 10-20,  1961.  50 TABLE 20. Number o f c o r p o r a l u t e a and embryos i n D i c r o s t o n y x females a t Aberdeen Lake, summers l°60-6l.  GROUP  I PERIOD N  MEAN  I I PERIOD  1  SE  N  MEAN  I I I PERIOD SE  N  MEAN  1  5.00  2  2.50  SE  NUMBER OF CORPORA LUTEA Winter  Generation  I960 Primiparous  7  7.00  ±0.72  Multiparous  -  9  5.67  ±0.61;  1961 Primiparous Multiparous  -  2  U.00  ±1.00  9  U.00 £0.65  8  U.00  NUMBER OF EMBRYOS Winter  Generation  I960 Primiparous Multiparous 1961 Primiparous Multiparous 1  7  5.57 *0.57 -  U  5.50 -  -  ±0.61;  ±0.38  ±1.50  I n s e m i n a t i o n d a t e s * I P e r i o d ~ June 3-9, 1960j June 5-lk, 1961$ 11 P e r i o d — June 2 1 - J u l y 9, I960: June 2 6 - J u l y 7, 1961$ I I I P e r i o d J u l y 16-20, 1961.  51 d i f f e r e n c e s between t h e y e a r s i n e i t h e r v a r i a b l e f o r e i t h e r s p e c i e s . The  seasonal trend i n l i t t e r  s i z e c a r r i e s through i n t o t h e  w i n t e r , as f a r as o u r meager w i n t e r r e c o r d s i n d i c a t e . the pregnant  females  o b t a i n e d i n w i n t e r a r e l i s t e d below.  Date  Number o f Embryos Lemmus  November 18, 1959 January 17, I960 March 2U, I960 A p r i l 25, I960  Dicrostonyx: 3  3 3  U  May 21, i960 May 2U, I960  5?  3  A p r i l 16, 1961 May 3, 1961 In  L i t t e r sizes of  3  h  c o n c l u s i o n , t h e r e seemed t o be no s i g n i f i c a n t change o v e r t h e  cycle i n either ovulation rate or l i t t e r  s i z e i n Lemmus o r D i c r o s t o n y x .  There was a s e a s o n a l t r e n d i n these v a r i a b l e s independent  of the cycle i n  numbers. Pregnancy Rates Given a summer b r e e d i n g season o f a c e r t a i n l e n g t h , we may e n q u i r e what p r o p o r t i o n o f mature f e m a l e s i s pregnant  a t v a r i o u s times i n  t h i s b r e e d i n g season and s u b s e q u e n t l y whether t h e r e a r e d i f f e r e n c e s between years i n t h i s v a r i a b l e . that of Leslie e t a l .  The a n a l y s i s o f pregnancy r a t e s used here f o l l o w s  (1952).  Table 21 g i v e s t h e crude  (observed) pregnancy r a t e s f o r Lemmus  and T a b l e 22 t h e r a t e s f o r D i c r o s t o n y x f r o m a l l a r e a s d u r i n g t h i s  study.  S i n c e animals i n v e r y e a r l y pregnancy w i l l n o t be c l a s s i f i e d as pregnant m a c r o s c o p i c a l l y , these crude pregnancy r a t e s tend t o underestimate t h e a c t u a l pregnancy r a t e s such t h a t a for  D i c r o s t o n y x and a  0.750  0.762 pregnancy  e q u i v a l e n t t o e v e r y female  crude pregnancy r a t e ( i . e .  rate ( i . e .  16/21) f o r  i n t h e p o p u l a t i o n b e i n g pregnant  These r a t e s a r e expressed p e r female  >20.5 grams  15/20)  Lemmus w i l l be a l l the time.  f o r Lemmus and >30.5 grams  52 TABLE 21. Crude pregnancy r a t e s p e r 1 6 lemmus, summers  1  days p e r f e m a l e >20.5  grams,  1959-61.  LOCATION MD TIME PERIOD  WINTER GENERATION  SUMMER YOUNG Y  N  PREG. RATE  1  N  LITTER PREG.  Y » LITTER  BITE  N  PREG. RATE  MAIN STUDY AREA 1959 June 18-30 July August  U+ 3 1+  0.711+  0.667 0.500  1 12  1.000 0.583  U7 1+9  1  0.000 0.231+ 0.020 0.000  3  1.000  1  0.000  1.000  I960 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  H+O 31 21 16 26 20 1+  0.007 0.516 0.667 0.812 0.538  1961 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  6 20 5 2 6 3 -  0.000 0.150  0.050 0.000  13  n 20  0.000 0.000  0.800  1.000 0.333 0.333  0.000 0.000  Total l i t t e r Production f o r June, J u l y and August 1959  2.81+1  1960  2.613  1961  2.996  1.568  0.253 1.1+69  0.813 0.000 0.000  ABERDEEN LffiKE AREA  I960 May 27-June 2 June 15-16 J u l y 10-18 1961 June 2-5 June 22 J u l y 26-29  9 it 12  0.000 1.000  3 1  0.000 1.000  7  8  0.375  0.750  0.1+29  E s t i m a t e d p o r t i o n o f t h e 21 day g e s t a t i o n can be r e c o g n i z e d m a c r o s c o p i c a l l y .  0.200 p e r i o d f o r which p r e g n a n c y  55 TABLE 21 ( c o n t i n u e d ) .  Lemmus crude pregnancy r a t e s .  LOCJfflON AND TIME PERIOD  WINTER GENERATION  SUMMER YOUNG  Y  ±  N  PREG. RATE  N  LITTER  T • LITTER  PREG. RATE  N  PREG. RATE  OTHER AREAS  1961 Long I s l a n d J u l y 17-20 Second I s l a n d  J u l y 21+-27  Prince River August lU-17 Nine M i l e I s . August LU-19 Ten M i l e I s . August lii-19 Thelon River August U4.-I9  0,250  6  O.667  0.667  6  0.833  1  0.000  U  0.000  1.000  2  0.000  1  0.000  1.000  3  0.333  2  0.000  U  0.000  mm  5  TABLE 22.  4  Crude pregnancy r a t e s p e r  Dicrostonyx,  15"*" days  p e r female  >30.5  grams,  summers 1959-61.  LOCATION AND TIME PERIOD  T  WINTER GENERATION N  PREG. RATE  June 15-30 July August  3 9 3  0.667 0.889 0.333  I960 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  22. 5 10 6 7 10  6  1961 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  50 25 10 5 h 1+  SUMMER YOUNG  !f  PREG. RATE  0.000 O.364 1.000 1.000 0.833  2  0.H|3  5 13  0.000 0.000 0.000  14  0.214  1  0.000  MAIN STUDY AREA  1959  3  TOTAL LITTER PRODUCTION FOR JUNE, JULY AND AUGUST 1959 I960 1961  0.000 0.000 0.060 0.760 0.600 0.800 0.250 0.000  3.190 3.394 2-524  ? 0.000 0.228  ABERDEEN LAKE AREA  I960  May 27-June 2 June 15-16 J u l y 10-18  4 7 26  0.000 1.000 0.3U7  1961 May 28-June 7 June 15-22 J u l y 10-19  5 11 17  0.200  J u l y 26-29  17  O.364 0.412: 0.176  0.000  E s t i m a t e d p o r t i o n o f t h e 20 d a y g e s t a t i o n p e r i o d f o r which p r e g n a n c y can be r e c o g n i z e d m a c r o s c o p i c a l l y .  f o r D i c r o s t o n y x because t h e s e a r e t h e w e i g h t s  above which a m a j o r i t y o f  females  F i n a l l y , t h e s e d a t a are  can be mature under good c o n d i t i o n s *  g i v e n i n terms o f crude pregnancy r a t e s i n s t e a d o f s t a n d a r d i z e d pregnancy r a t e s ( L e s l i e e t a l . 1952) because a f t e r a complete s t a n d a r d i z a t i o n o f the d a t a t h e r e was h a r d l y any change i n t h e r a t e s and c o n s e q u e n t l y t h e r e was no need t o i n c l u d e the s t a n d a r d i z e d r a t e s . I f we examine t h e s e d a t a ( T a b l e s 21, 22) we see t h a t t h e r e i s a g e n e r a l r i s e i n t h e pregnancy r a t e from zero i n May t o h i g h v a l u e s b y June 15 and a subsequent d e c l i n e i n August.  We a r e n o t i n t e r e s t e d  here  i n t h e t i m i n g o f t h i s r i s e and f a l l because t h i s has been t r e a t e d under the p r e v i o u s s e c t i o n on t h e l e n g t h o f t h e b r e e d i n g season.  What we a r e  i n t e r e s t e d i n i s t h e p e r i o d o f midsummer when b r e e d i n g i s n e i t h e r s t a r t i n g up n o r b e g i n n i n g t o stop, and we w i s h t o e n q u i r e whether there are s i g n i f i c a n t d i f f e r e n c e s between t h e y e a r s i n t h e r a t e s d u r i n g t h i s p e r i o d . Pregnancy r a t e s d u r i n g midsummer (June l 5 - J u l y 31) were compared  b f o r t h e Main Study A r e a and f o r Aberdeen Lake i n JBbth Lemmus and D i c r o s t o n y x . Chi-square t e s t s  (Snedecor,  1956, p 228) were made w i t h t h e f o l l o w i n g  r e s u l t s f o r the winter generation animals: Significant Differences Lemmus  Dicrostonyx  -  *  ( l ) Between y e a r s : Main Study A r e a , 1959-61  Aberdeen Lake, 1960-61 (2) Between a r e a s :  Main Study A r e a v s . Aberdeen Lake  1960  -  1961  -  ( - * P >.10: * = P <.025, >,01; ** = P <.005) While  Lemmus showed no d i f f e r e n c e s i n pregnancy r a t e s whatever e i t h e r  over  56 the c y c l e or between d i f f e r e n t a r e a s , D i c r o s t o n y x showed a s i g n i f i c a n t l o w e r i n g o f t h e midsummer pregnancy r a t e i n t h e d e c l i n e Lake b u t no d i f f e r e n c e s on the Main Study A r e a .  i960  and  1961  (l°6l)  Furthermore,  a t Aberdeen  i n each y e a r  t h e pregnancy r a t e s were s i g n i f i c a n t l y lower a t Aberdeen  Lake compared w i t h Baker Lake. L o o k i n g more c l o s e l y a t the Aberdeen Lake D i c r o s t o n y x d a t a ,  we  see t h a t t h e s e d i f f e r e n c e s can be e x p l a i n e d by a c u r t a i l m e n t of b r e e d i n g i n e a r l y J u l y , i . e . t h a t the in  I960  mid-June o f 1961  and 1961.  1961  The  r a t e of  summer b r e e d i n g season was  shortened  drastically  one p o i n t t h a t i s then l e f t t o be e x p l a i n e d i s the  O.36I4., b u t  t h i s may  be due  to the l a t e s p r i n g phenology  such t h a t these animals were j u s t b e g i n n i n g t o b r e e d . L e a v i n g the w i n t e r g e n e r a t i o n a d u l t s and l o o k i n g a t t h e pregnancy  r a t e s f o r the summer young, we  find  c o n s i d e r a b l e v a r i a t i o n between y e a r s .  However, the p e r t i n e n t f a c t o r s i n v o l v e d here are changes i n t h e l e n g t h o f t h e b r e e d i n g season and r e l a t e d changes i n the age a t s e x u a l m a t u r i t y o f these young; the former was below. these  Consequently,  t r e a t e d above, the l a t t e r w i l l be d e a l t w i t h  a s t a t i s t i c a l a n a l y s i s was  not done on the d a t a f o r  summer young. One  a s p e c t o f T a b l e s 21  and 22  figures f o r t o t a l l i t t e r production. d e s c r i b e d by L e s l i e e t a l .  (1952) b y  has not been d i s c u s s e d y e t , the  These are o b t a i n e d i n the manner a p p l y i n g t h e observed  r a t e s t o the  l e n g t h o f t h e i r p a r t i c u l a r time p e r i o d and summing the r e s u l t s .  Unfortunately  i t i s not p o s s i b l e t o a t t a c h s t a t i s t i c a l c o n f i d e n c e l i m i t s t o these numbers because t h e y are sums o f weighted  averages.  These f i g u r e s are h y p o t h e t i c a l  i n t h a t t h e y i n d i c a t e the number o f l i t t e r s an average  mature female  would  produce i f she l i v e d o v e r the e n t i r e p e r i o d between June 1 and August 31 f o r the w i n t e r g e n e r a t i o n , o r i n the case o f the summer young o v e r the p e r i o d bejsween r e a c h i n g a mature weight and the end  o f August.  Furthermore,  these  57 production f i g u r e s are rather a r b i t r a r i l y l i m i t e d because most o f the sampling  was done a t t h i s  t o June 1 t o August 31  time.  F o r Lemmus t h e r e i s a s l i g h t d e p r e s s i o n o f t o t a l p r o d u c t i o n i n the peak summer o f  I960, b u t t h i s  i s s m a l l i n view o f t h e  f a c t t h a t t h i s summer had a s h o r t e n e d b r e e d i n g season. on t h e o t h e r hand, there i s an apparent in  litter  For Dicrostonyx,  increase i n t o t a l l i t t e r  production  t h e peak summer, a f a c t t h a t seems t o c l a s h w i t h t h e p r e v i o u s o b s e r v a t i o n  t h a t t h i s summer was c h a r a c t e r i z e d b y a shortened b r e e d i n g season. anomaly i s e x p l a i n e d i n p a r t b y t h e f a c t t h a t i n e a r l i e r than i n e i t h e r 1959  I960 summer  This  b r e e d i n g began  o r 1961 and i n p a r t by random sampling  variations  i n t h e observed pregnancy r a t e s . I n c o n c l u s i o n , on the Main Study A r e a the midsummer pregnancy r a t e s d i d n o t change s i g n i f i c a n t l y from y e a r t o y e a r i n e i t h e r s p e c i e s o f lemming.  The same was t r u e a t Aberdeen Lake f o r Lemmus but n o t f o r  D i c r o s t o n y x w h i c h seemed t o show d e p r e s s e d pregnancy r a t e s i n t h e summer o f the d e c l i n e .  A l l o t h e r observed  changes i n pregnancy r a t e s were  on changes i n t h e l e n g t h o f the b r e e d i n g Age  a t Reproductive The  i n determining  reflections  season o r the age a t s e x u a l m a t u r i t y .  Maturity  age a t which r e p r o d u c t i o n b e g i n s i s o f t h e utmost importance the i n t r i n s i c r a t e o f i n c r e a s e o f a p o p u l a t i o n ( C o l e , 195U a ) .  S i n c e we do n o t know t h e age o f the specimens o b t a i n e d i n t h i s study, i t i s n e c e s s a r y t o use body weight as an i n d e x o f age. As mentioned above, t h e c r i t e r i a o f m a t u r i t y were the presence  o f c o r p o r a l u t e a i n females  and o f  v i s i b l e e p i d i d y m a l t u b u l e s i n males. Tables  23-26  g i v e t h e percentage  o f mature a n i m a l s i n t h e  v a r i o u s weight c l a s s e s f o r Lemmus and D i c r o s t o n y x males and f e m a l e s . The  method o f L e s k e ,  P e r r y , and Watson  (19U5) was  body weight a t m a t u r i t y f o r t h e v a r i o u s groups.  used t o determine t h e median In b r i e f t h i s technique  involves  TABLE  23.  Weight a t r e p r o d u c t i v e m a t u r i t y i n Lemmus males, summers  SUMMER GENERATION  WINTER GENERATION  WEIGHT CLASS (g)  1959-61.  Y »  YOUNG  1959 11-20.5 g .  mm  21-30.5  mm  I960  1961  1959  I960  1961  -  0  0 (2)  0 (12)  0  (9)  0 (11)  (5)  0 (2)  0 (10)  0 (12)  50  0 (16)  0 (27)  -  80  0  0  100 (1)  0  0 (3)  -  1961  (28)  0*  0 (2)  10 (20)  0 (1)  100  58 (210  Ui-50.5  100  81  100  5i-6o.5  100  98  (59)  100 (12)  61-70.5  100 (2)  99  100  -  71-80.5  100 (3)  96  100  -  81-90.5  100 (1)  96 (72)  100 (1)  (5) (9)  91-100.5  -  ioi-no.5  -  (3D  (98)  (65)  97 (29) 100 (10)  YOUNG  1959  I960  31-U0.5  (5)  x  (6)  (lit) (9)  mm  100 (1)  (*)  -  (65)  mm  Percentage mature; sample s i z e i n p a r e n t h e s e s .  (28)  0  -  -  0  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  mm  mm  mm  -  -  -  -  -  (19)  -  (9)  (6)  (5) mm  mm*  0 (2)  mm  -  -  2lu  TABLE  Weight a t r e p r o d u c t i v e m a t u r i t y i n Lemmus females, summers  WEIGHT CLASS (g)  11-20,5 21-30.5 31-U0.5 Ul-50.5 51-60.5 61-70.5  71-80.5  81-90.5 91-100.5 101-110.5  SUMMER GENERATION  WINTER GENERATION Y  1959  -  100  (1) 50 (6)  100 (6) 100 (U) 100 (U) 100 (1)  -  mm  -  i960 0  *  (19) 0  1961 0  1959  (2)  5  50 (6)  100  6U (11)  100 (2)  ( U 8 )  52  (52) 92 U8)  100 (25) 100 (26) 100 (16) 100 (3)  I960  33: ( 8 )  100 (1) ( 8 )  100 (16)  -  100  -  (1U)  8 8  -  UO ( 1 0 )  39 ( 6 1 )  8 0  (15) 1 0 0  (7)  1 0 0  (2)  ( 8 )  100 (1)  -  -  -  -  -  * Percentage maturej sample s i z e i n  Y^  YOUNG  mm  (12) (21) 21  x  (1) 0  1959-61.  ses.  1 9 6 1  0  (18) 75 ( 1 6 )  1 0 0  (7)  1 0 0  (2)  1 0 0  (15  -  1959 0  (2) 1 0 0  (3)  1 0 0  (2)  -  YOUNG I 9 6 0  0  (7) 0  (U) 0  (25) 0  1 9 6 1  0  (3) 32 (19) 1 0 0 ( 1 )  -  ( 1 )  -  -  -  -  -  -  25.  TABLE  Weight a t r e p r o d u c t i v e m a t u r i t y i n D i c r o s t o n y x males, summers  SUMMER GENERATION  WINTER GENERATION  WEIGHT GLASS  1959-61.  Y « YOUNG  YOUNG  (g)  1959  I960  1961  0* (7)  0 (3)  11-20.5  -  21-30.5  50  (2)  0 (U)  0 (12)  31-40.5  78 (9)  Uo (5)  71  Ul-50.5  100 (7)  75 (U)  69 (13)  51-60.5  86 (7)  77  97 (31)  61-70.5  -  83  71-80.5  -  80  81-90.5  -  91-100.5  -  101-110.5  -  •a P e r c e n t a g e  (13)  (12)  (15)  (1U)  96 (27) 100 (10)  100 (7)  100 (5)  100 (3)  100 (2)  67 (3)  mm  1959  0 (1) mm  mm  -  I960  1961  1959  I960  1961  0 (U)  0 (2)  0 (2)  0 (1)  0 (5)  0 (3)  0 (7)  0 (3)  0 (U)  0 (3)  0 (3)  0 (1)  0 (6)  0  0 (12) 0 (3)  -  mm  mm  (1)  0 (2)  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  mature j sample s i z e i n p a r e n t h e s e s .  mm  mm  mm  mm  TABLE 26,  Weight a t r e p r o d u c t i v e m a t u r i t y i n D i c r o s t o n y x females, summers 1959-61,  SUMMER GENERATION  WINTER GENERATION  WEIGHT GLASS (g)  Y  1  Y-L' YOUNG  YOUNG  1959  I960  1961  1959  I960  1961  1959  i960  1961  11-20.5  0 * (1)  0 (6)  0 (11)  0 (2)  0 (1)  7 (15)  0 (1)  0 (10)  0 (3)  21-30.5  25 (U)  0 (3)  9 (11)  0 (1)  0 (2)  33 (3)  0 (7)  0 (6)  0 (2)  31-1+0.5  100 (3)  0 (U)  10 (10)  0 (9)  100 (1)  0 (7)  0 (1)  ia-5o.5  100 (5)  60 (5)  61+  -  -  -  51-60.5  100 (5)  89  80 (25)  -  -  -  -  61-70.5  100 (3)  71-80.5  -  81-90.5  -  91-100.5  -  101-110.5  mm  * Percentage  (9)  (25)  100 (19)  93 (28)  100 (11)  78 (7)  100 (9)  100 (2)  100 (U)  100 (1)  100 (1)  100 (1)  -  12 (17) 0 (2)  mm  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  mm  mature; sample s i z e i n p a r e n t h e s e s .  mm  62 c o n v e r t i n g the weight d a t a i n t o l o g a r i t h m s and p e r c e n t mature d a t a i n t o p r o b i t s , f i t t i n g a s t r a i g h t l i n e t o t h i s , and t h e n c a l c u l a t i n g t h e $0% p o i n t and i t s s t a n d a r d e r r o r .  The r e s u l t s are summarized i n Table 27.  Some  o f t h e d a t a were n o t s u f f i c i e n t t o c a l c u l a t e t h e median body weight a t m a t u r i t y and i n these c l a s s e s t h e upper o r lower l i m i t s p o s s i b l e f o r t h e median were i n d i c a t e d . were grouped  D a t a from t h e Main Study A r e a f o r the whole summer  i n t h i s a n a l y s i s , but i n the a c t u a l c a l c u l a t i o n s the winter  g e n e r a t i o n r e s u l t s are based m a i n l y on t h e May and June samples and t h e summer g e n e r a t i o n r e s u l t s on J u l y and August i n f o r m a t i o n . These d a t a show s t r i k i n g changes i n t h e median body weight a t m a t u r i t y between t h e d i f f e r e n t y e a r s .  I n e v e r y case i n t h e peak summer o f  1960  t h e r e was an i n c r e a s e i n t h e median body weight  1961  summer o f d e c l i n e t h r e e p a t t e r n s c o u l d be found:  remained  I n the  ( l ) median weights  t h e same as I960, as i n w i n t e r g e n e r a t i o n D i c r o s t o n y x o f both  sexes and Lemmus males.; (2) median weights between 1959 and and  at maturity.  i960  declined t o a p o s i t i o n intermediate  l e v e l s , as i n the w i n t e r g e n e r a t i o n Lemmus f e m a l e s ;  (3) median w e i g h t s d e c l i n e d t o the same l e v e l s as 1959, as i n t h e summer  young Lemmus and D i c r o s t o n y x f e m a l e s .  M i s s i n g from t h i s  classification  a r e t h e summer young males o f both s p e c i e s because none o f t h e s e became s e x u a l l y m a t u r e . i n e i t h e r the summer o f I960 o r t h e summer o f 1961. The  summer o f 1959 seems t o r e p r e s e n t the most r a p i d r a t e s o f  m a t u r a t i o n found i n both s p e c i e s .  20-25  grams and males a t  Thus Lemmus females were mature a t  25-35 grams,  U-5 weeks o f age r e s p e c t i v e l y .  representing roughly  3-U weeks  and  D i c r o s t o n y x males and females were mature  a t about 30 grams, r e p r e s e n t i n g r o u g h l y U-5 weeks o f a g e .  In neither of  the o t h e r y e a r s were these r a p i d r a t e s o f m a t u r a t i o n found, w i t h the e x c e p t i o n of t h e 1961 young f e m a l e s , and i t i s t h e s e d e v i a t i o n s from t h e p o s s i b l e r a t e s o f m a t u r a t i o n t h a t must be e x p l a i n e d .  65  TABLE 27.  Median body w e i g h t s a t m a t u r i t y f o r Lsmmus and D i c r o s t o n y x  males and f e m a l e s , 1959-61.  GROUP £ND YEAR  LEMMUS WINTER GENERATION  1959 1960 1961  Yn SUMMER YOUNG  1959  1960 1961  MALES  FEMALES.  MEDIAN WEIGHT #  95% CONF. LIMITS  <30.5 36.7  3U.2 - 39.U  <26 51.6  33.8 > 61 > Ul  21+.6 - U6.6 -  < 26  31-Ul  -  MEDIAN WEIGHT  I4I.2  95% CONF. LIMITS  U9.8 - 53.6  37.U - k5.h  -  29.1 21.3  26.6 - 31.8 20.1 - 22.6  -  20-25 > hx 2U.U  22.6 - 26.U  30.5 U9.9 U9.3  23.2 - Uo.3 1*3.5 - 57.3 U6.9 - 51.7  Y-,' SUMMER YOUNG  1959 1960 1961  26.5 > 51 > Ul  DICROSTONYX WINTER GENERJSTION  195J9 1960 1961  28.3  U3.3  38.8  Y i SUMMER GENERATION  1959 1960 1961  > 51 > 31  * W e i g h t s i n grams.  ?  23.2 - 3U.5 38.5 - U8.8 36.7 - Ul.O -  ? > Ul 2U.0  22.3 - 27.1  6U To at  sum up, t h e r e  are s t r i k i n g changes i n t h e median weight  s e x u a l m a t u r i t y over t h e c y c l e .  These changes c o n s i s t i n a g e n e r a l  i n c r e a s e i n t h e median weight a t m a t u r i t y i n t h e peak summer i n a l l sexes and  generations,  and a complete l a c k o f m a t u r a t i o n  o f a l l summer young  males i n b o t h t h e peak summer and t h e summer o f d e c l i n e .  I n o n l y one  summer o u t o f t h r e e d i d e i t h e r s p e c i e s show maximal r a t e s o f m a t u r a t i o n . Embryo Rates It  i s convenient  t o have one f i g u r e which sums up most of t h e  components o f r e p r o d u c t i o n t o g i v e some assessment o f t o t a l p r o d u c t i v i t y . T h i s c a n be done b y t h e use o f embryo r a t e s , f o l l o w i n g t h e method o f L e s l i e et  a l . (1°£2).  if  w  e  observe a sample o f N mature females,  P o f which a r e  pregnant and which c o n t a i n a t o t a l o f E l i v e : embryos, we have f o r t h e embryo r a t e E/N  =  P/N  X  E/P  t h a t i s , t h e embryo r a t e i s t h e p r o p o r t i o n o f mature females pregnant m u l t i p l i e d b y t h e mean l i t t e r  size.  changes i n t h e l e n g t h o f t h e b r e e d i n g maturity,  S i n c e t h e pregnancy r a t e  reflects  season, changes i n t h e weight a t  and changes i n t h e p r o p o r t i o n pregnant d u r i n g t h e b r e e d i n g  season, t h i s e q u a t i o n e f f e c t i v e l y sums up most o f t h e components o f reproduction  ( c f . Figure $ ) ,  T a b l e s 28 and 29 g i v e t h e crude embryo r a t e s f o r Lemmus and Dicrostonyx.  Crude r a t e s are g i v e n here i n s t e a d o f s t a n d a r d i z e d  because t h e r e i s v e r y l i t t l e d i f f e r e n c e between t h e two. p r o d u c t i o n was o b t a i n e d  rates  T o t a l embryo ,  i n the same way as t o t a l l i t t e r p r o d u c t i o n  above,  by a p p l y i n g t h e observed r a t e s t o t h e l e n g t h o f t h e i r p a r t i c u l a r time p e r i o d and summing t h e r e s u l t s . for  the winter  These p r o d u c t i o n f i g u r e s are h y p o t h e t i c a l ;  g e n e r a t i o n t h e y give t h e number o f embryos a mature female  would produce i f she l i v e d throughout the whole summer and b r e d a t the  6 TABLE 28. Lemmus,  5  Crude embryo r a t e s p e r  16^  days p e r female  >20.5 grams,  1959-61.  LOCATION AND TIME PERIOD  WINTER GENERATION  SUMMER GENERATION YOUNG  N  EMBRYO RATE  N  EMBRYO RATE  Y « x  N  YOUNG EMBRYO RATE  MAIN STUDY AREA  1959  June 15-30 July August  llj. 3 4  I960  U.57 4.67 3.50  I4O  0.02  31 21 16 26. 20 4  3. 94 U.86 5.69 3.46 0.25 0.00  May 16-31 June 1-15 June 16-30  6 20  0.00 0.90  J u l y 1-15 J u l y 16-31  2 6 3  May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  1961  5  August 1-15 August 16-31  -  T o t a l Embryo P r o d u c a t i o n f o r June, J u l y and August  1959  I960 1961  6.00  8.50  2.16 2.00  -  19.5U 17.27 22.39  1 12  1 47  U9 13  _  3 —  1  6.00 3.17  0.00 1.17 0.10 0.00  3.80  11  20 _  mm mm  -  -  4.00  -  0.00  3 6  mm  -  8.76 1.27 5.88  0.00 0.00  _ _ _ _  0.00 0.00  3.09 0.00 0.00  ABERDEEN LAKE I 9 6 0  May  27-June  June 15-16 July 1 0 - 1 8  2  9  4 1 2  0.00  8.75 4.75  2.25  1 9 6 1  June 2-5 June 2 2 July 26-29  3 1 7  0.00  9.00  2.57  1.20  E s t i m a t e d p o r t i o n o f t h e 21 d a y g e s t a t i o n p e r i o d can be r e c o g n i z e d m a c r o s c o p i c a H y .  f o r which pregnancy  66  29.  TABLE  Crude embryo r a t e s p e r 1$^ days p e r female  Dicrostonyx,  >30.5 grams  1959-61.  LOCATION JND TIME PERIOD  WINTER GENERATION N  EMBRYO RATE:  SUMMER GENERATION N  EMBRYO RATE  MALTS STUDY AREA  1959  June 15-30 July August  I960  May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  1961 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31 TOTAL EMBRYO PROD. June - August  3 9 3 6 22 5 10 6 7 10 3 50 25 10 5 ii U  li.00  5.22  0.33  0.00 2.23  6.U0 5.10 1+.00 0.57 0.00  2 5 13  0.00 0.00 0.00  U.00  lU  0.71  2.00 0.00  1  0.00 0.U8  3.96 2.70  15.U8 18.56 13.U1  1959 I960 1961  0.00  ?  0.00 0.76  ABERDEEN LAKE AREA  I960  May 27-June 2 June 15-16 J u l y 10-18  1961 May 28-June 7 June July July  15-22 10-19  26-29  U 7  26  5 11 17 17  0.00  5.57 1.28  0.60 2.00 1.59 0.59  0.00  E s t i m a t e d p o r t i o n o f t h e 20 day g e s t a t i o n p e r i o d f o r which p r e g n a n c y can be r e c o g n i z e d m a c r o s c o p i c a l l y .  67 observed  r a t e s , and f o r the summer g e n e r a t i o n t h e number o f embryos  a female would produce from t h e time  o f r e a c h i n g minimum mature weight t o  the end o f August. The most s t r i k i n g changes i n t o t a l r e p r o d u c t i v e e f f o r t occur i n Lemmus.  I f we  assume f o r t h e moment t h a t female m o r t a l i t y r a t e s were the  same i n a l l y e a r s , t h e r a t i o of  1959-61  1.53.  i s 31.1+  As we  -  18.5  -  of t o t a l r e p r o d u c t i v e output f o r the  28.3  summers  1.70 - 1.00 -  embryos, o r a p p r o x i m a t e l y  have seen, these d i f f e r e n c e s a r i s e p r i n c i p a l l y because of  changes i n t h e l e n g t h o f the b r e e d i n g  season and i n the age a t m a t u r i t y  i n t h e young. I n D i c r o s t o n y x , on t h e o t h e r hand, t o t a l embryo p r o d u c t i o n a p p a r e n t l y h i g h e s t i n the peak summer o f i s brought about i n p a r t by b i o l o g i c a l  I960.  was  T h i s c o n t r a s t w i t h Lemmus  d i f f e r e n c e s ( i . e . young D i c r o s t o n y x  mature a t an o l d e r age t h a n lemmus and Y-j_' young D i c r o s t o n y x n e v e r seem t o mature i n t h e i r was  first  summer; the b r e e d i n g  apparently s h o r t e r than t h a t of  difficulties  ( i . e . 1959 and  September was  insufficient  stopped  o r young matured).  I96I  i960)  season o f 1961  and i n p a r t by  D i c r o s t o n y x sampling  statistical  i n August  and  t o determine a c c u r a t e l y i f and when b r e e d i n g F o r these r e a s o n s  I do not p l a c e t o o much  c o n f i d e n c e i n t h e t o t a l embryo p r o d u c t i o n f i g u r e s o f Dicrostonyx.  i n Dicrostonyx  1959  and  I96I  for  There i s a s u g g e s t i o n o f d e c r e a s e d p r o d u c t i v i t y d u r i n g the  d e c l i n e on the Main Study A r e a ; t h e d a t a from Aberdeen Lake a l s o  suggest  this. T o t a l embryo p r o d u c t i o n f o r t h e e n t i r e y e a r cannot be  determined  because t h e r e are i n s u f f i c i e n t w i n t e r and s p r i n g d a t a , and i t i s n e c e s s a r y t o bear i n mind t h a t t h e above f i g u r e s are o n l y summer p r o d u c t i o n . Comparison of t o t a l summer p r o d u c t i o n o f Lemmus w i t h t h a t o f D i c r o s t o n y x shows t h a t the p o t e n t i a l r a t e of i n c r e a s e o f Lemmus i s about  68 t w i c e as g r e a t as t h a t o f IDicrostonyx under good c o n d i t i o n s . To was h i g h i n  sum up, t o t a l embryo p r o d u c t i o n d u r i n g t h e summer i n Lemmus  1959  and 1961 and low i n  be lower i n 1961 t h a n i n  I960,  b u t i n D i c r o s t o n y x seemed t o  i960.  SUMMARY AND CONCLUSIONS (1)  There was no s i g n i f i c a n t  change i n l i t t e r  s i z e o r midsummer pregnancy  r a t e s over t h e c y c l e . (2)  There were two changes i n t h e l e n g t h o f the b r e e d i n g season:  first,  e x t e n s i v e w i n t e r b r e e d i n g o c c u r r e d o n l y i n t h e w i n t e r o f i n c r e a s e j and second, t h e r e was a s h o r t e n e d  summer b r e e d i n g season i n t h e peak summer and a l s o t o  some degree i n the d e c l i n e . (3) in  The median weight a t s e x u a l m a t u r i t y was h i g h e r i n t h e peak summer  a l l groups and remained h i g h i n most groups i n t h e summer o f t h e d e c l i n e  ( e x c e p t f o r young f e m a l e s ) .  Young Lemmus males d i d n o t mature i n e i t h e r t h e  peak o r t h e d e c l i n e , whereas young females n o t i n the peak summer.  d i d mature i n the d e c l i n e b u t  MORTALITY  The  second major f a c t o r w h i c h causes changes i n p o p u l a t i o n  density i s mortality.  T h i s f a c t o r begins  i t s o p e r a t i o n a t o v u l a t i o n and  may be c o n v e n i e n t l y s u b d i v i d e d i n t o p r e - n a t a l m o r t a l i t y and p o s t - n a t a l mortality.  The purpose o f t h i s s e c t i o n i s t o a s s e s s t h e importance o f  t h e s e components. METHODS P r e n a t a l m o r t a l i t y i s a s s e s s e d b y comparing counts made o f 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 , and l i v i n g embryos.  The methods o f  c o l l e c t i n g these d a t a were d i s c u s s e d i n the s e c t i o n on r e p r o d u c t i o n . D a t a on p o s t - n a t a l m o r t a l i t y were o b t a i n e d from l i v e The  trapping.  methods used i n l i v e t r a p p i n g were d i s c u s s e d i n t h e s e c t i o n on  population  density. RESULTS  Prenatal Mortality P r e n a t a l m o r t a l i t y was a s s e s s e d o f B r a m b e l l and M i l l s  (1947, 1 9 U 8 ) .  as f a r as p o s s i b l e b y t h e methods  P r e n a t a l m o r t a l i t y may be s u b d i v i d e d  as f o l l o w s : 1.  2.  P a r t i a l p r e n a t a l l o s s ( a t l e a s t one embryo s u r v i v e s parturition) a.  pre-implantation  b.  post-implantation  until  Total l i t t e r loss a.  pre-implantation  b.  post-implantation  P a r t i a l pre-implantation m o r t a l i t y i s estimated  from d i s c r e p a n c i e s between  c o r p o r a l u t e a counts and i m p l a n t a t i o n s i t e counts.  From these  the amount o f l o s s o f ova i n l i t t e r s t h a t s u r v i v e i m p l a n t a t i o n .  d a t a we  estimate  Partial  70 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 s e s t i m a t e d from d i s c r e p a n c i e s between the number of i m p l a n t a t i o n s i t e s and the number o f l i v i n g embryos i n i t h e uterus.  From t h e s e d a t a we  e s t i m a t e the number o f i m p l a n t e d embryos which  f a i l t o s u r v i v e , and t h i s may lost.  i n c l t i d e whole l i t t e r s i n the p r o c e s s o f b e i n g  T h i s e s t i m a t e i s always an underestimate  because t h e females  come f r o m v a r y i n g s t a g e s between i m p l a n t a t i o n and b i r t h . s h o u l d be made o n l y on females i n t h e v e r y l a t e t o o few were o b t a i n e d i n t h i s  Ideally  counted  counts  stages o f pregnancy, but  study.  T a b l e s 30 and 31 summarize t h e s e d a t a on p a r t i a l p r e n a t a l m o r t a l i t y i n Lemmus and B i c r o s t o n y x . these e s t i m a t e s . (Snedecor,  A l l d a t a from each summer were grouped  to obtain  D i f f e r e n c e s between the y e a r s were t e s t e d by c h i - s q u a r e  1956,  p 228)  between y e a r s was  and a l l found t o be n o n - s i g n i f i c a n t .  Variation  s l i g h t ; i n Lemmus the t o t a l l o s s o f ova amounted t o  and i n D i c r o s t o n y x t o 23-26$.  k-9%  D i c r o s t o n y x s u f f e r s c o n s i d e r a b l y more p a r t i a l  p r e n a t a l l o s s than does Lemmus. No i n f o r m a t i o n on the l o s s o f whole l i t t e r s b e f o r e o r d u r i n g i m p l a n t a t i o n i s g i v e n b y the above a n a l y s i s . complete  No l i t t e r s were found  r e s o r p t i o n i n middle o r l a t e pregnancy i n t h i s study.  i n d i r e c t evidence  young a n i m a l s .  However,  suggests t h a t under some c o n d i t i o n s i n Lemmus  r e s o r p t i o n of l i t t e r s  undergoing  complete  j u s t a f t e r i m p l a n t a t i o n does o c c u r e s p e c i a l l y i n  I n l a t e J u l y I960 young Lemmus  25-35 grams  i n weight  with  v e r y f a i n t p l a c e n t a l s c a r s , s m a l l c o r p o r a a l b i c a n t i a , and no a c t i v e mammary t i s s u e began t o appear i n the samples.  S i n c e i t was  quite impossible f o r  t h e s e animals t o have weaned a l i t t e r a l r e a d y ( t h e y were o n l y k-5 and  weeks o l d )  s i n c e t h e s c a r s were so f a i n t , a r e a s o n a b l e i n t e r p r e t a t i o n i s t h a t t h e s e  animals l o s t t h e i r e n t i r e l i t t e r s  just after implantation.  Although  some o f  t h e s e animals were p r o b a b l y missed d u r i n g autopsy because o f the v e r y s m a l l s i z e o f t h e s e s c a r s and c o r p o r a , a m i n i m a l assessment o f the f r e q u e n c y o f  71  TABLE 30.  19$°-6lj  P a r t i a l p r e n a t a l m o r t a l i t y d a t a f o r Lemmus f e m a l e s , summers Main Study A r e a .  1959  TYPE OF LOSS  Pre-implantation % litters loss  16.7 N ° 18  % ova l o s t  3.9 N =»  % litters loss  l?6l  31.3 N - 67  lk.7 N - 3k  loss  showing  Post-implantation  I960  102  5.3  H -  U76  2*3  H • 216  loss  showing  % embryos r e s o r b i n g  5.6 N - 18 2*0 N «  ?8  17»9 N - 67 U»2 N =  U5l  8,8 N - 3k 1»9 N -  211  72  TABIE 31. summers  P a r t i a l p r e n a t a l m o r t a l i t y data f o r Dicrostonyx  1959-61,  Main Study A r e a .  195°  TYPE O F LOSS  Pre-implantation % litters loss  females,  loss  showing  kk.o  6 3 . 6  N  % ova l o s t  -  1 1  1 6 . 9  N Post-implantation  1961  I960  7 7  N =  25  19.7 N = 178  N  55*9 = 3k 18.2  N -  21+2  loss  % litters loss  showing  % embryos  resorbing  N  18.2 - 11  20.0 N =  7 . 8  N  -  6k  25  32.3 3U  N «  6.3 N = ll|3  N  9.1 198  -  73 this total l i t t e r  l o s s may be made from t h e snap t r a p p i n g samples.  No D i c r o s t o n y x have y e t been seen w i t h these c h a r a c t e r i s t i c s .  No w i n t e r  g e n e r a t i o n Lemmus showing these p e c u l i a r i t i e s have been found, and thus the c o n d i t i o n appears  t o be found o n l y i n Lemmus summer young.  The  f o l l o w i n g samples c o u l d have c o n t a i n e d t h i s type o f young Lemmus:  No.  young females > 20.5 grams  No. mature (with c o r p o r a lutea)  No. showing evidence o f total litter  loss  Main Study A r e a  1959 August-Sept. 10  16  16  0  la 52  15 16  6 6  2 3 10  2  I960 J u l y 16-31 August 1-15 1961 J u l y 16-31 August 1-15 August 16-31 If  1  U  0 1 3  these d a t a a r e a p p r o x i m a t e l y c o r r e c t , we reach t h e c o n c l u s i o n t h a t o f  a l l t h e summer young which matured i n I960 about 3$-h0% l o s t t h e i r l i t t e r s j u s t a f t e r i m p l a n t a t i o n , and i n loss of their l i t t e r s .  196l  about  50-75$ s u f f e r e d  Thus t h e s e d a t a show a sharp  entire total  contrast t o the p a r t i a l  p r e n a t a l l o s s d a t a g i v e n above b y s u g g e s t i n g a c o n s i d e r a b l e i n c r e a s e i n total litter  l o s s i n Lemmus summer young o v e r t h e c y c l e .  • To sum up our assessment o f p r e n a t a l m o r t a l i t y :  p a r t i a l prenatal  m o r t a l i t y i n Lemmus and D i c r o s t o n y x showed no r e l a t i o n s h i p t o the c y c l i c d e n s i t y changes.  T o t a l l i t t e r l o s s a f t e r i m p l a n t a t i o n d i d n o t seem t o  o c c u r i n D i c r o s t o n y x o r i n a d u l t Lemmus, b u t i n Lemmus summer young i t seemed t o be h i g h i n t h e peak summer and i n the summer o f t h e d e c l i n e . Total l i t t e r  l o s s b e f o r e i m p l a n t a t i o n c o u l d n o t be a s s e s s e d i n t h i s  study.  Post-Natal Mortality (a)  Adults: A d u l t m o r t a l i t y here i n c l u d e s a l l w i n t e r m o r t a l i t y as w e l l as  7k the  summer m o r t a l i t y o f w i n t e r g e n e r a t i o n a n i m a l s .  The s p e c i f i c  c o n c l u s i o n s made here a p p l y t o t h e l i v e t r a p p i n g a r e a i n p a r t i c u l a r and p r o b a b l y Type G d e c l i n e s i n g e n e r a l . Some g e n e r a l o b s e r v a t i o n s on a d u l t m o r t a l i t y may be made from snap t r a p p i n g r e c o r d s . q u a n t i t a t i v e estimates be made.  U n f o r t u n a t e l y snap t r a p p i n g d a t a cannot g i v e v a l i d o f m o r t a l i t y r a t e s but q u a l i t a t i v e observations  There i s an annual o v e r t u r n i n p o p u l a t i o n .  may  A d u l t s o f the winter  g e n e r a t i o n , which comprise the e n t i r e p o p u l a t i o n a t t h e s t a r t o f the summer breeding,  are g r a d u a l l y r e p l a c e d t h r o u g h t h e summer b y t h e i r own young,  and by l a t e August and September t h e r e are v e r y few o l d a d u l t s l e f t .  This  i s r e f l e c t e d i n the snap t r a p p i n g samples as f o l l o w s :  % o f w i n t e r g e n e r a t i o n a d u l t s i n snap t r a p samples Lemmus  1959-61  combined June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31 September 1-15  Dicrostonyx  100 100 79 60 23 11 2  100 100 93 U2 35 17 8  Q u a n t i t a t i v e o b s e r v a t i o n s on a d u l t m o r t a l i t y may be made from the l i v e t r a p p i n g d a t a . Tables  1961,  32  and  33  No m o r t a l i t y e s t i m a t e s were o b t a i n e d i n  give the minimum s u r v i v a l r a t e s f o r Lemmus i n  and T a b l e s 3k and  35  f o r Dicrostonyx i n  i960  and  1961.  s u r v i v a l r a t e s are o b t a i n e d b y marking a c o h o r t o f animals and d e t e r m i n i n g  Minimum  a t time t  the t r u e s u r v i v a l r a t e and  care must be e x e r c i s e d i n i n t e r p r e t i n g them. comparisons t h e observed  (Dicrostonyx).  To f a c i l i t a t e  1952).  consequently direct  minimum s u r v i v a l r a t e s were c o n v e r t e d  a s t a n d a r d 28 day p e r i o d , and t h e s e  and 37  i960 and  t h e number known t o be a l i v e a t time t + w ( C h i t t y ,  These r a t e s always underestimate  to  1959.  logarithmically  d a t a are g i v e n i n T a b l e 36  (Lemmus)  75  TABLE 3 2 .  Minimum s u r v i v a l r a t e e s t i m a t e s f o r Lemmus, summer I960.  TIME PERIOD  WINTER GENERATION N *  N  t  N  t+1—  SUMMER GENERATION N  M.S.R.  h  t-&.»  M.S.!  Quadrat # 1 June 18 - 20  19  10  2  6-8  16  5  3  J u l y 28-30  9  6  0  July  August  25-27  8  —  —  0.63 0.50 0.67  8  2  0  20  5  0  50  —  —  0  1  17  0  0.25 0.25  Quadrat # 2 June 29 -  1$  July 2 J u l y 20 - 23  1U  August U - 6  6  8  0 0  mm  -  o.53  0.36  1 hS  69  -  -  •H- N = number r e l e a s e d , N-fc » number known t o be a l i v e n e x t t i m e , K  t+1-0B  number i n c l u d e d f r o m l a t e r sampling, M.S.R. « minimum  s u r v i v a l r a t e o v e r t h e t i m e p e r i o d shown.  1.00 0.38  76  TABLE 33•  Minimum s u r v i v a l r a t e e s t i m a t e s f o r Lemmus, summer  1961,  Quadrat # 1,  TIME PERIOD N #  \  June 5 -  2  2:  0  June 12 -  U  1  2  June 19 -  5  June 26 -  u  3  1  July 3 -  3  2  I  J u l y 10 -  3  0  1  J u l y 17 -  3  1  0  J u l y 2U -  2  0  0  J u l y 31 August 7 -  SUMMER GENERATION  WINTER GENERATION N  t + l - o o M.S.R.,  0  -  N  N  A  oo  mm  1,00 0.75 1.00 1.00 1.00 0.33 0.33  0.00  mm  1  1  August 21 -  -  August 28 -  _  _  * N » number r e l e a s e d ,  = number known t o be a l i v e n e x t t i m e ,  Sept. 1  N  t + l - a f numb©  1,  i n c l u d e d f r o m l a t e r sampling, M.S.R. » minimum  s u r v i v a l r a t e o v e r t h e time p e r i o d shown.  0.00  mm  -  August I i i -  M.S.R.  0.00  77  TABLE 3u»  Minimum s u r v i v a l r a t e e s t i m a t e s f o r D i c r o s t o n y x ,  summer  I960, Quadrat # 2.  TIME PERIOD  WINTER GENERATION N *  N  t  N  t l-«> +  June 29 July 2 J u l y 20 - 23  6  k  1  8  U  0  August U - 6  8  •*  M  SUMMER GENERATION  ' ' S  R  N  0.83  0.50  See Table 33 f o r e x p l a n a t i o n o f symbols.  N  5  t  ^t+l-00  2.  M.S.R.  0 O./4O  TABLE 35 •  Minimum s u r v i v a l r a t e e s t i m a t e s f o r D i c r o s t o n y x ,  summer  1961, Quadrat # 3 .  TIME PERIOD  WINTER GENERATION t + l - c o M.S.R.  N *  N  2  0  1  -  16  7  k  June 19 -  Hi  6  5  June 26 -  12  2  9  July 3  11  9  2  -  11  5  3  J u l y 17 -  8  k  2  J u l y 2li -  6  2  1  J u l y 31 -  3  2  0  2  0  0  June 5  -  June 12  -  J u l y 10  August 7  -  -  t  N  SUMMER GENERATION N  Nt  Nt+l-oo  2  0  0  5  1  2-i  k  2  2  6  2  1  o.5o  0.69  -  0.79 mm  0.92 1.00  -  0.73 0.75 0.50 0.67 0.00  August Ik  -  August 21  -  mm  6  2  0  August 28 Sept. 1  mm  2  -  »  #  M.S.:  See T a b l e 33 f o r e x p l a n a t i o n o f symbols  0.00 0.60 1.00 o.5o 0.33  79  TABLE 36.  Minimum s u r v i v a l r a t e s f o r Lemmus c o n v e r t e d t o a 28 day b a s e .  O r i g i n a l d a t a i n t a b l e s 32 and 33.  MINIMUM SURVIVAL RATE PER 28 DAIS WINTER GENERATION  SUMMER GENERATION  I960 Quadrat June July July  # 1 18-July 6 6 - J u l y 28 28-Aug. 25  Quadrat # 2 June 29-July 20 J u l y 20-Aug. 1;  0.U9  O.I4I  0.68  0.17 0.26  0.ii3 0.15  1.00 0.17  0.56 1.00 0.11 0.00 0.00  0.00  1961 .  Quadrat #1 June 5-20 June 2 1 - J u l y I4. J u l y 5-18 J u l y 19-31 Aug. 1-31  0.00  80  TABLE 37.  Minimum s u r v i v a l r a t e s  28 day b a s e .  f o r Dicrostonyx converted t o a  O r i g i n a l d a t a i n t a b l e s 3k and 35 •  MINIMUM SURVIVAL RATE PER 28 DAYS WINTER GENERATION  SUMMER GENERATION  I960 Quadrat # 2 June 2 9 - J u l y 20 J u l y 20-Aug. k 1961 Quadrat June June July July Aug. Aug.  # 3 5-20 21-July k 5-18 19-31  1-15  16-31  0.78 0.27  0.18  0.13  0.53 0.53 O.lU  0.00  0.00 0.36 0.03  81 C o n s i d e r i n g o n l y the w i n t e r g e n e r a t i o n , we all  summer s u r v i v a l appears  both species.  see f i r s t t h a t o v e r -  t o have been b e t t e r i n 1°60  than i n l°6l f o r  I n l°6l a f t e r m i d - J u l y s u r v i v a l seems t o decrease  moderately  i n D i c r o s t o n y x and c o n s i d e r a b l y i n Lemmus, r e s u l t i n g i n a complete absence of a d u l t s be e a r l y t o mid-August.  These d i f f e r e n c e s between  i960  1961  and  seem t o be r e a l , a l t h o u g h i t i s i m p o s s i b l e t o e s t i m a t e t h e i r magnitude these  from  data. Overwinter m o r t a l i t y cannot be e s t i m a t e d f o r  b r e e d i n g was  o c c u r r i n g , b u t we  a  because  can o b t a i n a b l o c k estimate f o r the  61 w i n t e r because no b r e e d i n g o c c u r r e d . d e n s i t y changes, there was  1959-60  90-95%  As was  decrease  shown above i n d i s c u s s i n g  i n Lemmus and a 70-80%  d e c r e a s e i n D i c r o s t o n y x over the p e r i o d from August I960 t o June There was  1961.  no b r e e d i n g o v e r t h i s p e r i o d (the few animals b o r n i n May  e x c l u d e d f r o m t h e s e e s t i m a t e s ) and no major movements o c c u r r e d . a p p r o x i m a t i o n we  may  I96O-  As  are an  e n q u i r e what mean monthly m o r t a l i t y r a t e would produce  the observed d e c l i n e s o v e r t h i s 10 month p e r i o d w i t h no r e c r u i t m e n t o r migration.  F o r D i c r o s t o n y x an  11-15%  monthly m o r t a l i t y would produce  70-80% d e c l i n e o v e r t h i s p e r i o d , and f o r Lemmus a would produce the observed  90-95%  20-25%  a  monthly m o r t a l i t y  reduction.  There i s some i n d i r e c t e v i d e n c e t h a t t h e w i n t e r m o r t a l i t y r a t e i n 1960-61 was  not c o n s t a n t i n Lemmus but may  The l o c a l Eskimos brought  have been so i n D i c r o s t o n y x .  i n a l l lemmings t h e y f o u n d d u r i n g t h e w i n t e r ,  and t h e s e were r e c o r d e d as "caught  a l i v e " o r "found dead".  There was  a  sharp drop i n t h e number o f l i v e Lemmus found by t h e Eskimos about December 15-31  and t h e r e a f t e r almost a l l specimens were found dead.  d i d not seem t o o c c u r i n D i c r o s t o n y x .  T h i s same change  F i g u r e s o b t a i n e d were as f o l l o w s :  P r o p o r t i o n o f w i n t e r specimens caught b e f o r e December 31 Lemmus  30 o f 98  alive  a f t e r January 3 of  99  1  82 Proportion  o f w i n t e r specimens caught  b e f o r e December 31 Dicrostonyx  around December. one  We c a n i n t r o d u c e  month w i t h an i n c r e a s e d  a constant r a t e .  a f t e r January 1  9 o f 20  These d a t a suggest a p e r i o d o f i n c r e a s e d  Then a  alive  8 o f 23 m o r t a l i t y f o r Lemmus sometime  t h e s e d a t a i n t o o u r model b y adding  m o r t a l i t y rate of  15-20$ monthly  5>0$, a l l  o t h e r months h a v i n g  m o r t a l i t y w i t h one month  t o $0% p r e d i c t s a d e c l i n e i n Lemmus s i m i l a r t o t h a t o b s e r v e d .  increased  The  interesting  t h i n g t o note i s t h a t t h e magnitude o f t h i s i n c r e a s e d  rate during  one month has v e r y l i t t l e  mortality  e f f e c t on t h e f i n a l p r e d i c t e d  decline;  f o r example,  = 89% d e c l i n e o v e r 10 mo.  20$ monthly m o r t a l i t y 20$  "  20$ The  "  «  ,r  p l u s one month a t 33% - 93$  "  "  "  " 50$ - 93$  »  "  "  "  K  " "  r e a s o n s f o r t h i s apparent mid-winter sharp d e c l i n e i n Lemmus are n o t  known.  The above h y p o t h e t i c a l model s u g g e s t s , however, t h a t u n l e s s  increased  m o r t a l i t y extended o v e r a c o n s i d e r a b l e  e x c e p t i o n a l l y severe i t need have l i t t l e The is  "  conclusion  o f time o r was  e f f e c t on s p r i n g d e n s i t i e s i n 196l.  t o t h i s d i s c u s s i o n on w i n t e r m o r t a l i t y i n 1960-61  that although the d e c l i n e i n both species  numerically,  length  this  over t h i s time i s v e r y  great  t h e average monthly m o r t a l i t y r a t e s which c o u l d produce t h e  o b s e r v e d d e c l i n e s a r e r e a s o n a b l e f o r s m a l l mammals ( L e s l i e , C h i t t y , and Chitty,AGoLley,  196l)j i n d e e d i f a n y t h i n g t h e y seem t o be low r a t h e r than  high. I n summary, m o r t a l i t y o f t h e w i n t e r g e n e r a t i o n h i g h e r i n the summer o f 1961 t h a n i n the summer o f rates during  1960-61 were  i960.  appeared s l i g h t l y Winter mortality  moderate t o low b u t produced a g r e a t  numerical  83 d e c l i n e because o f t h e l a c k o f b r e e d i n g over t h i s 9-10 Nothing  i s known o f summer m o r t a l i t y i n 1°5°  195>9-60.  month p e r i o d .  or winter m o r t a l i t y i n  The d a t a are n o t s u f f i c i e n t t o i n v e s t i g a t e p o s s i b l e d i f f e r e n c e s  i n m o r t a l i t y between t h e sexes, (b)  Juveniles: J u v e n i l e m o r t a l i t y r e f e r s t o the m o r t a l i t y o f summer young d u r i n g  t h e summer o f b i r t h , and t h u s i n c l u d e s b i r t h - w e a n i n g m o r t a l i t y and e a r l y post-weaning m o r t a l i t y . F o r a g e n e r a l i d e a o f changes i n j u v e n i l e m o r t a l i t y we may r e t u r n  32-37 f o r  to Tables  t h e summer g e n e r a t i o n d a t a .  These t a b l e s show f o r Lemmus  t h a t a p p a r e n t l y no young s u r v i v e d on t h e l i v e t r a p p i n g a r e a i n 1961, w h i l e a t l e a s t some s u r v i v e d i n I960 on t h e same a r e a .  F o r Dicrostonyx the  i960  d a t a a r e n o t v e r y e x t e n s i v e , b u t i n I96I t h e r e was a p p a r e n t l y no s u r v i v a l o f young u n t i l a f t e r August 1 and even t h e n s u r v i v a l was n o t v e r y good l i t t e r young s h o u l d have been i n t h e t r a p s b y 1$ J u l y 1961). suggest t h a t j u v e n i l e  (first  These d a t a  s u r v i v a l was poor i n b o t h s p e c i e s d u r i n g the summer  of the d e c l i n e . A more r e f i n e d e s t i m a t e o f t h i s m o r t a l i t y may be made as f o l l o w s . Knowing from t h e r e p r o d u c t i v e d a t a g i v e n p r e v i o u s l y t h e mean t i m i n g o f b r e e d i n g p e r i o d s and the mean l i t t e r  s i z e , and knowing from l i v e t r a p p i n g  the number o f a d u l t females l i v i n g on the quadrat  a t t h e v a r i o u s t i m e s , we  may e s t i m a t e t h e number o f young born on t h e quadrat f o r each b r e e d i n g period. ensure  A t a subsequent t r a p p i n g p e r i o d ( l a t e enough a f t e r weaning t o a l l t h e young b e i n g t r a p p a b l e ) we g e t a t a l l y o f how many o f these  young a r e a l i v e on the q u a d r a t ,  and b y comparing t h i s w i t h t h e c a l c u l a t e d  number b o r n we can e s t i m a t e t h e j u v e n i l e m o r t a l i t y r a t e d i r e c t l y .  We  assume i n t h i s a n a l y s i s ( l ) t h a t the females breed a t t h e average  rates  determined  p r e v i o u s l y , (2) t h a t a l l females have t h e i r l i t t e r s on o r  81* a d j a c e n t t o t h e quadrat,  (3) t h a t a l l t h e young on the quadrat have been  caught, as w e l l as a l l t h e females, and (U) t h a t t h e r e i s no n e t i m m i g r a t i o n or  e m i g r a t i o n o f young.  and assumption  Assumptions ( l ) ,  (1+) c o u l d n o t be e v a l u a t e d .  ( 2 ) , and (3) a r e p r o b a b l y v a l i d , These c a l c u l a t i o n s were done f o r  b o t h s p e c i e s i n I960 and 1961 and t h e r e s u l t s a r e p r e s e n t e d i n T a b l e s 38-ij.l. These s u r v i v a l e s t i m a t e s a r e a composite and a v a r i a b l e l e n g t h o f e a r l y post-weaning  o f birth-weaning  mortality  m o r t a l i t y , and hence some c a u t i o n  must be e x e r c i s e d i n comparing t h e s u r v i v a l r a t e s c o n v e r t e d t o t h e s t a n d a r d 28 day base. decline. litter  These d a t a show v e r y low s u r v i v a l r a t e s o f summer young i n t h e  There was some f u r t h e r s u g g e s t i o n t h a t t h e second D i c r o s t o n y x survived b e t t e r than the f i r s t l i t t e r .  T h i s suggestion i s  c o n f i r m e d i n t h e snap t r a p p i n g d a t a i n which t h e l a t e August samples o f both s p e c i e s a r e dominated b y I-, young w i t h almost no 1  young,  young and o n l y a few  ( s i n c e t h e b r e e d i n g a d u l t s are d y i n g out through t h e  summer, one  would expect t o g e t many Y-^ young, fewer Y^' and v e r y few Y ^ " ) .  There i s no  s t r i k i n g d i f f e r e n t i a l m o r t a l i t y between the sexes i n these d a t a .  Extensive  snap t r a p p i n g d a t a support these r e s u l t s o b t a i n e d from l i v e t r a p p i n g and r e n d e r improbable due  any s u g g e s t i o n t h a t these d i f f e r e n c e s between y e a r s a r e  t o e m i g r a t i o n o f young from t h e l i v e t r a p p i n g a r e a . B i r t h - w e a n i n g m o r t a l i t y c o u l d n o t be s e p a r a t e d from e a r l y p o s t -  weaning m o r t a l i t y i n t h e s e e s t i m a t e s .  I f much l o s s o c c u r r e d a t b i r t h o r  s h o r t l y a f t e r , p a r t i c u l a r l y l o s s e s o f whole l i t t e r s , in  t h i s s h o u l d show up  a r e g r e s s i o n o f a c t i v e mammary t i s s u e i n b r e e d i n g f e m a l e s .  However,  t h e r e was no d i f f e r e n c e m a c r o s c o p i c a l l y between l a c t a t i n g females i n i960 and 1961.  D u r i n g t h e b r e e d i n g season o f b o t h y e a r s v i r t u a l l y e v e r y female  showed  a c t i v e mammary t i s s u e , and t h e r e was no evidence t h a t l a c t a t i o n had stopped in  any o f t h e females such as o c c u r s a t t h e end o f the b r e e d i n g  season.  85  TABLE 38, summer  S u r v i v a l e s t i m a t e s f o r j u v e n i l e Lemmus on Quadrat # 1,  I960. PERIOD OF BREEDING • I  Mean Date a t which weaning i s complete  July 9  size  C a l c u l a t e d No. o f young b o r n  III  J u l y 28  8  No* a d u l t f e m a l e s alive then Mean l i t t e r  II  7.37 51.5  August 18  6  yg.  ad.,  5.00  k ad.,  6.23  7*11 56.9  39*9 25-27  25-27  Date o f subsequent trapping  J u l y 28-30  Aug.  No. o f t h e s e j u v e n i l e s i n t r a p s then  25  20  1U  Estimated s u r v i v a l r a t e from b i r t h t o trapping  0.U9  0.35  0.35  ( p e r 3k days)  ( p e r k3 days) ( p e r 22 d a y s )  Estimated s u r v i v a l r a t e converted t o 28 d a y base  0.56  0.50  Pregnancy r a t e o f young » o.50; for adults.  August  0.25  c o n t i n u o u s b r e e d i n g assumed  yg,  86  TABLE 39.  S u r v i v a l e s t i m a t e s f o r j u v e n i l e Lemmus on Quadrat # 1,  summer 1961.  PERIOD OF BREEDING I  II  Mean date a t which J u l y Ik weaning i s complete  .  I l l  August k  No. a d u l t f e m a l e s a l i v e then  1  Mean l i t t e r  7.00  7.80  C a l c u l a t e d No. o f young b o r n  7.00  7.80  0  Date o f subsequent trapping  J u l y 13-15  Aug. 10-12  Aug. 29-31  size  No. o f t h e s e j u v e n i l e s i n t r a p s then Estimated s u r v i v a l r a t e from b i r t h t o t r a p p i n g time  Estimated s u r v i v a l rate converted t o a 28 d a y base  1  August 27  2  0.29 ( p e r I4 days)  0.09  6.75  1  0.13 ( p e r 21 days)  0.07  0  1  87  TABLE UO.  S u r v i v a l e s t i m a t e s f o r j u v e n i l e D i c r o s t o n y x on Quadrat  # 2, summer I960.  PERIOD OF BREEDING I  II  Mean date a t which weaning i s complete  July 7  July  No. a d u l t f e m a l e s a l i v e then  3  3  Mean l i t t e r  6.11  5.00  C a l c u l a t e d No. o f young b o r n  18.3  15.0  Date o f subsequent trapping  J u l y 20-23  No. o f t h e s e j u v e n i l e s i n traps then  7  -  Estimated s u r v i v a l r a t e from b i r t h t o t r a p p i n g time  0.38 ( p e r 28 days)  ?  Estimated s u r v i v a l rate converted t o a 28 d a y base  0.38  ?  size  29  88  TABLE Ip..  S u r v i v a l e s t i m a t e s f o r j u v e n i l e D i c r o s t o n y x on Quadrat #  summer 1961. PERIOD OF BREEDING I  II  III  J u l y 12  August 7  August 27  5.61  5.29  8.00  C a l c u l a t e d No. o f young b o r n  39.3  10.6  0  Date o f subsequent trapping  July  Mean d a t e a t which weaning i s complete No. o f a d u l t f e m a l e s a l i v e then Mean l i t t e r  size  27-29  Aug. 3 -  5  Aug. 2U-26  No. o f these j u v e n i l e s i n traps then Estimated s u r v i v a l rate from b i r t h t o t r a p p i n g time  Estimated s u r v i v a l rate converted t o 28 d a y base  0.05  0.38  0.05  0.I4  (per 29 days)  ( p e r I4 days)  ?  3,  89 T h i s i n d i r e c t e v i d e n c e suggests t h a t t h e l o s s o f whole l i t t e r s a t b i r t h o r i n e a r l y s u c k l i n g s t a g e s i s not t h e cause o f the observed p o o r s u r v i v a l of j u v e n i l e s .  More d i r e c t e v i d e n c e on t h i s p o i n t i s needed.  I n summary, j u v e n i l e m o r t a l i t y between b i r t h and 1-k weeks a f t e r weaning was v e r y h i g h i n t h e summer o f d e c l i n e on t h e l i v e a r e a (Type G d e c l i n e ) . and  trapping  Almost no young o f the f i r s t l i t t e r seemed t o s u r v i v e  o n l y moderate numbers o f the second and t h i r d l i t t e r s .  This  high  m o r t a l i t y was p r o b a b l y not due t o the l o s s o f whole l i t t e r s a t b i r t h o r i n the  e a r l y s u c k l i n g stages,  but probably occurred  j u s t s h o r t l y before or  s h o r t l y a f t e r weaning. SUMMARY WD CONCLUSIONS (1) P a r t i a l p r e n a t a l m o r t a l i t y showed no change i n e i t h e r o v e r t h e c y c l e , but the complete l o s s o f l i t t e r s for  just after  species  implantation  summer young Lemmus seemed t o be h i g h i n t h e peak summer and i n t h e  summer o f d e c l i n e . (2)  A d u l t m o r t a l i t y seemed t o i n c r e a s e  d e c l i n e i n both s p e c i e s , moderate i n b o t h s p e c i e s . the  s l i g h t l y i n the summer o f the  and w i n t e r m o r t a l i t y r a t e s i n 1960-61 were low t o N o t h i n g i s known about m o r t a l i t y d u r i n g  1959 o r  1959-60 w i n t e r . (3) J u v e n i l e m o r t a l i t y between b i r t h and s h o r t l y a f t e r weaning was  v e r y h i g h i n the summer o f the d e c l i n e p a r t i c u l a r l y f o r t h e f i r s t summer litter.  T h i s c o n c l u s i o n p r o b a b l y a p p l i e s o n l y t o Type G d e c l i n e s - as w i l l  be shown i n a l a t e r  section.  MOVEMENTS AND  The  MIGRATIONS  t h i r d f a c t o r which can cause changes i n p o p u l a t i o n d e n s i t y  is dispersal.  D i s p e r s a l may  t a k e the form o f s m a l l l o c a l movements o r  mass movements ("migrations")  of the whole p o p u l a t i o n .  d i s p e r s a l can a f f e c t d e n s i t y t h r o u g h i m m i g r a t i o n areas immigration  u s u a l l y balances  On  small areas  or emigration.  e m i g r a t i o n and c o n s e q u e n t l y  On  large  dispersal  a f f e c t s d e n s i t y o n l y t h r o u g h r e p r o d u c t i o n o r m o r t a l i t y changes. METHODS Almost a l l d a t a on l o c a l movements were o b t a i n e d by l i v e and t h e s e methods have been d i s c u s s e d i n a p r e v i o u s s e c t i o n .  trapping,  A few movements  were o b t a i n e d by snap t r a p p i n g animals which had p r e v i o u s l y been l i v e  trapped.  RESULTS L o c a l Movements The  l i v e t r a p p i n g program used i n t h i s s t u d y was  p r i m a r i l y t o s t u d y movements, and c o n s e q u e n t l y desired.  The  attempt t o e s t i m a t e  p e r i o d o f t h r e e days; a t l e a s t 5-7 The  (1953),  Stickel  (195U),  (1956).  and Brown  or t h r e e times d u r i n g any one  r e c a p t u r e s are n e c e s s a r y f o r home range  (Brown, 1956)  because t h i s a l l o w s us t o  captured only twice during a t r a p p i n g p e r i o d .  T h i s type  distances  use  of a n a l y s i s  i s c o n f i n e d t o s h o r t term movements w i t h i n t r a p p i n g p e r i o d s . T a b l e s J+2 and i+3 g i v e t h e l e n g t h o f e v e r y movement w i t h i n t r a p p i n g p e r i o d s f o r Lemmus i n I 9 6 0 and 1961, g i v e t h e same i n f o r m a t i o n f o r D i c r o s t o n y x . were t e s t e d by c h i - s q u a r e  few  trapping  a p p r o p r i a t e method f o r the lemming d a t a i s t o analyze  between s u c c e s s i v e c a p t u r e s animals  t h e d a t a l e a v e much t o be  a c t u a l home range s i z e s w i l l be made because v e r y  animals were r e c a p t u r e d more t h a n two  estimates.  designed  many problems o f measuring home ranges and movements o f s m a l l  mammals have been d i s c u s s e d b y D a v i s No  not  (Snedecor, 1956)  recorded  and T a b l e s UU  and  D i f f e r e n c e s between the and b o t h s p e c i e s showed a  U5  years  91  TABLE U2.  L e n g t h o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e  #2.  t r a p p i n g f o r Lemmus, summer  I960,  LENGTH OF MOVEMENT  WINTER GENERATION MALES  < 5o« 51-100' 101-150' 151-200' 201-300! 301-U00I U01-500! 501-700! 701-900!  8 6 5 3 3 5 0  mm  Quadrat  FEMALES  17 13 10 9 1 2 1  -  N = 30 N o 53 x « Ui3.3' x - io5.l«  Y  1  SUMMER YOUNG  MALES  15 7 U  3 3 0 1  -  -  N - 33 X - 9U.6'  FEMALES  12 10 7 6 3 2 0  -  N = UO  x « 111,  92  TABLE U3.  Length o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e  t r a p p i n g f o r Lemmus, summer 1961, Quadrat # 1, and v i c i n i t y .  WINTER GENERATION  LENGTH OF MOVEMENT  MALES  ^5o« 51-100' 101-150' 151-200' 201-300!  301-U00! U01-500! 501-700' 700-900!  1 2 6  2  3  0 1 1 1  N •  17  x - 229.3'  FEMALES  I  x  MALES  SUMMER GENERATION FEMALES  9?  TABLE Ui*  L e n g t h o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e  t r a p p i n g f o r D i c r o s t o n y x , summer  LENGTH OF  I960,  Quadrats #  2 and # 3«  WINTER GENERATION  Y  x  SUMMER YOUNG  MOVEMENT  MALES  ^ 50« 51-100' 101-150' 151-200! 201-300' 301-UOO! U01-500! 501-600!  15 6 1 1 2 2 -  FEMALES  MALES  2  8 3  1  2 2 0 -  -  N =  27  N •  16  N  x -  86  x -  8U  x -  1  -U 62  FEMALES  1  0 1 -  N =  1  2 - 71  9  TABLE U5«  4  Length o f e v e r y movement r e c o r d e d w i t h i n p e r i o d s o f l i v e  trapping f o r Dicrostonyx  summer 1961, Quadrat # 3 and v i c i n i t y .  3  LENGTH OF MOVEMENT  WINTER GENERATION MALES  ^ 50' 51-100'  ioi-i5o«  151-200! 201-300.'  301-UOO!  U01-J00! 501-600!  1 1 5 0 1 0 1 1  FEMALES  Y-L SUMMER  MALES  1 6 2 3 1  0 1  N - 1  N -  10  N - 13  x =  201  x - 112  71  YOUNG  FEMALES  N = 0  95 s i g n i f i c a n t l y g r e a t e r number o f l o n g movements r e c o r d e d i n 1961  1960  ^.05,  (Lemmus w i n t e r g e n e r a t i o n males, P  males,  P<.005;  >.025;  and D i c r o s t o n y x w i n t e r females, P  s u g g e s t i o n i s t h a t the low d e n s i t y of 1961  was  m o b i l i t y o f the a d u l t s a t l e a s t , compared t o were o b t a i n e d i n  Dicrostonyx  < .05,>.025).  i960.  No d a t a on movements  t h e s e r e s u l t s are r e a s o n a b l e , t h e r e are s e v e r a l r e s e r v a t i o n s  s p a c i n g o f the t r a p s was  The p r i m a r y  not i d e n t i c a l i n the two  years.  difficulty In  i n p a r t i c u l a r , l i v e t r a p s were s c a t t e r e d a t i r r e g u l a r i n t e r v a l s o u t s i d e  the quadrat b o u n d a r i e s , l o n g e r movements. and t h i s a r e a had  and t h i s i n c r e a s e d t h e p r o b a b i l i t y of d e t e c t i n g  Furthermore, many o f t h e so few lemmings i n 1961  i960  d a t a come from Quadrat #  t h a t i t was  Observed range l e n g t h s ( S t i c k e l , 195U) a few  The  accompanied by a g r e a t e r  which r e n d e r t h e i r s i g n i f i c a n c e somewhat q u e s t i o n a b l e .  1961,  winter  1959*  Although  i s t h a t the  than i n  not  2  trapped.  c o u l d be e s t i m a t e d f o r o n l y  Lemmus w i n t e r g e n e r a t i o n males w i t h t h e f o l l o w i n g r e s u l t s :  1960  N - U  x - 286  feet  ± 63  feet  ( l SE)  1961  N = 3  x = 631  feet  +170  feet  ( l SE)  These d a t a conform t o the s u g g e s t i o n o f g r e a t e r m o b i l i t y i n t h e summer o f 1961  made above, but a g a i n r e s e r v a t i o n s must be made about t h e i r  Observed range l e n g t h s c o u l d not be  e s t i m a t e d f o r any o t h e r group  significance. except  t h i s because o n l y animals h a v i n g f i v e o r more r e c a p t u r e s w i t h i n one p e r i o d can be  trapping  used.  Whether lemmings occupy a d e f i n i t e t e r r i t o r y o r home range i s not known.  The  g e n e r a l i m p r e s s i o n I have gathered from l i v e t r a p p i n g i s  t h a t the males o f b o t h s p e c i e s are wide r a n g i n g and almost the move.  c o n t i n u a l l y on  Untagged a d u l t males c o n t i n u a l l y appeared on the l i v e t r a p p i n g  a r e a s through  the summer.  T h i s e f f e c t was  particularly striking i n  i960  on  96 t h e Lemmus quadrat  (see Table  7) where l/l+ t o l/3 o f t h e a d u l t s were  i n a d v e r t a n t l y k i l l e d each t r a p p i n g p e r i o d , and y e t t h e a d u l t p o p u l a t i o n on t h e quadrat almost  through  the summer d e c l i n e d a t a v e r y low r a t e .  completely o f f s e t the a r t i f i c i a l m o r t a l i t y .  Net i m m i g r a t i o n  T h i s same o b s e r v a t i o n  a p p l i e s t o a l e s s e r degree t o t h e summer young males and f e m a l e s . a d u l t females  The  o f b o t h s p e c i e s seem t o move around l e s s t h a n t h e males,  but even so t h e y range over r a t h e r l a r g e a r e a s .  Thus a n y complete  study  o f movements under these c o n d i t i o n s must i n v o l v e v e r y l a r g e l i v e t r a p p i n g a r e a s , p o s s i b l y as b i g as 15-20  a c r e s , i n o r d e r t o be c e r t a i n o f r e c o r d i n g  most o f an i n d i v i d u a l ' s movements. Movements o f i n d i v i d u a l s f r o m one week o r month t o t h e next d u r i n g t h e summer w i l l n o t be a n a l y z e d i n d e t a i l because t h e d a t a are t o o fragmentary. A few examples w i l l be g i v e n t o i n d i c a t e t h e s o r t o f movements t h a t can o c c u r .  D i s t a n c e between capture points (feet)  D a t e s o f capture  I960 Lemmus a d u l t male Lemmus Y, female Lemmus Y £ male  1960-61  June 2-July 8 J u l y 7-28 J u l y 23-August 1+  525  ( t h e f o l l o w i n g a r e i960 summer young r e c a p t u r e d a l i v e as a d u l t s i n June 1961)  Lemmus female Lemmus female Lemmus female D i c r o s t o n y x female  The  3700 2600  21+00  Aug.  365  - June  196l  11  2500  "  165  "  s i g n i f i c a n c e o f t h e s e movements i s s i m p l y n o t known.  t h e y may be e x t r e m e l y  i960  u  u M  On the one hand,  abnormal samples b i a s e d toward l o n g movements; on t h e  o t h e r hand, t h e y may r e p r e s e n t t h e normal s o r t o f movements which go on i n these p o p u l a t i o n s . a f t e r having  I am i n c l i n e d t o b e l i e v e more i n t h e l a t t e r  seen movements o f  500-800  alternative  f e e t t a k e p l a c e i n l e s s than  w i t h i n a t r a p p i n g p e r i o d (see T a b l e s 1+3 and 1+5)•  21+ hours  97 Migrations Perhaps t h e one t h i n g most p e o p l e know about lemmings i s t h a t p e r i o d i c a l l y t h e y a l l march down t o t h e s e a and drown t h e m s e l v e s .  Obviously  i f t h i s i s t r u e i t must have a p r o f o u n d e f f e c t on t h e p o p u l a t i o n  dynamics  of t h e lemmings. L o c a l movements o f i n d i v i d u a l animals can be v e r y pronounced a t c e r t a i n t i m e s o f the y e a r .  A t Baker Lake i n the s p r i n g o f  I960  lemmings  began t o appear i n p a r t i c u l a r a r e a s as t h e m e l t - o f f proceeded, as each l o c a l c e n t e r o f d e n s i t y began t o be a f f e c t e d b y t h e snow m e l t i n g . and  Individuals  'groups' o f Lemmus were r e p o r t e d on t h e l a k e i c e i n f r o n t o f t h e s e t t l e m e n t  on May 26, and t h e major a c t i v i t y o c c u r r e d d u r i n g t h e n i g h t hours a l l n i g h t . a t t h i s time o f y e a r ) .  (twilight  From 2 AM t o U AM on June 2 I observed  2$ lemmings moving i n d i v i d u a l l y on the , l a k e : i c e i n f r o n t o f t h e s e t t l e m e n t . F i f t e e n o f t h e s e were caught and t a g g e d ( l D i c r o s t o n y x male; 7 Lemmus males; 7 Lemmus f e m a l e s ) , and a l l were i n b r e e d i n g c o n d i t i o n .  None o f these animals  seemed t o do a n y t h i n g on t h e i c e except move i n a s t r a i g h t l i n e , toward t h e n e a r e s t l a n d , r u n n i n g a t t o p speed. when caught.  usually  A l l were v e r y a g g r e s s i v e  I t was n o t p o s s i b l e t o determine whether t h e lemmings on t h e  i c e came from t h e o p p o s i t e s i d e o f t h e l a k e (3-5 m i l e s ) o r whether t h e y had moved o u t from t h e a r e a o f t h e s e t t l e m e n t onto t h e i c e and t h e n l a t e r moved back a g a i n .  One o f t h e Lemmus males tagged was l a t e r r e c o v e r e d on t h e l i v e  t r a p p i n g a r e a f i v e weeks l a t e r a f t e r h a v i n g moved  3700  feet  (see a b o v e ) .  Most o f t h i s movement on t h e i c e was o v e r b y June u, h a v i n g l a s t e d 9 days.  about  I never saw any e v i d e n c e o f group movements on the i c e , and never  saw even two lemmings moving t o g e t h e r .  An Eskimo brought i n a b u c k e t f u l  o f 70 Lemmus which he k i l l e d on t h e i c e d u r i n g t h e n i g h t o f May 26, b u t whether these r e p r e s e n t e d a r e a l group o r merely a l o t o f i n d i v i d u a l s c o u l d n o t be d e t e r m i n e d .  V e r y few dead lemmings were found on t h e i c e .  98 A p p a r e n t l y these Mr.  S. Lunan, who  was  f o r about 30 y e a r s  s p r i n g movements are not common a t Baker Lake,  manager o f the Hudson Bay  ( u n t i l 193>7) t o l d me  Company p o s t a t Baker Lake  t h a t o n l y once had he  seen lemmings  so abundant t h a t t h e y were common on the i c e i n the s p r i n g . Many o t h e r areas around Baker Lake r e p o r t e d movements o f lemmings on the i c e i n the  I960:  spring of  P o i n t , Aberdeen Lake, and  C h e s t e r f i e l d I n l e t , Rankin I n l e t , Eskimo  S c h u l t z Lake.  These s p r i n g movements are  thus  q u i t e common i n p a r t i c u l a r y e a r s o f h i g h e r t h a n average p e a k ' d e n s i t i e s . Many o f the people see a l i v e lemming.  l i v i n g i n the N o r t h , even the Eskimos,  Thus when s p r i n g movements do o c c u r , t h e r e i s a tendency  t o exaggerate t h e i r s i z e .  A few t e n s of lemmings q u i c k l y become a  hundreds i n the mind, and t o t h e next p e r s o n t h e number i s i n the Another l o c a l movement o f brown lemmings was August I960 b y an Eskimo at the e a s t end  o f Baker Lake.  o f the o b s e r v a t i o n s c o u l d not be e s t a b l i s h e d . o f f a l l movements f r o m the No i n the  rarely  few thousands.  reported i n l a t e The  reliability  There are no o t h e r  records  area.  o t h e r " m i g r a t i o n s " were observed  d u r i n g e i t h e r 1959  or  1961  area. The  g e n e r a l c o n c l u s i o n r e g a r d i n g these  " m i g r a t i o n s " o f lemmings  i s t h a t t h e y assume a m e n t a l s t a t u s d i s p r o p o r t i o n a t e t o t h e i r b i o l o g i c a l s i g n i f i c a n c e f o r t h e lemmings. t h a n the l e g e n d ,  The  a c t u a l events  are f a r l e s s  and not a l l peak p o p u l a t i o n s even show these SUMMARY JND  striking events.  CONCLUSIONS  (1) Whether lemmings occupy a d e f i n i t e t e r r i t o r y o r home range i s not know. (2) There i s a s u g g e s t i o n t h a t the average d i s t a n c e moved between t r a p s was  g r e a t e r d u r i n g the d e c l i n e o f 1961  of I960 i n a d u l t s o f both  species.  compared t o the peak summer  99 (3) I n d i v i d u a l animals can move over l a r g e d i s t a n c e s ; i n s t a n c e s o f animals h a v i n g moved 800 f e e t i n l e s s t h a n one day have been r e c o r d e d . (U)  True group movements ("migrations") are v e r y r a r e l y i f e v e r r e c o r d e d ,  but t h e s p r i n g m e l t - o f f d u r i n g t h e peak y e a r may be accompanied by c o n s i d e r a b l e l o c a l movement o f i n d i v i d u a l s . (0)  There i s no evidence t h a t " m i g r a t i o n s " o r even t h e s e  movements a r e a n e c e s s a r y p a r t o f t h e c y c l e i n numbers.  spring  CHANGES IN EXTRINSIC FACTORS  F a c t o r s which a f f e c t r e p r o d u c t i o n and m o r t a l i t y may be b r o a d l y c l a s s i f i e d as i n t r i n s i c  or extrinsic factors.  E x t r i n s i c factors include  weather, p r e d a t o r s , d i s e a s e , p a r a s i t e s , and f o o d . n o r m a l l y s t u d i e d as d i s t i n c t and independent e f f e c t on t h e p o p u l a t i o n from the o u t s i d e .  These f a c t o r s are  v a r i a b l e s which e x e r t an They t h u s r e p r e s e n t t h e  first  and s i m p l e s t l e v e l o f e n q u i r y i n t o t h e causes o f p o p u l a t i o n d e n s i t y changes, and we must e n q u i r e whether e x t r i n s i c f a c t o r s c a n a d e q u a t e l y e x p l a i n the o b s e r v e d d e n s i t y changes o f lemmings. Weather The w i n t e r o f  1959-60,  when t h e lemmings i n c r e a s e d , began w i t h  a d r y f r e e z e - u p and a q u i c k b u i l d u p o f snow c o v e r .  The w i n t e r o f  1960-61,  when t h e y d e c l i n e d , began w i t h a wet f r e e z e - u p and a slow b u i l d u p o f snow c o v e r u n t i l December. tendency  However, because o f t h e d r i f t i n g o f t h e snow and the  o f lemmings t o seek out t h e more d e e p l y d r i f t e d a r e a s , p r o b a b l y  t h e r e were some areas i n in  1959-60.  1960-61 t h a t  were as f a v o r a b l e weather-wise as areas  Yet no. w i n t e r b r e e d i n g was found i n  1960-61,  which  suggests  t h a t bad w i n t e r weather was not s u f f i c i e n t t o cause t h e observed  absence o f  breeding. One was  o f the most s t r i k i n g f a c t s about t h i s  i t s synchrony  1959-61  o v e r a v e r y l a r g e a r e a o f the c e n t r a l Canadian a r c t i c .  does n o t appear t o be a simple c o i n c i d e n c e .  This  I t was i m p o s s i b l e t o f i n d a  p o p u l a t i o n around Baker Lake which was not a t a peak i n synchrony  lemming c y c l e  i960.  If this  i s more t h a n a mere c o i n c i d e n c e , t h e agent a c t i n g over t h e s e l a r g e  areas would most l i k e l y be weather.  We do n o t know i f good w i n t e r weather  i s the o n l y t h i n g needed f o r an i n c r e a s e i n numbers, o r whether some o t h e r f a c t o r must a l s o be p r e s e n t .  101 Summer weather seemed t o b e a r no r e l a t i o n s h i p t o t h e The  summer of 1959  was  to increase l o c a l l y .  i960  and y e t i n  v e r y wet The  cycle.  and c o l d and y e t t h e p o p u l a t i o n was  beginning  summers of I960 and l°6l were both warm and  t h e p o p u l a t i o n remained a t a peak whereas i n 1961  dry  i t declined.  There are a s u f f i c i e n t number o f c l i m a t i c v a r i a b l e s t h a t i f we i n v e s t i g a t e enough o f them we w i t h t h i s lemming c y c l e . always be  suspect.  s h a l l s u r e l y f i n d one  Because o f t h i s p o s t hoc  o r more c l o s e c o r r e l a t i o n s c l i m a t i c c o r r e l a t i o n s must  O n l y by r e p l i c a t i n g and d i v e r s i f y i n g our  on the a s s o c i a t i o n between t y p e s  observations  of weather and c y c l i c changes can we  t o o b t a i n a b e t t e r i d e a o f i t s r o l e u n t i l e x p e r i m e n t a l work can be To sum  hope  done.  up, f a v o r a b l e d e v i a t i o n s from the average w i n t e r weather  were a s s o c i a t e d w i t h a l a r g e i n c r e a s e i n d e n s i t y , and u n f a v o r a b l e were a s s o c i a t e d w i t h a d e c l i n e i n numbers.  deviations  Summer weather seemed o f  little  importance. Predators A v i a n p r e d a t o r s were not v e r y numerous near Baker Lake compared w i t h the numbers r e p o r t e d f o r n o r t h e r n A l a s k a ( P i t e l k a , Tomich, and  1955)•  Only three l o n g - t a i l e d jaegers ( S t e r c o r a r i u s longicaudus), three  p a r a s i t i c j a e g e r s (S. p a r a s i t i c u s ) , one one  Treichel,  s h o r t - e a r e d owl  rough-legged hawk (Buteo l a g o p u s ) ,  ( A s i o flammeus) were seen i n  1959»  I n I960 the  and  first  j a e g e r appeared on June 7, and b i r d s of p r e y were s t i l l v e r y s c a r c e d u r i n g t h i s summer i n s p i t e of t h e dense lemming p o p u l a t i o n s . t o census t h e s e b i r d s i n I960.  No  attempt was  made  Three p a r a s i t i c j a e g e r n e s t s were found on  the  Main Study A r e a i n I960, and t h i s seemed t o r e p r e s e n t most i f not a l l o f t h e j a e g e r s n e s t i n g on t h i s a r e a . one  snowy owl, two  I n 1961  a v i a n p r e d a t o r s were a g a i n s c a r c e .  l o n g - t a i l e d j a e g e r s , and two  t h e Main Study A r e a j no n e s t s were f o u n d .  p a r a s i t i c j a e g e r s were seen  Only on  L o n g - t a i l e d j a e g e r s were much more  common d u r i n g a l l t h r e e y e a r s on t h e i s l a n d s i n Baker Lake which  support  102 c o n s i d e r a b l e numbers o f n e s t i n g b i r d s . O n l y one mammalian p r e d a t o r was Area —  t h e weasel o r ermine  a t a l l abundant  (Mustela erminea).  such as t h e a r c t i c f o x (Alopex l a g o p u s ) , wolves (Gulo l u s c u s ) , were v i r t u a l l y a b s e n t . none was  on the Main Study  Other l a r g e r p r e d a t o r s , ( C a n i s l u p u s ) , and w o l v e r i n e  Weasels were v e r y s c a r c e i n  195>9  and  seen; t h e y were s t i l l uncommon i n I960 and o n l y two were caught by  the E s k i m o s .  I n 1961 w e a s e l s were v e r y numerous.  One was  caught by an  Eskimo on 1 F e b r u a r y 1961, a n o t h e r on May 17, another on June 29, August 7 on weasels were seen everywhere.  Over 70 specimens were caught by  the end o f August and many more i n e a r l y September. done on 22 o f the August specimens. l e n g t h ) and o n l y one was  and almost a l l were m o d e r a t e l y f a t . empty, 7', b i r d f e a t h e r s and bones, 3j c a r i b o u meat ( ? ) , hi  Complete  Of t h e s e 21 were males  a female (261 mm  and from  t o t a l length).  a u t o p s i e s were  (286—3Ul mm  total  None was b r e e d i n g ,  Stomach c o n t e n t s were c l a s s e d as f o l l o w s : lemming f u r and bones, 2 j f i s h  ( ? ) , 3',  b e r r i e s and p l a n t m a t t e r , 2j u n i d e n t i f i a b l e m a t t e r , 1.  I t i s c l e a r t h a t not a l l t h e s e w e a s e l s c o u l d have l i v e d on lemmings d u r i n g t h e e a r l y summer because o f the v e r y sparse lemming p o p u l a t i o n . which weasels began t o appear commonly (August 7) a l l the young b i r d s were f i n a l l y a b l e t o f l y ,  The date a t  c o i n c i d e d w i t h t h e time when  and t h i s s u g g e s t s t h a t t h e  weasels may have f e d on b i r d s d u r i n g much o f t h e summer. We  must now  see whether t h e s e weasels c o u l d have been  f o r t h e m o r t a l i t y changes o f t h e lemming p o p u l a t i o n .  responsible  I t seems d o u b t f u l whether  w e a s e l s were h a v i n g an important e f f e c t on the lemming p o p u l a t i o n o f the  live  trapping area f o r three reasons:  traps  ( l ) no w e a s e l s were caught i n t h e l i v e  u n t i l August k and s i x w e a s e l s were caught i n t h e s e t r a p s d u r i n g the r e s t o f August. it  I f weasels were p u r s u e i n g lemmings on t h i s a r e a d u r i n g June and  July  seems i n c o n c e i v a b l e t h a t one o r more o f them would n o t have been caught,  s i n c e t h e a r e a was  covered w i t h l i v e t r a p s .  (2) There i s no e v i d e n c e o f h i g h  103 death rates i n the adults during June and J u l y such as would be expected i f weasel predation was common.  (3) The s u r v i v a l of the second l i t t e r of  summer young (August) was r e l a t i v e l y better than that of the f i r s t  litter,  even though the weasels should have exerted more predation pressure on t h i s second l i t t e r . There i s thus no evidence that weasel predation d i d account f o r the observed mortality changes. Disease and Parasites No d e t a i l e d studies on disease or parasites were made i n t h i s research program, but i n the course of autopsying some 2500-3000 lemmings only eight specimens have been found with any gross abnormalities such as cysts i n the l i v e r and spleen.  There was no macroscopic evidence that most  of the animals were not healthy. low and there was no evidence  Parasite loads were s u p e r f i c i a l l y quite  of d e b i l i t a t i o n even i n the few specimens with  considerable numbers of stomach and i n t e s t i n a l p a r a s i t e s . About 50 Dicrostonyx were shipped to Toronto and Ottawa i n August I960.  Most of these specimens died e i t h e r on route or just a f t e r a r r i v a l  i n spite of rapid transport and apparently adequate food and bedding (Fisher, pers. comm.j Manning and Macpherson, pers. comm.).  The question arises  whether these animals died because of a latent disease which could be responsible f o r the d e c l i n e .  There i s no f i e l d evidence to support t h i s view. C e r t a i n l y  there was no spectacular m o r t a l i t y i n e i t h e r Dicrostonyx or Lemmus during August, September, or October I960.  As we have seen previously, the winter  mortality i n Dicrostonyx over 1960-61 was not excessive f o r a population i n which no recruitment was occurring.  We seem to have the alternative of  ascribing most of t h i s winter decline t o an epidemic and assuming a l l other mortality f a c t o r s to be almost n e g l i g i b l e , or of p l a c i n g disease on a par with many other m o r t a l i t y f a c t o r s which comprise the winter mortality.  ioh Furthermore,  even i f we could ascribe a l l t h i s winter mortality to disease,  we would be l e f t without an explanation f o r most of the observed changes In reproduction or mortality described previously. There i s thus l i t t l e evidence that disease or parasites can adequately explain the c y c l i c events. Food Forage production was assessed by clipping the standing crop of green vegetation at the end of each summer (September 1-10) Quadrats, one of which was open and the other enclosed.  on 15 p a i r s of  Each open quadrat  was paired as c l o s e l y as possible with an enclosed quadrat to reduce variation.  sampling  A l l clipped vegetation was dried i n an oven at 225°F to constant  weight and a l l weights given here are dry weights.  The quadrats were 2 sq.  meters i n size and one-fourth of t h i s t o t a l was clipped each year. of quadrats were set out i n 1959  Ten p a i r s  and the other f i v e i n I960. Each enclosed  quadrat was surrounded by 3/8" hardware c l o t h screening which was buried 8-12 i n the ground and extended 2l[-28 tt  u  above ground.  There was no evidence  that any lemmings got inside any of these enclosed quadrats during the period of study.  This general approach was the same as that of Thompson (1955  b).  Table I4.6 gives the standing crop measurements at the end of the 1959,  I960, and 1961  growing seasons on the Main Study Area.  be considered i n two parts. years; quadrats # 11-15 on the 1960-61 changes.  Quadrats # 1-10  These data  may  were present during a l l three  were i n s t a l l e d i n i960 and serve as a further check In the analysis of these data we are interested i n  the differences between the p a i r s of open and enclosed quadrats. Because there were very few lemmings i n 1959 we may adopt the data as our base and relate a l l changes to i t . deviations from t h i s base —  Two major e f f e c t s cause  weather effects and lemming effects — ,  problem i s to separate these.  1959  This was done i n the following way.  and the  *  * 1 am indebted to Dr. Monte Lloyd, Bureau of Animal Population, f o r t h i s s t a t i s t i c a l technique.  105 TABLE 1+6.  S t a n d i n g f o r a g e i n grams p e r 0.5 s q . meter d r y w e i g h t a t  t h e end o f summer.  1 2 3 U 5 6 7 8 9 10 11 12  I960  1959  QUADRAT  ENCLOSED  OPEN  ENCLOSED  OPEN  ENCLOSED  OPEN  H+.6 118.3 36.7  20.0 70.0 29.9  19.5 11*9.8  27.9 90.5 37.1 36.5  1+1+.7  57.8  1+8.9 28.7 28.1 2U.7  37.1 71+.5  1+6.5  31.9  26.0 27.0  1*7.6  73.2 37.3  Hi 15 l-10  " ll-l5 x  51.2 58.8 50.9  31.5  1*3.0 57.0 115.8 57.1 62.2 1*7.1  1+5.8  38.8  31.1  1*3.8  35.1 53.7 86.2 1*0.9  157.2 72.8 100.8 72.3 36.2  105.6  79. h  1+9.5  1+2.0  109.3  51+.9  75.1 97.6  1+9.1  68.3 1+2.3  35.1 32.8  90.1? 53.5  57.1 75.3  55.6 53.2 101.9 63.7 89.5  63.5  1+8.3  77.0  58.5  66.6  72.8  91.U  83.8  91.2  13  x  1961  101+.3 86.1+  93.6  67.1+  83.1 95.5 79.9 93.2  106 The d i f f e r e n c e between each I960 e n c l o s e d quadrat and t h e same quadrat i n 19^9  must be caused o n l y by weather d i f f e r e n c e s .  S i m i l a r l y , the d i f f e r e n c e  between each I960 open quadrat and the same quadrat i n 1959 by the i n t e r a c t i o n of lemming and weather e f f e c t s .  But s i n c e we  weather e f f e c t s alone from the e n c l o s e d q u a d r a t s , we may element t o e s t i m a t e t h e lemming e f f e c t s be independent  and a d d i t i v e ) .  We  caused  know the  subtract t h i s  (we assume these two e f f e c t s t o  can a p p l y t h e t - t e s t t o t h e s e d i f f e r e n c e s  and t h e r e b y t e s t t h e s i g n i f i c a n c e o f these e f f e c t s . be a p p l i e d t o the 1961  must be  The  same procedure  may  data.  The weather e f f e c t s a r e s i g n i f i c a n t between a l l t h r e e y e a r s (P < » 0 l ) , the p r o g r e s s i o n i n t h e s i z e o f t h e s t a n d i n g crop b e i n g  1959 < I960 <196l#  Thus i n terms o f the q u a n t i t y of f o o d , more was  available  a t t h e end o f the summer o f d e c l i n e t h a n e i t h e r the summer o f i n c r e a s e o r the peak summer. 1960  - 1391 1961  The -  1959  r e l a t i v e changes i n s t a n d i n g f o r a g e were:  (P <.05,  >.01).  There i s no d i f f e r e n c e i n t h e lemming e f f e c t  i960  q u a d r a t s # 1-10  between  I96I  quadrats #  on the new  and  11-15  1961,  on b o t h s e t s o f q u a d r a t s . o c c u r i n the absence  1961  lu.5%  in  i960  on Quadrats # The  I f we  i960  and  on  and the lemming e f f e c t shows up i n  as would be e x p e c t e d  (P<.0l).  e f f e c t o f the leirmrings on t h e f o r a g e i s v e r y n e a r l y the same i n  crop by  100j  168.  Lemmings s i g n i f i c a n t l y d e p r e s s e d the s t a n d i n g crop i n b o t h 1961  -  The d e p r e s s i n g  I960  and  1961  s e t t h e o r e t i c a l s t a n d i n g crop a t what would  of lemmings, the lemmings are f o u n d t o d e p r e s s s t a n d i n g and 16.k%  in  1961  on q u a d r a t s # 1-10,  and by lk»l% i n  11-15.  l i m i t a t i o n s o f t h e s e q u a d r a t d a t a must be s t r e s s e d .  q u a d r a t s are not a random sample o f t h e whole a r e a .  They are put  These almost  i n v a r i a b l y i n sedge marsh t y p e , i n the g r e e n e s t , d e n s e s t v e g e t a t i o n where one might e x p e c t h i g h u t i l i z a t i o n from p r i o r knowledge.  As such t h e y are n o t  1 0 7  even a random sample o f sedge marsh, and t h u s the c o n c l u s i o n s f r o m d a t a can s t r i c t l y be a p p l i e d o n l y t o t h e a r e a on which the a c t u a l occur.  such quadrats  A f u r t h e r d i f f i c u l t y a r i s e s f r o m t h e e n c l o s u r e s ' subsequent  of the microclimate o f the quadrat.  We must assume t h a t t h e s e m i c r o c l i m a t i c  changes are n e g l i g i b l e , b u t t h i s may n o t be t r u e . i n t e r p r e t i n g quadrat  (19$5 b ) .  altering  These d i f f i c u l t i e s i n  d a t a do n o t appear t o have been a p p r e c i a t e d b y Thompson  I f we l o c a t e q u a d r a t s i n t h e b e s t h a b i t a t s where maximum u t i l i z a t i o n  i s expected, we s h o u l d n o t be s u r p r i s e d t o f i n d h i g h u t i l i z a t i o n and d e p r e s s e d forage production.  However, w h i l e t h i s does g i v e us an e s t i m a t e o f maximal  e f f e c t s , i t t e l l s us v e r y l i t t l e  about the r e l a t i o n s h i p o f lemmings t o t h e i r  food supply i n general. Forage u t i l i z a t i o n was e s t i m a t e d i n t h e s p r i n g o f 1 9 6 1 by s y s t e m a t i c sampling  along l i n e t r a n s e c t s .  A 3 ' b y 1 ' r e c t a n g l e was dropped  e v e r y t e n f e e t a l o n g these t r a n s e c t s u n t i l t h e l i n e s r a n out o f t h e w e t t e r habitats.  The h a b i t a t was c l a s s i f i e d  a t each s t a t i o n .  A l l the cut grass  and moss was removed from t h e 3 s q . f e e t and a v i s u a l e s t i m a t e was made o f t h e p r o p o r t i o n o f t h e f o r a g e t h a t had been e a t e n .  T r a n s e c t s were done o n l y  i n t h e w e t t e r h a b i t a t s and u t i l i z a t i o n was e s t i m a t e d s e p a r a t e l y f o r mosses, and heaths  on each p l o t .  sedges,  No t r a n s e c t s were done i n t h e d r y h a b i t a t s  because u t i l i z a t i o n was so low as t o be unmeasurable w i t h t h i s t e c h n i q u e . A l l these e s t i m a t e s were made b e f o r e t h e new season's  growth o f p l a n t s had begun,  i . e . when t h e q u a n t i t y and q u a l i t y o f the f o o d s u p p l y was a t i t s lowest p o i n t f o r the y e a r . Table i i 7 g i v e s t h e s e f o r a g e u t i l i z a t i o n e s t i m a t e s made i n t h e spring of the d e c l i n e .  No t r a n s e c t s were made i n e i t h e r 19$9 o r  i 9 6 0  because t o t a l u t i l i z a t i o n was t o o s m a l l t o be c o n v e n i e n t l y measured. d a t a f o r 1 9 6 1 show average habitats.  These  u t i l i z a t i o n o f 30$ a t t h e most f o r t h e w e t t e r  I t was r a t h e r d i f f i c u l t t o e s t i m a t e the moss u t i l i z a t i o n b u t t h i s  108  TABIE I4.7. the line  Estimate o f percentage forage u t i l i z a t i o n i n the s p r i n g o f  d e c l i n e , June 1961,  F i g u r e s r e p r e s e n t a t o t a l o f seven d i f f e r e n t  transects.  HABITAT TYPE  Heath-sedge hummock Sedge hummock Sedge marsh  No. QUADRATS  52 171  53  % WITH RUNWAYS  % WITH WINTER CUTTINGS  % ESTIMATED UTILIZATION SEDGES  MOSS  HEATH  65.3  55.8  15.2  7.2  U.1  86.5  67.8 26.U  25.3  29.3  2.1  39.6  6.0  5.0  -  109 was  attempted  because moss i s a v e r y important f o o d i t e m d u r i n g the w i n t e r .  The p a t t e r n o f f o r a g e u t i l i z a t i o n was 2-6  v e r y s p o t t y i n 1961.  Small areas  f e e t i n diameter would be c o m p l e t e l y d e v a s t a t e d o f a l l l i v e p l a n t s  down t o the r o o t s , and these areas were surrounded by untouched There was  no evidence t h a t t h e b o u n d a r i e s  c o i n c i d e d w i t h packed  vegetation.  of these small f e e d i n g p l a c e s  snow, i c e , o r any v e g e t a t i o n o r t o p o g r a p h i c a l changes.  I n no case d i d these d e v a s t a t e d areas c o a l e s c e over l a r g e a r e a s ; no p l a c e was  more t h a n k-S f e e t from r e l a t i v e l y untouched  t o see how  vegetation.  f o o d s u p p l y c o u l d be s h o r t under t h e s e  It i s difficult  conditions.  The d r y t u n d r a areas were h a r d l y u t i l i z e d a t a l l d u r i n g the w i n t e r o f 1960-61.  S m a l l l o c a l areas were d e v a s t a t e d but on the whole  u t i l i z a t i o n must have been l e s s t h a n 5%,  S i n c e t h e p l a n t s o f the d r y  t u n d r a s grow v e r y s l o w l y (heath r e c o l o n i z a t i o n may any widespread afterward. was  take 50 y e a r s o r more),  d e s t r u c t i o n o f t h i s v e g e t a t i o n would be e v i d e n t f o r decades  The  very l i t t l e  same p o i n t may  be made about dwarf b i r c h and w i l l o w s .  There  g i r d l i n g of these shrubs d u r i n g 1960-61 e i t h e r on t h e Main  Study A r e a or on the o u t l y i n g a r e a s where t h e y are more common. A l l the p r e v i o u s p o i n t s have been concerned w i t h f o o d q u a n t i t y . Food q u a l i t y may  a l s o be i m p o r t a n t .  o f t h e f o o d i n t h i s study.  There was  d i s e a s e s such as o c c u r i n domestic (Maynard and L o o s l i ,  No attempt was  made t o a n a l y z e the  no evidence o f any obvious  quality  deficiency  animals having v i t a m i n or m i n e r a l shortages  1956).  W i t h a l l the d i f f i c u l t i e s i n v o l v e d i n measuring  f o r a g e changes  d i r e c t l y , i t seems e a s i e r t o t u r n the problem u p s i d e down and t o l o o k a t the a n i m a l as a measure o f the adequacy o f the f o o d s u p p l y (Bandy e t a l . ,  1956).  I have used a f a t i n d e x t o measure t h i s , and these d a t a w i l l be p r e s e n t e d i n the n e x t  section.  Thus t h e r e was  no e v i d e n c e o f a q u a n t i t a t i v e shortage o f f o o d o v e r  110 t h i s lemming c y c l e .  Nor was t h e r e any o b v i o u s e v i d e n c e o f a d e f i c i e n c y  d i s e a s e a s s o c i a t e d w i t h changes i n the q u a l i t y o f t h e f o o d . SUMMARY AND CONCLUSIONS (1) F a v o r a b l e w i n t e r weather was a s s o c i a t e d w i t h t h e i n c r e a s e i n numbers, and u n f a v o r a b l e w i n t e r weather was a s s o c i a t e d w i t h t h e d e c l i n e . Summer weather showed no c o r r e l a t i o n w i t h d e n s i t y changes. (2) A v i a n p r e d a t o r s were uncommon throughout appear t o p l a y a n e c e s s a r y r o l e i n i t .  the c y c l e and d i d n o t  The weasel o r ermine was the o n l y  important mammalian p r e d a t o r and t h e s e were n o t common u n t i l 1961, d u r i n g August and September.  However, t h e r e was no e v i d e n c e t h a t  especially weasel  p r e d a t i o n accounted f o r t h e m o r t a l i t y changes observed i n t h e lemmings. (3) There was no e v i d e n c e t h a t d i s e a s e o r p a r a s i t e s p l a y e d any n e c e s s a r y role i n this  cycle.  (1+) Lemmings s i g n i f i c a n t l y reduced t h e s t a n d i n g c r o p o f f o r a g e i n b o t h  I960  and  1961  b y about  1$%,  Forage u t i l i z a t i o n averaged  wetter h a b i t a t s j u s t a f t e r the winter o f s c a r c e l y touched.  1960-61,  30%  o r l e s s i n the  and t h e d r y h a b i t a t s were  There was no evidence o f q u a n t i t a t i v e f o o d s h o r t a g e , n o r  any c l e a r s u g g e s t i o n o f a d e f i c i e n c y i n f o o d q u a l i t y o v e r t h e c y c l e .  CHANGES IN INTRINSIC FACTORS  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 may a l s o r e s u l t from i n f a c t o r s i n t r i n s i c t o t h e p o p u l a t i o n , as w e l l a s i n t h e e x t r i n s i c just discussed.  changes factors  Other animals o f t h e same k i n d may produce b e h a v i o r a l and  p h y s i o l o g i c a l changes i n t h e i n d i v i d u a l organism.  The i n t r i n s i c  factors  are b e h a v i o r and p h y s i o l o g y ; these may be s t u d i e d d i r e c t l y i n themselves o r i n d i r e c t l y by their e f f e c t s .  I n t h i s s e c t i o n we s h a l l a n a l y z e some  changes  which o c c u r over the c y c l e i n t h e f o l l o w i n g p r o p e r t i e s o f i n d i v i d u a l s :  weight  d i s t r i b u t i o n s and. mean body w e i g h t s ; organ w e i g h t s ; f a t i n d e x ; and s o c i a l r e l a t i o n s h i p s. METHODS  Age D e t e r m i n a t i o n Many d i f f e r e n t t e c h n i q u e s f o r measuring c h r o n o l o g i c a l age have been proposed, b u t t h e m a j o r i t y o f s m a l l mammal workers s t i l l use body weight as a c r i t e r i o n o f age ( e . g . C h i t t y , 1952; Hoffmann, 1958).  F r a n k and  Zimmermann (1957) found t h a t t h e body weight - age r e l a t i o n s h i p i n M i c r o t u s a r v a l i s was g r e a t l y a f f e c t e d b y b o t h i n h e r e n t v a r i a b i l i t y and s e a s o n a l changes i n growth.  Body weight i s more a c r i t e r i o n o f p h y s i o l o g i c a l age t h a n  c h r o n o l o g i c a l age, and a s such i t i s more u s e f u l f o r our purposes t h a n c h r o n o l o g i c a l age would be. as a n age i n d i c a t o r  An attempt was made t o use the l e n s o f t h e eye  (Lord, 1959) i n t h i s  study b u t a n a l y s i s showed t h a t  weight was n o r m a l l y p r o p o r t i o n a l t o body w e i g h t .  lens  Whatever caused t h e body  weight t o change a l s o caused t h e l e n s weight t o change, and so no a d d i t i o n a l i n f o r m a t i o n a c c r u e d from w e i g h i n g l e n s e s (one e x c e p t i o n t o t h i s i s d i s c u s s e d below).  Body w e i g h t s were used r a t h e r t h a n t o t a l l e n g t h s because t h e r e i s  much l e s s v a r i a b i l i t y b o t h w i t h i n and between workers when u s i n g measurements t h a n when u s i n g t o t a l l e n g t h measurements.  112 Mean Body Weights F i g u r e 6 gives a g e n e r a l i z e d chronology l i t t e r s and g e n e r a t i o n s  o f both  discussion that follows. phases o f t h e c y c l e . animals The  and c l a s s i f i c a t i o n o f t h e  s p e c i e s o f lemmings and i s n e c e s s a r y f o r t h e  T h i s b a s i c p a t t e r n v a r i e s s l i g h t l y i n the d i f f e r e n t  The w i n t e r g e n e r a t i o n c o n s i s t s e i t h e r o f o v e r w i n t e r e d  (1959 ? and lp6l) o r o f animals  born d u r i n g t h e f a l l  and w i n t e r  s p r i n g g e n e r a t i o n appears each y e a r b e f o r e t h e snow m e l t s b u t i s n o t v e r y  l a r g e n u m e r i c a l l y ; i t e s s e n t i a l l y behaves l i k e t h e w i n t e r g e n e r a t i o n the  (i960).  summer.  during  The summer l i t t e r s f o l l o w i n r a p i d sequence; i t i s p r o b a b l e  some a d u l t females produce o n l y two l i t t e r s and o t h e r s f o u r l i t t e r s ,  and t h i s  may v a r y w i t h the c y c l i c phase, b u t t h e g e n e r a l p a t t e r n i s about t h r e e l i t t e r s p e r adult female.  By f a l l  summer  o n l y summer b o r n young a r e l e f t and these  f o r m next y e a r ' s w i n t e r g e n e r a t i o n . their first  that  Summer young females may breed i n  summer and add a f u r t h e r g e n e r a t i o n t o t h e f a l l p o p u l a t i o n , b u t  t h i s c o m p l i c a t i o n has been l e f t  out o f t h i s diagram.  I n computing mean body w e i g h t s we would l i k e t o f o l l o w d i s c r e t e generations  so t h a t t h e r e s u l t i n g means have a c l e a r b i o l o g i c a l  significance,  r a t h e r t h a n b e i n g a mere s t a t i s t i c a l c o l l e c t i o n o f d a t a from d i v e r s e groups o f animals.  There i s no problem i n s e p a r a t i n g summer-born animals  from w i n t e r o r  s p r i n g a n i m a l s , but t h e d i f f i c u l t y a r i s e s i n t r y i n g t o keep t h e s p r i n g g e n e r a t i o n (born A p r i l - M a y )  separate from t h e w i n t e r g e n e r a t i o n .  arose o n l y f o r t h e May 16-31 and June  1-15, I960  I n Lemmus t h i s  samples.  Since  difficulty  breeding  o c c u r r e d throughout t h i s w i n t e r i t was somewhat a r b i t r a r y t o d i s t i n g u i s h a w i n t e r g e n e r a t i o n and a s p r i n g g e n e r a t i o n , b u t t h i s was done f o r t h e above two  samples on t h e b a s i s o f b r e e d i n g v s . non-breeding animals, t h e b r e e d i n g  animals in  b e i n g r e f e r r e d t o as t h e w i n t e r g e n e r a t i o n .  These s p r i n g  animals  Lemmus a r e absorbed i n t o t h e r e s t o f t h e w i n t e r g e n e r a t i o n a d u l t s b y t h e  end o f June and cannot be r e c o g n i z e d as a d i s t i n c t element o f the samples a f t e r then.  115  FIGURE 6.  G e n e r a l i z e d annual c h r o n o l o g y o f g e n e r a t i o n s and l i t t e r s f o r  Lemmus and D i c r o s t o n y x .  T — I — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — r  i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r JAN.  MAR.  MAY  JUL.  SEPT.  NOV.  JAN.  MAR.  MAY  Ilk I n D i c r o s t o n y x the problem i s much more d i f f i c u l t .  The  spring  g e n e r a t i o n appears i n a l l t h r e e y e a r s and p e r s i s t s as. a d i s t i n c t e n t i t y even i n t o August.  D a t a on body weight, t o t a l l e n g t h , l e n s w e i g h t , and r e p r o d u c t i v e  c o n d i t i o n were u t i l i z e d i n t r y i n g t o s e p a r a t e the w i n t e r from the s p r i n g born animals.  I n 1961  t h e two groups were e a s i e s t t o d i s t i n g u i s h because  a l t h o u g h t h e body weights o v e r l a p p e d t h e r e was  a gap i n t h e l e n s w e i g h t s  between the w i n t e r g e n e r a t i o n ( b o r n i n the summer o f I960) and the s p r i n g generation  (born A p r i l - M a y 1961).  animals had l e n s w e i g h t s over 6.0 l e s s than l+.O mg. as break p o i n t s .  In  i960  I n 1959  F o r example,  i n t h e June 1-15  sample w i n t e r  mg w h i l e s p r i n g a n i m a l s had l e n s w e i g h t s o f  gaps i n t h e body weight d i s t r i b u t i o n s were u t i l i z e d  gaps i n body weight and t o t a l l e n g t h d i s t r i b u t i o n s  were m a i n l y u t i l i z e d f o r s e p a r a t i n g these groups.  While t h e r e i s a c o n s i d e r a b l e  s u b j e c t i v e element i n v o l v e d i n t h e s e s e p a r a t i o n s ( p a r t i c u l a r l y f o r 1959)  I  b e l i e v e the r e s u l t s a r e b i o l o g i c a l l y more m e a n i n g f u l t h a n t h e y would be i f t h e s e two groups were mixed. Organ  Weights A l l organs were p r e s e r v e d i n 10%  n e u t r a l f o r m a l i n and weighed i n  the w i n t e r a f t e r c o l l e c t i o n e i t h e r on an e l e c t r i c b a l a n c e o r on a t o r s i o n balance.  Organs w e i g h i n g more t h a n 200  organs w e i g h i n g l e s s t h a n 200  mg were weighed t o the n e a r e s t 5  mg were u s u a l l y weighed  t o the n e a r e s t 0.1  mgj mg.  A l l organs were c l e a n e d o f s u r f a c e f a t under a b i n o c u l a r microscope and r o l l e d d r y on f i l t e r paper b e f o r e w e i g h i n g . a c c u r a c y o f ± 3% i n normal w e i g h i n g s . f r e s h i n the f i e l d d u r i n g  Repeated w e i g h i n g s i n d i c a t e d  an  Some o f the l a r g e r organs x^ere weighed  1961.  The e x p r e s s i o n o f organ w e i g h t s n o r m a l l y used by p h y s i o l o g i s t s as w e l l as most e c o l o g i s t s i s t h a t o f m i l l i g r a m s o f organ weight p e r gram o f body w e i g h t .  However, few workers have heeded the a d v i c e o f C h e s t e r Jones  (1957,  t h a t such f i g u r e s ' m a y be m i s l e a d i n g when d i f f e r e n t body weight  p  6-7)  115 groups a r e compared.  There a r e o n l y two c i r c u m s t a n c e s under which t h e above  e x p r e s s i o n may be used v a l i d l y : s i m i l a r body w e i g h t s ; o r (2)  ( l ) i f a l l t h e a n i m a l s compared a r e o f v e r y  i f t h e a r i t h m e t i c r e g r e s s i o n o f organ weight on  body weight i s l i n e a r and p a s s e s t h r o u g h t h e o r i g i n .  I know o f no i n s t a n c e  i n which t h e l a t t e r i s t r u e , and t h e former i s n o t t r u e i n t h i s s t u d y . The problem, however, s t i l l remains o f c o r r e c t i n g f o r d i f f e r e n c e s i n body weight and o b t a i n i n g a measurement o f organ weight which i s independent o f the p a r t i c u l a r body w e i g h t s i n t h e sample. by C h i t t y  (l?6l)  T h i s d i f f i c u l t y was overcome  b y u s i n g s t a n d a r d i z e d means ( H i l l ,  c a l c u l a t e d as f o l l o w s .  1959)*  These means a r e  A l l t h e d a t a are grouped and mean organ w e i g h t s f o r  each 10 g weight c l a s s were determined, as w e l l as a grand mean f o r t h e whole data.  The s t a n d a r d i z e d mean i s t h e n o b t a i n e d b y t h e f o r m u l a :  S = 0/E X where  G 0 E S  = « = =  G  grand mean o f t h e whole d a t a observed sample mean e x p e c t e d sample mean s t a n d a r d i z e d mean f o r t h e sample  The o b s e r v e d and expected sample means are c a l c u l a t e d i n the same way as i n c h i - s q u a r e problems.  One d i f f i c u l t y o f u s i n g s t a n d a r d i z e d means i s t h a t  c o n f i d e n c e l i m i t s cannot be p l a c e d on them and no s i g n i f i c a n c e t e s t s may be applied. The t e c h n i q u e used by C h r i s t i a n and D a v i s  (1956)  and a p p a r e n t l y i n  a l l o f C h r i s t i a n ' s work i s somewhat s i m i l a r t o t h e s t a n d a r d i z e d mean method but t h e f i n a l r e s u l t s a r e e x p r e s s e d i n p e r c e n t a g e s ( i . e . by s u b s t i t u t i n g  X 100 f o r X G i n the above e q u a t i o n t h e r e s u l t s would be e x p r e s s e d i n p e r c e n t a g e s ) The d i f f e r e n c e i s t h a t he does n o t weight t h e means o f t h e component body w e i g h t groups i n r e l a t i o n t o t h e i r sample  s i z e , i . e . a weight group w i t h o n l y  one a n i m a l i n i t c o n t r i b u t e s as much t o the mean a s a weight group w i t h 25  116 animals i n i t .  These means a r e t h u s l e s s r e l i a b l e t h a n t r u e s t a n d a r d i z e d  means which a r e used i n t h i s  study. RESULTS  Body Weight D i s t r i b u t i o n s Much i n f o r m a t i o n c a n be l e a r n e d from age or weight (Bodenheimer, 1938j L e s l i e and Ranson, 19u0). weight  distributions  T a b l e s 1+8 and 1+9 g i v e t h e  d i s t r i b u t i o n s f o r Lemmus and D i c r o s t o n y x males d u r i n g 1959-61 on t h e  Main Study A r e a .  The d a t a f o r t h e females  are v e r y s i m i l a r t o t h a t f o r t h e males.  a r e n o t g i v e n here because  A l l t h e weight  were o b t a i n e d from the snap t r a p p i n g samplesj weight  d a t a d i s c u s s e d here  d a t a from l i v e t r a p p i n g  are n o t p r e s e n t e d b u t t h e y show t h e same changes d e s c r i b e d h e r e . must be t a k e n i n comparing weight  they  Some c a r e  d i s t r i b u t i o n s between t h e y e a r s because  1959 was b i o l o g i c a l l y 2-3 weeks behind I960, and 1961 was about 1 week b e h i n d I960. S e v e r a l p o i n t s are shown b y t h e s e d a t a . 1960 was c h a r a c t e r i z e d b y h i g h e r a d u l t body weights  F i r s t , t h e peak summer o f t h a n e i t h e r 1959 o r 1961.  There were v e r y few Lemmus above 76 g i n e i t h e r 1959 o r 1961, m a j o r i t y o f t h e a d u l t s were above t h i s weight. between  b u t i n i960 a  In Dicrostonyx the difference  I960 and 1961 was n o t so w e l l marked b u t t h e same tendency was shown.  Second, i f we c o n s i d e r t h e w i n t e r d a t a , Lemmus d i d n o t appear t o i n c r e a s e i n weight  through t h e w i n t e r whereas t h e D i c r o s t o n y x weight  suggest t h a t t h e y d i d i n c r e a s e i n weight  distributions  a t l e a s t slowly during the w i n t e r .  A sudden s p u r t o f growth seems t o occur i n May f o r each s p e c i e s . was  a gap i n t h e 1961 summer weight  young s h o u l d be. Dicrostonyx.  d i s t r i b u t i o n s where t h e e a r l y summer  A g a i n t h i s was shown more c l e a r l y b y Lemmus t h a n b y  The l e f t p a r t o f F i g u r e 7 shows g r a p h i c a l l y the l a t e  Lemmus male weight  Third, there  distributions for  July  i960 and I 9 6 I on the Main Study A r e a  and i l l u s t r a t e s two o f these p o i n t s , i . e . t h e h i g h e r body weights  of  i960  TABLE 1+8.  summer g e n e r a t i o n from w i n t e r and s p r i n g  WEIGHT CLASS (grams)  111621-  JAN.  FEB. MAR.  APR.  MAY  MAY  JUNE  JUNE  JULY  JULY  AUG.  ••*•  1959  — '•  3 1 3 3  X  3  X  2  1 3 2 3  51566166-  NOV.. DEC.  1 1 1 1  2 1 1 1 1  N  mm  2  X  mm  X  1 3  7681-  ->  1 2 1 1  2 2  ^  71-  X  mm  1 W  1960  16212631-  1 -  1+651566166-  1  *  81869196101106-  1961  21-  5  mm  2 1 1  6  1 1 1  — **  1 1  76-  81-  869196101-  I T 12~  T  —a^c:  1 1 1 3 5 It  2 1 1  3 6 1+ 1 1 2  6T  5T  3  1  V  _ _  __  T~  7 2 3 11 35  lit  m  *  5 3 6  9 N 1 - \2 v  _  —  1 3 3  W  1 1  -  2 3 2  5 1+  W  x—X — X,  W  1 1 1+ 2 1 2  N  ii~  1~  iaL  11 5  X,  2 6 6 3  1 2 3  T  x  •M  *•»  X  2 N  ^9" 29"  1 1 2 3 X  1  -  3 1  X X _x  1  2  —  1 1  7 1  _  —  3 2 1+ 2 2 2 2 3 1  t  3-, 1  IOT  —  X  X  2 3  1  1  X  _  1 1  X  —  mm  1 2 1  v  -1  v  9 2 3  v  _  1  7 18  \  25 9 9 1+ 1  1  1  5  3 6 5 10 10 5 7  -1  16-  W  k  11 11 22 20 29 1+2 2?  297  3 6 6 2 1 1  T  7 21 11 7 10  1 1  Ul-  361+11+651566166-  OCT.  r  X  ui-  SEPT.  \  X  1+6-  2631-  AUG.  1-15 16-31 1-15 16-30 1-15 16-31 1-15 16-31  36-  11-  Dotted l i n e s separate  generations.  2631-  li-  1959-61.  Body weight d i s t r i b u t i o n s f o r Lemmus males on t h e Main Study A r e a ,  X  X  X  X,  IT  17  3T  TABLE 49.  Body weight d i s t r i b u t i o n s f o r D i c r o s t o n y x males on t h e Main Study A r e a , 1959-61.  s e p a r a t e summer g e n e r a t i o n f r o m w i n t e r and s p r i n g  generations.  WEIGHT JAN. FEB. MAR. CLASS (grams)  JUNE 16-30  11-  APR.  MAY 1-15  MAY JUNE 16-31 1-15  JULY 1-15  JULY 16-31  AUG. 1-15  AUG. 16-31  SEPT.  Dotted  OCT.  NOV.  lines  DEC,  1959 3  16-  •s.  21-  1 1 _3_ 1 1  263136^.  Ul46-  2.  _3_  1  2  2 2 1  3  5156-  1 2 1  1  1  61667110  I960  1 5 2  1116212631-  hi-  1  l  -  U651566166717681869196101106-  -  k  -  1  3  —  2  1 1  3  3 1  —  -  -  1 1  1 2 1 2  _  3 3 1 1 3 2 2 mm  -  v  \  1  \ x-  1  -  - \ 2  _  2  2 2 1  1 1 2  -  2  --  3  3  -  1 1 1  3 1 2  2  2 2 6  1 N—  \ —- X\  -  -  1 2  i  3  -  2  2  -  1 1  1 2  l  I  2 2 3  •BB  IM>  mm-s-  1  -  -  1  M  1  k  1  — —*  -  1 1  2  1  1  T 11162126-  U  1 1 1 2 2 2  20"  1961  1 1 2  71-  1  2T  1  1  -  31-  36hl4651566166-  IB"  1  1 1 1  2  -  1 1 1 1 1  —  -  1  _  _  2  76-  _  81-  1  869196-  T  T  T  T  rr  N  2  1* 6  2  3 7 2 6 10 7 8 2  U 2 1 1  1 1 _  -  X.  ~9~  w  1 1 1  5 mm  6  1 2  u  1 2 4 2 1 2  1 3  —  1  —  1 1 mm  2 2  1  1  1  _  2  v  1  l -  W  2 7 2 2 1  v  —  _  3 3  1  _  2 1  58" I T  TT  20"  w  T  "8~  119  FIGURE 7.  I960  and  Body weight d i s t r i b u t i o n s f o r Lemmus males, J u l y 16-31,  1961.  Winter, spring,  and summer i n d i c a t e  generations.  i ooH  to  WINTER AND SPRING  80H  WINTER  <  WINTER  6 0 H  I  o >  40H  SPRING SUMMER  o SUMMER  20H (  o-  ALL  _20%_  |  N= IO  N =8  N= 40  —I  T  DATA  FROM JULY 1 6 - 3 1  MAIN  STUDY AREA  1961  1961  I960 MAIN TYPE  STUDY G  AREA  DECLINE  SECOND TYPE  ISLAND H  DECLINE  120 and t h e m i s s i n g summer young o f 1961 f o r t h e Main Study A r e a . I t i s i n s t r u c t i v e t o compare t h e body weight d i s t r i b u t i o n s f o r Aberdeen Lake d u r i n g I960 and 1961 w i t h those o f t h e Main Study A r e a .  Tables  $0 and $1 g i v e t h e d a t a f o r Lemmus and D i c r o s t o n y x males a t Aberdeen Lake. The f i r s t p o i n t t o n o t i c e about these d a t a i s t h a t t h e r e i s v e r y  little  d i f f e r e n c e between t h e I960 and 1961 d i s t r i b u t i o n s i n e i t h e r s p e c i e s , c o n t r a r y t o t h e r e s u l t f o u n d on t h e Main Study A r e a .  High body w e i g h t s  are f o u n d i n  b o t h y e a r s and t h e r e does n o t seem t o be a m i s s i n g group o f summer young i n 1961.  T h i s d i f f e r e n c e between Aberdeen Lake and the Main Study A r e a does n o t  appear u n t i l 1961, as t h e I960 d i s t r i b u t i o n s on t h e two areas are v e r y s i m i l a r . W i t h these two d i f f e r i n g p a t t e r n s i n mind l e t us l o o k a t t h e w e i g h t d i s t r i b u t i o n s found on the o t h e r o u t l y i n g a r e a s i n 1961. are g i v e n i n Table $2.  These d a t a  O n l y d a t a f o r Lemmus males are g i v e n ; D i c r o s t o n y x  i s v e r y sparse on a l l these t r a p p i n g a r e a s .  New Lake, Lower TheIon R i v e r ,  Ten M i l e I s l a n d , and the P r i n c e R i v e r were sampled i n I960 a l s o , b u t these d a t a are n o t g i v e n here because t h e y a r e v i r t u a l l y i d e n t i c a l w i t h t h a t p r e v i o u s l y g i v e n f o r t h e Main Study A r e a i n I960.  These 1961 d a t a are based  on s m a l l numbers o f a n i m a l s , b u t i f we compare t h e s e samples w i t h t h e c o r r e s p o n d i n g ones from t h e Main Study A r e a we f i n d some s t r i k i n g  differences.  New Lake, Long I s l a n d , Second I s l a n d , Ten M i l e I s l a n d , and Nine M i l e I s l a n d shoxir the weight d i s t r i b u t i o n p a t t e r n f o u n d a t Aberdeen Lake and n o t t h a t found on the Main S t u d y A r e a .  The P r i n c e R i v e r shows t h e Main Study A r e a p a t t e r n .  These two d i f f e r e n t p a t t e r n s are shown g r a p h i c a l l y i n t h e r i g h t p a r t o f F i g u r e 7« We can summarize t h e s e r e l a t i o n s h i p s i n t h e f o l l o w i n g way:  121 TABLE 50.  Body weight d i s t r i b u t i o n s f o r Lemmus males a t Aberdeen Lake,  1960-61.  WEIGHT. CLASS  1961  I960  MAY 27JUNE 2  JULY 10-18  JUNE  15-16  1116212631-  N V  JUNE  1-5  JUNE 22- JULY J u l y 10 26-29  2 7  1 6 1 \—  \  —v-  \  iaU6515661-  \  1 2 1  66-  1  81869196101106-  2 1  3  1 1  2 2 ~~2~ 3 2 1  1 1  1 1  ~2~  1 2  2  T  SB-  2T"  122  TABLE J?l. Body weight d i s t r i b u t i o n s f o r D i c r o s t o n y x males a t Aberdeen Lake, 1960-61. and  spring  D o t t e d l i n e s separate  summer g e n e r a t i o n f r o m w i n t e r  generations.  I960  WEIGHT  1961  CLASS MAY 2 7 JUNE 2  -  JUNE  JULY  1 5 - 1 6  1 0 - 1 8  MAY 28JUNE 7  JUNE 13-22  16-  v  N  1  2 6 -  31-  Ul5 1 5 6 -  3 1  -  2  aw  2  _  1  1  mm  2  MB  5  _  i  3  mm  2  2  mm  6 1 -  1  -  7 6 -  2  1  2  81-  mm  —  1  91-  1  3  8 6 -  1  9 6 -  mm  6671-  wm  1 0 1 -  _  1 0 6 -  1  1 1 1 -  1  9  1  -  2 1  s —  2  N — N  wm  36U 6 -  JULY 2 6 - 2 9  1  1 1 -  2 1 -  JULY 10-19  1  mm N  x  -  1  3 mm  2  1  1  2  1  1  — N _ N  -—  2  1 1 2  \2 3 2 1  mm  2  2  2 1  1  1  2  2 1 2 2  1  1  5  2 9  7  1 1  8  26  v  125 TABLE 52.  Body weight d i s t r i b u t i o n s f o r Lemmus males on t h e o u t l y i n g  a r e a s , summer 1961. D o t t e d l i n e s separate and  spring  WEIGHT CLASS  summer g e n e r a t i o n from w i n t e r  generations. NEW LAKE J u l y U-12  LONG ISLAND J u l y 17-20  SECOND ISLAND J u l y 21+-27  1116212631-  1  U  26= Ul46-  51-  56616613768186-  2 mm  1 1  U  1 2  3  "cT WEIGHT CLASS  1116212631i6r UlU6-  LOWER THELON R. August lU-19  10" TEN MILE I S . Aug. lU-19  NINE MILE I S . Aug. 14-19  PRDJCE R. Aug. 14-17  1  3 h  k 4 1  1 1 2 2  51-  56616671= 76818691-  3 1  11  12U 1959 Weights Ad. d*  1. 2. 3.  u.  5.  6.  7. 8. 9.  +  ( * D i c r o s t o n y x and Lemmus.  I n the  summer of d e c l i n e those  Weights Ad. ci*  Recruit. Y young  Low  Main Study A r e a * Aberdeen Lake * New Lake TheIon R i v e r Ten M i l e I s . Prince River Nine M i l e I s . Long I s l a n d Second I s l a n d  1961  I960  High High High High High High  R e c r u i t Weights R e c r u i t . Y^ young Ad. cr* Y^ young  + +  a r e a s which show r e c r u i t m e n t are  r e c r u i t m e n t o f these e a r l y young are undergoing r e a c h two  *  + ?  -  -  + +  +  Others r e f e r t o Lemmus o n l y . )  a Type H d e c l i n e ( s l i g h t r e c o v e r y ) by d e f i n i t i o n , and t h o s e  Thus we  -  Low High High ? High Low High High High  + + +  undergoing  areas showing no  a Type G d e c l i n e (no r e c o v e r y ) .  c o n c l u s i o n s which apply t o both s p e c i e s :  ( l ) t h a t Type  H d e c l i n e s were a s s o c i a t e d w i t h h i g h body w e i g h t s and Type G d e c l i n e s w i t h low body weights;  and  (2) t h a t the a d u l t body weight change was  w i t h p o p u l a t i o n phenomena and was cycle.  I t i s c l e a r from t h e 1959  associated  not s i m p l y a s i d e e f f e c t i r r e l e v a n t t o the d a t a t h a t low body w e i g h t s p e r se are  not  s u f f i c i e n t t o cause a l a c k of r e c r u i t m e n t o f young, but t h a t something e l s e must a l s o be  necessary.  I t i s p e r t i n e n t t o e n q u i r e what d i f f e r e n c e s t h e r e are between t h e a r e a s showing no r e c o v e r y of numbers i n 1961 recovery.  There i s no apparent  and the areas showing some  r e l a t i o n s h i p w i t h e i t h e r t h e q u a l i t y of t h e  h a b i t a t o r the p o p u l a t i o n d e n s i t y o f the a r e a i n I960.  This i s i l l u s t r a t e d  i n the f o l l o w i n g t a b l e : Type o f Vegetation TheIon R i v e r New Lake Main S t u d y A r e a Ten M i l e I s .  thick thick sparse sparse  Density i n I960  Type of Decline  very very mod. mod.  no r e c o v e r y some r e c o v e r y no r e c o v e r y some r e c o v e r y  high high high high  125 I t i a a l s o c l e a r t h a t weather cannot be t h e o n l y cause because o p p o s i t e e f f e c t s were found w i t h i n 1-2 m i l e s o f each o t h e r . out o t h e r e x t r i n s i c  While we cannot r u l e  e f f e c t s s u c h as d i s e a s e , t h i s d i f f e r e n c e i n t h e q u a l i t y  o f t h e i n d i v i d u a l s as measured by body weight may be caused by d i f f e r e n c e s i n the i n t r i n s i o  f a c t o r s o f the various populations  absolute density. suggestion.  independent o f t h e  There i s no i n f o r m a t i o n from t h i s study t o t e s t  this  F i n a l l y , a l l t h e f o u r i s l a n d p o p u l a t i o n s sampled were undergoing  Type H d e c l i n e s .  The s i g n i f i c a n c e o f t h i s i s n o t understood.  To sum up t h e r e s u l t s o f a n a l y z i n g body weight d i s t r i b u t i o n s , we have seen t h a t t h e peak summer was c h a r a c t e r i z e d by a d u l t s o f h i g h body weight, and t h a t two p a t t e r n s appeared i n t h e d e c l i n e * ( l ) low body w e i g h t s and no recruitment o f  young, i n Type G d e c l i n e s ; and (2)  recruitment o f  young, i n Type H d e c l i n e s .  h i g h body weights and  Mean Body Weights We may q u a n t i f y the o b s e r v a t i o n t h a t h i g h body weights were a s s o c i a t e d w i t h t h e peak summer and Type H d e c l i n e s by computing mean body T a b l e s 55 and 54 g i v e t h e mean body weights f o r t h e  weights f o r t h e a d u l t s .  w i n t e r and s p r i n g g e n e r a t i o n s o f Lemmus and D i c r o s t o n y x males f o r We a r e m a i n l y concerned here w i t h t h e w i n t e r The  Lemmus d a t a ( T a b l e 55)  the peak summer o f and  significant.  generation.  are very c l e a r .  I960 showed mean body weights about  20% g r e a t e r t h a n 196*1.  On t h e Main Study Area 28% g r e a t e r than  1959  I n both c a s e s t h e d i f f e r e n c e s a r e c l e a r l y  F o r Aberdeen Lake the  i960 and I96I d a t a a r e 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 , h i g h body weights o c c u r r i n g i n b o t h y e a r s . in  1959-61.  The o t h e r areas  sampled  I96I a l l have h i g h mean body weights except f o r t h e P r i n c e R i v e r . The  Dicrostonyx  Area t h e peak summer o f  d a t a (Table 54)  a r e not so c l e a r .  On t h e M a i n Study  i960 showed mean body weights about 4o$ g r e a t e r  than 1959 and 11% g r e a t e r t h a n I96I.  The q u e s t i o n a r i s e s whether t h e l a t t e r  figure i s statistically significant.  A non-parametric r a n k i n g  test  126 TABLE 53.  Mean body w e i g h t s f o r Lemmus males o f t h e w i n t e r and s p r i n g  g e n e r a t i o n s , summers  LOCATION AND TIME PERIOD  1959-61. SPRING GENERATION  WINTER GENERATION N  WEIGHT  10  50.21  S.E.  N  WEIGHT  S.E.  ±1.83 ±3.07  5  31.38  ±1.00  Ml  mm  mm  1  49.40  56 8  22.23 36.00  2 1  19.U0 19.00  Main Study A r e a 1959 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  I960  May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  1961 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  a a  3  2U1 57 35 15 10 1U 7 2 22 11 2 7  56.60 mm  69.10  ±4.40 *7.7U  66.92  ±0.95 ±1.83  71.30  67.20 79.33 81.77 83.79 91.06 82.61  76.60 62.58 61.56 67.80 62.8a  ±2.52 ±4.53  ±2.50  ±2.20 ±3.30  +1.80 ±3.28 ±2.10 ±1.30  ±3.50  Other A r e a s I960 Aberdeen Lake May 29-June 2 June 15-16 J u l y 10-18  7 3 20  1961 Aberdeen Lake June 1-5 2 June 22-July 10 4 J u l y 26-29 8 New Lake J u l y 4-12  8  66.9U 66.4O  75.83  ±0.88 ±3.51  +7.56  +4.57  +4.00  70.45 85.17 79.80  ±1.35 ±4.78 ±2.96  75.99  ±3.14  ±4.80  127  TABLE 53.  (continued)  LOCATION JSND TIME PERIOD  WINTER GENERATION N  O t h e r Areas  Lemmus male mean body w e i g h t s .  WEIGHT  SPRING GENERATION  S.E.  N  WEIGHT  mm  mm  mm  mm  S.E.  (cont'd)  1961 Long I s l a n d J u l y 17-20 Second I s l a n d  J u l y 2U-27  Lower The Ion R. Aug. lU-19 Ten M i l e I s . Aug. 14-19 Nine M i l e I s . Aug. 1U-19 Prince River Aug. 14-17  6  79.97  ±3.14  5  8I.O4  ±1.52  mm  mm  2  75.75  1  84.4O  -  h  65.03  •1.1*5  -  ±5.74  -  " -  -  -  128 TABLE 51*.  Mean body w e i g h t s f o r D i c r o s t o n y x males o f t h e w i n t e r and  s p r i n g g e n e r a t i o n s , summers  LOCATION AND TIME PERIOD  1959-61.  WINTER GENERATION N  WEIGHT  S.E.  5 2 2 1  50.00 51.70 52.95 51.1*0  +2.22 ±2.60 +0.1+5  10  77.50 71.02 67.68 67.58 7U.95 70.73 106.00  tii.ll  SPRING GENERATION WEIGHT  S.E.  7  1* k 1  33.60 37.10 la. 80 U2.00  +1.98 ±1.68 ±3.00  8 8 1 1*  18.63 29.72 39.20 1+5.20  1*  33.22  N  M a i n Study A r e a  1959  June 15-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  I960  May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  17  8  12 2 3  1  1961 May 1-15 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  3  5  36 12 8  1* 5  -  59.77  66.1+0 67.33  70.32  61.71  62.95  63.1+6 mm  ±3.79 ±3.72 ±3.1*9 ±0.75  ±1.13  ±2.56 •2.08  +I*.l6  ±3.57 +6.78 +1.58 ±3.60 +2.95 ±2.70 ±2.59  39.28  1*5.95 53.03  ±6.95 +2.16 ±3.90 +2.62 ±0.1*5 ±6.25  3  12  23.10 50.77 l*i*.l*7  +1.1*9 +1.55  2 2 2 10  32.00 38.85 1+8.55 1*9.53  ±9 .60 +8.35 +1.05 ±2.1*6  32 7 9 2 3  35.9U 1+0.3U  Aberdeen Lake  I960  May 27-June 2 June 15-16 July 10-18  1961  May 28-June 2 June 13-22 July 10-19 July 26-29  8  2 12 5 9  5 10  81+.70  81.75 76.90  ±7.71 ±1*.85 +2.9U  75.51*  ±6.11+  75.58 70.76 80.90  +5.53 ±3.60 ±3.28  1  129 (Snedecor,  1956,  p 115)  was a p p l i e d t o t h e i 9 6 0 v s . 1961 d a t a f o r t h e  w i n t e r g e n e r a t i o n and gave a s i g n i f i c a n t d i f f e r e n c e F o r Aberdeen Lake t h e i 9 6 0 and 1961  (P<.05,  >.0l).  d a t a are n o t s i g n i f i c a n t l y  different,  as i n Lemmus. Growth o f a d u l t animals i s m o s t l y complete by e a r l y June i n both species.  I n d i v i d u a l a d u l t s c a p t u r e d i n t h e l i v e t r a p p i n g program from  June t o August show growth r a t e s a v e r a g i n g about 0.2% p e r day i n both i 9 6 0 and 1961  f o r both s p e c i e s .  I t t h u s appears t h a t t h e c r i t i c a l growth  p e r i o d f o r t h e a d u l t s i s A p r i l and May, before t h e snow m e l t s o r the new season's p l a n t growth b e g i n s . I n summary, an a n a l y s i s o f mean body weights f o r t h e w i n t e r g e n e r a t i o n c o n f i r m s q u a n t i t a t i v e l y t h e p r i o r o b s e r v a t i o n t h a t h i g h body weights  (x « 70-85 g) were found i n the peak summer on a l l a r e a s and i n  Type H d e c l i n e s i n 1 9 6 l .  Lower mean body weights  a t t h e o t h e r times f o r both s p e c i e s .  (x = 5 0 - 6 5 g ) p r e v a i l  Most o f t h e growth which produces  these d i f f e r e n c e s o c c u r s i n A p r i l and May b e f o r e t h e snow m e l t s , and a d u l t growth r a t e s d u r i n g t h e summer are low. Organ Weights Over 6000 organs from about 2I4OO lemmings were weighed i n the course o f t h i s study i n an attempt  t o f i n d a p h y s i o l o g i c a l i n d e x which i s  c o r r e l a t e d with the p r e v i o u s l y d e s c r i b e d p o p u l a t i o n processes. t h a t c e r t a i n p h y s i o l o g i c a l changes i n i n d i v i d u a l s cause p r o f o u n d  The i d e a changes  i n p o p u l a t i o n p r o c e s s e s i s v e r y w i d e l y h e l d , p a r t i c u l a r l y because o f t h e work o f C h r i s t i a n ( 1 9 5 0 , 1957,  1961).  The assumption  i s t h a t each i n d i v i d u a l  has a c e r t a i n i n t e r n a l p h y s i o l o g i c a l s t a t e which can be c o n v e n i e n t l y measured by weighing  one o r more o f s e v e r a l i n t e r n a l organs such as t h e  t e s t e s , a d r e n a l s , and s p l e e n .  The f u r t h e r assumption  i s made t h a t t h i s  p h y s i o l o g i c a l s t a t e causes changes i n p o p u l a t i o n p r o c e s s e s .  Thus we have  130 diagrammatically:  changes i n p o p u l a t i o n p r o c e s s e s (e.g. r e p r o d u c t i o n )  causes  'physiological state" \  r e f l e c t e d by changes i n organ w e i g h t s  Now i t i s o f course p o s s i b l e t h a t t h e r e a l " p h y s i o l o g i c a l s t a t e " i s n o t measured b y these upholders  organ w e i g h t s .  o f these  But the p o i n t here i s t h a t a l l t h e  " p h y s i o l o g i c a l " t h e o r i e s do r e l y on organ w e i g h t s and have  based t h e i r supposed c o n f i r m a t i o n s on organ w e i g h t s . using t h e i r  Hence we may b e g i n b y  assumption.  T a b l e s 55 and 56 g i v e t h e mean s t a n d a r d i z e d organ weights f o r Lemmus males and females,  and T a b l e s 57 and 58 g i v e t h e same d a t a f o r  D i c r o s t o n y x males and f e m a l e s .  The organs i n c l u d e d a r e :  t e s t e s , a d r e n a l s , and s p l e e n ; and f o r t h e females —  were performed f o r each organ: generations.  F o r t h i s reason  Four separate  males and females,  F a t i n d i c e s are  standardizations  and w i n t e r and summer  comparisons should be made down the columns  o n l y and n o t a c r o s s t h e rows o f t h e s e t a b l e s . for  —  a d r e n a l s and s p l e e n .  A l l t e s t e s and a d r e n a l w e i g h t s g i v e n a r e p a i r e d w e i g h t s . a l s o g i v e n and w i l l be d i s c u s s e d l a t e r .  f o r t h e males  To c o r r e c t t o some e x t e n t  v a r i a t i o n due t o r e p r o d u c t i v e status,".I have i n c l u d e d o n l y f e c u n d males  and pregnant o r l a c t a t i n g f e m a l e s f o r the w i n t e r g e n e r a t i o n f i g u r e s , and o n l y non-fecund males and n u l l i p a r o u s females f o r t h e summer g e n e r a t i o n The  groups o m i t t e d by these The  first  r e s t r i c t i o n s are s m a l l and d i s c o n t i n u o u s i n t i m e .  organ we may c o n s i d e r i s t h e a d r e n a l g l a n d , and we may  e n q u i r e whether p o p u l a t i o n d e n s i t y changes were a s s o c i a t e d w i t h weight changes. these  figures.  adrenal  I t i s d i f f i c u l t t o see any c o n s i s t e n t r e l a t i o n s h i p between  a d r e n a l d a t a and t h e p o p u l a t i o n changes.  There i s a s e a s o n a l change  TABLE 55.  S t a n d a r d i z e d mean organ w e i g h t s ( m i l l i g r a m s ) and f a t i n d e x f o r Lemmus males, 1959-61  LOCATION AND TIME PERIOD  ADRENALS W#  TESTES S*  N  WT.  12  25.2 26.0  WT.  ¥ N  -  -  11 2  12.0 10.2  ia a  N  SPLEEN  S  FAT INDEX  W WT.  W  S>  WT.  N  N  6a6 521 361  - - i aa - -  WT.  N  WT.  231  _  353  11 1  S  N  IND.  15  1.7  N  IND.  Main S t u d y A r e a  1959 June 16-30 JulyAugust Sept.-Oct. December  a 7  — 2  -  2a.8 16.9  I 9 6 0  Feb.-April May 1 6 - 3 1 June 1 - 1 5 June 16-30 July 1-15 July 16-31 August 1 - 1 5 August 1 6 - 3 1 Sept. 1 5 - 3 0 Oct.  27-Nov.  December 1 9 6 1  January February March April May 1 - 1 5 May 16-31 June 1 - 1 5 June 1 6 - 3 0 July 1-15 July 1 6 - 3 1 August 1 - 1 5 August 1 6 - 3 1  56 35 2 3  ia ia  23.2 26.2 27.5 28.8 2U.8 25.0  -- --- --  2 22 8 10 7  -  mm  -  --3 -- 8  11.0  mm  88  -  Ml  25.3  2a.a 21.8 26.7 25.7  -  -  38  86  50 27 17 32  19 12  a  3  -  1 7 17  7  11.9  186  2 89 58  15a  7.3 7.8 9 . 7  7.6  16.1 16.3  13.7 11.5 13.8  ia.i  -  mm  11.8  n.a  12.5  23  ia ia  5a5  512 5io a9o 380 305  - -- - -  mm  -  -  M*  ii  576 556  Ml  -  --  19 12  17 15  87 50 27 1 7  a  3  -  ia  31 _ _  63a  a37  Ml  32-  31  —  1 9 7 17  Mi  83  — a3  57 35 23  ia ia  66 109 199  -  2 22 11 10 6  iao  96 156 267 516  10  265  ia  -  Ml  85  27 17  31 19 12  a  3 12 _  —  a  33 7 7  72 56 69 38 35  30 31 aa aa a3 _ —  1 1 7 16  a5  _  _  59 90 7a  1.8  -  - -  _  Mi  2.0  11  1.9  3  2.a  3 3  2.1  3  1.0  207 57 35 22  7 M  18  Mi  a9  -  35  -- --  _  mm  63 58  Mi  2a9 aoi 28a  mm  -  _  6 33  - - -  - - 1  mm  88 30 15 16 15 13 15 20  7  635  -  7  2  Mi  2 22 10 7  86  Mi  2  31  11  2a3  Ml  ia 13 mm  mm.  i.a  1.2 1.2 1.1 1.5  -  Ml  -  -  _  _  Mi  Mi  —  _  -  — — 2 22 11 10 7  _  a.2 2.5 1.1 1.3 1.0  _  1.7  2.a  _  — 9  33  9a 52 27 17  31 19 12  a  3 13  • 1.5  1.3 1.8 2.0 2.7  i.a 1.5  1.5 1.2 1.8 2.0 1.9  _, _  _  1 1 7 17  1.5 1.2  i.a  2.1  Other A r e a s  I960 Aberdeen Lake  May 27-June 2 July  10-18  7  22.5  -  666 512  8  17  7 20  86  2a.7  8  8.2  7 20  8  23.6  ia  9.7  8  535  ia  21  8  ao3  6  2a.8  3  7.2  6  a86  3  25  6  5  25.7  5  8.a  5  576  5  a3  a  2a.3  - 3  12.8  a  399  3  1  22.1  a  7.9  1  a22  2  22.7  9  u.a  2  8  11.2  -  >0  mm  as  7 20  2.3 1.3  ia  60  8  375  2  52  5  a6a  5  9  a  67a  a  35  1  538  9  32  2  -  8  12  _  Mi  8  1.6  1.2  ia  1.7  6  0.9  3  i.a  55  5  0.9  5  i.a  3  6a  a  1.3  3  1.9  231  a  79  1  1.7  3  1.9  213  9  62  2  0.9  8  1.5  8  73  _  M  8  1.8  7  1961-Ji-r  Aberdeen Lake July 2 7 - 2 9 Long I s l a n d J u l y 17-20 Second I s l a n d J u l y 2U-27 Prince River August H t - 1 7 Nine M i l e I s l a n d August l U - 1 9 Ten M i l e I s l a n d August 1 U - 1 9 TheIon R i v e r August lk-19  -  -  •»• W = w i n t e r g e n e r a t i o n ( f e c u n d a n i m a l s o n l y ) . # S = summer g e n e r a t i o n (non-fecund a n i m a l s o n l y ) .  TABLE 56.  S t a n d a r d i z e d mean organ w e i g h t s ( m i l l i g r a m s ) and f a t i n d e x f o r lemmas f e m a l e s , 1959-  WINTER * WT. N  FAT INDEX  SPLEEN  ADRENALS  LOCATION AND TIME PERIOD  SUMMER *• WT. N  WINTER N WT.  SUMMER N WT.  WINTER IND. N  SUMMER IND. N  Main Study A r e a  1959  June 1 6 - 3 0 July August Oct.-Dec.  u  2  1*  -  38.0 32.3  29.0  -  -  3 3  I 9 6 0  April May 1 6 - 3 1 June 1-15 June 1 6 - 3 0 July July  1-15 16-31  August 1-15 August 16-31 Sept. 15-30 Oct.-Nov. December 1 9 6 1  January February March April May 1-15 May 16-31 June 1-15 June 1 6 - 3 0 J u l y 1-15 July 16-31 August 1-15 August 16-31  2 3  1 8  2 1 1 6 2 1  -  30.0 25.3  mm  38.1 36.5 32.7 32.2  —  3 2 . 7  -  mm  -  -  8 26  1*1 3 7  -  15 18 26  -  -  32 8 6 3 11 -'  5  27.3 37.3  -  -  2  6 3 -  29.7 33.2 26.5  -  -  mm  1* 8  10.7  H*.o -  -  12.3  7.1  7.2  10.2  6.8 13.1 15.U 13.3 15.1* 10.9  15.3 20.1* -  —  11.8 15.6  6 2 5 2  3 18 2 1 1 6  21* 11* _  -  1* 5 2  6 3  -  21*3 259 2 6 7  119 8 1  1 6 6  130 198 285 2 1 1  -  -  11*9 188 215 1*66 171*  -  3 1  1*7.6  9 2 -  21.9  -  -  -  -  mm  1*5  -  —  8 25 1*0 3 7  15 1 8  2 6  3 2  8 6 I* 11  -  mm  — mm  1* 8  —  2 1  —  53.9  -  1 1  108.0  2  59.6  6 -2 -  69.1*  50.1 ll*.3 27.7  mm  -  22.7 3U.8  _  20.3  26.0 1*1*.2 -  mm  1* 8 1*  mm  _ _  70.1  h k  1.7 1.1 1.1*  1  -  1.3 1.3 1.3  26 1*0  -  -  -  mm  2.5 1.8 1.8 _ _  -  -  _  mm  -  mm  _  Mft  62.1  1.8 2.1*  _  9  3 7  15 18 2 6  3 2  8 6 I* 13 _ _  1.7 l*.o 2.3 •m.  1.7 1.8 1.9 2.2 2.5 1.7 1.7 1.5 1.1 2.0 1.1* 1.3 m mm  _ _  1* 8  1.7 2.0  Other Areas  I960 Aberdeen Lake May 27-June 2 June 15-16 J u l y 10-18  U 12  35.1 28.6  7  8.9  7  30.3  16  h  28.0  3  30.9  1*  5 1*  1.5  12  157 179  10.7  7  371*  11*  77.7  16  1.8  5  9.5  1*  201  5  1*9.9  5  1.7  2  10.5  3  189  2  156.1  2  1.6  2  1U.7  2  51.0  2  2.1*  1  85.8  1  1.6  3  31*. 9  3  1.9  73.6  1*  2.0  32.8  1  1.0  1.5  1.6  1961 Aberdeen Lake J u l y 26-29 Long I s l a n d J u l y 17-20 Second I s l a n d J u l y 2l*-27 Prince River August U*-17 Nine M i l e I s . August ll*-19 Ten M i l e I s . August ll*-19 TheIon R i v e r August ll!.-19  1  22.3  1  8.1  1  27.6  3  13.3  -  -  I*  10.8 1*  * W i n t e r G e n e r a t i o n : a d r e n a l s and s p l e e n - pregnant o r l a c t a t i n g o n l y f a t i n d e x - pregnant o r p a r o u s o n l y (not l a c t a t i n g ) Summer G e n e r a t i o n : n u l l i p a r o u s animals o n l y .  TABLE 57.  S t a n d a r d i z e d mean organ w e i g h t s ( m i l l i g r a m s ) and f a t i n d e x f o r D i c r o s t o n y x males, 1959-61  ADRENMJS  LOCATION AND TIME PERIOD  W"# N  WT.  WT.  -  -  6 3  9.1 6.2  FAT  N  WT.  6 12 2 1  261 273 202 89  N  WT.  -  -  N  INDEX  w  w  W  S# N  SPLEEN  TESTES  WT.  N  WT.  -  -  6 3  N  IND.  N  IND.  6 2  1.6 2.a  2  2.7 2.6  Main Study A r e a  1959 June 15-30 July August November  I960 Jan.-Mar. April May 16-31 June 1-15 June 16-30 July 1-15 July 16-31 August 1-15 August 16-31 S e p t . 15-30 November December 1961 January February March May 1-15 May 16-31 June 1-15  Junel6-30  July 1-15 J u l y 16-31 August 1-15 August 16-31  6 12 2 2  I4.8 12.1* 13.8 11.1  - 8 2  1U.7 12.1  1U 13.7 9 15.5 13 2 3  1 7  U7  16 16 6  7  -  1U.9 12.7 33.6.  -  5 1 10 7  21*  -  -  8.3  mm  7.1 8.1 8.8  9.1*  -  9 2 8  7.3 10.8 11.2  mm  5  9.3 7.6 13.6 — —  mm  -  13 .1*  15.3 16.0 16.6 1U.6 12.3 11.8  -  1 3  — _ 2 12  -  7.6 9.3  3  11 2 Iii  9 13  2 1 1  -  -  69  115 210.  22U  196 230  -  12U  6  253  I69 217 186  235  2Ul  7.3  8 5 9  7.3  5  9.U  15.2 12.3 12.7  5  5 15.6  5  2 9  17 38  -  --  -  ia  51  1 139 ,11 132 7 100 2a 9 2 8  1 7 1*7 16 16 7  6 2  -  5  1 3 _  a5  --  ai  -  20 23 22  25  19 37  3a 36  56 _  — mm  MM  mm  _  — 3 12  6 12 2  a6  27  31.5 55.3 37.6 18.8  - a  2  ia  9 13 2 3 1  _  61.3 31.3 a9.2 60.8 77.a 73.1 132.3 121..0 _ _  _  — _  1 7 a6 16 16 6 6  _  22.6 35.6 U9.0 69.8 98.a  65.a  - -  112.0  -  mm  — 1 9 7 2a 9 2 8  5  1 3 _  1.5 2.0  25.3 23.7  7 12 2 1  -  3 11 2  2.6 1.9 2.3 1.1 1.0 1.2 1.2  mm  — ao.8 39.a  as.s  59.9 ai.7 37.0 a2.3 33.1 2a.9 28.2 _  MM  _  _ _  mm  MM  3 12  37.5 38.6 3l*.9  ia  9 12 2 3 1  1.7 1.8  9  1 11  7 2a 9 2 8  1.6 1.2 1.7 2.2  2.5 2.a 1.6  5  1.7 0.9 1.0  3  ia  2.0 1.6 1.7  1 3  1 7 a7 18 16 6 7  2.6 3.0  2.3 1.7 1.2  5  1.7  I.a  5  1.8  1U  25  250 22a 178  5  32  8 5 9  3U.2 a7.3 109.1  a  87.3  8 5 9  160  5  16  5  91.2  5  1*3.8  5  ia  1.9 1.2  2.a i.a 1.3  i.a 1.2  11  Other A r e a s I960 Aberdeen Lake May 27-June 2 June 15-16 J u l y 10-18 1961 Aberdeen Lake J u l y 26-29  7 5 11  -* W » w i n t e r g e n e r a t i o n ( f e c u n d a n i m a l s o n l y ) . S = summer g e n e r a t i o n (non-fecund animals o n l y ) .  58.  TABIE  S t a n d a r d i z e d mean organ w e i g h t s ( m i l l i g r a m s ) and f a t i n d e x f o r D i c r o s t o n y x f e m a l e s ,  WINTER # N  WT.  3 9 U  27.3  FAT INDEX  SPLEEN  ADRENALS  LOCATION AND TIME PERIOD  SUMMER # N  WT.  SUMMER  WINTER N  WT.  3 9  108 102  1959-61.  N  SUMMER  WINTER  WT.  IND,  N  IND.  N  u 6  1.1 2.2  -2 -  Main S t u d y A r e a  1959 June 15-30 July August Oct. - Nov.  I960 Jan.-March  1-15 16-30  J u l y 1-15 J u l y 16-31  1  11.9 29.U 25.6 23.6 25.3 2U.0 16.0  10  6 5  1  -  Sept.,. 15-30 Oct. - Nov. December  1961  -  January February March April  -  1  May 1-15 May 16-31  J u l y 1-15 J u l y 16-31  17.0  9 5  August 1-15 August 16-31  June 1-15 June 16-30  2  -  Apr.-May 15 May 16-31 June June  1  22.5 17.7 12.8  mm  7 19 9  August 1-15 August 16-31  -  -  11.2 25.9  23.5  -2 8  -  ---  mm  15 Hi  25 6 5 5  2 3 —  2  -  mm  8.3 9.0 mm  -mm  7.3 8.8 8.7 6.1 9.2 7.6  7.1 10.U  10.1 -  U  71  mm  2 8  1  65  -  -21  -  20  --  10 5  10 6 U  1  _  25 73  165  79 71 73 22  -  -  -  — 1 -  -  5  20 8  16 Ml  69  82  mm  2U  -  _ _  15 lU  58  6  29 33 33  26  5 5  2 3  2 -  ia  U8  -—  -  -  -  7  97  21.2  9  6.6  21  93  8  6U  21.3  10  95  11  U9  10  85  10  68  5  3 2  2.1 0.9  17  1.3 1.6 1.8 1.3 1.2 1.6 1.7  -  5  1 2 2 9 U _  -  25.0  10  5  3 3  U  56  2.9  52 -  28  7  5  7.3 7.0 8.0 9.0  3  —  5 3 3  U  mm  3U  22.2 16.9 20.2 15.7  11  106  -  mm  Ui  56 Ul UU 35  -  -1  -  _  6 11 7  -  15 13  27 6  — -  3 2 1  2 2 1  1.2 0.8 1.8  l.U  2.0 1.8  3.1 1.8 1.7  _  —  -  1 22 22  2.5  Ml  5 5  1.7 1.6 1.6 1.7  5  8 2  U  2  -  l.U  1.7 2.3  2.5  1.3 1.6  1.5  0.9 3.7 2.1 1.6  -  -  —  u 11 5 9  2.0 1.6 1.6 1.5  Other Areas  I960 Aberdeen Lake May 27-June 2 -  June'15-16 J u l y 10-18  -  21  -  -  U  1.9  7  1.1  11  1.1  -  -  9  1.6  13  1.8  1961 Aberdeen Lake J u l y 26-29  9  7.3  9  9  1.5  •* W i n t e r G e n e r a t i o n ( i n c l . S p r i n g Gen.) ? a d r e n a l s and s p l e e n - pregnant o r l a c t a t i n g o n l y . : f a t i n d e x - pregnant o r parous o n l y (not l a c t a t i n g ) Summer G e n e r a t i o n : n u l l i p a r o u s a n i m a l s o n l y .  135 i n a d r e n a l w e i g h t s , r i s i n g t o a peak i n June o r J u l y , s i m i l a r t o t h a t d e s c r i b e d b y C h i t t y (1961) f o r M i c r o t u s a g r e s t i s .  A t one p o i n t o r another  almost a l l t h e means o v e r l a p and t h u s i t i s n o t p o s s i b l e t o say c a t e g o r i c a l l y t h a t any one summer showed h i g h e r o r lower a d r e n a l weights t h a n another summer.  However, some y e a r s t e n d t o be h i g h e r o r lower t h a n o t h e r s , and we  may b r o a d l y c l a s s i f y t h e y e a r s as f o l l o w s :  Summer  Lemmus Adult Adult Young Young  males females males females  Dicrostonyx A d u l t males Adult females Young males Young females  Winter  1959  I960  1961  1960-61  Low Low 1 High • Low ?  High High Low Low  Low Low High High  High High  Low High Low Low  Low High Low Low  High Low Low Low  High High  The mean d i f f e r e n c e between " h i g h " and "low" a d r e n a l weights was $-lk% f o r the summer a d u l t s , and  23-33%  f o r t h e Lemmus summer young.  High a d r e n a l  weights were found i n a l l groups i n t h e w i n t e r o f 1960-61, b u t u n f o r t u n a t e l y comparative d a t a f r o m t h e p r e v i o u s w i n t e r are not a v a i l a b l e .  There i s a  c l e a r r e l a t i o n s h i p o f summer a d r e n a l weights t o the c y c l e i f we l o o k a t s i n g l e groups such as t h e Lemmus a d u l t males.  These r e l a t i o n s h i p s , however,  are n o t c o n s i s t e n t between groups, as can be seen, f o r example, by comparing a d u l t s and young.  Two c o n c l u s i o n s f o l l o w f r o m these d a t a :  (1) summer a d r e n a l  weights do n o t show a c o n s i s t e n t r e l a t i o n s h i p t o the phase o f the c y c l e ; and (2) w i n t e r a d r e n a l w e i g h t s seemed h i g h i n I96O-6I r e l a t i v e t o t h e summer weights. The  second organ t o be c o n s i d e r e d i s the t e s t i s .  There i s some  c o r r e l a t i o n between t e s t e s weights and d e n s i t y changes, which shows up as  136 follows:  Slimmer  195°  I960  1961  Lemmus A d u l t males Young males  High High  Low Low  High Low  Dicrostonyx A d u l t males Young males  High High  Low Low  Low Low  The mean d i f f e r e n c e s between " h i g h " and "low" t e s t e s w e i g h t s was lk-21% f o r t h e summer a d u l t s and  U9-78$ f o r  t h e summer young.  were h i g h e s t i n both s p e c i e s d u r i n g 1959.  T e s t e s weights  The peak summer was c h a r a c t e r i z e d  b y low t e s t e s weights i n a l l groups, but i n the d e c l i n e Lemmus a d u l t s showed a d i f f e r e n t t r e n d from the o t h e r s .  These r e s u l t s f o r t h e young agree w i t h  those d i s c u s s e d p r e v i o u s l y r e g a r d i n g r e p r o d u c t i o n . mature e i t h e r i n I960 o r 196l. mature i n t h e i r f i r s t  Young male Lemmus d i d n o t  Young male D i c r o s t o n y x d i d n o t ever seem t o  summer; n o n e t h e l e s s t h e y seem t o show t h e same type o f  changes i n t e s t e s w e i g h t s ( c f . August w e i g h t s ) as do Lemmus young. t h i s i n h i b i t i o n o f gonadal  Because  development i n young male lemmings o c c u r r e d i n  the summer o f d e c l i n e as w e l l as i n t h e peak summer, d e n s i t y p e r se cannot be t h e f a c t o r d i r e c t l y i n v o l v e d h e r e , b u t r a t h e r t h e important v a r i a b l e must be capable The  o f a c t i n g a t v e r y low d e n s i t i e s i n t h e d e c l i n e .  f i n a l organ t o be c o n s i d e r e d i s t h e s p l e e n .  lemmings v a r i e s c o n s i d e r a b l y i n s i z e , weighing and 5-1200 mg i n Lemmus. animals  ( l a t e July-August)  i n average weights.  The s p l e e n i n  from 5-600 mg i n D i c r o s t o n y x  The v e r y h e a v i e s t s p l e e n s are found o n l y i n midsummer and t h e r e i s thus a. v e r y s t r o n g s e a s o n a l v a r i a t i o n  There was l i t t l e  d i f f e r e n c e between t h e d i f f e r e n t y e a r s ,  and t h e s p l e e n w e i g h t s showed no c l e a r r e l a t i o n s h i p t o t h e c y c l e i n numbers. The  s t r i k i n g s e a s o n a l change i n s p l e e n s i z e i s n o t understood  b u t may be  137 a s s o c i a t e d w i t h b l o o d p a r a s i t e s t r a n s m i t t e d by mosquitoes d u r i n g the midsummer i n s e c t season  (Baker and C h i t t y , ms.)  or by mosquito o r o t h e r  e c t o p a r a s i t e b i t e s d i r e c t l y ( C h i t t y and Phipps, I 9 6 0 ) .  This hypothesis  i s c o n s i s t e n t w i t h the o b s e r v a t i o n t h a t Lemmus has a g r e a t e r s p l e e n enlargement t h a n D i c r o s t o n y x . because Lemmus l i v e s i n t h e w e t t e r p l a c e s where mosquitoes are more abundant. To  sum up,  summer a d r e n a l and  r e l a t i o n t o the c y c l e i n numbers. but comparative  s p l e e n weights  Winter  showed no  a d r e n a l w e i g h t s were h i g h i n 1960-61  d a t a from 1959-60 a r e l a c k i n g .  T e s t e s w e i g h t s tended  change s y s t e m a t i c a l l y over the c y c l e , b e i n g h i g h i n 1959, somewhat v a r i a b l e i n 1961. changes independent  clear  to  low i n I960,  and  A l l t h e s e organs showed a s e a s o n a l c y c l e of weight  o f the p o p u l a t i o n c y c l e .  F a t Changes The  amount o f f a t s t o r e d by lemmings may  of g e n e r a l p h y s i o l o g i c a l c o n d i t i o n . index s c a l e of 1-5, very f a t animal.  T h i s index was  a s s e s s e d by an a r b i t r a r y f a t  e s t i m a t e d p u r e l y s u b j e c t i v e l y by o b s e r v i n g  s k i n , between the s h o u l d e r s , around the h i n d l e g s ,  and around the v i s c e r a and gonads.  Animals w i t h no f a t t o be seen  around the gonads where i t i s almost The  (except  always p r e s e n t ) were always c l a s s e d as  s k i n s of animals o f f a t 3 or more were always g r e a s y  had t o be wiped o r s c r a p e d a f t e r d r y i n g . a n a l y z e d i n the same way  55-58  and  These d a t a on f a t changes were  as the organ weight d a t a .  F a t index d a t a f o r Lemmus and D i c r o s t o n y x males and females given i n Tables  index  1 b e i n g the v a l u e f o r an animal w i t h no f a t and 5 f o r a  the amount of f a t on the  f a t index 1.  T h i s was  be used as another  a l o n g w i t h the organ weight d a t a .  are  The f a t index shows  a s e a s o n a l v a r i a t i o n , b e i n g a t i t s lowest i n midsummer when b r e e d i n g i s i n t e n s e and h i g h e s t i n the w i n t e r  ( p a r t i c u l a r l y i n f a l l and s p r i n g ) .  I f we  compare the s p r i n g and summer d a t a o f the d i f f e r e n t y e a r s , t h e r e does not  138 seem t o be any d i f f e r e n c e between y e a r s i n t h i s i n d e x . the s p r i n g and summer o f 1961 e i t h e r I960 o r  1959•  In p a r t i c u l a r ,  have f a t i n d i c e s e q u a l t o o r g r e a t e r t h a n  There i s t h u s no i n d i c a t i o n o f u n d e r n o u r i s h e d  i n t h e s p r i n g o f the d e c l i n e .  Low  f a t i n d i c e s may  animals  have p r e v a i l e d i n the w i n t e r  of 1960-61, b u t d a t a from the p r e v i o u s w i n t e r are not s u f f i c i e n t f o r a good comparison. Social Relationships Very l i t t l e  i s p r e s e n t l y known about s o c i a l r e l a t i o n s h i p s i n  n a t u r a l p o p u l a t i o n s of c y c l i c r o d e n t s .  Some i n d i r e c t e v i d e n c e o f  social  r e l a t i o n s h i p s and a few d i r e c t o b s e r v a t i o n s w i l l be b r i e f l y p r e s e n t e d w i t h the c l e a r u n d e r s t a n d i n g  t h a t t h e y are v e r y  inadequate.  I n e a r l y June I960 both s p e c i e s of lemmings were e x t r e m e l y i n b e h a v i o r , a t l e a s t towards humans. a c r o s s the t u n d r a , I encountered O f t e n they were h e a r d squeaking  On s e v e r a l o c c a s i o n s w h i l e  l o u d squeaking l o n g b e f o r e one  lemmings(both  were a l s o v e r y a g g r e s s i v e .  walking  species).  20 f e e t away).  on the i c e i n t h i s s p r i n g o f I960  T h i s t y p e of b e h a v i o r c o n t r a s t s s h a r p l y w i t h  t h e i r b e h a v i o r l a t e r i n the summer when t h e y t r y t o hide as soon as approaches.  Collett  (1895)  hostile  c o u l d a c t u a l l y see them  ( i n one p a r t i c u l a r i n s t a n c e squeaking began when I was As mentioned p r e v i o u s l y , lemmings caught  here  one  and C u r r y - L i n d a h l (1961) a l s o r e p o r t t h i s c u r i o u s  b e h a v i o r f o r t h e Norwegian lemming.  The  s i g n i f i c a n c e of t h e s e  observations  i s not known. Some crude measure o f a g g r e s s i v e b e h a v i o r may i n c i d e n c e o f wounding i n a p o p u l a t i o n (Southwick,  be o b t a i n e d by  1958).  A l l lemming s k i n s  c o l l e c t e d i n t h i s s t u d y were examined and c l a s s i f i e d on an a r b i t r a r y as f o l l o w s :  no r e c e n t wounds o r obvious  the  scale  s c a r s showing on i n s i d e o f s k i n ;  l i g h t wounds; moderate wounds; o r severe wounds.  S k i n s were s e l e c t e d  s e t out as s t a n d a r d s f o r each o f these f o u r c a t e g o r i e s , and t h e s e were  and  139 c o n s t a n t l y u s e d . f o r comparisons.  A l l t h i s c l a s s i f i c a t i o n was done i n a  two week p e r i o d a t the end o f t h e study so as t o minimize  the s u b j e c t i v e  element. T a b l e s 59 and 60 g i v e the i n c i d e n c e o f wounding shown on s k i n s for  Lemmus and D i c r o s t o n y x males over the c y c l e .  F o r both species  1959  showed t h e lowest o v e r - a l l amount o f wounding b o t h f o r o l d and young a n i m a l s . The  I960  a d u l t s o f b o t h s p e c i e s showed a h i g h i n c i d e n c e of x^ounding i n l a t e  June and J u l y , and t h i s d e c l i n e d b y August when b r e e d i n g had c e a s e d .  i960  The  young showed a c o n s i d e r a b l e amount o f wounding as t h e y moved under the  snow i n the f a l l  o f I960.  The 1961  a d u l t s showed t h e h i g h e s t wounding  p e r c e n t a g e s f o r a l l y e a r s i n l a t e J u l y and e a r l y August.  U n l i k e I960 however,  t h e r e appeared t o be v e r y l i t t l e wounding u n t i l e a r l y J u l y i n 1961, j u s t a f t e r the f i r s t again.  a time  summer young had been b o r n and the females were b r e e d i n g  The s i g n i f i c a n c e o f t h e s e y e a r t o y e a r and s e a s o n a l d i f f e r e n c e s are  not u n d e r s t o o d , and much more d e t a i l e d work must be done on t h e s e p o i n t s . However, these crude d a t a do i l l u s t r a t e t h r e e g e n e r a l p o i n t s :  ( l ) there i s  a c o n s i d e r a b l e s e a s o n a l v a r i a t i o n i n the amount of f i g h t i n g which causes s k i n wounds; (2) t h i s f i g h t i n g was n o t a simple f u n c t i o n o f d e n s i t y because at  c e r t a i n t i m e s i n t h e summer o f the d e c l i n e wounding was more e x t e n s i v e t h a n  i n t h e peak summerj  (3) b o t h Lemmus and D i c r o s t o n y x showed t h e same g e n e r a l  p a t t e r n , a l t h o u g h t h e r e was l e s s wounding shown on D i c r o s t o n y x s k i n s t h a n on Lemmus s k i n s . M a t u r a t i o n o f summer young males d u r i n g t h e i r f i r s t a s s o c i a t e d w i t h a c o n s i d e r a b l e amount o f wounding.  summer was  Only one sample o f Lemmus  summer young c o n t a i n e d b o t h mature and immature a n i m a l s , and t h e d a t a on these are as f o l l o w s :  l4o TABLE 59•  Amount o f wounding  shown on s k i n s o f Lemmus males f r o m t h e  Main Study A r e a , 1959-61.  N  %  LIGHT  1959  7  June 16-30 July August  I960 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31 Sept. 15-30 Oct.-Nov. December 1961 January FebruaryMarch April May 1-15 May 16-31 June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August 16-31  SUMMER GENERATION  WINTER GENERATION  TIME PERIOD  5 7 138  65  3h  25 1U 1U  -  mm  -  k  21 12  8  -  18  -  28.6 20.0  %  MOD.  -  mm  28.6  U2.9  21.7 liO.o  0.7  . 58.8 U8.0 50.0 28.6  5.9 8.0  21.il  7.1  -  -  -  -  -  *M  mm  -  25.0  19.1  16.7 25.0 66.7  -  -  -  12.5  -5.6 -  %  N  -  -  1.5 5.9 12.0  %  %  LIGHT  MOD.  -  -  -  -  --  HEAVY  11  mm  9.1  -  -mm  8 36 85 55 25 16  36.1, 28.0 62.5  46.7  10.0  -  18 12  27.8  -  k  25.0  -  mm  -  mm mm  -  mm  mm  —  -  W i n t e r G e n e r a t i o n - mature a n i m a l s o n l y . Summer G e n e r a t i o n - immature a n i m a l s o n l y  30  2 1  -  mm  10  7  18  2.8 25.9  8.3  mm  28.6  38.9  1.2 1.8  8.0 —  25.0  -  mm  -  mm  —  %  HEAV3  -  -  mm  -1.2 3.3  -5.5 mm  -  mm  -  —  -  141  TABLE 60.  Amount o f wounding shown on s k i n s o f D i c r o s t o n y x males f r o m  the Main Study A r e a ,  1959-61.  TIME PERIOD  WINTER GENE!RATION N  %  LIGHT  1959 June 15-30 July August  I960 May June  1-15  June July  16-30 1-15  July 16-31 August 1-15 August 16-31 S e p t . 15-30 Oct.-Nov. December  1961 January Feb.-March May June 1-15 June 16-30 J u l y 1-15 J u l y 16-31 August 1-15 August  16-31  11  13  9.1  15.1*  2  5o.o  6  -  28  9 16  1* 1 -*  -  9  65  17 17 6 8  -  28.6  1*1*. 1*  37.5 25.0  -  -  mm  20.0 23.5 1*7.1 66.7 50.0 -  W i n t e r G e n e r a t i o n - a l l mature. Summer G e n e r a t i o n - a l l immature.  SUMMER GENERATION  1  %  MOD.  -9.1  % HEAVY  «•  mm  -  3  . -  -  -  -  10.7 11.2  -  -  -  -  11.1  --  16.7 mm  -  N  _ _  6  _ _  - 1 - 11 - 8 - 22 - 9 - 2 - 9  %  %  LIGHT  MOD.  -  -  —  -  mm  _  — mm  9.1 25.0 18.2  hh.k  50.0 22.2  —  mm mm aM  tea  11.1  -  5  _  _  _ _  —  —  mm  mm  mm  mm-  —  -  _ _ _  _  mm  3  - 2 - 12 -  13  60.0  8.1*  7.7  Mi  mm mm  mm mm  11+2  August  Sample Size  % showing wounds  Mean body weight  Range o f body weights  11  9.1  21.7  g  13.5  -  30.8  8  75.0  33.5  g  27.8  -  38.5  1959  Immature males Mature males  These d i f f e r e n c e s i n wounding  6.01+, d f 1, P <.05,>.0l).  are s i g n i f i c a n t  ( c h i - s q u a r e f o r independence =  Since t h e mean body w e i g h t s o f t h e s e samples  d i f f e r c o n s i d e r a b l y , some o f the d i f f e r e n c e i n wounding may be e x p l a i n e d on t h i s b a s i s . in  However, t h i s i s p r o b a b l y n o t t h e e n t i r e e x p l a n a t i o n because  I960 and 1961 none o f t h e samples o f immature a n i m a l s w i t h as h i g h or  h i g h e r body w e i g h t s showed as much wounding  as t h i s mature  sample from  1959*  T h i s suggests a r e a s o n why t h e r e seems t o be a g r e a t i n c r e a s e i n t h e amount of f i g h t i n g i n the August 1959  Lemmus.  I t a l s o suggests a f u n c t i o n f o r t h e  observed i n h i b i t i o n o f m a t u r a t i o n o f males i n the peak and d e c l i n e  summers,  i . e . i t p r e v e n t s a c o n s i d e r a b l e amount o f f i g h t i n g i n t h e s e p o p u l a t i o n s . Summer young Lemmus f e m a l e s d i d not seem t o show t h i s d i f f e r e n c e :  July  i960  =0.0  Immature f e m a l e s  N = 18  % showing wounds  Mature f e m a l e s  N = 11+  % showing wounds =  0.0  However more p o s i t i v e e v i d e n c e i s needed on t h i s p o i n t . I n d i r e c t e v i d e n c e from snap t r a p p i n g s u g g e s t s some s o r t o f antagonism between  o l d and young Lemmus d u r i n g the summer o f I960.  G i v e n the August  I960 snap t r a p p i n g d a t a , we may make the n u l l h y p o t h e s i s t h a t the p r o p o r t i o n of  a d u l t and young Lemmus i n the h a b i t a t t y p e s i s t h e same.  The  relevant  d a t a are as f o l l o w s :  Lichenheath August a d u l t s «  young  0 23  Heath-sedge and h-s hummock 21 188  Sedge Hummock  Sedge Marsh  Totals  20  9  50  102  53  366  H43 I f young animals  t e n d t o d i s p e r s e from t h e d e n s e l y o c c u p i e d h a b i t a t s t o  the l e s s d e n s e l y o c c u p i e d  ones, the p r o p o r t i o n o f young i n the  h a b i t a t s ( i . e . d r y f o r Lemmus) s h o u l d be c o n v e r s e l y f o r the b e t t e r h a b i t a t s . and the n u l l h y p o t h e s i s was (chi-square = 6 . 4 0 ,  d f 3,  fewer young i n the  poorer  g r e a t e r than t h a t of a d u l t s , and  These d a t a were t e s t e d by  chi-square  not r e j e c t e d , a l t h o u g h the r e s u l t i s c l o s e  P<  .10, >.0$).  Thus a l t h o u g h the d a t a  suggest  sedge hummock and more young i n t h e l i c h e n - h e a t h h a b i t a t s ,  compared t o t h e a d u l t s , t h e d i f f e r e n c e s are not s t a t i s t i c a l l y  significant.  I t i s c l e a r t h a t the d a t a a v a i l a b l e on s o c i a l r e l a t i o n s h i p s i s v e r y meagre and almost a l l i n d i r e c t .  We  know t h a t a t times lemmings were  v e r y a g g r e s s i v e , t h a t t h e r e were l a r g e changes i n the amount o f wounding shown on s k i n s , and t h a t t h e r e may and young Lemmus.  Taken t o g e t h e r t h i s i s enough t o p o i n t out t h a t b e h a v i o r a l  changes r e p r e s e n t the l a r g e s t gap o p e r a t i n g i n the  have been some antagonism between o l d  i n our knowledge of t h e i n t r i n s i c  factors  cycle. SUMMARY AND  CONCLUSIONS  (1) High body w e i g h t s were a s s o c i a t e d w i t h a l l peak p o p u l a t i o n s Type H d e c l i n e s .  These h i g h w e i g h t s were about 20-30% g r e a t e r on the  and average  t h a n t h e normal low body w e i g h t s found d u r i n g the p e r i o d o f low numbers  and  Type G d e c l i n e s . (2) Midsummer weight d i s t r i b u t i o n s f o r Type G d e c l i n e s showed a gap where the f i r s t  summer young should have been.  (3) Organ w e i g h t s d i d not appear t o give us any i n s i g h t i n t o causes o f the c y c l e .  conspicuous  Summer a d r e n a l w e i g h t s and  s p l e e n weights d i d not  show any c o n s i s t e n t r e l a t i o n s h i p t o the d e n s i t y changes. x*eights d u r i n g 1960-61 seemed t o be h i g h i n both  species.  Winter adrenal T e s t e s weights  showed a f a i r l y c o n s i s t e n t r e l a t i o n t o t h e c y c l e s i m i l a r t o the d e s c r i b e d under r e p r o d u c t i o n .  the  changes  (10  S p r i n g and summer f a t i n d i c e s showed no r e l a t i o n t o the c y c l e ,  and  lemmings i n the s p r i n g o f the d e c l i n e were as f a t as o r f a t t e r t h a n animals from t h e o t h e r y e a r s , t h u s e l i m i n a t i n g any doubt about the q u a n t i t y of f o o d a v a i l a b l e d u r i n g the d e c l i n e .  W i n t e r f a t i n d i c e s f o r 1960-61 may  have been  lower than t h o s e f o r the p r e v i o u s w i n t e r but not enough d a t a are a v a i l a b l e for  an adequate (5)  comparison.  I n t r a s p e c i f i c s t r i f e , as measured by the wounds on t h e s k i n , showed  s t r o n g s e a s o n a l and y e a r t o y e a r changes.  T h i s s t r i f e was  not a simple  f u n c t i o n of d e n s i t y because  the h i g h e s t amount of wounding was  the summer o f t h e d e c l i n e .  The  s k i n s are not  changes i n the amount o f s t r i f e  recorded i n shown by the  understood.  (6) S e x u a l l y mature young Lemmus males s u f f e r e d more wounding than immature young males i n t h e August  1959  sample.  DISCUSSION  We. have now p r e s e n t e d t h e r e s u l t s o f t h i s study on lemming c y c l e s and must i n t e g r a t e t h e s e r e s u l t s w i t h contemporary i d e a s and s t u d i e s b y o t h e r workers.  The amount o f l i t e r a t u r e p u b l i s h e d about " c y c l e s " i s t r u l y  voluminous, b u t t h e p r o p o r t i o n o f t h i s which p r e s e n t s o r i g i n a l thought o r s o l i d evidence  i s v e r y low i n d e e d .  w i l l be a s f o l l o w s .  The p a t t e r n f o l l o w e d i n t h i s d i s c u s s i o n  A f t e r a b r i e f h i s t o r i c a l review and some  methodological  d i s c u s s i o n we s h a l l c o n s i d e r t h e main,changes d i s c u s s e d i n the p r e v i o u s s e c t i o n s and i n t e g r a t e these w i t h t h e r e s u l t s o f o t h e r workers.  Finally,  we s h a l l c o n s i d e r t h e c u r r e n t t h e o r i e s about m i c r o t i n e c y c l e s and t h e i r s t a t u s i n the l i g h t o f these d a t a from lemmings. H i s t o r i c a l Approaches and Background The h i s t o r y o f " c y c l e s " i s n o t v e r y l o n g i n terms o f y e a r s , b u t we can r e c o g n i z e two g e n e r a l approaches t o t h e problem.  The o r i g i n a l  o b s e r v a t i o n ( E l t o n , 19210 was t h a t a n i m a l p o p u l a t i o n s f l u c t u a t e d i n s i z e and  i n some s p e c i e s t h e r e appeared t o be some r e g u l a r i t y t o t h i s change.  Given these i n i t i a l d a t a , some workers emphasized t h e r e g u l a r i t y o f t h e c y c l e s and c o n c e n t r a t e d much e f f o r t  on an attempt t o determine the p r e c i s e  p e r i o d o f t h e s e c y c l e s f o r each s p e c i e s . S i i v o n e n (19U8).  An example from t h i s group i s  Another approach emphasized " c y c l e s " as a p a r t i c u l a r  p r o b l e m o f p o p u l a t i o n r e g u l a t i o n and c o n c e n t r a t e d s t u d y on the f a c t o r s o p e r a t i n g on the p o p u l a t i o n t o cause t h e s e i n c r e a s e s and d e c l i n e s . of  A blending  b o t h t h e s e approaches i s i l l u s t r a t e d by t h e work o f E l t o n (192U, 1931,  19U2). The f i r s t (1951, found  approach was c h a l l e n g e d by Palmgren (19U9) and Cole  195U b , 1958) who demonstrated t h a t " c y c l e s " s i m i l a r i n l e n g t h t o t h o s e i n nature  c o u l d be i n t e r p r e t e d as e s s e n t i a l l y random f l u c t u a t i o n s w i t h  1U6 some s e r i a l c o r r e l a t i o n between s u c c e s s i v e y e a r s . u n d e r s t a n d C o l e ' s argument o r we he has shown.  I t i s essential to  r i s k a complete m i s u n d e r s t a n d i n g o f what  G i v e n a s e t of " c y c l i c  1 1  d a t a on p o p u l a t i o n s i z e f o r any  a n i m a l , C o l e has shown t h a t you can produce a s i m i l a r " c y c l e " i n random numbers by i n t r o d u c i n g some s e r i a l c o r r e l a t i o n . anything.  Now  t h i s does not p r o v e  I t s u g g e s t s t h a t , g i v e n o n l y t h e s e d a t a , we  c o u l d i n t e r p r e t the  " c y c l e " as a random f l u c t u a t i o n , and t h i s would be t h e s i m p l e s t i f no o t h e r d a t a were a v a i l a b l e .  I n o t h e r words, i f we w i s h t o u n d e r s t a n d  " c y c l e s " we must s t u d y something more t h a n changes i n numbers. statest  interpretation  Cole  (1958)  "¥e s h o u l d seek t o u n d e r s t a n d t h e causes o f each case o f p o p u l a t i o n  growth and d e c l i n e i n s t e a d o f l o o k i n g f o r some h y p o t h e t i c a l and phenomenon capable o f g e n e r a t i n g  cryptic  cycles".  The second approach i s the one noitf emphasized by a m a j o r i t y o f workers on c y c l e s .  A t t e n t i o n has t u r n e d away from the p e r i o d i c i t y and toward  the p o p u l a t i o n a s p e c t s o f c y c l e s .  A s u p p o s i t i o n of t h i s approach i s t h a t  the problem o f c y c l i c l e n g t h w i l l be s o l v e d once t h e mechanism o f t h e s e c y c l e s i s understood.  I n t h i s s t u d y I have f o l l o w e d t h e second approach.  Much o f the d i f f i c u l t y o f t a l k i n g about " c y c l e s " a r i s e s because s e v e r a l meanings are g i v e n t o t h e term, and f a i l u r e t o d i s t i n g u i s h between them ( e . g . S l o b o d k i n , 1961)  g i v e s r i s e t o much c o n f u s i o n .  .We must t h e r e f o r e  attempt t o d e l i m i t t h e p a r t i c u l a r phenomenon t o be d i s c u s s e d here from a l l other " c y c l e s " .  .Chitty  (1952, I960) has d i s c u s s e d t h i s problem and c l a i m s t h a t  a s p e c i f i c t y p e o f c y c l e may t h i s approach, we  may  of c y c l e s t u d i e d here:  be r e c o g n i z e d i n m i c r o t i n e r o d e n t s .  Using  adopt the f o l l o w i n g d e f i n i t i o n f o r the p a r t i c u l a r type i n t h i s paper a c y c l e i s d e f i n e d as a t y p i c a l l y 3-U  year  - f l u c t u a t i o n i n numbers i n m i c r o t i n e r o d e n t s c h a r a c t e r i z e d by h i g h body w e i g h t s o f a d u l t s i n the peak summer. is  I do n o t propose t h i s as a d e f i n i t i o n  supposed t o a c c e p t , but I am merely s t a t i n g the way  everyone  I s h a l l use the word  III? c y c l e i n t h i s paper. and  Chitty  (1?60) d e f i n e d the problem somewhat more w i d e l y  i n c l u d e s t h i s d e f i n i t i o n as o n l y a p a r t i c u l a r i n s t a n c e  problem o f why p o p u l a t i o n s  o f t h e more  general  f a i l t o maintain a high rate of increase.  I s h a l l assume, u n t i l t h e r e i s e v i d e n c e t o the c o n t r a r y ,  that  these c y c l e s ( a s d e f i n e d above) are a s i n g l e c l a s s o f events and have a common e x p l a n a t i o n . and  The two f a c t s ( l ) t h a t t h e y are u s u a l l y 3-U y e a r c y c l e s  (2) t h a t h i g h body weights seem t o be always a s s o c i a t e d w i t h peak  populations  present  a s t r o n g argument f o r t h i s working  hypothesis.  Furthermore, t h i s i s a sound m e t h o d o l o g i c a l approach t o t h e problem i n the present  s t a t e o f knowledge.  One a l t e r n a t i v e i s t o b e g i n w i t h t h e assumption  t h a t a l l these r o d e n t c y c l e s have a d i f f e r e n t e x p l a n a t i o n . t h a t each c y c l e i s unique, l o c a l event and t h a t s u c c e s s i v e  This  implies  cycles i n the  same l o c a l i t y o r d i f f e r e n t l o c a l i t i e s cannot be compared, and c o n s e q u e n t l y t h i s makes i t i m p o s s i b l e  t o t e s t hypotheses o r t o p r e d i c t f u t u r e phenomena.  A n o t h e r a l t e r n a t i v e i s t o d i s t i n g u i s h a l i m i t e d number o f d i f f e r e n t t y p e s o f c y c l e s based on, f o r example, groups o f s p e c i e s o r c l i m a t i c zones.  I  have n o t used t h i s approach because I do n o t f e e l i t i s t h e most f r u i t f u l one  i n the present  s t a t e o f knowledge.  I am t h u s i n t e r e s t e d p r i m a r i l y i n  t h e t h i n g s common t o a l l c y c l e s and o n l y s e c o n d a r i l y i n those t h i n g s  restricted  t o a given area o r circumstance. However, we must r e c o g n i z e a s i n g l e c l a s s o f events.  t h a t there  i s no guarantee t h a t t h i s i s  I t i s p o s s i b l e t h a t t h e c l a s s i s l a r g e r than we have  i n d i c a t e d , perhaps i n c l u d i n g the g a l l i n a c e o u s b i r d s and t h e snowshoe h a r e .  I  do n o t w i s h t o argue w i t h t h o s e who w i s h t o make t h e c l a s s l a r g e r , b u t i t does not  seem t o me t o be prudent t o e x t e n d t h e c l a s s beyond t h e l i m i t s s e t by the  body weight c h a r a c t e r i s t i c u n t i l more e v i d e n c e becomes a v a i l a b l e .  But I do  object t o a r e s t r i c t i o n of the c l a s s t o include l e s s than t h a t given  above.  However, i f one b e l i e v e s t h e body weight c h a r a c t e r i s t i c t o be unimportant, one  c a n d e f i n e t h e problem d i f f e r e n t l y .  1U8 I suggest t h e r e f o r e t h a t these c y c l e s a s d e f i n e d above seem t o r e p r e s e n t a s i n g l e c l a s s o f e v e n t s and have a common e x p l a n a t i o n .  Thus a  s i n g l e e x p l a n a t i o n may be sought f o r lemming c y c l e s a t Baker Lake, i n A l a s k a , S c a n d i n a v i a , and R u s s i a , and v o l e c y c l e s i n E n g l a n d T h i s i s e s s e n t i a l l y t h e same b e l i e f e x p r e s s e d by C h i t t y  and e/SQ/ewhere.  (1952).  Reproduction S e v e r a l authors have d e s c r i b e d w i n t e r b r e e d i n g i n lemmings. Thompson (1955 a) working i n n o r t h e r n A l a s k a on Lammus t r i m u c r o n a t u s  found  t h a t w i n t e r b r e e d i n g o c c u r r e d o n l y d u r i n g t h e p e r i o d o f i n c r e a s e which he c l a i m e d o c c u p i e d two w i n t e r s , a l t h o u g h e v i d e n c e f o r b r e e d i n g d u r i n g the second w i n t e r i s n o t v e r y c o n c l u s i v e (as we have seen, lemmings may b r e e d under t h e snow e v e r y s p r i n g ) .  Dunaeva and Kucheruk (19IP-) found w i n t e r  b r e e d i n g i n both D i c r o s t o n y x t o r q u a t u s and Lemmus s i b i r i c u s i n R u s s i a d u r i n g the p e r i o d o f i n c r e a s e .  S u t t o n and Hamilton  (1932) found w i n t e r b r e e d i n g  i n b o t h D i c r o s t o n y x g r o e n l a n d i c u s and Lemmus t r i m u c r o n a t u s on Southampton I s l a n d during the p e r i o d o f i n c r e a s e . Tyan-Shanskii  Nasimovich,  Novikov, and Semenov-  (19U8) b e l i e v e d t h a t w i n t e r b r e e d i n g o f t h e Norwegian Lemming  was l i m i t e d t o t h e phase o f i n c r e a s e .  R e c e n t l y , C u r r y - L i n d a h l (1961) and  Koponen, Kokkonen, and K a l e l a (1961) r e p o r t e d p r o b a b l e w i n t e r b r e e d i n g i n the Norwegian lemming d u r i n g t h e p e r i o d o f i n c r e a s e .  Thus i t i s c l e a r t h a t t h e  o n l y r e p o r t s o f w i n t e r b r e e d i n g i n lemmings are from the p e r i o d o f i n c r e a s e . However, d u r i n g t h e p e r i o d o f low numbers i t would be v e r y d i f f i c u l t t o detect winter breeding. There i s e q u a l l y good evidence  t h a t t h e summer b r e e d i n g  season  i n the peak y e a r i s shortened i n lemmings, compared w i t h the i n c r e a s e o r d e c l i n e summers. Alaska.  Thompson (1955 a) r e p o r t e d t h i s f o r L . t r i m u c r o n a t u s i n  Dunaeva and Kucheruk (19U1) r e p o r t e d t h a t b r e e d i n g had ceased by  August i n the peak summer f o r D. t o r q u a t u s .  Nasimovich  e t a l . (19U8) and  Ill Kalela  0  (1961) b o t h found t h i s shortened summer b r e e d i n g season i n peak  p o p u l a t i o n s of L. Lemmus*  Wildhagen  (1953) d i d not r e p o r t e i t h e r w i n t e r  b r e e d i n g o r a shortened summer b r e e d i n g season i n the peak y e a r f o r Lo lemmus i n Norway; h i s samples however are v e r y s c a t t e r e d and d i s c o n t i n u o u s . The  a v a i l a b l e d a t a on l i t t e r - s i z e changes and  changes o v e r the lemming c y c l e are v e r y s c a r c e .  pregnancy-rate  Thompson (1955  a) r e p o r t e d  no change i n l i t t e r - s i z e , and h i s d a t a seem t o i n d i c a t e no d i f f e r e n c e i n midsummer pregnancy r a t e s over the c y c l e . in  such a way  made. his at  t h a t no s t a t i s t i c a l assessment o r d e t a i l e d comparisons may  H i s d a t a seem t o agree w i t h what was  own  U n f o r t u n a t e l y the d a t a are p r e s e n t e d  found i n t h i s study, and  be  even  d a t a f a i l t o b e a r out h i s c o n c l u s i o n t h a t r e p r o d u c t i o n proceeded  a reduced  l e v e l i n summers o f low p o p u l a t i o n d e n s i t y but reached  great  peaks o f i n t e n s i t y i n the summers o f h i g h d e n s i t i e s . There i s a l s o l i t t l e  i n f o r m a t i o n from t h e s e other.lemming s t u d i e s  on the q u e s t i o n o f changes i n the age cycle.  or weight a t s e x u a l m a t u r i t y o v e r the  Nasiraovich e t a l . (191*8) s t a t e t h a t most of the summer young  females  do not mature i n the peak y e a r ; n o t h i n g comparable i s s a i d about young males. Wildhagen  (1953) s t a t e s t h a t b o t h male and female Lemmus lemmus become f e c u n d  during t h e i r f i r s t  summer i n the peak y e a r , but h i s c r i t e r i o n of m a t u r i t y  for  the males has been q u e s t i o n e d by Newson ( p e r s . comm.), and  his  samples are v e r y d i s c o n t i n u o u s .  furthermore  C o n s i d e r i n g o t h e r c y c l i c m i c r o t i n e s b e s i d e s lemmings, we f i n d close p a r a l l e l i n Kalela's Lapland. (1)  (1957) s t u d y o f C l e t h r i o n o m y s rufocanus i n F i n n i s h  From h i s d a t a on the r e p r o d u c t i o n o f t h i s c y c l i c v o l e he  i n peak p o p u l a t i o n s n e a r l y a l l the summer young males and  concluded:  some o f the  summer young females f a i l e d t o mature; (2) a shortened summer b r e e d i n g o c c u r r e d i n the peak and d e c l i n e y e a r s ; and s i z e over the c y c l e . for  The  a  (3) t h e r e was  season  no change i n l i t t e r  s i m i l a r i t y o f these r e s u l t s t o those g i v e n p r e v i o u s l y  t h i s study i s q u i t e impressive.  150 Chitty  (1952) r e p o r t e d a s h o r t e n e d summer b r e e d i n g season i n  t h e peak y e a r f o r Microtias a g r e s t i s i n E n g l a n d .  Godfrey  (1953) suggested  t h a t a d e l a y i n r e a c h i n g m a t u r i t y f o r M. a g r e s t i s young may o n l y o c c u r i n y e a r s o f peak p o p u l a t i o n .  Stein  (1957) found no change i n l i t t e r s i z e o v e r  the c y c l e f o r M. a r v a l i s and a decrease maturing  d u r i n g t h e peak summers.  i n t h e percentage  o f young  females  Adams, B e l l , and Moore (quoted by  C h r i s t i a n , 1961) found i n M. montanus t h a t b r e e d i n g ceased e a r l y i n t h e peak summer and a p p a r e n t l y most o f t h e summer young males d i d n o t mature e i t h e r i n the peak or the d e c l i n e . peak b r e e d i n g season and a f a i l u r e summer f o r C l e t h r i o n o m y s  Z e j d a (1961) r e p o r t e d a s h o r t e n e d of summer young t o mature i n t h e peak  glareolus i n Czechoslovakia.  T h i s s e r i e s o f p o s i t i v e i n s t a n c e s suggests t h a t c y c l e s of t h e type d e f i n e d p r e v i o u s l y a r e a s s o c i a t e d w i t h a f a i r l y s p e c i f i c r e p r o d u c t i v e changes.  s e t of  I t i s important t o l o o k f o r n e g a t i v e i n s t a n c e s t o  see how f a r t h i s g e n e r a l i z a t i o n h o l d s .  Hamilton  accelerated breeding rate, increased l i t t e r  (1937  a, 19Ul)  s i z e , and l o n g e r r e p r o d u c t i v e  season i n i n c r e a s i n g and peak p o p u l a t i o n s o f M. p e n n s y l v a n i c u s . s t a t i s t i c a l d a t a were g i v e n f o r the l i t t e r to  t e l l i f t h e y are s i g n i f i c a n t .  r e p o r t e d an  No  s i z e changes so i t i s not p o s s i b l e  A l s o , Hamilton does n o t d i s c o u n t p o s s i b l e  body weight o r p a r i t y e f f e c t s and h i s i n c r e a s e d l i t t e r  s i z e s might be  e x p l a i n e d by the h e a v i e r animals i n h i s h i g h p o p u l a t i o n s (Hamilton, 1937  b).  He found w i n t e r b r e e d i n g o n l y i n t h e peak y e a r and no c u r t a i l m e n t o f t h e peak summer b r e e d i n g season. of  post-partum  H i s d a t a a l s o show an i n c r e a s e i n t h e amount  b r e e d i n g i n the peak y e a r .  Hamilton's  o b s e r v a t i o n s are a t  complete v a r i a n c e w i t h those d e s c r i b e d above f o r lemmings, and t h e y have never been r e p e a t e d . Hoffmann  (1958) s t u d i e d r e p r o d u c t i o n and m o r t a l i t y i n M. montanus  and M. c a l i f o r n i c u s .  He d e f i n e d t h e phases o f t h e c y c l e i n terms o f changes  151 i n f a l l population i f we  consider  d e n s i t i e s , and  a c o m p l e t e l y d i f f e r e n t p a t t e r n i s seen  changes i n h i s s p r i n g d e n s i t i e s , which C h i t t y and  (I960 a) c o n s i d e r e d  t o be the  i n d i c a t o r o f c y c l i c phase.  view he has no d a t a f o r the p e r i o d o f i n c r e a s e s p e c i e s o r f o r the p e r i o d o f d e c l i n e f o r M. e s s e n t i a l l y r e f e r to populations age  at maturity  and  o n l y minor changes i n l i t t e r  reproductive  or i n c i d e n c e  From t h i s p o i n t  c a l i f o r n i c u s , and  at peak phase o n l y .  He  found not "change i n  ovulation rate.  important changes must have been i n m o r t a l i t y .  He  T h i s i s the  exact  that  and  the  antithesis  but p a r t of t h i s apparent c o n f l i c t o f views  may  Hoffmann's  not. Much more c r i t i c a l d a t a on r e p r o d u c t i o n  e v e n t s i s needed.  i n r e l a t i o n to  cyclic  There i s c l e a r e v i d e n c e f r o m the more n o r t h e r l y lemmings  and v o l e s t h a t at l e a s t some c y c l e s are accompanied by the  studied  concluded t h a t  c y c l e and  a r i s e because Hamilton's d a t a c o v e r the p e r i o d of i n c r e a s e do  his data  o f p o s t - p a r t u m b r e e d i n g over the p e r i o d s i z e and  of  i n spring densities f o r either  changes were a r e l a t i v e l y minor p a r t of the  of Hamilton's conclusions,  Chitty  l e n g t h o f the b r e e d i n g season and  age  at maturity.  s t r i k i n g changes i n We  must now  whether t h i s i s a u n i v e r s a l c h a r a c t e r i s t i c o f these c y c l e s .  ask  Hamilton's  (193? a) and Hoffmann's (1958) d a t a suggest t h a t i t i s not and t h a t o t h e r patterns  are p o s s i b l e .  do these  patterns  i s p e r t i n e n t t o enquire what c o u l d have been the  cause of  d i f f e r from one It reproductive factors.  I f t h i s i s t r u e , how  and why  another?  changes observed i n t h i s s t u d y .  L e t us f i r s t  I t seems u n l i k e l y t h a t changes i n the f o o d  cause of these r e p r o d u c t i v e  changes.  Maynard and  consider  the  extrinsic  s u p p l y were the d i r e c t  Loosli  (1956, p387) p o i n t  out t h a t the n u t r i t i v e r e q u i r e m e n t s of b r e e d i n g females are g r e a t e r than those for (cf.  males and y e t i n t h i s s t u d y males were a f f e c t e d much more t h a n females T a b l e 27),  which suggests t h a t the f a c t o r s i n v o l v e d are not  M i l d w i n t e r weather may  have been n e c e s s a r y f o r the e x t e n s i v e  nutritional.  winter breeding  15a. t o o c c u r , b u t t h e r e was no evidence caused the midsummer b r e e d i n g summer o f d e c l i n e . evidence  t h a t any c l i m a t i c f a c t o r c o u l d have  changes f o u n d i n t h e peak summer o r t h e  I f we t u r n t o t h e i n t r i n s i c f a c t o r s , t h e r e i s no  t h a t these r e p r o d u c t i v e  changes were a f u n c t i o n o f d e n s i t y p e r se  because t h e y p e r s i s t e d i n t o the d e c l i n e i n some cases and a f f e c t e d the sexes d i f f e r e n t i a l l y .  There i s a l s o no evidence  t h a t these  reproductive  changes were c a u s e d by s t r e s s as d e f i n e d by C h r i s t i a n ( 1 9 5 9 ) j and, a l t h o u g h one  can o b t a i n r e p r o d u c t i v e  changes b y s t r e s s i n g animals,  t h e r e are o t h e r  ways t o do t h i s as x r e l l (e.g. Parkes and Bruce, 1961), and we are t h e r e b y no c l o s e r t o knowing what happens i n t h e f i e l d .  Nevertheless,  t h i s i s not  t o say t h a t t h e r e p r o d u c t i v e changes observed i n t h i s study do n o t have a p h y s i o l o g i c a l explanation.  I conclude t h a t these  reproductive  changes  were n o t caused p r i m a r i l y b y e x t r i n s i c f a c t o r s o r b y s t r e s s o r d e n s i t y p e r s e , b u t r a t h e r were caused b y some i n t r i n s i c change i n t h e p o p u l a t i o n , probably  associated with i n t r a s p e c i f i c To  sum up, a t l e a s t some c y c l e s are accompanied by a s e t o f  s p e c i f i c reproductive a shortened  strife.  changes i n v o l v i n g w i n t e r b r e e d i n g  summer b r e e d i n g  during the i n c r e a s e ,  season a t t h e peak, and a l a c k o f m a t u r a t i o n i n  young males and t o some e x t e n t i n young females d u r i n g the peak summer. The  a v a i l a b l e evidence  suggests t h a t w h i l e t h i s i s a common p a t t e r n i t may  not be found i n a l l c a s e s , and i t i s important  t o seek i n f o r m a t i o n on  c o n t r a r y i n s t a n c e s such as d e s c r i b e d by H a m i l t o n (1937 a ) .  The r e p r o d u c t i v e  changes d e s c r i b e d here cannot be e x p l a i n e d by e x t r i n s i c f a c t o r s b u t seem t o be  caused b y i n t r i n s i c  changes i n t h e p o p u l a t i o n .  Mortality V e r y l i t t l e work has been done on q u a n t i t a t i v e m o r t a l i t y assessments f o r cyclic microtines.  T h i s i s an important  p o i n t because t h e r e  i s a tendency  t o d i s r e g a r d v a r i a b l e s which have n o t been s t u d i e d i n t e n s i v e l y , o r e l s e t o p o s i t reasonable  but u n v e r i f i e d explanations  f o r the c y c l i c m o r t a l i t y which  153 would not be t e n a b l e  i f q u a n t i t a t i v e d a t a were a v a i l a b l e .  P a r t i a l p r e n a t a l m o r t a l i t y does n o t seem t o p l a y a n e c e s s a r y p a r t i n the c y c l e .  K a l e l a (1957) r e p o r t e d  no o b v i o u s change i n p r e n a t a l  m o r t a l i t y f o r Glethrionomys r u f o c a n u s , and Hoffmann (1958) found o n l y a s l i g h t change i n p a r t i a l p r e n a t a l m o r t a l i t y between peak and d e c l i n i n g populations  o f M. montanus.  I n f o r m a t i o n on t o t a l l i t t e r microtines  T h i s agrees w i t h t h e r e s u l t s o f t h i s  study.  l o s s i s almost c o m p l e t e l y l a c k i n g f o r c y c l i c  because t h i s t y p e o f l o s s i s d i f f i c u l t  t o measure.  I'fe may  conclude t h a t p r o b a b l y p a r t i a l p r e n a t a l m o r t a l i t y does n o t change over the c y c l e , b u t whether there  i s some change i n t o t a l l i t t e r  among young animals (as shown i n t h i s s t u d y ) , litter this  losses, particularly  i s n o t y e t known.  l o s s e s among a d u l t s are p r o b a b l y n o t s i g n i f i c a n t  Total  (Hoffmann, 1 9 5 8 ;  study). Hoffmann (1958) found t h a t w e a n l i n g and j u v e n i l e  increased  considerably  i n M. montanus d u r i n g  t h a t t h i s change was the k e y t o t h e d e c l i n e .  mortality  a d e c l i n e , and he suggested G o d f r e y (1955) found t h a t  h i g h m o r t a l i t y o f j u v e n i l e s was a s s o c i a t e d w i t h the d e c l i n e o f two M. a g r e s t i s populations,  and j u v e n i l e male m o r t a l i t y was a l s o h i g h  E l t o n , D a v i s , and F i n d l a y  (1935) have r e c o r d e d another i n s t a n c e  j u v e n i l e m o r t a l i t y i n a d e c l i n e o f M. a g r e s t i s . high  i n the peak summer.  C h i t t y (1952) found t h a t  j u v e n i l e m o r t a l i t y was a s s o c i a t e d w i t h peak and d e c l i n i n g  o f the same  of high  populations  species.  The r e s u l t s o f t h i s study agree w i t h those on M i c r o t u s and i n d i c a t e that a high  j u v e n i l e m o r t a l i t y rate occurred  a t l e a s t i n a l l the  d e c l i n i n g populations  which showed no r e c o v e r y (G).  Juvenile mortality i n  those d e c l i n e s which showed some r e c o v e r y (H) must be l e s s than i n Type G d e c l i n e s , b u t no q u a n t i t a t i v e  d a t a are a v a i l a b l e .  Some workers found  high  j u v e n i l e m o r t a l i t y a l s o i n t h e peak summer, b u t t h e r e was no s u g g e s t i o n o f  151* t h i s i n the p r e s e n t study, and t h i s c h a r a c t e r i s t i c may n o t be a c o n s t a n t feature  o f the c y c l e . C h i t t y (1952) r e p o r t e d  increased  adult m o r t a l i t y i n the spring  of t h e d e c l i n e , and the d a t a o f G o d f r e y (1955) suggest the same t h i n g . This increased  m o r t a l i t y however may be c o n f i n e d  t o a s h o r t p e r i o d i n the  s p r i n g when b r e e d i n g b e g i n s ( C h i t t y and C h i t t y , I960 a ) .  V e r y few e x t e n s i v e  measurements o f a d u l t m o r t a l i t y r a t e s have been made ( L e s l i e e t a l . , 1 9 5 3 ; C h i t t y and C h i t t y , I960 a ) , and we must be c a r e f u l n o t t o e x t r a p o l a t e from o b s e r v a t i o n s  t o o much  on midsummer a d u l t m o r t a l i t y such as were made i n t h i s  study. We may conclude from t h e above d a t a t h a t j u v e n i l e m o r t a l i t y changes are most important over t h e c y c l e and exceed any changes i n a d u l t which may o c c u r . mice b y B e n d e l l  mortality  T h i s i s n o t an u n u s u a l s i t u a t i o n , f o r s t u d i e s on n o n - c y c l i c (1959) and M a r t i n  (1956) a l s o p o i n t e d  t o t h e importance o f  j u v e n i l e m o r t a l i t y i n d e t e r m i n i n g d e n s i t y changes, and Lack (195U)  concluded  t h a t i n a l l a n i m a l s t h e d e a t h r a t e i s h i g h e r i n t h e j u v e n i l e s than i n t h e adults. Migrations Migrations  o f lemmings have been r e p o r t e d  p a r t i c u l a r b u t a l s o from v a r i o u s p a r t s  from S c a n d i n a v i a i n  o f N o r t h America.  I n view o f t h e  p r e o c c u p a t i o n o f many people w i t h t h e s e m i g r a t i o n s i t may be p r o f i t a b l e t o e n q u i r e how t h e s e m i g r a t i o n s d i f f e r from t h e s p r i n g u n r e s t  and wandering  found a t Baker Lake i n I960. Thompson (1955 c ) has d e s c r i b e d  a brown lemming e m i g r a t i o n a t  P o i n t Barrow, A l a s k a t h a t seems t o resemble c l o s e l y my o b s e r v a t i o n s previously.  given  F o r about s i x days a t t h e b e g i n n i n g o f June o f t h e peak summer,  when the snow was m e l t i n g  and summer b r e e d i n g had j u s t begun, i n d i v i d u a l  lemmings moved h a p h a z a r d l y t h r o u g h the camp and out onto the s e a i c e .  Only  155 a s m a l l percentage  o f the t o t a l p o p u l a t i o n took p a r t i n t h i s  Thompson remarked t h a t t h i s e m i g r a t i o n was m i g r a t i o n s of S c a n d i n a v i a n  emigration.  v e r y d i f f e r e n t from the mass  lemmings.  There i s no good evidence  f o r the North American a r c t i c t h a t  any  o t h e r type of lemming movements o c c u r s b e s i d e s t h a t d e s c r i b e d by Thompson. The  r e p o r t o f Gavin  (19U5)  o f a t e n day mass m i g r a t i o n of brown lemmings  at a d e n s i t y of one p e r sq. y a r d i s h a r d l y c r e d i b l e .  There i s no  question  t h a t lemmings do move i n d i v i d u a l l y on sea i c e , l a k e s , and the l a n d d u r i n g the s p r i n g m e l t - o f f i n peak y e a r s and t h a t t h e y may on i c e . a time may  There i s no q u e s t i o n t h a t one may  on the bare p a t c h e s  move q u i t e l o n g d i s t a n c e s  see t e n o r f i f t e e n lemmings a t  of ground d u r i n g the m e l t - o f f , and t h a t s l e d dogs  engorge themselves on lemmings w h i l e t r a v e l l i n g a c r o s s c o u n t r y .  i s a complete myth t o e x t r a p o l a t e such e v e n t s , as G a v i n  (l U5) Q  But i t  did, into  s o l i d mass of lemmings marching i n a p a r t i c u l a r d i r e c t i o n f o r days on I have been s e r i o u s l y t o l d by people  a t Baker Lake t h a t d u r i n g the  a  end.  I960  s p r i n g t h e r e were " m i l l i o n s " o f lemmings marching a c r o s s the t u n d r a toward Hudson Bay,  and t h a t t h e r e were "thousands" o f lemmings a l l over the l a k e i c e  when i n f a c t fewer t h a n 50  lemmings were a c t u a l l y seen by the persons  involved.  I t h e r e f o r e r e j e c t the s u g g e s t i o n t h a t mass m i g r a t i o n s o f lemmings occur i n North  America. L e t us now  (l895» 1911j  l o o k a t the S c a n d i n a v i a n  summarized by E l t o n ,  d e s c r i p t i o n s o f these movements. seems t o be  as f o l l o w s :  19U2) The  lemming m i g r a t i o n s .  has g i v e n one  o f the most e x t e n s i v e  evidence f o r m i g r a t i o n s which he  ( l ) lemmings are found i n the lowlands  numbers d u r i n g some y e a r s ;  (2)  Collett  i n d i v i d u a l lemmings may  gives  i n great  appear on c i t y  streets,  swimming i n the ocean, o r o t h e r abnormal p l a c e s d u r i n g the peak y e a r s ; (3)  v a r i o u s o b s e r v e r s r e p o r t " m i g r a t i n g swarms".  and  But i t has never been  shown t h a t lemmings do not i n h a b i t the woodland and  lowland  zones as a normal  h a b i t a t even i n low y e a r s , and y e t t h i s i s a c r i t i c a l p o i n t r e g a r d i n g whether  156 o r n o t a m i g r a t i o n i s n e c e s s a r y t o account Collett  f o r t h e presence  o f these  animals.  {189$, p 17) s t a t e s t h a t one r a r e l y sees lemmings even i n the b e s t  h a b i t a t s d u r i n g normal y e a r s , and y e t the b u l k o f t h e evidence t h a t lemmings do n o t i n h a b i t the xiroodland and l o w l a n d there.  zones i s t h a t t h e y a r e never seen  A g a i n t h e r e i s no doubt t h a t i n d i v i d u a l leiranings do move i n t o  abnormal p l a c e s d u r i n g a peak y e a r such as C o l l e t t d e s c r i b e s .  Collett  (1895),  p o r t r a y i n g t h e type o f movement, s t a t e s : "They a r e n o t s o c i a b l e i n the sense o f s e v e r a l i n d i v i d u a l s d e l i b e r a t e l y j o i n i n g company f o r l o n g d i s t a n c e s . . . T h e r e f o r e t h e y seldom, i f e v e r , advance i n c l o s e ranks as g e n e r a l l y d e p i c t e d i n drawings..." (p U3)  and again (1911): "They migrate c h i e f l y b y n i g h t , b u t a l s o p a r t l y b y d a y l i g h t , always s i n g l y o r some few near t o g e t h e r , never i n c l o s e f o r m a t i o n . . " ( t r a n s . ) I f t h i s i s t r u e , t h e n how do we d e c i d e when a " m i g r a t i o n " i s o c c u r r i n g ? There i s n o t a s i n g l e q u a n t i t a t i v e o b s e r v a t i o n on the e x t e n t o f t h e s e movements.  K a l e l a (19U9) s t a t e s t h a t t h e Norwegian lemming extended i t s  range b y more t h a n 100 k i l o m e t e r s over t h r e e subsequent c y c l e s , b u t t h e r e i s no e v i d e n c e why the s i m p l e r e x p l a n a t i o n o f permanent low d e n s i t y p o p u l a t i o n s i n t h e "invaded" areas i s n o t a c c e p t a b l e .  Nasimovich e t a l . (19U8) r e c o r d e d  the f o l l o w i n g o b s e r v a t i o n s on Norwegian lemmings: " I n s p r i n g . . . . t h e lemmings r a n s i n g l y on the i c e , n e v e r f o r m i n g groups, and o n l y i n a few c a s e s were more t h a n t h r e e animals seen s i m u l t a n e o u s l y ... On an e x c u r s i o n on the i c e o f ( a l a k e ) from end t o end (about 18 km) 20-32 r u n n i n g lemmings were counted. Thus t h e s p r i n g m i g r a t i o n s observed b y us are f a r d i f f e r e n t from t h e p i c t u r e o f mass 'flows' d e s c r i b e d b y other w r i t e r s . . . " ( t r a n s l a t i o n page 27) R e c e n t l y K a l e l a (1961) and Koponen e t a l . (1961) have d i s c u s s e d lemming m i g r a t i o n s i n n o r t h e r n L a p l a n d .  They d i s t i n g u i s h  (1) s p r i n g m i g r a t i o n s  which go on f o r about 1 week, and (2) f a l l m i g r a t i o n s which may go on f o r 2-3 months.  Koponen e t a l . (1961) have d e s c r i b e d a s p r i n g m i g r a t i o n on l a k e  i c e which was e x t r e m e l y  s i m i l a r t o t h a t d e s c r i b e d above f o r t h i s  t h a t d e s c r i b e d b y Thompson (1955 c ) .  study and  Each lemming on t h e i c e moved  157 i n d e p e n d e n t l y , and o n l y v e r y s m a l l numbers o f lemmings were i n v o l v e d .  The  b e g i n n i n g o f these movements c o i n c i d e d w i t h the s t a r t o f the s p r i n g b r e e d i n g season and seemed t o be a s s o c i a t e d w i t h a s e a s o n a l change o f h a b i t a t . Kalela of  (1961)  habitat.  has d e s c r i b e d f a l l m i g r a t i o n s a s s o c i a t e d w i t h a s e a s o n a l change  Not a s i n g l e a n i m a l was marked and r e c a p t u r e d i n t h i s work, and  a l l t h e e v i d e n c e f o r m i g r a t o r y movements c o n s i s t s o f t h e f a c t s t h a t ( l ) lemmings were found i n farmyards lemmings were  and o t h e r u n u s u a l p l a c e s a t t h e peak, and (2)  seen i n an a r e a a t one time  were t r a p p e d t h e r e .  and a t a l a t e r date  no  some lemmings  No one doubts t h a t i n d i v i d u a l lemmings do wander  into  s t r a n g e p l a c e s a t t i m e s o f peak d e n s i t i e s , and no evidence i s p r e s e n t e d why l o c a l r e p r o d u c t i o n a l o n g w i t h movements o f s e v e r a l hundred meters a t the most between s e a s o n a l h a b i t a t s would n o t be enough t o account f o r a l l t h e changes o b s e r v e d b y K a l e l a  (1961).  I reject h i s claim that f a l l  migrations  o c c u r i n t h e Norwegian lemming because the d a t a p r e s e n t e d have a much s i m p l e r explanation. It for  i s i n d e e d s u r p r i s i n g t o f i n d t h a t t h e r e i s no o b j e c t i v e evidence  mass m i g r a t i o n s o f t h e Norwegian lemming.  The s p r i n g " m i g r a t i o n s " d e s c r i b e d  seem t o be no d i f f e r e n t from t h e l o c a l movements found by Thompson (1955 and b y t h i s s t u d y .  T h e r e i s no good evidence t h a t the f a l l  c)  "migrations"  are a n y t h i n g b u t l o c a l movements o f i n d i v i d u a l s a t h i g h d e n s i t i e s .  No  o r i e n t e d l o n g d i s t a n c e movements o f groups have ever been demonstrated. U n t i l evidence t o t h e c o n t r a r y becomes a v a i l a b l e , i t seems b e s t t o r e g a r d mass lemming m i g r a t i o n s as a f i c t i o n and t o c o n f i n e our a t t e n t i o n t o t h e i n d i v i d u a l movements found sometimes a t peak d e n s i t i e s . Thompson (1955  c ) s t a t e s t h a t t h e mass u n r e s t i n t h e s p r i n g o f  the peak y e a r a t P o i n t Barrow was p r o b a b l y caused b y changes i n a v a i l a b l e f o o d and cover and seemed t o have a v e r y minor e f f e c t on l o c a l p o p u l a t i o n densities.  He does not c o n s i d e r the f a c t t h a t t h i s u n r e s t marks t h e onset  158 o f summer b r e e d i n g . m e l t - o f f may  The  sudden e n v i r o n m e n t a l changes a s s o c i a t e d w i t h the  not even by a n e c e s s a r y cause o f t h i s u n r e s t , because the  same t y p e o f s h u f f l e i s a l s o found i n v o l e s a t t h e s t a r t o f summer b r e e d i n g ( C h i t t y and P h i p p s , 1 9 6 l ) .  I t would be d i f f i c u l t  o f course t o f i n d a n a t u r a l  s i t u a t i o n i n the lemmings t o t e s t t h i s h y p o t h e s i s t h a t the onset o f summer b r e e d i n g i s a s u f f i c i e n t cause  o f the mass u n r e s t observed, and  so we  must l e t the matter r e s t f o r the moment. To sum up, t h e r e i s no c o n v i n c i n g evidence t h a t mass m i g r a t i o n s o f lemmings o c c u r e i t h e r i n N o r t h America  or i n Scandinavia.  The  descriptions  p u b l i s h e d f o r s p r i n g " m i g r a t i o n s " can be e x p l a i n e d as s m a l l l o c a l e m i g r a t i o n s of i n d i v i d u a l s such as d e s c r i b e d by Thompson (1955 may  These l o c a l e m i g r a t i o n s  be caused by the onset o f summer b r e e d i n g a c t i v i t y i n dense p o p u l a t i o n s  perhaps The  c).  c o u p l e d w i t h the s t r o n g e n v i r o n m e n t a l changes d u r i n g the m e l t - o f f .  f a l l " m i g r a t i o n s " seem t o be n o t h i n g but l o c a l movements o f i n d i v i d u a l s  caused i n p a r t by h i g h d e n s i t i e s . Weather and Synchrony The problem  o f synchrony o f c y c l e s over l a r g e a r e a s o f c o u n t r y  has l o n g i n t r i g u e d workers.  I do n o t propose  t o d i s c u s s any o f the  cosmic  t h e o r i e s t h a t have a t one time o r another been p u t f o r w a r d t o e x p l a i n synchrony.  Weather seems t o be t h e o n l y r e a s o n a b l e v a r i a b l e which c o u l d  account f o r t h i s synchrony.  Thus we  o r d i n a r y weather phenomena — etc. —  must attempt  t o d i s c o v e r what e f f e c t  deep snow c o v e r s , warm s p r i n g s , wet  have on r e p r o d u c t i o n and m o r t a l i t y o f c y c l i c As C h i t t y  numbers was  (1952)  summers,  animals.  has p o i n t e d out, i f e v e r y t h i n g about a c y c l e i n  e x p l a i n e d by an i n t r a s p e c i f i c p r o c e s s ( o r , f o r t h a t matter,  by  the f o o d s u p p l y h y p o t h e s i s ) , we would expect non-synchronous f l u c t u a t i o n s , which i s not what we p a r t i n the c y c l e .  observe a t a l l .  Thus weather must p l a y a n e c e s s a r y  I t seems c l e a r a t the o t h e r extreme t h a t weather changes  159 cannot be a s u f f i c i e n t cause o f the c y c l i c i n c r e a s e  o r d e c l i n e because  we would n o t n o r m a l l y g e t 3-4 y e a r c y c l e s i f t h i s were t r u e there  a r e no weather c y c l e s o f t h i s p e r i o d i c i t y ) .  We conclude t h a t  weather changes must be a n e c e s s a r y cause o f i n c r e a s e in  these c y c l i c  (assuming  or d e c l i n e or both  s p e c i e s b u t cannot be the e n t i r e cause.  (19U3) c o n c l u d e d t h a t D i c r o s t o n y x p o p u l a t i o n s a t  Shelford  C h u r c h i l l tended t o i n c r e a s e w i t h average o r above average s n o w f a l l which gave p r o t e c t i o n over the e n t i r e w i n t e r and w i t h warm temperatures i n J u l y and August, and t e n d e d t o d e c l i n e over c o l d w i n t e r s is  c l e a r from h i s d a t a ,  not r e s p o n s i b l e winters  with l i t t l e  snow. I t  however, t h a t weather changes alone were p r o b a b l y  f o r t h e i n c r e a s e s o r d e c l i n e s observed because some f a v o r a b l e  were n o t accompanied b y i n c r e a s e s and a t l e a s t one f a v o r a b l e  was f o l l o w e d b y a d e c l i n e . summer weather had l i t t l e  Collett  winter  (1895) p o i n t e d out t h a t s p r i n g and  e f f e c t on L. lemmus p o p u l a t i o n s .  The  suggestion,  t h e r e f o r e , f o r lemmings i s t h a t summer weather n o r m a l l y has l i t t l e  o r no  e f f e c t on the c y c l e , b u t t h a t w i n t e r weather may be a p a r t i a l cause o f t h e i n c r e a s e s and d e c l i n e s . Chitty populations  (1952, i960) has r e p o r t e d i n s t a n c e s o f asynchrony i n  v e r y c l o s e t o each o t h e r  and c o n c l u d e d f r o m t h i s t h a t bad  weather alone was n o t s u f f i c i e n t t o cause a d e c l i n e i n M i c r o t u s a g r e s t i s populations.  There a r e few o t h e r r e p o r t s o f p o p u l a t i o n s  l o c a l i t y f l u c t u a t i n g out o f phase such as C h i t t y found. r e p o r t s some i n s t a n c e s  from n o r t h e r n  Alaska  c l e a r t h a t i t i s not easy t o f i n d these,  i n the same P i t e l k a (1961)  o f asynchrony, b u t i t seems  and t h e a r e a s i n v o l v e d seem t o  come back i n t o phase r a t h e r q u i c k l y . Pitelka in  northern  (1957 b) has d i s c u s s e d  Alaska.  some a s p e c t s o f r e g i o n a l synchrony  He has c o n c l u d e d f r o m t h e a v a i l a b l e d a t a t h a t the  s h o r t term c y c l e i s not a normal c h a r a c t e r i s t i c o f t u n d r a m i c r o t i n e s  everywhere,  160 and that c y c l i c fluctuations among several microtines i n the same area are not t y p i c a l l y i n phase.  Now  i t i s probably true that very strong  cycles such as occur at Point Barroxtf do not occur i n the areas more toward the i n t e r i o r of Alaska, but t h i s does not mean that the same phenomenon may not be occurring there to a l e s s e r extent.  In other words, the absolute  densities at the "peak" and the "low" may be very d i f f e r e n t from area to area ( t h i s i s one problem to be explained) while the same c y c l i c process may occur i n a l l these areas (and t h i s i s another problem).  To map  the  extent of c y c l i c "highs" by means of airplane observations on the abundance of predators and d r i f t e d winter-cut vegetation, such as was done f o r northern Alaska, seems to me to miss the whole point at issue.  I f we applied t h i s  same technique to the Canadian Barren Grounds we would conclude some very misleading things about c y c l i c "highs" ( i . e . that the lemming cycle was confined to a very small part of the t o t a l area, i n habitats of very thick marshy vegetation), and yet the vegetation of the F o o t h i l l s sector of northern Alaska (Britton, 1957) Grounds.  i s rather s i m i l a r to that of the Barren  Even casual observations on the ground can be very misleading  i n these respects.  I t seems premature to decide whether cycles are or are  not c h a r a c t e r i s t i c of tundra microtines everywhere.  However, we must look  f o r instances of non-cyclic populations as i t would be most i n t e r e s t i n g to compare t h i s type of population with a normal c y c l i c population. A second problem regarding synchrony treated by P i t e l k a (1957 i s whether sympatric microtines cycle i n phase.  E l t o n (191+2, p U39)  that both species of lemmings probably fluctuated i n phase.  stated  Watson (1957)  believed that when Lemmus and Dicrostonyx were sympatric they tended to fluctuate i n phase, although t h i s synchrony was never exact. given i n t h i s paper support t h i s b e l i e f .  b)  The data  No intensive work has yet been  done on areas where three or more c y c l i c microtines commonly occur.  161 The r o l e of weather i n c y c l i c f l u c t u a t i o n s remains very poorly understood, and t h i s generalization probably applies to almost a l l animal populations  (Andrewartha and Birch, 195U).  We cannot study a natural  population i n the absence of weather and we have not yet learned to set up laboratory populations which are comparable t o f i e l d populations.  It i s  c e r t a i n l y possible to ascribe almost a l l population-changes to weather changes by ad hoc hypotheses (e.g. Schindler, l ° 6 o ) , but t h i s hardly furthers our understanding of the changes. To sum up, weather must be regarded as a necessary cause of the c y c l i c increase or decline because ( l ) i f weather was not necessary, cycles would not tend to be synchronous, and (2) i f weather was s u f f i c i e n t to cause the increase or decline, cycles would not tend to be 3-h years i n length.  To  date the only p l a u s i b l e explanation of t h i s r o l e of weather i s that of Chitty (1952, 1955 b, I960) which i s i l l u s t r a t e d i n a model by L e s l i e ( 1 9 5 9 ) . Predators Very few workers today support the idea that the cycle i s caused by predators  (Lack, W9$hy p 213; P i t e l k a et a l . , 1 9 5 5 ) .  There i s no doubt  that under c e r t a i n conditions predators do k i l l many lemmings, and P i t e l k a (1959) believes that they may dampen the fluctuations of the lemmings at least i n northern Alaska.  A sharp spring decline i n Lemmus occurred i n i 9 6 0  i n t h i s study i n the v i r t u a l absence of predators;  s i m i l a r spring declines  were described by Thompson (1955 a) and P i t e l k a (1957 a) and attributed to predators.  While i t i s reasonable to suppose that predation might a l t e r  the length and pattern of the cycle, mere association of events must be viewed critically. Disease and Parasites E l t o n (19U2, p 201) and C h i t t y (195U,  I960)  have shown that disease  cannot be regarded as a s u f f i c i e n t condition f o r a decline i n numbers.  Disease  162 i s b e l i e v e d t o be a l o c a l f a c t o r o f v a r i a b l e i n t e n s i t y and occurrence n o t an e s s e n t i a l p a r t o f the c y c l i c p r o c e s s . opposes t h e s e  N o t h i n g from t h i s  and  study  ideas.  Body Weight Changes H i g h body w e i g h t s i n t h e peak summer have been described!, by Chitty  M.  (19$2) f o r M i c r o t u s a g r e s t i s , Zimmermann (19$$) and S t e i n (1957) f o r  a r v a l i s , Thompson  (1955 a) f o r Lemmus t r i m u c r o n a t u s , K a l e l a (1957) f o r  Clethrionomys rufocanus. S t e i n and by t h i s  (1956) and Z e j d a (196l) f o r C. g l a r e o l u s ,  s t u d y f o r Lemmus t r i m u c r o n a t u s and D i c r o s t o n y x g r o e n l a n d i c u s .  I t i s i m p o r t a n t t o e n q u i r e why  t h i s weight change o c c u r s .  A change i n mean body weight  o f a d u l t s may  be produced i n two  ( l ) b y a change i n the growth r a t e o f i n d i v i d u a l s ; o r (2)  g e n e r a l ways:  a change i n the normal age c l a s s s t r u c t u r e o f t h e p o p u l a t i o n . would be a r e a l e f f e c t , t h e second a s t a t i s t i c a l e f f e c t .  The  by  first  Zimmermann  (1955)  b e l i e v e d t h a t t h e s e weight changes d i d n o t r e p r e s e n t mere changes i n the p r o p o r t i o n s o f the age c l a s s e s but were p a r t l y caused by changes i n the growth r a t e o f i n d i v i d u a l s .  He  s t a t e s t h a t p r o b a b l y the same e x t r i n s i c  f a c t o r s cause the d e n s i t y changes and the growth changes ( e . g . f a v o r a b l e weather).  Stein  (1956) found t h a t the lower age groups were m i s s i n g from  peak and d e c l i n i n g p o p u l a t i o n s o f C l e t h r i o n o m y s g l a r e o l u s i n Germany and t h i s caused the mean body weight populations.  i n t h e s p r i n g t o be g r e a t e r i n peak  He b e l i e v e d t h a t these changes were not produced by e x t r i n s i c  v a r i a b l e s which caused a change i n growth r a t e , but r a t h e r t h a t t h e y were due  t o a s e l e c t i v e e l i m i n a t i o n o f the younger animals by some form o f  intraspecific  strife  ( i . e . t h a t t h e e f f e c t was  statistical).  Zejda  (1961) o f f e r e d a d i f f e r e n t i n t e r p r e t a t i o n of S t e i n ' s r e s u l t s based on his  won work on C. g l a r e o l u s , i . e . t h a t these lower age groups were  m i s s i n g because  o f t h e s h o r t e n e d r e p r o d u c t i v e season i n the peak y e a r .  163 Thus p o p u l a t i o n s descended  from normal  spring to f a l l breeding  seasons  would have on the average normal body w e i g h t s , but those descended c u r t a i l e d summer b r e e d i n g seasons would i n c l u d e o n l y the l a r g e r animals and not t h e s m a l l e r f a l l  from  spring  a n i m a l s , and t h o s e from which t h e s p r i n g  l i t t e r s are e l i m i n a t e d would produce  on t h e average below-normal s i z e  animals. It  i s d i f f i c u l t t o r e c o n c i l e any o f t h e s e hypotheses w i t h the  r e s u l t s found i n t h i s s t u d y .  Presumably  lemmings born d u r i n g t h e w i n t e r  s h o u l d have t h e lowest growth r a t e s , and y e t i t was formed the b u l k o f the h i g h weight  these a n i m a l s t h a t  a d u l t s o f summer 1°60.  As we have  a d u l t lemmings do not grow much a f t e r the f i r s t week o f June. the anomalous s i t u a t i o n i n which t h e 1°60  seen,  Thus we  a d u l t animals were produced  time o f t h e y e a r when t h e r e i s no v e g e t a t i v e growth and had reached h i g h weights b e f o r e any summer p l a n t growth began.  Furthermore,  have at a  their  a 3-4  year  c y c l i c a l change o f e x t r i n s i c f a c t o r s would be needed t o v e r i f y Zimmermann's hypothesis f o r c y c l i c a l species.  These c o n s i d e r a t i o n s seem t o r u l e  Zimmermann's e x p l a n a t i o n as s u f f i c i e n t . season o f 1°60  produced  A l s o , the c u r t a i l e d summer b r e e d i n g  s m a l l a d u l t s the f o l l o w i n g y e a r on some a r e a s  G d e c l i n e ) and l a r g e a d u l t s on o t h e r a r e a s (Type H d e c l i n e s ) . to  r u l e out t h e i n t e r e s t i n g h y p o t h e s i s o f Z e j d a .  of  S t e i n i s e x c l u d e d because  (winter generation  out  the youngest  (Type  T h i s seems  F i n a l l y , the h y p o t h e s i s  a d u l t s were a l s o t h e l a r g e s t  I960), and the g r e a t e s t amount o f s t r i f e seemed t o produce  s m a l l e r animals ( w i n t e r g e n e r a t i o n 1961)  not l a r g e r ones.  Furthermore,  t h e l a r g e animals o f the peak lemming p o p u l a t i o n s are b i g g e r animals i n e v e r y way  t h a n t h o s e from d e c l i n i n g o r low p o p u l a t i o n s .  occupy weight c l a s s e s which are not even approached  These l a r g e  animals  i n the low y e a r s , and  t h u s t h i s change r e p r e s e n t s more t h a n a s t a t i s t i c a l change o f p r o p o r t i o n s within certain size  groups.  16U C h i t t y and C h i t t y (I960 b) found t h a t weather d i f f e r e n c e s not  account f o r the d i f f e r e n c e s i n growth s i n c e o p p o s i t e  o b s e r v e d on two  a r e a s s u b j e c t t o the  a l s o found t h a t h i g h p o p u l a t i o n an adequate e x p l a n a t i o n .  could  e f f e c t s were  same weather w i t h i n one  season.  d e n s i t y a t the time of poor growth was  There i s no i n d i c a t i o n t h a t age  the low weight animals o f the Type G d e c l i n e s were a t l e a s t 3-4 average t h a n the h i g h weight a n i m a l s of the  peak.  t h i s s i z e change a s s o c i a t e d w i t h d e n s i t y changes might i n v o l v e  from d e c l i n i n g p o p u l a t i o n s i n the f i e l d . was  not  Newson and  study  months o l d e r  S t e i n (1956) seems t o have been the f i r s t t o p o s t u l a t e  changes i n the p o p u l a t i o n .  not  differences  are r e s p o n s i b l e f o r the o b s e r v e d d i f f e r e n c e s ; i n d e e d i n the p r e s e n t  on the  They  that  genetic  C h i t t y (1962) found t h a t some v o l e s  would grow i f brought i n t o the l a b but none grew  T h i s demonstrated t h a t the  a s u f f i c i e n t explanation  intrinsic  c o n d i t i o n of the  o f low body w e i g h t s d u r i n g the  hence t h a t some e n v i r o n m e n t a l v a r i a b l e must be  animals  decline,  i n v o l v e d , p r o b a b l y some  and  aspect  of behavior. The  c o n c l u s i o n which emerges from t h i s d i s c u s s i o n i s t h a t  weight changes a s s o c i a t e d w i t h c y c l i c f l u c t u a t i o n s r e p r e s e n t the Me  growth r a t e s of i n d i v i d u a l s and do not know why  change the  change o v e r t h e c y c l e .  One  growth r a t e o f l a b o r a t o r y animals i s t o modify the d i e t  way  to  (e.g.  i t i s p o s s i b l e t o change growth r a t e s i n  o t h e r ways as w e l l ( e . g . MacArthur, 1949; we  a change i n  thus a change i n p o p u l a t i o n q u a l i t y .  growth r a t e s should  Osborne and Mendel, 1926), but  the  C r o w c r o f t and Rowe, 196l)  and  so  cannot conclude what f a c t o r s are n e c e s s a r i l y i n v o l v e d i n these weight  changes u n t i l f u r t h e r study i s made.  I t i s p o s s i b l e t h a t these body weight  changes d i s c u s s e d here are not a s i n g l e c l a s s of events but several different factors. b e l i e v e t h a t we  should  But,  u n t i l we  l o o k f o r a common  are produced  have e v i d e n c e t o the c o n t r a r y , explanation.  by I  165 I n summary, we know t h a t h i g h body weights are a s s o c i a t e d w i t h peak p o p u l a t i o n s o f s e v e r a l d i f f e r e n t t y p e s o f m i c r o t i n e s . understand  ¥ e do not  why animals do not grow d u r i n g c e r t a i n phases o f the  cycle,  except t h a t these d i f f e r e n c e s seem t o be a r e s u l t o f i n t r a s p e c i f i c  interactions.  The r e l e v a n t change i s i n the growth r a t e s o f i n d i v i d u a l animals, and g e n e t i c changes may be i n v o l v e d . Three C u r r e n t Hypotheses I would now l i k e t o c o n s i d e r t h r e e c u r r e n t hypotheses which attempt to  e x p l a i n these c y c l i c a l f l u c t u a t i o n s .  (1) Food Supply  Hypothesis;  The h y p o t h e s i s supported b y P i t e l k a from h i s work and t h a t o f Thompson at P o i n t Barrow, A l a s k a on the brown lemming i s shown i n F i g u r e 8. The  essence  o f t h e change i n v o l v e d i n the d e c l i n e i s a q u a l i t a t i v e  and  q u a n t i t a t i v e change i n the f o r a g e ; p r e d a t i o n does not seem t o be an e s s e n t i a l p a r t o f the d e c l i n e ( P i t e l k a e t a l . ,  1955)•  h y p o t h e s i s f i t s the o b s e r v a t i o n s of t h i s  We may enquire whether t h i s  study.  F i r s t , t h e r e was no e x t e n s i v e f o r a g e u t i l i z a t i o n a t Baker Lake and t h i s would seem t o c r i p p l e t h i s h y p o t h e s i s a t the s t a r t  (Table U?)«  Second, t h e r e was no evidence o f s t a r v a t i o n i n animals a l i v e i n the s p r i n g of  t h e d e c l i n e ( T a b l e s 55-58).  to  the h y p o t h e s i s because Thompson (1955 a) r e p o r t e d no evidence o f  However, t h i s i s a p p a r e n t l y not an o b j e c t i o n  m a l n u t r i t i o n i n P o i n t Barrow lemmings i n the s p r i n g of the d e c l i n e e i t h e r . We are l e f t w i t h the q u a l i t a t i v e f o r a g e change as t h e supposed cause of t h e decline.  Yet t h e r e was no evidence  o f d e f i c i e n c y d i s e a s e s i n t h e young o f  t h e d e c l i n e ; i n d e e d , the whole d i f f i c u l t y i s t o account f o r t h e l o s s of v e r y normal l o o k i n g young.  Thus the d e f i c i e n c i e s must be such t h a t t h e y are  noticeable macroscopically.  They must p r e v e n t t h e young males from  i n the d e c l i n e but a l l o w t h e young f e m a l e s t o mature.  not  maturing  Furthermore, t h e y must  166  FIGURE 8.  P i t e l k a * s food supply  hypothesis.  Peak numbers  Very extensive  forage  utilization  Qualitative food deficiency ( i . e . phosphate d e f i c i e n c y ? )  Quantitative food deficiency ("starvation")  'Weakening o f a n i m a l s (e.g. d e c l i n e i n reproductive rate)  I  Decrease i n p o p u l a t i o n  density  C e s s a t i o n o f heavy f o r a g e  1  utilization  Q u a l i t a t i v e and q u a n t i t a t i v e r e c o v e r y of v e g e t a t i o n  increase i n population  peak numbers  etc.  density  Depletion of cover  Predation  167 account f o r an i n c r e a s i n g decline be  the  compared t o the  s u r v i v a l r a t e i n the  first  litter.  r e s u l t o f q u a l i t a t i v e forage  Such e f f e c t s seem h i g h l y u n l i k e l y t o  changes.  F o r these r e a s o n s I r e j e c t the adequate e x p l a n a t i o n explained  o f the  l a t e r summer l i t t e r s o f the  f o o d s u p p l y h y p o t h e s i s as an  Baker Lake lemming c y c l e .  The e v e n t s t o be  are m a i n l y i n t r i n s i c changes i n v o l v i n g b o t h r e p r o d u c t i o n and  m o r t a l i t y and are o f such a g e n e r a l n a t u r e t h a t ad hoc hypotheses e x t r i n s i c f a c t o r s i n l o c a l s i t u a t i o n s are Yet the  same e f f e c t o f the  the v e g e t a t i o n the  entirely insufficient.  lemmings on the  There i s no doubt t h a t lemmings e x e r t but  s t a n d i n g crop o f  b y Thompson (1955 b) a t P o i n t Barrow were a l s o found  f o r a g e as were r e p o r t e d i n t h i s study.  regarding  t h i s i s h a r d l y e v i d e n c e f o r the  a strong  e f f e c t on  above h y p o t h e s i s . A l l  e v i d e n c e f o r t h i s h y p o t h e s i s c o n s i s t s o f an observed a s s o c i a t i o n between  lemming d e c l i n e s and e x t e n s i v e f o r a g e u t i l i z a t i o n 1957 a ) , and u n t i l more c o n c l u s i v e  (Thompson, 1955 b; P i t e l k a ,  evidence i s a v a i l a b l e i t i s necessary  t o remain s k e p t i c a l o f t h i s i n t e r p r e t a t i o n . Rausch (1950) s t a t e s t h a t t h e r e was n o t h i n g t o i n d i c a t e t h a t d e c l i n e i n numbers i n 19U9 a t P o i n t C h i t t y (1952,  I960)  came t o the  starvation.  has p r e s e n t e d h i s reasons f o r r e j e c t i n g the  s u p p l y h y p o t h e s i s as an e x p l a n a t i o n (1957)  Barrow r e s u l t e d from  same c o n c l u s i o n  e t a l . (19U8) s t a t e t h a t the  of Microtus agrestis cycles. f o r Clethrionomys rufocanus.  f o o d s u p p l y was not  the  responsible  food Kalela Nasimovich  f o r Lemmus lemmus  fluctuations. (2) C h r i s t i a n ' s S t r e s s The be was  associated  Hypothesis:  i d e a t h a t c y c l e s were caused b y s t r e s s and t h a t d e c l i n e s w i t h changes i n a d r e n a l - p i t u i t a r y f u n c t i o n s  proposed b y C h r i s t i a n (1950) from the  b a s i c h y p o t h e s i s has not  could  and shock d i s e a s e  b a s i c work o f S e l y e  (19U6).  The  changed much s i n c e t h e n , w i t h the e x c e p t i o n o f the  added e f f e c t s o f s t r e s s on l a t e r g e n e r a t i o n s ( C h r i s t i a n and Lamunyan, 1 9 5 7 ) ,  168 and Figure 9 outlines the stress hypothesis summarized i n C h r i s t i a n  (1961).  A long series of papers presents the evidence f o r t h i s idea (Christian, 1955  a, 1955  b, 1956,  1957,  1959,  1961,  and others).  Me must d i s t i n g u i s h a general and a s p e c i f i c aspect of Christian's ideas.  His general thesis i s that a l l mammals l i m i t t h e i r own densities  by a combination of behavioral and p h y s i o l o g i c a l changes.  His s p e c i f i c  thesis i s that the mechanism of t h i s l i m i t a t i o n involves the General Adaptation Syndrome and i s purely phenotypic.  I s h a l l not discuss the  a p p l i c a b i l i t y of t h i s scheme to a l l mammals, but w i l l l i m i t my discussion to the c y c l i c rodents. There are two conditions which must be f u l f i l l e d to v e r i f y t h i s s p e c i f i c hypothesis: (1)  there must be increased adrenal a c t i v i t y and decreased reproductive a c t i v i t y at high densities;  (2) t h i s increased adrenal a c t i v i t y must cause an increased death r a t e . It i s not s u f f i c i e n t merely to f i n d increased adrenal a c t i v i t y at high densities and to claim that the hypothesis has been confirmed. Christian has amassed a large amount of data to support h i s hypothesis.  The sheer bulk of data from at least p a r t l y controlled  laboratory studies i s considered by some to be the strongest point of t h i s hypothesis, but C h i t t y ( I 9 6 0 ) considers t h i s the weakest point.  There i s  no evidence that these laboratory situations correspond to anything that goes on i n nature, and thus the extrapolation from the lab to the f i e l d i s not j u s t i f i e d . No consistent relationship between summer adrenal weights and the phases of the cycle was found i n t h i s study (Tables 55-58), and C h i t t y (196l) reported the same r e s u l t from Microtus a g r e s t i s.  Data from M. montanus  given by C h r i s t i a n (196l) shows no relationship between adrenal weights and  169  FIGURE 9.  C h r i s t i a n ' s stress hypothesis.  The system i s p u r e l y p h e n o t y p i c  and o p e r a t e s t h r o u g h t h e g e n e r a l a d a p t a t i o n syndrome.  Increase i n p o p u l a t i o n d e n s i t y  Increased  s o c i a l pressure  S t r e s s ( G e n e r a l A d a p t a t i o n Syndrome)  Decrease i n p o p u l a t i o n d e n s i t y  v  Decrease i n s o c i a l p r e s s u r e  v  Increase i n p o p u l a t i o n d e n s i t y  V etc.  170 p o p u l a t i o n s i z e , c o n t r a r y t o what C h r i s t i a n says. seem t o have two  choices.  We  G i v e n these  data,  we  can r e j e c t C h r i s t i a n ' s h y p o t h e s i s , o r we  can  save the h y p o t h e s i s by s a y i n g t h a t a d r e n a l w e i g h t s are not always a v a l i d index o f the G e n e r a l A d a p t a t i o n we  Syndrome.  I f we  must a l s o q u e s t i o n t h e m a j o r i t y of the evidence  s i n c e i t i s m o s t l y based oh a d r e n a l w e i g h t s . is  v e r y f a v o r a b l e t o the c u r r e n t  (l 6l) Q  Munday may  has  i n f a v o r of t h e  N e i t h e r horn of t h e  hypothesis, dilemma  hypothesis.  c r i t i c a l l y reviewed the e v i d e n c e  e x p l a i n c y c l i c d e c l i n e s and has  evidence  concluded  that stress  t h a t t h e r e i s as y e t  no  t h a t normal s t r e s s o r s can induce d i s e a s e i n e i t h e r normal i n d i v i d u a l s  o r succeeding same t h i n g . of  accept the second a l t e r n a t i v e  generations. Chitty  Turner  (1959) has  shock d i s e a s e i n n a t u r e .  (i960, p 265)  has  concluded  shed c o n s i d e r a b l e doubt on the  There i s l i t t l e  much t h e existence  c o n c l u s i v e evidence  c o r r e l a t i o n i n n a t u r a l p o p u l a t i o n s between a d r e n a l h y p e r t r o p h y  of  and  any a  r e g r e s s i o n of r e p r o d u c t i v e f u n c t i o n , and f i n a l l y t h e i d e a t h a t s t r e s s has an e f f e c t on subsequent g e n e r a t i o n s has To  sum  received l i t t l e  up, t h e r e i s no e v i d e n c e  (sensu S e l y e ) p l a y e d any major r o l e s p e c i f i c h y p o t h e s i s was  support  (Munday,  196l).  from a d r e n a l w e i g h t s t h a t s t r e s s  i n t h i s lemming c y c l e and thus C h r i s t i a n ' s  r e j e c t e d as an  explanation.  (3) C h i t t y ' s H y p o t h e s i s ;  (1952) found  Chitty produced l i t t l e appeared t o be  that i n t r a s p e c i f i c  s t r i f e d u r i n g the peak summer  e f f e c t on the a d u l t s but r a t h e r the p r o g e n y o f these less viable.  He  animals  emphasized the i n d i r e c t e f f e c t on the progeny  r a t h e r t h a n the d i r e c t e f f e c t s on the a d u l t s , and p o i n t e d out t h a t C h r i s t i a n ' s views  (1950) c o u l d  (Chitty,  1955  a).  not e x p l a i n the l o n g c o n t i n u e d d e c l i n e s which may Chitty  (1952, 1957, I960) p r o p o s e d  the f o l l o w i n g  occur concept:  t h a t mutual antagonism a s s o c i a t e d w i t h h i g h b r e e d i n g d e n s i t i e s b r i n g s about a change i n the p r o p e r t i e s o f the contemporary p o p u l a t i o n , and of the  subsequent  171 g e n e r a t i o n s , w h i c h become l e s s r e s i s t a n t t o t h e normal sources It  i s important  t o d i s t i n g u i s h t h i s concept of a change i n q u a l i t y o f  p o p u l a t i o n from the e x p l a n a t i o n p 106).  of m o r t a l i t y .  1951,  (mechanism) of t h i s concept (Conant,  C h i t t y (l°60) r e v i e w e d the evidence  f o r t h i s concept and  no  demonstrated.  C h r i s t i a n ' s (l°6l) g e n e r a l i d e a s have much i n common w i t h  to  a mechanism had not y e t been  concept. The  two  against i t , although  concluded  t h a t t h e r e was  this  evidence  the  r e l e v a n t changes produced by mutual antagonism might i n v o l v e  mechanisms:  ( l ) changes i n m a t e r n a l p t r y s i o l o g y which are  t h e o f f s p r i n g ( i . e . s i m i l a r t o the  (2)  changes i n the g e n e t i c c o m p o s i t i o n  The  f i r s t e x p l a n a t i o n was  s t r e s s hypothesis  not  A t t e n t i o n was  t e s t e d e x t e n s i v e l y i n t h e l a b o r a t o r y by C h i t t y and striking  effects  a d u l t s by mutual antagonism, t h e i r o f f s p r i n g d i d  t h u s t u r n e d t o the  f i r s t p o s s i b i l i t y i n v e s t i g a t e d was  1956; C h i t t y , 1957)-  or  selection.  show the changes i n q u a l i t y found i n n a t u r a l p o p u l a t i o n s  i960). The  of C h r i s t i a n ) ;  o f the p o p u l a t i o n b y  r e j e c t e d as an adequate e x p l a n a t i o n because, a l t h o u g h c o u l d be produced i n the  transmitted  (Chitty,  second p o s s i b i l i t y , g e n e t i c  1957,  changes.  h e r e d i t a r y s p l e n i c anemia (Dawson,  T h i s has been r e j e c t e d as an e x p l a n a t i o n of the  r e c u r r i n g d e c l i n e s by Newson and C h i t t y (1962). C h i t t y has thus m o d i f i e d h i s views r e g a r d i n g the mechanism involved, while agent i n these is  r e t a i n i n g the p r i m a c y o f mutual i n t e r f e r e n c e as a declines.  shown i n F i g u r e 10  H i s c u r r e n t view on the mechanism of the  cycle  ( C h i t t y , p e r s . comm.).  E v i d e n c e from t h i s study f u l l y supports populations  necessary  the g e n e r a l concept t h a t  change i n q u a l i t y d u r i n g changes i n abundance.  showed these  q u a l i t a t i v e d i f f e r e n c e s by  (1)  reproductive  changes w h i c h c a r r i e d over i n t o t h e d e c l i n e .  Peak p o p u l a t i o n s  h i g h body w e i g h t s and The  (2)  high j u v e n i l e  m o r t a l i t y d u r i n g the d e c l i n e c o u l d not be p r e d i c t e d from the p o p u l a t i o n d e n s i t y  172  FIGURE 10. primarily  C h i t t y s hypothesis. 1  The system i s p a r t l y g e n e t i c and  behavioral.  I n c r e a s e i n numbers  Low  Numbers  V No i n t e r f e r e n c e  I n c r e a s e i n numbers  etc.  173 at the time.  F i n a l l y , the d i f f e r e n t t y p e s of d e c l i n e s c o u l d n o t be e x p l i t o a d  by d i f f e r e n c e s i n e x t r i n s i c  factors.  There i s no d i r e c t e v i d e n c e from t h i s study t o t e s t the mechanism proposed b y C h i t t y ( F i g u r e 1 0 ) .  We  have seen t h a t a c o n s i d e r a b l e amount o f  wounding o c c u r s i n t h e s e lamming p o p u l a t i o n s ( T a b l e s may range o v e r v e r y l a r g e areas ( T a b l e s  U2-U5)  H > 9 - 6 0 ) .  A d u l t males  and t h e most p r o b a b l y h y p o t h e s i s  about the d i s a p p e a r i n g young o f t h e d e c l i n e i s t h a t t h e s e a d u l t males k i l l them.  None o f t h e s e p o i n t s i s good evidence f o r t h i s mechanism, and n e i t h e r  are t h e y good e v i d e n c e a g a i n s t t h i s Birch  ( i 9 6 0 )  mechanism.  has d i s c u s s e d the f a c t t h a t n a t u r a l s e l e c t i o n a c t s  t o b r i n g t h e a c t u a l r a t e o f i n c r e a s e r t o a maximum.  I f t h i s i s the c a s e ,  we may assume t h a t i n peak and d e c l i n i n g lemming p o p u l a t i o n s t h e r e i s some s u r v i v a l v a l u e i n a f a i l u r e t o mature ( f o r males a t l e a s t ) .  I t i s very  i m p o r t a n t t o determine whether t h i s change i n m a t u r a t i o n i s g e n o t y p i c o r p h e n o t y p i c ; as y e t we do n o t know.  I n e i t h e r c a s e , i t would seem p o s s i b l e  t h a t a v e r y h i g h r a t e o f s e l e c t i o n a g a i n s t t h e e a r l y - m a t u r i n g young c o u l d o c c u r because o f .increased f i g h t i n g a s s o c i a t e d w i t h s e x u a l m a t u r a t i o n , and we c o u l d t h e r e b y get a complete change i n q u a l i t y o f t h e p o p u l a t i o n over a v e r y s h o r t p e r i o d o f time a t h i g h d e n s i t i e s .  I f the m a t u r a t i o n change were  p h e n o t y p i c , i t would be secondary i n importance the o t h e r hand, i f t h i s change x i r e r e importance  t o aggressive behavior.  g e n o t y p i c , i t c o u l d be o f p r i m a r y  i n the c y c l e . One o f t h e i n t e r e s t i n g p o i n t s t h a t has come from t h i s  lemming  work i s t h e s i m i l a r i t y between t h i s lemming c y c l e and t h e c y c l e s i n M i c r o t u s a g r e s t i s d e s c r i b e d i n d e t a i l by C h i t t y .  That s i m i l a r t y p e s o f  e v e n t s s h o u l d o c c u r i n two such d i f f e r e n t e c o l o g i c a l s i t u a t i o n s  argues  On  17U q u i t e s t r o n g l y f o r a u n i f i e d view o f c y c l i c To sum  up,  processes.  C h i t t y ' s concept o f a change i n q u a l i t y o f the  d u r i n g changes i n d e n s i t y i s f u l l y supported the mechanism i n v o l v e d i s b e h a v i o r a l and  by t h i s  study.  population  H i s view t h a t  g e n e t i c i s not r e f u t e d b y my  which suggest t h a t b e h a v i o r a l changes may  c o n s t i t u t e the  crux of  data  the  lemming c y c l e . Conclusions I have attempted t o give a semi-complete d e s c r i p t i o n of a s i n g l e lemming c y c l e , and w i t h t h i s s i n g l e o b s e r v a t i o n on a v e r y complex n a t u r a l event have attempted t o examine the c u r r e n t i d e a s on p o p u l a t i o n c y c l e s . The w i d e r our h o r i z o n o f f a c t s has become, the l e s s and the c o n v e n t i o n a l i d e a s .  As l o n g as we  and  seek o n l y c o n f i r m a t o r y evidence  and  conventional We  We  we  l e s s adequate seem  s t i c k t o s m a l l p a r t s o f l o c a l problems s h a l l be content w i t h ad hoc  explanations  ideas.  have t r i e d t o p e n e t r a t e  t o the core o f the phenomenon s t u d i e d .  have seen t h a t e x t r i n s i c f a c t o r s c o u l d not e x p l a i n the c y c l e .  i n t r i n s i c f a c t o r s we  Of  the  d i s c a r d e d the p u r e l y p h y s i o l o g i c a l i d e a s because the  i n t e r a c t i o n s of i n d i v i d u a l s which c o u l d produce p h y s i o l o g i c a l changes were severe enough t h a t a t t e n t i o n was  t u r n e d d i r e c t l y t o the u n d e r l y i n g  and p o s s i b l e s e l e c t i v e f o r c e s t h a t might  behavior  result.  F u t u r e work on the mechanism o f c y c l i c a l f l u c t u a t i o n s should c o n s i d e r the r o l e of b e h a v i o r past.  The  suggestion  i n f a r more d e t a i l t h a n has  o f C h i t t y t h a t these  f l u c t u a t i o n s may  g e n e t i c polymorphism deserves c o n s i d e r a b l e a t t e n t i o n . f a r from b e i n g  solved.  been done i n the  The  represent  a  problem remains  SUMMARY  1.  A t h r e e y e a r study c o v e r i n g one c y c l e i n numbers o f the brown lemming (Lemmus t r i m u c r o n a t u s ) and t h e v a r y i n g lemming ( D i c r o s t o n y x g r o e n l a n d i c u s ) has been c a r r i e d out a t Baker Lake, N.W.T. i n an attempt lemming c y c l e from the Canadian  B a r r e n Grounds and t o see what  e x p l a n a t i o n s would f i t the observed 2.  t o describe a  events.  I n c r e a s e began from v e r y low numbers i n 1959 and tremendous p o p u l a t i o n growth o c c u r r e d over the w i n t e r o f 1959-60. o c c u r r e d i n the peak summer o f i960.  L i t t l e f u r t h e r increase  A g r e a t d e c l i n e o c c u r r e d over  the w i n t e r o f I96O-6I, and t h i s d e c l i n e c o n t i n u e d through the summer o f 1961  on the Main Study A r e a .  T h i s c y c l e was synchronous i n both s p e c i e s  over a wide zone o f t h e c e n t r a l 3.  arctic.  Two major changes i n r e p r o d u c t i o n o c c u r r e d over the c y c l e .  A lengthened  summer b r e e d i n g season and w i n t e r b r e e d i n g o c c u r r e d d u r i n g the i n c r e a s e i n 1959-60, b u t no w i n t e r b r e e d i n g and a s h o r t e n e d summer b r e e d i n g c h a r a c t e r i z e d the peak and d e c l i n e .  season  Young male Lemmus d i d not mature  i n e i t h e r t h e peak o r d e c l i n e summers, n o r d i d young females i n the peak. No changes i n midsummer pregnancy r a t e s o r l i t t e r s i z e o c c u r r e d . k»  P a r t i a l p r e n a t a l m o r t a l i t y d i d not change over the c y c l e .  Adult m o r t a l i t y  •may have been s l i g h t l y h i g h e r i n t h e summer o f d e c l i n e t h a n i n the peak summer.  J u v e n i l e m o r t a l i t y was v e r y h i g h i n the summer o f d e c l i n e ,  p a r t i c u l a r l y f o r the f i r s t 5.  summer l i t t e r .  S p r i n g movements o f i n d i v i d u a l lemmings on t h e i c e were found i n t h e peak y e a r . for  The e x i s t e n c e o f mass lemming m i g r a t i o n s i s q u e s t i o n e d  North America and S c a n d i n a v i a .  both  There i s no good e v i d e n c e o f any  o r i e n t e d l o n g d i s t a n c e group movements o f lemmings. 6,  F a v o r a b l e f a l l and w i n t e r weather was a s s o c i a t e d w i t h the i n c r e a s e , and  unfavorable  f a l l and e a r l y w i n t e r weather was a s s o c i a t e d w i t h t h e  declineo 7.  A v i a n p r e d a t o r s were uncommon throughout t h e c y c l e .  The w e a s e l was  the o n l y common mammalian p r e d a t o r b u t c o u l d not have accounted f o r the observed m o r t a l i t y changes.  D i s e a s e s and p a r a s i t e s d i d n o t seem  t o p l a y any s i g n i f i c a n t r o l e i n the c y c l e . 8.  Lemmings reduced t h e f o r a g e  crop by about 1$% i n the peak and d e c l i n e .  Forage u t i l i z a t i o n averaged 30% o r l e s s i n t h e wet h a b i t a t s and was n e g l i g i b l e i n t h e d r y h a b i t a t s a f t e r the c r i t i c a l w i n t e r  o f 1960-61.  There was no evidence o f q u a n t i t a t i v e f o o d shortage n o r any s u g g e s t i o n of d e f i c i e n c i e s i n f o o d q u a l i t y o v e r t h e c y c l e .  Lemmings i n the s p r i n g  of t h e d e c l i n e were as f a t as u s u a l . 9.  High body w e i g h t s (20-30% above 'normal') were a s s o c i a t e d w i t h a l l peak p o p u l a t i o n s .  10.  Organ weights ( a d r e n a l s , s p l e e n ) d i d n o t give any c l u e t o what was causing t h e c y c l e .  Summer a d r e n a l w e i g h t s showed no c o n s i s t e n t  r e l a t i o n s h i p t o t h e d e n s i t y changes. 11.  I n t r a s p e c i f i c s t r i f e , as measured b y wounds on s k i n s , showed s t r o n g seasonal  12.  and y e a r l y changes which were n o t a simple  Three c u r r e n t hypotheses were c o n s i d e r e d  function of density.  i n the l i g h t o f these  data.  The Food Supply H y p o t h e s i s o f P i t e l k a was r e j e c t e d as an adequate explanation.  The S t r e s s H y p o t h e s i s o f C h r i s t i a n was a l s o r e j e c t e d .  C h i t t y ' s g e n e r a l concept t h a t p o p u l a t i o n s  change i n q u a l i t y d u r i n g  changes i n d e n s i t y i s supported b y t h i s s t u d y . e x p l a n a t i o n t h a t these c y c l e s r e p r e s e n t aggressive  Chitty's specific  a g e n e t i c polymorphism i n v o l v i n g  b e h a v i o r was n o t r e f u t e d b y these d a t a , which suggest t h a t  b e h a v i o r a l changes may c o n s t i t u t e t h e crux o f the c y c l e .  LITERATURE CITED Andrewartha, H.G. and L.C. B i r c h . 19$k» The d i s t r i b u t i o n and abundance of animals. Univ. o f Chicago P r e s s , Chicago. 782 p p . Baker, J.R. and D. C h i t t y . (ms). The i n c i d e n c e o f p r o t o z o a n b l o o d p a r a s i t e s i n a w i l d population o f Microtus a g r e s t i s i n England, u n p u b l i s h e d ms. Bandy, P.J., I . McT. Cowan, W.D. K i t t s , and A . J . Wood. 1 9 5 6 . A method f o r t h e assessment o f the n u t r i t i o n a l s t a t u s o f w i l d u n g u l a t e s . Can. J . Z o o l . 3U« U8-52. B e n d e l l , J.F. 1 9 5 9 . Food as a c o n t r o l o f a p o p u l a t i o n o f w h i t e - f o o t e d mice, Peromyscus leucopus noveboracensis ( F i s c h e r ) . Can. J . Zool. 3 7 : 173-209. B i r c h , L.C.  9U:  I960.  The g e n e t i c f a c t o r i n p o p u l a t i o n e c o l o g y . 5-24.  Bodenheimer, F.S. Oxford. Brambell,  1938.  Problems o f animal e c o l o g y .  Amer. N a t .  Clarendon  Press,  F.W.R. and I.H. M i l l s . 1947• S t u d i e s on s t e r i l i t y and p r e n a t a l m o r t a l i t y i n w i l d r a b b i t s . P a r t I I I . The l o s s o f o v a b e f o r e implantation. J . Exp. B i o l . 21+: 192-210.  B r a m b e l l , F.W.R. and I.H. M i l l s . I948. S t u d i e s on s t e r i l i t y and p r e n a t a l m o r t a l i t y i n w i l d r a b b i t s . P a r t IV. The l o s s o f embryos a f t e r implantation. J . Exp. B i o l . 2 5 : 21+1-269. B r i t t o n , M.E. 1 9 5 7 . V e g e t a t i o n o f the a r c t i c t u n d r a , pp. 26-61 i n A r c t i c B i o l o g y , B i o l o g y C o l l o q u i u m , Oregon State C o l l e g e , 1957. Brown, L.E. 1 9 5 6 . Movements o f some B r i t i s h s m a l l mammals. E c o l . 25:- 54-71.  J . Anim.  Calhoun, J.B. 19U8. N o r t h American Census o f S m a l l Mammals. Release No. 1. Announcement o f Program. Rodent E c o l o g y P r o j e c t , Johns Hopkins Univ., B a l t i m o r e . 9 pp. mimeo. C h e s t e r Jones, I . 1 9 5 7 . 316 p p .  The a d r e n a l c o r t e x .  U n i v e r s i t y P r e s s , Cambridge.  C h i t t y , D. 1 9 5 2 . M o r t a l i t y among v o l e s ( M i c r o t u s a g r e s t i s ) a t Lake Vyrnwy, Montgomeryshire i n 1 9 3 6 - 9 . P h i l . T r a n s . Roy. S o c . London, Ser. B, 236: 5 0 5 - 5 5 2 . C h i t t y , D. 1 9 5 U « T u b e r c u l o s i s among w i l d v o l e s : w i t h a d i s c u s s i o n o f o t h e r p a t h o l o g i c a l c o n d i t i o n s among c e r t a i n mammals and b i r d s .  Ecology 35:  227-237.  C h i t t y , D. 1955 a . Adverse e f f e c t s o f p o p u l a t i o n d e n s i t y upon the v i a b i l i t y of l a t e r g e n e r a t i o n s , p p . 57-67 i n the Numbers o f Man and A n i m a l s , e d . b y J.B. Cragg and N.W. P i r i e . O l i v e r and Boyd L t d . , E d i n b u r g h .  178 C h i t t y , D. 1955 b . Allgemeine GedandengSnge liber d i e Dichteschwankungen b e i der Erdmaus ( M i c r o t u s a g r e s t i s ) . Z e i t . Saugetierkunde 20: 55-60. C h i t t y , D. 1957. S e l f - r e g u l a t i o n o f numbers through changes i n v i a b i l i t y . C o l d S p r i n g Harbor Symp. Quant. B i o l . 22: 277-280. C h i t t y , D.  1959. A note on shock d i s e a s e .  E c o l o g y 1+0:  728-731.  C h i t t y , D. i960. P o p u l a t i o n p r o c e s s e s i n the v o l e and t h e i r r e l e v a n c e to general theory. Can. J . Z o o l . 38: 99-113. C h i t t y , D. and H. C h i t t y . I960 a . P o p u l a t i o n t r e n d s among the v o l e s a t Lake Vyrnwy, 1932-60. Symposium t h e r i o l o g i c u m , B r n o , i960. C h i t t y , D. and E . P h i p p s . I960. The e f f e c t o f f l e a s on s p l e e n s i z e i n voles. J . P h y s i o l . 151: 27-28. C h i t t y , D. and E . P h i p p s . 1961. E c o l . 30: 1+90.  A d e c l i n i n g vole p o p u l a t i o n .  J . Anim.  C h i t t y , H. 1961. V a r i a t i o n s i n the weight o f the a d r e n a l g l a n d s o f the f i e l d vole, Microtus a g r e s t i s . J . E n d o c r i n . 22: 387-393. C h i t t y , H. and D. C h i t t y . i960 b. Body weight i n r e l a t i o n t o p o p u l a t i o n phase i n M i c r o t u s a g r e s t i s . Symposium t h e r i o l o g i c u m , Brno, I960. C h r i s t i a n , J . J . 1950. The a d r e n o - p i t u i t a r y system and p o p u l a t i o n i n mammals. J . Mammal. 31: 21+7-259.  cycles  C h r i s t i a n , J . J . 1955 a. E f f e c t o f p o p u l a t i o n s i z e on t h e a d r e n a l g l a n d s and r e p r o d u c t i v e organs o f male mice i n p o p u l a t i o n s o f f i x e d s i z e . Amer. J . P h y s i o l . 182: 292-300. C h r i s t i a n , J . J . 1955 b . E f f e c t o f p o p u l a t i o n s i z e on the weights o f the r e p r o d u c t i v e organs o f male white mice. Amer. J . P h y s i o l . 181: 1+77-1+80. C h r i s t i a n , J . J . 1956. A d r e n a l and r e p r o d u c t i v e responses t o p o p u l a t i o n s i z e i n mice f r o m f r e e l y growing p o p u l a t i o n s . E c o l o g y 37: 258-273. C h r i s t i a n , J . J . 1957. A review o f the endocrine responses i n r a t s and mice t o i n c r e a s i n g p o p u l a t i o n s i z e i n c l u d i n g d e l a y e d e f f e c t s on offspring. N a v a l Med. Res. I n s t . , L e c t . Rev. S e r . No. 57-2:  1+1+3-1+62.  C h r i s t i a n , J . J . 1959. The r o l e s o f e n d o c r i n e and b e h a v i o r a l f a c t o r s i n the growth o f mammalian p o p u l a t i o n s , p p . 71-97 o f Comparative E n d o c r i n o l o g y , ed. b y A. Gorbman. J . W i l e y and Sons, Inc., New York. C h r i s t i a n , J . J . 1961. Phenomena a s s o c i a t e d w i t h p o p u l a t i o n d e n s i t y . P r o c . Nat. Acad. S c i . 1+7: 1+28-1+1+9. C h r i s t i a n , J . J . and D.E. D a v i s . 1956. The r e l a t i o n s h i p between a d r e n a l weight and p o p u l a t i o n s t a t u s o f urban Norway r a t s . J . Mammal. 37:  1+75-U86.  179 C h r i s t i a n , J.J. and C D . Lemunyan. 1 9 5 7 . Adverse e f f e c t s o f crowding on r e p r o d u c t i o n and l a c t a t i o n o f mice and two g e n e r a t i o n s o f t h e i r progeny. N a v a l Med. Res. I n s t . Res. P e r . V o l . 1 5 : 925-936. C o l e , L.C.  1951. P o p u l a t i o n c y c l e s and random o s c i l l a t i o n s . Mgmt. 15s 2 3 3 - 2 5 2 .  C o l e , L.C.  195U a . The p o p u l a t i o n consequences o f l i f e h i s t o r y phenomena. Q u a r t . Rev. B i o l . 2 9 : 103-137.  C o l e , L.C,  195U b. Mgmt. 18:  C o l e , L.C.  1958.  J. W i l d l .  Some f e a t u r e s o f random p o p u l a t i o n c y c l e s . 2-21; Population f l u c t u a t i o n s .  v o l . 2 , 1956:  639-6U7.  J. W i l d l .  P r o c . Tenth I n t e r . Cong. E n t .  C o l l e t t , R. 1 8 9 5 . Myodes lemmus, i t s h a b i t s and m i g r a t i o n s i n Norway. C h r i s t i a n i a V i d e n s k a b s - S e l e k a b s F o r h a n d l i n g e r 1895 No. 3. 62 p p . 1911.  C o l l e t t , R. Conant, J.B.  Norges P a t t e d y r .  195l«  On u n d e r s t a n d i n g  lUU p p .  Kristiania. science.  7hh p p .  Mentor Books, New York.'  Conaway, C H . 1955* Embryo r e s o r p t i o n and p l a c e n t a l s c a r f o r m a t i o n i n t h e r a t . J. Mammal. 36: 5 1 6 - 5 3 2 . Crowcroft,  P. and F.P. Rowe. 1961. The weights o f w i l d house mice Mus Musculus l i v i n g i n c o n f i n e d c o l o n i e s . P r o c . Z o o l . Soc. London 136 (2) : 1 7 7 - 1 8 5 .  C u r r y - L i n d a h l , K. 1961. F j a l l a m m e l n (Lemmus lemmus) i S V e r i g e under i 9 6 0 . F a k t a och t e o r i e r . Fauna och F l o r a 55. T - 2 7 . D a v i s , D.E. 1 9 5 3 .  31*: Dawson, J.  A n a l y s i s o f home range from r e c a p t u r e d a t a .  352-358.  1956.  Splenic hypertrophy  i n voles.  Nature 1?8:  J. Mammal.  1183-1181;.  Dunaeva, T.N. and V.V. Kucheruk. 1 9 u l . M a t e r i a l on the e c o l o g y o f the t e r r e s t r i a l v e r t e b r a t e s o f the t u n d r a o f South Yamal. M a t e r i a l on the f a u n a and f l o r a o f the U.S.S.R. No. k ( 1 9 ) , pp. 1-80. ( i n Russian) E l t o n , C.  E l t o n , C.  E l t o n , C.  1921;. P e r i o d i c f l u c t u a t i o n s i n the numbers o f animals: causes and e f f e c t s . B r i t . J. Exp. B i o l . 2 : 119-163. 1931.  The study o f epidemic d i s e a s e s among w i l d  J. Hyg. 3 1 :  U35-Ji56.  I9I4.2. V o l e s , mice and lemmings. pp.  1|96 E l t o n , C.  Clarendon  their  animals.  Press,  Oxford.  D.H.S. D a v i s , and C M . F i n d l a y . 1 9 3 5 . An epidemic among v o l e s ( M i c r o t u s a g r e s t i s ) on the S c o t t i s h b o r d e r i n the s p r i n g o f 1931).. J. Anim. E c o l . 4:  277-288  180 Emlen, J.T. J r . , R.L. Hine, W.A. F u l l e r , and P. A l f o n s o . 1957Dropping boards f o r p o p u l a t i o n s t u d i e s o f s m a l l mammals. J . W i l d l . Mgmt. 21: 300-3H*. Enzmann, E.V. 1935. I n t r a u t e r i n e growth of a l b i n o mice i n normal and i n d e l a y e d pregnancy. Anat. Rec. 62: 31-1*5* F r a n k , F. and K. Zimmermann. 1957* Uber d i e Beziehungen zwischen L e b e n s a l t e r und morphologischen Merkmalen b e i der Feldmaus, M i c r o t u s arvalis (Pallas). Z o o l . J a h r . S y s t . 85* 283-300. Gavin, A.  191*5. Notes on mammals observed i n the P e r r y R i v e r d i s t r i c t , Queen Maud Sea. J . Mammal. 26: 226-230.  Godfrey, G.K. 1953. F a c t o r s a f f e c t i n g the s u r v i v a l , movements, and i n t r a s p e c i f i c r e l a t i o n s d u r i n g e a r l y l i f e i n p o p u l a t i o n s of s m a l l mammals w i t h p a r t i c u l a r r e f e r e n c e t o the v o l e , u n p u b l i s h e d D. P h i l , t h e s i s , Bureau o f Animal P o p u l a t i o n , Oxford. 127 pp. Godfrey, G.K. 1955* O b s e r v a t i o n s on the nature of the d e c l i n e i n numbers of two M i c r o t u s p o p u l a t i o n s . J . Mammal. 3 6 : 209-211*. G o l l e y , F.B. 1961. I n t e r a c t i o n o f n a t a l i t y , m o r t a l i t y and movement d u r i n g one annual c y c l e i n a M i c r o t u s p o p u l a t i o n . Amer. M i d i . Nat. 66:  152-159.  Hamilton,  W.J.  51*:  Jr.  1937  779-790.  Hamilton, W.J. J r . 1937 Amer. Nat. 71:  a.  The  b i o l o g y of microtine c y c l e s .  b. Growth and l i f e 500-507.  J. Agric.  Res.  span o f the f i e l d mouse.  Hamilton, W.J. J r . 19l*l. R e p r o d u c t i o n o f the f i e l d mouse M i c r o t u s p e n n s y l v a n i c u s ( O r d ) . C o r n e l l Univ. A g r i c . Exp. S t a . Mem. 23 pp.  237.  Henneberg, B. 1937* Normentafeln zur E n t w i c k l u n g s g e s c h i c h t e der Wanderratte (Rattus n o r v e g i c u s E r x l e b e n ) . Normentaf. W i r b e l t . No. 1 5 , 160 pp. Jena. Hill,  A.B.  1959. London.  P r i n c i p l e s of m e d i c a l 311* pp.  statistics.  The  Lancet  Ltd.,  Hoffmann, R.S. 1958. The r o l e o f r e p r o d u c t i o n and m o r t a l i t y i n p o p u l a t i o n f l u c t u a t i o n s of v o l e s ( M i c r o t u s ) . E c o l . Monographs 28: 79-109. Jameson, E.W.  Jr.  1950.  J . Mammal. 3 1 :  Determining  1+33-1*36.  f e c u n d i t y i n male s m a l l mammals.  Kalela, 0.  19i*9» Uber Fjeldlemming-Invasionen und andere i r r e g u l a r s Tierwanderungen. Ann. Z o o l . Soc. 'vanamo' 13: 1-90.  Kalela, 0.  1957. Regulation of reproduction rate i n s u b a r c t i c populations of the v o l e Clethrionomys r u f o c a n u s (Sund.). Ann. Acad. S c i . Fenn. S e r . A, IV, 31*:  1-60.  181 K a l e l a , 0.  l°6l. S e a s o n a l change o f h a b i t a t i n the Norwegian lemming, lemmus lemmus ( L . ) . Ann. Acad. S c i . Fenn. S e r . A, IV, 55: 1-72.  Koponen, T., A. Kokkonen, and 0. K a l e l a . 1961. On a case of s p r i n g m i g r a t i o n i n the Norwegian lemming. Ann. Acad. S c i . F e n n . S e r . A. IV, 5 2 : 1-30. Lack, D .  195U* The n a t u r a l r e g u l a t i o n of a n i m a l numbers. Oxford. 3U3 pp.  Clarendon  Press,  L e s l i e , P.H. 1959• The p r o p e r t i e s o f a c e r t a i n l a g type o f p o p u l a t i o n growth and the i n f l u e n c e o f an e x t e r n a l random f a c t o r on a number o f such p o p u l a t i o n s . P h y s i o l . Z o o l . 3 2 : l5l-l59» L e s l i e , P.H., D. C h i t t y , and H. C h i t t y . 1953. The e s t i m a t i o n o f p o p u l a t i o n parameters from d a t a o b t a i n e d by means o f the c a p t u r e - r e c a p t u r e method. I I I . An example o f the p r a c t i c a l a p p l i c a t i o n s of the method. B i o m e t r i k a U O : 137-169. L e s l i e , P.H., J.S. P e r r y , and J.S. Watson. 19U5« The d e t e r m i n a t i o n of the median body-weight a t which female r a t s r e a c h m a t u r i t y . P r o c . Z o o l . Soc. London 115: 473-U88. L e s l i e , P.H. and R.M. Ranson. 1940. The m o r t a l i t y , f e r t i l i t y , and r a t e of n a t u r a l i n c r e a s e of the v o l e ( M i c r o t u s a g r e s t i s ) as observed i n the l a b o r a t o r y . J . Anim. E c o l . 9: 27-527 L e s l i e , P.H., U.M. Venables, and L.S.V. V e n a b l e s . 1952. The f e r t i l i t y and p o p u l a t i o n s t r u c t u r e of t h e brown r a t (Rattus n o r v e g i c u s ) i n c o r n - r i c k s and some o t h e r h a b i t a t s . P r o c . Z o o l . Soc. London 122: 187-238. L o r d , R.B.  1959. The l e n s as an i n d i c a t o r of age i n c o t t o n t a i l r a b b i t s . J . W i l d l . Mgmt. 23: 358-360.  MacArthur, J.W. 1949. house mouse.  S e l e c t i o n f o r s m a l l and l a r g e body s i z e i n t h e G e n e t i c s 34: 194-209.  Manning, T.H. 195U* Remarks on t h e r e p r o d u c t i o n , sex r a t i o , and l i f e expectancy o f the v a r y i n g lemming, D i c r o s t o n y x g r o e n l a n d i c u s , i n nature and c a p t i v i t y . A r c t i c 7: 36-48. M a r t i n , E.P. 1956. A p o p u l a t i o n study o f the p r a i r i e v o l e ( M i c r o t u s o c h r o g a s t e r ) i n n o r t h e a s t e r n Kansas. Univ. Kansas Mus. Nat. H i s t . P u b . V o l . 8, No. 6: 361-106. Maynard, L.A. and J.K. McGraw H i l l ,  L o o s l i , 1956. Animal N u t r i t i o n . I n c . New York. 484 pp.  Fourth  Ed.  Munday, K.A. 1961. A s p e c t s o f s t r e s s phenomena, pp. 168-189 i n Mechanisms i n B i o l o g i c a l C o m p e t i t i o n , Symp. Soc. Exp. B i o l . No. XV. Cambridge, at the U n i v e r s i t y P r e s s . Nasimovich, A., G. Novikov, and 0. Semenov-Tyan-Shanskii. 1948. The Norwegian lemming: i t s e c o l o g y and r o l e i n the nature complex of the Lapland Reserve. M a t e r i a l y po gryzunam 19U8, 3: 203-262. (in  Russian)  182  Newson, J . and D. C h i t t y . 1962. Haemoglobin l e v e l s , growth and s u r v i v a l i n two Microtus populations. Ecology, i n press. Osborne, T.B. and L.B. Mendel. 1926. to d i e t . J . B i o l . Chem 69:  The r e l a t i o n of the rate of growth 661-673.  Palmgren, P. 19U9* Some remarks on the short-term f l u c t u a t i o n s i n the numbers of northern b i r d s and mammals. Oikos 1: llU-121. Parkes, A.S. and H.M. Bruce. 1961. Olfactory stimuli i n mammalian reproduction. Science 13U: 10U9-105U* P i t e l k a , F.A. 1907 a. Some aspects of population structure i n the shortterm cycle of the brown lemming i n northern Alaska. Cold Spring Harbor Symp. Quant. B i o l . 22: 237-251. P i t e l k a , F.A. 1957 b.  Some c h a r a c t e r i s t i c s of microtine cycles i n the  A r c t i c , pp. 73-88 i n A r c t i c Biology, Biology Colloquium, Oregon State College, 1957. P i t e l k a , F.A. 1959. Population studies of lemmings and lemming predators i n northern Alaska. XV t n International Cong. Zool., Sect. X, Paper 5« 3 pp. P i t e l k a , F.A. I96I. Ecology of lemmings and other microtines i n northern Alaska. Progress report f o r 1960-61 submitted to A r c t i c Inst, of North America. 18 pp. mimeo. P i t e l k a , F.A., P.Q. Tomich, and G.W. T r e i c h e l . 1955. E c o l o g i c a l r e l a t i o n s of jaegers and owls as lemming predators near Barrow,Alaska. E c o l . Monographs 25: 85-117. Quay, W.B.  i960.  The reproductive organs of the c o l l a r e d lemming under  diverse temperature and l i g h t conditions.  J . Mammal. U l : 7U-89*  Quay, W.B. and J.F. Quay. 1956. The requirements and biology of the c o l l a r e d lemming, Dicrostonyx torquatus P a l l a s , 1778, i n c a p t i v i t y . Saugerierkundliche Mitteilungen U: 17U-180. Rausch, R. 1950. Observations on a c y c l i c decline of lemmings (Lemmus) on the a r c t i c coast of Alaska during the spring of 19U9* A r c t i c 3: 166-177. Schindler, U. i960. Auftreten und Bekampfung der Rbtelmaus (Clethrionomys glareolus Schreber) im Durrejahr 1959 i n Nordwestdeutschland.  Z. angew. Zool. U7:  U31-UU7.  Selye, H. 19U6. The general adaptation syndrome and the diseases of adaptation. J . C l i n . Endocrin. 6: 117-230. Shelford, V.E. 19U3. The abundance of the c o l l a r e d lemming (Dicrostonyx groenlandicus (Tr.) v a r . richardsoni Mer.) i n the C h u r c h i l l area, 1929-to 19U0. Ecology 2k: U72-U8U.  183 S i i v o n e n , L . 19U8. S t r u c t u r e of mammals and b i r d s hemisphere. F i n n i s h on Game Research No.  o f s h o r t - c y c l i c f l u c t u a t i o n s i n numbers i n t h e n o r t h e r n p a r t s o f the n o r t h e r n F o u n d a t i o n f o r Game P r e s e r v a t i o n , Papers 1, 166 pp.  S l o b o d k i n , L.B. l°6l. Growth and r e g u l a t i o n o f a n i m a l p o p u l a t i o n s . R i n e h a r t , and Winston, New York. 18U pp. Snedecor, G.W. 1956. S t a t i s t i c a l Methods, P r e s s , Ames. 53U pp. Southwick, C H . 1958.  21:  1956.  density relationships.  P r o c . Z o o l . Soc.  N a t u r l i c h e Auslese b e i d e r RBtelmaus.  8U-100.  S t e i n , G.H.W. 1957. a r v a l i s P.  Iowa State C o l l e g e  P o p u l a t i o n c h a r a c t e r i s t i c s o f house mice l i v i n g  i n E n g l i s h corn r i c k s : London 131: 163-175. S t e i n , G.H.W.  F i f t h Ed.  Holt,  Z.Saugetierkunde  M a t e r i a l i e n z u r K e n n t n i s d e r Feldmaus, M i c r o t u s Z. Saugetierkunde 22: 117-135.  S t i c k e l , L.F. 195U» A comparison of c e r t a i n methods o f measuring ranges of small animals. J . Mammal. 35s 1-15* S u t t o n , G.M. and W.J. Hamilton J r . 1932. The mammals o f Southampton I s l a n d . Mem. Carnegie Museum 12 ( 2 ) : 1-111. Thompson, D . C  1955 a.  The e c o l o g y and p o p u l a t i o n dynamics o f t h e brown  lemming (Lemmus t r i m u c r o n a t u s ) a t P o i n t Barrow, A l a s k a , unpublished Ph.D. t h e s i s , Department o f Zoology, U n i v e r s i t y of M i s s o u r i . 138 pp. Thompson, D.Q. 1955 b . The r o l e o f f o o d and cover i n p o p u l a t i o n f l u c t u a t i o n s of the brown leniming a t P o i n t Barrow, A l a s k a . T r a n s . 20th No. Amer. W i l d l . .Conf., 166-176. Thompson, D.Q. 1955 c. The 1953 lemming e m i g r a t i o n a t P o i n t Barrow, A l a s k a . A r c t i c 8: 37-U5. Turner, C D . i960. G e n e r a l E n d o c r i n o l o g y . P h i l a d e l p h i a . 511 pp.  Third Ed.  W.B. Saunders Co.,  Watson, A. 1956. E c o l o g i c a l n o t e s on t h e lemmings Lemmus t r i m u c r o n a t u s and D i c r o s t o n y x g r o e n l a n d i c u s i n B a f f i n I s l a n d . J . Anim. E c o l . 25: 289-302. Wildhagen, A. 1953. On the r e p r o d u c t i o n o f v o l e s and lemming i n Norway. S t a t e n s V i l t u n d e r s ^ k e l s e r 1953, l - 6 l . Zejda, J .  196l. Age s t r u c t u r e i n p o p u l a t i o n s o f the Bank V o l e , Clethrionomys g l a r e o l u s Schreber 1780. Z o o l o g i c k e L i s t y 10: 21+9-261+.  Zimmermann, K. 1955* KBrpergrBsse und B e s t a n d s d i c h t e b e i Feldmlusen (Microtus a r v a l i s ) . Z. S a u g e t i e r k . 20: III+-II8.  

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