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Adjustments of the Northwestern muskrat (ondatra zibethicus spatulatus) to a Northern environment Stevens, Ward Earl 1955

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ADJUSTMENTS OF THE NORTHWESTERN MUSKRAT (ONDATRA ZIBETHICUS SPATULATUS) TO A NORTHERN ENVIRONMENT  hy Ward E a r l Stevens  A Thesis Submitted i n P a r t i a l Fulfilment of the Requirements f o r the Degree of DOCTOR OF PHILOSOPHY in the Department of Zoology  We accept this thesis as conforming to the standard required from candidates for the degree of Doctor of Philosophy  Members of the Department of Zoology  THE UNIVERSITY OF BRITISH COLUMBIA August 1955  ABSTRACT  The northwestern muskrat (Ondatra zlbethlcus spatulatus) i s nowhere more abundant than i n the d e l t a of the Mackenzie R i v e r in northern Canada.  An investigation  was undertaken to assess the adjustments resident animals have made in order to inhabit a region more than one hundred miles inside the A r c t i c C i r c l e . The rigours of the physical environment demand that these animals l i v e i n burrows rather than In lodges of vegetation, as i n more southern areas.  During summer  the breeding p a i r s may occupy shallow temporary habitat which i s not suitable f o r tenure during winter.  These  summer sites must be vacated before i c e seals the lakes and prevents escape from a r a p i d l y deteriorating environment.  That a l l muskrats do not desert t h i s  temporary  summer habitat was indicated by the f a c t that only h a l f as many marked animals from such s i t e s were encountered subsequently.  I t may be assumed that a depressed survival  was the rule i n such l o c a t i o n s .  Only deeper lakes with  adequate submerged food plants constituted s a t i s f a c t o r y wintering  environment. Normal movement of muskrats i n the Mackenzie  d e l t a i s an average distance of about 300 yards i n summer and 100 yards i n winter.  Winter a c t i v i t y i s supported by  an extensive system of feeding stations or "push-ups"  constructed on the lake i c e .  These s t r u c t u r e s are a  necessary p a r t of the d a l l y l i f e of the i n d i v i d u a l muskrat because the dispersed nature of the food plants demands a r e l a t i v e l y great radius of a c t i v i t y .  The number of muskrats  using each push-up v a r i e s from three to t h i r t e e n w i t h an average of s i x . The r e l a t i v e l y short period of open water i n t h i s l a t i t u d e so shortens the breeding season t h a t prlmlparous females probably produce only one l i t t e r of young t h e i r f i r s t year of l i f e .  However, they can, by maturing s e x u a l l y at  an e a r l i e r date t h e i r second year, produce two  litters.  Inasmuch as the l a t e winter population i s comprised of f o u r y e a r l i n g s to each adult female, the delay i n breeding i n duced by the l a t e removal of i c e on the lakes and channels i s s i g n i f i c a n t In reducing the rate of population  Increase.  The r e s t r i c t i v e e f f e c t s of climate on breeding a c t i v i t y are compensated f o r hy the b i r t h of l a r g e r l i t t e r s (8.3 young) and by a very s a t i s f a c t o r y s u r v i v a l of these young to y e a r l i n g s t a t u s .  Intolerance between a d u l t s i s  noted during the e a r l y p a r t of the breeding season but does not p e r s i s t during the r e a r i n g of the young. there are few l o s s e s from depredations  As a consequence  of adults upon young  animals as has been reported i n other areas. D e n s i t i e s of animals per u n i t area are low when compared w i t h races of muskrats from other regions.  In  a d d i t i o n the s i z e of the i n d i v i d u a l animal i s s m a l l , and  the m a j o r i t y do not s u r v i v e long past t h e i r second year of l i f e .  These observations support the view that the  Mackenzie d e l t a provides marginal h a b i t a t f o r muskrats. I t i s suggested t h a t p h y s i c a l f a c t o r s induced by the s e v e r i t y of the c l i m a t e represent the major influence l i m i t i n g p o p u l a t i o n growth.  The f u r industry i s another  s i g n i f i c a n t d r a i n on animal numbers but other f a c t o r s appear to be l e s s important.  A l l m o r t a l i t y f a c t o r s taken  together, however, have suppressed or e l i m i n a t e d any  tendency  f o r muskrat numbers to f l u c t u a t e i n a c y c l i c manner as has been reported by several authors f o r other parts of North America. Reference i s made throughout the t e x t to races of muskrats i n h a b i t i n g more southern l a t i t u d e s .  Httitorsitg of iritis!} Columlria F a c u l t y  o f  G r a d u a t e  P R O G R A M M E  S t u d i e s  O F  T H E  Sinai ©r&l ^xmnination for ttye Jbgra of Jtottot* of  |Jl)tIoso^Ii]j  of  W A R D EARL STEVENS B . S c .  ( U t a h  M . S c .  S t a t e A g r i c .  ( I o w a  S E P T E M B E R I N  T H E  I . M c T . J.  S t a t e C o l l e g e ) 2 3 r d ,  1 9 5 5 , a t  B I O L O G I C A L  187  C O M M I T T E E  I N  3 : 0 0  p . m . B U I L D I N G  A C H A R G E  F . ANGUS,  Chairman  COWAN  J. S . ALLEN  A . ADAMS  V . C . BRINK  M .  D . F . UDVARDY  A . J.  W .  S . HOAR  T . S . E .  External  1 9 4 2  1 9 4 7  S C I E N C E S  Room  H .  C o l l e g e )  Examiner  U n i v e r s i t y  M .  WOOD C .  READ —  o f  DR. W . ROWAN  A l b e r t a  TAYLOR  ABSTRACT  [Ondatra  T h e n o r t h w e s t e r n m u s k r a t  i s n o w h e r e R i v e r .  m o r e  A n  a b u n d a n t  t h a n  i n v e s t i g a t i o n w a s  i n  u n d e r t a k e n  t h e s p e c i e s h a s  m a d e  h u n d r e d  i n s i d e o f t h e A r c t i c  m i l e s  T h e  i n o r d e r  l i v e , a t  s h o r e s o f d e e p e r  l e a s t i n  activity.  i n c r e a s e d  i n f o o d  A single  f e m a l e s t h o u g h  t w o  n u m b e r  s u r v i v e T h e  l o n g  m o d e a n d  o f  a r e g i o n  c o n c l u s i o n  p a s t  t h e i r s e c o n d  b e  b o r n e  t h a t m o s t  t h o u g h  i n  i n  s u p e r i o r  D e l t a  t h e i n  r e p r o d u c primiparous  t h e s e c o n d  r a t e o f  t h e s e  a d j u s t m e n t s  for  y e a r .  s u r v i v a l  b i r t h . m u s k r a t s  A n t o  T a g g i n g d o  n o t  o f l i f e .  f u r i n d u s t r y r e p r e s e n t so n e o f t h e m a j o r  r a t n u m b e r s  o n e  t h a t  c o n s t r u c t e d  is normal  l i t t e r a n d  y e a r  t h a n  d e m a n d  o f l i f e i n d u c e s  f o r t h e r e d u c e d  t o t h e  a d j u s t m e n t s  m o r e  f e e d i n g h a b i t s , a n d  p e r  M a c k e n z i e  t o a s s e s st h e  b u r r o w s  litter yearly  y o u n g  t h e  spatulat  C i r c l e .  i n  l i t t e r s m a y  s t a t u s c o m p e n s a t e  s t u d i e s l e d  w i n t e r ,  l a k e s . T h i s  r a d i u s o f m o v e m e n t ,  a d u l t  t o i n h a b i t  d e l t a o f  r i g o u r s o f t h e p h y s i c a l e n v i r o n m e n t  a n i m a l s  tive  t h e  zibcthicus  t h e s e v e r i t y o f t h e c l i m a t e  d r a i n s o n  i s t h e p r i m e  m u s k f a c t o r  l i m i t i n g p o p u l a t i o n o f  t h e  M a c k e n z i e  n a t u r e .  d e l t a  D e n s i t i e s  o f  C o m p a r i s o n s m u s k r a t s  g r o w t h .  u n d e r g o  a n i m a l s  a r e  i n h a b i t i n g  T h e r e  m a d e m o r e  i s n o  e v i d e n c e  p o p u l a t i o n p e r  t h a t  c h a n g e s  m u s k r a t s  o f a c y c l i c  u n i t  a r e a  g e n e r a l l y  a r e  l o w .  t h r o u g h o u t  t h e  t e x t w i t h  r a c e s  o f  s o u t h e r l y l a t i t u d e s .  GRADUATE STUDIES F i e l d o f S t u d y :  Z o o l o g y  H i s t o l o g y  -  -  -  -  C . C .  W , S.  C o m p a r a t i v e P h y s i o l o g y B i o l o g i c a l M e t h o d s-  W .  O t h e r  HOAR  A . CLEMENS a n d I . M c T . COWAN  P o p u l a t i o n D y n a m i c s A d v a n c e d S t u d i e s ( Z o o l o g y )  NICHOL  P . A . I . M c T .  LARKIN COWAN  S t u d i e s : P r i n c i p l e s o f G e n e t i c s  A . H .  V . C.  B i o m e t r y  D i r e c t e d S t u d i e s ( B o t a n y ) P h i l o s o p h i c a l P r o b l e m s  HUTCHINSON  T .  M .  C .  BRINK  TAYLOR B . SAVF.RY  TABLE OF CONTENTS page INTRODUCTION  1  REVIEW OF LITERATURE  4  Taxonomic Status of the Northwestern Muskrat Resume o f Habitats  .  THE STUDY AREA  9 11 21  P h y s i c a l Features of t h e Mackenzie D e l t a Climate o f the Area  ...  METHODS OF PROCEDURE  21 36 39  Duration of t h e Study Location and D e s c r i p t i o n o f t h e Study Areas . . Methods of T r a v e l . . Methods of L i v e Trapping Methods of Tagging Methods of Handling Live Animals THE INVESTIGATION  39 41 51 52 55 56 59  E x t r i n s i c o r P h y s i c a l Adjustments Summer Habitat Winter Habitat Winter Push-up Studies Yearly Radius o f A c t i v i t y Food and Feeding Habits P o p u l a t i o n Density  61 64 66 71 78 85 96  I n t r i n s i c o r B i o l o g i c a l Adjustments 100 Size and Growth Rates of Muskrats 100 Longevity o f Northern Muskrats 109 Breeding P o t e n t i a l and Breeding Habits . . . I l l Age Ratios 133 Sex R a t i o s 137 L i t t e r s per Year 143 Number of Animals per L i t t e r 1^7 Factors i n M o r t a l i t y ... I n t r a s p e c i f i c Intolerance Adult -Young Ass oc i a t i on ( 11 )  TABLE OF CONTENTS (cont inued) page Cannibalism Diseases and Parasites Predatory Animals Hunting and Trapping  .  156 158 160 163  DISCUSSION  167  SUMMARY  176  LITERATURE CITED  181  APPENDICES  190  ACKNOWLEDGEMENTS  195  (  H i  )  LIST OF TABLES Page  Table Number o f D i f f e r e n t Animals Trapped p e r Push-up on Grassy L a k e , November, 1947  765  Catch o f A n i m a l s by Sex and Age on G r a s s y Lake I n November, 1 9 4 7 .  77  Number o f M u s k r a t s Tagged by Months i n the Mack e n z i e D e l t a D u r i n g 1947-50 I n c l u s i v e . . . .  79  Resume o f Food P l a n t s E a t e n by M u s k r a t s i n t h e Mackenzie D e l t a D u r i n g Each Month  91  5.  P r o d u c t i v i t y of S e l e c t e d Areas i n M u s k r a t s per A c r e  97  6.  A Comparison o f Weights and S t a n d a r d Measuremerits' o f M u s k r a t s From D i f f e r e n t P a r t s o f North America  1.  2.  3.  ,  7.  Weights and Measurements o f a Sample o f M u s k r a t s Known t o be A d u l t , Mackenzie D e l t a , 1947-49 .  8.  Average D a i l y Weight Gains of M u s k r a t s i n t h e Mackenzie D e l t a and i n Three Other Areas . .  9.  C a l c u l a t e d Ages o f R e c a p t u r e d M u s k r a t s I n t o F i f t y - D a y Age C l a s s e s  B r e e d i n g C o n d i t i o n o f Female M u s k r a t s C o l l e c t e d i n Spring  11.  Dates and Accumulated D a y l i g h t I n d i c e s o f E v e n t s i n t h e R e p r o d u c t i v e Cycle o f M u s k r a t s .... G r e a t e s t D i f f e r e n c e i n Length of Day f o r F i v e Areas'-of N o r t h America ( iv )  1QJ  ^  1°°  Grouped  10.  12.  1 0 1  1 1 0  120  128  130  LIST OF TABLES (continued) Table  Page  13.  Age Ratios of Trapped Muskrats i n the Mackenzie Delta, 1947-50  133  14.  Comparison of Age Ratios of Muskrats From Various Areas of North America  135  15.  Muskrats Tagged and Subsequently Recaptured After a Period of at Least Three Months . .  137  16.  Number of Young Muskrats per L i t t e r Deduced From Summer Den Trapping Studies, 1947-49 .  140  The Breeding A c t i v i t i e s of Five Races of Muskrats at Several Locations i n North America  145  Evidence of L i t t e r Size i n Muskrats From Post-Mortem Examination  149  17.  18.  ( v )  LIST OF FIGURES Figure 1.  Rage D i s t r i b u t i o n of S i x Geographical Races of Muskrats i n North America  7  2.  A e r i a l View of the Caribou H i l l s . . . .  23  3.  Outcropping of Limestone at Campbell Lake Arcuate and Oxbow Lakes i n the Mackenzie River Delta  25 29  5.  Map of Study Area No. 1  42  6.  Depth Contours i n Grassy Lake  44  7.  Map of Study Area No. 2  45  8.  U t i l i z a t i o n of Shoreline Vegetation by Muskrats, Study Area No. 2  46  4.  9. Map of Study Area No. 3  48  10.  Juvenile Muskrats i n Live Traps  11.  Adult Muskrat i n Holding Cone  56  12.  Method of Weighing Live Muskrats . . . .  57  13.  Relationship Between Snow and Ice Depths  14.  Fate of Push-ups on Grassy Lake  15.  Segregation of Muskrats by Zones i n  16.  Grassy Lake During Winter . . . . . . . Monthly Average Measurements of Male Muskrat Reproductive Organs  ^°  Rate of Increase i n Uterine Width i n Adult and Sub adult Muskrats . . . .  118  17.  ( vi )  . . . .  . . . .  52  70 75 83  INTRODUCTION The for  f u r industry represents  the major souroe of income  the r e s i d e n t s of Northwest Canada.  c r a t i v e areas from t h i s standpoint River.  And one of the most l u -  i s the d e l t a of the Mackenzie  Therein i s concentrated the l a r g e s t t r a p p i n g  population  i n the D i s t r i c t of Mackenzie, a l l of whom depend almost e x c l u s i v e l y upon the revenue they derive from, the s a l e of muskrat furs.  Within t h i s vast d e l t a w e l l w i t h i n the A r c t i o C i r c l e  t h r i v e s the most n o r t h e r l y population of muskrats i n North America.  The modifying i n f l u e n c e of the great r i v e r has allowed these  animals t o become e s t a b l i s h e d t h e r e i n but t h e i r existence  often  i s precarious and they have had to make some changes i n t h e i r usual mode of l i f e i n order t o s u r v i v e . I n the t i t l e of t h i s paper these changes were c a l l e d "adjustments", which designation i s t o be preferred t o "adaptat i o n s " , a more i n c l u s i v e term. cal  Adaptation connotes a p h y s i o l o g i -  r e a c t i o n having genetic o r i g i n whereas adjustment, as used  h e r e a f t e r , r e f e r s to more immediate responses to e x t e r n a l s t i m u l i . I t i s suggested that there i s enough l a t i t u d e w i t h i n any  genetic  complement f o r a f a i r l y wide range of adjustments t o environmental factors. Where t o draw the l i n e between adaptation may  not i n a l l oases be c l e a r .  and adjustment  An example of the d i f f e r e n c e i s  provided by the reindeer or A s i a t i o caribou.  These animals were  2  introduced i n t o northern Canada and even a f t e r twenty years of establishment have r e t a i n e d the same breeding season they had i n t h e i r native range.  As a oonsequenoe t h e i r young are born  almost two months e a r l i e r than those of the North American o a r i bou i n the same l a t i t u d e , and at a time when the ambient condit i o n s u s u a l l y are severe.  I t might be s a i d that the reindeer  beoame "adapted" through genetio s e l e c t i o n t o the climate of i t s o r i g i n a l homeland and when t r a n s f e r r e d t o a new environment i t was not able t o adjust t o the more severe c o n d i t i o n s encountered. I t i s p o s s i b l e t h a t the d i s t i n c t i o n between adaptation and adjustment i s one of degree only, that on a time scale adaptation i s a long term and adjustment a short term change. C e r t a i n l y the l a t t e r i s more circumscribed i n i t s e f f e c t s and more profound changes must be of genetic o r i g i n . Medawar ( 1 ° 5 D has approached the problem by accepting Lotka's "rather portentous-sounding words" of "endosomatic" and "exosomatio" adaptations.  The former r e f e r s t o adaptations whioh  are a part of the animal, and the l a t t e r r e f e r s t o those which are an e x t e n s i o n of the animal and increase i t s f i t n e s s under a given s e t of circumstances. Thus the powerful i n c i s o r t e e t h of a beaver are endosomatic adaptations f o r c u t t i n g wood, but the lodges and dams the animal b u i l d s by employing these teeth are exosomatio adaptations a l l o w i n g i t t o make the most of i t s aquatio surroundings. The p a r a l l e l between exosomatio adaptations and our term of adjustments i s obvious, though the l a t t e r does have a wider ' meaning.  I t , more than Lotka»s term, i s a measure of the  3  a v a i l a b l e genetic d i v e r s i t y w i t h i n a s p e c i e s , of i t s p o t e n t i a l i t y f o r ohange. As Medawar (op. c i t . ) explained, when speaking of adapta t i o n s , whether endosomatic or exosomatic, we c o n s c i o u s l y or unconsciously i n f e r a comparison, e i t h e r between populations w i t h respect t o a p a r t i c u l a r environment, or between environments w i t h respect t o a given population.  Herein the w r i t e r w i l l  deal  w i t h a p a r t i c u l a r s p e c i e s , the muskrat, and w i l l i n v e s t i g a t e the degree of adjustment i t d i s p l a y s I n each of s e v e r a l h a b i t a t s . Thereby an attempt w i l l be made t o e x p l a i n the presence of an i s o l a t e d population of muskrats i n an environment n e a r l y a r c t i c in i t s severity. For the purpose of making v a l i d comparisons w i t h other regions the w r i t e r has not h e s i t a t e d t o draw upon and i n t e r p r e t data from various sources.  I t i s hoped t h a t no inaccuracies or  ambiguities i n r e p o r t i n g the f i n d i n g s of other i n v e s t i g a t o r s have r e s u l t e d .  A l l references used are acknowledged i n the t e x t  and l i s t e d i n the b i b l i o g r a p h y attached.  4  REVIEW OF LITERATURE The muskrat i s so w i d e l y d i s t r i b u t e d i n North Amerioa and has survived so w e l l the impact of c i v i l i z a t i o n and a t t e n dant a g r i c u l t u r e t h a t i t s presence g e n e r a l l y i s evident i n any wet areas which provide f o r i t s h a b i t a t demands. Most people who have v i s i t e d such areas have seen muskrat lodges r i s i n g as small mounds of plant m a t e r i a l i n an expanse of shallow water and waving  cattails.  The demands of the muskrat are f u l f i l l e d not o n l y by these extensive marsh areas, but as E r r i n g t o n (1937, 1941) has recorded, by i n t e r m i t t e n t streams, drainage t i l e s , highway c u l v e r t s , and a v a r i e t y of other s i t e s .  Other i n v e s t i g a t o r s have  found them i n mountain streams and tundra l a k e s , i n c i t y parks and n a v i g a t i o n c a n a l s . The wide v a r i e t y of oircumstances under which they can endure and t h r i v e i s proof of t h e i r a b i l i t y t o adjust t o t h e i r environment  and t o t h r i v e i n the face of seem-  i n g l y adverse c o n d i t i o n s . In the past the greatest amount of study has been d i rected toward the more eooncmioally important races of muskrats, e s p e c i a l l y those e x h i b i t i n g p e c u l i a r i t i e s i n h a b i t s and h a b i tats.  G e n e r a l l y , the i n o e n t i v e f o r these i n v e s t i g a t i o n s was a  desire t o understand the l i f e processes of the animals so t h a t through h a b i t a t c o n t r o l a l a r g e r f u r crop could be produced.  As  a consequence the l i t e r a t u r e concerning muskrats has grown v o l u minous and i n v e s t i g a t o r s c o n t i n u a l l y have attacked more b a s i c  5 problems i n l i f e h i s t o r y and h a b i t a t requirements.  Thus a v a i l -  able information has become more q u a n t i t a t i v e as study t e c h niques have beoome more exact.  At the present time there i s  no doubt that we know more about the h a b i t s and ecology of the muskrat than about any other w i l d f u r bearer. In s p i t e of the acknowledged v a r i a b i l i t y of h a b i t a t , eaoh area occupied by the s e v e r a l geographical races of muskrats has c e r t a i n features which are unique.  For the purpose of t h i s  study these p e c u l i a r c h a r a c t e r i s t i c s w i l l be enumerated and compared. The geographical raoe of muskrats which occupies the northwestern p o r t i o n s of North America was the object of t h i s investigation.  A m a j o r i t y of the f i e l d work was conducted i n  the d e l t a of the Mackenzie R i v e r . Herein f l o u r i s h e s a l a r g e but i s o l a t e d p o p u l a t i o n of muskrats, the most n o r t h e r l y representatives of the species on t h i s hemisphere.  I n order t o gauge  the adjustments which t h i s population has had to make i n order to s u r v i v e , comparisons w i l l be drawn w i t h other geographical races i n North America. The type subspecies z i b e t h i c u s and the c l o s e l y r e l a t e d cinnamomlnus w i l l be used as the norm f o r comparison because they have been accorded more thorough study than any other raoes and occupy a range reasonably c e n t r a l w i t h i n the species range. In the f o l l o w i n g d i s c u s s i o n s concerned w i t h h a b i t s and h a b i t a t s they w i l l be considered as a s i n g l e e n t i t y . The geographical races occupying the b r a c k i s h t i d a l marshes of the A t l a n t i c seaboard and the Gulf of Mexico have  much i n common from a h a b i t a t standpoint but w i l l be dealt w i t h s e p a r a t e l y because of d i f f e r e n c e s i n plant growth and general c o n d i t i o n s obtaining i n each area.  The Hudson Bay muskrat i s  included because of i t s s i m i l a r i t y t o and i n t e g r a t i o n w i t h the northwestern muskrat.  Lack of s u f f i c i e n t published data on the  muskrat o f the Alaska peninsula has precluded i t s i n c l u s i o n i n t h i s study, but i t i s postulated that i t w i l l have greater a f f i n i t i e s w i t h other c o a s t a l races than w i t h those occupying a more c o n t i n e n t a l type o f h a b i t a t .  Seven races were omitted  e i t h e r because they represent r e l a t i v e l y s m a l l populations or because t h e i r h a b i t s are not s u f f i c i e n t l y unique t o d i f f e r e n t i a t e them.  Furthermore, t h e i r i n c l u s i o n would tend t o make  comparisons u n n e c e s s a r i l y involved.  The subspeoies r e f e r r e d t o  i n t h i s study are therefore the f o l l o w i n g : 1.  Ondatra z i b e t h i c u s z i b e t h l o u s  -Common muskrat  2.  Ondatra z i b e t h i c u s maorodon  - V i r g i n i a muskrat  3.  Ondatra z i b e t h i o u s r i v a l i o i u s  - L o u i s i a n a muskrat  4.  Ondatra z i b e t h i o u s oinnamominus  -Great P l a i n s muskrat  5.  Ondatra z i b e t h i c u s albus  -Hudson Bay muskrat  6.  Ondatra z i b e t h i o u s spatulatus  -Northwestern muskrat  &  Relegated from s p e c i f i c t o s u b s p e c i f i c rank by Davis and Lowery (1940).  The d i s t r i b u t i o n of each of these geographical races of muskrats i s presented i n Figure 1 and the range of each may be kept i n mind as comparisons are made h e r e a f t e r between these subspecies. An attempt w i l l be made t o present much of the information gleaned from the l i t e r a t u r e i n a reasonably condensed form and to include only those data concerned w i t h the problems of  7  1  0  Ondatra z. zibethicus- Common Muskrat  2. Ondatra z. macrodon - Virginia Muskrat 3. Ondatra z. cinnamominus - Great Plains Muskrat 4. Ondatra z. albua- Hudson Bay Muskrat 5. Ondatra z, spatulatus - Northwestern Muskrat 6. Ondatra z. rivalicius - Louisiana Muskrat FIGURE 1 . DISTRIBUTION OF SIX GEOGRAPHICAL RACES OF MUSKRATS IN NORTH AMERICA DISCUSSED XN THE TEXT.  8 e c o l o g i c a l adjustment being considered.  On the other hand, be-  cause the northwest muskrat has received mueh l e s s a t t e n t i o n from i n v e s t i g a t o r s than have the more s o u t h e r l y r a c e s , o r i g i n a l m a t e r i a l concerning i t w i l l be given i n more d e t a i l .  Some phases  of the i n v e s t i g a t i o n s which have no counterpart i n other s t u d i e s w i l l be included t o complete the p i e t u r e of an animal l i v i n g at the extreme periphery of i t s geographical range.  9  Tazoncmio Status of the Northwestern Muskrat The animal w i t h which t h i s study deals i s the northwestern muskrat, Ondatra z i b e t h i o u s s p a t u l a t u s .  Osgood (1900),  who f i r s t described t h i s raoe, noted that i t was a smaller and darker form than the t y p i c a l subspecies, z i b e t h i c u s . of the Great P l a i n s , and had very small molar teeth and spatulate nasals by comparison.  The most complete systematic treatment  of the muskrats was that of H o l l i s t e r (1911) and most a u t h o r i 1  t i e s since that date have followed h i s c l a s s i f i c a t i o n . Anderson (1946) gave the d i s t r i b u t i o n of the northwest muskrat a s : "Northwestern North America, from Kowak R i v e r (east of Kotzebue.Sound) and Yukon V a l l e y i n A l a s k a , through the lower parts of Yukon t o the A r c t i c coast, north t o Richards Island i n Mackenzie d e l t a , Northwest T e r r i t o r i e s , south and east t o Anderson R i v e r , Great Bear, and Great Slave Lakes, and south i n t o northwestern B r i t i s h Columbia and n o r t h - c e n t r a l A l b e r t a ; probably also i n t o northwestern Saskatchewan". Osgood (op. o i t . ) was of the opinion that the raoe spatulatus was derived from a form east of the Rocky Mountains and H o l l i s t e r (op. c i t . ) thought that speoimens from the north of Great Bear Lake and from the lower Mackenzie v a l l e y were obviously approaohing the Hudson Bay form i n having s l i g h t l y l a r g e r t e e t h , l e s s broadly spreading zygomata and a tendency towards an increase i n r u s t y c o l o u r throughout the pelage. 1  The most recent synopsis concerned w i t h muskrats was that of H a l l and Cockrum (1953). The w r i t e r has followed the nomenc l a t u r e presented t h e r e i n .  10  Because of i t s d i s t r i b u t i o n i n a r e l a t i v e l y i s o l a t e d s e c t i o n of the continent, the northwestern muslcrat has i n the past reoeived very l i t t l e a t t e n t i o n from s c i e n t i f i c i n v e s t i gators.  11  Resume of H a b i t a t s At t h i s point the w r i t e r would l i k e t o supply a background f o r subsequent d i s c u s s i o n s by g i v i n g a b r i e f resume of the h a b i t a t occupied by each of the geographical races of muskr a t s under review.  This resume w i l l d e a l mainly w i t h conditions  of the h a b i t a t and only i n c i d e n t a l l y w i t h the r e l a t i o n s h i p o f muskrats t o t h e i r environment.  That information w i l l be i n -  cluded under the appropriate s e c t i o n i n the body of the t e x t . HABITAT RESUME OF THE COMMON MUSKRAT The t y p i c a l race z i b e t h i o u s i s an animal of the small i n l a n d lakes and watercourses w i t h i n the temperate eastern sections of North America south t o Delaware Bay. I t i s not, however, a c o a s t a l race because the r e l a t i v e l y abrupt nature of the northern A t l a n t i c coast l i n e precludes any extensive a s s o c i a t i o n w i t h t i d a l waters.  The animal s u f f e r e d when a g r i -  c u l t u r a l p r a c t i c e s destroyed much of i t s former h a b i t a t , but t h i s c o n d i t i o n has been a l l e v i a t e d t o some extent by the cons t r u c t i o n of impoundments along such major r i v e r s as the Ohio and the M i s s o u r i and by extensive use of drainage d i t c h e s i n Iowa, I l l i n o i s , and neighbouring s t a t e s .  Fewer attempts have  been made t o provide s u i t a b l e h a b i t a t f o r t h i s race than f o r the Gulf coast or the Hudson's Bay races.  However, z i b e t h i o u s has  benefited by an extensive scheme of waterfowl refuges b u i l t by the governments of United States and Canada.  Many of the s t u d i e s  to date have been done under the reasonably c o n t r o l l e d c o n d i t i o n s  12  provided by these refuges.  Dozier and h i s associates worked on  the Montezuma Refuge i n New York, B e l l r o s e , Brown and Low at the refuges along the Ohio R i v e r , and Beer and others at the Horicon Marsh W i l d l i f e Area i n Wisconsin. Floods and droughts both are harmful t o e s t a b l i s h e d popul a t i o n s of muskrats as E r r i n g t o n (1939) and B e l l r o s e and Low (1943) have demonstrated.  These f a c t o r s u s u a l l y do not produce any  d i r e c t m o r t a l i t y o f animals but have t h a t effect- i n d i r e c t l y by d i s t u r b i n g the h a b i t a t and p l a c i n g the insecure animals i n c i r cumstances more subject t o other d e s t r u c t i v e f a c t o r s . The type of ground or s o i l conditions associated w i t h populations  of muskrats seldom have been reported.  (1937) considered  Errington  that d a y s o i l s were more s u i t e d t o bank  denning muskrats since d a y would a l l o w c o n s t r u c t i o n of an ext e n s i v e burrow system.  Personal observation i n northern Iowa  showed the s o i l s t o be f a i r l y heavy and i n long-established sloughs o f t e n covered w i t h a loose l a y e r o f organic m a t e r i a l of v a r i a b l e depth.  Areas of deep peat deposits were not common i n  Iowa although they were reported f a r t h e r t o the north. HABITAT RESUME OF THE ATLANTIC TIDEWATER MUSKRATS The V i r g i n i a muskrat oocupies the c o a s t a l region from Delaware Bay south t o Pamlioo Sound, and maintains i t s greatest concentrations coastline.  of population i n the t i d a l marshes bordering t h i s  Here prime muskrat h a b i t a t seems t o be that offered  by the "ebb and flow" marshes.  The water t h e r e i n i s f r e s h o r  s l i g h t l y brackish and v a r i a t i o n s i n t i d a l depth average f i v e and one h a l f feet (Pancoast, 1937).  Bottom s o i l i n the optimum  13  h a b i t a t has a v a r i a b l e depth of organic m a t e r i a l above i t . Smith (1938) reported t h i s thickness o f peat t o be s i x inches and remarked that optimum water l e v e l s were at or near the surface of t h i s peat l a y e r . Dozier (1948) was of the opinion t h a t inundation by high t i d e s and f l o o d s increased the s a l i n i t y of the c o a s t a l marshes thereby d i s r u p t i n g the plant succession t o the p r e j u d i c e of the muskrat population.  Drought also increased the s a l i n i t y of the .  marshes and r e s u l t e d i n m o r t a l i t y t o muskrats, e s p e c i a l l y the young born during periods of hot dry weather. Quite a d i f f e r e n t f l o r i s t i c complex accompanied such an increase i n s a l i n i t y .  Very s a l i n e c o n d i t i o n s produced the cord  grasses (Spartina cynosuroides. S. a l t e r n i f o l i a . S. patens), plus s a l t grasses ( D i s t i o h l i s  spioata) and spike rushes (Juncus spp).  This plant community represented poor h a b i t a t f o r the V i r g i n i a muskrat. The ebb and flow marshes f u r t h e r i n l a n d e x h i b i t e d l e s s s a l i n i t y and henoe produced Typha, f l a g r e e d (Phragmites  communis),  and bulrushes (Scirpus Q l n e y i . S. amerioanus, and S. robustus). RESUME OF HABITAT CONDITIONS AT GULF TIDEWATER The coast marshes of L o u i s i a n a and eastern Texas are i n habited by a dark race of muskrat 0 . z. r i v a l i o i u s .  Very high  populations of t h i s animal are maintained i n those b r a c k i s h t i d a l areas t h i o k l y grown t o three-square bulrush, (Scirpus Qlneyi) and cord grass ( S p a r t i n a patens).  The former p l a n t furnished most  of the food whereas the l a t t e r provided the bulk of the b u i l d i n g m a t e r i a l f o r lodges.  This marsh vegetation grows on h i g h l y  organic s o i l s (Lynch e t . a l , 1947).  Wherever there i s an  14  admixture of c l a y w i t h the peat, "coco" marsh i s formed which has an i n t r u s i o n of s a l t grass ( D i s t i c h l i s s p l c a t a ) .  Although coco  marshes are e s s e n t i a l l y the same f l o r i s t i c a l l y as the t y p i o a l three-square marshes, t h e y cannot produce as many muskrats over as long a period of time.  The adjaoent f r e s h water h a b i t a t s pro-  duce i n f e r i o r vegetation and comparatively fewer muskrats. Great areas of the p r e f e r r e d braokish type are found w i t h i n the continuous r e g i o n o f c o a s t a l marshes.  Under-trapping of  animals t h e r e i n r e s u l t s not only i n d e t e r i o r a t i o n of the h a b i t a t by "eat-outs", but a l s o i n increased f i g h t i n g , predation, and emigration.  I c e i s a r a r i t y and though f r o s t s may k i l l some of the  emergent v e g e t a t i o n there i s u s u a l l y ample f o r food year-long. Growth of marsh p l a n t s i s l u x u r i a n t and under normal conditions proceeds r a p i d l y enough t o replace plant m a t e r i a l u t i l i z e d by muskrats and waterfowl.  F i r i n g of the marshes i s p r a c t i s e d t o  a r r e s t the hydrosere short of the olimax c o n d i t i o n (Penfound and Sohneidau, 1945) and t o f o s t e r the growth of p l a n t s favoured as food by muskrats.  The e x c l u s i o n of s a l t water from the marshes  i s discouraged because i t I n h i b i t s adequate growth of the desired plant a s s o c i a t i o n . HABITAT OF THE HUDSON BAY MUSKRAT A r e l a t i v e l y pale race of muskrat 0. z. albus i s found i n the  "waters d r a i n i n g i n t o Hudson Bay from the west, i n eastern  Saskatchewan and Keewatin; north t o the barren grounds" ( H o l l i s t e r , 1911).  This animal, the Hudson Bay muskrat, was described f i r s t  from specimens taken near Cumberland House, Saskatchewan.  In this  v i c i n i t y and the adjacent d e l t a of the Saskatchewan R i v e r , albus  15  a t t a i n s i t s greatest abundance.  Recently concentrations of  animals here have been bolstered by large f u r management proj e c t s developed both by the f e d e r a l and p r o v i n c i a l  governments.  J . A. MoLeod and h i s a s s o c i a t e s (1948, 1949, 1950) conducted studies of muskrat biology on the Manitoba Government Fur R e h a b i l i t a t i o n Blocks.  Most of t h e i r f i e l d work was confined t o  the Summer berry Fur B l o c k i n the Saskatchewan R i v e r d e l t a .  Here  a system of seventy dykes and c o n t r o l gates of v a r i o u s s i z e s have helped to s t a b i l i z e water l e v e l s over approximately one m i l l i o n acres of f l a t marsh area. Concerning these impoundments i n the Saskatchewan  delta  area, MoLeod (1948) states t h a t " s h o r t l y a f t e r i n i t i a l r a i s i n g of the water l e v e l a s h i f t i n the d i s t r i b u t i o n of the plants t o the new margin i s seen, and under s u i t a b l e water l e v e l conditions a new zonation i s e s t a b l i s h e d i n two or three y e a r s , and a maximum d e n s i t y i s reached i n about f i v e years.  One would expect  t h i s t o continue i n an unaltered s t a t e almost I n d e f i n i t e l y but under conditions o f consistent h i g h water a d e t e r i o r a t i o n sets i n , and i n about t e n years the area has reverted t o open water". S o i l sampling i n d i c a t e d t o those workers that " i n the absence of complete decomposition of plant m a t e r i a l i n the water and mud by b a c t e r i a and animal organisms the nitrogen would be found i n no simpler form than amino a c i d s " and therefore not a v a i l a b l e t o growing p l a n t s .  This s t a b i l i z a t i o n o f l e v e l s also discouraged  establishment from seeds o f seme major species of marsh p l a n t s which r e q u i r e d mud banks o r very shallow water f o r germination. I n l o n g term these d i f f i c u l t i e s from s t a b i l i z e d water l e v e l s were no doubt s i g n i f i c a n t but the f a o t o r s o f sudden and  16  profound flooding were also a problem i n marsh ecology.  During  May, 1949, the author (1949 unpubl.) v i s i t e d the Summerberry developmental area and examined the r e s u l t s of f l o o d i n g that had occurred the previous year. exceptionally high water.  The summer o f 1948 had been one of  Not only did the water levels r i s e  above previous marks but they stayed high throughout the summer. The whole of the Saskatchewan d e l t a was transformed into a vast lake and ample evidence of t h i s f l o o d i n g was r e f l e c t e d i n the altered marsh ecology as well as i n the broken dykes and flood gates.  Extensive areas of flag-reed were k i l l e d and growths of  c a t t a i l s , separated from t h e i r substratum, e i t h e r persisted as f l o a t i n g islands or were blown ashore.  Sedges adapted t o the  flooded conditions by producing stems as long as f i v e feet. Concomitant with the reduced vegetation, the number of muskrats harvested the following year was lower. Under normal circumstances water depths i n t h i s marsh seldom exceeded eight or ten f e e t and often were much l e s s .  The  s o i l s of more recent o r i g i n consisted of clay with l i t t l e humus incorporated t h e r e i n .  Some areas seen by the w r i t e r , however,  exhibited a layer of organic matter twelve to eighteen inches deep above t h i s hard stratum of clay.  During summer these de-  posits often were floated to the surface by decomposition gases and were seen t o be an accumulation of reed and sedge fragments. HABITAT RESUME OF THE NORTHWESTERN MUSKRAT The northwestern muskrat has received l i t t l e  intensive i n -  vestigation because i t s geographical range i s r e s t r i c t e d to the  17 l e s s s e t t l e d parts of the continent.  E a r l y references a l l u d e d  t o i t s g e n e r a l l y wide d i s t r i b u t i o n but s i n c e muskrats at that time were not an important  item i n the f u r trade, only b r i e f  mention was made of them.  Richardson (182°) gave a d e s c r i p t i o n  of the northern animals, probably albus and s p a t u l a t u s . and ascertained that they extended northward n e a r l y t o the mouth of the Mackenzie, i n l a t i t u d e 6 9 . 0  Rae (1888) and R u s s e l l (1898)  mentioned that muskrats were very abundant i n some places. R u s s e l l i n t r a v e l l i n g from Grand Rapids on Lake Winnipeg to Herschel I s l a n d , Yukon, traversed the d e l t a s t o the Saskatchewan, Athabaska, Slave, and Mackenzie Rivers and noted that muskrats abounded i n such l o c a t i o n s . Preble (I908) wrote t h a t "on the lower reaches of the Mackenzie and P e e l Rivers muskrats are exc e s s i v e l y abundant".  MaoFarlane (1908) found them "very common  on the lower Mackenzie R i v e r , and l e s s so on the same p o r t i o n of the Anderson R i v e r t o t h e i r o u t l e t s i n the p o l a r sea". Richardson lop. c i t . ) provided a general d e s c r i p t i o n of the habitat of Ondatra i n North America.  That most of h i s ex-  perience w i t h the species was i n Canada may be judged from h i s observation that the animals i n winter were almost e x t i r p a t e d i n c e r t a i n areas by f r e e z i n g of the swamps which they i n h a b i t . He f u r t h e r says that " t h e i r f a v o u r i t e abodes are small grassy lakes or swamps, or the grassy borders of slow-flowing streams where there i s a muddy bottom". Preble (1908) found spatulatus i n h a b i t i n g muskeg ponds and s m a l l streams i n the h i l l s near Fort Chipewyan.  Near the  mouth of the Peace R i v e r he saw them frequenting landlocked ponds i n dense spruce woods. He a l s o found them on the i s l a n d s and  18  along the shore of the North Arm of Great Slave Lake wherever marshy i n l e t s occurred.  As he journeyed northward, evidenoe  of muskrats dwindled and the l a s t "house" was seen a few m i l e s north of Lake H a r d i s t y . Muskrats were not enoountered on the south shore of Great Bear Lake hut were found near F o r t F r a n k l i n where they "were occupying burrows i n the banks and were l i v i n g mainly on coarse g r a s s , which was abundant on the marshy p a r t s of the shore". R u s s e l l (1898) commented upon t h i s use of bank d w e l l i n g s by muskrats.  He noted t h a t "they do not b u i l d winter houses as  e x t e n s i v e l y as those f a r t h e r south, but u s u a l l y burrow i n the banks of streams and ponds; they p r e f e r the streams t o the swamps, and are e s p e c i a l l y abundant i n the d e l t a s of the l a r g e r i v e r s , where they may be seen by day, but at night they f a i r l y swarm i n the s m a l l e r channels". Later d e t a i l e d studies of the northwestern muskrat i n cluded those by B a n f i e l d (1946) f o r the Mackenzie d e l t a , by Cowan (1948) i n the same r e g i o n , by F u l l e r (1951a, 1951h) i n Wood B u f f a l o Park, and by Law (1950) i n the Slave R i v e r d e l t a . The endeavours o f B a n f i e l d and Cowan were p r e l i m i n a r y t o the present i n v e s t i g a t i o n . F u l l e r (195D concentrated h i s e f f o r t s on s e v e r a l ponds and sloughs l y i n g along the l e f t bank o f the Athabaska R i v e r between Embarras l a n d i n g s t r i p and the settlement of Embarras Portage.  He noted that "the area contains three l a r g e sloughs,  a number of p o t h o l e s , which u s u a l l y hold water the year round, and several marshy areas which dry up i n l a t e summer and autumn". I t was found that "many of the best muskrat sloughs ... were  19  ponded s e c t i o n s of o l d stream beds" which s i l t a t i o n had closed at e i t h e r end. Three zones of v e g e t a t i o n were t y p i o a l of the ponds and sloughs i n F u l l e r s study area. 1  The emergent species found were  mainly c a t t a i l (Typha l a t i f o l l a ) . h o r s e t a i l (Equisetum). o a r i c e s . bulrush (Scirpus v a l i d u s ) , and bur-reed (Sparganium latum).  multlpeduncu-  Water t o depths of four feet supported dense mats of  yellow water l i l y (Nuphar variegatum) and beneath t h i s , grading i n t o deeper water, grew waterweed (Anaoharis), arrowhead (Sagitt a r i a ) , crowfoot (Ranunculus). and bur-reed (Sparganium).  Com-  p l e t e l y submerged p l a n t s i n the deeper p o r t i o n s of the l i t t o r a l zone included bladderwort ( U t r i o u l a r i a ) . c o o n t a i l (Ceratophyllum). m i l f o i l (Myriophyllum). and pondweeds (Potamogeton spp.).  The  amount of each species v a r i e d w i t h the water depths i n the ponds and sloughs. The northwestern muskrats b u i l t three types of s t r u c t u r e i n the Athabaska d e l t a .  Winter houses constructed mainly of cat-  t a i l and bulrush were prominent f e a t u r e s of the marsh.  Feeding  huts or "push-ups", erected a f t e r "freeze-up", were composed of waterweed, m i l f o i l , bladderwort, and pondweeds.  Burrows i n the  stream and pond banks were inhabited during summer and perhaps yearlong.  Brood nests were made i n the bank burrows except during  times of f l o o d i n g . A summer study by Law (1950) i n the d e l t a of the Slave R i v e r , only two hundred m i l e s t o the north of the Athabaska  delta  depicted some i n t e r e s t i n g changes i n choice of h a b i t a t by the resident muskrat p o p u l a t i o n . I n the f i r s t p l a c e , the animals oocupied bank dens or burrows yearlong and d i d not construct  20  lodges of emergent v e g e t a t i o n , even though, plants were abundant. Their burrows were dug i n s u i t a b l e c l a y banks bordering the deeper sloughs and the l e s s a c t i v e channels.  Neither steep banks  nor s h e l v i n g banks were u t i l i z e d although water f i l l e d  canals  often were dug through shallow areas t o reach the more s u i t a b l e banks.  The animals i n t h i s d e l t a had t o cope w i t h heavy s i l t a -  t i o n , which during the oourse of a year could render  previous  h a b i t a t s u n s u i t a b l e e i t h e r by making them too shallow, or by s h i f t i n g the channels and thus a l t e r i n g the flow o f water therein. Law reported t h i s d e l t a t o be unforested though high ground would support w i l l o w , a l d e r , and some poplar.  Cattail,  h o r s e t a i l , marsh reed grass (Calamagrostis), and bulrush comp r i s e d most of the emergent vegetation.  F l o a t i n g and submerged  p l a n t s were s i m i l a r t o those found by F u l l e r (op. c i t . ) at the Athabaska areas.  21  THE STUDY AREA P h y s i c a l Features of the Mackenzie R i v e r D e l t a LOCATION OF THE MACKENZIE DELTA Northeastern B r i t i s h Columbia, the northern h a l f of A l b e r t a , and northwestern Saskatchewan a l l f a l l w i t h i n the Mackenzie R i v e r drainage system as does the southeastern part of the Yukon T e r r i t o r y and most of the D i s t r i c t of Mackenzie. The water from t h i s vast area e v e n t u a l l y f i n d s i t s way i n t o the Maokenzie R i v e r by way of the Peace, the Athabaska, the Slave, and the L i a r d R i v e r s .  Three large bodies of f r e s h water,  Athabaska Lake, Great Slave Lake, and Great Bear Lake also cont r i b u t e to t h i s drainage. The Mackenzie R i v e r , as such, begins as the o u t l e t t o Great Slave Lake and flows westward and northward more than 1,000 m i l e s t o the Beaufort Sea. T h i s gradient of the stream i s s m a l l , about f i v e inches t o the m i l e , and consequently the flow of the r i v e r i s l e i s u r e l y and the channel devious and f i l l e d w i t h i s l a n d s and s h i f t i n g bars of mud and g r a v e l .  The  whole length of the waterway from the Fort Smith rapids t o the Beaufort Sea, a distance of 1,400 m i l e s , i s navigable by r i v e r boats during the season of open water.  I t forms the only prac-  t i c a l l i n k between what we h a p p i l y c a l l c i v i l i z a t i o n and the vast reaches of the western A r c t i c The r i v e r at present has a very r e s t r i c t e d f l o o d p l a i n and flows f o r most of i t s distance between banks one hundred t o  22  f i v e hundred f e e t h i g h .  The stream narrows t o about f i v e hun-  dred yards at the Ramparts above Good Hope but at other places i t i s two t o three m i l e s wide.  A few m i l e s below Point Separa-  t i o n , whioh l i e s at l a t i t u d e 67° 35  1  N., the r i v e r spreads out  i n t o a wide a l l u v i a l p l a i n which comprises i t s d e l t a . s p l i t s i n t o three main channels —  Here i t  the West Channel which j o i n s  a branch of the P e e l River and flows past A k l a v l k t o the Shoalwater Bay; the Middle Channel, which r a m i f i e s i n t o many streams near i t s mouth, flows i n t o both Shoalwater Bay and Mackenzie Bay; and the East Channel which begins a few m i l e s below Point Separation and flows along the foot of the Caribou H i l l s i n t o Kugmallit Bay. are  Numerous secondary channels and creeks which  active yearlong form i n t e r c o n n e c t i n g l i n k s between these  l a r g e r channels. The i s l a n d s formed by these main and secondary channels are dotted w i t h innumerable lakes and marshes. The settlement of A k l a v i k l i e s near the centre of the d e l t a area at 68° 13» N. 135°W.  The distance from the Richard-  son Mountains on the west t o the Caribou H i l l s on the east s i d e averages f o r t y m i l e s , though at i t s mouth the r i v e r spreads out to cover almost seventy m i l e s .  The length of the d e l t a from  Point Separation t o the s a l t water i s roughly one hundred m i l e s . The present a l l u v i a l p o r t i o n of the d e l t a , w i t h which we w i l l d e a l , comprises therefore an area of at l e a s t 4,000 square m i l e s . TOPOGRAPHY 0E THE DELTA AREA Conspicuous on the western s i d e of the D e l t a i s the steep f a u l t scarp of the Richardson Mountains that r i s e s abruptly out of the  p l a i n t o a height of about 5,000 f e e t .  Northward the  23  e l e v a t i o n of the h i l l s f l a n k i n g the d e l t a g r a d u a l l y decreases as the main range trends westward away from the c o a s t a l p l a i n . On the opposite side of the d e l t a the Caribou H i l l s extend from the v i c i n i t y of Campbell Lake northward t o the A r c t i c Ocean.  They are i n t e r s e c t e d at Tununuk by the East  Channel of the Mackenzie and continue from thence as the e l e vated portions of the Richards I s l a n d .  This range of h i l l s i s  nowhere more than 300 feet high ( F i g . 2) and marks the western edge of a r o l l i n g and poorly drained plateau that separates the Mackenzie d e l t a from the Eskimo Lakes.  Figure 2.  An a e r i a l view of the Caribou H i l l s i n the v i c i n i t y of the Reindeer S t a t i o n .  24  To a boat t r a v e l l e r the impression of the iJelta i s one of innumerable winding and interconnecting waterways t r e n d i n g i n a r a t h e r aimless f a s h i o n toward the sea.  The banks of the  channels vary i n height from f i v e t o twenty-five f e e t , though they are g e n e r a l l y higher i n the upper reaches of the d e l t a . I t i s not u n t i l one mounts t o commanding e l e v a t i o n s that the amazing abundance of lake and marsh h a b i t a t becomes apparent. The panorama i s one i n which the water area s u r e l y exceeds that of the l a n d .  I t may be seen that the watercourses have formed  n a t u r a l levees along t h e i r banks during times o f high water and that the i n t e r v e n i n g country i s lower and comprised o f lakes and marshes.  The sheets o f water are s m a l l , few a t t a i n i n g t o a  square m i l e of area. A m a j o r i t y of the lakes and channels are interconnected so that they f l o o d d u r i n g the s p r i n g and g r a d u a l l y subside during summer. However, the amount of drainage i s r e s t r i c t e d by the depth o f the s m a l l waterways that connect the lakes with the main channels.  Few of the lakes d r a i n out completely though  many l o s e enough water so that i n summer they a t t a i n a marshy character and during winter may q u i c k l y freeze t o the bottom. The l a k e s , which do not have access t o the channels, t a i n e d by a run-off of surface water.  are main-  Because evaporation i s  slow i n these l a t i t u d e s these lakes acquire a c e r t a i n s t a b i l i t y both i n water l e v e l s and i n the attendant  ecological conditions.  That such lakes are i n a m i n o r i t y was seen during an a e r i a l survey of the d e l t a i n mid-June of 1948, volume.  a year of unusual water  I t was estimated that at l e a s t ninety percent of a l l  25 lakes had been f l o o d e d , the c l e a r water of the remaining lakes standing out i n s t r i k i n g contrast against the sediment laden water from the r i v e r . GEOLOGY OF THE DELTA AND VICINITY Perhaps a l l t h a t need be said about the geology of the lower Mackenzie R i v e r Is that a l l of the exposed rock formations are of sedimentary o r i g i n and consist mostly of Cretaceous shales sandstones, and conglomerates.  There i s an i n t r u s i o n of lower  Devonian limestone at the south end of Campbell Lake ( F i g . 3) although Camsell 11921) otherwise d i d not record limestone frcxa the region of the Mackenzie d e l t a .  Figure 3«  A bold outcropping of limestone at the south end of Campbell Lake.  26  Hume (1924) and Camsell both asserted that the Mackenzie b a s i n was g l a c i a t e d i n the past.  They postulated that the i c e  from the Keewatin centre invaded the Mackenzie b a s i n from the east and pushed to the foot of the mountains where i t was met and o v e r l a i n by the C o r d i l l e r a n i c e f l o w i n g down from higher levels.  I n the lower Mackenzie there appeared to have been a  general northward flow o f a sheet of i c e 1,500 feet t h i c k which extended down the v a l l e y and out t o sea.  The disappearance of  t h i s i c e l e f t behind a poorly drained topography together with morainal mounds and eskers.  Probably the most s i g n i f i c a n t  e f f e c t of release from.the i c e load was an u p l i f t of the e n t i r e basin w i t h the i n s t i t u t i o n of a new c y c l e of e r o s i o n .  There  seems t o be l i t t l e doubt that the d e l t a of the Mackenzie has r i s e n w i t h i n the recent past and that the present stream now i s degrading i t s bed. PERMAFROST A c h a r a c t e r i s t i c feature of much of the Northwest T e r r i t o r i e s i s i t s permanently frozen s u b s o i l , or "permafrost". M u l l e r (1947) defined permafrost as "a thickness of s o i l or other s u p e r f i c i a l d e p o s i t , even of bedrock, at a v a r i a b l e depth beneath the surface of the earth i n whioh a temperature below f r e e z i n g has e x i s t e d f o r a long time, (from two t o tens of thousands of y e a r s ) " .  An " a c t i v e " l a y e r , d e l i n e a t e d by the depth o f  the summer thaws, l i e s above the p e r p e t u a l l y frozen s o i l .  The  thickness o f t h i s l a y e r depends upon the amount of i n s u l a t i o n by plant and other m a t e r i a l s , and upon the t e x t u r e and water content of the s o i l .  I n the r e g i o n of the Mackenzie d e l t a exposed banks  27  of sand may thaw t o a depth of s i z f e e t during l a t e summer. the  On  other hand, at A k l a v i k the s i l t - c l a y s o i l under a sparse  growth of grass (Poa arotioa) had thawed t o only t h i r t y - t w o inches i n e a r l y September of 194°, and the dry tundra above the  Reindeer Depot was frozen at depths below s i x inches on  28 August, 1947.  The l i n e d e l i n e a t i n g the southern boundary  of permafrost i n the Northwest T e r r i t o r i e s agrees roughly with the  minus 3°C  isotherm and passes through the v i c i n i t y of  Simpson, Providence, and Y e l l o w k n i f e . The depth of the permafrost l a y e r i n the Mackenzie d e l t a has never been a s c e r t a i n e d .  Any s h a f t s which have been  sunk f o r basements or i c e c e l l a r s have f a i l e d t o pass through the  frozen earth at depths to twenty-five f e e t .  T h i s i s not  s u r p r i s i n g since Jenness (1949) c i t e d depths of 230 metres i n S i b e r i a and over 130 metres i n the environs of Point Barrow, Alaska.  S h a f t s d r i l l e d f o r petroleum at Norman W e l l s , over two  hundred m i l e s t o the south of A k l a v i k , d i d not p i e r c e below permafrost u n t i l depths of 267 f e e t ( 8 l metres) had been reached. I t i s assumed that the beds of r i v e r s are f r e e from permafrost.  This f a c t has been demonstrated at Normal Wells and at  C h u r c h i l l , Manitoba (Jenness, op. c i t . ) .  At Norman Wells i t  was noted that s h a f t s d r i l l e d at one hundred f e e t , two hundred f e e t , and three hundred and f i f t y feet from the w a t e r s edge 1  e x h i b i t e d a thiokness of permafrost of s i x t y f e e t , one hundred and t h i r t y - f i v e f e e t , and two hundred and s i x t y - s e v e n feet r e spectively.  Yet i t also i s t r u e that i s l a n d s i n the Mackenzie  R i v e r and those forming i t s d e l t a show a permanently f r o z e n condition.  We may assume, t h e r e f o r e , that permafrost i s being  28  formed i n these l a t i t u d e s under the e x i s t i n g c l i m a t i c c o n d i t i o n s . DELTA BUILDING AND SCULPTURING The d e l t a of the Mackenzie i s of the estuarine type, that i s , the m a t e r i a l s forming i t were deposited i n a long narrow estuary or arm of the sea.  The d e l t a has been shaped by  the higher land along i t s margins and since.-, presumably, i t was l a i d down i n quiet water, there has been l i t t l e s o r t i n g of the m a t e r i a l s by coastwise currents and growth has been uninterrupted seaward.  Camsell (1921) noted that f o r i t s s i z e the Mackenzie  R i v e r o a r r i e s r e l a t i v e l y l i t t l e sediment and that i t s d e l t a i s not being b u i l t as r a p i d l y as i t s great counterpart, the M i s s i s s ippi.  The great lakes along the oourse of the Mackenzie R i v e r  system act not only as f l o o d c o n t r o l r e s e r v o i r s but a l s o as sediment basins.  I t i s only during the s p r i n g f l o o d that the  s i l t load i s heavy; at other times the water i s r e l a t i v e l y clear. Channel b r a i d i n g of the Mackenzie R i v e r at i t s d e l t a i s profound and most of the channels remain a c t i v e c o n t i n u a l l y . Thus, although the water current i s slow, e r o s i o n of the downstream face of meander curves along w i t h a d e p o s i t i o n on the opposite bank are o c c u r r i n g c o n s t a n t l y .  This water a c t i o n forms  an unusually large number of that type of lake which Lynch (1940) c a l l s "arcuate swales" ( E l g . 4 ) ; lakes between p a r a l l e l bars on the upstream face of meander curves.  Seme of these lakes may  t r a n s i t o r y although others, e s p e c i a l l y f l a n k i n g the Middle  and  East Channels, are of a more permanent nature. I n a d d i t i o n t o the "arcuate swale" i n the d e l t a i s the  be  29  FIGURE 4. ARCTUATE TYPE LAKES FORMED BY THE DEVELOPMENT OF A MEANDER IN ONE OF THE LARGER CHANNELS OF THE MACKENZIE RIVER DELTA. SCALE: 9 INCHES/6 MILES.  FIGURE 4 . OX-BOW LAKES FORMED BY THE CUTTING OFF OF A MEANDER SPUR IN ONE OF THE LARGER CHANNELS OF THE MACKENZIE RIVER DELTA. SCALE: 9 INCHES/6MILES.  30 ox-bow lake. Here the stream has cut through a meander spur and has s i l t e d up one or both ends of the former channel.  If  such lakes maintain communication with the adjacent stream, they s i l t i n r a p i d l y .  However, i f they become isolated from  the parent stream, they persist for a longer period and may so changed i n form by erosion of t h e i r banks and  be  subsequent  coalescence with other lakes, that their o r i g i n i s d i f f i c u l t to assess.  Many old channels, only partly s i l t e d i n , remain  as drainage ways between the island lakes and the main streams, and as such ,they become almost dry i n the late summer.  The  ox-  bow type represent some of the deepest lakes i n the Mackenzie delta. The t h i r d lake form comprises the larger bodies of water in the Mackenzie delta.  Through the deposition of natural  levees along the channels, the edges of islands are b u i l t up so that a depression i n the i n t e r i o r becomes a catchment basin for surface water.  Such lakes may extend for three or four  miles although some measure l e s s .  In the upper part of the  delta lakes of t h i s sort are smaller than those below Aklavik but t h i s no doubt i s a function of t h e i r r e l a t i v e ages and the degree of s t a b i l i t y attained. A l l three types mentioned above are subject to various forces which tend to a l t e r them; draining some, f i l l i n g i n others, and wearing away the banks of those which remain. It was mentioned previously that there i s a profound thrust of running water against the downstream channel banks and concomitant deposition of a l l u v i a l material on the upstream  31  banks.  I n both, lakes and channels, such a c t i o n i s a s s i s t e d by  s e v e r a l other f a c t o r s which are mostly seasonal i n nature. During spring f l o o d s the streams are f i l l e d from bank to bank w i t h great blocks of i c e which not i n f r e q u e n t l y jam and b u i l d up a head of water and i c e as high as t h i r t y feet above the usual l e v e l s .  When the impounded water and i c e are released  they not only deepen the ohannel but a l s o may overflow and s c a t t e r ice f a r back i n t o the timber.  I n s p i t e of the spectacular nature  of t h i s phenomenon, i t i s doubtful i f water-borne ice i s of major importance i n the erosion of stream banks.  At the time that the  r i v e r i s i n f l o o d , the banks, which have been exposed t o r l g o u r ous w i n t e r temperatures, are f r o z e n s o l i d l y below the f i r s t few surface inches.  Thus, though they become smoothed by the g r i n d -  ing of the i c e , the p e n e t r a t i o n i s s l i g h t , and because the flow of ice l a s t s l e s s than a week l i t t l e damage i s done.  Eardley  (1938) found the same i c e c o n d i t i o n s p r e v a i l i n g i n the Yukon R i v e r of A l a s k a , and concluded was  a l s o that bank e r o s i o n by ice  inconsequential i n i t s e f f e c t s . The permanently f r o z e n s o i l incorporates a great deal  of i c e e i t h e r i n t i m a t e l y associated w i t h the s o i l p a r t i c l e s or separated as c l e a r blue l e n s e s .  When t h i s ice melts  solifluction  ensues and the banks of the channels and lakes slough away as a consequence. When the banks are steep, and e s p e c i a l l y when they have a southern exposure, the r e f l e c t i o n of the sun from the water surface augments the thawing e f f e c t s of water and a i r . Hence the banks are r a p i d l y under-cut.  The surface l a y e r s of s o i l  are protected from i n s o l a t i o n by the mats of vegetation and melt  32 more s l o w l y than the exposed banks nearer water l e v e l .  As  t h i s process of undercutting continues, the surface l a y e r comes to overhang the stream and e v e n t u a l l y drops i n t o i t i n l a r g e angular ohunks.  Under s u i t a b l e c o n d i t i o n s i t i s probable that  r e f l e c t e d s u n l i g h t i s j u s t as important i n loosening the bank m a t e r i a l s as the a c t i o n of water.  I n two instances s u n l i g h t  caused high stream banks t o recede f i f t e e n and twenty-five f e e t r e s p e c t i v e l y one summer. Wind-driven i c e i s probably one of the main f a c t o r s i n a l t e r i n g lake c o n f i g u r a t i o n . Ice p e r s i s t s I n the lakes long a f t e r the r i v e r s are c l e a r .  When the water l e v e l s have been  r a i s e d at the time of f l o o d i n g , l a r g e pans of the i c e break loose and are blown from shore t o shore u n t i l they d i s i n t e g r a t e . Such gouging of the banks i s b o l s t e r e d by m e l t i n g of permafrost and as a consequence many of the lake shores are r a t h e r abrupt. Wind has another e f f e c t on the d e l t a i c deposits.  During  winter the exposed banks, though f r o z e n , become very dry and as a consequence dust i s c o n t i n u a l l y being blown from them.  The  e f f e c t i s s m a l l as compared w i t h some of the other f a c t o r s d i s cussed but i t i s noticeable because the snow i n the v i c i n i t y becomes mixed w i t h d i r t and i s the f i r s t to melt i n s p r i n g .  Wind  a l s o regulates the depth of i c e on d e l t a lakes t o the extent that i t removes the p r o t e c t i v e l a y e r s of snow. VEGETATIVE COVER AND PLANT SUCCESSION IN THE MACKENZIE DELTA There are two climax vegetative types represented Mackenzie d e l t a and i t s v i c i n i t y .  i n the  The f i r s t of these i s spruce  f o r e s t , a northward extension of the boreal f o r e s t , and the  33  second i s dry heath, or tundra.  P o r s i l d (1937) described these  zones as Hudsonian and A r o t i c r e s p e c t i v e l y but made a r e s e r v a t i o n concerning the l a t t e r .  Dry tundra, he s a i d , may have i t s o o n i -  ferous climax retarded by l a c k of r a i n f a l l and the extremely dry a i r of w i n t e r .  Under such oircumstanoes  dry tundra may be r e -  garded as t r u e p r a i r i e or steppe and i s not, s t r i c t l y speaking, w i t h i n the A r c t i c zone. The upper t w o - t h i r d s of the Mackenzie d e l t a i s f o r e s t e d with white spruce (Picea g l a u c a ) , but shows very r e s t r i c t e d amounts of black spruoe (Picea mariana) and tamarack ( L a r i x l a r i o i n a ) i n the moister h a b i t a t s . A point twenty m i l e s below A k l a v i k joined w i t h a point f i f t e e n m i l e s below the Reindeer Depot approximately d e l i n e a t e s the northern l i m i t of coniferous species.  Beyond that i s a zone of a l d e r (Alnus c r i s p a ) and w i l l o w  ( S a l i x spp).  Northward the a l d e r disappears u n t i l only w i l l o w i s  found at the coast.  There the extensive w i l l o w f l a t s represent  a preolimax s i t u a t i o n which no doubt w i l l become f o r e s t e d as the d e l t a i s b u i l t f u r t h e r seaward. The tundra h a b i t a t i s r e s t r i c t e d t o the higher land bordering the Mackenzie d e l t a .  Along the Caribou H i l l s a d i f f e r e n c e  i n e l e v a t i o n of no more than three hundred f e e t i s s u f f i c i e n t f o r the vegetation type t o change from f o r e s t t o tundra. Within the f o r e s t e d portions of the d e l t a there i s a progressive succession of plant types from the invaders on raw a l l u v i a l s o i l s t o the heath species on h i g h l y organic s o i l s .  Lynch  (1940) l i s t e d the species c o l o n i z i n g mud bars and f l a t s as horset a i l (Equlsetum variegatum and E. arvense) together w i t h water-oats ( A r c t o p h i l a f u l v a ) , r i v e r sedge (Carex a o u a t i l i s ) , and cotton sedge  34 (Erlophorum a n g u s t i f o l i u m ) .  These are followed subsequently by  w i l l o w s on the b e t t e r drained areas, and i n turn by a l d e r , popl a r (Populus taoamahacca), and f i n a l l y white spruce as the ground i s b u i l t p r o g r e s s i v e l y higher. edges the' inner portions of i s l a n d s may poor drainage.  Away from the channel show the e f f e c t s of  These low wet areas develop an accumulation of  organic m a t e r i a l l a r g e l y contributed by such mosses as Sphagnum and Hypnum. With these grow various ericaceous Labrador-tea,  species,  notably  (Ledum groenlandicum), crowberry (Empetrum nigrum),  oranberry (Vaocinium spp.). b i l b e r r y (V. uliginosum). and o a r i c e s , i n c l u d i n g cotton sedge.  Such plants form an  various  understory  f o r scattered white spruce, and the whole a s s o c i a t i o n surrounds these more permanent l a k e s .  Those lakes which o r d i n a r i l y d i d  not f l o o d were apt t o tend towards an a c i d c o n d i t i o n even though t h e i r waters d i d not show much organic s t a i n .  W i t h i n these lakes  were found suoh c h a r a c t e r i s t i c species as buokbean (Menyanthes t r i f o l i a t a ) , water arum ( C a l l a p a l u s t r i s )  t  marsh-marigold (Caltha  p a l u s t r i s ) . and m a r e s - t a i l (Hippuris v u l g a r i s ) . This type of 1  h a b i t a t was more common i n the southern h a l f of the d e l t a and was associated w i t h a mature climax o o n d i t i o n . Those lakes r e o e i v i n g a y e a r l y load of a l l u v i a l m a t e r i a l showed quite a d i f f e r e n t vegetative complement.  Suoh lakes were  a m a j o r i t y of those both above and below the l i n e of timber. They represented  the newer h a b i t a t , that i n more a c t i v e development.  These a l l u v i a l lakes e x h i b i t e d an emergent a s s o c i a t i o n c o n s i s t i n g of bur-reed (Sparganlum a n g u s t i f o l i u m ) , and h o r s e t a i l (Equisetum f l u v i a t i l e ) o f t e n mixed w i t h water-oats, and around  35  the periphery, v a r i a b l e amounts of r i v e r sedge.  The banks of  most of these lakes were ringed with a dense r i p a r i a n growth of w i l l o w and a l d e r .  The h o r s e t a i l , however, could not withstand  the r i g o r o u s conditions nearer the ooast and i n the l a t i t u d e of Tununuk was l a r g e l y replaced by r i v e r sedge and water-oats. There was a f a i r l y abundant growth of submerged p l a n t s i n the lakes throughout the d e l t a .  This included the pondweeds (Potamogeton  gramineus var. g r a m i n i f o l i u s . P. pectinatus, P. Riohardsonii) and others, m i l f o i l (Myriophyllum exalbesoens).  bladderwort  ( U t r i o u l a r i a v u l g a r i s var. amerioana), and, i n r e g u l a r l y flooded l a k e s , muskgrass (Ghara s p . ) . Other species worthy of mention i n a l l u v i a l l a k e s were duckweed (Lemna t r i s u l c a ) , starwort ( C a l l i t r i o h e autumnalis), spike-rush (Eleocharis a c i o u l a r i s ) , and creeping spearwort (Ranunculus r e p t a n s ) . countered  An alga of the genus Cladophora was  en-  i n s e v e r a l of the lakes i n the v i c i n i t y of A k l a v i k but  was seldom seen elsewhere. There were many lakes i n the poorly drained plateau east of the Mackenzie d e l t a .  These were a l l c o l d freshwater lakes  w i t h . v a r i a b l e amounts of organic s t a i n .  The s h o r e l i n e s were  u s u a l l y devoid of t r e e growth except i n more s h e l t e r e d s i t e s where there were a few low w i l l o w s .  The v e g e t a t i o n  immediately  surrounding these lakes was u s u a l l y one of the cotton sedges, otherwise the ground cover was  a t y p i c a l dry heath.  Some lakes  produced dense growths of yellow w a t e r - l i l y (Nuphar variegatum), though i n the r e g i o n of the d e l t a t h i s species d i d not extend i t s range north to the coast.  M a r e s - t a i l , m i l f o i l , and s e v e r a l !  species of pondweeds were found i n lakes shallow and warm enough  36  t o support t h e i r growth.  Otherwise, the tundra l a k e s , as com-  pared with those i n the d e l t a , were r e l a t i v e l y barren of vegetation. CLIMATE  A r c t i c North America may a t i c regions.  be d i v i d e d i n t o two major c l i m -  The f i r s t of these l i e s roughly to the north of a  l i n e j o i n i n g the mouth of the Mackenzie R i v e r w i t h C h u r c h i l l , Manitoba.  I t i s c h a r a c t e r i z e d by a climate i n which the average  temperature of the warmest month i s l e s s than 50°F., but i s higher than 32°F.  This i s the true A r c t i c r e g i o n .  The second r e g i o n , comprising most of the Yukon and'the remainder of the Northwest T e r r i t o r i e s , has a s u b - A r c t i c climate wherein the average temperatures of the three warmest months are above 30°F., and the averages f o r the coldest months are below freezing. The boundary between the A r c t i c and s u b - A r c t i c  regions  bears no r e l a t i o n s h i p t o the A r c t i o C i r c l e nor i s the d i v i d i n g l i n e always c o i n c i d e n t with the northern extent of tree s p e c i e s . I t may,  as explained elsewhere, l i e north of the t r e e l i n e .  S t r i c t l y speaking, t h e r e f o r e , most of the Mackenzie d e l t a l i e s w i t h i n the s u b - A r c t i c regions. At A k l a v i k the summer temperatures of the three warmest months —  June, J u l y , and August, have averaged 4 8 . 5 ° , 3 6 . 3 ° and  50.2°F. r e s p e c t i v e l y .  The y e a r l y average, however, i s only 13.5°F.  Average annual extremes of temperature range from 83.3°F. to minus 31.8°F. w i t h absolute extremes of 93°F. and minus 36°F. recorded. January i s the coldest month w i t h an average of minus 19°F.  and -  37  J u l y the warmest w i t h 56.3°F.; a range of 73.3°F. Robinson (1946) reported the average annual p r e o i p i t a t i o n i n the Northwest T e r r i t o r i e s t o be low because the  atmospheric  conditions which cause p r e c i p i t a t i o n do not often occur.  I n the  f i r s t place, c o l d A r c t i o a i r does not contain much water vapour and i n moving southward i t i s being warmed, not cooled.  Most of  the p r e c i p i t a t i o n t h a t does f a l l i s the r e s u l t of warm P o l a r P a c i f i c a i r being cooled as i t flows a l o f t over the colder a i r masses.  U s u a l l y the Mackenzie v a l l e y settlements record only  ten t o t h i r t e e n inches of moisture y e a r l y w i t h J u l y the wettest month. At A k l a v i k there has been an average of 10.03 inches of moisture y e a r l y , of which 6.20 inches f e l l as snow and the r e mainder as r a i n . season.  R a i n f a l l , e s p e c i a l l y , v a r i e s from season t o  Winter snows are f a i r l y w e l l d i s t r i b u t e d  throughout  the winter months, and because thawing temperatures  are unusual  during t h i s p e r i o d snow may c o l l e c t t o depths of three f e e t i n wooded areas.  I n more exposed s i t e s the snow may  be blown i n t o  compact d r i f t s of greater depth. Winds at A k l a v i k are predominantly from the north or the south.  Wind v e l o c i t i e s g e n e r a l l y are low d u r i n g summer but  become stronger i n autumn and w i n t e r .  Winds of gale force blowing  f o r s e v e r a l days are not frequent though they do occur, e s p e c i a l l y i n winter.  Periods of calm are more common i n w i n t e r than i n  summer because of s t a g n a t i o n a s s o c i a t e d w i t h i n v e r s i o n s of a i r masses.  L o c a l winds, associated w i t h c o l d a i r drainage, are  common along the base of the Caribou H i l l s .  Chinook or fohn  winds do occur i n w i n t e r but t h e i r e f f e c t i s l i m i t e d to the lower  38  slopes of the Richardson mountains. During summer there i s continuous d a y l i g h t from the l a t t e r part of May  to the end of J u l y .  In t h i s period the sun  c i r c l e s the sky, d i p p i n g near the northern h o r i z o n at midnight. A f t e r the f i r s t part of August the sun disappears below the horizon f o r an i n c r e a s i n g l y longer period d a i l y u n t i l by the f i r s t part of December i t no longer r i s e s and even at mid-day appears only as a ruddy s u f f u s i o n on the southern h o r i z o n .  This  c o n d i t i o n l a s t s u n t i l about 10 January at A k l a v i k when the rim of the sun again appears. as might be supposed.  The "dark" period i s not as profound  Though the sun i s gone the moon may  be  v i s i b l e f o r a l l 24 hours of the day and the dry atmosphere and r e f l e c t i o n s from the snow enhance the amount of l i g h t a v a i l a b l e . There i s a t w i l i g h t f o r about f i v e hours d a i l y but even when the sun i s at the meridian there i s not s u f f i c i e n t l i g h t f o r good photography. This d e s c r i p t i o n of the A k l a v i k r e g i o n has been given i n some d e t a i l .  The data supporting t h i s summary, together w i t h  that f o r other s e l e c t e d r e g i o n s , appears i n Appendix B.  39  METHODS OF PROCEDURE Duration of the Study F i e l d i n v e s t i g a t i o n s f o r t h i s study began on 7 June, 1947 when the w r i t e r , i n company w i t h I . McT. i n the Mackenzie d e l t a .  Cowan, a r r i v e d  Dr. Cowan remained u n t i l the f o l l o w i n g  17 August and presented t h e r e a f t e r a p r e l i m i n a r y report of the f i n d i n g s of the f i r s t summer (Cowan, 1948). The w r i t e r continued f i e l d i n v e s t i g a t i o n s throughout the f o l l o w i n g w i n t e r and u n t i l September of 1948.  F i e l d work was r e -  sumed i n June of 1949 and continued t h e r e a f t e r u n t i l September, 1950.  A l l periods of the year were not e q u a l l y productive of results.  L i v e t r a p p i n g could be c a r r i e d on best during the  months of J u l y , August, and the f i r s t h a l f of September.  The  m a j o r i t y of the animals tagged were caught during the three successive summer periods, I947-I948-I949.  Some l i v e t r a p p i n g , as d e t a i l -  ed h e r e a f t e r , was done i n e a r l y w i n t e r (November and December) but t h e r e a f t e r temperatures became too low f o r the animals to survive f o r long i n the t r a p s .  The opening of the l e g a l t r a p p i n g  season, on 1 March, gave an opportunity t o take animals w i t h s t e e l t r a p s , to inspect the carcasses f o r d i s e a s e , p a r a s i t e s , and breeding c o n d i t i o n , and t o a s c e r t a i n whether they had been tagged p r e v i o u s l y .  During l a t e May the lakes and streams began  to show open water around the edges.  Thereafter f o r the  succeeding  40  three weeks u n t i l 15 June, animals oould he taken l e g a l l y by the use of fire-arms.  At t h i s period numerous carcasses were  a v a i l a b l e from hunters at a time coincident with the highest breeding a c t i v i t y .  The remainder of June y i e l d e d poor l i v e  trapping r e s u l t s because the water i n lakes had not yet subsided f o l l o w i n g the s p r i n g f l o o d s and was so muddy that i t h i d den entrances.  The period from the f i r s t i c e formation, about  1 October, u n t i l i c e was again t h i c k enough f o r t r a v e l , about 1 November, y i e l d e d only meagre data because the animals were not e a s i l y a c c e s s i b l e .  41  Location and Description of the Study Areas Studies were centred at four widely dispersed areas within the Mackenzie d e l t a , Appendix A.  These f i e l d stations  were selected with a view t o giving f a i r l y adequate coverage to the different conditions to be met within the region. Intensive work was carried on at these locations and other areas were given only cursory attention.  I t was f e l t that t h i s would  give more valuable information than the same amount of e f f o r t spread over a wider portion of the d e l t a . AREA NO. 1 (LANG»S) The f i r s t area t o be considered was thirty-two miles up the Peel Channel from Aklavik and represented the twelve square mile trapping area of K. H. Lang.  Here some of the lakes had  received very l i g h t hunting u t i l i z a t i o n and were used p r i m a r i l y as reservoirs of breeding stock.  F i e l d work was conducted on  several lakes i n t h i s area, as detailed i n F i g . 5. Better stands of white spruce occurred here than i n any of the other three study areas.  This condition was character-  i s t i c of the upper delta and was as much a result of higher land to be found there as of the s l i g h t l y more favourable climatic conditions.  Three lakes W-l, W-2,  and Grassy Lake were given  most of the attention afforded t h i s area though the l a t t e r was the most intensively studied. Grassy Lake was ringed on two sides by thickets of alder  42  FIGURE 5 .  STUDY AREA MO. 1 (LANG'S). ARE NUMBERED OR NAMED.  LAKES INVESTIGATED  4? and willow and the immediate shoreline was grown to r i v e r sedge (Carex a q u a t i l i s ) , except where the bank was too abrupt to support emergent growth.  The south and west portions of the lake,  being more shelving, had luxuriant stands of h o r s e t a i l  (Equisetum  f l u v i a t i l e ) , growing i n six to twelve inches of water.  Beyond  that i n depths to two feet bur-reed (Spargenium angustifollum) appeared.  The centre of the lake supported growths of pondweeds  (Potamogeton spp.) and m i l f o i l (Myriophllum exalbesoens).  Depths  of ninety-four inches of water were recorded i n winter, ( E i g . 6), but would average two feet more during times of flood.  The char-  acter of the s o i l was l a r g e l y a l l u v i a l even though there were some areas of heath i n the v i c i n i t y . Lakes W-l and W-2  were d i s s i m i l a r i n that the former had  an abrupt shoreline throughout  i t s entire length while the l a t t e r  had emergent growths of sedge and h o r s e t a i l .  To one side of these  lakes, and quite d i s t i n c t from them, was a shallow pond no more than an acre i n size which supported a luxuriant growth of horsetail.  Well used "portages" into t h i s pond attested to the u t i l i -  zation i t received as a food source and the presence of two burrow systems along i t s shores marked i t as very successful breeding habitat.  Animals born i n t h i s pond normally were oaught i n W-2  during the succeeding winter or spring. AREA NO.  2 (BOXER'S) The second area of the study was located nine miles from  Aklavik on the trapping area of A. J . Boxer, ( F i g . 7 ) .  Originally  f i v e lakes i n t h i s area were designated by number and slated f o r intensive study but subsequently i t was found that only two of  44  FIGURE  6 . GRASSY LAKE IN STUDY AREA NO. 1 SHOWING THE DEPTH CONTOURS IN FEET DURING APRIL, 1948.  45  46  them, B - l and B-3  c o u l d be covered e f f e c t i v e l y .  Lake B - 3  was  of i n t e r e s t because i t was b e i n g c o n v e r t e d i n t o t h r e e i s o l a t e d ponds by t h e r a p i d g r o w t h of w i l l o w , s e d g e , and w a t e r - o a t s . Though n e a r l y u s e l e s s a s w i n t e r h a b i t a t , t h i s l a k e supported numerous muskrats i n s p r i n g a f t e r the i c e had d i s a p p e a r e d  and  t h e animals were moving about i n s e a r c h o f f o o d . Lake B - l , ( F i g . 7), great w i d t h .  w a  s  n e a r l y a m i l e l o n g but o f no  Most of i t s s h o r e l i n e was  r a t h e r abrupt except i n  the north-west c o r n e r where an o u t l e t c r e e k f l o w e d .  Here sedge  and h o r s e t a i l formed a s h o r e l i n e bank up t o f i f t y y a r d s i n w i d t h , w h i c h r e c e i v e d heavy u t i l i z a t i o n as muskrat food ( F i g . 8 ) . s o u t h e r n p o r t i o n of the l a k e was  The  l e s s u s e f u l t o muskrats t h a n the  main body because o f l a c k o f emergent food p l a n t s .  F i g u r e 8.  High u t i l i z a t i o n of s h o r e l i n e v e g e t a t i o n by muskrats on Study Area No. 2.  47 AREA NO.  3 (REINDEER DEPOT) At Reindeer Depot a small area on both sides of the  channel had been reserved as a trapping area f o r the reindeer herders.  The t h i r d f i e l d s t a t i o n was  constituting t h i s area. (R-l, R-2,  established on the  lakes  A s e r i e s of three interconnected  lakes,  and R-3), were chosen f o r intensive f i e l d work ( F i g . 9)  and other lakes i n t h e i r v i c i n i t y were afforded occasional examination. Lake R - l , though the smallest of any investigated, had the heaviest summer concentration of muskrats recorded. may  This  be ascribed i n part t o a high yield of such food plants as  sedges and  horsetail.  Lake R-2 was  larger and presented a more diverse habitat.  A northern and a southern portion communicated by a narrow waterway.  The northern  part supported white spruce along the channel  but the rest of the lake was surrounded by dense growths of w i l low and alder.  Depths were v a r i a b l e but s u f f i c i e n t i n both por-  tions successfully t o harbour muskrats i n winter. Lake R-3 vegetation.  resembled Lake R-2  in depth and i n shoreline  The southern shoreline was  i n d i s t i n c t because of  recent invasion by willows, and the r e s u l t i n g ragged outline had an understory of sedge and h o r s e t a i l which provided excellent summer habitat f o r both muskrats and waterfowl. AREA NO.  4 (TUNUNUK) The fourth area was  Johnny Alekuk, and was  part of the trap l i n e of an Eskimo,  situated about four miles from the old  camp-site of Tununuk, Appendix A.  The area was  unique i n that  48  FIGURE 9>.  STUDY AREA NO. 3 at. REINDEER DEPOT INDICATING THE TRAPPING SITES USED DURING THE INVESTIGATION  1947-50.  49  i t was beyond the limit of t r e e s , and though surrounded by tangles of willow and alder, had no coniferous growth of any kind. shallow lakes were investigated here.  Four  Two of these were probably  not deep enough to allow any muskrats to winter s u c c e s s f u l l y , a l though the other two may have had s u f f i c i e n t water f o r winter habitat over about cne-tenth of t h e i r areas.  The sedges, mostly  (Carex a q u a t i l i s ) , were the abundant emergent food plants, though during summer the largest of the four lakes was a f l o u r i s h i n g meadow of sedge and water-oats i n about equal amounts. H o r s e t a i l did not grow i n any of the lakes though i t was found sparingly i n the small channels connecting them.  Pond weeds and m i l f o i l repre-  sented the major winter foods a v a i l a b l e . No tagging was done on Area No. 4 because successive v i s i t s f a i l e d to reveal muskrats i n s u f f i c i e n t numbers to make such an effort worthwhile.  One interesting point regarding the muskrats  here was that most of them seen during summer had dens along the channel banks and were rearing young t h e r e i n .  This was not noted  at any other study area investigated, but. may be usual north of timber l i n e where the banks of the lakes are low and the lakes are shallow.  I t corresponds  to the condition found  in the d e l t a of the Slave River.  by Law (1950)  The channels were, however,  almost barren i n submerged food plants so i t was doubtful whether the animals could winter i n these l o c a t i o n s . A l l study areas except Tununuk were given attention during each of the four seasons of 1947-1950 i n c l u s i v e .  Work was begun  on Area No. 4 during the summer of 1948 and continued the following summer. As oie proceeded from Lang's toward Tununuk, there was a  50 progressive decrease in the heights of lake shores.  The south  shore of Grassy Lake measured twelve feet above water l e v e l though the north shore was about three feet lower.  The shore  height of Lake B - l at Boxer's was from four to s i x feet along the south side, less facing the willow f l a t s to the north. Reindeer Depot lakes shores were likewise variable.  At  None mea-  sured s i x feet so that during spring when flood waters had r i s e n four feet there was very l i t t l e land not inundated. Shorelines were low at Tununuk; the lakes were l i t t l e more than depressions surrounded by banks which rose gradually toward the channel edges.  Spring floods here l e f t exposed only  the higher land along these channels.  51  Methods of T r a v e l The distances involved i n t r a v e r s i n g the Mackenzie d e l t a were r e l a t i v e l y great.  From the most s o u t h e r l y study area (Lang's)  to A k l a v i k was t h i r t y - t w o miles by water.  Frcm A k l a v i k t o the  second area (Boxer's) was nine m i l e s and thence t o the Reindeer Depot was a water distance of some seventy m i l e s .  From Reindeer  Depot t o the f o u r t h area near Tununuk was t h i r t y - s i x m i l e s .  From  Lang's, t h e r e f o r e , one had t o t r a v e l one hundred and f o r t y miles by water t o the most n o r t h e r l y s t a t i o n . In summer these distances were u s u a l l y t r a v e r s e d by canoe or s k i f f propelled by an outboard motor.  However water transpor-  t a t i o n was p o s s i b l e only during June, J u l y , August and September. Thereafter t r a v e l was  accomplished by dog team.  Distances were  somewhat l e s s by t h i s method since i t was p o s s i b l e to take a more d i r e c t r o u t e , but t r a v e l was  also slower and l e s s comfortable be-  cause of low temperature and the load of equipment i t was  necessary  to transport. For t r a v e r s i n g the lakes and channels i n each study area an ordinary canoe was used i n summer and snow-shoes or dogteam i n winter.  S k i s were t r i e d but were found, to be i n e f f i c i e n t because  the snow assumed the texture of sand at low temperatures.  52  Methods of L i v e Trapping I n order that the fortunes of i n d i v i d u a l animals might be followed over a period of time, a system of l i v e t r a p p i n g and tagging was i n s t i t u t e d .  E a r l y i n t h e study, u n t i l proper  techniques were developed, t r a p p i n g endeavour produced few r e sults.  During the period when water was s t i l l high and sediment  laden, animals were trapped only at or near t h e i r feeding s i t e s . No b a i t or l u r e we could f u r n i s h would induce the animals t o enter cage traps s e t among the s h o r e l i n e vegetation.  Figure 10. J u v e n i l e muskrats taken i n l i v e t r a p s set i n runways through shoreline vegetation, September 194-9.  53 Once the water l e v e l s had subsided  to t h e i r summer norm,  l i v e traps were more successful in taking animals ( F i g . 10). These traps were set i n runways through marginal vegetation i n underground tunnels leading to bank dens. was  and  The l a t t e r method  rather arduous i n that i t involved f i r s t , probing the shore-  l i n e above the water l e v e l to find the runways; second, excavating s u f f i c i e n t of each runway that a trap could be set therein; and t h i r d , surrounding the trap with green plants. of vegetation was  This layer  important, not only i n excluding some of the  l i g h t and thus p a r t l y obscuring the trap, but also i n a f f o r d i n g material which the captured  animal could p u l l into the trap f o r  warmth, food, and protection from mosquitoes. During the early winter i t was  possible, by taking  due  precautions, to resume l i v e trapping on an intensive scale. t h i s time the muskrats had constructed stations on the lake i c e .  By  and were using feeding  These stations, which consisted of a  dome of vegetation b u i l t above a hole i n the i c e , were known throughout the T e r r i t o r i e s as "push-ups", and t h i s w i l l be employed hereafter. a s i t u a t i o n , i t was  designation  In order to set a l i v e trap i n such  necessary to cut away one side of the chamber,  place a trap next the hole thus made, and cover the trap and push-up with burlap.  Snow was  p i l e d to a depth of s i x to twelve  inches over both the trap and push-up.  At temperatures as  low  as minus 20°F. the animals were well insulated therein from the cold and could survive twelve hours of c a p t i v i t y without apparent ill  effect. Later i n the winter t h i s method proved less e f f i c i e n t  54 because the plunge-hole o f t e n froze a f t e r the push-up had been opened.  Coupled w i t h t h i s was the f a c t that the i n s u l a t i n g pro-  p e r t i e s of the snow had decreased a p p r e c i a b l y .  New snow o r y s t a l s  formed an i d e a l i n s u l a t i n g medium, but by January there had been a coalescence of o r y s t a l s i n the lower l a y e r s , and a coarse granul a r type of snow r e s u l t e d .  I n November there had been a recorded  d i f f e r e n c e of 19°3P. between the upper and lower l e v e l s of twelve inches of snow, but by mid-January t h i s d i f f e r e n c e was reduced t o as l i t t l e as 3°F. A seoond method of t a k i n g l i v e animals i s worthy of note. I t involved the use of a long-handled l a n d i n g net to capture swimming young muskrats or the animals of a l l ages as they l e f t the den entrance.  The method was only f a i r l y e f f i c i e n t as i t r e -  quired the e f f o r t s of two men —  one t o hold the net and the other  to induce the animals t o leave the den by whatever persuasion seemed appropriate.  55  Methods of Tagging The f i r s t tags employed were s e l f - p i e r c i n g monel metal f i n g e r l i n g tags, two mm. wide by eight mm. long when o l i n c h e d . These t a g s , a p p l i e d t o the animal's r i g h t ear, had a l i m i t e d value due t o t h e i r s m a l l s i z e .  Trappers t a k i n g a tagged muskrat  seldom were aware of such a s m a l l t a g hidden i n the f u r . Aluminium bands, commonly used i n waterfowl s t u d i e s , were applied to the base of t a i l s of adult animals.  Though the  use of these bands was r e s t r i c t e d to the adult age olass they had the advantage of being d i s c e r n i b l e  on released  animals.  A t h i r d and most s u c c e s s f u l kind of t a g was of monel metal which measured three mm. wide and f i f t e e n mm. long when olinched.  This tag was s e l f - p i e r c i n g and was closed with a  special clincher.  I t was applied by e n o i r c l i n g the tendon of  A c h i l l e s (tendo calcaneous) of the r i g h t hind limb as suggested by Cook (1943).  I t was found necessary t o enlarge the depth of  the t a g so as not t o c o n s t r i c t the tendon and enclosed s k i n .  If  the proper care was exercised there seemed no impairment of the animal's a c t i v i t y and inflammation  of the s i t e was almost n i l .  Unlike the aluminium bands, these tags could be applied e q u a l l y w e l l t o j u v e n i l e or adult animals, and no t a g - l o s s was noted. However, t h e i r greatest advantage l a y i n the f a c t that t h e i r s i z e and l o c a t i o n made.it impossible f o r them to be overlooked trapper who was s k i n n i n g a muskrat. i v e l y a f t e r June, 1949.  by the  These tags were used exclus-  56  Methods of Handling Live Animals A c e r t a i n amount of care had to be exercised i n handling adult muskrats.  To prevent p a i n f u l bites a holding cone of wire  mesh with a wooden base was used.  Animals were dropped head  f i r s t into t h i s and a s t i c k was inserted through the wire between the  t a i l and the hind legs (Fig.  11).  the  cage trap by the t a i l , and i f held suspended by this member,  could not reach t o bite the operator.  An animal was pulled from  I f the animal had been  tagged previously or i f only a s u p e r f i c i a l examination was  Figure 11.  Adult muskrat. l i v e trap.  Aldous holding cone, and National  necessary, i t was a simple matter to swing him under the l e f t arm, b e l l y up, and there hold him u n t i l the task was  finished.  The operator was w e l l advised to wear stout c l o t h i n g f o r t h i s procedure. During the l a s t h a l f of the study a l l animals captured were weighed by employing s m a l l s p r i n g s c a l e s and a stout piece of cord looped around the base of the t a i l ( F i g . 12).  These  s c a l e s , though very convenient and easy t o t r a n s p o r t , would supply weights only t o the nearest 25 grams. An examination of l i v e animals gave some i n f o r m a t i o n as to t h e i r reproductive s t a t u s .  Figure 12.  Females were examined f o r the  Method of weighing l i v e muskrats.  58 presence or the absence of m i l k i n the mammary glands, f o r a perforate or imperforate v a g i n a l opening, and f o r a v a g i n a l plug or other evidence of breeding. the males i t was  By p a l p a t i n g the t e s t e s of  p o s s i b l e t o a s c e r t a i n t h e i r s i z e and p o s i t i o n  and thence something of the breeding c o n d i t i o n of the  animal.  I n both sexes an examination of the p e l t f o r wounds i n f l i c t e d during the breeding season was made. Post-mortem sheets were completed f o r every dead a n i mal examined.  These inspections s u p p l i e d a d d i t i o n a l information  r e s p e c t i n g reproductive  s t a t u s , as w e l l as on diseases,  s i t e s , and general h e a l t h .  para-  Techniques employed were more or  l e s s standard though f o r the most part macroscopic.  59  THE INVESTIGATION As stated in the Introduction, the muskrats of the Mackenzie delta have had t o make c e r t a i n adjustments i n order to occupy a habitat characterized by a lack of emergent vegetation during summer and a long period of ice cover during winter.  The  adjustments which they have made, however, are largely adjustments i n degree, not in kind.  They faoe a longer winter and have a v a i l -  able a sparser vegetative f l o r a than do those races farther t o the  south.  In s p i t e of t h i s , the continuation of a t h r i v i n g popu-  l a t i o n of muskrats i n and adjacent to the Mackenzie d e l t a is v e r i f i c a t i o n of the f a c t that the necessary adjustments have been made. It w i l l be necessary f i r s t to describe these adjustments and thereafter to account f o r them.  In the d e s c r i p t i v e attempt  comparisons must be made with races of muskrats inhabiting other parts of the Continent. This w i l l help to c l a r i f y not only the kind of adjustment but a l s o i t s degree. justments i s more d i f f i c u l t .  To account f o r the ad-  There must be both common and pe-  c u l i a r factors in each habitat to which the animals have reacted in the manner described. tors and assess t h e i r role  We can attempt to delineate these faci n producing the adjustments noted.  The adjustment of the northwestern muskrat to the conditions of the Mackenzie d e l t a may be divided conveniently into two more or less a r b i t r a r y categories.  The f i r s t of these w i l l be  designated as e x t r i n s i c or physical adjustments —  those which  have to do with the r e l a t i o n s h i p of the animal t o i t s physical  6o environment.  This s e c t i o n : w i l l attempt to describe the hab-  i t a t and the use made of i t by the muskrats f o r denning, f o r food, and f o r other l i f e processes. The second category w i l l deal with those i n t r i n s i c or b i o l o g i c a l adjustments which manifest themselves within the animal i t s e l f .  Such considerations as size and growth rates,  breeding a c t i v i t y , and association with t h e i r own and other species f a l l within t h i s category.  I t was the i n t r i n s i c  fact-  ors which proved more rewarding i n exhibiting the effects of northern conditions upon the animals under investigation.  61  E x t r i n s i c or Physical Adjustments . A resume of the habitat preferences of the northwestern muskrat was given i n the Review of L i t e r a t u r e .  Therein i t was  noted that the animals i n the delta of the Athabaska River constructed houses or lodges of emergent vegetation and occupied them throughout the winter.  This propensity of house building  i s exhibited by a l l other races of muskrat under review.  There  i s no doubt, however, that where conditions are favourable they w i l l d i g and inhabit burrows i n stream and lake banks as w e l l . Johnson  (1925) i n New York found that i n watercourse environment  "the muskrats confine t h e i r dens mainly to bank burrows, and consequently s u p e r f i c i a l evidence of the animal's presence i s less conspicuous than i n other types of habitat".  In the large c a t t a i l  marshes which supported a greater density of muskrats he found that "the animals almost invariably build houses but where the marshes are crossed by railways a c e r t a i n number of them may make t h e i r dens i n burrows dug i n the embankments". The same s i t u a t i o n exists i n other parts of North America, and i n Europe as w e l l .  Errington (1939b) encountered animals i n  Iowa l i v i n g i n drainage ditches and t i l e drains, e s p e c i a l l y Warwick (1934) found that muskrats  during periods of drought.  b u i l t lodges or else l i v e d i n the banks of the numerous waterways of England and Scotland.  Their burrowing habits proved so de-  structive i n England and Europe that a determined e f f o r t was made to exterminate the animals.  In England t h i s venture was  measure successful, i n Europe less so.  i n large  62 In a l l these locations the muskrat either occupied a house or a burrow, at times both. i n part on l o c a l conditions.  Law  Their preference was  based  (1950), however, found that  muskrats i n the delta of the Slave River l i v e d i n the channel banks and did not b u i l d nor inhabit houses.  A l l conditions  seemed favourable f o r some houses t o be constructed.  There were  luxuriant growths of c a t t a i l and bulrush i n areas of permanent water but bank dens were the only permanent abode of the animals. This s i t u a t i o n prompted Law to suggest that animals be exchanged between the Slave and Athabaska deltas to ascertain whether they would continue to l i v e i n dens and lodges respectively or whether they would change t h e i r habits under t h e i r new  surroundings.  This experiment was never t r i e d . From the Slave delta northward to the Mackenzie delta muskrats generally have abandoned the construction of houses. Within the Mackenzie d e l t a there i s a general lowering of lake shorelines from the upper reaches to the lower end.  In some  places these shorelines are abrupt with no emergent plant growth and i n others they are shelving and produce abundant growth.  But  whether lake shores are steep or gradual the muskrats therein, without exception, l i v e i n the banks and do not build houses which they inhabit permanently.  Yet i t cannot be denied that  at house building were evident i n some of the lakes.  attempts  The struc-  tures b u i l t , however, were more of the nature of feeding lodges as described by Dozier (1948). merely a feeding platform.  They were small and had no nest, Generally they consisted of no more  than a hollowed'pile of vegetation over a tree root or clump of turf.  Materials for these "feeders" were l a r g e l y lakeside sedges  63 though, h o r s e t a i l and even b a l l s of filamentous algae occasionally were incorporated. Nearly a l l of the bank dens investigated during t h i s were those b u i l t into the more shallow lake shores.  study  These burrow  systems were usually of long standing and were very complex with several under-water entrances and many connecting tunnels above the water l i n e .  Signs of occupancy of such den systems consisted  of r o i l e d water i n the trench, mown vegetation i n the v i c i n i t y , and lack of vegetation i n a path from the open water of the lake to the den entrance.  There was  often a feeding platform nearby  and smoothed spots on the bank near the den entrance indicated where the animals had hauled themselves out of the water.  There-  fore, such locations received most of the attention during the trapping and tagging studies.  Dens i n steep banks were hard to  f i n d and almost impossible to excavate because of buried logs and tree roots. It may  be concluded, therefore, that muskrats i n the  lati-  tude of Aklavik constructed and inhabited shallow burrow systems out of necessity, perhaps also out of preference.  But there  was  d e f i n i t e l y not enough emergent vegetation i n any of the delta lakes to b u i l d s u f f i c i e n t lodges to accommodate a l l of the r e s i dent animals.  And even i f the animals had constructed lodges  and  endeavoured to l i v e i n them, the depth of winter ice was  such that  i t would have prevented  unfrozen  portions of the lake.  access to the food supply i n the  I t would require a l o t of vegetation to  build one muskrat lodge i n deep water, and unless such a structure i n the Mackenzie delta was soon would freeze out.  i n at least three feet of water, i t  Thus the habit of using burrows only i s  64 an adjustment to winter conditions that render houses dangerous. It seems probable that t h i s i s a habit"of s u f f i c i e n t s u r v i v a l value to have genetic influence and background. The choice of habitat by muskrats of the Maokenzie delta requires some a m p l i f i c a t i o n over that already given.  Winter habi-  tat was found to be much more circumscribed than was summer habitat because of the r e s t r i c t i v e e f f e c t s of i c e . A comparison of the s i t u a t i o n here with that i n other parts of North America w i l l be given only i n b r i e f .  The e a r l i e r section which reviewed the  habitat conditions of four other races of muskrats may be kept in mind as the reader proceeds with this section. SUMMER HABITAT Summer a c t i v i t y of the muskrats i s concerned l a r g e l y with reproduction.  Habitats suitable f o r t h i s a c t i v i t y are more numer-  ous than those which w i l l support muskrat populations  yearlong.  This i s fortunate because the animals seem t o exhibit t h e i r greatest intolerance to t h e i r own kind during this, period.  The spreading  out of the population into temporary but adequate summer s i t e s no doubt reduces i n t r a s p e c i f i c f i g h t i n g and may i n t h i s way contribute to reproductive e f f i c i e n c y . These temporary s i t e s were excellent summer habitat and many young animals were reared therein because food usually was abundant and s a t i s f a c t o r y burrowing areas could be found.  Animals  pushed into these areas after the spring floods and remained therein even though water l e v e l s receded during summer.  This same s i t u -  ation e x i s t s i n many other places where muskrats abound and therefore is not unique.  What i s of interest i n the Aklavik region i s the  65 great difference between s a t i s f a c t o r y summer and adequate winter habitats.  An example of a summer habitat may help to i l l u s t r a t e  t h i s difference. Study Lake R - l at the Reindeer Depot was no more than 550  yards long by about 75 yards wide with water depths to three  feet only at the north end.  Over much of i t s extent there was  only eighteen inches of water, or l e s s .  The banks were low and  flanked with willows over nearly the whole of the perimeter. The understory f o r the shoreline willows consisted of sedge (Carex a q u a t i l i s ) which grew to the water's edge and blended with the h o r s e t a i l which formed a band up t o 30 feet i n width a l l along the east side of the lake.  Submerged plant growth  consisted largely of m i l f o i l with l e s s e r amounts of pondweeds and bur-reed.  Food, therefore, was abundant i n and around the  lake, and though much of i t was eaten, the percentage of u t i l i zation was  low.  This lake connected both with Lake R-2 and with a second lake by small streams that became no more than a t r i c k l e by late summer.  There were well used t r a i l s or "portages" made by the  muskrats through the vegetation flanking these two waterways. The f i v e burrow systems i n t h i s lake are shown i n Fig., 9. Four of these were used regularly during the four summers of the study but the f i f t h was rendered unsuitable by setting cage traps therein, and was abandoned.  Because of the movement of the musk-  rats i n and out of the lake i n t h e i r feeding a c t i v i t i e s and wanderings, i t was resident.  impossible to ascertain how many animals were  But some idea of the usage of the area by muskrats  may be gained from the fact that f o r t y - f i v e animals were tagged  66 there i n eight days of late August and early September of  1947,  these i n addition to four which previously had been caught there. Similar r e s u l t s were obtained  i n the f i r s t ten days of September,  1 9 4 9 , when forty-one animals were tagged i n seven days of trapping. Almost a l l of these animals had moved to other lakes by the time ice  covered the area, t h e i r concentration here seemed mainly i n  quest f o r food.  Nearly eighty per cent of the animals  captured  during these periods were juveniles. Muskrats usually vacated t h e i r summer habitat i n shallow lakes after the f i r s t f r o s t s of autumn (late August) had blighted the emergent growths of sedge and h o r s e t a i l which had t h e i r food and cover requirements to that time.  fulfilled  F a i l u r e to seek  more permanent water bodies resulted almost c e r t a i n l y i n freezing out i n e a r l y winter. WINTER HABITAT Summer abode f o r muskrats i n the Mackenzie d e l t a required abundant emergent food plants, burrows s a t i s f a c t o r y f o r rearing young animals, and only enough water to cover and protect the den entrance. ing  Winter habit, on the other hand, was more exact-  and had two primary r e q u i s i t i e s which had to be f u l f i l l e d  i f the animals were to survive the long period beneath the i c e . The f i r s t of these was  a s u f f i c i e n t depth of water so that the  lake would not freeze s o l i d l y , and the second was  an adequate  supply of submerged plants i n water which remained unfrozen. It was  stated previously that winter habitat was more re-  structed i n i t s extent than summer habitat.  I t should not  be  inferred that t h i s involved two separate sets of lakes, one for  67  each season.  Rather, many of the lakes inhabited i n summer  ^  were also inhabited i n winter even though the accretion of ice caused a decrease i n the volume of water available to muskrats. Accessible lake bottom area was very important because therein lay the winter food.  Lake gradient also was  a factor in muskrat  s u r v i v a l because the growth of an extra foot of ice i n a V-shaped lake rendered a greater percentage of lake f l o o r unavailable than i n a U-shaped lake.  Other factors being equal, steep-sided lakes  were more valuable as wintering lakes than those with a shallow gradient. Depths of a l l of the study lakes, and of numerous others as w e l l , were measured during the course of winter investigations. These depths varied over a wide range and i t was found that the deeper lakes were the newer ones —  those arcuate lakes formed  by the s h i f t i n g of r i v e r channels.  Depths t'o 38 feet were r e -  corded at one such lake and 23 feet i n another, but these depths were extreme.  Measured depths of a l l types of lakes known success-  f u l l y to harbour muskrats during winter varied from three and h a l f to sixteen feet with an average of just over six f e e t .  oneThe  deepest areas were not always located i n the centre of a lake; often they were to one side, e s p e c i a l l y near the steeper shore. But f o r the most part the lakes were rather regularly saucer-like. Lakes with only three and one-half feet of water could not be depended upon to support muskrats every winter because ice was sometimes as much as four feet thick.  The average measured water depth  of s i x feet was considered nearly optimum. Even though i t has been mentioned that lake depths were important, yet the prime factor governing muskrat s u r v i v a l was  68  the depth or thickness of w i n t e r i c e .  I t was  t h i s one f a c t o r  which governed the success of any w i n t e r h a b i t a t . Freezing out does not appear to be a problem i n the Gulf States and even In lew York Johnson (1925) asserted t h a t he  had  "learned of no p a r t i c u l a r winter d i f f i c u l t i e s that had been known to a f f e c t muskrats".  He conceded t h a t "the w i n t e r conditions i n  the marshes and swamps of New York must be of v e r y nature t o the muskrats.  favourable  The surface of these h a b i t a t s i s gener-  a l l y covered w i t h a dense mat o f f l a g and other v e g e t a t i o n , and does not become f r o z e n t o any depth before i t r e c e i v e s a blanket of snow as s t i l l f u r t h e r p r o t e c t i o n .  Under t h i s cover the deeper  waterways of the muskrats remain open and the animals are able t o move about f r e e l y " . The c o a s t a l race maorodon. i n h a b i t i n g the s a l i n e marshes of Maryland, u s u a l l y remains a c t i v e i n an unfrozen h a b i t a t yearlong.  However, Smith (1938) recorded t h a t when the peat l a y e r s In  which they oonstruct t h e i r runways f r o z e to depth of s i x inches i n 1934 many of t h e animals perished because t h e y could not d i g out to the surface. Aldous (1947) i n South Dakota knew of the danger of f r e e z i n g out shallow h a b i t a t and recommended t h a t suoh areas be trapped  be-  tween 15 December and 15 January t o remove muskrats before they were forced out by i c e . Henderson (192 3) mentioned a high w i n t e r death r a t e among muskrats o f the Peace R i v e r d i s t r i c t of A l b e r t a during the winter of 1916-17.  He recorded "thousands" of muskrats i n the f a l l , yet  i n spring a f t e r i n t e n s i v e hunting he was able t o secure l e s s than one hundred p e l t s .  This high r a t e of m o r t a l i t y he ascribed d i r e c t l y  69  t o severe w i n t e r c o n d i t i o n s . Fuller  (1951a)  r e f e r r i n g t o c o n d i t i o n s i n the d e l t a of  the Athabaska R i v e r noted t h a t "winter f r o s t s s e a l the l a k e s and sloughs c o n t a i n i n g the choioest foods and may t u r n t h e shallow ones t o s o l i d blocks of i c e , f o r c i n g e v i c t i o n of the muskrats l i v i n g i n them". I t may be s t a t e d , t h e r e f o r e , t h a t f r e e z i n g out i s not unique i n t h e muskrat h a b i t a t s of the c e n t r a l and northern p a r t s of the c o n t i n e n t .  What i s important i s the degree of such f r e e -  z i n g as governed by the type of h a b i t a t and the depth of i c e . I n v e s t i g a t i o n s i n A l a s k a , Buckley (1954- unpubl.) recorded i c e depths of 6 6 inches under three inches of snow i n March of 1 9 5 3 . He pointed cut, however, that t h i s was an extreme c o n d i t i o n . I n any area where f r e e z i n g temperatures p e r s i s t ioe depths are regulated by the t h i c k n e s s of the i n s u l a t i n g blanket of snow as w e l l as by t h e mean d a i l y temperature.  Snow depths i n the Mac-  kenzie d e l t a v a r i e d w i t h the time of year and the exposure of any lake surface t o wind a c t i o n . T h i s exposure t o wind was determined by the height of the t r e e s and banks surrounding the l a k e , by the s i z e of t h e l a k e , and by i t s o r i e n t a t i c n r e l a t i v e t o t h e p r e v a i l i n g n o r t h e r l y winds.  A graphic representation of i c e measurements  beneath varying depths of snow, demonstrated t h a t t h e r e was an i n verse r e l a t i o n s h i p between the two, F i g . 1 3 .  Heavy blankets of  snow c o u l d , however, unless adequately protected, be removed i n a s i n g l e day of high winds. Barnes  (1906)  remarked t h a t "the p r o t e c t i v e a c t i o n of snow  on the ground oannot be overestimated, and i n the dry s t a t e i n which i t u s u a l l y e x i s t s during the severest weather, i t i s equal  70  ICE DEPTHS IN INCHES FIGURE 13> RELATIONSHIP BETWEEN SNOW AND ICE DEPTHS ON LAKES IN MACKENZIE DELTA DURING APRIL - 1948.  71  to the best non-oonductor of heat".  He suggested (1928) that the  main f a o t o r i n i o e a c c r e t i o n was the l o s s of heat hy conduction through the i c e and i n t o t h e a i r . Tutton (1927) recorded the cond u c t i v i t y of ioe as t e n times that of snow —  an i n c h of snow,  t h e o r e t i c a l l y , o f f e r i n g t h e same p r o t e c t i o n as t e n inches of i c e . But since t h e c o n d u c t i v i t y of snow v a r i e s as the square of i t s d e n s i t y , the i n s u l a t i n g p r o p e r t i e s of wind-packed snow on northern lakes f a l l f a r short of t h a t of newly deposited snow. Another phenomenon r e l a t e d t o snow and i o e cover, known l o c a l l y as "over-flow", oonsisted o f a l a y e r of water interposed between t h e snow and i c e cover on a l a k e .  Such a c o n d i t i o n o f t e n  succeeded a heavy f a l l o f snow, t h e weight of whioh f o r c e d the water from below t o cover t h e lake t o a depth of three o r f o u r inches.  T h i s water wetted t h e snow above but soon f r o z e and was  incorporated w i t h t h e i c e cover.  I t thereby reduced the l a y e r of  snow, increased the depth of I c e , and hastened t h e formation o f more i c e . During the w i n t e r of 194-7-48 successive l a y e r s of overflow added as much as nine Inches t o ibhe i c e cover and thereby e f f e c t i v e l y sealed many muskrat "push-ups".  Over-flow seldom  occurred a f t e r December because by t h e n t h e i c e was t h i c k enough to bear a g r e a t e r weight of snow. WINTER "PUSH-UP" STUDIES Perhaps cne of t h e most c h a r a c t e r i s t i c f e a t u r e s o f w i n t e r muskrat h a b i t a t s i n t h e north i s the presence of feeding s t a t i o n s or push-ups b u i l t upon the surface of the i o e .  John Rae (1888)  was one of the f i r s t to describe these s t r u c t u r e s which he c a l l e d " l i t t l e huts o f mud and weeds". Dozier (1948) termed them "breathers"  72  and remarked that they "are s t a r t e d by muskrats c u t t i n g a f o u r or f i v e inch hole through the ioe and pushing up through t h i s a twelve t o eighteen inch p i l e o f f i n e f i b r o u s r o o t s , water weed, or other submergent v e g e t a t i o n " .  F u l l e r (1951a) i n northern A l -  berta recorded t h a t "the dome o f the push-up i s made o f r e j e c t e d portions o f food items, most o f whioh are parts o f submerged aquatic p l a n t s .  Ana char i s . Myriophyllum, U t r i o u l a r i a . and  s e v e r a l species o f Potamogeton are the u s u a l c o n s t i t u e n t s " . P o r s i l d (1945) speaking o f the Mackenzie d e l t a s t a t e d t h a t "the p r i n c i p a l m a t e r i a l used i n push-ups i s the leafy-stemmed Richardson's pondweed (Potamogeton R i c h a r d s o n i i ) and w a t e r - m i l f o i l (Myriophyllum exalbesoens)".  There i s not much a d d i t i o n a l i n f o r -  mation i n p r i n t concerning these very common and very  necessary  winter feeding houses because s t u d i e s i n other areas u s u a l l y have been conducted i n summer.  However, the c o n s t r u c t i o n o f push-ups  appears t o be l i m i t e d t o the mare n o r t h e r l y portions o f the cont i n e n t where i c e cover i s continuous  f o r long periods eaoh w i n t e r .  C e r t a i n l y they are not constructed i n the Gulf States by r i v a l l o i u s nor on the A t l a n t i c ooast by maorodon. I t i s safe t o a s s e r t that spatulatus b u i l d s more push-ups and i s mare dependent upon them than i s any of the other races o f muskrat under c o n s i d e r a t i o n .  This i s even more the case w i t h t h e  f a r northern r e p r e s e n t a t i v e s o f that r a c e .  Animals whioh i n h a b i t  houses or ledges during w i n t e r u s u a l l y construct them i n f a i r l y deep water i n o r near stands of vegetation which w i l l serve as winter food.  Thus t h e i r food stores are r e l a t i v e l y a c c e s s i b l e  from the house and the need f o r an extensive system o f push-ups i s  73  obviated.  But those animals which b u i l d no houses but i n h a b i t  bank dens instead may r e q u i r e s e v e r a l push-ups t o extend t h e i r r a d i u s of a c t i v i t y t o t h e i r food sources i n t h e deeper water. Thus muskrats of t h e Mackenzie d e l t a , and those i n s i m i l a r l a t i tudes i n A l a s k a , are unique i n t h e i r degree of dependence upon these s t r u c t u r e s f o r t h e i r w i n t e r s u r v i v a l . F u l l e r (1951a) considered t h a t "there were two p o s s i b l e explanations f o r the o r i g i n of push-ups:  ( 1 ) , h o l e s are kept  open from the time of ioe f o r m a t i o n , and ( 2 ) , new holes are gnawed through the i c e , probably where an a i r pocket reduces the t h i c k ness of ioe to be gnawed and supplies a source of oxygen".  This  observation was v e r i f i e d i n part i n the Mackenzie d e l t a where i t . was noted t h a t a number of small h o l e s p e r s i s t e d i n the newly formed i c e . These holes d i d not c l o s e r e a d i l y because t h e y were kept open by the c o n t i n u a l r i s e and b u r s t i n g of bubbles of gas. When they f i n a l l y d i d freeze a done-shaped area beoame f i l l e d w i t h the gas and t h e i c e above i t remained t h i n .  The source of the gas  seemed t o be from the decomposition of organic matter.  There was  a considerable mound of vegetative d e b r i s , a by-product o f f e e d i n g , which accumulated below each push-up.  The g e n e r a l l y slow r a t e of  decomposition of t h e m a t e r i a l produced a slow but c o n t i n u a l r e lease of gas and i n this way push-ups were perpetuated i n the same place from year to year. Any plant m a t e r i a l s a v a i l a b l e t o t h e animal were apt t o be incorporated i n i t s push-up.  Speoies seldom used f o r feed often  proved u s e f u l i n the b u i l d i n g of these s t r u c t u r e s .  Thus an alga  (Cladophora) f r e q u e n t l y appeared in push-ups i n the v i c i n i t y of A k l a v i k and there was a general use of hornwort (Chara) and of  74 duckweed (Lemna t r l a u l o a ) wherever these occurred.  The m a j o r i t y  of push-ups, however, were made from the stems and leaves of pondweeds and m i l f o i l , and might even i n c l u d e mua and pieces of wood as w e l l .  Lakes which showed a l a r g e proportion of mud and r o t t e d  vegetable d e b r i s i n the push-ups were i n most cases devoid of adequate submerged vegetation and presented poor w i n t e r i n g h a b i t a t f o r the animals . I t was found t h a t mare push-ups were constructed i n autumn than were kept open w i n t e r l o n g .  I f the s t r u c t u r e was  not v i s i t e d  f r e q u e n t l y by muskrats d u r i n g cold weather, access was by f r e e z i n g of the plunge-hole.  prevented  The p r o p o r t i o n of these f r o z e n  push-ups y e a r l y v a r i e d w i t h the s e v e r i t y of the weather, the amount of "over-flow", and t h e depth of the i n s u l a t i n g snow. and A p r i l of 1°48 there were 48 per cent (382) s t i l l i n use.  I n March  of 835 push-ups  Figures f o r A p r i l of 1950 revealed a lower success,  i n t h i s case 33 per cent (507)  of 1,522  push-ups.  Winds of g a l e  force which removed the snow cover from most lakes were h e l d r e sponsible f o r t h i s lessened  success.  As a p a r t i c u l a r oase we might c i t e the f a t e of the push-ups on Grassy Lake of Area ftl. I n e a r l y November, 1947, were found and charted, F i g . 14.  72 push-ups  By e a r l y March only 35 of these  push-ups were i n use; the r e s t had f r o z e n .  During the A p r i l musk-  r a t harvest only 23 of these s t i l l were a c t i v e .  I n view of t h e  f a c t that t h e t o t a l muskrat population of the lake was s e v e r a l times t h i s number, i t would appear that more than one animal used each f e e d i n g s t a t i o n and t h i s repeated usage helped keep the plunge hole open.  As the weather became c o l d e r and the number of a c t i v e  push-ups decreased, mare i n t e n s i v e use of each s t r u c t u r e was  75  ©  Trapping sites Nov., 1947  £2> Known muskrat  den3  S Trapping s i t e s , Apr. 1949 • Push-ups charted Nov. 1947  FIGURE 14. FATE OF PUSH-UPS ON GRASSY LAKE DURING WINTER 1947-48  76  occasioned, thereby l e s s e n i n g the ohanoes of f u r t h e r f r e e z i n g . There had been 45 muskrats tagged and r e l e a s e d i n Grassy Lake during August, 1947.  Continued trapping on the i c e the  f o l l o w i n g November r e v e a l e d that 26 of these animals were s t i l l i n the lake but the remaining nineteen were not, nor were they subsequently contacted anywhere.  An a d d i t i o n a l 43 tagged animals,  however, were r e l e a s e d i n the l a k e , making a t o t a l of 69.  Twenty-  four of these were handled only once, s i x t e e n were captured t w i c e , s i x t e e n t h r e e times, and e i g h t f o u r times.  Of the remaining f i v e  animals, three e v e n t u a l l y d i e d i n t h e l i v e t r a p s and two were taken a t the outset as specimens f o r examination.  Most animals  confined t h e i r a c t i v i t i e s t o a s i n g l e push-up; thus, 41 (65 per cent) were taken from one push-up, eighteen (28 per cent) from two, and the remaining seven per oent from t h r e e . The number of animals u s i n g each push-up v a r i e d from one to t h i r t e e n w i t h an average of s i x as may be adduced roughly from the trapping r e s u l t s i n Table 1. TABLE 1 NUMBER OF DIFFERENT ANIMALS TRAPPED PER PUSH-UP ON GRASSY LAKE, NOVEMBER, 1947 Push-up No. 1  Animals Taken  Push-up No.  Animals Taken  3  5 6 7 8  10 13 10 6  10 11  4  la  2 2a  5 1 3  3  3  4 4a  13 6  9  I  77  I f there were as many as t h i r t e e n d i f f e r e n t animals taken at a s i n g l e f e e d i n g s t a t i o n , there must have been an even l a r g e r number u s i n g i t because not a l l animals i n the lake were caught.  The minimum number o f animals necessary t o keep a plunge-  hole open d u r i n g severe weather was not a s c e r t a i n e d , though i t must have r e q u i r e d mare t h a n three because t r a p p i n g s t a t i o n s showing t h i s minimum l a t e r became f r o z e n and abandoned. Animals were trapped i n proportion t o t h e r e p r e s e n t a t i o n of t h e i r sex and age groups i n the l a k e .  There were 139  animals  taken i n t r a p s ; t h i s included those caught more than once. s u r v i v i n g complement  of 67 tagged animals e v i d e n t l y was  The  captured  at random because the f o l l o w i n g table of r e s u l t s (Table 2) i n d i cates t h a t n e i t h e r sex nor age group was dominant i n the catch. TABLE 2 CATCH OF ANIMALS BY SEX AND AGE ON GRASSY LAKE I N NOVEMBER, 1947 ADULT Male Female T o t a l take ( i n c l . reoaptures)  11  17  JUVENILE Male Female  60  31  =  139  =  67  Known complement of lake  5  6  30  26  Pro-rated take per 67 animals  3  8  29  25  67  I t i s b e l i e v e d t h a t push-ups are constructed by muskrats during e a r l y wint er wherever there i s a hole i n the i c e through which plant m a t e r i a l s can be pushed t o form a dome. Those i n the  78 )  TV v i c i n i t y of a more or l e s s permanent food supply w i l l be kept open by c o n t i n u a l use, t h e r e s t w i l l be abandoned and f r e e z e . In the w r i t e r ' s view the elaborate system of push-ups constructed by spatulatus i s the key t o r a o i a l s u r v i v a l under the very rigorous c l i m a t i c c o n d i t i o n s north of the A r c t i c C i r c l e .  Herein  l i e s evidence of one of the important e x t r i n s i c adjustments whioh the Mackenzie p o p u l a t i o n has made t o i t s environment. THE YEARLY RADIUS OE ACTIVITY OF MUSKRATS The  preceding s e c t i o n dealt w i t h the summer and w i n t e r  h a b i t a t of muskrats i n the Maokenzie d e l t a .  I n order t o d e f i n e  more c l e a r l y the r e l a t i o n s h i p of t h a t h a b i t a t to the  animals  themselves,it i s d e s i r a b l e t h a t some l i m i t be set t o the amount of h a b i t a t r e q u i r e d per animal.  This w i l l be defined by g i v i n g  the radius of a c t i v i t y of muskrats during both summer and w i n t e r periods.  A l l data i n t h i s regard were obtained through the cap-  ture and tagging o f l i v e animals.  Information concerning w i n t e r  movements w i l l be g i v e n i n some d e t a i l because such data are not available  elsewhere.  The number o f animals tagged each month i s shown i n Table 3.  I t w i l l be noted t h a t mare than h a l f of the t o t a l of 621 were  taken d u r i n g August w i t h September ranking second.  Young animals  were captured w i t h ease during these periods and a d u l t s thereby were handled l e s s o f t e n . Of t h e 621 animals tagged 173  (28 per cent) were again  recaptured a f t e r a period of at l e a s t three months. Sumrner Movements A m a j o r i t y of animals (141 of the 173 recovered) were  79  TABLE 3 NUMBER OF MUSKRATS TAGGED BY MONTHS IN THE MACKENZIE DELTA DURING THE PERIOD 1947-50 INCLUSIVE  Adults Male Female -  Month  Juveniles Kale Female  Totals  J&ne  18  7  0  0  25  July.-  24  15  1  5  45  August  47  51  145  95  338'  September  9  13  59  46  127  October  0  0  0  0  0  November  3  4  28  20  55  December  0  1  8  5  14  January  0  0  5  9  14  February  0  0  0  0  0  March  0  3  0  0  3  94  246  180  621  Totals:  101  Males/100 Female s:  108:100  137:100  8o taken a g a i n i n the same lake where they had been tagged.  For  those whioh had moved l e s s t h a n one-half m i l e , and yet were found i n another lake , the average r a d i u s o f movement was 300 yards w i t h a range from 100 t o 800 yards.  This average movement 1300 yards)  represented t h e d i s t a n c e many animals would cover i n t h e i r normal a c t i v i t i e s w i t h i n a l a k e , t h e r e f o r e any e x c e p t i o n a l movements probably exceeded t h a t f i g u r e . Animals d i s p e r s i n g more than 800 yards from t h e i r t a g g i n g s i t e s e x h i b i t e d an average movement of one and one-half m i l e s 12640 yards) w i t h a range o f one-half (880 yards) t o f o u r and one-ha I f mils s (7920 y a r d s ) .  An extreme case involved a muskrat  that t r a v e l l e d t h i r t e e n m i l e s between 19 August and t h e time of i c e formation (1 October).  Because these ionger movements i n -  volved only f i f t e e n animals we may regard them as unusual f o r muskrats of the Mackenzie d e l t a . E r r i n g t o n (1944) found that the m a j o r i t y of muskrats he tagged i n northwestern Iowa moved an average o f 125 yards though there were a few t h a t wamdered two m i l e s away.  Takos (1944) be-  l i e v e d that a r a d i u s of 25 feet defined t h e average movement i n Maine w i t h l e s s than one-third of the animals v e n t u r i n g beyond a distance of 100 yards.  Dorney and Rusch (195 3) demonstrated that  these f i g u r e s were e s s e n t i a l l y c o r r e c t f o r southern Wisconsin as well.  Aldcus (1947) working i n South Dakota found t h a t only f i f -  teen per cent of the animals under i n v e s t i g a t i o n moved more than 31 rods (170 y a r d s ) , w i t h a maximum recorded d i s t a n c e of 200 rods (1100  yards).  F u l l e r (1951b) would concur i n these f i g u r e s from  South Dakota because t h e y agree w i t h h i s f o r the d e l t a of the  81  Athabaska R i v e r .  He i n f e r r e d , however, t h a t movements during  the spring d i s p e r s a l period might exceed the autumn r a d i u s of 170 yards. A l l of the movements d e t a i l e d above demonstrated an act i v i t y range of l e s s t h a n h a l f the average of 300 yards recorded i n the Mackenzie d e l t a .  I n t h e southern areas the muskrats l i v e d  i n r e l a t i v e l y food r i c h h a b i t a t s i n which c a t t a i l was the prominent food and b u i l d i n g m a t e r i a l .  The l a r g e r movements of the animals  i n 1Jiis study were a s c r i b e d to two major f a c t o r s .  The f i r s t of  these was the neoessity t o vacate temporary summer quarters f o r more favourable winter h a b i t a t .  The second was the more dispersed  nature o f t h e food resource which demanded t h a t muskrats undertake more extensive f o r a y s to f u l f i l l t h e i r d a i l y food requirements. There i s no doubt that these longer journeys, o f t e n across land or open water, predisposed t h e animals t o depredations by t h e i r n a t u r a l enemies.  But extensive t r a i l s and "portages" through the  vegetation between water bodies i n d i c a t e d that these movements, though p o t e n t i a l l y dangerous, were q u i t e normal. Winter Movements No information i s a v a i l a b l e i n the l i t e r a t u r e concerning the amount of movement necessary f o r existence beneath a cover o f ice.  I n the l a t i t u d e of the Mackenzie d e l t a the muskrats are con-  f i n e d beneath i c e from October t o the f o l l o w i n g June.  Live t r a p -  ping i n November and December was undertaken, as d e t a i l e d e a r l i e r , In order t o assess t h e fortunes and chart t h e movements of animals in t h e i r daily activit ies.  L i t t l e information was c o l l e c t e d con-  cerning the wanderings of animals which might escape from shallow  82  frozen l a k e s .  I t was apparent that such unfortunate i n d i v i d u a l s  perished q u i c k l y because they were not equipped t o w i t h s t a n d the severe ambient temperatures. Discussions w i t h trappers i n d i c a t e d that such animals on occasion were found f r o z e n i n a w i l l o w copse or sedge clump and t h a t t h e i r t r a c k s showed that t h e y had not wandered f a r from where they had dug out.  Their chances o f enter-  i n g another lake were almost n i l because of the depth of f r o s t i n a l l exposed surfaces. The map  ( F i g . 14) i n d i c a t e s the l o c a t i o n of seventy-two  push-ups as they appeared d u r i n g the month of November 1947 on Grassy Lake.  The distance between each push-up averaged about  f o r t y yards.  The average distance from shore f o r t h e fourteen  t r a p - s i t e s shown was e i g h t y yards, w i t h a range of 32 t o 132 yards. To reach the t r a p p i n g s i t e the animals f i r s t of a l l had t o t r a v e r s e that d i s t a n c e .  Then i f t h e y v i s i t e d more t h a n one push-up, and  35 per oent of them d i d so, t h e y would t r a v e l a probable d i s t a n c e of 120 yards.  I f t h e y v i s i t e d three push-ups, and f i v e animals  were caught i n t h r e e d i f f e r e n t s i t e s , the d i s t a n c e might be 160 yards.  By actual measurement i t was found t h a t these movements  varied greatly.  Without accounting f o r the d i s t a n c e of the push-  up from the bank den, and i t was not possible i n a l l cases t o decide i n which den an animal was l i v i n g , the appearance of animals at d i f f e r e n t push-ups i n v o l v e d distances of 21 t o 231 yards (average 103 y a r d s ) . I n s p i t e of the f a c t that the animals showed t h i s a b i l i t y to move about under t h e i c e , t h e r e was a d e f i n i t e d i v i s i o n of t h e lake i n t o zones, each used by a f a i r l y d i s c r e t e group of animals.  83  3) Number of muskrats crossing zone boundaries  YARDS  ®  FIGURE 15. SEGREGATION OF MUSKRATS BY ZONES ^ IN GRASSY LAKE, WINTER OF 1948. •  Number of tagged animals caught per zone K n o w n  d e n s  Trapping sites  84 The diagram  ( F i g . 15) i l l u s t r a t e s how 67 tagged animals probably-  d i s t r i b u t e d themselves among the nine occupied burrews and t h e fourteen t r a p p i n g s i t e s .  Nine zones are o u t l i n e d and t h e number  of animals u s i n g each appears t h e r e i n .  Some of these zones were  closed and showed no i n t e r c o u r s e with any o t h e r ; they are del i m i t e d by s o l i d l i n e s .  Others shewed l i m i t e d movement of animals  across the boundaries, i n whioh case t h e l i n e s a r e broken and the included f i g u r e denotes t h e number of animals i n v o l v e d . Zone l i m i t s seemed l e s s r i g i d i n t h e eastern part o f t h e lake b u t , i f we discount the movements o f two animals whioh showed a prop e n s i t y f o r wandering, they were as d i s c r e t e as those i n the western h a l f . Such zonatian suggests a l i m i t e d t e r r i t o r i a l i t y even i n winter.  I t has other i m p l i c a t i o n s , however, i n v o l v i n g j u x t a -  p o s i t i o n of dens, push-ups, and food supply.  Habits o f f e e d i n g ,  formed e a r l y i n w i n t e r , would tend t o keep the animals i n the same general a r e a .  How much these animal a s s o c i a t i o n s would break  down w i t h a decrease i n food supply was not e s t a b l i s h e d . However, animals trapped i n A p r i l were i n the same g e n e r a l l o c a t i o n as they had been i n November, and three animals occupied t h e same dens i n J u l y as they had t h e previous August.  No doubt there i s some r e -  s t r i c t i o n o f movement a t any time during the year.  I n the south-  eastern corner of the l a k e two animals i n very poor c o n d i t i o n were trapped i n s p r i n g , and each had been p a r t i a l l y eaten before i t could be removed from the t r a p .  These animals d i d not appear to  be separated by d i s t a n c e from an adequate food supply and hence may have been r e s t r i c t e d i n movement by the animals i n surrounding areas.  Such i n t o l e r a n c e t o f r e e movement of animals d u r i n g w i n t e r  85  would help t o e x p l a i n mid-winter though not a prevalent s i t u a t i o n .  f i g h t i n g which was an occasional Depletion o f food s u p p l i e s i n  l a t e w i n t e r could induce ma 1 adjustments i n the population as a n i mals moved about i n search f o r new sources.  This would a f f e c t  not only the s e c u r e l y s i t u a t e d animals but also the wanderers. This range of movement beneath the i c e of a t l e a s t eighty yards i s as great as t h a t recorded f o r muskrats i n other areas during summer.  I t was assumed t h a t the movements between push-  ups were d i r e c t , as they seemed t o be, and not at random.  Also,  i t should be pointed cut that only f o u r t e e n out of a p o s s i b l e 7 2 push-ups were sampled and, t h e r e f o r e , the a c t u a l movements might have been even greater.  No comparisons with w i n t e r movements  elsewhere are p o s s i b l e but the information regarding l i f e beneath the ice was t o the w r i t e r one of the most i n t e r e s t i n g phases of the i n v e s t i g a t i o n . FOOD AND FEEDING HABITS I t i s g e n e r a l l y conceded t h a t muskrats do not show any great preference i n the foods which they w i l l accept.  Takos  (1947)  stated t h a t the animals u t i l i z e d the most a v a i l a b l e plants i n the environment as f o o d , e s p e c i a l l y when the abundant species occurred i n dense stands.  He noted t h a t there was same seasonal preference  i n plant food when one species was e s p e c i a l l y a t t r a c t i v e at a time vflien other species were i n a l e s s edible developmental stage. Errington  (1941)  drew a t t e n t i o n to the wide v a r i e t y of m a t e r i a l s  which c o n s t i t u t e d the yearlong foods of the Iowa muskrats t h a t he studied.  He found that most green vegetation was more or l e s s  adequate as food I n warm weather so t h a t muskrats were not o f t e n  86  subjected t o r a p i d or o u t r i g h t s t a r v a t i o n . However, shortages or u n a v a i l a b i l i t y of food i n combination w i t h w i n t e r v i c i s s i t u d e s could produce s t a r v a t i o n .  B e l l r o s e (19.50) i n d i c a t e d that the  animals he studied i n I l l i n o i s d i d show d e f i n i t e food  preferences  and "although there was a great d e a l o f i n d i v i d u a l v a r i a t i o n , i t was evident t h a t muskrats s e l e c t e d c e r t a i n p l a n t s and c e r t a i n parts of plants f o r food". In s p i t e of these o p p o r t u n i s t i c tendencies i n feeding there i s nevertheless  a marked change i n p l a n t foods from the Gulf  t o the A r c t i c Coast concomitant w i t h the f l o r a l complex and the h a b i t a t conditions enoountered.  Both the a v a i l a b i l i t y o f food  and the type of food have a d i r e c t bearing upon the h a b i t s of the animal and i t s w e l l being.  A b r i e f resume of the food h a b i t s of  the various races under consideration w i l l provide an i n s i g h t i n t o the type of h a b i t a t each occupies and t h e s t a t u s of food as a l i m i t ing f a c t o r t h e r e i n . Common and p l a i n s muskrats appeared t o favour the same veget a t i o n f o r food and f o r cover.-  I n New Tork, Dozier (1950) l i s t e d  the p r e f e r r e d foods as Typha glauca and T. l a t i f o l i a w i t h s e v e r a l , species o f Scirpus r a n k i n g second.  Takos (1947) i n Maine l i s t e d  sweet f l a g (Aoorus calamus) i n a d d i t i o n to c a t t a i l s .  Errington  (1941) observed t h a t muskrats were very c a t h o l i o i n t h e i r f e e d i n g h a b i t s but was of t h e opinion ( E r r i n g t o n , 1948) that i n Iowa, catt a i l marshes had double t h e "carrying c a p a c i t y " of any other type of vegetation.  Many of t h e animals he s t u d i e d were from stream  dwelling populations t h a t i n summer subsisted upon a v a r i e t y of green vegetation, and i n w i n t e r upon whatever presented i t s e l f , whether h i g h l y n u t r i t i o u s e a r corn s t o r e d i n burrows or on d r y  87  grass and weeds along stream banks.  B e l l r o s e (1950) l i s t e d c a t -  t a i l s (Typha a n g u s t i f o l i a and T. l a t i f o l i a ) as the most p r e f e r r e d food i n I l l i n o i s w i t h pickerelweed ( P o n t e d e r i a ) , b u l r u s h (Scirpus aoutus and S. v a l Idus) , and marsh smartweed (Polygonum Muhlenbergii) , ranking i n t h a t order. I f any s i n g l e plant species may be designated as the mainstay of the muskrats of t h e c e n t r a l p l a i n s areas t h a t one would be the c a t t a i l .  This plant i s d i s t r i b u t e d abundantly over a wide  area and supports t h r i v i n g populations of muskrats.  Because i t  w i l l not grow and maintain i t s e l f i n bodies of deep permanent water, the d i s t r i b u t i o n of muskrats dependent upon i t i s r e s t r i c t e d t o s m a l l shallow ponds and lakes i n the p r a i r i e and r i v e r basin areas. Smith (1939) reported t h a t 75 per cent of the food of the V i r g i n i a muskrat maorodon i n the Maryland marshes was composed of three-square rush (Scirpus Olneyi) and c a t t a i l .  I n summer the  leaves and s t a l k s were eaten and i n w i n t e r the roots and r o o t stalks.  E i t h e r high onshore winds or p r o t r a c t e d periods of drought  increased the s a l i n i t y of the c o a s t a l marshes and caused the t h r e e square rush and c a t t a i l marshes t o d e t e r i o r a t e t o a l e s s valuable Spartina - D i s t i c h l i s a s s o c i a t i o n which would not support an equal density of muskrats.  D o z i e r (1947) reported on t h i s s i t u a t i o n at  some l e n g t h . The c o a s t a l marshes of L o u i s i a n a and e a s t e r n Texas are the heme of the L o u i s i a n a muskrat r i v a l l c i u s .  I n t h i s area very high  concentrations of t h i s animal are supported by the brackish t i d a l flats.  Nearby f r e s h water areas produce an i n f e r i o r type of vege-  t a t i o n w i t h fewer muskrats.  I n these t i d a l areas, as i n those of  88 Maryland, the three-square rush (Scirpus Qlneyi) i s t h e p r e f e r r e d food.  Cordgrass ( S p a r t l n a patens) i s a l s o abundant here but f i n d s  more use as b u i l d i n g m a t e r i a l than as food.  The s o i l i s h i g h l y  organic but where storm t i d e s deposit a l a y e r of s i l t or c l a y over the marshes an i n t r u s i o n o f s a l t g r a s s ( D i s t l o h l i s spioata) renders the h a b i t a t much l e s s acceptable to muskrats. There i s no dearth o f plant food i n t h i s r e g i o n .  Winter  f r o s t s and i c e are r a r e and t h e danger i n t h i s area i s t h a t muskrats w i l l increase t o the point where they w i l l mow o f f a l l of the veget a t i o n and d i g out i t s roots over wide areas.  When t h i s happens  there i s a d r i f t i n g of t h e d i s p l a c e d animals w i t h r e s u l t i n g i n t o l erance and damage t o p e l t s through f i g h t i n g . The Hudson Bay muskrat albus I n h a b i t s the area adjacent to the west shore of Hudson Bay and a t t a i n s i t s greatest numbers i n the d e l t a region of the North Saskatchewan R i v e r .  Because p r i -  vate and government sponsored f u r r e h a b i l i t a t i o n endeavours have been undertaken i n t h i s d i s t r i c t  there has been a s a t i s f a c t o r y  amount of i n v e s t i g a t i o n made of the food h a b i t s of t h e muskrats tbs r e i n . MoLeod (1949) s t a t e d that "there i s a d e f i n i t e r e l a t i o n ship between water depth and v e g e t a t i o n d i s t r i b u t i o n , and our f i e l d observations have proved that the emergent p l a n t s suoh as bulrush, c a t t a i l , and f l a g g r a s s e t c . on which the muskrat i s l a r g e l y dependent are found normally i n water o f a depth v a r y i n g from zero t o t h i r t y inches.  This i s a f o r t u n a t e coincidence as the  muskrat p r e f e r s t o b u i l d i t s lodge, on the average, i n s i x t e e n inches of water".  Therefore, the yearlong h a b i t a t was much more  circumscribed than normally supposed because emergent v e g e t a t i o n  89  was f a i r l y s p e c i f i c as t o water depths.  Those p l a n t s used as  winter food, i n descending order of importance were:  cattail,  bulrushes (Soixpus f l u v i a t i l i s and S. v a l i d us) . f l a g r e e d , sedges, and duck potato ( S a g i t t a r l a spp.).  Besides s u p p l y i n g winter food,  the emergent v e g e t a t i o n o f f e r e d some p r o t e c t i o n against winds which would sweep the snow from the marshes.  Mcl>eod (op. c i t . )  continued,  "There can be l i t t l e doubt that f a r the q u a l i t y and q u a n t i t y of muskrats supported per aore Typha l a t i f o l i a i s without a near r i v a l as a native plant t y p e " .  I n summer he found t h a t young captive  muskrats "showed a decided preference f o r the s t a l k s of Equisetum and would take i t even though an abundance of other plant m a t e r i a l was present".  These f e e d i n g experiments i n d i c a t e d t h a t "each  muskrat would consume about two hundred and f i f t y pounds of food m a t e r i a l per year.  Examinations shewed t h a t about twelve f e e t o f  Typha rhizome weighed one pound; thus during one year a muskrat could consume approximately 3,060 f e e t or .57 m i l e s of rhizome". B u t l e r (1°40) attempted to c a l c u l a t e the number o f muskr a t s that could be supported adequately on the experimental marsh area at Mafeking, Manitoba.  H i s method was e s s e n t i a l l y a compari-  son between t h e d a i l y food consumption by penned muskrats and the d e n s i t y per square y a r d of food p l a n t s i n the marsh.  Although  t h i s approach to the problem may be questioned, he has s u p p l i e d a reasonably d e t a i l e d resume of the p l a n t s t o be found i n t h a t marsh area. B u t l e r (op. c i t . ) enumerated s e v e r a l p l a n t s , not p r e y i o u s l y reported as muskrat f o o d , which c o n t r i b u t e d t o the food and cover requirements of the Maf eking muskrats.  The f i r s t of these was  h o r s e t a i l (Equisetum f l u v i a t i l e ) which grew i n two to f o u r f e e t  90 of water and was favoured by t h e animals as summer f e e d .  Another  was the buckbean (Menyanthes t r i f o l i a t a ) which f l o u r i s h e d i n f o u r to f i v e feet  of water.  I n summer i t s l e a v e s and shoots were  u t i l i z e d as food and i n w i n t e r i t s t h i o k stems were e a t e n . Of a l l t h e p l a n t s a v a i l a b l e at t h i s e x p e r i m e n t a l area, cattail  (Typha l a t i f o l i a ) and s o f t s t e m b u l r u s h ( S c i r p u s v a l l d u s )  were l i s t e d as t h e most important f l a g r e e d r a n k i n g next i n o r d e r ,  f o o d p l a n t s w i t h h o r s e t a i l and i t was estimated t h a t these f o u r  p l a n t s p e c i e s would support twenty-three, seventeen, e i g h t e e n , and t w e n t y - s i x muskrats per a c r e r e s p e c t i v e l y . I t w i l l be noted t h a t t h e f o o d of t h e r a c e s of muskrats d i s c u s s e d above i s comprised of emergent s p e c i e s growing  i n com-  p a r a t i v e l y s h a l l o w water, u s u a l l y l e s s t h a n three f e e t i n depth. Because i c e c o n d i t i o n s g e n e r a l l y were not severe w i t h i n these h a b i t a t s , o r because p e r s i s t e n t emergent v e g e t a t i o n caught and held w i n t e r snows a s i n s u l a t i o n , t h e muskrats s u b s i s t e d on more or l e s s the same plant  s p e c i e s throughout  the year.  When we come t o a c o n s i d e r a t i o n of t h e northwestern muskr a t we f i n d t h a t summer foods u s u a l l y d i f f e r markedly from w i n t e r foods.  F u l l e r (1951a) wrote t h a t i n many cases "the water l e v e l s  had dropped  so low t h a t emergent p l a n t s a r e completely u n a v a i l a b l e  a f t e r i c e has formed and t h e degree determines  of abundance of submerged f l o r a  s u r v i v a l o r death of t h e muskrat p o p u l a t i o n " .  foods he l i s t e d as c a t t a i l , h o r s e t a i l (Equisetum) (Scirpus).  and b u l r u s h  Winter foods i n c l u d e d none of these but c o n s i s t e d i n -  stead o f waterweed (Ana char i s ) , w a t e r - m i l f o i l bladderwort  Summer  (Myriophyllum) ,  ( U t r i c u l a r l a ) , and s e v e r a l of t h e pondweeds (Potamogeton).  91  Information concerning food i n the Mackenzie d e l t a was supplied hy P o r s i l d (1945) who wrote t h a t the "food of the muskr a t , at l e a s t during autumn, w i n t e r and spring c o n s i s t s of the f r u i t s , rhizomes, and winterbuds o f pondweeds (Potamogeton spp.), rhizomes, and tubers o f h o r s e t a i l (Equisetum  arvensej, the rhizomes  and f r u i t s of water arum ( C a l l a p a l u s t r i s ) . and duckweed (Lemna trisuloa).  The r o o t s , rhizane stems, and f r u i t s of a number of  other p l a n t s , notably sedges and grasses are, no doubt, eaten as well". Eoods eaten yearlong by muskrats i n the Mackenzie d e l t a were d i v i d e d r a t h e r s h a r p l y between emergent v e g e t a t i o n i n summer and submerged p l a n t s during the remainder of the year.  Table 4  i s a compilation of s p e c i e s noted at feeding s i t e s a l o n g lake TABLE 4 A RESUME OF THE PLANT EOODS EATEN BY MUSKRATS IN THE MACKENZIE DELTA DURING EACH MONTH J • Emergent Species  F  M  A  M J J A S O N Percentages of t o t a l food  v  Equisetum spp.  T  0  0  T  25  Carex aquat i l l s  0  0  0  0  15  15  10  5  Other ±  D  20 55  40  20 15  10  T  5  10  15  30 40  T  0  T  15  35  15  T  5  T  10 10  30 25  10  10  20 40  30 60  Submerged Spe c l es Potamogeton spp. 50 50  20 20 15  Myriophvllum Other &  60 75  ft  15  15  15  10  10  T  T  5  5  T  15  5  T  T  10  30 30 10 10  10  15  ± - A r c t o p h i l a , Menyanthes, C a l l a , S a l i x , e t . a l . T - l e s s than && - Sparganium. U t r i c u l a r i a . Lemna. e t . a l . 5 per cent  92  shares,  i n food s t o r e s a t nest dens, and from other  observations  i n summer; and  from f o o d remnants found  i n s i d e push-ups i n w i n t e r .  These data are  c h i e f l y f o r a l l u v i a l l a k e s i n the t h r e e main  study  areas though observations from o t h e r l o c a l i t i e s a r e i n c l u d e d . By f a r t h e most important  summer food was  v a r i o u s car i c e s ranking second i n importance. s p e c i e s included water-oats ( A r o t o p h i l a ) . and  horsetail, with  Other emergent a v a r i e t y of l e a f y  s h o r e l i n e v e g e t a t i o n from woody p l a n t s such as a r c t i c (Rubus a r o t i c u s ) , and w i l l o w P y r o l a and Ranunculus. w e l l represented  raspberry  ( S a l i x spp.) , to t h e more herbaceous  Submerged and  f l o a t i n g s p e c i e s were not  i n the summer food though i t was  not uncommon  t o see muskrats d i v i n g f o r the rhizomes o f pondweeds and  milfoil.  They a l s o f e d a t the s u r f a c e on duckweed (Lemna t r i s u l c a ) ,  and  the f r u i t s of sago pondweed (Potamogeton p e o t i n a t u s ) . The u s u a l w i n t e r foods were much l e s s d i v e r s e as t o s p e c i e s but i n c l u d e d r e p r e s e n t a t i v e s from almost a l l submerged p l a n t s t h a t were a v a i l a b l e i n a lake .  A c c r e t i o n of i c e v i r t u a l l y  e l i m i n a t e d t h e submerged p o r t i o n s of n o r m a l l y emergent p l a n t s so that  only those s p e c i e s s u r v i v i n g i n more t h a n t h r e e f e e t  remained a v a i l a b l e as f o o d .  T h e r e f o r e , the  s e v e r a l s p e c i e s of  pondweeds formed the b u l k of t h e w i n t e r d i e t and these p l a n t s were found  of water  a l l p o r t i o n s of  on the f e e d i n g ledges of push-ups.  Mil-  f o i l and duckweed a l s o were u t i l i z e d f r e q u e n t l y as f o o d , but animal f o o d s seldom were r e p r e s e n t e d . Growth of pondweeds s t a r t e d b e f o r e the w i n t e r i c e c o v e r (  was  removed so that d u r i n g A p r i l and  comprised most of t h e f o o d .  e a r l y May  these new  shoots  During t h e l a t t e r h a l f of May  s p r i n g f l o o d s had r a i s e d t h e l a k e i c e and  once again the  the  underwater  93 portions of emergent vegetation were accessible.  During t h i s  period the rhizomes o f h o r s e t a i l formed the bulk of the residue found at lake shore feeding s i t e s .  Flood conditions prevailed  during the f i r s t two weeks i n June and forced the animals back into higher land where they subsisted upon a variety of both heath and shoreline species.  A w i n d f a l l for them at t h i s period  was the large number of new white shoots of pondweed which had been gouged loose from the lake bottom by d r i f t i n g ice and f l o a t e d t o shore i n windrows.  As the flood waters subsided the new  shoots of h o r s e t a i l were exposed along lake shores and channel hanks and the f i r s t green appeared i n the old clumps of Carex. The muskrats turned t o these species as t h e i r chief sustenance far the remainder of the summer. In summation, i t may be said t h a t the Mackenzie d e l t a muskrats ate whatever was most available i n abundance.  Their  preferences were roughly i n proportion to the representation of the food species i n the lake f l o r a , though no accurate measurements of t h e a v a i l a b i l i t y of plants and their u t i l i z a t i o n by the animals was attempted.  There were no c a t t a i l s or bulrushes i n the  delta f l o r a but the muskrats managed very w e l l by turning to other plants.  They d i d exhibit preference f o r species with which they  were f a m i l i a r .  In an endeavour to induce the animals to enter  the cage traps we t r i e d various baits — and even onions.  apples, carrots, potatoes,  But muskrats d i d not seem t o recognize  these  items as food because they quite nonchalantly sat atop the traps and consumed t h e i r usual meal of Equisetum or Carex. So f a r only casual mention has been made of the amount of  94 winter food whioh was a v a i l a b l e t o support the muskrat population of the Mackenzie d e l t a .  I t was s t a t e d p r e v i o u s l y t h a t the lake  depth i n l a t e w i n t e r averaged s i x f e e t and t h a t up t o t h r e e o r even four f e e t of t h a t might be i c e .  Therefore, the lake had t o  be at l e a s t three and one-half or four f e e t deep over much of i t s area to support any muskrats.  A l l the w i n t e r food l a y i n the un-  frozen portions of t h e l a k e , but i t should not be assumed that a l l of the lake bottom was e q u a l l y productive  of plant food.  The de-  mands of the plants f o r s u n l i g h t and warmth appeared t o be quite critical.  Pond weeds grew l u x u r i a n t l y i n water up t o four or even  s i x f e e t deep, but beyond that depth fewer plants were e s t a b l i s h e d and at twelve f e e t very l i t t l e plant food was able t o grow. This s i t u a t i o n set up f a i r l y r i g i d l i m i t s t o the area of any lake which would support p l a n t l i f e s u i t a b l e for muskrats. Thus, depths from three t o s i x f e e t had t h e greatest value, from six t o twelve f e e t p r o g r e s s i v e l y l e s s , and beyond twelve f e e t very l i t t l e value t o w i n t e r i n g animals.  The i n t e r - r e l a t i o n s h i p of water  l e v e l s and a v a i l a b l e foo d was recognized by Buckley (1954 unpubl.) i n Alaska who wrote "that as ioe increases i n t h i c k n e s s , l e s s and l e s s area i s a v a i l a b l e f o r f o r a g i n g .  Muskrats are f o r c e d t o leave  shallow ponds i n searoh of food, ard i n deeper ponds, a l l feeding is r e s t r i c t e d t o t h e deeper portions of the pond. that i n t e n s i f y the e f f e c t s of t h i c k i c e are:  Other f a c t o r s  that deep ponds and  channels o f t e n have l e s s aquatic vegetation than shallower ponds, and thus supply l e s s food and can support fewer muskrats; and low f a l l water l e v e l s provide l e s s i n i t i a l w i n t e r h a b i t a t which r e s u l t s i n even greater r e s t r i c t i o n of h a b i t a t s as ice formation  continues.  95 As the muskrats are squeezed into the deeper areas, the food supplies are mare r a p i d l y depleted.  Often t h i s may  r e s u l t in  exhaustion of the food supply and subsequent migration, f i g h t i n g or starvation of the muskrats". Another factor which must be cons idered as influencing the food available to muskrats is the very short growing season compared with the long period of ice cover.  In the space of about  three months the submerged plant species must make s u f f i c i e n t growth to provide food for muskrats during the eight months of vegetative dormancy.  It i s true that the pondweeds, and perhaps other  species as w e l l , do produce some.nsw growth before the i c e cover is removed.  However, the production o f green portions of the  plants i s delayed by the s i l t load l e f t i n the lakes by spring floods of early June.  Though i t i s J u l y before most lakes have  cleared, yet the f r u i t s of sago pondweed (Potamogeton pectinatus) have been collected at Study Area #1 the t h i r d week i n August. Summer with i t s continual daylight sponsors such rapid growth but should the season be cold and overcast, as was 194-7> the amount of plant growth can be r e s t r i c t e d s e r i o u s l y . It i s apparent, therefore, that winter food, or the a v a i l a b i l i t y of wint er food, d e f i n i t e l y was area under investigation.  a l i m i t i n g f a c t o r in the  Reports of other authors demonstrate  that food i s less and l e s s a direct l i m i t i n g influence as one proceeds from north to south , u n t i l i n Texas and Louisiana i t was never a problem associated with climatic f a c t o r s .  In the Mackenzie d e l t a  the food resources governed the survival, of animals which were produced and thereby acted as a check upon the number of individuals which could be supported per unit of water area.  96  POPULATION DENSITY I t p r e v i o u s l y has been i n f e r r e d t h a t i n the Mackenzie d e l t a there was a lew d e n s i t y of muskrats i n comparison with areas f u r t h e r t o the south.  Inasmuch as population d e n s i t y  appears t o be an important f a c t o r i n the b i o l o g y o f t h i s animal, i t might be w e l l t o c o n s i d e r i t b r i e f l y before proceeding t o the section concerned w i t h i n t r i n s i c  adjustments.  Too few s t u d i e s of t o t a l populations have appeared i n print to make adequate comparisons of muskrat numbers per unit area.  However, there i s a s a t i s f a c t o r y r e c o r d of the number of  animals harvested from marsh areas o f known s i z e so t h a t an e v a l u a t i o n may be made on t h i s b a s i s (Table 5 ) . I t i s apparent that the highest concentrations of muskrats to be found are those maintained i n the c o a s t a l swamps of L o u i s iana and Texas.  The average harvest o f f u r f o r the whole r e g i o n  would produce between one and s i x animals per a c r e , depending upon the type o f marsh area i n v o l v e d .  However, 23 animals per  acre have been trapped i n p a r t s of Louisiana (Cameron Parish) though such d e n s i t i e s p r e j u d i c e the subsequent maintenance of the population by destroying the h a b i t a t .  Texas has recorded a  per-acre p r o d u c t i v i t y only s l i g h t l y lower t h a n t h a t e x h i b i t e d i n Louisiana. The c o a s t a l marshes of t h e A t l a n t i c States i n h a b i t e d by macrodon were able t o support an annual k i l l o f four t o f i v e a n i mals p e r surface acre though favourable areas produced s i x t o eight or even t e n animals. Some of t h e c o n t i n e n t a l marsh areas ocoupied by z i b e t h i c u s were as productive as the c o a s t a l environment.  Yeager (194-5)  97 TABLE 5 PRODUCTIVITY OE SELECTED AREAS IN MUSKRATS PER ACRE  Author  Region  Muskrats per Acre  Remarks  rivalicius Lay & O ' N e i l l (1942)  Texas coast  1.35  O'Neill(1949)  Louisiana  1.0 5.8  Dozier (195?)  Cameron Parish, La.  23  20 year State average 20 year average best brackish marsh u n u s u a l l y high y i e l d  maorcdon Dozier (195 3)  Maryland  Panooast (1937) New Jersey B a i l e y (1937)  4 - 5  long term average y i e l d high l o c a l populations  6 3.5  a r t i f i c i a l ponds harvest from native marsh  3 - 4  6-8  average y i e l d maximum harvest  5.0  average f o r 1938 only  1.7 2.4 7.1  r i v e r marsh type g l a c i a l marsh drainage d i t c h e s  3.6  lake marsh area  1.0  1943-48 average  1.1  I946-48  2.7 5.5  1945-48 average high y i e l d of 1948  5.2  y i e l d i n 1944  10  Maryland  D o z i e r , Markley, Blackwater and Llewellyne Refuge, Md. H948)  zibethious Yeager (1945)  Illinois  Anderson (1947) Ohio Dozier (1950)  New York  Gashwiler (1948]I Maine Beer and Truax (1950)  Wisconsin  Errington (1946.b)  Central Iowa  average  98  re carded seven muskrats per acre from drainage d i t c h h a b i t a t i n I l l i n o i s and Beer and Truax (1950) counted 5.5  per acre from, the  Haricon Marshes of Wisconsin. Averages over a p e r i o d of time v a r i e d from about one animal per acre i n Maine and New York t o three animals per acre i n p a r t s of Ohio and Wisconsin, i n Iowa E r r i n g t o n (1948) estimated t h a t the best c a t t a i l marsh might have a density of 35 animals f o r each aore though he d i d not suggest the  average number harvested from any such area.  Even h i s f i g u r e  of 5.2 animals per acre from L i t t l e W a l l Lake i n centtral Iowa exceeds the average f o r t h e surrounding S t a t e s . When we come to the more n o r t h e r l y r a c e s of muskrats populations per u n i t area are lower and i t i s more meaningful t o give d e n s i t y i n terms o f acres per muskrat r a t h e r than muskrats per  acre.  I n the d e l t a of t h e Saskatchewan R i v e r an average of  150 thousand muskrats have been removed annually from approximately one m i l l i o n acres of improved marsh area (1940-48).  The  y e a r l y average, t h e r e f o r e , was one muskrat f o r each seven aores. This i s i n accord w i t h d e n s i t i e s recorded by F u l l e r (1951a) f o r the  Athabaska-Peaoe R i v e r d e l t a wherein i t required nearly nine  acres t o produce one muskrat. These d e n s i t i e s are higher t h a n those f o r the A k l a v i k region.  There, over a period of 24 years (1930-53), an average of  195 thousand muskrats was taken from the two and cne-half m i l l i o n acres of d e l t a h a b i t a t , i . e . t h i r t e e n aores per animal. There have been v a r i a t i o n s i n t h i s number which ranged from 42 acres per muskrat  i n 1933-34 t o seven acres i n 1945-46. Mare exact information has been made a v a i l a b l e by the  r e g i s t r a t i o n of d i s c r e t e t r a p p i n g u n i t s i n the Mackenzie d e l t a .  99  In 1949-50 the harvest o f muskrats was 167 low normal.  thousand, s l i g h t l y be-  The number of p e l t s from 3 2 randomly s e l e c t e d r e g i s -  tered t r a p p i n g areas v a r i e d from one per three acres t o one per 29 a c r e s , w i t h an average of one per t h i r t e e n acres. The d e n s i t i e s recorded f o r albus and s p a t u l a t u s are lower than most of those which appear i n Table 5 because i n the three areas under review the d e n s i t y has been computed on a r e g i o n a l basis rather than f o r a p a r t i c u l a r marsh area.  These r e g i o n a l  blocks include a c e r t a i n amount of unproductive country which tends to b r i n g down the average c o n s i d e r a b l y .  Quite s a t i s f a c t o r y den-  s i t i e s of muskrats could be found w i t h i n d i s c r e t e areas of t h e . Mackenzie d e l t a .  I t already has been s t a t e d that Grassy Lake i n  Study Area No. 1 had a f a l l population of 69 muskrats, almost three per surface acre.  And subsequently f i f t y of these animals  were removed during s p r i n g , thereby s u p p l y i n g a harvest of almost two animals per a c r e .  But Grassy Lake was unique i n m a i n t a i n i n g  a high yearlong population o f animals.  Most d e l t a l a k e s had a  much lower capacity t o support muskrats. A review of the information concerning d e n s i t i e s of animals reve als the d e f i c i e n c i e s of the northern environment. :  Even account  ing f o r y e a r l y v a r i a t i o n s i n population l e v e l s , the d i f f e r e n c e between Maryland and A k l a v i k may be of the o r d e r o f s i x t y t o one (4.5 x 13).  L o u i s i a n a showed an even g r e a t e r p r o p o r t i o n a l d i f f e -  rence i n p r o d u c t i v i t y .  100  I n t r i n s i c or B i o l o g i c a l Adjustments SIZE AND GROWTH RATES OF DELTA MUSKRATS A l l e e e t . a l . (1949) stated that "homoiothermal animals from c o l d e r climates tend t o be l a r g e r i n s i z e and hence t o have l e s s surface i n p r o p o r t i o n t o body weight than do t h e i r r e l a t i v e s from warmer regions.  This phenomenon oocurs w i d e l y even though  not u n i v e r s a l l y among b i r d s and mammals and i s u s u a l l y i n t e r p r e t e d i n r e l a t i o n t o heat conservation i n the n o r t h and heat r a d i a t i o n i n the south.  This i s Bergmann's r u l e .  Allen's r u l e i s correlated  w i t h i t and i s conoerned w i t h t h e marked tendenoy toward the l e s s e n i n g of e x t r e m i t i e s i n c o l d e r c l i m a t e s " . The North American hares (Le pus) demonstrate both Bergmann's and A l l e n ' s r u l e s r a t h e r w e l l , and other examples could be g i v e n . But a l l mammal populations do not conform t o these general r u l e s , the muskrat being a good i l l u s t r a t i o n . The a d a p t a b i l i t y of t h e muskrat a l l o w s i t t o l i v e i n a v a r i e t y of h a b i t a t types and as a consequence  i t often i s t h r u s t  i n t o r e l a t i v e l y unfavourable circumstances. I n these undesirable surroundings i t e x h i b i t s greater p r o p o r t i o n a l growth responses than do those species with more p r e c i s e h a b i t a t demands.  The r e -  sponse of muskrats t o t h e i r h a b i t a t was w e l l demonstrated by D o z i e r (1950) on the Montezuma marshes i n c e n t r a l New  York.  Over a period  of s i x years d u r i n g which 29,651 muskrats were handled, he noted average weight. d i f f e r e n c e s of seventeen per cent i n both males and females i n response to v a r i a b i l i t y i n the h a b i t a t they occupied. There can be no doubt that weights w i l l show a greater response t o  101 TABLE 6 A COMPARISON OF WEIGHTS AND STANDARD MEASUREMENTS ft OF MUSKRATS FROM DIFFERENT PARTS OF NORTH AMERICA AUTHORITY and RACE  SAMPLE SEX SIZE  Weight  STANDARD MEASUREMENTS Tail Total Hind Length Foot Length  zibethicus Alexander and Radway (1951) New York  M F  17703 17008  1454 1344  Anderson (194-7) Ohio  M F  652 494  1087 1024  Buss (1941) Wisconsin  M F  352 237  1016 1058  D o z i e r (1950) New York  M F  14785 14866  1474 1361  Grimm and Roberts (1950) Pennsylvania  M F  9 8  1139  H o l l i s t e r (1911)  B ftft  7  Sooter (1946) California  M F  96 106  Anonymous (19 51) West V i r g i n i a  M F  M F  567 547  268 255  80.2 78.6  563  254  81  1170 1068  606 595  297 290  84 83  5 7  1429 1102  593 570  268 255  84 81  99 99  IO69  rival icius Lay (194 5) Texas  1041 •  spatulatus M F u l l e r (1951b) Athabaska D e l t a F  41 40  1132 1053  572 562  256 250  75.6 74.5  B  15  932  531  246  73.5  Slave D e l t a  (Table c o n t i n u e d on the f o l l o w i n g page)  102  TABLE 6 (CONTINUED)  AUTHORITY and RACE  SEX  SAMPLE SIZE  STANDARD MEASUREMENTS Total Tail Hind Weight Length Length Foot •  s p a t u l a t u s (cont'd) Osgood (1900) Yukon  F  1  495  170  73  P r e b l e (1908) Athabaska-Slave Lake Area  B  5  546  264  75  M  1  540  244  80  B  5  544  251  76  M  1  1020  584  260  80  M F  77 81  1065 962  539 533  232 232  76.3 75.4  spatulatus P r e b l e (1908) Franklin.NWT McPherson,  NWT  Soper (1948) Peace R i v e r Stevens (unpubl.) Macke nz ie D e l t a (see Table 7)  ± ±k -  Weights i n grams and measurements i n m i l l i m e t r e s . B r e p r e s e n t s both sexes of a n i m a l s .  103 habitat conditions than w i l l other standard measurements, but animals growing to maturity under unfavourable circumstances can be expected to show a proportional decrease i n a l l s i z e measurements.  Thus, the size of an animal can be not only a  r a c i a l character but a l s o a r e f l e c t i o n of the adequacy of the habitat i t occupies.  In sampling any animal population, therefore,  i t i s necessary to r e l y upon a large enough sample under average conditions to obtain r e l i a b l e me an me asureme nt s for comparison. Too often the size data available s u f f e r from the fact that they have sampled too small a portion of the t o t a l population. Table 6 sets f o r t h the weights and flesh measurements of several races of muskrats from various parts of North America. These data do not include a l l of the muskrats i n North America nor a l l those included i n t h i s survey.  The adequacy of small  samples such as those provided by the e a r l i e r investigators i s open to question.  They are included, however, to support the  more complete data they accompany. From the table i t may be noted that the northwestern muskrat (spatulatus) i s a small race, and that the animals from the Mackenzie d e l t a are the smallest representatives of that race yet studied.  Data f o r animals of the Slave River d e l t a presented  by F u l l e r (1951b) showed them also t o be small.  Only f i f t e e n a n i -  mals were included i n F u l l e r ' s measurements but i f these are r e presentative of the whole population then they are s i g n i f i c a n t l y smaller than the animals i n the Athabaska delta only 200 miles to the south.  Law (1950), who studied the Slave River animals,  suggested that they were occupying a habitat which was ating beoause of excessive s i l t  deterior-  deposition i n the lakes and waterways.  TABLE 7 WEIGHTS AND MEASUREMENTS OF A SAMPLE OF MUSKRATS KNOWN TO BE ADULT, MACKENZIE DELTA, 1947-49 ft  Weight  Total Length  Length of Tail  Length of Hind Foot  ADULT MALE Average  IO65.3  Number  119  77  77  Standard Deviation  137.8  19.4  19.0  Range  539.4  (740-1580) (489-589)  232.4  (208-254)  76.3 77  I.65 (72-80)  ADULT FEMALE Average  962.0  532.9  Number  124  8l  83  Standard Deviation  127.7  23.4  14.8  Range  ft -  (725-1395)  (466-575)  231.9  (198-270)  75.4 84 2.2 (71-79)  Weights i n grams, measurements i n m i l l i m e t r e s .  Measurements for the Mackenzie delta muskrats appear i n Table 7 as w e l l as i n Table 6.  The former presents the v a r i a b i l i t y  in s i z e and weights which can be expeoted i n adult animals of the Aklavik area.  On the strength of these data i t can be asserted  that there were no constant differences i n weight or standard measurements which w i l l d i f f e r e n t i a t e adult male from adult female animals.  Though d i f f e r e n t i a t i o n of adults from sub-adults could be  105  done on t h e b a s i s of w e i g h t s and measurements up u n t i l about t h e f i r s t p a r t o f A p r i l , t h e r e a f t e r o n l y t a g g e d animals or t h o s e accorded post-mortem examination c o u l d be p l a c e d i n t h e  A l l t h o s e animals i n c l u d e d i n T a b l e 7 were  p r i a t e age c l a s s . known t o be Two  appro-  adult. l i n e s of r e a s o n i n g may be f o l l o w e d t o account f o r t h e  s m a l l e r s i z e of muskrats i n the Mackenzie d e l t a a r e a . of t h e s e would  The  first  i n f e r t h a t a s m a l l race o r i g i n a l l y c o l o n i z e d the  n o r t h e r n a r e a s as t h e y were r e l e a s e d f r o m P l e i s t o c e n e  glaciation.  H a v i n g a t t a i n e d t h e Mackenzie d e l t a a r e a t h e y were s h i e l d e d by t h e i r a q u a t i c h a b i t a t from t h e e v o l u t i o n a r y p r e s s u r e e x e r t e d by lower temperatures.  T h e r e f o r e , they remained  s m a l l and no doubt  w i l l c o n t i n u e t o be s o . A l t e r n a t i v e l y , the i n v a d i n g a n i m a l s from more s o u t h e r n a r e a s f o l l o w e d t h e n o r t h e r n d r a i n a g e and a r r i v e d a t the  Mackenzie  d e l t a when c o n d i t i o n s were e v e n l e s s f a v o u r a b l e t h a n now.  Because  the h a b i t a t t h e y e n c o u n t e r e d was n o t adequate t o meet t h e i r demands t h e y never were a l l o w e d t o e x e r t t h e i r f u l l p o t e n t i a l i t y f o r g r o w t h , and over a p e r i o d of time e i t h e r d e t e r i o r a t e d i n s i z e o r never grew to f u l l s i z e .  They p e r s i s t i n t h a t o o n d i t i o n even now.  A t the  p r e s e n t t i m e a s e v e r e w i n t e r can e x e r t a d e l e t e r i o u s e f f e c t  on  t h e whole p o p u l a t i o n , e s p e c i a l l y thos e animals i n the l o w e r  one-  t h i r d of t h e d e l t a w h i c h l i e s beyond t h e t i m b e r l i n e .  As a c o n -  sequence t h e s i z e of many a n i m a l s i s s m a l l and t h e p e l t s so l i g h t t h a t t h e y command a low p r i c e i n the f u r m a r k e t . As a f u r t h e r i l l u s t r a t i o n of the e f f e c t s of  environment  c o n s i d e r t h e p e l t s of a n i m a l s f r o m the t u n d r a l a k e s e a s t of t h e Mackenzie R i v e r . the d e l t a proper.  I n most y e a r s , they, a r e s m a l l e r t h a n t h o s e from  *  106 i  It i s the writer's opinion that this second a l t e r n a t i v e comes nearer to explaining the reason f o r the small muskrats l i v i n g i n the lower reaches of the Mackenzie River.  The muskrat,  l i k e the raccoon (Prooyon) , i s an animal which had invaded less favourable habitat i n extending i t s range northward and persists therein as a smaller animal.  In the Mackenzie delta the a v a i l -  a b i l i t y o f winter food as governed by a rigorous climate i s considered to be the f a o t o r responsible f o r the small s i z e and r e duced growth rates encountered. TABLE 8 AVERAGE DAILY WEIGHT GAINS OF MUSKRATS IN THE MACKENZIE DELTA AND IN THREE OTHER AREAS. (ALL WEIGHTS IN GRAMS) JUVENILES  AUTHORITY  Male s Snead (1950 unpubl.)  Aldous (1947)  Errington (1944)  St evens (unpubl.)  ADULTS  Fern ales  116 5.65  No. Gain/day  137 5.86  No. Gain/day  6 4.52  No. Gain/day  6 4.63  8 4.10  No. Gain/day  10 1.97  10 2.25  Males Females  62 0.34  3  0.24  65 0.57  4 0.19  At the beginning of the investigation i n the Mackenzie delta i t was surmised that the shorter growing season and longer day length for feeding might acoelerate growth rates of young muskrats i n comparison with mare southerly races.  An opportunity for checking the  107  v a l i d i t y of t h i s a s s u m p t i o n was  provided  by growth measurements  of tagged animals t h a t s u b s e q u e n t l y were r e c a p t u r e d .  These d a t a  are not c o n c l u s i v e because i n s t a n c e s o f r e c a p t u r e w e r e t o o  few,  but t h e y are  period.  i n d i c a t i v e of growth a o h i e v e d over a v a r i a b l e  T a b l e 8 p r e s e n t s t h i s i n f o r m a t i o n t o g e t h e r w i t h data g l e a n e d from o t h e r authors who conditions.  have s t u d i e d muskrat p o p u l a t i o n s under f i e l d  As t h e  sadly neglected  t a b l e d e m o n s t r a t e s , t h i s phase has been r a t h e r  i n past s t u d i e s , though the d e f i c i e n c y stems i n  p a r t from t h e d i f f i c u l t y o f o b t a i n i n g r e c a p t u r e d measurement.  specimens f o r  E r r i n g t o n (1939a) c o m p i l e d growth c u r v e s f o r n e s t -  l i n g muskrats up t o t h i r t y days of age, but come independent of t h e  once t h e  animals  p a r e n t a l c a r e and the heme n e s t , he  found i t v e r y d i f f i c u l t t o w e i g h them p e r i o d i c a l l y .  betoo  Measurements  of growth under l a b o r a t o r y c o n d i t i o n s would g i v e more complete d a t a , t h o u g h the r e s u l t s might be The but  atypical.  growth r a t e s g i v e n r e p r e s e n t  n o t a t r u e growth c u r v e  o n l y an amount of growth over a p e r i o d o f time.  are s e g r e g a t e d a s t o t h e s e x and  age of t h e animals concerned  show o n l y the average f o r t h e sample. t h e i n f o r m a t i o n f r o m the  These d a t a  They a c c o r d t h e r e b y  l i t e r a t u r e but  give l i t t l e  and  with  indication  t h e r a t e o f growth d u r i n g any p a r t i c u l a r p e r i o d of an a n i m a l ' s The was  amount of growth of t h e muskrats i n the d e l t a  e r n animals was l o s t 125  apt t o be  G a i n i n weight of a d u l t s i n the  zero or e v e n n e g a t i v e ; one  grams between August and A p r i l .  life.  population  low when compared w i t h d a t a f r o m Iowa (Sneed and E r r i n g t o n )  f r o m South Dakota ( A l d o u s ) .  of  and  north-  a d u l t female  Rates o f g a i n f o r young  animals were l a r g e s t f o r t h e s h o r t e r time i n t e r v a l s .  Three  108 j u v e n i l e f e m a l e s t a g g e d i n August and c a p t u r e d  f o r t y days l a t e r  had g a i n e d weight a t the r a t e o f between s e v e n and e i g h t per day.  Most of the o t h e r animals r e p r e s e n t e d  were t a g g e d i n August and not r e c a p t u r e d s p r i n g (about 250  days).  i n the t a b l e  u n t i l the  following  D u r i n g t h i s p e r i o d t h e r a t e o f growth  natur a l l y would decrease as a d u l t s i z e was c e l e r a t i o n no doubt was  grams  a s s i s t e d by t h e  approached.  This  de-  sudden appearanoe of i c e  cover and the a d j u s t m e n t s necessary f o r l i f e beneath i t ,  adjust-  ments not o n l y i n f e e d i n g h a b i t s but a l s o i n a v a i l a b l e f o o d . Whatever t h e r e a s o n , the muskrats o f the M a c k e n z i e d e l t a d i d not appear t o grow as r a p i d l y as t h o s e s t u d i e d by gators  i n the mid-Western U n i t e d S t a t e s .  investi-  In the f i r s t place  average of i n d i v i d u a l s o f t h i s p o p u l a t i o n was  s m a l l e r (Table  the 7),  so t h a t t h e animals d i d n o t have t o grow a s r a p i d l y t o a t t a i n t h e i r terminal weight.  I n t h e second p l a c e , t h e i r w i n t e r  habits  and d i e t were s u f f i c i e n t l y r e s t r i c t e d t h a t t h e y c o u l d not be  ex-  pected t o a t t a i n t h e r a t e o f growth shown by t h e i r more s o u t h e r l y relatives.  And  f i n a l ly,..the. muskrats o f the Mackenzie by m a t u r i n g  l a t e r i n t h e y e a r c o u l d grow f o r a l o n g e r p e r i o d w i t h o u t  the i n -  h i b i t i n g i n f l u e n c e of sexual a c t i v i t y . Too few d a t a are a v a i l a b l e i n t h i s or any o t h e r from w h i c h t o draw v a l i d c o n c l u s i o n s of m u s k r a t s .  c o n c e r n i n g r a t e s of growth  F u r t h e r i n v e s t i g a t i o n s w i l l be r e q u i r e d b e f o r e a  comparison o f growth r a t e s i n one a r e a another.  One  study  can be made w i t h those i n  u l t i m a t e o b j e c t i v e of t h e s e s t u d i e s would be  to  measure t h e e f f e c t of h a b i t a t c o n d i t i o n s upon those a t t r i b u t e s normally  used by s y s t e m a t i s t s i n d i f f e r e n t i a t i n g s p e c i f i c and  s p e c i f i c rank.  sub-  109 LONGEVITY OF NORTHERN MUSKRATS There i s l i t t l e i n f o r m a t i o n the adult l i f e of m u s k r a t s .  concerning  the  J o h n s o n (1925) was  l e n g t h of  "not  aware o f  any i n s t a n c e s where muskrats have been kept i n c a p t i v i t y l o n g enough t o f u r n i s h any d a t a o f value i n t h i s c o n n e c t i o n " ,  but  thought i t p r o b a b l e t h a t a muskrat might l i v e a t l e a s t t e n  years.  But t h e l e n g t h of time w h i c h the animals can l i v e i n c a p t i v i t y p r o b a b l y b e a r s l i t t l e r e l a t i o n s h i p t o the time they can s u r v i v e under n a t u r a l c o n d i t i o n s .  Gould and K r e e g e r (194 8) s t a t e d t h a t  "the l e n g t h of t h e a d u l t l i f e i s unknown". c a l c u l a t e d two animals t o be 5&1  and  552  E r r i n g t o n (1944)  days of age when r e c a p -  t u r e d , though most o f h i s r e c o r d s were f o r s h o r t e r p e r i o d s time.  of  There i s a r e c o r d of one a n i m a l r e l e a s e d i n L o u i s i a n a as  a young a d u l t b e i n g caught t h r e e y e a r s l a t e r . A few d a t a are a v a i l a b l e f r o m t h e M a c k e n z i e d e l t a w h i c h g i v e some i n d i c a t i o n of t h e l e n g t h of l i f e i n those l a t i t u d e s . markably short l i f e old  age  of t h e muskrats  animals t h e r e had a r e -  span, w h e t h e r they were e l i m i n a t e d by v i o l e n c e ,  or o t h e r f a c t o r s . I t was  covered i n the writer.  They s u g g e s t t h a t the  study  The  b e l i e v e d t h a t most cf the t a g s from animals r e y e a r l y c a t c h o f t r a p p e r s were r e t u r n e d t o t h e  n o v e l t y of f i n d i n g a tagged a n i m a l was  t h a t t h e I n d i a n or Eskimo t r a p p e r was in reporting i t .  sufficient  e s p e c i a l l y conscientious  White t r a p p e r s a l s o c o - o p e r a t e d f u l l y and  even  went t o the a d d i t i o n a l e f f o r t of w e i g h i n g t a g g e d animals t h a t t h e y recovered. The ages o f 169  r e c a p t u r e d a n i m a l s were c a l c u l a t e d by  assuming t h a t a l l animals were b o r n on 15th  June of any y e a r ,  and  110  TABLE 9 CALCULATED AGES OF RECAPTURED MUSERATS GROUPED INTO FIFTY-DAY AGE CLASSES.  Frequency of o c c u r r e n c e Female Male  Age Group (days)  Tot a l  150-200  6  3  9  200-250  2  1  3  250-300  15  12  27  300-350  42  27  69  350-400  7  6  13  400-450  0  0  0  450-500  0  0  0  500-550  0  2  2  550-600  1  2  3  600-650  8  6  14  650-700  8  12  20  700-750  4  4  8  750-800  1  . 0 •  1  Totals:  94  75  169  Ill  that the animals tagged as adult d u r i n g summer had been born the previous summer and were, t h e r e f o r e , j u s t a year o l d . Using these c r i t e r i a , one animal recaptured was assumed to be 776 days o l d and may have l i v e d longer f o l l o w i n g i t s r e l e a s e from the cage t r a p . The ages of eight other animals were c a l c u l a t e d to be over 700 days but the remaining 16 0 were of l e s s e r age (Table 9 ) .  The  last  muskrat was tagged 15th September, 1949, and the l a s t t a g was r e turned i n June, 1950.  Thereafter, i n s p i t e of continued contact  w i t h the t r a p p e r s , no more tagged muskrats were reported and i t i s assumed t h a t no more were found. I t seemed s i g n i f i c a n t that so few animals were recaptured during Hie second  simmer a f t e r they had been tagged, and of those  nine which were, none had been tagged as an a d u l t .  I n l a k e s which  had been given intensive a t t e n t i o n , and i n which large numbers of tagged animals had been r e l e a s e d , seme animals were recovered the first  succeeding summer but almost ncne the second.  I t may  be  assumed e i t h e r that the animals had wandered away, or t h a t t h e y were no longer l i v i n g .  I f they had made t h e i r way to a d i f f e r e n t  lake t h e y had almost as much chance t o be taken there as at the tagging s i t e .  A l t e r n a t i v e l y , t h e r e f o r e , the animals must have  had a v e r y b r i e f l i f e span w i t h the r e s u l t that most had succumbed to l e t h a l f a c t o r s by the end of t h e i r second year of l i f e  (about  730 days). BREEDING POTENTIAL. AND BREEDING HABITS The maintenance of a population i n space and time i s dependent upon the a b i l i t y of the animals i n v o l v e d to reproduce t h e i r numbers at a s u f f i c i e n t rate to overcome the depressant  112  f a c t o r s o f nature.  I n a long term view t h e g r e a t e r the accu-  m u l a t i v e m o r t a l i t y t h e g r e a t e r must be t h e r e p r o d u c t i v e p o t e n t i a l o f the s p e c i e s .  C o n v e r s e l y , a l o w e r m o r t a l i t y u s u a l l y w i l l be r e -  f l e c t e d i n a decreased r a t e o f r e p r o d u c t i o n .  The b r e e d i n g  habits  ard b r e e d i n g r a t e o f a s p e c i e s o r p o p u l a t i o n s h o u l d , t h e r e f o r e , r e f l e c t i n some measure t h e " e n v i r o n m e n t a l upon i t .  resistance" acting  Few p o p u l a t i o n s , however, a r e i n b a l a n c e w i t h t h e i r en-  vironment over any p e r i o d o f t i m e .  The o r g a n i c system w h i c h  pro-  duces a d d i t i o n a l numbers of i n d i v i d u a l s i s s e t over a g a i n s t t h a t complex o f f a c t o r s w h i c h r e d u c e these numbers. balance  The r e s u l t i n g  f l u c t u a t e s about a mean w h i c h i s a r e l a t i v e l y s t a b l e  though l a r g e l y t h e o r e t i c a l f i g u r e . T h i s s e c t i o n w i l l present rat  the v i t a l  p o t e n t i a l of t h e musk-  p o p u l a t i o n found w i t h i n t h e Mackenzie d e l t a and endeavour t o  compare t h i s p o t e n t i a l f o r p o p u l a t i o n i n c r e a s e w i t h t h a t found i n other areas of North America. As was p o i n t e d out under t h e s e c t i o n on c l i m a t e , t h e i c e f r e e p e r i o d i n t h e M a c k e n z i e d e l t a l a s t s o n l y f r o m e a r l y June u n t i l l a t e September.  During t h i s p e r i o d t h e m u s k r a t s must b r e e d ,  r a i s e t h e i r young, and s e t t l e t h e r e a f t e r i n l o c a t i o n s adequate for  t h e i r winter requirements.  By the time t h e animals  are con-  f i n e d beneath the i c e , b r e e d i n g a c t i v i t y long s i n c e has d e c l i n e d and t h e r e p r o d u c t i v e organs have r e g r e s s e d t o a q u i e s c e n t s t a t e . The  Oestrus C y c l e I t i s of i n t e r e s t t o compare t h e l e n g t h o f t h e b r e e d i n g  p e r i o d o f m u s k r a t s throughout t h e i r range i n North A m e r i c a .  For  the t i d e - w a t e r marshes of t h e Gulf c o a s t of Texas and L o u i s i a n a ,  113  Lay (1945) discovered pregnancies yearlong with the highest i n cidence of b i r t h s i n November and March, and the lowest i n January and i n mid-summer.  Because of the favourable conditions and  the long period of growth seme animals were said t o breed during the year of t h e i r b i r t h . In the eastern coastal marshes of Maryland and Delaware investigators recorded a d e f i n i t e anoestral period.  Forbes (1942)  found that there were some corpora lutea i n the ovaries of female muskrats before the middle of February.  These bodies, however,  were mare common after the end of that month and were found thereafter u n t i l the l a s t of October. almost yearlong, but was  Spermatogenesis i n males lasted  most common from early January to October.  Smith (1938) considered that the peak of the breeding season was between mid-April and mid-September. Inland, in the same l a t i t u d e s , the period of oestrus was found to be shorter.  In Iowa, E r r i n g t o n (1937) encountered l i t t e r s  born from mid-April to late August and was that the main breeding season in Iowa was  of the opinion (1947) three to f i v e weeks l a t e r  than that reported by Smith (op. c i t . ) for Maryland.  In northern  Utah, Marshall (1937) reported l i t t e r s during the last ten days of March, ani Dalquest  (1948)  ary i n Washington.  Aldous (1947) noted that i n South Dakota musk-  recorded l i t t e r s as e a r l y as late  Febru-  rats were breeding during the f i r s t two weeks of A p r i l but gave no more information on how  early t h i s a c t i v i t y began.  Adequate information for Wisconsin was given by Beer  (1950).  He indicated that the capacity of the male for breeding was r e s t r i c t e d to a period s t a r t i n g i n March and ending in August.  Ovu-  l a t i o n in females began about the f i r s t of A p r i l and had ceased  by  MoLeod (194 8) examined 28 l i t t e r s  the  f i r s t part of July.  of  the  Hudson Bay muskrat i n Manitoba and found that the f i r s t  litters  arrived from May 2°th to June 17th. Data concerning the habits of the northwestern muskrat (spatulatus) are scarce indeed i n l i t e r a t u r e .  Law (1950) found  a pregnant female with seven embryos i n late May Park, Alberta.  i n Wood Buffalo  He suggested that "the f i r s t l i t t e r i s probably  bora during late June, the second l i t t e r i n J u l y " . working i n the Mackenzie delta was  Cowan (1948)  of the opinion that f i r s t  l i t t e r s arrived i n early June, w i t h a second, at least f o r some females, i n August. Marsh (1948) suggested a spring and a f a l l l i t t e r but was no more e x p l i c i t than that i n his observations. P o r s i l d (1945) quoted K. H. Lang of Aklavik as saying that during average years the  animals r a i s e d only one l i t t e r , but during favourable years,  perhaps  two. Inasmuch as the physiological condition of the animals i s  important i n a s s i s t i n g us to understand t h e i r habits as Beer and Meyer (1951) have point out, a l l animals taken during t h i s study were examined t o assess endocrine levels as r e f l e c t e d i n the cond i t i o n of the reproductive organs.  Several methods were employed  to ascertain the breeding condition of animals. both macroscopic examination under f i e l d  These Involved  conditions and, where  possible, subsequent laboratory examination.  I t was not always  f e a s i b l e to carry out the second mare accurate a n a l y s i s but r e l i a b l e information nevertheless could be obtained i n the  field.  Animals were c o l l e c t e d at intervals throughout the year i n order t o assess the condition of the reproductive organs.  115  A f t e r t h e y e a r l y o p e n i n g o f t h e t r a p p i n g season on M a r c h 1 c a r casses were a v a i l a b l e i n q u a n t i t y .  Field  examination c o n s i s t e d  i n measurements o f volume o f t e s t e s and a c c e s s o r y organs i n t h e male, and i n r e c o r d i n g t h e growth of f o l l i c l e s and c o r p o r a l u t e a , s i z e and v a s c u l a r i z a t i o n o f t h e u t e r u s , p r e s e n c e o f v a g i n a l membrane o r v a g i n a l p l u g , and c o n d i t i o n o f t h e mammary g l a n d s i n t h e female.  U t e r i n e s c a r s m a r k i n g past s i t e s o f i m p l a n t a t i o n were  c r i t e r i a f o r d i f f e r e n t i a t i n g m u l t i p a r o u s f r o m n u l l i p a r o u s animals and f o r e s t i m a t i n g numbers o f young b o r n e .  These p l a c e n t a l  sites  p e r s i s t e d i n t h e q u i e s c e n t u t e r u s f o r s e v e r a l months p a s t t h e end of t h e b r e e d i n g season. Male muskrats i n t h e M a c k e n z i e d e l t a e x h i b i t e d s e a s o n a l development o f s e x u a l organs a t an e a r l i e r d a t e t h a n d i d t h e f e males. F i g u r e 16 r e p r e s e n t s g r a p h i c a l l y t h e development o f t h e t e s t e s and concomitant growth of a c c e s s o r y o r g a n s , t h e s e m i n a l v e s i c l e s and t h e musk g l a n d s , of a s e r i e s of a n i m a l s c o l l e c t e d throughout t h e y e a r .  The onset o f t h e b r e e d i n g c o n d i t i o n may be  deduced from t h e s e d a t a but t h e d a t e f o r l o s s o f f e c u n d i t y i s less well defined. The e a r l i e s t date on w h i c h spermatozoa appeared i n t h e t e s t e s a n d e p i d i d y m i d e s o f male m u s k r a t s was 23rd March ( 1 9 4 7 ) . A n i m a l s t a k e n i n A p r i l were i n m o s t c a s e s found t o have some s t o r e d spermatozoa though smears t a k e n o f t e n were s c a n t y . mid-May spermatozoa were abundant  By  and remained so u n t i l r e c e s s i o n  o f t h e r e p r o d u c t i v e organs began i n J u l y . The volume o f t e s t e s and a s s o c i a t e d organs d e c r e a s e d s t e a d i l y f r o m the h i g h i n May and June.  By August t h e y were about  116  -3.0 ml.  t/3  1000 mm.  B-i  53 W 2 W  a  r-*2.0 300  "6bo W  < K  W f>  Li.o 400  O Volume right seminal vesicle © Volume right testis * Measurement right musk gland (length X width) Apr. May Jun. I  M 0 N TH FIGURE 16.  Jly. !_  MONTHLY AVERAGE MEASUREMENTS OF MALE MUSKRAT REPRODUCTIVE ORGANS IN THE MACKENZIE DELTA DURING THE PERIOD 1947-50. (ALL MEASUREMENTS IN MILLIMETRES & MILLILlTRES)  Aug  117 h a l f their former volume, and by September had returned to a quiescent state.  Beer and Meyer (op. c i t . )  stated that "the  adult males seem to come into f u l l breeding condition about a month e a r l i e r than the f i r s t - y e a r males.  The increase i n s i z e  of the adult testes i s also much more rapid than that o f the young animals".  Such an assertion could also be made f o r the  Mackenzie muskrats.  This d i f f e r e n t i a l attainment o f sexual matur-  i t y was, however, mare prominent  i n the females studied.  The onset of the reproductive cycle was r e l a t i v e l y more sudden i n female muskrats than i n males.  About mid-April the  uterus, which i n i t s dormant state was t h i n and translucent, began t o show an increased v a s c u l a r i z a t i o n .  There was also a notice-  able increase i n the diameter of the blood vessels over the surface of the ovary so that i t began to assume a pinkish colour rather than the dead white hue i t had presented a l l winter.  Meanwhile  the growth of f a t bodies surrounding the ovary had been progressing u n t i l by the time of the ripening of the f o l l i c l e s , the ovary was p a r t i a l l y embedded. In A p r i l and May adult females developed more r a p i d l y t o ward sexual maturity than d i d the younger females.  This f i e l d ob-  servation was deduced from the examination of tagged animals taken in A p r i l , May and June.  In a l l cases these animals were of known  ages and had been tagged during the previous summer and early winter. It i s d i f f i c u l t to give quantitative data concerning the various q u a l i t a t i v e changes occurring at the onset of reproductive cycles. Such indications as thickening and vascularization of the uterus, ripening of the f o l l i c l e s , and growth of the f a t bodies, while cert a i n l y diagnostic, are changes i n degree and not i n number.  118  FIGURE 17.  REGRESSION LINES INDICATING RATE OF INCREASE IN UTERINE WIDTH FOR ADULT AND SUBADULT MUSKRATS DURING THE PERIOD MARCH TO JUNE.  119 I n s t e a d , a measurement of u t e r i n e w i d t h 1 cm. above t h e c e r v i x was used a s a c r i t e r i o n o f u t e r i n e growth, assuming t h a t development of t h e o v a r i e s and a c c e s s o r y organs would be c o n comitant w i t h t h e growth of the u t e r u s . I t was f o u n d i n t h e r e s t i n g u t e r u s t h a t t h e w i d t h of t h e u t e r i n e horns a t 1 cm. above t h e c e r v i x was  2.5 t o 3 mm.  These  w i d t h s were a t t a i n e d by September and p e r s i s t e d d u r i n g most o f the a n o e s t r u s p e r i o d .  By the l a s t week i n March a few a d u l t f e -  males were b e g i n n i n g t o show an i n c r e a s e i n w i d t h o f u t e r i n e  horns  t o g e t h e r w i t h i n c r e a s i n g v a s c u l a r i z a t i o n of t h e u t e r u s and o v a r i e s and an i n c r e a s e i n the f a t bodies a t t a c h e d to the o v a r i e s . f a t b o d i e s , however, were e x t r e m e l y v a r i a b l e i n s i z e and  These  probably  r e f l e c t e d g e n e r a l n u t r i t i o n a l as w e l l a s e n d o c r i n e l e v e l s .  Figure  17 i n d i c a t e s t h a t up u n t i l t h e middle of June t h e u t e r i n e horns of a d u l t animals had d e v e l o p e d more r a p i d l y t h a n had those of subadults.  T h e r e i n the d a t a f o r t h e l e a s t squares r e g r e s s i o n l i n e s  are p l o t t e d from t h o s e p o i n t s a p p e a r i n g w i t h i n the square.  Those  d a t a appearing above the square r e p r e s e n t u t e r i n e w i d t h s of p r e g nant females and t h o s e t o t h e r i g h t , w i d t h s of u t e r i r e g r e s s i n g t o wards q u i e s c e n c e .  I t i s s i g n i f i c a n t t h a t among t h e younger animals  l i t t l e development had t a k e n p l a c e by t h e end o f A p r i l but t h e r e a f t e r t h e a t t a i n m e n t o f s e x u a l m a t u r i t y was  so r a p i d t h a t by mid-  June some of t h e f e m a l e s were c a r r y i n g young. D u r i n g t h e s p r i n g s of 1948 and 1949 examine a t o t a l of 674 not t h e y had been b r e d .  i t was  possible to  female muskrats and t o a s c e r t a i n whether or I n each i n s t a n c e shown i n Table 10 the  animals examined had b e e n k i l l e d w i t h i n the two-weeks p e r i o d p r i o r t o 12th June, the end of l e g a l h u n t i n g i n the M a c k e n z i e  delta.  The  120  animals were a l l t a k e n i n t h e v i c i n i t y o f A k l a v i k . TABLE 1 0 BREEDING CONDITION OE FEMALE MUSKRATS COLLECTED I N SPRING ANIMALS BRED V a g i n a l Implant- N e i t h e r plug at i o n s  NOT BRED  REMARKS  0  1  2J  7  A n i m a l s shot 4 t o 7 June 1949 Boxer's Area  7  16  37  106  Animals s h o t 2 9 May t o 9 June 1 9 4 9 , Boxer's Area  14  11  44  138  Animals s h o t 29 May t o 1 2 June 1949, Lang's A r e a  0  0  26  27  Animals shot 1 t o 1 0 J u n e , 1 9 4 8 B o x e r ' s Area  0  0  13  33  A n i m a l s shot 2 2 - 2 7 May, 1 9 4 8 Wiedemann's A r e a  0  0  6  40  A n i m a l s s h o t 2 3 t o 31 May, 1 9 4 8 Lang's A r e a  0  8  23  63  A n i m a l s shot 1 t o 8 June, 1 9 4 8 Lang's A r e a  0  9  4  18  A n i m a l s shot 9 J u n e , 1 9 4 8 Lang's A r e a  21  45  176  432  Totals  I t w i l l be noted t h a t animals w h i c h had been b r e d were d i v i d e d i n t o t h r e e c l a s s e s , t h o s e w i t h a v a g i n a l p l u g , t h o s e showing  i m p l a n t e d f o e t u s e s , and t h o s e bred b u t shewing n e i t h e r .  l a s t c a t e g o r y r e q u i r e s some  This  clarification.  I n the female m u s k r a t , as i n many o t h e r r o d e n t s , a v a g i n a l p l u g i s formed f o l l o w i n g c o p u l a t i o n by c o a g u l a t i o n o f t h e s e m i n a l  121  fluid.  Because t h e  p l u g i s p e r s i s t e n t and  c l o s e l y applied to  the.rugae o f the v a g i n a , i t l o c k s s e m i n a l and o t h e r f l u i d s i n the u t e r u s .  The r e s u l t  i s t h a t the accumulated f l u i d s s t r e t c h  the w a l l s of t h e u t e r i n e horns so t h a t e v e n a f t e r t h e p l u g  has  d i s i n t e g r a t e d and t h e u t e r u s d r a i n e d , the g e n e r a l shape of  the  organ remains somewhat a s i n i t s t u r g i d s t a t e . n o s t i c was  the  apex o f each u t e r i n e h o r n .  Especially diag-  I n the  quiescent  s t a t e the apex o f e a c h h o r n e x h i b i t e d a r a t h e r i n d e f i n i t e j u n c t ure w i t h t h e E a l l o p i a n t u b u l e s .  A f t e r f l u i d s had  u t e r u s , a d e f i n i t e pouch or b u r s a was  s t r e t c h e d the  formed at t h e apex o f each  h o r n d i s t a l t o t h e e n t r y of the t u b u l e s .  This  characteristic,  t o g e t h e r w i t h t h e o b v i o u s t h i n n i n g o f the u t e r i n e w a l l s , c o u l d be s e e n w i t h f a c i l i t y and b o t h were c o n s i d e r e d d i a g n o s t i c of successful breeding having  occurred.  D u r i n g t h e l a s t two weeks of the h u n t i n g season i n the Mackenzie d e l t a (May 674  21 - June 7 ) ,  o n l y 24-2 o r 36 p e r c e n t  female m u s k r a t s examined had been b r e d .  of  I f t h i s sample  was  r e p r e s e n t a t i v e of the whole p o p u l a t i o n , 64 p e r cent o f  litters  could not be b o r n p r i o r t o t h e p e r i o d 20th June t o 7th  July  (assuming a g e s t a t i o n of 25  days).  Of t h e number w h i c h had been b r e d , o n l y 45  19  or about  per c e n t (6 per c e n t of t h e t o t a l sample) e x h i b i t e d d e f i n i t e plantations.  The o t h e r 197  im-  had been bred r e c e n t l y enough t h a t  i m p l a n t a t i o n s i t e s were e v i d e n t from g r o s s a n a l y s i s .  Erom t h e s e  d a t a i t i s p o s s i b l e t o e s t i m a t e t h a t b r e e d i n g a c t i v i t y was g i n n i n g d u r i n g the l a s t week i n May  no  be-  and became g e n e r a l d u r i n g t h e  f i r s t week i n J u n e , assuming t h a t t h e  implanted  dent w i t h i n t e n t o f o u r t e e n days a f t e r b r e e d i n g .  ova would be This period  evi-  122  coincided very w e l l with the usual dates of inundation of the delta by spring floods and the rapid disappearance cover from most delta lakes.  of the i c e  The flooding of densites, the i n -  crease i n water temperature, and the general movement of animals into emergency s h e l t e r a l l helped to e l i c i t a general unrest among the population at a time which coincided with the commencement of sexual a c t i v i t y . If the data i n Table 10 are representative, i t i s suggested t h a t the females which showed e a r l y implantations were adults. They had arrived at breeding condition sooner than sub-adult f e males and had been bred before extensive areas of open water appeared around  ttoe  lake edges.  A review of available information indicates that the r e productive period of the muskrat is of shorter duration as one proceeds northward.  In the Texas and Louisiana marshes young  are born during a l l months of the year.  In the coastal areas  of Maryland and Delaware there i s a long breeding period but an ancestral s t a t e i s encountered.  Nearby i n New  York the period  of fecundity i s again shortened and so on westward and northward u n t i l i n the Mackenzie delta reproductive a c t i v i t y may  be con-  densed into a period of as l i t t l e as seven weeks. The f a c t o r s i n the environment which govern the onset of reproductive a c t i v i t y have not been demonstrated i n f u l l though many investigators have given t h e i r attention to the problem. Bullough (1951) has summarized the information available to that date concerning the sexual cycles of vertebrates.  He noted that  there was an ultimate cause for these cycles, that of having the young produced at the time of year most favourable to the species.  123  There were i n addition three groups of proximate causes. f i r s t of these was i n question. cycle was  the i n t e r n a l reproductive  The second was  The  rhythm of the  species  the external variant t o which the  attuned, and the t h i r d was  that group of factors i n  the immediate neighbourhood of the i n d i v i d u a l (the breeding area, the mate, the s o c i a l group, and  so f o r t h ) .  It may  be noted that  t h i s t h i r d group i s l a r g e l y of a psychological nature and is necessary to trigger f u l l reproductive a c t i v i t y .  often  In the Mac-  kenzie delta t h i s complex of f a c t o r s reached i t s summation at the time of the spring floods. Light and temperature have been assigned major r o l e s as factors regulating sexual cycles, and other have been investigated also.  For a r c t i c mammals, however, the influence of light  and  temperature i s r e l a t i v e l y unknown, even though the v a r i a t i o n i n both i s extreme.  It probably can be stated a p r i o r i that temper-  ature i n an aquatic habitat, such as that frequented  by muskrats,  w i l l have less p h y s i o l o g i c a l e f f e c t than w i l l light because the v a r i a t i o n throughout the year w i l l not be great.  Light, on the  other hand, can range from the continuous daylight of midsummer to the continuous t w i l i g h t or darkness of winter and i t s influence w i l l be considered  hereafter.  It was established by Rowan (1925) that l i g h t i s a causal 'factor i n the sexual development and migration of b i r d s .  He  as-  cribed the sexual development of juncos exposed to an increasing l i g h t regimen t o the opportunity f o r more exercise under those conditions.  Bissonnette was  able t o eliminate exercise as a oausal  factor and established that l i g h t was  responsible per se.  work (1938b) with f e r r e t s indicated that this e f f e c t was  His transmitted  12^  through t h e o p t i c nerve t o the  a n t e r i o r l o b e of t h e  pituitary.  Hypophysectomy a b o l i s h e d s e x u a l c y c l e s though b l i n d n e s s o n l y r e moved them from p h o t i c c o n t r o l .  M a r s h a l l (1940) a l s o worked w i t h  f e r r e t s and as a r e s u l t of h i s e x p e r i m e n t s was  able t o assert that  "the a c c e l e r a t i o n of t h e o e s t r u s c y c l e , g e n e r a l l y s p e a k i n g , c o r r e l a t e d w i t h t h e degree o f ( l i g h t ) i n t e n s i t y " . were i n a c c o r d w i t h B i s s o n n e t t e  was  These r e s u l t s  (1938a) w h e r e i n he summarized a  s e r i e s o f e x p e r i m e n t s concerned w i t h p h o t o - p e r i o d i c i t y . H i s r e s u i t s i n d i c a t e d t h a t t h e wave l e n g t h o f l i g h t as w e l l as i t s i n t e n s i t y was  a f a c t o r i n promoting s e x u a l development.  w i t h s t a r l i n g s showed t h a t r e d l i g h t was  very s t i m u l a t i n g s e x u a l l y  even at l o w i n t e n s i t JBS, w h i l e g r e e n l i g h t and not s t i m u l a t i n g at a l l .  Research  p u r p l e l i g h t were  Subsequent s t u d i e s of f e r r e t s by M a r s h a l l  and Bowden (1934) proved a l l t h e v i s i b l e spectrum and a s h o r t cont i g u o u s p a r t o f t h e u l t r a - v i o l e t t o be almost e q u a l l y e f f e c t i v e i n s t i m u l a t i o n t o the breeding s t a t e . L i g h t , however, i s not u n i v e r s a l l y e f f e c t i v e i n s t i m u l a t i n g anoestrus  a n i m a l s t o b r e e d i n g a c t i v i t y and a p p a r e n t l y i s most e f f e c t -  i v e i n a n i m a l s f r o m r e g i o n s of extreme s e a s o n a l change i n l i g h t i n Dempsey e t . a l . (1934) found t h a t guinea  tensity.  respond t o v a r i a t i o n s i n day l e n g t h .  p i g s would  And Moore and h i s  (1934) had s i m i l a r r e s u l t s w i t h ground s q u i r r e l s .  cohorts  I n t h e second  case the r e f r a c t o r y h i b e r n a t i o n p e r i o d at low t e m p e r a t u r e s was necessary  not  a  p r e c u r s o r t o s e x u a l s t i m u l a t i o n by e x t e r n a l f a c t o r s .  To the b e s t o f the w r i t e r ' s knowledge no i n v e s t i g a t i o n of the e f f e c t s o f l i g h t ducted.  on t h e s e x u a l c y c l e s o f muskrats has been con-  However, the e f f i c a c y of r a d i a t i o n i n s t i m u l a t i n g o t h e r  125 mammals, including Mjorotus. as well as birds and insects has been well established and i t may be assumed that muskrats react in a s i m i l a r manner. In a l l of the investigations detailed above the method consisted i n a progressive increase i n illumination which eventu a l l y brought about reproductive  a c t i v i t y of varying degree.  They  shewed that up to a point an increase i n the i n t e n s i t y of i l l u m i n ation correspondingly accelerated sexual development.  I t was  assumed that the increase i n day length was the prime  stimulus  prompting gonadotropic a c t i v i t y of the p i t u i t a r y .  No attempt  was made to measure c r i t i c a l l y the amount of l i g h t necessary t o bring about t h i s e f f e c t .  For example, assuming the f a c t o r s of  i n t e n s i t y , wave length and so f o r t h to be equal, would daylight of eighteen hours acting for one month be equivalent to 24 hours of l i g h t f o r two-thirds  as long?  Or stated i n another way,  there a minimum and a maximum rate at which the threshold of illumination can be achieved? further investigation of t h i s  is level  There appears to be a need for  question.  The sequel to reproductive  a c t i v i t y is a regression of  the sexual organs and eventually an anoestrus state.  Marshall  (193 6) stated "that in a l l the higher animals sexual p e r i o d i c i t y , while conditioned by the environment, i s regulated i n i t s successive phases by the combined integrative action of the nervous endocrine systems".  and  However, whether the animal i s monoestrus,  like the f e r r e t , or polyoestrus  like the f i e l d mouse, this y e a r l y  internal p e r i o d i c i t y u s u a l l y ceases eventually and a refractory period follows. What prompts this cessation of sexual a c t i v i t y has  not  126  been demonstrated (1938b)  s a t i s f a c t o r i l y with, any animal.  Bissonnette  found that while decreasing length of day normally may  help i n the induction of anoestrus, and may yet i t was  even accelerate i t ,  not a necessary faotor i n f e r r e t s because blinded  animals ceased breeding without functional l i g h t receptors. same author (1938a)  The  i n r e f e r r i n g to his studies with s t a r l i n g s  noted that the males underwent sexual regression i n May  and that  by mid-June no spermatozoa were found i n the testes of autopsied birds.  Yet day length normally increases u n t i l 21 June and l i g h t  intensity for a period thereafter.  In animals with a gestation  period extending over the major part of the year the f a c t of pregnancy terminates oestrus.  But i n polyoestrus animals with short  gestation and recurrent heat periods other faotors i n addition to l i g h t maybe important, f o r example, temperature, food, and the crowding of the habitat induced by the pressure of earlier-born young. In the s e c t i o n which i s to follow, the w r i t e r has used accumulative daylight as the measure of the l i g h t factor i n nature. This i s merely another way to measure an increase i n day length i n any p a r t i c u l a r area.  I t w i l l be found that the rate of increase  and deorease i s greater in the northern areas than i n the southern ones because t o t a l length of daylight varies more widely.  Over  the course of a year, however, t o t a l daylight w i l l be more or less the same i n any  locality.  In the f i v e races of muskrats under consideration we have representatives i n bands of latitude from 30°N. to nearly 70°N. For the purpose of comparison f i v e general l a t i t u d e bands were chosen at 30°N. (Texas and Louisiana), 40°N. (Maryland, Wisconsin  127  and Iowa approx.), 53°N. (The Pas, Manitoba), 60°N. (AthabascaPeace River d e l t a s ) , and 68°N. (Mackenzie River d e l t a ) .  The t o t a l  accumulative l i g h t index was calculated for each of these areas ' using morning and evening c i v i l twilight as datum points.  The  cumulative t o t a l originated at the autumnal equinox (23 September) which date was  chosen because f o r most races of muskrats repro-  ductive a c t i v i t y had almost or e n t i r e l y ceased by that date.  The  time o f s i g n i f i c a n t events i n the reproductive a c t i v i t y of muskrats i n the several l a t i t u d e bands was correlated with the accumulated hours of daylight f o r each date or period of time.  In that  way i t was hoped that some generalizations could be drawn between the timing of the event(s) and the amount of daylight which had been acting upon the animals since the autumnal equinox.  The r e -  s u l t s of t h i s endeavour are presented in tabular form i n Table 11 and at Appendix  C.  In some cases i t was necessary to interpret the statements of the authors c i t e d i n the table and in some cases to be more s p e c i f i c than the published data.  The dates given, however, can  be taken as averages f o r the phenomena observed.  More accurate  recording by these authors of s p e c i f i c events i n the reproductive behaviour of the widely d i s t r i b u t e d raoes of muskrats would have allowed a better correlation of l i g h t and other factors i n the environment with this a c t i v i t y .  However, some observations may  be  made concerning the information a v a i l a b l e . A study of these accumulated daylight t o t a l s , corresponding to events i n the reproductive a c t i v i t y of muskrats  i n various areas,  w i l l r e v e a l that the f a r t h e r north the species i s found the more daylight i s accumulated by the time the event occurs.  The b i r t h  TABLE 11 DATES AND AC CUMULATED DAYLIGHT INDICES OF EVENTS IN THE REPRODUCTIVE CYCLE OF MUSKRATS IN NORTH AMERICA EVENT Aklavik Reproductive Organs  Ft.. Smith  LOCALITY The Pas  Wiscons in  Maryland  (male)  Recession primary organs to Primary organs dormant until Adult male capable of breeding Mating occurs  15-9 to 20-10 to 01-3 until?20-1 until 20-4 (2100)^01-4 (1930) 01-4 (2050) 01-6 (3030) 01-5 (2450) 20-4 (2350)  10-11 30-12 ±k 10-3 (1900) 10-1 (1250) 01-4 (2200) 20-3 (2050)  15-•4 (2200) 20--3 (1850) 10-•5 (2550) 20--4 (2300) 20-•6 (3500) 01--6 (3050) 01-6 (3050)  01-3 (1900) 01-4 (2150) 01-5 (2600)  20-5 (2800) 20-4 (2300) 10-4 (2200) 01-6 (3050) 10-5 (2650) 01-5 (2500)  20-2 (1650) 20-3 (2050)  20-5 (2800) 10-6 (3300) 10-5 (2650) 01-6 (3050) 10-5 (2650) 20-4 (2300)  20-2 (1650) 01-5 (2600)  2  A  Reproductive Organs (female ) Beginning of uterine enlarge] Females capable of breeding Few young born p r i o r t o Movement Movement of animals begins Movement well established Fighting Seme f i g h t i n g Peak of f i g h t i n g Spring break-up  A  Date  &ft Accumulated daylight i n hours  15-3 (1950)  15-4  (2400)  129  of l i t t e r s i s a s a t i s f a c t o r y and e a s i l y observed event.  In Mary-  land, Iowa and Wisconsin young muskrats seldom are born prior t o the  first  of May, at which time there i s a daylight index of  about 2600 hours.  In Manitoba the date was about 20 May with a  corresponding daylight t o t a l of 2850.  In northern Alberta l i t t e r s  generally were not produced prior to the f i r s t of June at a light index of 3050 hours.  And l a s t l y , i n the Mackenzie d e l t a the ave-  rage date was about 20 June and the accumulated daylight at that time 3500 hours. Some explanation f o r this variation i n timing of births of the f i r s t  l i t t e r s can be given.  I t was stated by F u l l e r (1951a)  and noted by other investigators that "the time of mating seems to be determined by the time of break-up of the i c e cover".  In those  continental areas under scrutiny the spring break-up varied from mid-March i n Iowa and Wisconsin to late A p r i l i n central Manitoba and northern Alberta and early June i n the Mackenzie d e l t a .  There-  by the mating a c t i v i t y of the muskrats was delayed u n t i l they were able to emerge from t h e i r winter habitat. There is some evidence that animals were capable of breeding prior to removal of the i c e cover. (195U  In Wisconsin Beer and Meyer  pronounced adult males f e r t i l e by the tenth of March, though  mating probably d i d not occur u n t i l about the f i r s t of A p r i l .  In  northern Alberta male muskrats were sexually mature by about the f i r s t of A p r i l ( F u l l e r , op. c i t . ) the  end of A p r i l .  and i n the Mackenzie d e l t a by  Central Manitoba studies revealed the males i n  breeding condition by about the f i r s t of A p r i l .  The accumulative  daylight indices f o r a l l of these dates were 1900, 1950, 2300 and 2050 r e s p e c t i v e l y .  This would indicate that daylight per se may  130  have some influence on the sexual development of muskrats and that inasmuch as i t is an invariable factor dependent only upon the sun's angular  distance above the horizon, the amount of sun-  light required should be more or l e s s comparable in a l l l o c a t i o n s . The figures for Maryland appear t o be an exception because there spermatogenesis has been recorded e a r l y i n January at a daylight index of only 1250.  I t i s supposed that the lack of continuous  ice cover there exposes the animals to greater amounts of dayl i g h t than otherwise would be the case. This explanation  involving the amount of i c e cover  be given further consideration.  may  Female muskrats i n the areas a l -  ready mentioned reveal a greater d i s p a r i t y than the males i n the times they arrive at fecundity.  Daylight  indices were 2000 for  Maryland, 2200 for Wisconsin, 2300 for northern Alberta, and 2800 for Aklavik.  Once again the Maryland t o t a l was the lowest  the Mackenzie delta t o t a l the  and  highest.  TABLE 12 GREATEST DIFFERENCE IN LENGTH OF DAY FOR FIVE AREAS OF NORTH AMERICA. Location  Length of daylight i n hours Longest day Shortest day Difference  Louisiana, 30°N.  l^.O  11.0  4.0  Iowa, 40°N.  16.3  10.0  6.3  Manitoba, 33°N.  19.0  9.0  10.0  22.0  7.5  14.5  24.0  5.0  19.0  Ft. Smith, NWT., Aklavik, NWT.,  60°N. 68°N.  131 No mention yet has been made of the e f f e c t of sunlight on the reproductive a c t i v i t i e s of the muskrats of the Texas and Louisiana coastal marshes.  It may be noted in Table 1 2 that  there i s a difference of only four hours between the length of the longest day and the length of the shortest day of the year i n those l a t i t u d e s .  This f a c t , coupled with the high mean  annual temperature, may period e x i s t s there.  account f o r the f a c t that no anoestral  I t i s possible that there i s not  v a r i a t i o n i n day length throughout the year to e l i c i t  sufficient those  changes in the physiology observed in muskrats f a r t h e r north. In Louisiana Svihla and Svihla ( 1 9 3 D "found young muskrats or embryos every month of the year, yet the heaviest breeding appeared t o take place frcm November to A p r i l i n c l u s i v e " .  It has been  notedthat there is some decrease of breeding a o t i v i t y during January.  Inasmuch as t h i s i s the coldest month of the year in  these regions, the hiatus observed may be ascribed to low rather than high t emper atures.  This would confine the breeding  mainly to the cooler months of the year  (see Appendix B)  activity and,  therefore, temperature and not l i g h t may have been the prime f a c tor  governing reproductive capacity. I f we examine the daylight indices for various events i n  the sexual a c t i v i t y of muskrats i n the Mackenzie delta, i t i s apparent that more daylight must accumulate there before the occurrence  of a given event than i n other areas studied.  Two  possible reasons are suggested for t h i s . The f i r s t that l a t i t u d e .  involves the extremely low angle of the sun at  This would allow less penetration of light into  the substratum (ice or water) and would also f i l t e r out some of  132 the light components, notably those towards the v i o l e t and u l t r a v i o l e t end of the spectrum.  The low angle of incidence c e r t a i n l y  would decrease the amount of r a d i a t i o n available to wintering muskrats though the wave length of that r a d i a t i o n might be a l e s s important  consideration.  In any case, i t i s assumed that the low  intensity of radiation in the f a r north, together with the f i l t e r ing cut of the shorter wave lengths, would have a retarding effect on northern  animals.  The second factor involves the r e l a t i v e l y greater thickness of ice interposed between the radiant energy of the sun and the animals beneath.  I t seems possible that the d e l t a muskrats  live i n a nearly dark environment during part of the winter and that the time of year at which a c t i v a t i n g r a d i a t i o n i s available to them w i l l vary with the thickness of the i c e and the depth of the snow cover above i t .  Temperature then becomes a factor i n  determining the l i g h t regime of muskrats because i t , in part, determines the amount and persistence of the i c e and snow cover. Depth of snowfall also i s an important  consideration.  I f there i s a correlation between the physiological cond i t i o n of muskrats and the amount of sunlight they receive, as there seems t o be, then the f a c t o r s of snow and i c e depths become more important latitudes.  as one moves towards the higher (and colder)  These f a c t o r s , and others, should be given consider-  ation i n assessing the r e l a t i o n s h i p of l i g h t per se to the sexual a c t i v i t y of muskrats. On the b a s i s of t h i s study i t i s not possible to state whether far northern muskrats are more or less susceptible to solar r a d i a t i o n than t h e i r counterparts i n more temperate regions.  133  To answer t h i s q u e s t i o n would have r e q u i r e d a more d e t a i l e d i n v e s t i g a t i o n t h a n t h e w r i t e r was i n a p o s i t i o n t o u n d e r t a k e a t the t i m e .  I t has been assumed t h a t t h e y were n e i t h e r more nor  l e s s s e n s i t i v e t o a c t i v a t i n g r a d i a t i o n t h a n are o t h e r muskrats. It follows, therefore,  that t h e i r apparent l a g i n reproductive  development can be a s c r i b e d  to factors i n t h e i r physical  ment w h i c h s h i e l d them f r o m t h e r a p i d l y i n c r e a s i n g  environ-  radiation  f o l l o w i n g t h e v e r n a l equinox. TABLE 13 AGE RATIOS OF TRAPPED MUSKRATS I N THE MACKENZIE DELTA 1947-50. T r a p p i n g Area  Date  Adult  Sub a d u l t  11  56  1948  4  8  Mar.  1948  7  33  Gully  Mar.  1949  10  59  Cockney  Mar.  1949  18  103  Owen A l l e n  Mar.  1949  36  161  B. Wiedemann  Mar.  1949  16  45  K. H. Lang  Apr.  1950  18  49  120  514  G r a s s y Lake  Nov.  - Dec. 1947  R e i n d e e r Depot  Feb.  A. J . Boxer Joe Bob  Totals Proportion  100 A d u l t s 428 S u b a d u l t s  Age R a t i o s I n t h e M a c k e n z i e d e l t a d a t a f o r age c o m p o s i t i o n o f muskr a t s i n t h e sampled a r e a s were o b t a i n e d e i t h e r from, animals examined  13*  d u r i n g the s p r i n g t r a p p i n g season or by l i v e t r a p p i n g d u r i n g e a r l y w i n t e r , Table  13.  A l l a n i m a l s were d i f f e r e n t i a t e d as t o age  by post-mortem e x a m i n a t i o n  or by the f a c t t h a t t h e y bore a numbered  From 634- animals g i v e n t h i s c r i t i c a l e x a m i n a t i o n  tag.  either  514- ( 8 l per  cent) were j u v e n i l e s , the re by s u p p l y i n g a j u v e n i l e - a d u l t r a t i o 428:100.  of  S i m i l a r i n f o r m a t i o n from n o r t h e r n A l b e r t a gave an e v e n  h i g h e r r a t i o : 655  j u v e n i l e s t o 100 a d u l t s , F u l l e r (1951a).  A com-  p a r i s o n o f age r a t i o s i n other, p a r t s of n o r t h e r n N o r t h America a v a i l a b l e from the p u b l i s h e d i n f o r m a t i o n of t h o s e listed  i n Table  14.  I f we  is  investigators  assume an a p p r o x i m a t e l y e q u a l sex r a t i o  f o r b r e e d i n g a d u l t s t h e n t h e number o f s u r v i v i n g young p e r a d u l t female  i s h i g h e r i n n o r t h e r n A l b e r t a and the M a c k e n z i e D i s t r i c t  t h a n i n most o t h e r r e g i o n s l i s t e d . S t a t e s the r a t i o s v a r y f r o m 202 a d u l t s i n M i c h i g a n , t o 359: The  Far the n o r t h c e n t r a l U n i t e d 100  j u v e n i l e s (or s u b - a d u l t s ) per  100 i n W i s c o n s i n , and 436:100 i n Iowa.  f i g u r e s f o r Iowa a r e h i g h e r t h a n t h o s e f o r the Mackenzie d e l t a ,  but i f we  c o n s i d e r t h a t t h e number o f young b o r n per female i n Iowa  i s about t w i c e t h a t f o r the A k l a v i k r e g i o n , t h e n we  can a s s e r t  e i t h e r a s u p e r i o r s u r v i v a l o f a l l ages of n o r t h e r n m u s k r a t s ,  or  e l s e a d e p r e s s e d s u r v i v a l of a d u l t a n i m a l s . I t may  be s i g n i f i c a n t t h a t o n l y two  g e o g r a p h i c a l r a c e s are  b e i n g compared h e r e i n , z i b e t h i c u s i n the s o u t h , and s p a t u l a t u s i n the n o r t h .  A l o w e r s u r v i v a l of j u v e n i l e a n i m a l s may  be a c h a r a c t e r -  i s t i c of the more s o u t h e r l y r a c e , d i c t a t e d e i t h e r by i n h e r e n t c h a r a c t e r i s t i c s o f t h e a n i m a l s t h e m s e l v e s , or by t h e p r e s s u r e of t h e i r environment.  E r r i n g t o n (1946a) i n d i s c u s s i n g t h e e f f e c t s o f p r e -  d a t i o n on m u s k r a t s s t a t e d t h a t " i n t h e event o f p r e d a t o r p r e s s u r e b e i n g i n c o n s e q u e n t i a l , t h e g r e a t e r p a r t of the b a l a n c i n g sooner  or  135  TABLE 14 A COMPARISON OF THE AGE RATIOS OF MUSKRATS FROM VARIOUS AREAS OF NORTH AMERICA DURING AUTUMN AND WINTER  Author i t y  Period  McCann (1944) Minnesota Baumgartner & B e l l r o s e (1943) Illinois Baumgartner & B e l l r o s e (1943) Michigan G a s h w i l e r (195 0b) Maine  Fall 15  to  1  564  409  155  264  1147  784  318  243  7511  4866  2645  202  951  681  270  252  24082  18860  5222  359  Nov.  15 J a n .  to  Sample NUMBERS PROPORTION Size S u b a d u l t A d u l t S.ad.:100 Ad,  Nov.  31 J a n .  Nov.  Beer & Truax (195 0 )  Wiscons i n  Fall  A l e x a n d e r (1951) New York  1 Jan. t o 20 Mar.  541  392  149  263  (1947)  Nov. and Dec.  4652  3785  867  436  Errington Iowa  F u l l e r (1951a) Alberta  Dec. t o May  417  361  56  655  S t e v e n s (unpubl.) M a c k e n z i e R.  Nov . t o Apr.  634  514  120  428  136  l a t e r took the form of l e t h a l f r i c t i o n among the muskrats themselves".  As indicated previously, the muskrat populations i n  the l a t i t u d e of Iowa assume densities of several times those i n the Mackenzie d e l t a .  Therefore, i t can be assumed that the Iowa  muskrats normally maintain a population much nearer to the density threshold, beyond which f r i c t i o n among animals begins, than do those i n northern Canada. The question of the s u r v i v a l of adult animals i s one which was mentioned  previously as capable of a l t e r i n g age r a t i o s .  However, there is only one reason to suspect that adults do not survive as well as younger animals from autumn to spring.  The  section concerning the longevity of animals indicated that the l i f e span of northern muskrats was r e l a t i v e l y short, thereby r e quiring continual recruitment of young animals i n order to maintain the population.  I f adult animals are going to succumb from natural  causes, they no doubt w i l l do so during the c r i t i c a l winter period. In spite of the p o s s i b i l i t y of d i f f e r e n t i a l survival of adults and juveniles i t i s the writer's opinion that the younger animals i n the Mackenzie  d e l t a reached adulthood i n greater numbers than those  i n most other areas because the population was always well dispersed i n the habitat, thus avoiding " l e t h a l f r i c t i o n " .  Furthermore,  the losses from disease and the action of predatory animals both were low, as w i l l be discussed subsequently.  137  TABLE 15 MUSKRATS TAGGED AND SUBSEQUENTLY RECAPTURED AETER A PERIOD OF AT LEAST THREE MONTHS. .. • ADULT Male Fema le  JUVENILE Male Female  Number captured and tagged ft  100  96  246  180  Males per 100 females  104  100  137  100  Percentage of t o t a l tagged  16%  15%  40%  29%  Number of animals recaptured ftft  17  24  75  51  Percentage of t o t a l recaptured  10%  14%  45f.  30%  ft - T o t a l number tagged 621  ftft  - Total number recaptured 167  Sex Ratios Throughout this t r e a t i s e i t has been necessary to compare the northwestern muskrat (spatulatus) with races of muskrat i n other parts of North America.  Having discussed the age r a t i o s  of muskrats i n the Mackenzie delta and elsewhere as influencing the breeding population, i t i s considered necessary also t o deal b r i e f l y with the male-female r a t i o s as a f f e c t i n g the same phenomenon.  Two factors w i l l be discussed, f i r s t the actual sex r a t i o  observed and, second the probable e f f e c t s of an unbalanced r a t i o . Many authors have published information concerning sex r a t i o s of muskrats. Beer and Truax  (1950)  One of the more recent of these was that of who summarized information a v a i l a b l e to that  138 date.  I t was  t h e i r c o n t e n t i o n t h a t a s h i f t i n sex r a t i o s  p r o g r e s s i v e l y from b i r t h t o adulthood. more n e a r l y balanced  occurred  t  M a l e - f e m a l e r a t i o s were  at b i r t h and d u r i n g t h e b r e e d i n g s e a s o n , w i t h  males i n the ascendancy at a l l o t h e r p e r i o d s . not quote E r r i n g t o n (1947) who  presented  sex r a t i o i n f o r m a t i o n i n Iowa, and  These a u t h o r s  did  a d e t a i l e d summary o f  i n other a r e a s as w e l l .  His  d a t a , however, i n d i c a t e d t h a t t h e r e i s a v a r i a b l e m o r t a l i t y f a c t o r a c t i n g t o the detriment  of male a n i m a l s , r a t h e r t h a n t h e  females.  I n t h e M a c k e n z i e d e l t a the a d u l t a n i m a l s tagged d u r i n g t h e b r e e d i n g season ( J u l y - September) e x h i b i t e d a male-female r a t i o of 104 males t o each 100 f e m a l e s , T a b l e 15.  T h i s was v a s t l y  d i f f e r e n t t o t h e r a t i o s encountered i n the months j u s t A sample of 5,054 muskrats t r a p p e d 14 May 118  was  previous.  d u r i n g t h e p e r i o d 2 March t o  comprised o f 2,732 males and 2,322 females - a r a t i o  males per 100 f e m a l e s .  e x a m i n a t i o n was made o f 752 d u r i n g e a r l y June.  of  S u b s e q u e n t l y , a f t e r open w a t e r appeared, animals  shot by t h r e e d i f f e r e n t  hunters  T h i s sample c o n s i s t e d of l 8 l males f o r e v e r y  100 f e m a l e s , e v i d e n c e  of a s e l e c t i v e m o r t a l i t y d u r i n g t h e p e r i o d  o f water h u n t i n g . S e v e r a l f a c t o r s were r e s p o n s i b l e f o r t h e i n c r e a s e d m o r t a l i t y t o males j u s t p r i o r t o the b r e e d i n g s e a s o n . was  The  f i r s t of  these  the f a c t t h a t h u n t i n g c o i n c i d e d w i t h t h e a r r i v a l of open w a t e r  which f l o o d e d most o f t h e a v a i l a b l e h a b i t a t and ousted most a n i mals f r o m t h e i r w i n t e r dens. at once o c c u p i e d  The m a l e s , a l r e a d y s e x u a l l y mature,  themselves w i t h m a t i n g a c t i v i t i e s .  Their state  of u n r e s t r e s u l t e d i n t h e i r becoming not o n l y v e r y b e l l i g e r e n t t o t h e i r own  sex but a l s o much l e s s wary t k a n u s u a l .  They were f o u n d ,  t h e r e f o r e , i n t h e more exposed s i t e s around l a k e edges and  along  139  the r i v e r channels.  At that time of year male muskrats could be  called within easy r i f l e range when the hunter made a squeaking sound between compressed l i p s . the mating cries of the female.  This sound was said to simulate In any event, i t was  attractive  to male animals and they would respond promptly by swimming toward the hunter with an impressive chattering of exposed  incisors.  As the male became more venturesome the female often exh i b i t e d quite the opposite q u a l i t i e s .  She became wary and con-  fined her a c t i v i t i e s to the v i c i n i t y of the nest or den which she was  occupying.  In several cases females which showed these secre-  t i v e c h a r a c t e r i s t i c s were found to be gravid. The hunter himself often was Not only was  selective, i n h i s hunting.  i t easier t o shoot males because of t h e i r v u l n e r a b i l i t y ,  but i f an animal was thought t o be a female i t was sometimes spared to rear i t s young.  A l l hunters, however, were not so provident and  would shoot males and females indiscriminately. A sample of 32 animals taken from the creeks and r i v e r channels 8 to 12 June showed seventeen t o be males and f i f t e e n females.  This indicated that both sexes undertook these spring  migrations i n quest of suitable breeding and rearing s i t e s .  The  females showed lacerations from f i g h t i n g but the males more often were wounded during t h i s period, probably because they had t o oompete not only f o r denning s i t e s but f o r mates as w e l l .  There  no doubt i s evidence here of s e l e c t i v e m o r t a l i t y acting against the male animals. The sex r a t i o s of juvenile animals were i n a l l cases found to be unbalanced i n favour of the young males.  During  summer investigations from mid-July to mid September, 137 males  I*t0  TABLE 16 NUMBER OF YOUNG MUSKRATS PER LITTER DEDUCED FROM SUMMER DEN TRAPPING STUDIES, 1947-49.  LAKE & DEN NUMBER  ADULT Male  DEN COMPLEMENT Female  JUVENILE Male Female  G r a s s y L a k e . Aug. '47 Den. No. 1 2 4 6 7 8 9 10  1 1 1 1 1 1 1 1  1 1 2 1 1 0 1 0  2 4 4 2 4 3 1 1  0 0 1 0 3 3 4 1  1  0  1  3  1 0 1 1 1 1  0 1 1 1 1 1  1 1 4 3 3 3  4 3 1 2 3 3  1 1 1  1 2 2  4 3 2  1 4 4  1 1 1 1 1 1 1 1 1 1 1 1 1  1 0 0 1 0 1 2 1 1 1 1 1 1  0 4 3 3 1 5  p  1 0 ? 0  15 1 6 1 3 1  n u 0 4 2 4 3  G r a s s y Lake, J u l y ' 4 8 Den No. H Grassy Lake. Aug. ' 4 9 Den No. G - l G-2 G-3 G-3 G-6 G-7 Lake 2-W. Aug. ' 4 9 Den No. WS 1 3 R e i n d e e r Depot, J l y . - A u g . '49 Den No. 1-1 2-1 2-2 2-3 2-4 2-6 JF-1 JF-2 3-4 3-7 3-8 3-10 3-11  s  T X  l*fl  were tagged f o r e v e r y 100 f e m a l e s .  T h i s r a t i o agreed f a i r l y w e l l  w i t h F u l l e r (1951a) though he e n c o u n t e r e d an even g r e a t e r i m b a l ance.  Dorney and Rusch (1953) i n W i s c o n s i n noted 125 males t o  each 100 female k i t s and E r r i n g t o n (1939a) r e c o r d e d 120 males t o 100 female n e s t l i n g s .  A l l a u t h o r s agreed t h a t even i n t h e v e r y  young a n i m a l s more males are e n c o u n t e r e d t h a n f e m a l e s .  T h i s would  appear t o be a c h a r a c t e r i s t i c of t h e s p e c i e s w h i c h a c t s t o ensure s u f f i c i e n t males i n t h e b r e e d i n g p o p u l a t i o n . The n e c e s s i t y f o r a r o u g h l y even s e x r a t i o i n t h e b r e e d i n g p o p u l a t i o n was demonstrated i n t h e c o u r s e of t h e summer den t r a p p i n g s t u d i e s , t h e r e s u l t s of w h i c h a r e d e t a i l e d i n T a b l e 16. The 47 dens i n w h i c h l i v e t r a p s were p l a c e d , and i n w h i c h t r a p p i n g was c o n t i n u e d u n t i l o n l y t a g g e d a n i m a l s were t a k e n , p r e s e n t e d i n f o r m a t i o n c o n c e r n i n g den complement d u r i n g summer.  It  may be noted t h a t 41 o f t h e s e dens s u p p l i e d more t h a n one c a p t i v e muskrat.  Of t h e s e , t h e r e were t h i r t y i n w h i c h t h e r e were e i t h e r  an a d u l t male and young a n i m a l s , o r b o t h a male and female p l u s young.  There was o n l y one den i n w h i c h a female but no male was  captured.  M o r e o v e r , i t i s s i g n i f i c a n t t h a t a l l o f t h e males were  t a k e n a t o n l y one den. The p a r t t h e male p l a y e d i n t h e r e a r i n g o f t h e young was not demonstrated e x c e p t t h a t sometimes he g a t h e r e d f o o d and c a r r i e d i t t o t h e den.  About t h e t i m e t h e f i r s t young o f t h e y e a r were  b e i n g encountered den e n t r a n c e s and runways o f t e n were plugged w i t h f r e s h green v e g e t a t i o n , u s u a l l y sedge o r h o r s e t a i l .  Animals  which c o l l e c t e d t h i s g r e e n e r y and swam w i t h i t t o a s p e c i f i c den, u s u a l l y proved t o be males.  The use made of t h i s p l a n t m a t e r i a l  was n o t e n t i r e l y c l e a r though i t was employed t o s t o p h o l e s i n  1^2  the runways and to seal off that part of the burrow system not in use.  I t may  also have been eaten by the nursing female and  the young, as w e l l as by the male himself. The  conclusion reached, by consideration of the available  evidence, was  that normally a male and a female were i n attendance  at one nest den, and that there was  a f a i r l y s t r i c t monogamy which  kept the male with the female from the time she was  bred u n t i l the  young had grown to the age of independence from parental care. With such a r e l a t i o n s h i p the same male no doubt would s i r e the second l i t t e r i f one were produced. The result of the d i f f e r e n t i a l k i l l i n g of male muskrats may  not have been to the advantage of the breeding population.  Fuller (1951a)  stated the case s u c c i n c t l y when he said that "assum-  ing a low incidence of promiscuity, we must conclude that a s u f f i cient supply of males i s probably necessary for maximum production. A l l schemes and arguments, therefore, aimed only at the protection of females as e s s e n t i a l to reproductive garded with suspicion".  success should be re-  This necessity f o r an even sex r a t i o  was more important i n the Mackenzie delta where muskrats were t h i n l y and evenly dispersed throughout the habitat than i n areas where many animals may  be crowded into small areas of favoured  habitat. The  large number of males found among the youngest ani-  mals examined i n the Mackenzie d e l t a and i n northern Alberta would ensure that subsequently s u f f i c i e n t breeding males would be available f o r e f f i c i e n t reproduction. number of males conceived i n the environment.  It i s doubtful i f the  is under the influence of any f a c t o r  More l i k e l y , itfc i s an a t t r i b u t e with a  1*3  genetic background just as i s the larger number of young born at each partus. the  The demands of the habitat have, however, "selected"  genetic complex which ensures both s u f f i c i e n t young animals  and s u f f i c i e n t males among them to provide for r a c i a l continuity. L i t t e r s Per Year In the Mackenzie d e l t a a few muskrats produced l i t t e r s by mid-June, but most did not do so u n t i l early J u l y .  Assuming  a post-partum oestrus, as Errington (1937b) demonstrated, i t would be possible f o r second l i t t e r s t o be produced from mid-July to mid-August.  Newly born young were collected on the f i r s t and  fourth of August, 1949, but i n four years these were the latest births a c t u a l l y encountered.  There was one small female l i v e -  trapped on 9th November, 1948, that weighed  only 300 grams.  She  probably was born about mid-September but such late b i r t h s were considered quite exceptional. In spite of the short period i n which breeding a c t i v i t y was possible there was an opportunity f o r some females to produce two l i t t e r s  i n one summer.  Evidence that t h i s had occurred was  provided by the appearance of uterine scars or implantation s i t e s of two ages i n adult females.  There were too few such specimens  collected to supply any indication of the frequency of second l i t t e r s but they did demonstrate that such l i t t e r s occurred.  A  female taken 13th July, 1947, exhibited seven reddish purple p l a cental scars and s i x very e a r l y implantations as w e l l . instance was recorded on 21st July of the same year.  A similar The p e r s i s t -  ence of placental s i t e s varied with the i n d i v i d u a l animal but generally they had disappeared by the end of December.  One female  1**  taken 7th November, 1947, had twelve uterine scars of two ages, and another examined 12th December, 1947, had f i v e old and i n d i s t i n c t scars and four which were l a r g e r and darker. These data indicated that two l i t t e r s  of young were born  to some females but that the incidence of such births was r e l a t i v e l y low. the  Nulliparous animals usually matured sexually too late i n  spring to produce a second l i t t e r .  Therefore, most of the  second l i t t e r s were from multiparous adult females that arrived at breeding condition r e l a t i v e l y e a r l y i n the year.  They e v i -  dently had mated before a l l the ice cover was removed from the lakes and produced young during the l a s t h a l f of June.  That  being the case, a second l i t t e r s t i l l could have been born by August 1.  On the other hand, a nulliparous female that was not  bred u n t i l mid-June could not have a second l i t t e r before midAugust.  Because very few late l i t t e r s were encountered, i t was  evident that most year-old females raised only one  litter.  Table 17 i s a resume of information from several areas concerning the breeding season for muskrats i n each. the  Therein  reproductive period was taken t o be the time from the f i r s t  matings to the birth of the l a s t subsequent l i t t e r s .  Average  conditions are given, not exceptionally late or e a r l y occurrences. Information f o r the race r i v a l i c i u s i s d i f f i c u l t  to assess because  births have been recorded on the Gulf Coast during a l l months of the  year.  The h i s t o r y of any p a r t i c u l a r female, however, i s  lacking. The table demonstrates rather w e l l the v a r i a b i l i t y i n reproductive a c t i v i t y which is exhibited w i t h i n the range of the species.  Therein i t may be noted that the Mackenzie delta animals  1*5 TABLE 17 THE BREEDING ACTIVITIES OF F I V E RACES OF MUSKRATS AT SEVERAL. LOCATIONS I N NORTH AMERICA. . Race and Local i t y  First Mating  zibethicus  Last B r e e d i n g Number L i t t e r s Period of (wks.) Litters  Average Young  Tot a l Young  early W i s c o n s i n (1) Apr.  midJiy.  14  2-3 &  7.4  15  Iowa (2)  early Apr.  early Aug.  15  2-3  6.8  14-20  early Apr.  late Jly.  15  2-3  7.1  14  yearlong  52  4-5  4.0  16-20  Lou i s i ana (5) y e a r long  52  4-5  3.8  14-19  4.4  13-18  Maine (3)  ±  rivalicius Texas (4)  macro den Maryland (6)  late Mar.  midAug.  18  3-4  late Apr.  late Jly.  13  2-3 &  Athabaska D e l t a (8)  late Apr.  late Jly.  12  2  8.7  Ma eke nz ie Delta  early June  early Aug.  1-2  8.3  A l a s k a (9)  midMay  early Aug.  2  7.7  albus Manitoba  (7)  - 6.8  14  spatulatus  (1) (2) (3 (4)  Beer and Truax (1950) E r r i n g t o n (194?) G a s h w i l e r (1950a) Lav (1945)  9 11  (5) (6) (7) (8)  17 8-17 15  & - Rare o c c u r r e n c e S v i h l a & S v i h l a (1931) S m i t h (1938) McLeod (1950) (9) B u c k l e y (1954 F u l l e r (1951a) unpubl.)  1*6 have a f a r shorter breeding season than any other population listed. As stated previously, the breeding a c t i v i t y of some races of muskrats i s delayed u n t i l open water appears and they are able to escape from their ice-covered winter habitat.  The-timing of  the yearly break-up i s governed by the weather not only during the spring but also throughout the previous winter period.  There-  by breeding dates may vary from year t o year, sometimes by as much as two weeks.  In s p i t e of that, the f a c t remains that ani-  mals of some inland areas which experience a continuous  ice cover  may produce fewer young than they are b i o l o g i c a l l y capable of doing.  The decrement from t h e i r reproductive p o t e n t i a l w i l l be  a function of the l a g between sexual maturity and s p r i n g break-up. In northern United States there does not appear t o be any significant delay i n breeding a c t i v i t y which may be ascribed t o r e s t r i c t i o n by ice cover.  But i n central Manitoba McLecd  (1950)  found animals capable of breeding by late A p r i l , at a time coincident with the disappearance of winter i c e .  The same author  (1948) found that f i r s t l i t t e r s "arrived from May 29th to June 17th, while second l i t t e r s from June 26th t o J u l y 28th with a time i n t e r v a l between l i t t e r s -of from twenty-five t o t h i r t y - f o u r days".  He l a t e r (1950) reported f i n d i n g four l i t t e r s per year.  This would mean that l i t t e r s appeared from the 29th May to the end of September.  This information, i f true, i s viewed as being  exceptional for that area.  His e a r l i e r estimate of two l i t t e r s  no doubt comes nearer to the normal s i t u a t i o n . The latitude of central Manitoba may be the d i v i d i n g l i n e beyond which ice cover i s a s i g n i f i c a n t factor i n r e s t r i c t i n g  1*7 productivity.  F u l l e r (1951a) concluded that muskrats were capable  of breeding by the f i r s t to the middle of A p r i l , but the ice cover was not'removed u n t i l the end of the month, or l a t e r . the delay was  of the order of about two weeks.  that "an unusually early spring break-up may  Therefore,  He stated, however,  ... induce the onset  of breeding at a s u f f i c i e n t l y early date to allow some at least of the females t o bear a t h i r d l i t t e r " .  In t h i s statement he r e -  cognized the f a c t that ice removal was the f a c t o r governing the onset of breeding a c t i v i t y and that the e a r l i e r i t was  accomplished,  the e a r l i e r breeding began. In the Mackenzie delta the delay i n breeding a c t i v i t y extended by ice conditions to as long as one month.  was  This delay  represented the time necessary for the production of one  litter,  which thereby was l o s t t o the population. For northern races of muskrats, therefore, the climate becomes a l i m i t i n g f a c t o r not only i n s u r v i v a l of animals but also i n the rate at which they may be replaced in the population.  As  a r e s u l t of t h i s climatic pressure muskrats must maintain t h e i r numbers even though allowed to r a i s e a lesser number of l i t t e r s . In the following section i t w i l l be shown that they have compensated f o r this r e s t r i c t i o n by producing more young per l i t t e r . Number of Animals  per L i t t e r  It has been demonstrated  that the yearly number of l i t t e r s  produced by muskrats decreases from south to north. might be expected that seme compensating  Therefore, i t  f a c t o r s would appear to  offset this curb on population growth. In Louisiana Svihla and Svihla (1931) found young muskrats during every month of the year.  From a sample of 66 females examined  1*8  an average of three embryos was found i n utero with extremes of one and s i x . Arthur (193U  placed the average young per partus  s l i g h t l y higher at 3 . 8 , and Lay (1945) studying the same race i n Texas found average l i t t e r sizes from 3.8 to 4.6 with four i n dividuals being very close to the average over a period of time. Muskrats i n the tidewater marshes of Maryland were reported by Smith (1938)  and Delaware  to produce 4.4 young at each b i r t h  with three to four l i t t e r s being born each year, (see Table 17). This represents a breeding capacity only s l i g h t l y i n f e r i o r t o the muskrats of the Texas coast. The inland races of muskrat appear to be just as  prolific  as t h e i r coastal r e l a t i v e s in spite of a shorter breeding season. Average l i t t e r s Maine 7.1  from Iowa were 6 . 8 , from Wisconsin 7.4,  young at a b i r t h .  and from  These figures were comparable to  those f o r central Manitoba. In the Athabaska-Peace d e l t a of northern A l b e r t a , F u l l e r (1951a) recorded 8.7 young as an average  l i t t e r size, the largest  authentic mean figure i n the abundant l i t e r a t u r e concerning musk- . rats.  The average of 8.3 for the Mackenzie delta was  only s l i g h t l y  lower than the northern Alberta f i g u r e . In the Mackenzie delta the number of young per b i r t h  was  deduced f i r s t of a l l from the number of embryos or implantations found _in utero.  From 68 females a t o t a l of 571 implantations were  recorded f o r an average of 8.4 young, Table 18. quite i n agreement with F u l l e r ' s data. which had produced s i t e s , an average  This figure i s  U t e r i from eleven females  only one l i t t e r shewed a t o t a l of 82 placental of 7.5 per female, range 6 to 13.  I f these data  be combined with the embryo counts, a t o t a l of 633 embryos from  1*9 TABLE 18 EVIDENCE OF LITTER SIZE I N MUSKRATS OF THE MACKENZIE DELTA FROM PLACENTAL SCARS, EMBRYOS, AND CORPORA LUTEA OF PREGNANCY. Date  Implantations B  13-6-47 16-6-47  13- 7 21- 7 22- 7 6- 8 7- 8 14- 8 3- 9  L  10 8 6 7 6 7  R  3  6  5 2 3  3  0  B  10-6  15- 6 25-3 t o 8-6  21-6-49  L  R  4  6  Corpora L u t e a B  L  10  5 8 2 8 3 5 7 3  5 6  7-11 4 - 12  3-2-48 19-2-48 2-6 9-6  P l a c e n t a l Scars  6 7 6 8 7 13 13  4  9  9 9  6  3  R 5 6 f t 4  12 7 5 (two ages) 9 (two ages) 8 7 10 10 10 8 10 10 8 7 9 10 9 10 7 10 8 11 12 8 11 8 7 8 11 10  9  8 4 3 5 4 4 2 9  1  7 4 4 4 6 6  2 6 3 5 6 4 5 6  7  3 3 5 6 3 4 5  (Table c o n t i n u e d f o l l o w i n g ft - Two embryos b e i n g r e s o r b e d .  page)  TABLE 18 (CONTINUED)  Date  21-6-49  Implantations  P l a c e n t a l Scars  Corpora Lute a  B  B  B  L  11 11 11  3 3 3 7 2  8 8 8 2 7  4  4  L  R  L  R  R  8 L O  6  9  8 7 I8-5-49 t o  9-6-49  9 8 L  O  2 8  9 9 4  24-8  10 9 8 11  27-3-50 4-6-50  7-6 9-6 11-6  9 9 9 8 9 6 8 8  2 2 3 6 2 0  9  2' 3 1 5 5 6  9 10 9 9  Total Total Total Total  4 5  4  9 9  7 7 6 2 7 6  9 9  4 3  7  4 7  4 5 3 5  •  8 8 6 9 8 8 9 10 9  ?  5 2 2  4  1 5 5 6  4  3  4 7  4 7  4  5 3 5  i m p l a n t a t i o n s - 571, N = 68, Average - 8.4 (Range 2 - 12) l e f t u t e r i n e h o r n - 132, N = 35, Average - 3.8 (Range 0 - 8 ) r i g h t u t e r i n e h o r n - 170, N = 35, Average - 4 . 9 , (Range 2 - 7 ) p l a c e n t a l s c a r s - 82, N = 11, Average - 7.5 (Range 6 - 13)  Grand t o t a l - 653, N = 79, Average - 8.3  151  79 females gives an average of 8.3 young per female. An i n t e r e s t i n g side issue of the study of embryos i n utero was the observation recorded in Table 16" that the right uterine horn bore more implantations than the l e f t .  In 35 i n -  stances recorded the right horn had an average of 4.9 embryos (range 2 - 7 )  and the l e f t horn 3.8 (range 0 - 8 ) .  No particu-  l a r significance i s attached to t h i s d i f f e r e n t i a l placentation though i t agrees with data f o r the house mouse (Mus mus cuius) presented by Laurie (1946). Table 17, which presents the breeding capacity of seyeral races of muskrats, i s of interest at t h i s point.  It shows that,  in spite of the wide d i s p a r i t y i n breeding season throughout the range of the muskrat, f o r each female the t o t a l productivity d i d not  vary g r e a t l y .  The compensating f a c t o r was the increase i n  l i t t e r s i z e as the number of l i t t e r s  decreased.  Excluding the  Mackenzie d e l t a the yearly productivity varied from 13 to 20 individuals with an average f o r a l l areas of about 16 animals. It would appear that the "environmental resistance" i n each of the  various habitats i s roughly comparable because the "breeding  p o t e n t i a l " dees not vary appreciably from one to the next. the  In  Aklavik region the force of the environment may even be less  than elsewhere because fewer young were produced there. As depicted i n Table 17, the Mackenzie d e l t a exhibited the  lowest per capita productivity because only one l i t t e r  born to most of the females.  was  In the previous section i t was  stated that adult females might have two l i t t e r s  per year but  that f i r s t year females no doubt produced only one l i t t e r .  Table  12 presents the age r a t i o s of muskrats i n the Mackenzie d e l t a and  152  therein i t may  be seen that there was  every four sub-adult  about one adult female f o r  females in the breeding population.  as the l a t t e r had only one l i t t e r and the former two, adults were producing two-thirds third. ing  Inasmuch  the  sub-  of the young and the adults  one-  Therefore, the average number of young produced by a breed-  female i n a season was  about ten.  Had a l l the animals produced  two l i t t e r s , as they were able to do i n northern Alberta, then the t o t a l productivity would have increased by two-thirds *  The  late  maturing of the f i r s t year females and the retarded break-up i n the Mackenzie d e l t a , therefore, reduced the breeding p o t e n t i a l by f o r t y per cent. FACTORS IN MORTALITY Attention already has been directed toward those physical factors acting t o reduce the number of muskrats i n the Mackenzie delta population.  The following sections w i l l concern those  agents of mortality that are biological i n t h e i r o r i g i n . Intraspecifio  Intolerance  Beer and Meyer (195D  wrote concerning  the muskrats of  Wisconsin that "the gonadotropic a c t i v i t y of the p i t u i t a r y i s cont r o l l e d by the season and that t h i s a c t i v i t y controls the reproductive cycle and either d i r e c t l y or i n d i r e o t l y through the gonadotropic hormones the s o c i a l relationships of the animals".  "Thus,  when the muskrat i s developing re productively, i t is also doing the maximum amount of t r a v e l l i n g and f i l l i n g i n vacant habitats". This general unrest  associated with the breeding season is mani-  fested c l e a r l y in the f i g h t i n g which takes place i n spring and e a r l y summer, not only between the more pugnacious males but also  153  between animals i n secure locations and those i n search of new denning  sites. At the time of the spring break-up In the Mackenzie  delta  there was a general f l o o d i n g of a l l habitat so that few dens were habitable. ing  Animals were forced to b u i l d nests of sedges on f l o a t -  debris or i n the willow and alder brush r i n g i n g the lakes.  This period during which a l l animals i n the flooded lakes were forced into a temporary and rather precarious habitat was  short,  l a s t i n g about a week during which time the ice i n the main channels broke up and moved out. among muskrats  It was, however, attended by intolerance  concomitant with the f i r s t mating a c t i v i t y .  Movement of individuals to new habitat was quite dramatic at this time.  A l l through the evening of 30 May,  1948, muskrats  could be seen moving from Study Area #3 across the heaving pans of i c e running in the East Channel.  Animals were found subse-  quently i n the warehouses of the Reindeer Depot, i n the swollen spring freshets from the Caribou H i l l s , and i n the shallow basins of melt water l e f t  along the base of the h i l l s .  Three muskrats  were k i l l e d that evening as they invaded the dog lines.,: and the sight of other such morsels wandering the  by just out of reach kept  sled dogs howling most of the night.  The same performance  was repeated the following night though there were fewer animals moving. the  By the next night the exodus of muskrats had ceased,  ice i n the channels was gone and the water l e v e l s had receded  by almost three feet. With t h i s drop i n water levels the bank dens once again became habitable.  Mating a c t i v i t y had proceeded during the period  of high water and even before dens were reclaimed a l i m i t e d area  15*  had been defended.  From several instances observed i t was i n -  ferred that t h i s t e r r i t o r y was i n the environs of the temporary nest occupied by the male and female. As the water subsided j there was a shrinking of the habitat and as a consequence  animals were thrown into closer association.  At t h i s time the incidence of f i g h t i n g increased as displaced animals wandered from one burrow system to the next.  Limited, though  unequivocal observations, indicated that males fought f o r females not only i n the water but also through the underbrush along the shore.  But whether both members of a breeding pair shared i n  the defence of the denning s i t e was not established.  The pelted  skins of females and males taken during t h i s period showed bites and lacerations, and the view expressed by the l o c a l trappers was that both members of the pair cooperated to repulse interlopers. If t h i s was the case, then the. den area was being defended most a c t i v e l y by the male, but a l s o by the female, and any trespassers of either sex were driven away.  No doubt the female had been bred  by t h i s time so that most contacts with other animals were made by the male. The incidence of mortality i n animals attributable to spring f i g h t i n g was d i f f i c u l t to ascertain.  Fighting d i d not  last f o r as long a period as that recorded by Beer and Meyer (op. c i t . ) and by Errington (1943) nor, evidently, was i t as v i o l e n t . The shortness of t h e breeding season i n the l a t i t u d e of Aklavik, the sudden onset of breeding a c t i v i t y , and the r e l a t i v e l y large amount of temporary summer habitat allowed the f i g h t i n g among adult animals t o reach a rapid climax and then almost to cease as displaced animals either were forced into unoccupied areas or  155  were harvested by the trappers. F i e l d notes and post-mortem records revealed that the f i r s t f i g h t i n g in spring was noted  25 A p r i l but that holes i n  muskrat pelts were not general u n t i l about a month l a t e r . the l a s t week i n May  During  and the f i r s t week i n June, at a time coin-  cident with open water and mating a c t i v i t y , many pelts showed bites, some amounting t o serious wounds.  No dead animals were  found though there oan be no doubt that animals could be and were k i l l e d by t h e i r f i g h t i n g a c t i v i t y .  A barren female taken  22 June, 1947 had 54 bite wounds, one of which opened the abdomen and coelom.  A male c o l l e c t e d 28 May, 1948 was outwardly i n good  condition but was found t o have a bite 34 mm.  long that opened  the body cavity and had bisected the right kidney.  Other examples  could be cited, most of them of less serious consequence, but a l l demonstrating the marked intolerance among animals early in the breeding season. It i s doubtful whether the two isolated cases of f i g h t i n g recorded during A p r i l , and three subsequently i n November and December can be ascribed to other than t e r r i t o r i a l considerations. During both periods there was  a cover of ice on the lakes and  f i g h t i n g was more l i k e l y for food and dens than f o r mates.  As  mentioned previously, there was an automatic adjustment among animals sealed beneath lake ice wherein a discrete group normally used a common feeding ground.  U n t i l feeding zones had  been established i n f a l l , or after they had broken down from food scarcity i n late winter, the p o s s i b i l i t y of s t r i f e  remained.  Adult -Young Association Compared with the s i t u a t i o n mentioned  by Errington (1943)  156  wherein "losses of young from attacks by older animals ... were s u f f i c i e n t p r a c t i c a l l y t o n u l l i f y continued reproductive e f f o r t " , the young animals i n the Mackenzie delta had a very secure existence.  During the whole period of the study there were almost  instances of adult attacks upon young animals.  no  The only excep-  tions noted occurred when an adult and a juvenile animal were confined over night i n the same cage trap. Young animals, tagged i n one burrow system sometimes were caught in a nearby burrow. assumed that there was  From t h i s observation i t can be  some intermingling of the juvenile popu-  l a t i o n , though probably not an interchange of young between f e males.  The whole picture was one of tolerance by adults of both  sexes towards the young animals and of older juveniles f o r those newly weaned.  We may  assume, therefore, on the basis of E r r i n g -  ton's premises concerning "inter-compensation", that the habitat was adequate f o r the summer muskrat population and that no overcrowding occurred. .Cannibalism Cannibalism i n muskrats i s usually a direct r e s u l t of stress born of physical factors i n the environment.  The term  should be d i f f e r e n t i a t e d from s t r i f e associated with the breeding season because i t r e f e r s only to purposeful k i l l i n g of other members of the same species f o r food.  It operates in environments  where animals are imprisoned by winter ice to a p a r t i c u l a r marsh or lake with no opportunity to escape therefrom. The phenomenon previously has been recorded by t r a v e l l e r s in northern regions of North America.  Hearne (1795) remarked  157  that " i t sometimes happens i n very cold winters that the holes i n t h e i r houses freeze over, in spite of a l l t h e i r e f f o r t s to keep them open.  When this i s the case, and they have no provision  l e f t in the house, the strongest prey upon the weakest, u n t i l by degrees only one i s l e f t i n the whole lodge".  P o r s i l d (1945)  stated for the Mackenzie d e l t a that "the Eskimo, who  are keen  observers, deny that rats eat f i s h , but say that r a t s , when 'frozen-up  T  i n t h e i r houses, often resort to cannibalism".  Other authors, i f they do. not .mention cannibalism, at least accurately describe conditions which predispose i t . During most winters such conditions are found on some parts of the Summerberry Eur Block i n Manitoba and they are not unknown elsewhere.  Henderson (1923) reported high winter death  rates from the Peace River region and E u l l e r (1951a) alludes to similar conditions i n the Athabaska-Peace d e l t a area. The muskrats of the Mackenzie delta have not been able to adjust to their environment s u f f i c i e n t l y that they w i l l not freeze out under adverse conditions.  Indeed, the lower t h i r d of  the delta suffers yearly from animals f r e e z i n g i n shallow lakes and succumbing to s t a r v a t i o n . Actual instances of cannibalism were r a r e l y encountered i n t h i s area, either because i t was  diffi-  cult to f i n d the remains of animals eaten by others, or the  inci-  dence of such predation was  low.  Most instances of cannibalism  recorded were during the muskrat harvest when trapped  animals  were p a r t i a l l y devoured by other muskrats. Because the number of animals f r e e z i n g i n push-ups i s greater than the number of cases of reported oannibalism., i t i s assumed that cannibalism often may  be a scavenging of dead animals  158  by those s t i l l a l i v e .  One reason i t i s not more wide-spread i s  that by the time animals  have begun to die of s t a r v a t i o n or are  at extremes of hunger s u f f i c i e n t to impel them to prey on weaker animals, t h e i r radius of a c t i v i t y i s circumscribed by the same ice and frozen s o i l that keeps them from t h e i r usual plant food. Furthermore, many of the dead animals have become sealed i n frozen push-ups and are not a v a i l a b l e to s u r v i v i n g muskrats. True cannibalism more l i k e l y i s experienced where animals  are  sealed i n bank dens by i c e , i n which case i t passes unnoticed. Mortal ity from Diseases and Parasites A hemorrhagic disease of unknown etiology has been described in some d e t a i l by Errington (1946b).  This disease has  since found favour with other authors to account for large-scale "die-of f s " of animals  i n various parts of t h e i r range.  McLeod  (1950) asserted that "every major, decline i n the past has been accompanied by an epizootic of Errington*s virus e n t e r i t i s —  or  by some s i m i l a r disease" and that " s u f f i c i e n t data are presently at hand to indicate strongly that i t recurs a t regular i n t e r v a l s of 4 to 5 years regardless of population density". MacFarlane (1908 ) spoke of an epidemic l i v e r disease that k i l l e d thousands of muskrats.  He reported that i n one year the  returns at Cumberland House, Saskatchewan, declined two-thirds as a r e s u l t .  No such disease was encountered i n the Mackenzie  muskrats though there were persistent opinions expressed by the trappers of the area that disease was numbers.  a factor controlling muskrat  The year 1933-34 often was quoted as one with a high  rate of mortality among muskrats.  Seme trappers who  remembered  159 the s i t u a t i o n at that time mentioned the l i v e r s of many animals.  f i n d i n g "white spots" i n  Therefore, during the whole course  of the investigation a l l available animals were examined f o r this condition. It was found that a coenurus type cyst of an unidentif i e d cestode was not uncommon in the muskrats examined.  These  cysts appeared as b l i s t e r s , usually at least one cm. i n diameter, and though most often concentrated in the l i v e r also were found i n the spleen and i n t e s t i n a l mesentery.  The number of cysts  u s u a l l y was less than seven though i n a few cases they p r a c t i c a l l y covered the whole surface of the l i v e r .  What depressant  effect they exerted on the muskrats was not determined.  In a  sample of 453 adult animals examined during the spring of 1948 only nine per cent were so parasitized.  Degree of i n f e s t a t i o n  was f i f t e e n per cent f o r 97 males and seven per cent f o r 356 f e males.  In nearly a l l animals in t h i s sample, and in those exam-  ined subsequently, the parasite appeared t o be doing no harm to i t s host; the animals were i n s a t i s f a c t o r y condition and had progressed as r a p i d l y toward sexual maturity as those not so a f f l i c t e d . Cowan (1948) collected samples of this cestode coenurus during the summer of 1947 and lacking adult stages t e n t a t i v e l y placed i t i n the genus Cladotaenla. He was the d e f i n i t i v e host was  of the opinion that  one of the larger r a p t o r i a l b i r d s .  The  writer suggests that the horned owl (Bubo virginianus) was the most l i k e l y candidate among birds resident i n the delta and able to prey on muskrats. It seems probable, therefore, that the " l i v e r disease" of MacFarlane  and the "white spots" seen by the l o c a l trappers  160 were the Cladotaenia larvae mentioned by Cowan.  It is e n t i r e l y  possible that during years of high incidence of these tapeworm cysts the a f f l i c t e d animals may  be e s p e c i a l l y vulnerable t o some  less evident condition which a c t u a l l y i s the l e t h a l factor. That disease and parasites were not important  limiting factors  i n the delta muskrats may be asserted from the findings of t h i s investigation.  However, rumours of "rat sickness" persisted  among the native trappers and there i s a p o s s i b i l i t y that i t has existed and may appear again despite i t s absence during the period covered by t h i s  study.  M o r t a l i t y from Predatory Animals Errington (1946a) has said that "regardless of the countless individuals or the large percentages  of populations that  may  annually be k i l l e d by predators, predation looks i n e f f e c t u a l as a l i m i t i n g factor to the extent that i n t r a s p e c i f i c s e l f - l i m i t i n g mechanisms b a s i c a l l y determine the population l e v e l s by the prey". may  maintained  He stated further that most agencies of mortality  be looked upon as merely harvesting surplus animals which are  insecurely established i n a given habitat.  We may  i n f e r , there-  fore, that the s e l f - l i m i t i n g factors which determine population levels are density dependent and only at the upper l e v e l s of popul a t i o n numbers do they begin to exert t h e i r e f f e c t .  I f animals  are amply provided for by the habitat which produced them, then u n t i l the upper asymptote i s reached wherein density, dependent factors begin t o exert themselves, predation is not important  and  probably takes off a r e l a t i v e l y constant proportion of animals, dependent upon the predator-prey r a t i o and the number of contacts possible between each.  161 I n t h e Mackenzie d e l t a t h e p o t e n t i a l f o r p r o d u c i n g muskrats was r e s t r i c t e d by the s h o r t b r e e d i n g s e a s o n but not t o any e x t e n t by the h a b i t a t .  There was a s u r f e i t o f .summer  f o o d and adequate denning s i t e s f o r t h e young animals  born.  Animals were s p r e a d throughout a l l h a b i t a t t y p e s t h a t w o u l d p o r t them at t h a t p e r i o d of the y e a r .  I f a l l of t h e s e  sup-  muskrats  had the s e c u r i t y of a good burrow system t h e n the r e l a t i v e l y  low  d e n s i t y of p r e d a t o r s i n t h e d e l t a t o the end o f summer had o n l y m i n i m a l e f f e c t on t o t a l numbers. The f i r s t f r o s t s of l a t e August or e a r l y September markedl y changed t h e g e n e r a l a s p e c t of t h e d e l t a muskrat h a b i t a t . mals i n s h a l l o w s l o u g h s suddenly found the E q u i s e t u m and which  once had been t h e i r c h i e f s o u r c e of sustenance  by f r o s t .  Ani-  Carex  blighted  This f a c t , together w i t h the g r a d u a l lowering of the  water l e v e l s i n a l l t h e l a k e s and s l o u g h s , i n d u c e d a s h i f t  of  p o p u l a t i o n towards mare permanent h a b i t a t where submerged p l a n t s were a v a i l a b l e as f o o d .  A p a r t of t h e p o p u l a t i o n t h e r e b y  was  made l e s s secure as i t moved i n t o a r e a s o c c u p i e d by w e l l e s t a b l i s h e d animals. "may  These d i s p l a c e d a n i m a l s , t o quote F u l l e r (1951a)  have t o f i g h t  f u l l y occupied".  t h e i r way  i n t o more f a v o u r a b l e h a b i t a t s a l r e a d y  A t t h i s time t h e r e was a " b i o l o g i c a l s u r p l u s "  as a n i m a l s searched f o r s a t i s f a c t o r y burrows and were h a r a s s e d and wounded i n t h e p r o c e s s .  U n t i l t h e y found adequate dens t h e y  remained v u l n e r a b l e and p r e d a t o r s had f i n d and overcome them.  increased opportunity t o  F u r t h e r m o r e , t h e p o p u l a t i o n of p r e d a t o r s  by t h a t t i m e had been augmented by n a t u r a l i n c r e a s e and was  in a  most f a v o u r a b l e p o s i t i o n t o take advantage of t h e f o o d s o u r c e p r e s e n t e d by t h e s e excess p r e y a n i m a l s .  T h e i r o p p o r t u n i t y t o do  so,  162  however, was  short-lived.  The movement of animals began i n  l a t e August and by e a r l y Oetober the l a k e s were f r o z e n and r e mained  so u n t i l t h e f o l l o w i n g May.  During t h i s time  muskrats  were withdrawn from a v a i l a b i l i t y of most t e r r e s t r i a l p r e d a t o r y animals.  That being t h e c a s e , unless p r e d a t i o n was  s w i f t and  sure d u r i n g the s h o r t p e r i o d when i t was a b l e t o a c t , i t would not  be e f f e c t i v e m a t e r i a l l y i n r e d u c i n g t h e number o f muskrats. Some i n s i g h t i n t o t h e e x t e n t of m o r t a l i t y caused by t h e  v i c i s s i t u d e s o f autumn displacement accrued from t h e t a g g i n g of live  muskrats.  As d e s c r i b e d e a r l i e r ,  j u v e n i l e muskrats were cap-  t u r e d f o r s t u d y i n two types of h a b i t a t ; temporary or summer s i t e s and permanent or yearlong s i t e s .  These marked animals were d i s -  t r i b u t e d t h e r e i n as f o l l o w s : 110  from t h e former and J l 6 from the  latter.  There i s j u s t i f i c a t i o n f o r assuming  i t y of animals from each type of h a b i t a t . mals r e c o v e r e d i n e a c h case would each c l a s s  an e q u a l r e c o v e r a b i l -  Thus t h e number of a n i -  s u p p l y an index o f s u r v i v a l i n  of s i t e .  I t was found that oraly n i n e t e e n (17  per cent) of t h e 110  .  animals from temporary s i t e s were r e c o v e r e d by the w r i t e r or any of t h e t r a p p e r s .  On the other hand 107  ( 3 4 p e r cent) of t h e  316  animals from r e a s o n a b l y permanent s i t e s were a g a i n encountered. would  appear, t h e r e f o r e , t h a t those animals e s t a b l i s h e d  lakes had about t w i c e the chance of s u r v i v a l to y e a r l i n g The  i n permanent status.  i n f e r i o r s u c c e s s of temporary h a b i t a t s as r e a r i n g  s i t e s may be a s c r i b e d not o n l y t o t h e i r  progressive  deterioration  but a l s o t o the t h r e a t of i c e and the dangers a t t e n d i n g movement to more permanent  quarters.  It  163 Hunting and Trapping The trapping e f f o r t of the human residents of the Mackenzie d e l t a was the most evident, though not necessarily the most important b i o l o g i c a l factor l i m i t i n g the growth of the muskrat population. During the winter and spring of 1945-4-6, 344 thousand animals were harvested and exported.  The following  two years the numbers f e l l progressively to 179 thousand, then jumped again to 333 thousand. was low (167  and  The t o t a l for 1949-50 again  thousand), and since that date the harvests have  increased (see Appendix D). Numerous authors have investigated the periodic abundance and scarcity of mammal populations i n northern regions. The study of E l t o n and Nicholson (1942) dealt s p e c i f i c a l l y with muskrats i n Canada.  Therein a 10-year cycle was asserted though  i t was admitted that the highs and lows of population were not synchronous  over the whole of the Dominion.  I t i s not the  writer's intention t o disagree with the 10-year cycle of E l t o n and Nicholson.  Their findings must be accepted as f a c t u a l u n t i l  more information concerning animal cycles i s made a v a i l a b l e .  But  something of the h i s t o r y of muskrat trapping i n the Mackenzie delta might assist i n illuminating the seemingly i r r e g u l a r f l u c tuations i n the number of animals harvested there over a period of time. Toward the end of World War pelts was exceptionally high.  II the price paid f o r muskrat  Naturally, the large population of  muskrats at the mouth of the Mackenzie River attracted trappers from a wide area.  In the s p r i n g of 1946 they arrived by a i r , by  boat, and by canoe from various areas of the Mackenzie D i s t r i c t  164 to share i n the lucrative spring hunt.  As a r e s u l t , the largest  number of animals ever taken from the delta was shared among the greatest number of hunters ever assembled there. year the harvest was  appreciably smaller.  The following  This was the r e s u l t ,  not so much of the mounting hunting pressure, but rather of the very severe winter with l i t t l e snow. Many of the lakes were frozen to the bottom and trappers reported many animals dead i n their push-ups.  In spite of t h i s , a s a t i s f a c t o r y harvest was r e -  ported, e s p e c i a l l y during the period of open water when the a n i mals l e g a l l y could be shot with small-bore r i f l e s . The winter of 1947-48 was reasonably mild with adequate snow-fall, but the harvest of muskrats f o r that year was only about h a l f what i t had been two years previously. two l o c a l causes contributing to t h i s d e c l i n e .  There were  The f i r s t was a  severe epidemic of influenza among the native trappers which so disrupted t h e i r early spring trapping a c t i v i t y that very few animals were taken prior to open water. floods did a r r i v e the temperature  And a f t e r the spring  remained unusually low so that  ice re-formed n i g h t l y on the lakes and t r a v e l by canoe was impossible.  almost  As a consequence, the hunting pressure was much lower  than i t previously had been.  After t h i s respite the muskrats en-  joyed a p a r t i c u l a r l y favourable summer during 1948 and the following spring 333 thousand animals were harvested. It may  be noted that i n 1949-50 the harvest again was  low.  This was f o r almost the same reasons as those outlined e a r l i e r . In the f i r s t place, though the winter had been mild, most of the lakes had been blown clear of snow and i c e depths were unusually great.  Coupled with t h i s was a widespread epidemic of measles  165 w h i c h amounted t o a v i r t u a l c a t a s t r o p h e f o r many f a m i l i e s .  The  n a t i v e t r a p p e r s were d i s c o u r a g e d by t h i s bout w i t h d i s e a s e t r a p p i n g a g a i n was d e l a y e d .  And a s i n 1948, the  and  ice conditions  were severe up u n t i l t h e end of the l e g a l muskrat s e a s o n i n midJune . S i n c e t h a t time the number of a n i m a l s h a r v e s t e d has f l u c t u a t e d s o g r e a t l y nor has t h e r e b e e n any emergency t o the e x t e n t o f t r a p p i n g e f f o r t .  Furthermore,  not  affect  the t a k i n g o f musk-  r a t s has been s t a b i l i z e d by t h e i n s t i t u t i o n of a system o f r e g i s t e r e d t r a p p i n g areas, e a c h r e s e r v e d f o r one s m a l l group o f i n d i v i d u a l s .  i n d i v i d u a l or a  T h i s has had the e f f e c t o f s t a n -  d a r d i z i n g h u n t i n g p r e s s u r e so t h a t any f l u c t u a t i o n s i n t o t a l muskrat numbers w i l l become more  apparent.  Even though t h e s i z e of the h a r v e s t d i d not always r e f l e c t the numbers o f muskrats i n the Mackenzie d e l t a , y e t t h e r e was no doubt a g e n e r a l c o r r e l a t i o n between the f u r h a r v e s t e d t h e animals a v a i l a b l e .  and  Inasmuch as v a r i a t i o n s i n the h a r v e s t  appear t o be i r r e g u l a r , r a t h e r t h a n p e r i o d i c , i t i s r e a s o n a b l e to suppose t h a t i n t e n s i v e t r a p p i n g , a s one of the major f a c t o r s i n m o r t a l i t y , i s an i m p o r t a n t i n f l u e n c e governing  population  levels. P r i o r to 1941 t r a d e as i t now  the muskrat was n o t as i m p o r t a n t i n the f u r  i s , and t r a p p i n g e f f o r t was  valuable " f i n e " fur s p e c i e s . consequence was ditions.  The  d i r e c t e d t o w a r d more  d e l t a muskrat p o p u l a t i o n as a  l e f t much more t o the i n f l u e n c e of n a t u r a l con-  W i t h t h e decrease  i n t h e market v a l u e of o t h e r f u r s ,  the t r a p p i n g p r e s s u r e d i r e c t e d t o w a r d m u s k r a t s i n c r e a s e d .  1&3S  Thereby the b i o l o g i c a l system became increasingly a r t i f i c i a l . The muskrat population  never was  allowed to b u i l d up to a point  where disease or other i n t r i n s i c factors were exerted. situation has persisted to the -present time.  That  I f the Mackenzie  delta muskrats ever d i d exhibit profound periodic  fluctuations  in numbers, the contributing causes appear to have been suppressed by intensive trapping and  an honest attempt on the part of the  trappers to produce a sustained y i e l d of animals from t h e i r registered trapping areas.  167  DISCUSSION The  e c o l o g i c a l factors facing muskrats in the Mackenzie  delta have been recounted i n some d e t a i l .  From that  discussion  i t i s apparent that no one factor of the environment i s p a r t i c u l a r l y unique but that a l l f a c t o r s taken together comprise a habitat nearly marginal f o r the resident animals.  I t i s evident  that no organism can survive where any single f a c t o r necessary for  i t s existence  i s lacking.  But organisms usually can and do  survive with only p a r t i a l f i t n e s s to each e c o l o g i c a l component in a complex environment.  Such i s the case with the population  of muskrats under investigation. The  f i r s t adjustment to the nor the rn environment con-  sidered was that concerned with choice o f habitat.  Therein i t  was pointed out that abandoning lodge b u i l d i n g and resorting t o burrows as a permanent abode i s f o r muskrats an adjustment of survival value induced by the climate.  Lodge building may be a  learned s k i l l or habit because muskrats i n the Slave River d e l t a have abandoned lodges e n t i r e l y even though i t appears that the animals could construct  and inhabit them s u c c e s s f u l l y .  It is  l i k e l y that the Mackenzie muskrats through long disuse have lost e n t i r e l y the a b i l i t y t o erect and occupy these structures.  On  the other hand, i t is evident that they could not do so i f they t r i e d because the vegetation  i s too sparse and the ice depths too  great. Choice of habitat i n summer may be a r e f l e c t i o n of the  168  inherent tendency, exhibted by muskrats i n more moderate climates, to choose to l i v e i n shallow marsh areas r i c h i n food and cover. The evidence of a depressed rate of s u r v i v a l i n such surroundings i n the f a r north indicates that the animals have had to make the necessary s h i f t towards areas of more permanent water or perish. This adjustment then, has been of value i n the continuation of the population. The dispersed nature of the lake environment  no doubt has  formed certain a c t i v i t y patterns i n the muskrats i n question.  For  instance, there i s no assurance that food of -sufficient volume w i l l be contiguous to adequate denning s i t e s .  Therefore, it i s  necessary for some animals normally to embark upon extensive forays i n search of food, not only during summer, but also i n winter beneath a heavy covering of i c e . As a consequence the radius of a c t i v i t y of most animals i s greater than that recorded for other areas of North America.  During winter t h i s movement i s abetted  by an extensive system of push-ups which replace the feeding lodges characteristic of house-dwelling animals.  Yet there i s a l i m i t t o  the amount of area which can be u t i l i z e d by one animal during winter.  The majority evidently use no more than two or three such  push-ups i n the v i c i n i t y of the heme den.  It appears therefore  that the muskrat i s by nature a sedentary animal confined to a familiar heme range.  The  size of t h i s range i n the Aklavik region  is enlarged only cut of necessity. But the fact that this extension has been made has allowed the animals to invade the area and persist therein. Food eaten by northern muskrats i s divided rather sharply  169  between emergent species in summer and submerged species i n winter.  That they abandon one f o r the other has l i t t l e s i g n i f i -  cance aside from the fact that they are forced to do so by the ambient conditions.  After the a e r i a l portions of the food plants  have been frozen they are no longer a t t r a c t i v e as food and the muskrats then t u r n t o other sources.  This progressive s u b s t i t u t i o n  as food sources dwindle in winter or burgeon i n summer is a r e f l e c t i o n of the a d a p t a b i l i t y of the species with respect t o the sustenance i t w i l l accept. Food per se is a l i m i t i n g f a c t o r , e s p e c i a l l y when i t s a v a i l a b i l i t y is r e s t r i c t e d .  Submerged plants in a lake could  be almost eliminated by muskrats grubbing  out the roots and r h i -  zomes because the pondweeds and r e l a t e d submerged plants reproduce only by vegetative means i n the l a t i t u d e of Aklavik. an instance was not observed during t h i s study but gleaned from the trappers was  Such  information  to the effect that a lake which  had "frozen cut" took several years to regain i t s productivity. It i s probable that t h i s delay i s the result of a s i g n i f i c a n t  de-  struction of the food plants. In several places in the previous discussion the question of a defended home range or t e r r i t o r y was mentioned.  E a r l y i n the  breeding season, at a time coincident with f l o o d i n g of lake-side habitat, there i s a zonation of lake perimeters territory.  into defended  But once the population has become sorted out into  pairs and more dens become habitable as the water recedes, the incidence of f i g h t i n g drops sharply.  Thereafter the size of the  t e r r i t o r y shrinks to include only the home den as animals extend their radius of a c t i v i t y i n search of food.  170 Food, however, i s not a b a s i s f o r d e f e n c e of any a r e a , d u r i n g summer.  A n i m a l s appear t o share a v a i l a b l e f o o d r e s o u r c e s  w i t h no e v i d e n c e of s t r i f e .  I n w i n t e r , on the o t h e r hand, t h e r e  i s some s o r t of s e g r e g a t i o n of a n i m a l s i n t o f a i r l y groups.  discrete  D i r e c t o b s e r v a t i o n of a n i m a l s beneath t h e i c e c o v e r i s  almost i m p o s s i b l e but t e r r i t o r i a l i t y i s a s s e r t e d f o r t h i s z o n a t i o n of l a k e a r e a s f o r want of another e x p l a n a t i o n . Perhaps the most i m p o r t a n t adjustments t o the n o r t h e r n environment is l i t t l e  a r e t h e i n t r i n s i c or b i o l o g i c a l a d j u s t m e n t s .  There  doubt t h a t some of t h e s e a d j u s t m e n t s are of g e n e t i c  o r i g i n though o t h e r s may be merely a f u n c t i o n of the e n v i r o n m e n t . One  such phe nomenon - i n the l a t t e r c a t e g o r y i s the s i z e of t h e i n -  d i v i d u a l a n i m a l s of the Mackenzie p o p u l a t i o n .  When compared w i t h  m u s k r a t s e l s e w h e r e , e v e n members of t h e same s u b - s p e c i e s , t h e y are smaller i n s i z e  and w e i g h t .  T h i s r e d u c e d s i z e most l i k e l y i s t h e  r e s u l t of t h e m a r g i n a l h a b i t a t c o n d i t i o n s .  But the  possibility  e x i s t s t h a t under such a s i t u a t i o n n a t u r a l s e l e c t i o n has f a v o u r e d s m a l l animals demanding l e s s sustenance from each u n i t a r e a of habit a t . I n t r i n s i c adjustme nt s c e r t a i n l y of g e n e t i c o r i g i n a r e those cone erned w i t h r e p r o d u c t i o n .  I t p r e v i o u s l y was s t a t e d t h a t  ice conditions tend to delay reproductive a c t i v i t y i n spring as a r e s u l t the f e m a l e s .  and  o n l y one l i t t e r was t h e norm f o r a l a r g e percentage of This s i t u a t i o n i s o f e n v i r o n m e n t a l o r i g i n b u t t h e  l a r g e r number of young produced a t a b i r t h i s c e r t a i n l y g e n e t i c . So a l s o i s t h e sex r a t i o w h i c h i s unbalanced i n favour o f male animals at b i r t h .  Because n a t u r a l m o r t a l i t y a c t s t o t h e d e t r i -  ment of males i t i s n e c e s s a r y t h a t t h e y be i n the ascendency  so  171  that there i s a roughly even sex r a t i o by the time breeding a c t i v i t y commerces. The observation was made that males occupy nest burrows •with females and young, that i s , that there i s a f a i r l y s t r i c t monogamy.  Inasmuch as monogamy i s a method of ensuring repro-  ductive e f f i c i e n c y , e s p e c i a l l y necessary where populations are sparse or discontinuous, i t may have developed  i n the Mackenzie  delta as a r e s u l t of the dispersed nature of the h a b i t a t . The extent of a monogamous r e l a t i o n s h i p between male and female muskrats has not been determined with any degree of f i n a l i t y , and appears to vary with the race and the region.  If the  north-  ern muskrats exhibit a more s t r i c t monogamy than other populations, as they evidently do, t h i s i s an adaptation that w i l l ensure a male in attendance when a female is receptive for mating. The s o c i a l r e l a t i o n s h i p between a l l animals, both adult and young, was observed to be amicable  during most of the year.  The general f i g h t i n g , characteristic of the breeding season, was of short duration and thereafter was of l o c a l occurrence  only.  I t i s suggested that lack of intolerance i s l a r g e l y the r e s u l t of space a v a i l a b l e to animals rather than any reticence on the part of individuals t o engage i n offensive a c t i v i t y .  The w r i t e r does  not dismiss the p o s s i b i l i t y that populations of muskrats may show inherent differences i n the i r tolerance to t h e i r own kind.  Cer-  t a i n l y where the reproductive rate i s high and the environment r e s t r i c t e d , an increased i r r i t a b i l i t y on the part of individual animals w i l l prevent  overcrowding.  On the other hand, where the  danger from crowding is obviated by a lower reproductive rate and ample space, intolerance on the part of representatives of the  172  population w i l l be t o the detriment  of r a c i a l s u r v i v a l .  We have recounted some of the adjustments which the Mackenzie d e l t a muskrats have made to t h e i r environment.  In  conclusion i t i s proposed to mention some of t h e factors t o which the animals have not been able to adjust, factors which may l i m i t population growth. There i s no doubt that the long winter i s the c r i t i c a l period for the muskrats i n question though climate helps t o shape their a c t i v i t i e s throughout the year. Ice depth during winter governs the amount of water and food available to animals.  I t is suggested t h a t i t also tends  to r e s t r i c t the transmission of radiation which activates the endocrine  system t o promote sexual development.  After belated  sexual maturity i s achieved, ice r e s t r i c t s breeding u n t i l the time i t i s l i f t e d by spring floods and the animals are able t o escape from t h e i r depleted winter habitat.  I f those  animals  that do survive t o breed are small i n s i z e and often i n poor condition, then the s e v e r i t y of the climate i s the chief contributing cause. On the other side of the ledger may be c i t e d some components of the environment that are favourable to the Mackenzie delta muskrats.  The l e v e l of predation from species other than  human i s only an incidental factor i n summer and i s almost absent in winter.  The amount of disease and parasitism i s low, no doubt  p a r t i a l l y as a result of the cleansing action of spring floods and t h e low winter temperatures.  Low population d e n s i t i e s also  act t o prevent the spread of diseases and parasites.  173  I n o t h e r a r e a s under r e v i e w t h e f a c t o r s l i m i t i n g  growth  of muskrat p o p u l a t i o n s u s u a l l y were o t h e r t h a n c l i m a t i c . as E r r i n g t o n (1946a) e x p l a i n e d , " l o s s t y p e s may  However,  s u b s t i t u t e natu-  r a l l y f o r each o t h e r " and " v i c t i m s of one agency s i m p l y m i s s coming v i c t i m s o f a n o t h e r " .  be-  W i t h i n any p a r t i c u l a r r e g i o n t h e  a g e n c i e s of l o s s are apt t o be p r e d o m i n a n t l y o f one k i n d .  But  from one r e g i o n to a n o t h e r we can expect them t o change as t h e e n v i r o n m e n t a l complex changes. Thus i n L o u i s i a n a and Texas , favour a b l e c o n d i t i o n s t h r o u g h out t h e year a l l o w muskrats t o b u i l d up such d e n s i t i e s of popul a t i o n t h a t t h e y l i t e r a l l y consume t h e i r h a b i t a t . type i s so i m p o r t a n t t h a t O ' N e i l l (1949) was  T h i s one  loss  prompted t o s t a t e  " t h a t a l l work t h u s f a r t e n d s t o i n d i c a t e t h a t food c o n d i t i o n s form t h e p r i m a r y b a s i s f o r c o n t r o l of muskrat p o p u l a t i o n s " . c l i m a t e e x e r t s a n e g a t i v e e f f e c t o n l y i n times of drought storms w i t h s e v e r e t i d a l a c t i o n .  The  or  E s s e n t i a l l y t h e same f a c t o r s  govern the muskrat numbers i n t h e c o a s t a l marshes of M a r y l a n d and Delaware,  though t h e s e a r e a s have s u f f e r e d i n a d d i t i o n from  o c c a s i o n a l severe The  frosts.  i n l a n d w at erways and marshes of t h e n o r t h e r n U n i t e d  S t a t e s and s o u t h e r n Canada s u p p o r t l e s s e r d e n s i t i e s of t h a n do t h e c o a s t a l marshes.  muskrats  These a n i m a l s , however, do not seem  to l i v e so amicably w i t h i n t h e i r h a b i t a t  as do t h e c o a s t a l r a c e s .  E r r i n g t o n (op. c i t . ) has documented the o f t e n d r a m a t i c n a t u r e o f t h i s l e t h a l f r i c t i o n w h e r e i n b o t h a d u l t and  j u v e n i l e a n i m a l s were  e l i m i n a t e d from a p o p u l a t i o n as i t s d e n s i t y i n c r e a s e d .  He  found  " i n t r a s p e c i f i c s t r i f e becoming d o m i n a n t l y o p e r a t i v e when predat i o n and the u s u a l r u n o f m i s c e l l a n e o u s l o s s e s a l l happened t o  174  be  low". I f comparisons are made between the coastal races, i n -  cluding the Mackenzie delta population, on the other hand, and the animals other, one pounded.  of the p r a i r i e marshes and inland waterways on the probable-reason  f o r t h i s r a c i a l s t r i f e can be pro-  The former populations a l l occupy very extensive areas  of continuous habitat while the l a t t e r are crowded into r e l a t i v e l y small though often very favourable water areas.  This discontinuous  nature of the inland habitat with i t s sharp d i s t i n c t i o n between dry and wet  areas would discourage emigration overland to  new  areas and thereby would foster population d e n s i t i e s wherein l e t h a l intolerance among resident animals would be achieved. Drought i n the home range of the i n l a n d muskrats has an occasional influence but of late disease has been given more notoriety as a population depressant.  Errington and h i s associates  at Iowa State College have documented a hemorrhagic e n t e r i t i s that at times has assumed epizootic proportions.  The severity  of the outbreaks appeared t o be a function of the population density, Errington (194 6b)  stating t h a t , "during l e t h a l e p i -  zootics those places c h a r a c t e r i s t i c a l l y accommodating low dens i t i e s may  indeed afford the s a f e s t refuges".  As mentioned e a r l i e r , McLecd (1950) i n Manitoba has accorded a prominent place to e p i z o o t i c disease as a control of muskrat numbers.  He, however, discounts the role of " i n t r a -  s p e c i f i c predation" and wrote that "no pronounced m o r t a l i t y from this cause has been noted l o c a l l y " .  He considers winter  food to be a l i m i t i n g f a c t o r and holds the view (1948) that " i n  175  the. presence of i n s u f f i c i e n t water and the lack of emergent vegetation to hold the snow, the f r o s t may penetrate into the bottom and cut o f f the food supply". It may be said i n summary that beginning with the Man! toba marshes and proceeding northward, climate exerts an i n creasing influence upon the various populations of muskrats as such other factors as disease, intolerance, and predation decrease i n importance.  176  SUMMARY  1.  F i e l d studies f o r t h i s i n v e s t i g a t i o n were conducted during  the period June 194-7 t o September 1950 i n the delta of the MacKenzie River i n northwestern Canada. 2.  The northwestern muskrat (Ondatra zibethicus spatulatus) was  the object of t h i s investigation and special attention was paid to the adjustments i n i t s a c t i v i t i e s and l i f e processes  occasion-  ed by i t s l o c a t i o n within the A r c t i c C i r c l e . 3.  Attention was accorded f i v e other races o f muskrats i n North  America, two of which are coastal and three of which occupy i n land regions.  An attempt was made to outline how these races re-  acted to factors i n t h e i r p a r t i c u l a r habitats and t o compare this reaction to that exhibited by the Mackenzie River population. 4.  For convenience of reference the adjustments of the northwes-  tern muskrat were divided i n t o those which were e x t r i n s i c or physical and those which were i n t r i n s i c or b i o l o g i c a l .  Generally  these adjustments were i n degree only. 5.  The muskrats of the Mackenzie d e l t a occupied bank dens ex-  clusively.  They could not build lodges because there was not  s u f f i c i e n t vegetation and because i c e depths i n winter were too great. 6.  Summer habitat was more abundant than winter habitat. The  l a t t e r oomprised that portion of lakes between three and twelve feet deep wherein there was an abundant growth of submerged plant material. 7.  S u r v i v a l of young r a i s e d upon lakes habitable only i n summer  177 was half that of young raised on permanently habitable lakes. 8.  Wintering muskrats depended, upon a system of feeding stations  or push-ups to extend t h e i r radius of a c t i v i t y to adequate supplies of food.  Spatulatus was more dependent upon these push-ups f o r  i t s winter s u r v i v a l than was any other race of muskrats under r e view. 9.  The number of animals using each push-up varied from three  to t h i r t e e n , with an average  of s i x .  Individual animals frequented  only one to three push-ups. but i n doing so they were forced t o t r a v e l from 32 t o 132 yards beneath the i c e . 10.  The distances t r a v e l l e d by muskrats both summer and winter  were greater i n the Mackenzie d e l t a than i n other areas under consideration.  This was because the bank dens were not always  near t o food plants and because there was a s h i f t i n g of animals from summer to winter habitat. 11.  In winter there appeared  to be a d e f i n i t e zonation of lakes  with each zone being used by a' discrete group of animals.  This  denoted either a l i m i t e d t e r r i t o r i a l i t y or was the r e s u l t of a n i mals from adjacent dens using a common food supply. 12.  Food u t i l i z e d by muskrats was divided sharply between emer-  gent shore-line species i n summer and submerged species i n winter. Equisetum and Carex constituted the summer foods with Potamogeton and Myriophyllum 13..  replacing them i n winter.  The muskrats of the Mackenzie d e l t a assumed only f r a c t i o n a l  densities of population when compared with t h e i r counterparts in southern areas.  Densities were more nearly equal to those i n the  deltas of the Saskatchewan and the Peace-Athabaska Rivers.  178 14.  As a r e s u l t o f t h e u n f a v o u r a b l e nature of t h e h a b i t a t t h e  muskrats of the Mackenzie d e l t a were of s m a l l e r s i z e and t o o k l o n g e r to r e a c h t h e i r t e r m i n a l g r o w t h t h a n d i d muskrats i n more optimum e n v i r o n m e n t s . 15.  Tagging s t u d i e s l e d t o the c o n c l u s i o n t h a t the m a j o r i t y o f  muskrats i n the Mackenzie d e l t a d i d not s u r v i v e l o n g beyond t h e i r second y e a r o f l i f e . 16.  M u s k r a t s i n t h e M a c k e n z i e p o p u l a t i o n had a v e r y s h o r t  breeding season.  Females produced o n l y one l i t t e r t h e i r  first  year b u t might have two t h e second y e a r when t h e y were s e x u a l l y mature at a n e a r l i e r d a t e . 17.  B r e e d i n g a c t i v i t y i n b o t h sexes was d e l a y e d u n t i l t h e i c e  cover was removed e a r l y i n June.  I t was c o n s i d e r e d t h a t  d e l a y might be as l o n g a s one month.  this  D e l a y e d b r e e d i n g appears  t o be a phenomenon p e c u l i a r t o n o r t h e r n r e g i o n s . 18; of  A r e v i e w of accumulated d a y l i g h t i n d i c e s i n s e v e r a l bands l a t i t u d e i n d i c a t e d that from south t o north p r o g r e s s i v e l y  more d a y l i g h t was r e q u i r e d b e f o r e a n i m a l s became mature s e x ually.  I t was assumed t h a t p h o t i c c o n t r o l of s e x u a l  cycles  was i n h i b i t e d by g r e a t e r depths of i c e and a lower l i g h t  angle  i n the h i g h e r l a t i t u d e s . 19.  For most p l a c e s i n N o r t h America the number o f young musk-  r a t s produced per a d u l t female v a r i e d f r o m t h i r t e e n t o t w e n t y w i t h an average o f s i x t e e n .  Where t h e number o f l i t t e r s was r e -  duced t h e compensating f a c t o r was an i n c r e a s e i n the number o f young a n i m a l s p e r l i t t e r .  I n the M a c k e n z i e d e l t a , however, t h e r e  was a n average of o n l y t e n young p e r f e m a l e .  179  20.  The r a t i o o f young to adult animals i n winter and spring was  as high i n the Mackenzie d e l t a as i n most other areas notwithstanding the lower number of young produced per breeding female.  This  was ascribed either to superior survival of young animals or t o a reduced s u r v i v a l of adults. 21.  Adult sex r a t i o s in delta muskrats did not d i f f e r from those  found i n other areas.  Males usually were more numerous than females  at any time of year and l 8 l males were harvested every 100 females.  i n spring for  In s p i t e of the f a c t that the animals e x h i b i t -  ed a f a i r l y s t r i c t monogamy, most females were bred and r a i s e d young during the summer months. 22.  There was l i t t l e  evidence of l e t h a l f r i c t i o n between muskrats  in the Mackenzie population.  Fighting f l a r e d b r i e f l y at the time  of spring break-up but was not common thereafter.  This s i t u a t i o n  was i n contrast to that reported from the north-central United States wherein r a c i a l s t r i f e was s u f f i c i e n t to be a major check on population growth. 23.  The incidence of disease and parasitism was low in the Ak-  l a v i k region and these f a c t o r s were considered t o be minor ones i n the c o n t r o l of population growth. 24.  Predatory animals exerted a l i m i t e d e f f e c t upon the d e l t a  muskrats which f o r eight months of the year were shielded from t h e i r enemies by the i c e cover on the lakes and channels.  The  number of predatory animals was small because of intensive t r a p ping within the area. 25.  The fur industry yearly removed a s i g n i f i c a n t portion of the  muskrat population  i n the Mackenzie d e l t a .  this had the e f f e c t of suppressing  I t was surmised that  those f a c t o r s contributing to  c y c l i c fluctuations i n numbers reported from other northern  regions.  180  26.  Factors l i m i t i n g population growth vary from one region t o  •the next.  In northwestern Canada these f a c t o r s were of a physical  sect induced by the s e v e r i t y of the climate.  Therefore, most of  the adjustments made by muskrats were i n response to t h i s severe climatic regime.  181  LITERATURE CITED Aldous, Shaler E. 1947 Muskrat trapping on Sand Lake National W i l d l i f e Refuge, South Dakota. Jour. W i l d l . Mgt. 1 1 ( 1 ) : 7 7 - 9 0 . Alexander, M. M. 1951 The aging of muskrats on the Montezuma National W i l d l i f e Refuge. Jour. W i l d l . Mgt. 1 5 ( 2 ) : 1 7 5 - l 8 6 . Alexander, M. M., and M. Radway 1951 The d i s t r i b u t i o n and production of muskrats on the Montezuma National W i l d l i f e Refuge. Jour. W i l d l . Mgt. 1 5 ( 4 ) : 3 6 0 - 3 6 7 . A l l e e , W. C., et..al. 1949 P r i n c i p l e s of animal ecology. W. B. Saunders Co., 837 pp.  P h i l a . and London,  Anderson, J . M. 1947 Sex r a t i o and weights of southwestern Lake E r i e muskrats. Jour. Mamm. 2 8 ( 4 ) : 3 9 1 - 3 9 5 . Anderson, R. M. 1946 Catalogue of Canadian recent mammals. Nat.-Mus. Canada Bui. 1 0 2 , B i o l . Ser. 3 1 , 238 pp., maps. Anonymous 1951  A survey of West V i r g i n i a mammals. Cons. Comm. W. V i r g i n i a , P-R Project 22-R, 126 pp. mimeo.  Arthur, S. C. 1931 The f u r mammals of Louisiana. Bui. 1 8 , pp. 9-439.  La. Dept. of Cons.  Bailey, Vernon 1937 The Maryland muskrat marshes. 18:350-354.  Jour. Mamm.  Banfield, A. W. i ' . 1946 A preliminary investigation of the muskrat resources of the Mackenzie d e l t a . Manuscript report, Canadian W i l d l i f e Service, Ottawa. 22 pp., map. Barnes, H. T. 1906 Ice formation with s p e c i a l reference to anchor-ice and f r a z i l . New York, J . Wiley and Sons, 260 pp. Barnes, H. T. 1928 Ice engineering. 364 pp., 5 p l .  Montreal, Renour Publ. Co.  182  Baumgartner, L. B., and F. C. B e l l r o s e , J r . 194-3 D e t e r m i n a t i o n o f sex and age i n m u s k r a t s . W i l d l . Mgt. 7 U ) : 7 7 - 7 9 .  Jour.  B e e r , J * R. 1950 The r e p r o d u c t i v e c y c l e o f m u s k r a t s i n W i s c o n s i n . J o u r . W i l d l . Mgt. 14(2):151-156. B e e r , J . R., and R. K. Meyer 1951 S e a s o n a l changes i n t h e e n d o c r i n e organs and b e h a v i o r p a t t e r n s of m u s k r a t s . J o u r . Mamm. 32(2):173-191. B e e r , J . R., and W. Truax 1930 Sex and age r a t i o s of W i s c o n s i n m u s k r a t s . W i l d l . Mgt. 14(3):323-331.  Jour.  B e l l r o s e , F. C. 1950 The r e l a t i o n s h i p of muskrat p o p u l a t i o n s t o v a r i o u s marsh and a q u a t i c p l a n t s . J o u r . W i l d l . Mgt. 1 4 ( 3 ) : 299-315. B e l l r o s e , F. C , and J . B. Low 1943 The i n f l u e n c e of f l o o d and low water l e v e l s on the s u r v i v a l of m u s k r a t s . J o u r . Mamm. 24(2):173-191. B l s s o n n e t t e , T. H. 1938a Experimental control of sexual p h o t o - p e r i o d i c i t y i n animals and p o s s i b l e a p p l i c a t i o n s t o w i l d l i f e management. J o u r . W i l d l . Mgt. 2 ( 3 ) : 1 0 4 - l l 8 . 193ob  I n f l u e n c e o f l i g h t on t h e h y p o p h y s i s . Effects of l o n g - c o n t i n u e d " n i g h t l i g h t i n g " on h y p o p h y s e c t o mized female f e r r e t s and t h o s e - w i t h o p t i c n e r v e s c u t . E n d o c r i n o l o g y 22(1):92-103.  B u c k l e y , J . L. 1954 P r e l i m i n a r y r e p o r t o f muskrat i n v e s t i g a t i o n s i n A l a s k a . A l a s k a Coop. W i l d l . Res. U n i t Report 18 pp. u n p u b l . Buss, I . 0. 1941 Sex r a t i o s and w e i g h t s of m u s k r a t s (Ondatra z. z i b e t h i c a ) from W i s c o n s i n . J o u r . Mamm. 22:403-40oT B u t l e r , L. 1940 A q u a n t i t a t i v e s t u d y of muskrat f o o d . Nat. -54(3) :37-40. Camsell, Charles 1921 The M a c k e n z i e R i v e r b a s i n . Memoir 108, 151 pp.  Canad. F i e l d  G e o l . S u r v . Canada,  183  Cook, A. H. 1943 A t e c h n i q u e f o r m a r k i n g mammals. 24(l):45-47.  J o u r . Mamm.  Cowan, I . McT. 1948 P r e l i m i n a r y s u r v e y o f t h e Mackenzie D e l t a w i t h s p e c i a l r e f e r e n c e t o t h e muskrat. Manuscript r e p o r t t o Dept. M i n e s and Res., Ottawa - 79 pp. i l l u s t r . D a l q u e s t , W. W. 1948 Mammals o f Washington. U n i v . Kans. P u b l . , Mus. N a t . H i s t . v o l . 2, 444 pp., 140 f i g s . D a v i s , W. M., and G. H. Lowery, J r . 1940 The s y s t e m a t i c s t a t u s of t h e L o u i s i a n a m u s k r a t . J o u r . Mamm. 21(2):212. Dempsey, E. W., e t . a l . Absence of l i g h t and t h e r e p r o d u c t i v e c y c l e i n the guinea p i g . Amer. J o u r . P h y s i o l . 109:307-311. Dorney, R. S., and A. J . Rusch 1953 M u s k r a t growth and l i t t e r p r o d u c t i o n . Dept. Tech. W i l d l . B u i . 8, 32 pp.  Wise. Cons.  D o z i e r , H. L. 1947 S a l i n i t y as a f a c t o r i n A t l a n t i c coast t i d e w a t e r muskrat p r o d u c t i o n . N o r . Amer. W i l d l . Conf. T r a n s . 12:398-420. 1948  E s t i m a t i o n o f muskrat p o p u l a t i o n s by house Nor. Amer. W i l d l . Conf. Trans. 13:372-392.  1950  M u s k r a t t r a p p i n g on t h e Montezuma N a t i o n a l W i l d l i f e Refuge, New York 1943-48. J o u r . W i l d l . Mgt. 1 4 ( 4 ) : 403-412.  1953  M u s k r a t p r o d u c t i o n and management. U. S. Dept. I n t . E i s h and W i l d l . S e r v . C i r c u l a r 18, 42 pp., p i s .  counts.  D o z i e r . H. L., M. H. M a r k l e y , and L. M. L l e w e l l y n 1940 Muskrat i n v e s t i g a t i o n s on t h e B l a c k w a t e r N a t i o n a l W i l d l i f e Refuge 1941-43, J o u r . W i l d l . Mgt. 12(2):177-190. E a r d l e y , A. J . 1938 Yukon c h a n n e l s h i f t i n g . 343-358.  G e o l . S o c . Amer. B u i . 49:  E l t o n , C , and M. N i c h o l s o n 1942 F l u c t u a t i o n s i n numbers o f muskrats (Ondatra z i b e t h i c a ) i n Canada. J o u r . An. E c o l o g y 11:96-126.  184 E r r i n g t o n , P. L. 1937 H a b i t a t r e q u i r e m e n t s o f stream d w e l l i n g m u s k r a t s . Nor.. Amer. W i l d l . Conf. Trans. 2:411-416. a  1937b  The ^ b r e e d i n g s e a s o n of t h e muskrat  Iowa.  1939  a  1939b  J o u r . Mamm. 1 8 ( 3 ) : 3 3 3 - 3 3 7 .  i n northwest  Observations on young muskrats i n Iowa. Mamm. 20(4): 465-478.  Jour.  R e a c t i o n s o f muskrat p o p u l a t i o n s t o d r o u g h t . Ecology 2012):l68-l86.  1941  V e r s a t i l i t y i n f e e d i n g and p o p u l a t i o n maintenance of t h e muskrat. J o u r . W i l d l . Mgt. 5 ( 1 ) : 6 8 - 8 9 .  1943  An a n a l y s i s o f mink p r e d a t i o n upon m u s k r a t s i n n o r t h c e n t r a l U n i t e d S t a t e s . Iowa Agr. Exp. S t n . Res. B u i . 320:797-924.  1944  A d d i t i o n a l s t u d i e s o f tagged young m u s k r a t s . W i l d l . Mgt. 8 ( 4 ) : 3 0 0 - 3 0 6 .  1946a  P r e d a t i o n and v e r t e b r a t e p o p u l a t i o n s . Q u a r t . B i o l . 2 1 ( 2 ) : 1 4 4 - 1 7 7 , 21(3):221-245.  1946b  S p e c i a l r e p o r t on muskrat d i s e a s e . Iowa Coop. W i l d l . Res. U n i t Q u a r t . Rept. J u l y - S e p t . 1946:34-51.  1947  S p e c i a l r e p o r t on c e r t a i n v i t a l s t a t i s t i c s of Iowa m u s k r a t s . Iowa Coop. W i l d l . Res. U n i t . Q u a r t . Rept. Oct.-Dec. 1 9 4 7 .  I948  E n v i r o n m e n t a l c o n t r o l f o r i n c r e a s i n g muskrat p r o d u c t i o n . Nor. Amer. W i l d l . Conf. T r a n s . 1 3 : 5 9 6 - 6 0 9 .  Jour.  Rev.  E o r b e s , T. R. 1942 The p e r i o d o f gonadal a c t i v i t y i n t h e M a r y l a n d muskrat, Science 95(2467):382-383. P u l l e r , W. A. 1951a N a t u r a l h i s t o r y and economic importance of t h e muskrat i n t h e Athabaska-Peace d e l t a , Wood B u f f a l o P a r k . Canad. W i l d l . Serv. W i l d l i f e Mgt. B u i . S e r . 1 No. 2, 80 pp., i l l u s t r .  185 F u l l e r , W. A. 1951b Measurement s and weight s of n o r t h e r n m u s k r a t s . Mamm. 32(3):360-362. G a s h w i l e r , J . S. 1948 Maine muskrat i n v e s t i g a t i o n s . l a n d F i s h and Game Augusta.  Jour,  B u i . Maine Dept. I n 38 pp.  1950a  A s t u d y of t h e r e p r o d u c t i v e c a p a c i t y of Maine muskr a t s . J o u r . Mamm. 31( 2) :l8o-185 .  1950b  Sex r a t i o s and age c l a s s e s of Maine m u s k r a t s . W i l d l . Mgt. 14(4):384-389.  Jour.  Gould, H. N., and F. B. Kreeger 1948 The s k u l l of t h e L o u i s i a n a muskrat (Ondatra z i b e t h i o a r i v a l i o i a Bangs) : I . The s k u l l i n advanced age. Jour. Mamm. 29(2):138-149. Grimm, W. C , and H. A. R o b e r t s 195 0 Mammal survey of s o u t h w e s t e r n P e n n s y l v a n i a . Game Comm. H a r r i s b u r g , 99 pp., i l l u s t r .  Penna.  H a l l , E . R., and E. L. Cockrum 1953 A s y n o p s i s of the N o r t h American m i c r o t i n e r o d e n t s . U n i v . o f Kans. P u b l . , Mus. Nat. H i s t . 5(27):373-498. Hearne, S. 1795 A j o u r n e y from P r i n c e of Wales F o r t on Hudson s B a y t o the n o r t h e r n ocean i n the y e a r s 17&9, 1770, 1771, and 1772. London. T  Henderson, A . D. 1923 C y c i e s of abundance and s c a r c i t y i n c e r t a i n mammals and b i r d s . J o u r . Mamm. 4:264-265. x  H o l l i s t e r , N. 1911 A s y s t e m a t i c s y n o p s i s of the muskrat. U. S. Dept. Agr., B i o l . Surv., Nor. Amer. Fauna 32. 47 pp., i l l u s t r . Hume, G. S. 1924 Mackenzie r i v e r a r e a , D i s t r i c t of M a c k e n z i e , N o r t h west T e r r i t o r i e s . Summary Rept. 1923 P t . B, Canada G e o l . S u r v . , 15 pp. Jenness, J . L. 1949 P e r m a f r o s t i n Canada.  A r c t i c 2 (1)-.13-28.  Johnson, C. E. 1925 The muskrat i n New York.  3:199-320.  Roosevelt W i l d l . B u i .  186  Laur ie , E. M. 0 . 1946 The reproduction of the house mouse (Mus mus cuius) l i v i i n different environments. Proc. Roy. Soc. B, 133:248-281. Law, C. E. 1950 Ecology of the muskrat of the Slave River d e l t a , N.W.T. Univ. of B. C., unpubl. B.A. t h e s i s , 120 pp. Lay, D. W. 1945 Muskrat investigations i n Texas, Jour. W i l d l . Mgt. 9(l):56-76. Lay, D. W., and T. O'Neill 1942 Muskrats on the Texas coast. 6(4):301-311.  Lynch, J . 1940  Jour. W i l d l . Mgt.  Origin and natural maintenance of some a r c t i c waterfowl habitats. U.S. Fish and W i l d l . Serv. unpubl. manuscript report, 34 pp.  Lynch, J . , T. O'Neill, and D. W. Lay 1947 Management significance of damage by geese and muskrats t o gulf coast marshes. Jour. W i l d l . Mgt. 11(1):50-76.  McCann, L. 1944 Notes on growth , sex and age r a t i o s , and suggested management of Minnesota muskrats. Jour. Mamm. 25(1):59-63.  MacFarlane, R. 1908 Notes on mammals c o l l e c t e d and observed i n the northern Mackenzie River D i s t r i c t , Northwest T e r r i t o r i e s of Canada. In Mair 1908, Through the Mackenzie Basin. Toronto, Wm. Briggs. pp. 153-494. McLeod, J . A. 1948 Preliminary studies of muskrat biology i n Manitoba. Trans. Roy. Soc. Canada 42, Ser. 3 , Sect. 5:81-95. 1949  Some aspects of muskrat management., i n Manitoba. Conv. Int. Assoc. Game, Fish and Cons. Comm. 39:64-71.  1950  A consideration of muskrat populations and population trends i n Manitoba. Trans. Roy. Soc. Canada 44, Ser. Sect. 5:69-79.  Marsh, D. B. 1948 Ratting i n the d e l t a .  Beaver 278:32-35.  187  M a r s h a l l , F. H. A. 1936 ' S e x u a l p e r i o d i c i t y and t h e c a u s e s w h i c h d e t e r m i n e i t . Roy. Soc. London P h i l o s . Trans. S e r . 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R e c o r d 60:279-289. M u l l e r , S. W. 1947 P e r m a f r o s t or p e r m a n e n t l y f r o z e n ground and r e l a t e d e n g i n e e r i n g problems. Ann A r b o r , M i c h . , J . W. Edwards I n c . 231 pp., i l l u s t r . O ' N e i l l , T. • 1949 The muskrat i n the L o u i s i a n a c o a s t a l marshes. New O r l e a n s , L a . Dept. W i l d l i f e and F i s h e r i e s , Game and F i s h D i v . , 132 pp., p i s . and maps. Osgood, W. 1900 R e s u l t s of a b i o l o g i c a l r e c o n n a i s s a n c e of t h e Yukon R i v e r r e g i o n . U. S. Dept. A g r . , B i o l . S u r v . , N o r . Amer. Fauna 19. Pancoast, 1937  J . M. M u s k r a t i n d u s t r y i n s o u t h e r n New J e r s e y . W i l d l . Conf . T r a n s . 2:527-530.  Nor. Amer.  Penfound, W. T., and J . D. S c h n e i d a u 1945 The r e l a t i o n of land r e c l a m a t i o n t o a q u a t i c w i l d l i f e r e s o u r c e s of s o u t h e a s t e r n L o u i s i a n a . N o r . Amer. W i l d l . C o n f . T r a n s . 10:308-318. P o r s i l d , A. E . 1937 F l o r a o f t h e Northwest T e r r i t o r i e s . I n Canada's Western N o r t h l a n d , Dept. Mines and R e s o u r c e s , Ottawa, pp. 130-141.  188  P o r s i l d , A. E . 1945 Mammals of the Mackenzie d e l t a . 59(D:4-22.  Canad.  Field-Nat.  P r e b l e , E. A. I9O0 A b i o l o g i c a l i n v e s t i g a t i o n of the A t h a b a s c a M a c k e n z i e r e g i o n . U. S. Dept. A g r . B i o l . Surv. , Nor. Amer. Fauna 27. Rae, J o h n 1888  N o t e s on some of t h e b i r d s and mammals o f t h e Hudson's Bay Company's t e r r i t o r y and t h e a r c t i c c o a s t of A m e r i c a . J o u r . L i n n . S o c , Z o o l o g y 20:136-145.  Richardson, J . 1829 Fauna b o r e a l i - a m e r i c a n a . P a r t F i r s t , London, I . M u r r a y , 300 pp., 24 p i s .  Quadrupeds.  Robinson, J . L. 1946 Weather and c l i m a t e o f the Northwest T e r r i t o r i e s , Canad. Geog. J o u r . 32:124-139. Rowan, W. 1925  R e l a t i o n of l i g h t t o b i r d m i g r a t i o n and d e v e l o p m e n t a l changes. N a t u r e 115:494-495.  R u s s e l l , F. 1898 E x p l o r a t i o n s i n the f a r n o r t h . 290 pp., 21 p i s . , map.  Iowa C i t y , U n i v . Iowa,  S m i t h , F. R. 1938 Muskrat i n v e s t i g a t i o n s i n D o r c h e s t e r County, M a r y l a n d 1930-34, U. S. Dept. A g r i c . C i r c . 474, 24 pp. Snead, I . E . 195 0 Muskrat i n v e s t i g a t i o n s . report.  Iowa S t a t e C o l l e g e u n p u b l .  S o o t e r . C. A. 194b M u s k r a t s of the Tule Lake Refuge, C a l i f o r n i a . W i l d l . Mgt. 1 0 ( l ) : 6 8 - 7 0 .  Jour.  Soper, J . D. 1948 Mammal notes f r o m t h e Grande P r a i r i e - P e a c e R i v e r r e g i o n o f A l b e r t a . Jour . Mamm. 2 9 ( l ) : 4 9 - 6 4 . S v i h l a , A., and R u t h D. S v i h l a 1931 The L o u i s i a n a m u s k r a t .  J o u r . Mamm. 1 2 ( l ) : 1 2 - 2 8 .  Takos, M. J . 1944 Summer movements o f banded m u s k r a t s . Mgt. 8(4) : 3 0 7 - 3 H .  Jour. W i l d l .  189  Takos, M. J . 1944 A semi-quantitative study of muskrat food habits. Jour. W i l d l . Mgt. 11(4) : 331-3 39. Tut ton, A. E. H. 1927 A natural h i s t o r y of ice and snow. London. Paul, Trench, Tubne r and Co., 319 pp.  Kegan,  Warwick, T. 1934 The d i s t r i b u t i o n of the muskrat (Fiber zibethicus) i n the B r i t i s h I s l e s . Jour. An. E c o l . 3(2) :250-267. Yeager, L. E. 1945 Capacity of I l l i n o i s land types to produce f u r s . Nor. Amer. W i l d l . Conf. Trans. 10:79-86.  190  APPENDIX A 136°  134°  1320  191  APPENDIX m WEATHER-DATA FOR SELECTED AREAS OF NORTH AMERICA OCCUPIED BY SIX DIFFERENT RACES OF MUSKRATS Locality  Month": J » F » M * A » M « J * J» A • S * 0 « N « D » Ave. Monthly and Year!Ly Averages o f D a i l y Mean Temo. (K°)  Aklavik N.W.T.  -21 -15  F t . Smiths N.W.T.  -16  The Pas Man.  -7  10  34  53  58  53  39  19  -7 •-16  16  -9  3  26  44  54  60  56  45  30  10  -8  25  -9  -2  11  33  48  59  65  61  49  35  17  1  31  Des Moines Iowa  20  24  37  50  61  70  76  73  65  54  38  26  50  Chesapeake Bay, Md.  36  36  44  53  62  70  75  74  68  58  46  38  55  New Orleans La.  55  57  63  69  75  81  82  82  80  71  62  56  70  Monthly and Annual Snowfall i n 'Inches Aklavik N.W.T.  6.0 4.7 3.9 6.2 2.7 1.2  0  T 3.1 8.0 6.8 6.0 48.6  F t . Smith NiW.T.  5.4 6.0 4.9 4.1 1.3 0.1  0  T 0.8 4.7 8.3 6.7 42.3  The Pas Man.  6.1 5.0 7.0 4.6 1.0  0  0  0 0.1 5.1 9.1 7.9 44.9  Des Moines Iowa  8.6 7.3 5.5 1.2  T  0  0  0  0 0.3 2.1 7.5 32.5  Chesapeake Bay, Md.  3.3 4.3 2.0 0.3  0  0  0  0  0  0  0  0  0  0  New Orleans 0.1 0.2 La.  Ti:  0  0.5 2.2 12.6. 0  TI  T  0.3  192  APPENDIX  C  ACCUMULATED HOURS OF DAYLIGHT IN FIVE LATITUDE BANDS INMNORTH AMERICA USING THE AUTUMNAL EQUINOX AS THE DATUM POINT 1  Date  Total Hours Accumulated Daylight 3o°m  40°Ni  53PN?  6o°m  68 N.  Sep. 30  130  130  132  135  140  Oct. 10  256 379 499  256 377.' 493  257 375 487  261 378 485  267 382 482  617 733 846  605 714 820  592 692 788  583 675 762  570 648 717  958 1067 1177  923 1024 1126  880 969 1058  843 920 997  776 827 879  1288 1401 1516  1231 1337 1445  1149 1243 1341  1077 1162 1253  937 1003 1078  1632 1751 1873  1556 1671 1791  1444 1554 1670  1350 1455 1569  1163 1261 1370  1998 2126 2257  1915 2044 2178  1792 1921 2058  1691 1822 1963  1491 1625 1772  2391 2528  2668  2316 2459 2608  2203 2356 2516  2114 2276 2448  1937 2119 2323  10 20  2811 2956 3104  2761 2918 3078  2683 2857 3036  2631 2825 3030  2552 2792 3032  Jian. 10  3253 3403  3241 3405  3220 3407  3247 3478  3272 3512  20  30  Nov. 10  20  30  Dec. 10  20  30  Jan; 10  20  30  Feb. 10  20  28  Mar. 10  20  30  Apr. 10  20  30  May  30  20  Gulf Coast  Maryland Iowa  The Pas Peace R. Man. Delta  G  Aklavik N.W.T.  193  APPENDIX  D  STATEMENT OF THE NUMBER AND KINDS OF SOME OF THE FURS EXPORTED) FROM THE MACKENZIE DELTA DURING THE PERIOD 1930-31 TO 1953-54  Kind of Fur Year  Fox Coloured  Mink'  1930-31 1931-32 1932-33 1933-34 1934-35  537 1277 1294 1420 2390  686 22111 4465 4673 2417  93,963 200,992 170,139 60,680 82,339  1271 3641 2691 755 963  1935-36 1936-37 1937-38 1938-39 1939-40  2582 3216 2144 1431  908 629 112 216 813  54,370 64,798 117,802 190,740 216,115  566 1551 1658 3076 2010  1940-41 1941-42 1942-43 1943-44 1944-45  2148 3468 2893 3552 1114  1345 2668 1213 792 1094  256,195 237,708 298,787 192,123 229,753  2133 5455 2343 558 486  1945-46 1946-47 1947-48 1948-49 1949-50  1656 1112 462 321 380  2282 1907 1245 10165 1000  344,393 234,475 178,956 332,619 167,318  2022 1672 1069 1578 2318  1950-51 1951-52 1952-53 1953-54  323 649 168 58  1495 1270 1343 1590  245,300 303,831 193,392 206,603  2762 3190 1978 2170  36622  37395  4,673,931  47916  2027  Totals  Muskrat  Weasell  ANNUAL NUMBER OFMUSKRAT PELTS TRADED IN THE MACKENZIE DELTA DURING THE PERIOD 1930-54  INCLUSIVE  195  ACKNOWLEDGEMENT S The w r i t e r w i s h e s t o p o i n t o u t t h a t t h e d a t a i n the preceding  presented  pages were g a t h e r e d w h i l e was employed a s a  b i o l o g i s t w i t h t h e Canadian W i l d l i f e S e r v i c e .  Some o f t h e i n f o r -  m a t i o n h e r e i n may be found i n r e p o r t s t o t h a t agency. There a r e many p e r s o n s who c o n t r i b u t e d i n one way o r a n o t h e r t o t h i s study.  Dr. I a n McT"£Cowan-deserves f i r s t men-  t i o n f o r h i s continued  guidance and encouragement from t h a t JJune  day when f i r s t we stepped a s h o r e i n A k l a v i k from t h e f l o a t - e q u i p p ed a i r c r a f t w h i c h had c a r r i e d us n o r t h from Norman W e l l s . A f t e r o u r a r r i v a l many r e s i d e n t s o f t h e Mackenzie d e l t a a s s i s t e d us by t h e i r numerous i&ndnesses and s p l e n d i d h o s p i t a l i t y . Most o f t h e t r a p p e r s o f t h e a r e a c o n t r i b u t e d a d v i c e and a s s i s t a n c e but s p e c i a l t h a n k s s h o u l d be e x p r e s s e d t o Khud Lang, B e r t Boxer, and Bruno Wiedemann.  I t was on t h e i r t r a p p i n g grounds t h a t much  o f t h e f i e l d work was done and they always were w i l l i n g t o g i v e f r e e l y o f t h e i r t i m e , and o f t e n t h e i r r equipment a s well!. personnel  The  a t t h e Government R e i n d e e r Range S t a t i o n d e s e r v e m e n t i o n  not o n l y f o r p r o v i d i n g l i v i n g q u a r t e r s b u t a l s o f o r t h e i r a s s i s t ance i n s e v e r a l phases o f t h e f i e l d work.  The w r i t e r i s espec-  i a l l y g r a t e f u l t o Mr. and M r s . Lee P o s t , Mr. and Mrs.  F i n l e y Mclnnes,  and Andrew P i l o n . D.R. F l o o k a s a s t u d e n t a s s i s t a n t made t h e summer o f 1949 much more p r o f i t a b l e by h i s h e l p and c h e e r f u l companionship.  Dur-  i n g t h e s u c c e e d i n g summer C.E. Law, who s e r v e d i n t h e same c a p a c i t y , provided the b e n e f i t o f h i s experience  w i t h muskrats i n t h e d e l t a  196  of .the Slave River. of both these  I t is a pleasure to acknowledge the  help  men.  Plant specimens c o l l e c t e d during the course of the i n vestigation were i d e n t i f i e d by A. E . Porsild of the  National  Herbarium of Canada and by Dr. T. M . C. Taylor of the U n i v e r s i t y of B r i t i s h Columbia.  In conclusion the w r i t e r wishes t o express  his appreciation to Dr. W. A. Clemens, past head of the Department of Zoology at the U n i v e r s i t y of B r i t i s h Columbia, f o r his help i n o u t l i n i n g the objectives of t h i s study.  

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