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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 Earl Stevens A Thesis Submitted in Partial Fulfilment of the Requirements for 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) is nowhere more abundant than in the delta of the Mackenzie River 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 Arctic Circle. The rigours of the physical environment demand that these animals live in burrows rather than In lodges of vegetation, as in more southern areas. During summer the breeding pairs may occupy shallow temporary habitat which is not suitable for tenure during winter. These summer sites must be vacated before ice seals the lakes and prevents escape from a rapidly deteriorating environ-ment. That a l l muskrats do not desert this temporary summer habitat was indicated by the fact that only half as many marked animals from such sites were encountered subsequently. It may be assumed that a depressed survival was the rule in such locations. Only deeper lakes with adequate submerged food plants constituted satisfactory wintering environment. Normal movement of muskrats in the Mackenzie delta is an average distance of about 300 yards in summer and 100 yards in winter. Winter activity is supported by an extensive system of feeding stations or "push-ups" constructed on the lake i c e . These structures are a necessary part 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 varies from three to thirteen with 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 that 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 sexually at an e a r l i e r date t h e i r second year, produce two l i t t e r s . Inasmuch as the late winter population i s comprised of four yearlings to each adult female, the delay i n breeding i n -duced by the l a t e removal of ice 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 effects 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 larger l i t t e r s (8.3 young) and by a very satisfactory survival of these young to yearling status. Intolerance between adults i s noted during the early part of the breeding season but does not p e r s i s t during the rearing of the young. As a consequence there are few losses from depredations of adults upon young animals as has been reported i n other areas. Densities of animals per u n i t area are low when compared with races of muskrats from other regions. In addition the size of the in d i v i d u a l animal i s small, and the majority do not survive 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 habitat for muskrats. I t i s suggested that physical factors induced by the severity of the climate represent the major influence l i m i t i n g population 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 factors appear to be les s important. A l l mortality factors taken together, however, have suppressed or eliminated any tendency f o r muskrat numbers to fluctuate 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 text to races of muskrats inhabiting 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 S t u d i e s P R O G R A M M E O F T H E Sinai ©r&l ^xmnination for ttye Jbgra of Jtottot* of |Jl)tIoso^Ii]j of WARD EARL STEVENS B . S c . ( U t a h S t a t e A g r i c . C o l l e g e ) 1 9 4 2 M . S c . ( I o w a S t a t e C o l l e g e ) 1 9 4 7 S E P T E M B E R 2 3 r d , 1 9 5 5 , a t 3 : 0 0 p . m . I N T H E B I O L O G I C A L S C I E N C E S B U I L D I N G R o o m 1 8 7 A C O M M I T T E E I N C H A R G E H . F . A N G U S , Chairman I . M c T . C O W A N J . S . A L L E N J . A . A D A M S V . C . B R I N K M . D . F . U D V A R D Y A . J . W O O D W . S . H O A R T . M . C . T A Y L O R S . E . R E A D External Examiner — D R . W . R O W A N U n i v e r s i t y o f A l b e r t a ABSTRACT T h e n o r t h w e s t e r n m u s k r a t [Ondatra zibcthicus spatulatus) i s n o w h e r e m o r e a b u n d a n t t h a n i n t h e d e l t a o f t h e M a c k e n z i e R i v e r . A n i n v e s t i g a t i o n w a s u n d e r t a k e n t o a s s e s s t h e a d j u s t m e n t s t h e s p e c i e s h a s m a d e i n o r d e r t o i n h a b i t a r e g i o n m o r e t h a n o n e h u n d r e d m i l e s i n s i d e o f t h e A r c t i c C i r c l e . T h e 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 d e m a n d t h a t t h e s e a n i m a l s l i v e , a t l e a s t i n w i n t e r , i n b u r r o w s c o n s t r u c t e d i n t h e s h o r e s o f d e e p e r l a k e s . T h i s m o d e o f l i f e i n d u c e s a d j u s t m e n t s i n r a d i u s o f m o v e m e n t , i n f o o d a n d f e e d i n g h a b i t s , a n d i n r e p r o d u c -t i v e a c t i v i t y . A s i n g l e l i t t e r y e a r l y i s n o r m a l f o r p r i m i p a r o u s f e m a l e s t h o u g h t w o l i t t e r s m a y b e b o r n e t h e s e c o n d y e a r . A n i n c r e a s e d n u m b e r o f y o u n g p e r l i t t e r a n d s u p e r i o r s u r v i v a l t o a d u l t s t a t u s c o m p e n s a t e f o r t h e r e d u c e d r a t e o f b i r t h . T a g g i n g s t u d i e s l e d t o t h e c o n c l u s i o n t h a t m o s t D e l t a m u s k r a t s d o n o t s u r v i v e l o n g p a s t t h e i r s e c o n d y e a r o f l i f e . T h e f u r i n d u s t r y r e p r e s e n t s o n e o f t h e m a j o r d r a i n s o n m u s k -r a t n u m b e r s t h o u g h t h e s e v e r i t y o f t h e c l i m a t e i s t h e p r i m e f a c t o r l i m i t i n g p o p u l a t i o n g r o w t h . T h e r e i s n o e v i d e n c e t h a t m u s k r a t s o f t h e M a c k e n z i e d e l t a u n d e r g o p o p u l a t i o n c h a n g e s o f a c y c l i c n a t u r e . D e n s i t i e s o f a n i m a l s p e r u n i t a r e a g e n e r a l l y a r e l o w . C o m p a r i s o n s a r e m a d e 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 m u s k r a t s i n h a b i t i n g m o r e 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 . N I C H O L C o m p a r a t i v e P h y s i o l o g y W , S. H O A R B i o l o g i c a l M e t h o d s - W . A . C L E M E N S a n d I . M c T . C O W A N P o p u l a t i o n D y n a m i c s P . A . L A R K I N A d v a n c e d S t u d i e s ( Z o o l o g y ) I . M c T . C O W A N O t h e r 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 . H U T C H I N S O N B i o m e t r y V . C . B R I N K D i r e c t e d S t u d i e s ( B o t a n y ) T . M . C . T A Y L O R P h i l o s o p h i c a l P r o b l e m s B . S A V F . R Y TABLE OF CONTENTS page INTRODUCTION 1 REVIEW OF LITERATURE 4 Taxonomic Status of the Northwestern Muskrat . 9 Resume of Habitats 11 THE STUDY AREA 21 Physical Features of the Mackenzie Delta . . . 21 Climate of the Area 36 METHODS OF PROCEDURE 39 Duration of the Study 39 Location and Description of the Study Areas . . 41 Methods of Travel . . 51 Methods of Live Trapping 52 Methods of Tagging 55 Methods of Handling Live Animals 56 THE INVESTIGATION 59 E x t r i n s i c or Physical Adjustments 61 Summer Habitat 64 Winter Habitat 66 Winter Push-up Studies 71 Yearly Radius of A c t i v i t y 78 Food and Feeding Habits 85 Population Density 96 I n t r i n s i c or B i o l o g i c a l Adjustments 100 Size and Growth Rates of Muskrats 100 Longevity of Northern Muskrats 109 Breeding P o t e n t i a l and Breeding Habits . . . I l l Age Ratios 133 Sex Ratios 137 L i t t e r s per Year 143 Number of Animals per L i t t e r 1^7 Factors in M o r t a l i t y . . . Int 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 156 Diseases and Parasites 158 Predatory Animals . 160 Hunting and Trapping 163 DISCUSSION 167 SUMMARY 176 LITERATURE CITED 181 APPENDICES 190 ACKNOWLEDGEMENTS 195 ( H i ) LIST OF TABLES Table 1. 2. 3. 5. 6. 7. 8. 9. 10. 11. 12. Page Number of D i f f e r e n t Animals Trapped per Push-up on Grassy Lake, November, 1947 765 Catch of Animals by Sex and Age on Grassy Lake I n November, 1 9 4 7 . 7 7 Number of Muskrats Tagged by Months i n the Mac-kenzie D e l t a During 1947-50 I n c l u s i v e . . . . 79 Resume of Food P l a n t s Eaten by Muskrats i n the Mackenzie D e l t a During Each Month 91 P r o d u c t i v i t y of Selected Areas i n Muskrats per Acre 97 A Comparison of Weights and Standard Measure- , merits' of Muskrats From D i f f e r e n t P a r t s of North America 1 0 1 Weights and Measurements of a Sample of Muskrats Known t o be A d u l t , Mackenzie D e l t a , 1947-49 . 1 Q J ^ Average D a i l y Weight Gains of Muskrats i n the Mackenzie D e l t a and i n Three Other Areas . . 1 ° ° C a l c u l a t e d Ages of Recaptured Muskrats Grouped In t o F i f t y - D a y Age Classes 1 1 0 Breeding C o n d i t i o n of Female Muskrats C o l l e c t e d i n S p r i n g 1 2 0 Dates and Accumulated D a y l i g h t Indices of Events i n the Reproductive Cycle of Muskrats . . . . 128 Greatest D i f f e r e n c e i n Length of Day f o r F i v e Areas'-of North America 1 3 0 ( i v ) LIST OF TABLES (continued) Table Page 13. Age Ratios of Trapped Muskrats in the Mac-kenzie 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 17. The Breeding Activities of Five Races of Muskrats at Several Locations in North America 145 18. Evidence of Litter Size in Muskrats From Post-Mortem Examination 149 ( v ) LIST OF FIGURES Figure Rage 1. Distribution of Six Geographical Races of Muskrats i n North America 7 2. Aerial View of the Caribou H i l l s . . . . 23 3. Outcropping of Limestone at Campbell Lake 25 4. Arcuate and Oxbow Lakes in the Mackenzie River Delta 29 5. Map of Study Area No. 1 42 6. Depth Contours in 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 9. Map of Study Area No. 3 48 10. Juvenile Muskrats in Live Traps . . . . 52 11. Adult Muskrat in Holding Cone 56 12. Method of Weighing Live Muskrats . . . . 57 13. Relationship Between Snow and Ice Depths 70 14. Fate of Push-ups on Grassy Lake . . . . 75 15. Segregation of Muskrats by Zones in Grassy Lake During Winter . . . . . . . 83 16. Monthly Average Measurements of Male Muskrat Reproductive Organs ^ ° 17. Rate of Increase in Uterine Width i n Adult and Sub adult Muskrats . . . . 118 ( v i ) INTRODUCTION The fur industry represents the major souroe of income for the residents of Northwest Canada. And one of the most l u -crative areas from t h i s standpoint i s the delta of the Mackenzie River. Therein i s concentrated the largest trapping population i n the D i s t r i c t of Mackenzie, a l l of whom depend almost exclu-s i v e l y upon the revenue they derive from, the sale of muskrat f u r s . Within t h i s vast delta w e l l w i t h i n the A r c t i o C i r c l e thrives the most northerly population of muskrats i n North Ameri-ca. The modifying influence of the great r i v e r has allowed these animals to become established therein 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 to survive. In 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 to be preferred to "adapta-t i o n s " , a more inclusiv e term. Adaptation connotes a physiologi-c a l reaction having genetic o r i g i n whereas adjustment, as used hereafter, refers to more immediate responses to external 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 within any genetic complement f o r a f a i r l y wide range of adjustments to environmental factors. Where to draw the l i n e between adaptation and adjustment may not i n a l l oases be clear. An example of the difference i s provided by the reindeer or A s i a t i o caribou. These animals were 2 introduced into northern Canada and even af t e r twenty years of establishment have retained 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 condi-tions usually are severe. I t might be said that the reindeer beoame "adapted" through genetio selection to the climate of i t s o r i g i n a l homeland and when transferred t o a new environment i t was not able to adjust to the more severe conditions encountered. I t i s possible that 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. Certainly the l a t t e r i s more circumscribed i n i t s effects 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 refers to adaptations whioh are a part of the animal, and the l a t t e r refers to those which are an extension of the animal and increase i t s f i t n e s s under a given set of circumstances. Thus the powerful i n c i s o r teeth of a beaver are endosomatic adaptations f o r cuttin g wood, but the lodges and dams the animal builds by employing these teeth are exosomatio adaptations allowing i t to 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 available genetic d i v e r s i t y within a species, of i t s p o t e n t i a l -i t y for ohange. As Medawar (op. c i t . ) explained, when speaking of adapt-ations, whether endosomatic or exosomatic, we consciously or un-consciously i n f e r a comparison, either between populations with respect to a p a r t i c u l a r environment, or between environments with respect to a given population. Herein the w r i t e r w i l l deal with a p a r t i c u l a r species, the muskrat, and w i l l investigate the degree of adjustment i t displays In each of several habitats. Thereby an attempt w i l l be made to explain the presence of an isolat e d population of muskrats i n an environment nearly a r c t i c i n i t s severity. For the purpose of making v a l i d comparisons with other regions the w r i t e r has not hesitated to draw upon and interpret data from various sources. I t i s hoped that no inaccuracies or ambiguities i n reporting the findings of other investigators have resulted. A l l references used are acknowledged i n the text and l i s t e d i n the bibliography attached. 4 REVIEW OF LITERATURE The muskrat i s so widely di s t r i b u t e d i n North Amerioa and has survived so well the impact of c i v i l i z a t i o n and atten-dant agriculture that i t s presence generally i s evident i n any wet areas which provide f o r i t s habitat 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 material i n an expanse of shallow water and waving c a t t a i l s . The demands of the muskrat are f u l f i l l e d not only by these extensive marsh areas, but as Errington (1937, 1941) has recorded, by intermittent 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 investigators have found them i n mountain streams and tundra lakes, i n c i t y parks and navigation canals. The wide v a r i e t y of oircumstances under which they can endure and thrive i s proof of t h e i r a b i l i t y to adjust to t h e i r environment and to thrive i n the face of seem-ingly adverse conditions. In the past the greatest amount of study has been d i -rected toward the more eooncmioally important races of muskrats, esp 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 habits and habi-t a t s . Generally, the inoentive for these investigations was a desire to understand the l i f e processes of the animals so that through habitat control a larger fur crop could be produced. As a consequence the l i t e r a t u r e concerning muskrats has grown volu-minous and investigators continually have attacked more basic 5 problems i n l i f e h i s t o r y and habitat requirements. Thus a v a i l -able information has become more quantitative as study tech-niques have beoome more exact. At the present time there i s no doubt that we know more about the habits and ecology of the muskrat than about any other w i l d fur bearer. In spite of the acknowledged v a r i a b i l i t y of habitat, eaoh area occupied by the several 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 peculiar c h a r a c t e r i s t i c s w i l l be enumerated and com-pared. The geographical raoe of muskrats which occupies the northwestern portions of North America was the object of t h i s investigation. A majority of the f i e l d work was conducted i n the delta of the Mackenzie River. Herein flourishes a large but i s o l a t e d population of muskrats, the most northerly repre-sentatives of the species on t h i s hemisphere. In order to gauge the adjustments which t h i s population has had to make i n order to survive, comparisons w i l l be drawn with other geographical races i n North America. The type subspecies zibethicus and the closely related cinnamomlnus w i l l be used as the norm for comparison because they have been accorded more thorough study than any other raoes and occupy a range reasonably central w i t h i n the species range. In the following discussions concerned with habits and habitats they w i l l be considered as a single e n t i t y . The geographical races occupying the brackish 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 habitat standpoint but w i l l be dealt with separately because of differences i n plant growth and general conditions 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 to and integration with the northwestern muskrat. Lack of s u f f i c i e n t published data on the muskrat of 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 with other coastal races than with those occupying a more continental type of habitat. Seven races were omitted either because they represent r e l a t i v e l y small populations or because t h e i r habits are not s u f f i c i e n t l y unique to d i f f e r e n -t i a t e them. Furthermore, t h e i r inclusion would tend to make comparisons unnecessarily involved. The subspeoies referred to i n t h i s study are therefore the following: 1. Ondatra zibethicus zibethlous -Common muskrat 2. Ondatra zibethicus maorodon - V i r g i n i a muskrat 3 . Ondatra zibethious r i v a l i o i u s -Louisiana muskrat 4. Ondatra zibethious oinnamominus -Great Plains muskrat 5. Ondatra zibethicus albus -Hudson Bay muskrat 6 . Ondatra zibethious spatulatus -Northwestern muskrat & Relegated from s p e c i f i c to subspecific 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 hereafter between these subspecies. An attempt w i l l be made to 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 with the problems of 7 10 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 ecological adjustment being considered. On the other hand, be-cause the northwest muskrat has received mueh less attention from investigators than have the more southerly races, o r i g i n a l material concerning i t w i l l be given i n more d e t a i l . Some phases of the investigations which have no counterpart i n other studies w i l l be included to complete the pieture 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 with which t h i s study deals i s the north-western muskrat, Ondatra zibethious spatulatus. 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, zibethicus. 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 authori-1 t i e s since that date have followed his 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 as: "Northwestern North America, from Kowak River (east of Kotzebue.Sound) and Yukon Valley i n Alaska, through the lower parts of Yukon to the A r c t i c coast, north to Richards Island i n Mackenzie de l t a , Northwest T e r r i t o r i e s , south and east to Anderson River, Great Bear, and Great Slave Lakes, and south into northwestern B r i t i s h Columbia and north-central Alberta; probably also into 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 larger teeth, less broadly spreading zygomata and a tendency towards an increase i n rusty colour throughout the pelage. 1 The most recent synopsis concerned with muskrats was that of H a l l and Cockrum (1953). The w r i t e r has followed the nomen-clature presented therein. 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 section of the continent, the northwestern muslcrat has i n the past reoeived very l i t t l e attention from s c i e n t i f i c i n v e s t i -gators. 11 Resume of Habitats At t h i s point the w r i t e r would l i k e to supply a back-ground for subsequent discussions by giving a b r i e f resume of the habitat occupied by each of the geographical races of musk-rats under review. This resume w i l l deal mainly with conditions of the habitat and only i n c i d e n t a l l y with the relationship of muskrats to t h e i r environment. That information w i l l be i n -cluded under the appropriate section i n the body of the text. HABITAT RESUME OF THE COMMON MUSKRAT The t y p i c a l race zibethious i s an animal of the small inland lakes and watercourses w i t h i n the temperate eastern sections of North America south to Delaware Bay. I t i s not, however, a coastal 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 association with t i d a l waters. The animal suffered when a g r i -c u l t u r a l practices destroyed much of i t s former habitat, but t h i s condition has been a l l e v i a t e d to some extent by the con-str u c t i o n of impoundments along such major r i v e r s as the Ohio and the Missouri and by extensive use of drainage ditches i n Iowa, I l l i n o i s , and neighbouring states. Fewer attempts have been made to provide suitable habitat for t h i s race than for the Gulf coast or the Hudson's Bay races. However, zibethious 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 studies to date have been done under the reasonably controlled conditions 12 provided by these refuges. Dozier and his associates worked on the Montezuma Refuge i n New York, Be l l r o s e , Brown and Low at the refuges along the Ohio River, 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 established popu-lati o n s of muskrats as Errington (1939) and Bellrose and Low (1943) have demonstrated. These factors usually do not produce any direct m o rtality of animals but have that effect- i n d i r e c t l y by disturbing the habitat and placing the insecure animals i n c i r -cumstances more subject to other destructive factors. The type of ground or s o i l conditions associated with populations of muskrats seldom have been reported. Errington (1937) considered that d a y s o i l s were more suited to bank denning muskrats since d a y would allow construction of an ex-tensive burrow system. Personal observation i n northern Iowa showed the s o i l s to be f a i r l y heavy and i n long-established sloughs often covered with a loose layer of organic material of variable depth. Areas of deep peat deposits were not common i n Iowa although they were reported farther to the north. HABITAT RESUME OF THE ATLANTIC TIDEWATER MUSKRATS The V i r g i n i a muskrat oocupies the coastal region from Delaware Bay south to Pamlioo Sound, and maintains i t s greatest concentrations of population i n the t i d a l marshes bordering t h i s coastline. Here prime muskrat habitat seems to be that offered by the "ebb and flow" marshes. The water therein i s fresh or s l i g h t l y brackish and var 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 habitat has a variable depth of organic material above i t . Smith (1938) reported t h i s thickness of peat to be s i x inches and remarked that optimum water l e v e l s were at or near the sur-face of t h i s peat layer. Dozier (1948) was of the opinion that inundation by high tides and floods increased the s a l i n i t y of the coastal marshes thereby disrupting the plant succession to the prejudice of the muskrat population. Drought also increased the s a l i n i t y of the . marshes and resulted i n m o r t a l i t y to muskrats, especially the young born during periods of hot dry weather. Quite a different 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 saline conditions 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 habitat for the V i r g i n i a muskrat. The ebb and flow marshes further inland exhibited less s a l i n i t y and henoe produced Typha, flagreed (Phragmites communis), and bulrushes (Scirpus Qlneyi. S. amerioanus, and S. robustus). RESUME OF HABITAT CONDITIONS AT GULF TIDEWATER The coast marshes of Louisiana 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 brackish t i d a l areas t h i o k l y grown to three-square bulrush, (Scirpus Qlneyi) and cord grass (Spartina patens). The former plant furnished most of the food whereas the l a t t e r provided the bulk of the building material f o r lodges. This marsh vegetation grows on highly organic s o i l s (Lynch et. a l , 1947). Wherever there i s an 14 admixture of clay with the peat, "coco" marsh i s formed which has an in t r u s i o n of s a l t grass ( D i s t i c h l i s splcata). 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, they cannot produce as many muskrats over as long a period of time. The adjaoent fresh water habitats pro-duce i n f e r i o r vegetation and comparatively fewer muskrats. Great areas of the preferred braokish type are found with-i n the continuous region of coastal marshes. Under-trapping of animals therein r e s u l t s not only i n deterioration of the habitat by "eat-outs", but also i n increased f i g h t i n g , predation, and emi-gration. Ice i s a r a r i t y and though frosts may k i l l some of the emergent vegetation there i s usually ample for food year-long. Growth of marsh plants i s luxuriant and under normal conditions proceeds r a p i d l y enough to replace plant material u t i l i z e d by muskrats and waterfowl. F i r i n g of the marshes i s practised to arrest the hydrosere short of the olimax condition (Penfound and Sohneidau, 1945) and to foster the growth of plants favoured as food by muskrats. The exclusion of s a l t water from the marshes i s discouraged because i t Inhibits adequate growth of the desired plant association. 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 draining into Hudson Bay from the west, i n eastern Saskatchewan and Keewatin; north to 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 t h i s v i c i n i t y and the adjacent delta of the Saskatchewan River, albus 15 attains i t s greatest abundance. Recently concentrations of animals here have been bolstered by large fur management pro-jects developed both by the federal and p r o v i n c i a l governments. J . A. MoLeod and his associates (1948, 1949, 1950) con-ducted studies of muskrat biology on the Manitoba Government Fur Reh 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 to the Summer berry Fur Block i n the Saskatchewan River delta. Here a system of seventy dykes and control gates of various sizes have helped to s t a b i l i z e water levels 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 that "shortly 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 to the new margin is seen, and under suitable water l e v e l conditions a new zonation i s established i n two or three years, and a maxi-mum density i s reached i n about f i v e years. One would expect t h i s to continue i n an unaltered state almost I n d e f i n i t e l y but under conditions of consistent high water a deterioration sets i n , and i n about ten years the area has reverted to open water". S o i l sampling indicated to those workers that " i n the absence of complete decomposition of plant material i n the water and mud by bacteria and animal organisms the nitrogen would be found i n no simpler form than amino acids" and therefore not available to growing plants. This s t a b i l i z a t i o n of le v e l s also discouraged establishment from seeds of seme major species of marsh plants which required mud banks or very shallow water for germination. In long 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 faotors of sudden and 16 profound flooding were also a problem i n marsh ecology. During May, 1949, the author (1949 unpubl.) visited the Summerberry de-velopmental area and examined the results of flooding that had occurred the previous year. The summer of 1948 had been one of exceptionally high water. Not only did the water levels rise above previous marks but they stayed high throughout the summer. The whole of the Saskatchewan delta was transformed into a vast lake and ample evidence of this flooding was reflected in the altered marsh ecology as well as in the broken dykes and flood gates. Extensive areas of flag-reed were k i l l e d and growths of cattails, separated from their substratum, either persisted as floating islands or were blown ashore. Sedges adapted to the flooded conditions by producing stems as long as five feet. Concomitant with the reduced vegetation, the number of muskrats harvested the following year was lower. Under normal circumstances water depths i n this marsh seldom exceeded eight or ten feet and often were much less. The soils of more recent origin consisted of clay with l i t t l e humus incorporated therein. Some areas seen by the writer, however, exhibited a layer of organic matter twelve to eighteen inches deep above this hard stratum of clay. During summer these de-posits often were floated to the surface by decomposition gases and were seen to 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 in-vestigation because i t s geographical range is restricted to the 17 less s e t t l e d parts of the continent. E a r l y references alluded to i t s generally wide d i s t r i b u t i o n but since 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 description of the northern animals, probably albus and spatulatus. and ascertained that they extended northward nearly to the mouth of the Mackenzie, i n l a t i t u d e 6 9 0 . Rae (1888) and Russell (1898) mentioned that muskrats were very abundant i n some places. Russell i n t r a v e l l i n g from Grand Rapids on Lake Winnipeg to Herschel Island, Yukon, traversed the deltas to the Saskatchewan, Athabaska, Slave, and Mackenzie Rivers and noted that muskrats abounded i n such locations. Preble (I908) wrote that "on the lower reaches of the Mackenzie and Peel Rivers muskrats are ex-cessively abundant". MaoFarlane (1908) found them "very common on the lower Mackenzie River, and less so on the same portion of the Anderson River to t h e i r outlets i n the polar sea". Richardson lop. c i t . ) provided a general description of the habitat of Ondatra i n North America. That most of his ex-perience with the species was i n Canada may be judged from h i s observation that the animals i n winter were almost extirpated i n certain areas by freezing of the swamps which they inhabit. He further says that " t h e i r favourite 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 inhabiting muskeg ponds and small streams i n the h i l l s near Fort Chipewyan. Near the mouth of the Peace River he saw them frequenting landlocked ponds i n dense spruce woods. He also found them on the islands 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 miles north of Lake Hardisty. Muskrats were not enoountered on the south shore of Great Bear Lake hut were found near Fort Franklin where they "were occupying burrows i n the banks and were l i v i n g mainly on coarse grass, which was abundant on the marshy parts of the shore". Russell (1898) commented upon t h i s use of bank dwellings by muskrats. He noted that "they do not b u i l d winter houses as extensively as those farther south, but usually burrow i n the banks of streams and ponds; they prefer the streams to the swamps, and are espe c i a l l y abundant i n the deltas of the large 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 smaller channels". Later detailed studies of the northwestern muskrat i n -cluded those by Banfield (1946) for the Mackenzie d e l t a , by Cowan (1948) i n the same region, by F u l l e r (1951a, 1951h) i n Wood Buffalo Park, and by Law (1950) i n the Slave River d e l t a . The endeavours of Banfield and Cowan were preliminary to the pre-sent investigation. F u l l e r (195D concentrated h i s e f f o r t s on several ponds and sloughs l y i n g along the l e f t bank of the Athabaska River be-tween Embarras landing s t r i p and the settlement of Embarras Portage. He noted that "the area contains three large sloughs, a number of potholes, which usually hold water the year round, and several marshy areas which dry up i n late summer and autumn". I t was found that "many of the best muskrat sloughs ... were 19 ponded sections of old stream beds" which s i l t a t i o n had closed at either end. Three zones of vegetation were t y p i o a l of the ponds and sloughs i n F u l l e r 1 s study area. 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). oarices. bulrush (Scirpus v a l i d u s ) , and bur-reed (Sparganium multlpeduncu-latum). Water to depths of four feet supported dense mats of yellow water l i l y (Nuphar variegatum) and beneath t h i s , grading into deeper water, grew waterweed (Anaoharis), arrowhead (Sagit-t a r i a ) , crowfoot (Ranunculus). and bur-reed (Sparganium). Com-pl e t e l y submerged plants i n the deeper portions 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 oontail (Ceratophyllum). m i l f o i l (Myriophyllum). and pondweeds (Potamogeton spp.). The amount of each species varied with the water depths i n the ponds and sloughs. The northwestern muskrats b u i l t three types of structure i n the Athabaska d e l t a . Winter houses constructed mainly of cat-t a i l and bulrush were prominent features of the marsh. Feeding huts or "push-ups", erected after "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 flooding. A summer study by Law (1950) i n the delta of the Slave River, only two hundred miles to the north of the Athabaska del t a depicted some in t e r e s t i n g changes i n choice of habitat by the resident muskrat population. In the f i r s t place, the animals oocupied bank dens or burrows yearlong and did not construct 20 lodges of emergent vegetation, even though, plants were abundant. Their burrows were dug i n suitable clay banks bordering the deeper sloughs and the l e s s active channels. Neither steep banks nor shelving 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 to reach the more suitable banks. The animals i n t h i s delta had to cope with heavy s i l t a -t i o n , which during the oourse of a year could render previous habitats unsuitable either 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 of water there-i n . Law reported t h i s d e l t a to be unforested though high ground would support willow, alder, and some poplar. C a t t a i l , h o r s e t a i l , marsh reed grass (Calamagrostis), and bulrush com-prised most of the emergent vegetation. Floating and submerged plants were s i m i l a r to those found by F u l l e r (op. c i t . ) at the Athabaska areas. 21 THE STUDY AREA Physical Features of the Mackenzie River Delta LOCATION OF THE MACKENZIE DELTA Northeastern B r i t i s h Columbia, the northern half of Alberta, and northwestern Saskatchewan a l l f a l l within the Mackenzie River 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 eventually finds i t s way into the Maokenzie River by way of the Peace, the Athabaska, the Slave, and the Li a r d Rivers. Three large bodies of fresh water, Athabaska Lake, Great Slave Lake, and Great Bear Lake also con-tribu t e to t h i s drainage. The Mackenzie River, as such, begins as the outlet to Great Slave Lake and flows westward and northward more than 1,000 miles to the Beaufort Sea. This gradient of the stream i s small, about f i v e inches to the mile, 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 with islands and s h i f t i n g bars of mud and gravel. The whole length of the waterway from the Fort Smith rapids to the Beaufort Sea, a distance of 1,400 miles, 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 happily 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 flood p l a i n and flows for most of i t s distance between banks one hundred to 22 f i v e hundred feet high. The stream narrows to about f i v e hun-dred yards at the Ramparts above Good Hope but at other places i t i s two to three miles wide. A few miles 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 into 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 . Here i t s p l i t s into three main channels — the West Channel which joins a branch of the Peel River and flows past Aklavlk to the Shoal-water Bay; the Middle Channel, which ramifies into many streams near i t s mouth, flows into both Shoalwater Bay and Mackenzie Bay; and the East Channel which begins a few miles below Point Separation and flows along the foot of the Caribou H i l l s into Kugmallit Bay. Numerous secondary channels and creeks which are active yearlong form interconnecting l i n k s between these larger channels. The islands formed by these main and second-ary channels are dotted with innumerable lakes and marshes. The settlement of Aklavik l i e s near the centre of the delta area at 68° 13» N. 135°W. The distance from the Richard-son Mountains on the west to the Caribou H i l l s on the east side averages f o r t y miles, though at i t s mouth the r i v e r spreads out to cover almost seventy miles. The length of the delta from Point Separation to the s a l t water i s roughly one hundred miles. The present a l l u v i a l portion of the delta, with which we w i l l deal, comprises therefore an area of at leas t 4,000 square miles. TOPOGRAPHY 0E THE DELTA AREA Conspicuous on the western side of the Delta 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 to a height of about 5,000 feet. Northward the 2 3 elevation of the h i l l s flanking the delta gradually decreases as the main range trends westward away from the coastal p l a i n . On the opposite side of the delta the Caribou H i l l s extend from the v i c i n i t y of Campbell Lake northward to the A r c t i c Ocean. They are intersected at Tununuk by the East Channel of the Mackenzie and continue from thence as the e l e -vated portions of the Richards Island. This range of h i l l s i s nowhere more than 300 feet high (Fig. 2) and marks the western edge of a r o l l i n g and poorly drained plateau that separates the Mackenzie delta 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 Station. 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 trending i n a rather aimless fashion 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 generally higher i n the upper reaches of the delta. I t is not u n t i l one mounts to commanding elevations that the amazing abundance of lake and marsh habitat becomes apparent. The panorama i s one i n which the water area surely exceeds that of the land. I t may be seen that the watercourses have formed natural levees along t h e i r banks during times of high water and that the intervening country is lower and comprised of lakes and marshes. The sheets of water are small, few a t t a i n i n g to a square mile of area. A majority of the lakes and channels are interconnected so that they flood during the spring and gradually subside during summer. However, the amount of drainage i s r e s t r i c t e d by the depth of the small waterways that connect the lakes with the main channels. Few of the lakes drain out completely though many lose enough water so that i n summer they a t t a i n a marshy character and during winter may quickly freeze to the bottom. The lakes, which do not have access to the channels, are main-tained by a run-off of surface water. Because evaporation i s slow i n these l a t i t u d e s these lakes acquire a ce r t a i n s t a b i l i t y both i n water levels and i n the attendant ecological conditions. That such lakes are i n a minority was seen during an a e r i a l sur-vey of the delta i n mid-June of 1948, a year of unusual water volume. I t was estimated that at least ninety percent of a l l 25 lakes had been flooded, the clear 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 that need be said about the geology of the lower Mackenzie River 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 int r u s i o n of lower Devonian limestone at the south end of Campbell Lake (Fig. 3) although Camsell 11921) otherwise did 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 basin was glaciated i n the past. They postulated that the ice from the Keewatin centre invaded the Mackenzie basin from the east and pushed to the foot of the mountains where i t was met and overlain by the Cordilleran ice flowing down from higher l e v e l s . In the lower Mackenzie there appeared to have been a general northward flow of a sheet of ice 1,500 feet t h i c k which extended down the v a l l e y and out to sea. The disappearance of th i s ice 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 effect of release from.the ice load was an u p l i f t of the entire basin with the i n s t i t u t i o n of a new cycle of erosion. There seems to be l i t t l e doubt that the delta of the Mackenzie has ri s e n within 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". Muller (1947) defined permafrost as "a thickness of s o i l or other s u p e r f i c i a l deposit, even of bedrock, at a variable depth beneath the surface of the earth i n whioh a temperature below freezing has existed for a long time, (from two to tens of thou-sands of years)". An "active" layer, delineated by the depth of the summer thaws, l i e s above the perpetually frozen s o i l . The thickness of t h i s layer depends upon the amount of i n s u l a t i o n by plant and other materials, and upon the texture and water content of the s o i l . In the region of the Mackenzie delta exposed banks 27 of sand may thaw to a depth of s i z feet during l a t e summer. On the other hand, at Aklavik the s i l t - c l a y s o i l under a sparse growth of grass (Poa arotioa) had thawed to only thirty-two inches i n early 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 delineating the southern boundary of permafrost in 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 Yellowknife. The depth of the permafrost layer i n the Mackenzie delta has never been ascertained. Any shafts which have been sunk for basements or ice c e l l a r s have f a i l e d to pass through the frozen earth at depths to twenty-five feet. This i s not surprising since Jenness (1949) c i t e d depths of 230 metres i n Siberia and over 130 metres i n the environs of Point Barrow, Alaska. Shafts d r i l l e d for petroleum at Norman Wells, over two hundred miles to the south of Aklavik, did not pierce below permafrost u n t i l depths of 267 feet (8 l metres) had been reached. I t i s assumed that the beds of r i v e r s are free from perma-f r o s t . This fact 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 shafts d r i l l e d at one hundred feet, two hundred feet, and three hundred and f i f t y feet from the water 1s edge exhibited 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 feet, and two hundred and sixty-seven feet re-spectively. Yet i t also i s true that islands i n the Mackenzie River and those forming i t s delta show a permanently frozen con-d i t i o n . We may assume, therefore, that permafrost i s being 28 formed i n these lat i t u d e s under the e x i s t i n g climatic conditions. DELTA BUILDING AND SCULPTURING The delta of the Mackenzie i s of the estuarine type, that i s , the materials forming i t were deposited i n a long narrow estuary or arm of the sea. The delta 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 materials by coastwise currents and growth has been uninterrupted seaward. Camsell (1921) noted that for i t s s i z e the Mackenzie River oarries r e l a t i v e l y l i t t l e sediment and that i t s delta 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 -i p p i . The great lakes along the oourse of the Mackenzie River system act not only as flood control reservoirs but also as sediment basins. I t i s only during the spring flood 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 c l e a r . Channel braiding of the Mackenzie River at i t s delta i s profound and most of the channels remain active continually. Thus, although the water current i s slow, erosion of the down-stream face of meander curves along with a deposition on the opposite bank are occurring constantly. This water action forms an unusually large number of that type of lake which Lynch (1940) c a l l s "arcuate swales" (Elg. 4); lakes between p a r a l l e l bars on the upstream face of meander curves. Seme of these lakes may be t r a n s i t o r y although others, e s p e c i a l l y flanking the Middle and East Channels, are of a more permanent nature. In addition to the "arcuate swale" i n the delta i s the 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 silte d up one or both ends of the former channel. If such lakes maintain communication with the adjacent stream, they s i l t in rapidly. However, i f they become isolated from the parent stream, they persist for a longer period and may be so changed in form by erosion of their banks and subsequent coalescence with other lakes, that their origin is d i f f i c u l t to assess. Many old channels, only partly sil t e d in, remain as drainage ways between the island lakes and the main streams, and as such ,they become almost dry in the late summer. The ox-bow type represent some of the deepest lakes in the Mackenzie delta. The third 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 built up so that a depression in the interior becomes a catchment basin for surface water. Such lakes may extend for three or four miles although some measure less. In the upper part of the delta lakes of this sort are smaller than those below Aklavik but this no doubt is a function of their relative 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 alter them; draining some, f i l l i n g in others, and wearing away the banks of those which remain. It was mentioned previously that there is 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. In both, lakes and channels, such action i s assisted by several other factors which are mostly seasonal i n nature. During spring floods the streams are f i l l e d from bank to bank with great blocks of ice which not infrequently jam and build up a head of water and ice as high as t h i r t y feet above the usual l e v e l s . When the impounded water and ice are released they not only deepen the ohannel but also may overflow and scatter ice f a r back into the timber. In spite 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 to rlgour-ous winter temperatures, are frozen s o l i d l y below the f i r s t few surface inches. Thus, though they become smoothed by the grind-ing of the i c e , the penetration i s s l i g h t , and because the flow of ice l a s t s less than a week l i t t l e damage i s done. Eardley (1938) found the same ice conditions p r e v a i l i n g i n the Yukon River of Alaska, and concluded also that bank erosion by ice was inconsequential i n i t s e f f e c t s . The permanently frozen s o i l incorporates a great deal of ice either intimately associated with the s o i l p a r t i c l e s or separated as clear blue lenses. When t h i s ice melts s o l i f l u c t i o n 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 effects of water and a i r . Hence the banks are r a p i d l y under-cut. The surface layers 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 slowly than the exposed banks nearer water l e v e l . As t h i s process of undercutting continues, the surface layer comes to overhang the stream and eventually drops into i t i n large angular ohunks. Under suitable conditions i t i s probable that reflected sunlight i s just as important i n loosening the bank materials as the action of water. In two instances sunlight caused high stream banks to recede f i f t e e n and twenty-five feet respectively one summer. Wind-driven ice i s probably one of the main factors i n al t e r i n g lake configuration. Ice persists In the lakes long after the r i v e r s are clear. When the water levels have been raised at the time of flooding, large pans of the ice break loose and are blown from shore to shore u n t i l they disintegrate. Such gouging of the banks i s bolstered by melting of permafrost and as a consequence many of the lake shores are rather abrupt. Wind has another effect on the d e l t a i c deposits. During winter the exposed banks, though frozen, become very dry and as a consequence dust i s continually being blown from them. The effect i s small as compared with some of the other factors d i s -cussed but i t i s noticeable because the snow i n the v i c i n i t y be-comes mixed with d i r t and i s the f i r s t to melt in spring. Wind also regulates the depth of ice on delta lakes to the extent that i t removes the protective layers of snow. VEGETATIVE COVER AND PLANT SUCCESSION IN THE MACKENZIE DELTA There are two climax vegetative types represented i n the Mackenzie delta and i t s v i c i n i t y . The f i r s t of these i s spruce for e s t , a northward extension of the boreal forest, and the 33 second i s dry heath, or tundra. P o r s i l d (1937) described these zones as Hudsonian and Arotic respectively but made a reservation concerning the l a t t e r . Dry tundra, he s a i d , may have i t s ooni-ferous climax retarded by lack of r a i n f a l l and the extremely dry a i r of winter. Under such oircumstanoes dry tundra may be re-garded as true p r a i r i e or steppe and i s not, s t r i c t l y speaking, within the A r c t i c zone. The upper two-thirds of the Mackenzie delta i s forested with white spruce (Picea glauca), but shows very r e s t r i c t e d amounts of black spruoe (Picea mariana) and tamarack (Larix l a r i o i n a ) i n the moister habitats. A point twenty miles below Aklavik joined with a point f i f t e e n miles below the Reindeer De-pot approximately delineates the northern l i m i t of coniferous species. Beyond that i s a zone of alder (Alnus crispa) and willow (Salix spp). Northward the alder disappears u n t i l only willow i s found at the coast. There the extensive willow 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 forested as the delta i s b u i l t further seaward. The tundra habitat is r e s t r i c t e d to the higher land bor-dering the Mackenzie d e l t a . Along the Caribou H i l l s a difference i n elevation of no more than three hundred feet is s u f f i c i e n t for the vegetation type to change from forest to tundra. Within the forested portions of the delta there i s a pro-gressive succession of plant types from the invaders on raw a l l u -v i a l s o i l s to the heath species on highly organic s o i l s . Lynch (1940) l i s t e d the species colonizing mud bars and f l a t s as horse-t a i l (Equlsetum variegatum and E. arvense) together with water-oats (Arctophila 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 angustifolium). These are followed subsequently by willows on the better drained areas, and i n turn by alder, pop-l a r (Populus taoamahacca), and f i n a l l y white spruce as the ground i s b u i l t progressively higher. Away from the channel edges the' inner portions of islands may show the e f f e c t s of poor drainage. These low wet areas develop an accumulation of organic material l a r g e l y contributed by such mosses as Sphagnum and Hypnum. With these grow various ericaceous species, notably Labrador-tea, (Ledum groenlandicum), crowberry (Empetrum nigrum), oranberry (Vaocinium spp.). b i l b e r r y (V. uliginosum). and various oarices, including cotton sedge. Such plants form an understory for scattered white spruce, and the whole association surrounds these more permanent lakes. Those lakes which o r d i n a r i l y did not flood were apt to tend towards an acid condition even though t h e i r waters did not show much organic s t a i n . Within 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 (Cal 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 1 s - t a i l (Hippuris v u l g a r i s ) . This type of habitat was more common in the southern h a l f of the d e l t a and was associated with a mature climax oondition. Those lakes reoeiving a yearly load of a l l u v i a l material showed quite a d i f f e r e n t vegetative complement. Suoh lakes were a majority of those both above and below the l i n e of timber. They represented the newer habitat, that i n more active development. These a l l u v i a l lakes exhibited an emergent association consisting of bur-reed (Sparganlum angustifolium), and h o r s e t a i l (Equisetum f l u v i a t i l e ) often mixed with water-oats, and around 35 the periphery, variable 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 willow and alder. The h o r s e t a i l , however, could not withstand the rigorous 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 plants i n the lakes throughout the delta. This included the pondweeds (Potamogeton  gramineus var. graminifolius. 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 vulgaris var. amerioana), and, i n r e g u l a r l y flooded lakes, muskgrass (Ghara sp.). Other species worthy of mention i n a l l u v i a l lakes 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 reptans). An alga of the genus Cladophora was en-countered i n several of the lakes i n the v i c i n i t y of Aklavik 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 cold freshwater lakes with.variable amounts of organic s t a i n . The shorelines were usually devoid of tree growth except i n more sheltered s i t e s where there were a few low willows. The vegetation immediately surrounding these lakes was usually 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 region of the delta t h i s species did 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 several species of pondweeds were found in lakes shallow and warm enough 36 to support t h e i r growth. Otherwise, the tundra lakes, 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 vege-t a t i o n . CLIMATE A r c t i c North America may be divided into two major clim-a t i c regions. 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 River with C h u r c h i l l , Manitoba. I t i s characterized 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 is the true A r c t i c region. The second region, comprising most of the Yukon and'the remainder of the Northwest T e r r i t o r i e s , has a sub-Arctic climate wherein the average temperatures of the three warmest months are above 30°F., and the averages for the coldest months are below freezing. The boundary between the A r c t i c and sub-Arctic regions bears no r e l a t i o n s h i p to 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 coincident with the northern extent of tree species. I t may, as explained elsewhere, l i e north of the tree l i n e . S t r i c t l y speaking, therefore, most of the Mackenzie delta l i e s w ithin the sub-Arctic regions. At Aklavik the summer temperatures of the three warmest months — June, J u l y , and August, have averaged 4 8 . 5 ° , 36.3° and 50.2°F. respectively. The yearly average, however, i s only 13.5°F. Average annual extremes of temperature range from 83.3°F. to minus 31.8°F. with absolute extremes of 93°F. and minus 36°F. recorded. January is the coldest month with an average of minus 19°F. and -37 J u l y the warmest with 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 to 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. In the f i r s t place, cold 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 that does f a l l i s the result of warm Polar 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. Usually the Mackenzie v a l l e y settlements record only ten to t h i r t e e n inches of moisture yearly with J u l y the wettest month. At Aklavik there has been an average of 10.03 inches of moisture yearly, of which 6.20 inches f e l l as snow and the re-mainder as r a i n . R a i n f a l l , e s p e c i a l l y , varies from season to season. Winter snows are f a i r l y well 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 period snow may c o l l e c t to depths of three feet i n wooded areas. In more exposed s i t e s the snow may be blown into compact d r i f t s of greater depth. Winds at Aklavik are predominantly from the north or the south. Wind v e l o c i t i e s generally are low during summer but become stronger i n autumn and winter. Winds of gale force blowing for several 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 in winter than i n summer because of stagnation associated with inversions of a i r masses. Local winds, associated with cold 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 winter but t h e i r effect i s l i m i t e d to the lower 38 slopes of the Richardson mountains. During summer there i s continuous daylight from the l a t t e r part of May to the end of July . In t h i s period the sun c i r c l e s the sky, dipping near the northern horizon at midnight. After the f i r s t part of August the sun disappears below the horizon for an increasingly 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 suffusion on the southern horizon. This condition l a s t s u n t i l about 10 January at Aklavik when the rim of the sun again appears. The "dark" period i s not as profound as might be supposed. 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 for 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 description of the Aklavik region has been given i n some d e t a i l . The data supporting t h i s summary, together with that f o r other selected regions, appears i n Appendix B. 39 METHODS OF PROCEDURE Duration of the Study F i e l d investigations for t h i s study began on 7 June, 1947 when the w r i t e r , i n company with I . McT. Cowan, arrived i n the Mackenzie delta. Dr. Cowan remained u n t i l the following 17 August and presented thereafter a preliminary report of the findings of the f i r s t summer (Cowan, 1948). The w r i t e r continued f i e l d investigations throughout the following winter and u n t i l September of 1948. F i e l d work was re-sumed in June of 1949 and continued thereafter u n t i l September, 1950. A l l periods of the year were not equally productive of r e s u l t s . Live trapping could be carried 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 majority of the animals tagged were caught during the three success-ive summer periods, I947-I948-I949. Some l i v e trapping, as d e t a i l -ed hereafter, was done i n early winter (November and December) but thereafter temperatures became too low for the animals to survive for long i n the traps. The opening of the l e g a l trapping season, on 1 March, gave an opportunity to take animals with s t e e l traps, to inspect the carcasses for disease, parasites, and breeding condition, and to ascertain whether they had been tagged previously. During la 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 available from hunters at a time coincident with the highest breeding a c t i v i t y . The remainder of June yielded poor l i v e trapping r e s u l t s because the water i n lakes had not yet sub-sided following the spring floods and was so muddy that i t hid den entrances. The period from the f i r s t ice formation, about 1 October, u n t i l ice was again t h i c k enough for t r a v e l , about 1 November, yielded only meagre data because the animals were not e a s i l y accessible. 41 Location and Description of the Study Areas Studies were centred at four widely dispersed areas within the Mackenzie delta, Appendix A. These f i e l d stations were selected with a view to giving f a i r l y adequate coverage to the different conditions to be met within the region. In-tensive work was carried on at these locations and other areas were given only cursory attention. It was f e l t that this would give more valuable information than the same amount of effort spread over a wider portion of the delta. AREA NO. 1 (LANG»S) The f i r s t area to 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 light hunting u t i l i z a t i o n and were used primarily as reservoirs of breeding stock. Field work was conducted on several lakes in this area, as detailed in Fig. 5. Better stands of white spruce occurred here than in 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 sl 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 this area though the latter 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). LAKES INVESTIGATED ARE NUMBERED OR NAMED. 4? and willow and the immediate shoreline was grown to river sedge (Carex aquatilis), except where the bank was too abrupt to sup-port emergent growth. The south and west portions of the lake, being more shelving, had luxuriant stands of horsetail (Equisetum  f l u v i a t i l e ) , growing in six to twelve inches of water. Beyond that in 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 in winter, (Eig. 6), but would average two feet more during times of flood. The char-acter of the s o i l was largely a l l u v i a l even though there were some areas of heath in the v i c i n i t y . Lakes W-l and W-2 were dissimilar in that the former had an abrupt shoreline throughout i t s entire length while the latter had emergent growths of sedge and horsetail. To one side of these lakes, and quite distinct from them, was a shallow pond no more than an acre in size which supported a luxuriant growth of horse-t a i l . Well used "portages" into this 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 in this pond normally were oaught in 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, (Fig. 7 ) . Originally five lakes in this area were designated by number and slated for 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 could 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 being converted i n t o three i s o l a t e d ponds by the r a p i d growth of w i l l o w , sedge, and water-oats. Though n e a r l y useless as w i n t e r h a b i t a t , t h i s lake supported numerous muskrats i n s p r i n g a f t e r the i c e had disappeared and the animals were moving about i n search of food. Lake B - l , ( F i g . 7), w a s n e a r l y a mile long but of no great w i d t h . 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 corner where an o u t l e t creek flowed. 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 yards i n w i d t h , which received heavy u t i l i z a t i o n as muskrat food ( F i g . 8). The southern p o r t i o n of the l a k e was l e s s u s e f u l to muskrats than the main body because of l a c k of emergent food p l a n t s . Figure 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 for the reindeer herders. The third f i e l d station was established on the lakes constituting this area. A series of three interconnected lakes, (R-l, R-2, and R-3), were chosen for intensive f i e l d work (Fig. 9) and other lakes i n their 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. This may be ascribed in part to 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 water-way. The northern part supported white spruce along the channel but the rest of the lake was surrounded by dense growths of wil-low and alder. Depths were variable but sufficient in both por-tions successfully to harbour muskrats in winter. Lake R-3 resembled Lake R-2 in depth and i n shoreline vegetation. The southern shoreline was indistinct because of recent invasion by willows, and the resulting ragged outline had an understory of sedge and horsetail which provided excellent summer habitat for both muskrats and waterfowl. AREA NO. 4 (TUNUNUK) The fourth area was part of the trap line of an Eskimo, Johnny Alekuk, and was situated about four miles from the old camp-site of Tununuk, Appendix A. The area was unique in that 48 FIGURE 9>. STUDY AREA NO. 3 at. REINDEER DEPOT INDICATING THE TRAPPING SITES USED DURING THE INVESTIGATION 1947-50. 49 it was beyond the limit of trees, and though surrounded by tangles of willow and alder, had no coniferous growth of any kind. Four shallow lakes were investigated here. Two of these were probably not deep enough to allow any muskrats to winter successfully, a l -though the other two may have had sufficient water for winter habitat over about cne-tenth of their areas. The sedges, mostly (Carex aquatilis) , were the abundant emergent food plants, though during summer the largest of the four lakes was a flourishing meadow of sedge and water-oats in about equal amounts. Horsetail did not grow in any of the lakes though i t was found sparingly in the small channels connecting them. Pond weeds and m i l f o i l repre-sented the major winter foods available. No tagging was done on Area No. 4 because successive v i s i t s failed to reveal muskrats in sufficient 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 therein. This was not noted at any other study area investigated, but. may be usual north of timber line where the banks of the lakes are low and the lakes are shallow. It corresponds to the condition found by Law (1950) in the delta of the Slave River. The channels were, however, almost barren i n submerged food plants so i t was doubtful whether the animals could winter in these locations. A l l study areas except Tununuk were given attention during each of the four seasons of 1947-1950 inclusive. Work was begun on Area No. 4 during the summer of 1948 and continued the follow-ing 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 level though the north shore was about three feet lower. The shore height of Lake B-l at Boxer's was from four to six feet along the south side, less facing the willow f l a t s to the north. At Reindeer Depot lakes shores were likewise variable. None mea-sured six feet so that during spring when flood waters had risen 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 to-ward the channel edges. Spring floods here l e f t exposed only the higher land along these channels. 51 Methods of Travel The distances involved i n traversing the Mackenzie delta were r e l a t i v e l y great. From the most southerly study area (Lang's) to Aklavik was thirty-two miles by water. Frcm Aklavik to the second area (Boxer's) was nine miles and thence to the Reindeer Depot was a water distance of some seventy miles. From Reindeer Depot to the fourth area near Tununuk was t h i r t y - s i x miles. From Lang's, therefore, one had to t r a v e l one hundred and f o r t y miles by water to the most northerly s t a t i o n . In summer these distances were usually traversed by canoe or s k i f f propelled by an outboard motor. However water transpor-t a t i o n was possible 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 less by t h i s method since i t was possible to take a more direct route, but t r a v e l was also slower and less comfortable be-cause of low temperature and the load of equipment i t was necessary to transport. For traversing 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. Skis 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 Live Trapping In 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 trapping and tagging was i n s t i t u t e d . Early i n the study, u n t i l proper techniques were developed, trapping endeavour produced few re-s u l t s . 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 bait or lure we could furnish would induce the animals to enter cage traps set among the shoreline vegetation. Figure 10. Juvenile muskrats taken i n l i v e traps set i n runways through shoreline vegetation, September 194-9. 53 Once the water levels had subsided to their summer norm, live traps were more successful in taking animals (Fig. 10). These traps were set in runways through marginal vegetation and in underground tunnels leading to bank dens. The latter method was rather arduous in that i t involved f i r s t , probing the shore-line above the water level to find the runways; second, excavat-ing sufficient of each runway that a trap could be set therein; and third, surrounding the trap with green plants. This layer of vegetation was important, not only in excluding some of the light and thus partly obscuring the trap, but also in affording material which the captured animal could pull into the trap for warmth, food, and protection from mosquitoes. During the early winter i t was possible, by taking due precautions, to resume live trapping on an intensive scale. By this time the muskrats had constructed and were using feeding stations on the lake ice. These stations, which consisted of a dome of vegetation built above a hole in the ice, were known throughout the Territories as "push-ups", and this designation w i l l be employed hereafter. In order to set a live trap in such a situation, i t was 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 piled to a depth of six 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 captivity without apparent i l l effect. Later in the winter this method proved less efficient 54 because the plunge-hole often froze a f t e r the push-up had been opened. Coupled with t h i s was the fact that the i n s u l a t i n g pro-perties of the snow had decreased appreciably. New snow orystals formed an ideal i n s u l a t i n g medium, but by January there had been a coalescence of orystals i n the lower layers, and a coarse granu-l a r type of snow resulted. In November there had been a recorded difference 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 difference was reduced to as l i t t l e as 3°F. A seoond method of taking l i v e animals i s worthy of note. I t involved the use of a long-handled landing 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 re-quired the e f f o r t s of two men — one to hold the net and the other to induce the animals to 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 olinched. These tags, applied to the animal's r i g h t ear, had a l i m i t e d value due to t h e i r small s i z e . Trappers taking a tagged muskrat seldom were aware of such a small tag hidden i n the f u r . Aluminium bands, commonly used i n waterfowl studies, 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 discernible on released animals. A t h i r d and most successful kind of tag 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 spec i a l clincher. I t was applied by en o i r c l i n g the tendon of A c h i l l e s (tendo calcaneous) of the ri g h t hind limb as suggested by Cook (1943). I t was found necessary to enlarge the depth of the tag so as not to co n s t r i c t the tendon and enclosed s k i n . I f 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 equally w e l l to juvenile or adult animals, and no tag-loss was noted. However, t h e i r greatest advantage lay i n the fact that t h e i r s i z e and l o c a t i o n made.it impossible for them to be overlooked by the trapper who was skinning a muskrat. These tags were used exclus-i v e l y a f t e r June, 1949. 56 Methods of Handling Live Animals A certain amount of care had to be exercised in handling adult muskrats. To prevent painful bites a holding cone of wire mesh with a wooden base was used. Animals were dropped head f i r s t into this and a stick was inserted through the wire between the t a i l and the hind legs (Fig. 11). An animal was pulled from the cage trap by the t a i l , and i f held suspended by this member, could not reach to bite the operator. If the animal had been tagged previously or i f only a superficial examination was Figure 11. Adult muskrat. Aldous holding cone, and National live trap. 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 clothing for t h i s procedure. During the l a s t half of the study a l l animals captured were weighed by employing small spring scales and a stout piece of cord looped around the base of the t a i l ( F ig. 12). These scales, though very convenient and easy to transport, would supply weights only to the nearest 25 grams. An examination of l i v e animals gave some information as to t h e i r reproductive status. Females were examined for the Figure 12. Method of weighing l i v e muskrats. 58 presence or the absence of milk i n the mammary glands, f o r a perforate or imperforate vaginal opening, and for a vaginal plug or other evidence of breeding. By palpating the testes of the males i t was possible to ascertain t h e i r size and p o s i t i o n and thence something of the breeding condition of the animal. In both sexes an examination of the pelt for wounds i n f l i c t e d during the breeding season was made. Post-mortem sheets were completed for every dead ani-mal examined. These inspections supplied additional information respecting reproductive status, as w e l l as on diseases, para-s i t e s , and general health. Techniques employed were more or less standard though for the most part macroscopic. 59 THE INVESTIGATION As stated in the Introduction, the muskrats of the Mac-kenzie delta have had to make certain 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 in degree, not in kind. They faoe a longer winter and have avail-able a sparser vegetative flora than do those races farther to the south. In spite of thi s , the continuation of a thriving popu-lation of muskrats in and adjacent to the Mackenzie delta is v e r i -fication of the fact 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 for them. In the descriptive 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 also i t s degree. To account for the ad-justments i s more d i f f i c u l t . There must be both common and pe-culiar factors in each habitat to which the animals have reacted in the manner described. We can attempt to delineate these fac-tors and assess their role in producing the adjustments noted. The adjustment of the northwestern muskrat to the condi-tions of the Mackenzie delta may be divided conveniently into two more or less arbitrary categories. The f i r s t of these w i l l be designated as extrinsic or physical adjustments — those which have to do with the relationship of the animal to i t s physical 6o environment. This section:will attempt to describe the hab-itat and the use made of i t by the muskrats for denning, for food, and for other l i f e processes. The second category w i l l deal with those intrinsic or biological adjustments which manifest themselves within the animal i t s e l f . Such considerations as size and growth rates, breeding activity, and association with their own and other species f a l l within this category. It was the in 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 Extrinsic or Physical Adjustments . A resume of the habitat preferences of the northwestern muskrat was given in the Review of Literature. Therein i t was noted that the animals in the delta of the Athabaska River con-structed houses or lodges of emergent vegetation and occupied them throughout the winter. This propensity of house building is exhibited by a l l other races of muskrat under review. There is no doubt, however, that where conditions are favourable they w i l l dig and inhabit burrows in stream and lake banks as well. Johnson (1925) in New York found that in watercourse environment "the muskrats confine their dens mainly to bank burrows, and con-sequently superficial evidence of the animal's presence is less conspicuous than in 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 certain number of them may make their dens in burrows dug in the embankments". The same situation exists in other parts of North America, and in Europe as well. Errington (1939b) encountered animals in Iowa li v i n g in drainage ditches and t i l e drains, especially during periods of drought. Warwick (1934) found that muskrats built lodges or else lived in the banks of the numerous waterways of England and Scotland. Their burrowing habits proved so de-structive in England and Europe that a determined effort was made to exterminate the animals. In England this venture was in large measure successful, in Europe less so. 62 In a l l these locations the muskrat either occupied a house or a burrow, at times both. Their preference was based in part on local conditions. Law (1950), however, found that muskrats in the delta of the Slave River lived in the channel banks and did not build nor inhabit houses. A l l conditions seemed favourable for some houses to be constructed. There were luxuriant growths of c a t t a i l and bulrush in areas of permanent water but bank dens were the only permanent abode of the animals. This situation prompted Law to suggest that animals be exchanged between the Slave and Athabaska deltas to ascertain whether they would continue to live in dens and lodges respectively or whether they would change their habits under their new surroundings. This experiment was never tri e d . From the Slave delta northward to the Mackenzie delta muskrats generally have abandoned the construction of houses. Within the Mackenzie delta there is 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 in others they are shelving and produce abundant growth. But whether lake shores are steep or gradual the muskrats therein, without exception, live in the banks and do not build houses which they inhabit permanently. Yet i t cannot be denied that attempts at house building were evident in some of the lakes. The struc-tures built, however, were more of the nature of feeding lodges as described by Dozier (1948). They were small and had no nest, merely a feeding platform. 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 largely lakeside sedges 63 though, horsetail and even balls of filamentous algae occasionally were incorporated. Nearly a l l of the bank dens investigated during this study were those built into the more shallow lake shores. These burrow systems were usually of long standing and were very complex with several under-water entrances and many connecting tunnels above the water line. Signs of occupancy of such den systems consisted of roiled water in the trench, mown vegetation in the v i c i n i t y , and lack of vegetation in 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 in steep banks were hard to find and almost impossible to excavate because of buried logs and tree roots. It may be concluded, therefore, that muskrats in the l a t i -tude of Aklavik constructed and inhabited shallow burrow systems out of necessity, perhaps also out of preference. But there was definitely not enough emergent vegetation in any of the delta lakes to build sufficient 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 live in them, the depth of winter ice was such that i t would have prevented access to the food supply in the unfrozen portions of the lake. It would require a lot of vegetation to build one muskrat lodge in deep water, and unless such a structure in the Mackenzie delta was in at least three feet of water, i t soon would freeze out. Thus the habit of using burrows only i s 64 an adjustment to winter conditions that render houses dangerous. It seems probable that this is a habit"of sufficient survival value to have genetic influence and background. The choice of habitat by muskrats of the Maokenzie delta requires some amplification over that already given. Winter habi-tat was found to be much more circumscribed than was summer habi-tat because of the restrictive effects of ice. A comparison of the situation here with that in other parts of North America w i l l be given only in brief. The earlier 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 activity of the muskrats is concerned largely with reproduction. Habitats suitable for this a c t i v i t y are more numer-ous than those which w i l l support muskrat populations yearlong. This is fortunate because the animals seem to exhibit their great-est intolerance to their own kind during this, period. The spreading out of the population into temporary but adequate summer sites no doubt reduces intraspecific fighting and may in this way contribute to reproductive efficiency. These temporary sites were excellent summer habitat and many young animals were reared therein because food usually was abundant and satisfactory burrowing areas could be found. Animals pushed into these areas after the spring floods and remained there-in even though water levels receded during summer. This same situ -ation exists i n many other places where muskrats abound and therefore is not unique. What is of interest in the Aklavik region is the 65 great difference between satisfactory summer and adequate winter habitats. An example of a summer habitat may help to illustrate this 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 its extent there was only eighteen inches of water, or less. The banks were low and flanked with willows over nearly the whole of the perimeter. The understory for the shoreline willows consisted of sedge (Carex aquatilis) which grew to the water's edge and blended with the horsetail which formed a band up to 30 feet in 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 lesser amounts of pondweeds and bur-reed. Food, therefore, was abundant in 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 trickle 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 five burrow systems in this lake are shown in 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 in and out of the lake in their feeding activities and wan-derings, i t was impossible to ascertain how many animals were resident. But some idea of the usage of the area by muskrats may be gained from the fact that forty-five animals were tagged 66 there in eight days of late August and early September of 1947, these in addition to four which previously had been caught there. Similar results were obtained in the f i r s t ten days of September, 1 9 4 9 , when forty-one animals were tagged in seven days of trapping. Almost a l l of these animals had moved to other lakes by the time ice covered the area, their concentration here seemed mainly in quest for food. Nearly eighty per cent of the animals captured during these periods were juveniles. Muskrats usually vacated their summer habitat in shallow lakes after the f i r s t frosts of autumn (late August) had blighted the emergent growths of sedge and horsetail which had f u l f i l l e d their food and cover requirements to that time. Failure to seek more permanent water bodies resulted almost certainly in freezing out in early winter. WINTER HABITAT Summer abode for muskrats in the Mackenzie delta required abundant emergent food plants, burrows satisfactory for rearing young animals, and only enough water to cover and protect the den entrance. Winter habit, on the other hand, was more exact-ing and had two primary requisities 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 ice. The f i r s t of these was a sufficient depth of water so that the lake would not freeze solidly, and the second was an adequate supply of submerged plants in water which remained unfrozen. It was stated previously that winter habitat was more re-structed in i t s extent than summer habitat. It should not be inferred that this involved two separate sets of lakes, one for 67 each season. Rather, many of the lakes inhabited in summer ^ were also inhabited in winter even though the accretion of ice caused a decrease in 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 survival because the growth of an extra foot of ice in a V-shaped lake rendered a greater percentage of lake floor unavailable than in 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 well, 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 shifting of river channels. Depths t'o 38 feet were re-corded at one such lake and 23 feet in 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 one-half to sixteen feet with an average of just over six feet. The deepest areas were not always located in the centre of a lake; often they were to one side, especially near the steeper shore. But for the most part the lakes were rather regularly saucer-like. Lakes with only three and one-half feet of water could not be de-pended upon to support muskrats every winter because ice was some-times as much as four feet thick. The average measured water depth of six feet was considered nearly optimum. Even though i t has been mentioned that lake depths were important, yet the prime factor governing muskrat survival was 68 the depth or thickness of winter i c e . I t was t h i s one f a c t o r which governed the success of any winter habitat. Freezing out does not appear to be a problem i n the Gulf States and even In lew York Johnson (1925) asserted that 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 affect muskrats". He conceded that "the winter conditions i n the marshes and swamps of New York must be of very favourable nature to the muskrats. The surface of these habitats i s gener-a l l y covered with a dense mat of f l a g and other vegetation, and does not become frozen t o any depth before i t receives a blanket of snow as s t i l l further protection. Under this cover the deeper waterways of the muskrats remain open and the animals are able to move about f r e e l y " . The c o a s t a l race maorodon. inhabiting the saline marshes of Maryland, usually remains active i n an unfrozen habitat year-long. However, Smith (1938) recorded that when the peat layers In which they oonstruct t h e i r runways froze to depth of s i x inches i n 1934 many of the animals perished because they could not dig out to the surface. Aldous (1947) i n South Dakota knew of the danger of freezing out shallow habitat and recommended that 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 winter death rate among muskrats of the Peace River d i s t r i c t of Alberta during the winter of 1916-17. He recorded "thousands" of muskrats i n the f a l l , yet i n spring after intensive hunting he was able to secure less than one hundred p e l t s . This high rate of mortality he ascribed d i r e c t l y 69 to severe winter conditions. F u l l e r ( 1 9 5 1 a ) r e f e r r i n g t o conditions i n the d e l t a of the Athabaska River noted that "winter f r o s t s seal the lakes and sloughs containing the choioest foods and may t u r n the shallow ones t o s o l i d blocks of i c e , forcing eviction of the muskrats l i v i n g i n them". I t may be stated, therefore, that freezing out i s not unique i n the muskrat habitats of the c e n t r a l and northern parts of the continent. What i s important i s the degree of such free-zing as governed by the type of habitat and the depth of i c e . Investigations i n Alaska, Buckley (1954- unpubl.) recorded ice 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 condition. In any area where freezing temperatures persist ioe depths are regulated by the thickness of the insu l a t i n g blanket of snow as w e l l as by the mean d a i l y temperature. Snow depths i n the Mac-kenzie d e l t a varied with the time of year and the exposure of any lake surface to wind a c t i o n . This exposure to wind was determined by the height of the trees and banks surrounding the lake, by the size of the lake, and by i t s orientaticn r e l a t i v e to the p r e v a i l -ing northerly winds. A graphic representation of i c e measurements beneath varying depths of snow, demonstrated that there 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 could, however, unless adequately protected, be removed i n a single day of high winds. Barnes ( 1 9 0 6 ) remarked that "the protective a c t i o n of snow on the ground oannot be overestimated, and i n the dry state i n which i t usually 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 faotor i n ioe accretion was the loss of heat hy conduction through the i c e and into the a i r . Tutton (1927) recorded the con-d u c t i v i t y of ioe as ten times that of snow — an inch of snow, t h e o r e t i c a l l y , offering the same protection as ten inches of i c e . But since the conductivity of snow varies as the square of i t s density, the i n s u l a t i n g properties of wind-packed snow on northern lakes f a l l f ar short of that of newly deposited snow. Another phenomenon r e l a t e d to snow and ioe cover, known l o c a l l y as "over-flow", oonsisted of a layer of water interposed between the snow and i c e cover on a lake. Such a condition often succeeded a heavy f a l l of snow, the weight of whioh forced the water from below to cover the lake t o a depth of three or four inches. This water wetted the snow above but soon froze and was incorporated with the i c e cover. I t thereby reduced the laye r of snow, increased the depth of Ice, and hastened the formation of more i c e . During the winter of 194-7-48 successive layers of over-flow added as much as nine Inches t o ibhe ice cover and thereby e f f e c t i v e l y sealed many muskrat "push-ups". Over-flow seldom occurred after December because by then the ice was t h i c k enough to bear a greater weight of snow. WINTER "PUSH-UP" STUDIES Perhaps cne of the most c h a r a c t e r i s t i c features of winter muskrat habitats i n the north i s the presence of feeding stations or push-ups b u i l t upon the surface of the ioe. John Rae (1888) was one of the f i r s t to describe these structures which he c a l l e d " l i t t l e huts of mud and weeds". Dozier (1948) termed them "breathers" 72 and remarked that they "are started by muskrats cutting a four 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 of fine fibrous roots, water weed, or other submergent vegetation". F u l l e r (1951a) i n northern A l -berta recorded that "the dome of the push-up i s made of rejected portions of food items, most of whioh are parts of submerged aquatic plants. Ana char i s . Myriophyllum, U t r i o u l a r i a . and several species of Potamogeton are the usual constituents". Por s i l d (1945) speaking of the Mackenzie d e l t a stated that "the p r i n c i p a l material used i n push-ups i s the leafy-stemmed Richard-son's pondweed (Potamogeton Richardsonii) and w a t e r - m i l f o i l (Myriophyllum exalbesoens)". There i s not much additional i n f o r -mation i n print concerning these very common and very necessary winter feeding houses because studies i n other areas usually have been conducted i n summer. However, the construction of push-ups appears t o be limi t e d to the mare northerly portions of the con-tinent where i c e cover i s continuous f o r long periods eaoh winter. Certainly 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 to assert that spatulatus builds more push-ups and i s mare dependent upon them than i s any of the other races of muskrat under consideration. This i s even more the case with the far northern representatives o f that race. Animals whioh inhabit houses or ledges during winter usually construct them i n f a i r l y deep water i n or 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 accessible from the house and the need f o r an extensive system of push-ups i s 73 obviated. But those animals which build no houses but inhabit bank dens instead may require several push-ups to extend t h e i r radius of a c t i v i t y to t h e i r food sources i n the deeper water. Thus muskrats of the Mackenzie d e l t a , and those i n s i m i l a r l a t i -tudes i n Alaska, are unique i n t h e i r degree of dependence upon these structures for t h e i r winter s u r v i v a l . F u l l e r (1951a) considered that "there were two possible explanations f o r the o r i g i n of push-ups: (1), holes are kept open from the time of ioe formation, and (2), new holes are gnawed through the i c e , probably where an a i r pocket reduces the thick-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 that a number of small holes persisted i n the newly formed i c e . These holes did not close r e a d i l y because they were kept open by the continual r i s e and bursting of bubbles of gas. When they f i n a l l y did freeze a done-shaped area beoame f i l l e d with the gas and the 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 debris, a by-product of feeding, which accumulated below each push-up. The generally slow rate of decomposition of the material produced a slow but continual 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 materials available t o the animal were apt t o be incorporated i n i t s push-up. Speoies seldom used f o r feed often proved useful i n the b u i l d i n g of these structures. Thus an alga (Cladophora) frequently appeared in push-ups i n the v i c i n i t y of Aklavik 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 majority of push-ups, however, were made from the stems and leaves of pond-weeds and m i l f o i l , and might even include mua and pieces of wood as w e l l . Lakes which showed a large proportion of mud and rotted vegetable debris i n the push-ups were i n most cases devoid of ade-quate submerged vegetation and presented poor wintering habitat for the animals . I t was found that mare push-ups were constructed i n autumn than were kept open winterlong. I f the structure was not v i s i t e d frequently by muskrats during cold weather, access was prevented by freezing of the plunge-hole. The proportion of these frozen push-ups yearly varied with the severity of the weather, the amount of "over-flow", and the depth of the i n s u l a t i n g snow. In March and A p r i l of 1°48 there were 48 per cent (382) of 835 push-ups s t i l l i n use. Figures for 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 gale force which removed the snow cover from most lakes were held 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 fate of the push-ups on Grassy Lake of Area ftl. In early November, 1947, 72 push-ups were found and charted, F i g . 14. By e a r l y March only 35 of these push-ups were i n use; the r e s t had frozen. During the A p r i l musk-rat harvest only 23 of these s t i l l were a c t i v e . In view of the fact that the t o t a l muskrat population of the lake was several times t h i s number, i t would appear that more than one animal used each feeding s t a t i o n and t h i s repeated usage helped keep the plunge hole open. As the weather became colder and the number of active push-ups decreased, mare intensive use of each structure was 75 © Trapping sites Nov., 1947 S Trapping sites, Apr. 1949 £2> Known muskrat den3 • Push-ups charted Nov. 1947 FIGURE 14. FATE OF PUSH-UPS ON GRASSY LAKE DURING WINTER 1947-48 76 occasioned, thereby lessening the ohanoes of fur t h e r freezing. There had been 45 muskrats tagged and released i n Grassy Lake during August, 1947. Continued trapping on the ice the following November revealed 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 ad d i t i o n a l 43 tagged animals, however, were released i n the lake, making a t o t a l of 69. Twenty-four of these were handled only once, sixteen were captured twice, sixteen three times, and eight four times. Of the remaining f i v e animals, three eventually died i n the l i v e traps and two were taken at 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 to a single 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 three. The number of animals using each push-up varied from one to th i r t e e n with 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. Animals Taken Push-up No. Animals Taken 1 3 l a 5 2 1 2a 3 3 3 4 13 4a 6 5 6 7 8 9 10 11 10 13 10 6 I 4 77 I f there were as many as thi r t e e n different animals taken at a single feeding s t a t i o n , there must have been an even larger number using i t because not a l l animals i n the lake were caught. The minimum number of animals necessary to keep a plunge-hole open during severe weather was not ascertained, though i t must have required mare than three because trapping stations show-ing t h i s minimum l a t e r became frozen and abandoned. Animals were trapped i n proportion t o the representation of t h e i r sex and age groups i n the lake. There were 139 animals taken i n traps; t h i s included those caught more than once. The surviving complement of 67 tagged animals evidently was captured at random because the following table of r e s u l t s (Table 2) i n d i -cates that neither sex nor age group was dominant i n the catch. TABLE 2 CATCH OF ANIMALS BY SEX AND AGE ON GRASSY LAKE IN NOVEMBER, 1947 ADULT JUVENILE Male Female Male Female Total take ( i n c l . reoaptures) 11 17 60 31 = 139 Known complement of lake 5 6 30 26 = 67 Pro-rated take per 67 animals 3 8 29 25 67 I t i s believed that push-ups are constructed by muskrats during early wint er wherever there i s a hole i n the ice through which plant materials can be pushed to form a dome. Those i n the 78 ) TV v i c i n i t y of a more or less permanent food supply w i l l be kept open by continual use, the r e s t w i l l be abandoned and freeze. In the writer's view the elaborate system of push-ups construct-ed by spatulatus i s the key to 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 conditions 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 population has made to i t s environment. THE YEARLY RADIUS OE ACTIVITY OF MUSKRATS The preceding section dealt w i t h the summer and winter habitat of muskrats i n the Maokenzie d e l t a . I n order to define more c l e a r l y the r e l a t i o n s h i p of that habitat to the animals themselves,it i s desirable that some l i m i t be set to the amount of habitat required per animal. This w i l l be defined by giving the radius of a c t i v i t y of muskrats during both summer and winter periods. A l l data i n t h i s regard were obtained through the cap-ture and tagging of l i v e animals. Information concerning winter movements w i l l be given i n some d e t a i l because such data are not available elsewhere. The number of animals tagged each month i s shown i n Table 3. I t w i l l be noted that mare than h a l f of the t o t a l of 621 were taken during August with September ranking second. Young animals were captured with ease during these periods and adults thereby were handled less often. Of the 621 animals tagged 173 (28 per cent) were again recaptured a f t e r a period of at least three months. Sumrner Movements A majority of animals (141 of the 173 recovered) were 7 9 TABLE 3 NUMBER OF MUSKRATS TAGGED BY MONTHS IN THE MACKENZIE DELTA DURING THE PERIOD 1947-50 INCLUSIVE Month Adults Juveniles Totals Male Female - Kale Female 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 Totals: 101 94 246 180 621 Males/100 Female s: 108:100 137:100 8o taken again i n the same lake where they had been tagged. For those whioh had moved less than one-half mile, and yet were found i n another lake , the average radius of movement was 300 yards with a range from 100 to 800 yards. This average movement 1300 yards) represented the distance 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 lake, therefore any exceptional movements probably exceeded that f i g u r e . Animals dispersing more than 800 yards from t h e i r tagging s i t e s exhibited an average movement of one and one-half miles 12640 yards) with a range of one-half (880 yards) t o four and one-ha I f mils s (7920 yards). An extreme case involved a muskrat that t r a v e l l e d t h i r t e e n miles between 19 August and the time of ice 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 for muskrats of the Mackenzie d e l t a . Errington (1944) found that the majority of muskrats he tagged i n northwestern Iowa moved an average of 125 yards though there were a few that wamdered two miles away. Takos (1944) be-lieved that a radius of 25 feet defined the average movement i n Maine with les s than one-third of the animals venturing beyond a distance of 100 yards. Dorney and Rusch (195 3) demonstrated that these figures were e s s e n t i a l l y correct for southern Wisconsin as w e l l . Aldcus (1947) working i n South Dakota found that only f i f -teen per cent of the animals under investigation moved more than 31 rods (170 yards), w i t h a maximum recorded distance of 200 rods (1100 yards). F u l l e r (1951b) would concur i n these figures from South Dakota because they agree with his for the d e l t a of the 81 Athabaska River. He i n f e r r e d , however, that movements during the spring d i s p e r s a l period might exceed the autumn radius of 170 yards. A l l of the movements detailed above demonstrated an ac-t i v i t y range of less than h a l f the average of 300 yards recorded i n the Mackenzie d e l t a . In the 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 habitats i n which c a t t a i l was the prominent food and building material. The la r g e r movements of the animals i n 1Jiis study were ascribed to two major fa c t o r s . The f i r s t of these was the neoessity to vacate temporary summer quarters f o r more favourable winter habitat. The second was the more dispersed nature of the food resource which demanded that muskrats undertake more extensive forays 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, often across land or open water, predisposed the animals t o depredations by t h e i r natural enemies. But extensive t r a i l s and "portages" through the vegetation between water bodies indicated that these movements, though p o t e n t i a l l y dangerous, were quite normal. Winter Movements No information i s available i n the l i t e r a t u r e concerning the amount of movement necessary for existence beneath a cover of ic e . In the l a t i t u d e of the Mackenzie delt a the muskrats are con-fined beneath ice from October to the following June. Live t r a p -ping i n November and December was undertaken, as detailed e a r l i e r , In order t o assess the fortunes and chart the movements of animals i n t h e i r d a i l y a c t i v i t i e s . L i t t l e information was co l l e c t e d con-cerning the wanderings of animals which might escape from shallow 82 frozen lakes. I t was apparent that such unfortunate i n d i v i d u a l s perished quickly because they were not equipped to withstand the severe ambient temperatures. Discussions with trappers indicated that such animals on occasion were found frozen i n a willow copse or sedge clump and that t h e i r tracks showed that they had not wandered far from where they had dug out. Their chances of enter-ing 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 (Fig. 14) indicates the location of seventy-two push-ups as they appeared during 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 the fourteen t r a p - s i t e s shown was eighty yards, w i t h a range of 32 to 132 yards. To reach the trapping s i t e the animals f i r s t of a l l had to traverse that distance. Then i f they v i s i t e d more than one push-up, and 35 per oent of them did so, they would t r a v e l a probable distance of 120 yards. I f they v i s i t e d three push-ups, and f i v e animals were caught i n three different s i t e s , the distance might be 160 yards. By actual measurement i t was found that these movements varied greatly. Without accounting for the distance of the push-up from the bank den, and i t was not possible i n a l l cases to de-cide i n which den an animal was l i v i n g , the appearance of animals at diff e r e n t push-ups involved distances of 21 t o 231 yards (average 103 yards). In spite 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 the i c e , there was a d e f i n i t e d i v i s i o n of the lake i n t o zones, each used by a f a i r l y discrete group of animals. 83 3) Number of muskrats crossing zone boundaries YARDS ® Number of tagged animals caught per zone FIGURE 15. SEGREGATION OF MUSKRATS BY ZONES ^ K n o w n d e n s IN GRASSY LAKE, WINTER OF 1948. • Trapping sites 84 The diagram (Fig. 15) i l l u s t r a t e s how 67 tagged animals probably-di s t r i b u t e d themselves among the nine occupied burrews and the fourteen trapping s i t e s . Nine zones are outlined and the number of animals using each appears therein. Some of these zones were closed and showed no intercourse with any other; they are de-lim i t e d by s o l i d l i n e s . Others shewed li m i t e d movement of animals across the boundaries, i n whioh case the l i n e s are broken and the included figure denotes the number of animals involved. Zone l i m i t s seemed l e s s r i g i d i n the eastern part of the lake but, i f we discount the movements of two animals whioh showed a pro-pensity f o r wandering, they were as discrete 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 implications, however, involving juxta-position of dens, push-ups, and food supply. Habits of feeding, formed early i n winter, would tend to keep the animals i n the same general area. How much these animal associations would break down with a decrease i n food supply was not established. However, animals trapped i n A p r i l were i n the same general location as they had been i n November, and three animals occupied the same dens i n July as they had the previous August. No doubt there i s some re-s t r i c t i o n of movement at any time during the year. In the south-eastern corner of the lake two animals i n very poor condition were trapped i n spring, 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 did not appear to be separated by distance 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 intolerance t o free movement of animals during winter 85 would help t o explain mid-winter f i g h t i n g which was an occasional though not a prevalent s i t u a t i o n . Depletion of food supplies i n lat e winter could induce ma 1 adjustments i n the population as ani-mals moved about i n search f o r new sources. This would a f f e c t not only the securely situated animals but also the wanderers. This range of movement beneath the ice of at least eighty yards i s as great as that recorded f o r muskrats i n other areas during summer. I t was assumed that the movements between push-ups were d i r e c t , as they seemed to be, and not at random. Also, i t should be pointed cut that only fourteen out of a possible 7 2 push-ups were sampled and, therefore, the actual movements might have been even greater. No comparisons with winter movements elsewhere are possible 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 in v e s t i g a t i o n . FOOD AND FEEDING HABITS I t i s generally conceded that muskrats do not show any great preference i n the foods which they w i l l accept. Takos ( 1 9 4 7 ) stated that the animals u t i l i z e d the most available plants i n the environment as food, especially when the abundant species occurred i n dense stands. He noted that 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 less edible developmental stage. Errington ( 1 9 4 1 ) drew attention to the wide v a r i e t y of materials which constituted the yearlong foods of the Iowa muskrats that he studied. He found that most green vegetation was more or less adequate as food In warm weather so that muskrats were not often 86 subjected to rapid or outright starvation. However, shortages or u n a v a i l a b i l i t y of food i n combination with winter v i c i s s i t u d e s could produce starvation. Bellrose (19.50) indicated that the animals he studied i n I l l i n o i s did show d e f i n i t e food preferences and "although there was a great deal of i n d i v i d u a l v a r i a t i o n , i t was evident that muskrats selected c e r t a i n plants and c e r t a i n parts of plants f o r food". In spite of these opportunistic tendencies i n feeding there i s nevertheless a marked change i n plant foods from the Gulf to the A r c t i c Coast concomitant with the f l o r a l complex and the habitat conditions enoountered. Both the a v a i l a b i l i t y of food and the type of food have a direct bearing upon the habits of the animal and i t s w e l l being. A b r i e f resume of the food habits of the various races under consideration w i l l provide an insight into the type of habitat each occupies and the status of food as a l i m i t -ing f a c t o r therein. Common and plains muskrats appeared to favour the same vege-t a t i o n for food and f o r cover.- In New Tork, Dozier (1950) l i s t e d the preferred foods as Typha glauca and T. l a t i f o l i a with several , species of Scirpus ranking second. Takos (1947) i n Maine l i s t e d sweet f l a g (Aoorus calamus) i n addition to c a t t a i l s . Errington (1941) observed that muskrats were very catholio i n t h e i r feeding habits but was of the opinion (Errington, 1948) that i n Iowa, cat-t a i l marshes had double the "carrying capacity" of any other type of vegetation. Many of the animals he studied were from stream dwelling populations that i n summer subsisted upon a variety of green vegetation, and i n winter upon whatever presented i t s e l f , whether highly nutritious ear corn stored i n burrows or on dry 87 grass and weeds along stream banks. Bellrose (1950) l i s t e d cat-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 preferred food i n I l l i n o i s w i t h pickerelweed (Pontederia), bulrush (Scirpus  aoutus and S. v a l Idus) , and marsh smartweed (Polygonum Muhlenbergii) , ranking i n that order. I f any single plant species may be designated as the main-stay of the muskrats of the c e n t r a l plains areas that 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 to small 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 that 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 . In summer the leaves and stalks were eaten and i n winter the roots and root-s t a l k s . E i t h e r high onshore winds or protracted periods of drought increased the s a l i n i t y of the coastal marshes and caused the three-square rush and c a t t a i l marshes to deteriorate to a less valuable Spartina - D i s t i c h l i s association which would not support an equal density of muskrats. Dozier (1947) reported on t h i s s i t u a t i o n at some length. The coastal marshes of Louisiana and eastern Texas are the heme of the Louisiana muskrat r i v a l l c i u s . In t h i s area very high concentrations of this animal are supported by the brackish t i d a l f l a t s . Nearby fresh water areas produce an i n f e r i o r type of vege-t a t i o n with fewer muskrats. In these t i d a l areas, as i n those of 88 Maryland, the three-square rush (Scirpus Qlneyi) is the preferred food. Cordgrass (Spartlna patens) i s also abundant here but finds more use as buil d i n g material than as food. The s o i l i s highly organic but where storm t i d e s deposit a layer of s i l t or clay over the marshes an intrusion of saltgrass ( D i s t l o h l i s spioata) renders the habitat much less acceptable to muskrats. There i s no dearth of plant food i n this region. Winter fr o s t s and ic e are rare and the danger i n this area i s that 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 vege-t a t i o n and dig 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 the displaced 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 Inhabits the area adjacent to the west shore of Hudson Bay and attains i t s greatest numbers i n the d e l t a region of the North Saskatchewan River. 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 this 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 investigation made of the food habits of the muskrats tbs r e i n . MoLeod (1949) stated 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 vegetation 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 plants suoh as bulrush, c a t t a i l , and flaggrass etc. on which the muskrat i s large-l y dependent are found normally i n water of a depth varying from zero to t h i r t y inches. This i s a fortunate coincidence as the muskrat prefers to bu i l d i t s lodge, on the average, i n sixteen inches of water". Therefore, the yearlong habitat was much more circumscribed than normally supposed because emergent vegetation 89 was f a i r l y s p e c i f i c as to water depths. Those plants used as winter food, i n descending order of importance were: c a t t a i l , bulrushes (Soixpus f l u v i a t i l i s and S. v a l i d us) . flagreed, sedges, and duck potato ( S a g i t t a r l a spp.). Besides supplying winter food, the emergent vegetation offered some protection 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 far the q u a l i t y and quantity 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 type". In summer he found that 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 material was present". These feeding experiments indicated that "each muskrat would consume about two hundred and f i f t y pounds of food material per year. Examinations shewed that about twelve feet of Typha rhizome weighed one pound; thus during one year a muskrat could consume approximately 3,060 feet or .57 miles of rhizome". Butler (1°40) attempted to calculate the number of musk-rats that could be supported adequately on the experimental marsh area at Mafeking, Manitoba. His method was e s s e n t i a l l y a compari-son between the d a i l y food consumption by penned muskrats and the density per square yard of food plants i n the marsh. Although t h i s approach to the problem may be questioned, he has supplied a reasonably detailed resume of the plants to be found i n that marsh area. Butler (op. c i t . ) enumerated several plants, not preyiously reported as muskrat food, which contributed to 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 four feet 90 of water and was favoured by the animals as summer feed. Another was the buckbean (Menyanthes t r i f o l i a t a ) which flourished i n four to f i v e feet of water. In summer i t s leaves and shoots were u t i l i z e d as food and i n winter i t s thiok stems were eaten. Of a l l the plants available at t h i s experimental area, c a t t a i l (Typha l a t i f o l i a ) and softstem bulrush (Scirpus valldus) were l i s t e d as the most important food plants with h o r s e t a i l and flagreed ranking next i n order, i t was estimated that these four plant species would support twenty-three, seventeen, eighteen, and twenty-six muskrats per acre r e s p e c t i v e l y . It w i l l be noted that the food of the races of muskrats discussed above i s comprised of emergent species growing i n com-paratively shallow water, usually less than three feet i n depth. Because ice conditions generally were not severe within these habitats, or because persistent emergent vegetation caught and held winter snows as in s u l a t i o n , the muskrats subsisted on more or less the same plant species throughout the year. When we come to a consideration of the northwestern musk-rat we f i n d that summer foods usually d i f f e r markedly from winter foods. F u l l e r (1951a) wrote that i n many cases "the water leve l s had dropped so low that emergent plants are completely unavailable a f t e r ice has formed and the degree of abundance of submerged f l o r a determines s u r v i v a l or death of the muskrat population". Summer 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) and bulrush (Scirpus). Winter foods included none of these but consisted i n -stead of waterweed (Ana char is) , w a t e r - m i l f o i l (Myriophyllum) , bladderwort ( U t r i c u l a r l a ) , and several of the pondweeds (Potamogeton). 91 Information concerning food i n the Mackenzie del t a was supplied hy P o r s i l d (1945) who wrote that the "food of the musk-r a t , at least during autumn, winter and spring consists of the f r u i t s , rhizomes, and winterbuds of pondweeds (Potamogeton spp.), rhizomes, and tubers of 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 t r i s u l o a ) . The roots, rhizane stems, and f r u i t s of a number of other plants, notably sedges and grasses are, no doubt, eaten as w e l l " . Eoods eaten yearlong by muskrats i n the Mackenzie d e l t a were divided rather sharply between emergent vegetation i n summer and submerged plants during the remainder of the year. Table 4 i s a compilation of species noted at feeding s i t e s along lake TABLE 4 A RESUME OF THE PLANT EOODS EATEN BY MUSKRATS IN THE MACKENZIE DELTA DURING EACH MONTH J F M A M J J A S O N D  • Percentages of t o t a l food  Emergent Species v Equisetum spp. T 0 0 T 25 20 55 40 20 15 10 T Carex aquat i l l s 0 0 0 0 5 10 15 30 40 T 0 T Other ± 15 15 10 5 15 35 15 T 5 T 10 10 Submerged Spe c l es Potamogeton spp. 50 50 60 75 30 25 10 10 20 40 30 60 Myriophvllum 20 20 15 10 10 T T 5 5 30 30 10 Other & ft 15 15 15 T 15 5 T T 10 10 10 15 ± - Arctophila, Menyanthes, Calla , S a l i x , et. a l . T - less than && - Sparganium. U t r i c u l a r i a . Lemna. et. a l . 5 per cent 92 shares, i n food stores at nest dens, and from other observations i n summer; and from food remnants found inside push-ups i n winter. These data are c h i e f l y f o r a l l u v i a l lakes i n the three main study areas though observations from other l o c a l i t i e s are included. By f a r the most important summer food was h o r s e t a i l , with various car ices ranking second i n importance. Other emergent species included water-oats (Arotophila). and a v a r i e t y of l e a f y shoreline vegetation from woody plants such as a r c t i c raspberry (Rubus aroticus) , and willow (Salix spp.) , to the more herbaceous Pyrola and Ranunculus. Submerged and f l o a t i n g species were not well represented i n the summer food though i t was not uncommon to see muskrats diving f o r the rhizomes of pondweeds and m i l f o i l . They also fed at the surface on duckweed (Lemna t r i s u l c a ) , and the f r u i t s of sago pondweed (Potamogeton peotinatus). The usual winter foods were much less diverse as to species but included representatives from almost a l l submerged plants that were available i n a lake . Accretion of ice v i r t u a l l y eliminated the submerged portions of normally emergent plants so that only those species s u r v i v i n g i n more than three feet of water remained available as food. Therefore, the several species of pondweeds formed the bulk of the winter d i e t and a l l portions of these plants were found on the feeding ledges of push-ups. M i l -f o i l and duckweed also were u t i l i z e d frequently as food, but animal foods seldom were represented. Growth of pondweeds started before the winter ice (cover was removed so that during A p r i l and e a r l y May these new shoots comprised most of the food. During the l a t t e r h a l f of May the spring floods had r a i s e d the lake ice and once again the underwater 93 portions of emergent vegetation were accessible. During this period the rhizomes of horsetail formed the bulk of the residue found at lake shore feeding sites. 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 windfall for them at this period was the large number of new white shoots of pondweed which had been gouged loose from the lake bottom by drifting ice and float-ed to shore in windrows. As the flood waters subsided the new shoots of horsetail 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 to these species as the i r chief sustenance far the remainder of the summer. In summation, it may be said that the Mackenzie delta muskrats ate whatever was most available i n abundance. Their preferences were roughly i n proportion to the representation of the food species in the lake flora, though no accurate measure-ments of the av 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 cattails or bulrushes i n the delta flora but the muskrats managed very well by turning to other plants. They did exhibit preference for species with which they were familiar. In an endeavour to induce the animals to enter the cage traps we tried various baits — apples, carrots, potatoes, and even onions. But muskrats did not seem to recognize these items as food because they quite nonchalantly sat atop the traps and consumed their usual meal of Equisetum or Carex. So far only casual mention has been made of the amount of 94 winter food whioh was available t o support the muskrat population of the Mackenzie d e l t a . I t was stated previously that the lake depth i n late winter averaged s i x feet and that up to three or even four feet of that might be i c e . Therefore, the lake had t o be at least three and one-half or four feet deep over much of i t s area to support any muskrats. A l l the winter food l a y i n the un-frozen portions of the lake, but i t should not be assumed that a l l of the lake bottom was equally productive of plant food. The de-mands of the plants for sunlight and warmth appeared to be quite c r i t i c a l . Pond weeds grew l u x u r i a n t l y i n water up to four or even si x f eet deep, but beyond that depth fewer plants were established and at twelve feet 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 to the area of any lake which would support plant l i f e suitable for muskrats. Thus, depths from three to s i x feet had the greatest value, from six t o twelve feet progressively l e s s , and beyond twelve feet very l i t t l e value t o wintering animals. The i n t e r - r e l a t i o n s h i p of water levels and available foo d was recognized by Buckley (1954 unpubl.) i n Alaska who wrote "that as ioe increases i n thickness, less and less area i s available for foraging. Muskrats are forced to 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 to the deeper portions of the pond. Other factors that i n t e n s i f y the effects of thick ice are: that deep ponds and channels often have less aquatic vegetation than shallower ponds, and thus supply less food and can support fewer muskrats; and low f a l l water l e v e l s provide less i n i t i a l winter habitat which r e s u l t s i n even greater r e s t r i c t i o n of habitats as ice formation continues. 95 As the muskrats are squeezed into the deeper areas, the food supplies are mare rapidly depleted. Often this may result in exhaustion of the food supply and subsequent migration, fighting 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 sufficient growth to provide food for muskrats during the eight months of vegetative dormancy. It is true that the pondweeds, and perhaps other species as well, do produce some.nsw growth before the ice cover is removed. However, the production of green portions of the plants is delayed by the s i l t load l e f t in the lakes by spring floods of early June. Though i t is July 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 third week in 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 restricted seriously. It i s apparent, therefore, that winter food, or the avail-a b i l i t y of wint er food, definitely was a limiting factor in the area under investigation. Reports of other authors demonstrate that food is less and less a direct limiting influence as one pro-ceeds from north to south , until i n Texas and Louisiana i t was never a problem associated with climatic factors. In the Mackenzie delta 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 previously has been inferred that i n the Mackenzie delta there was a lew density of muskrats i n comparison with areas further to the south. Inasmuch as population density appears t o be an important f a c t o r i n the biology of t h i s animal, i t might be w e l l t o consider i t b r i e f l y before proceeding to the section concerned with i n t r i n s i c adjustments. Too few studies of t o t a l populations have appeared i n print to make adequate comparisons of muskrat numbers per unit area. However, there is a s a t i s f a c t o r y record of the number of animals harvested from marsh areas of known s i z e so that an evalu-ation may be made on this basis (Table 5 ) . I t i s apparent that the highest concentrations of muskrats to be found are those maintained i n the coastal swamps of Louis-iana and Texas. The average harvest of f u r f o r the whole region would produce between one and s i x animals per acre, depending upon the type of marsh area involved. However, 23 animals per acre have been trapped i n parts of Louisiana (Cameron Parish) though such densities prejudice the subsequent maintenance of the population by destroying the habitat. Texas has recorded a per-acre productivity only s l i g h t l y lower than that exhibited i n Louisiana. The coastal marshes of the A t l a n t i c States inhabited by macrodon were able to support an annual k i l l of four to f i v e ani-mals per surface acre though favourable areas produced s i x to eight or even ten animals. Some of the continental marsh areas ocoupied by zibethicus were as productive as the coastal environment. Yeager (194-5) 97 TABLE 5 PRODUCTIVITY OE SELECTED AREAS IN MUSKRATS PER ACRE Author Region Muskrats per Acre Remarks r i v a l i c i u s Lay & O'Neill (1942) Texas coast 1.35 O'Neill(1949) Louisiana 1.0 5.8 20 year State average 20 year average best brackish marsh Dozier (195?) Cameron Parish, La. 23 unusually high y i e l d maorcdon Dozier (195 3) Maryland 4 - 5 10 long term average y i e l d high l o c a l populations Panooast (1937) New Jersey 6 3.5 a r t i f i c i a l ponds harvest from native marsh Bailey (1937) Maryland 3 - 4 6 - 8 average y i e l d maximum harvest Dozier, Markley, Blackwater and Llewellyne Refuge, Md. H948) 5.0 average for 1938 only zibethious Yeager (1945) I l l i n o i s 1.7 2.4 7.1 r i v e r marsh type g l a c i a l marsh drainage ditches Anderson (1947) Ohio 3.6 lake marsh area Dozier (1950) New York 1.0 1943-48 average Gashwiler (1948] I Maine 1.1 I946-48 average Beer and Truax (1950) Wisconsin 2.7 5.5 1945-48 average high y i e l d of 1948 Errington (1946.b) Central Iowa 5.2 y i e l d i n 1944 98 re carded seven muskrats per acre from drainage ditch habitat 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 period of time varied from about one animal per acre i n Maine and New York to three animals per acre i n parts of Ohio and Wisconsin, i n Iowa Errington (1948) estimated that the best c a t t a i l marsh might have a density of 35 animals for each aore though he d i d not suggest the average number harvested from any such area. Even his figure of 5.2 animals per acre from L i t t l e Wall Lake i n centtral Iowa ex-ceeds the average for the surrounding States. When we come to the more northerly races of muskrats populations per unit area are lower and i t i s more meaningful to give density i n terms of acres per muskrat rather than muskrats per acre. In the delta of the Saskatchewan River an average of 150 thousand muskrats have been removed annually from approxi-mately one m i l l i o n acres of improved marsh area (1940-48). The yearly average, therefore, was one muskrat f o r each seven aores. This i s i n accord w i t h densities recorded by F u l l e r (1951a) f o r the Athabaska-Peaoe River delta wherein i t required nearly nine acres to produce one muskrat. These densities are higher than those for the Aklavik 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 delta h a b i t a t , i . e . t h i r t e e n aores per animal. There have been variations i n t h i s number which ranged from 42 acres per musk-rat i n 1933-34 to seven acres i n 1945-46. Mare exact information has been made available by the r e g i s t r a t i o n of discrete trapping units i n the Mackenzie d e l t a . 99 In 1949-50 the harvest of muskrats was 167 thousand, s l i g h t l y be-low normal. The number of p e l t s from 3 2 randomly selected r e g i s -tered trapping areas varied from one per three acres to one per 29 acres, w i t h an average of one per t h i r t e e n acres. The densities recorded for albus and spatulatus are lower than most of those which appear i n Table 5 because i n the three areas under review the density has been computed on a regional basis rather than for a p a r t i c u l a r marsh area. These regional blocks include a certain amount of unproductive country which tends to bring down the average considerably. 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 discrete areas of the. Mackenzie delta. I t already has been stated 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 spring, thereby supplying a harvest of almost two animals per acre. But Grassy Lake was unique i n maintaining a high yearlong population of animals. Most delta lakes had a much lower capacity t o support muskrats. A review of the information concerning densities of animals reve :als the d e f i c i e n c i e s of the northern environment. Even account ing for yearly v a r i a t i o n s i n population l e v e l s , the difference be-tween Maryland and Aklavik may be of the order of s i x t y to one (4.5 x 13). Louisiana showed an even greater proportional d i f f e -rence i n productivity. 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 Allee et. a l . (1949) stated that "homoiothermal animals from colder climates tend t o be larger i n s i z e and hence to have less surface i n proportion 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 widely even though not u n i v e r s a l l y among bird s and mammals and i s usually interpreted i n r e l a t i o n to heat conservation i n the north and heat ra 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 with i t and i s conoerned with the marked tendenoy toward the lessen-ing of extremities i n colder climates". The North American hares (Le pus) demonstrate both Bergmann's and Allen's r u l e s rather w e l l , and other examples could be given. But a l l mammal populations do not conform to 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 the muskrat allows i t to l i v e i n a variety of habitat types and as a consequence i t often i s thrust into r e l a t i v e l y unfavourable circumstances. In these undesirable surroundings i t exhibits greater proportional growth responses than do those species with more precise habitat demands. The r e -sponse of muskrats to t h e i r habitat was w e l l demonstrated by Dozier (1950) on the Montezuma marshes i n ce n t r a l New York. Over a period of s i x years during which 29,651 muskrats were handled, he noted average weight. differences 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 habitat they occupied. There can be no doubt that weights w i l l show a greater response to 101 TABLE 6 A COMPARISON OF WEIGHTS AND STANDARD MEASUREMENTS ft OF MUSKRATS FROM DIFFERENT PARTS OF NORTH AMERICA AUTHORITY and RACE SEX SAMPLE SIZE Weight STANDARD Total Length MEASUREMENTS T a i l Length Hind Foot 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 Dozier (1950) New York M F 14785 14866 1474 1361 Grimm and Roberts (1950) Pennsylvania M F 9 8 1139 567 547 268 255 80.2 78.6 H o l l i s t e r (1911) B ftft 7 563 254 81 Sooter (1946) C a l i f o r n i a M F 96 106 1170 1068 606 595 297 290 84 83 Anonymous (19 51) West V i r g i n i a M F 5 7 1429 1102 593 570 268 255 84 81 r i v a l i c i u s Lay (194 5) Texas M F 99 99 IO69 1041 spatulatus • F u l l e r (1951b) Athabaska Delta M F 41 40 1132 1053 572 562 256 250 75.6 74.5 Slave Delta B 15 932 531 246 73.5 (Table continued on the following page) 102 TABLE 6 (CONTINUED) AUTHORITY STANDARD MEASUREMENTS and RACE SEX SAMPLE SIZE Weight T o t a l Length T a i l Length Hind Foot • sp a t u l a t u s (cont'd) Osgood (1900) Yukon F 1 495 170 73 Preble (1908) Athabaska-Slave Lake Area B 5 546 264 75 s p a t u l a t u s Preble (1908) Franklin.NWT M 1 540 244 80 McPherson, NWT B 5 544 251 76 Soper (1948) Peace R i v e r M 1 1020 584 260 80 Stevens (unpubl.) Macke nz ie Delt a M F 77 81 1065 962 539 533 232 232 76.3 75.4 (see Table 7) ± -±k -Weights i n grams and measurements i n m i l l i m e t r e s . B represents both sexes of animals. 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 size measurements. Thus, the size of an animal can be not only a racial character but also a reflection of the adequacy of the habitat i t occupies. In sampling any animal population, therefore, i t is necessary to rely 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 suffer from the fact that they have sampled too small a portion of the total population. Table 6 sets forth 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 this survey. The adequacy of small samples such as those provided by the earlier investigators is 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) is a small race, and that the animals from the Mackenzie delta are the smallest representatives of that race yet studied. Data for animals of the Slave River delta presented by Fuller (1951b) showed them also to be small. Only fifteen ani-mals were included in Fuller's measurements but i f these are re-presentative of the whole population then they are significantly 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 deterior-ating beoause of excessive s i l t 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 T a i l Length of Hind Foot ADULT MALE Average IO65.3 539.4 Number 119 77 Standard Deviation 137.8 19.4 Range (740-1580) (489-589) 232.4 77 19.0 (208-254) 76.3 77 I .65 (72-80) ADULT FEMALE Average 962.0 532.9 Number 124 8l Standard Deviation 127.7 23.4 Range (725-1395) (466-575) 231.9 83 14.8 (198-270) 75.4 84 2.2 (71-79) ft - Weights i n grams, measurements i n millimetres. Measurements for the Mackenzie delta muskrats appear in Table 7 as well as in Table 6. The former presents the v a r i a b i l i t y in size 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 in weight or standard measurements which w i l l differentiate adult male from adult female animals. Though differentiation of adults from sub-adults could be 1 0 5 done on the b a s i s of weights and measurements up u n t i l about t h e f i r s t part o f A p r i l , t h e r e a f t e r only tagged animals or those accorded post-mortem examination could be placed i n the appro-p r i a t e age c l a s s . A l l those animals i n c l u d e d i n Table 7 were known to be a d u l t . Two 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 the smaller s i z e of muskrats i n the Mackenzie d e l t a area. The f i r s t of these would 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 northern areas as they were released from P l e i s t o c e n e g l a c i a t i o n . Having a t t a i n e d t h e Mackenzie d e l t a area t h e y were s h i e l d e d by t h e i r aquatic h a b i t a t from the e v o l u t i o n a r y pressure exerted by lower temperatures. Therefore, they remained s m a l l and no doubt w i l l continue t o be so. A l t e r n a t i v e l y , the invading animals from more southern areas f o l l o w e d the northern drainage and a r r i v e d a t the Mackenzie d e l t a when conditions were even less favourable t h a n now. Because the h a b i t a t they encountered was not adequate t o meet t h e i r demands they never were allowed 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 growth, and over a period 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 or 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. At the present time a severe w i n t e r can exert a d e l e t e r i o u s e f f e c t on the 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 lower one-t h i r d of the d e l t a which l i e s beyond the timber l i n e . As a con-sequence the s i z e of many animals i s s m a l l and the p e l t s so l i g h t that they command a low p r i c e i n the f u r market. 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 consider the p e l t s of animals from the tundra l a k e s east of the Mackenzie R i v e r . I n most years, they, are s m a l l e r than those from the d e l t a proper. 106 * i It is the writer's opinion that this second alternative comes nearer to explaining the reason for the small muskrats l i v i n g in the lower reaches of the Mackenzie River. The muskrat, like the raccoon (Prooyon) , is an animal which had invaded less favourable habitat in extending i t s range northward and persists therein as a smaller animal. In the Mackenzie delta the avail-a b i l i t y of winter food as governed by a rigorous climate i s con-sidered to be the faotor responsible f o r the small size and re-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) AUTHORITY JUVENILES ADULTS Male s Fern ales Males Females Snead (1950 unpubl.) No. Gain/day 137 5.86 116 5.65 62 65 0.34 0.57 Aldous (1947) No. Gain/day 6 4.52 Errington (1944) No. Gain/day 6 4.63 8 4.10 St evens (unpubl.) No. Gain/day 10 1.97 10 2.25 3 4 0.24 0.19 At the beginning of the investigation in 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 com-parison with mare southerly races. An opportunity for checking the 107 v a l i d i t y of t h i s assumption was provided by growth measurements of tagged animals t h a t subsequently were recaptured. These data are not conclusive because instances o f recapture were too few, but they are i n d i c a t i v e of growth aohieved over a v a r i a b l e p e r i o d . Table 8 presents t h i s i n f o r m a t i o n together w i t h data gleaned from other authors who have stud i e d muskrat populations under f i e l d c o n d i t i o n s . As the t a b l e demonstrates, t h i s phase has been r a t h e r s a d l y neglected 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 part from t h e d i f f i c u l t y of o b t a i n i n g recaptured specimens f o r measurement. E r r i n g t o n (1939a) compiled growth curves f o r nest-l i n g muskrats up t o t h i r t y days of age, but once the animals be-come independent of the parental care and the heme nest, he too found i t very d i f f i c u l t t o weigh them p e r i o d i c a l l y . Measurements of growth under l a b o r a t o r y conditions would give more complete data, though the r e s u l t s might be a t y p i c a l . The growth r a t e s given represent not a t r u e growth curve but only an amount of growth over a p e r i o d of time. These data are segregated as to the sex and age of the animals concerned and show o n l y the average f o r the sample. They accord thereby w i t h the information from the l i t e r a t u r e but give l i t t l e i n d i c a t i o n of the r a t e o f growth duri n g any p a r t i c u l a r p e r i o d of an animal's l i f e . The amount of growth of t h e muskrats i n the d e l t a p o pulation was low when compared w i t h d a t a from Iowa (Sneed and E r r i n g t o n ) and from South Dakota (Aldous). Gain i n weight of a d u l t s i n the north-ern animals was apt t o be zero or even negative; one a d u l t female l o s t 125 grams between August and A p r i l . Rates of g a i n f o r young animals were l a r g e s t f o r the 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 females tagged i n August and captured f o r t y days l a t e r had gained weight a t the r a t e o f between seven and eight grams per day. Most of the other animals represented i n the t a b l e were tagged i n August and not recaptured u n t i l the f o l l o w i n g s p r i n g (about 250 d a y s ) . During t h i s period the r a t e of growth natur a l l y would decrease as a d u l t s i z e was approached. This de-c e l e r a t i o n no doubt was a s s i s t e d by the sudden appearanoe of i c e cover and the adjustments necessary for l i f e beneath i t , a d j u s t -ments not only 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 food. Whatever the reason, the muskrats o f the Mackenzie d e l t a did not appear t o grow as r a p i d l y as those s t u d i e d by i n v e s t i -gators i n the mid-Western United S t a t e s . I n the f i r s t place the average of i n d i v i d u a l s of t h i s population was smaller (Table 7), so t h a t t h e animals d i d not have t o grow as r a p i d l y t o a t t a i n t h e i r t e r m i n a l weight. I n the second place, t h e i r w i n t e r h a b i t s 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 could not be ex-pected t o a t t a i n the r a t e o f growth shown by t h e i r more s o u t h e r l y r e l a t i v e s . And f i n a l ly,..the. muskrats o f the Mackenzie by maturing l a t e r i n the year could grow fo r a longer p e r i o d without 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 data are a v a i l a b l e i n t h i s or any o t h e r study from which t o draw v a l i d conclusions concerning r a t e s of growth of muskrats. Further 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 before a comparison o f growth r a t e s i n one area can be made w i t h those i n another. One u l t i m a t e objective of these 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 conditions 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 sub-s p e c i f i c rank. 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 concerning the l e n g t h of the adult l i f e of muskrats. Johnson (1925) was "not aware of 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 long enough to f u r n i s h any data of value i n th i s connection", but thought i t probable t h a t a muskrat might l i v e at l e a s t ten years. But t h e l e n g t h of time which the animals can l i v e i n c a p t i v i t y probably bears 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 Kreeger (194 8) s t a t e d t h a t "the l e n g t h of the adult l i f e i s unknown". E r r i n g t o n (1944) c a l c u l a t e d two animals to be 5&1 and 552 days of age when recap-t u r e d , though most of h i s r e c o r d s were fo r s h o r t e r periods of time. There i s a r e c o r d of one animal 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 being caught three years l a t e r . A few data are a v a i l a b l e from the Mackenzie d e l t a study which g i v e some i n d i c a t i o n of the length of l i f e of the muskrats i n those l a t i t u d e s . They suggest t h a t the animals there had a r e -markably short l i f e span, whether they were e l i m i n a t e d by v i o l e n c e , old age or other f a c t o r s . I t was b e l i e v e d t h a t most cf the tags from animals r e -covered i n the y e a r l y catch o f t r a p p e r s were ret u r n e d to the w r i t e r . The novelty of f i n d i n g a tagged animal was s u f f i c i e n t t h a t the I n d i a n or Eskimo trapper was e s p e c i a l l y c o n s c i e n t i o u s i n r e p o r t i n g i t . White trappers a l s o co-operated f u l l y and even went to the a d d i t i o n a l e f f o r t of weighing tagged animals t h a t t h e y recovered. The ages of 169 recaptured animals were c a l c u l a t e d by assuming t h a t a l l animals were born on 15th June of any year, and 110 TABLE 9 CALCULATED AGES OF RECAPTURED MUSERATS GROUPED INTO FIFTY-DAY AGE CLASSES. Age Group (days) Frequency of occurrence Male Female 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 T o t a l s : 94 75 169 I l l that the animals tagged as adult during summer had been born the previous summer and were, therefore, just a year old. Using these c r i t e r i a , one animal recaptured was assumed to be 776 days old and may have l i v e d longer following i t s release from the cage tra p . The ages of eight other animals were calculated to be over 700 days but the remaining 16 0 were of lesser age (Table 9 ) . The l a s t muskrat was tagged 15th September, 1949, and the l a s t tag was r e -turned i n June, 1950. Thereafter, i n spite of continued contact with the trappers, no more tagged muskrats were reported and i t i s assumed that 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 after they had been tagged, and of those nine which were, none had been tagged as an adult. In lakes which had been given intensive attention, and i n which large numbers of tagged animals had been released, seme animals were recovered the f i r s t 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 that they 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 they had almost as much chance to be taken there as at the tagging s i t e . A l t e r n a t i v e l y , therefore, the animals must have had a very b r i e f l i f e span with the r e s u l t that most had succumbed to l e t h a l factors 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 de-pendent upon the a b i l i t y of the animals involved to reproduce th 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 he g r e a t e r the accu-mulative m o r t a l i t y the g r e a t e r must be the rep r o d u c t i v e p o t e n t i a l of the s p e c i e s . Conversely, a lower 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 of r e p r o d u c t i o n . The breeding h a b i t s ard breeding r a t e of a sp e c i e s o r population should, t h e r e f o r e , r e f l e c t i n some measure the "environmental r e s i s t a n c e " a c t i n g upon i t . Few pop u l a t i o n s , however, are i n balance w i t h t h e i r en-vironment over any pe r i o d of time. The organic system which 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 set over aga i n s t t h a t complex of f a c t o r s which reduce these numbers. The r e s u l t i n g balance f l u c t u a t e s about a mean which 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 the v i t a l p o t e n t i a l of the musk-r a t population found w i t h i n the 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 population increase with t h a t found i n other areas of North America. As was pointed out under the s e c t i o n on c l i m a t e , the i c e free p e r i o d i n the Mackenzie d e l t a l a s t s o n l y from e a r l y June u n t i l l a t e September. During t h i s period the muskrats must breed, 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 f o r t h e i r w i n t e r requirements. By the time the animals are con-f i n e d beneath the i c e , breeding a c t i v i t y long s i n c e has d e c l i n e d and the re p r o d u c t i v e organs have regressed to a quiescent s t a t e . The Oestrus Cycle I t i s of i n t e r e s t t o compare the l e n g t h of the breeding p e r i o d o f muskrats throughout t h e i r range i n North America. For the t i d e - w a t e r marshes of t h e Gulf coast of Texas and L o u i s i a n a , 113 Lay (1945) discovered pregnancies yearlong with the highest in-cidence of births in November and March, and the lowest in Janu-ary and in mid-summer. Because of the favourable conditions and the long period of growth seme animals were said to breed during the year of their birth. In the eastern coastal marshes of Maryland and Delaware investigators recorded a definite anoestral period. Forbes (1942) found that there were some corpora lutea in 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 there-after unti l the last of October. Spermatogenesis in males lasted almost yearlong, but was 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 latitudes, the period of oestrus was found to be shorter. In Iowa, Errington (1937) encountered l i t t e r s born from mid-April to late August and was of the opinion (1947) that the main breeding season in Iowa was three to five weeks later 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) recorded l i t t e r s as early as late Febru-ary in Washington. Aldous (1947) noted that in South Dakota musk-rats were breeding during the f i r s t two weeks of April but gave no more information on how early this 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 re-stricted to a period starting i n March and ending in August. Ovu-lation in females began about the f i r s t of A p r i l and had ceased by the f i r s t part of July. MoLeod (194 8) examined 28 l i t t e r s of the Hudson Bay muskrat i n Manitoba and found that the f i r s t l i t t e r s arrived from May 2°th to June 17th. Data concerning the habits of the northwestern muskrat (spatulatus) are scarce indeed i n literature. Law (1950) found a pregnant female with seven embryos in late May in Wood Buffalo Park, Alberta. He suggested that "the f i r s t l i t t e r is probably bora during late June, the second l i t t e r i n July". Cowan (1948) working i n the Mackenzie delta was of the opinion that f i r s t l i t t e r s arrived i n early June, with a second, at least for some females, in August. Marsh (1948) suggested a spring and a f a l l l i t t e r but was no more explicit than that i n his observations. Porsild (1945) quoted K. H. Lang of Aklavik as saying that during average years the animals raised 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 in assisting us to understand their habits as Beer and Meyer (1951) have point out, a l l animals taken during this study were examined to assess endocrine levels as reflected i n the con-dition of the reproductive organs. Several methods were employed to ascertain the breeding condition of animals. These Involved both macroscopic examination under f i e l d conditions and, where possible, subsequent laboratory examination. It was not always feasible to carry out the second mare accurate analysis but re-liable information nevertheless could be obtained in the f i e l d . Animals were collected at intervals throughout the year in order to assess the condition of the reproductive organs. 115 A f t e r the y e a r l y opening of the t r a p p i n g season on March 1 car-casses were a v a i l a b l e i n q u a n t i t y . F i e l d examination c o n s i s t e d i n measurements of volume of t e s t e s and accessory organs i n the male, and i n recording the growth of f o l l i c l e s and corpora 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 of the u t e r u s , presence of v a g i n a l mem-brane or v a g i n a l plug, and c o n d i t i o n of the mammary glands i n the female. U t e r i n e s c a r s marking past s i t e s of implantation 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 multiparous from 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 of young borne. These p l a c e n t a l s i t e s p e r s i s t e d i n t h e quiescent uterus f o r s e v e r a l months past the end of t h e breeding season. Male muskrats i n the Mackenzie d e l t a e x h i b i t e d seasonal development of sexual organs at an e a r l i e r date t h a n d i d the f e -males. Figure 16 represents g r a p h i c a l l y the development of the t e s t e s and concomitant growth of accessory organs, the seminal v e s i c l e s and the musk glands, of a s e r i e s of animals c o l l e c t e d throughout the year. The onset of t h e breeding c o n d i t i o n may be deduced from these data but the date f o r l o s s of f e c u n d i t y i s l e s s w e l l d e f i n e d . The e a r l i e s t date on which spermatozoa appeared i n t h e t e s t e s and epididymides o f male muskrats was 23rd March (1947). Animals taken i n A p r i l were i n m o s t cases found t o have some stored spermatozoa though smears t a k e n often were scanty. By mid-May spermatozoa were abundant and remained so u n t i l r e c e s s i o n of the r e p r o d u c t i v e organs began i n J u l y . The volume of t e s t e s and a s s o c i a t e d organs decreased s t e a d i l y from the high i n May and June. By August they were about 116 t/3 B-i 53 W 2 W a W < K W f> r-*2.0 300 -3.0 ml. 1000 mm. "6bo L i . o 400 O Volume right seminal vesicle © Volume right testis * Measurement right musk gland (length X width) Apr. I May Jun. Jly. !_ Aug M 0 N T H FIGURE 16. MONTHLY AVERAGE MEASUREMENTS OF MALE MUSKRAT REPRODUCTIVE ORGANS IN THE MACKENZIE DELTA DURING THE PERIOD 1947-50. (ALL MEASUREMENTS IN MILLIMETRES & MILLILlTRES) 117 half 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 earlier than the first-year males. The increase i n size of the adult testes i s also much more rapid than that of the young animals". Such an assertion could also be made for the Mackenzie muskrats. This differential attainment of sexual matur-ity was, however, mare prominent in the females studied. The onset of the reproductive cycle was relatively more sudden in female muskrats than in males. About mid-April the uterus, which in i t s dormant state was thin and translucent, be-gan to show an increased vascularization. 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 fa t bodies surrounding the ovary had been progress-ing 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 partially embedded. In A p r i l and May adult females developed more rapidly to-ward sexual maturity than did 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 is d i f f i c u l t to give quantitative data concerning the various qualitative 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 fat bodies, while cer-tainly 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 uterine width 1 cm. above the c e r v i x was used as a c r i t e r i o n of u t e r i n e growth, assuming t h a t development of the ovaries and accessory organs would be con-comitant w i t h the growth of the u t e r u s . I t was found i n t h e r e s t i n g uterus t h a t the width of the u t e r i n e horns a t 1 cm. above the c e r v i x was 2.5 t o 3 mm. These widths were a t t a i n e d by September and p e r s i s t e d during most of the anoestrus p e r i o d . By the l a s t week i n March a few adult f e -males were beginning to show an increase i n width of u t e r i n e horns together 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 the uterus and o v a r i e s and an increase i n the f a t bodies a t t a c h e d to the o v a r i e s . These f a t bodies, however, were extremely v a r i a b l e i n s i z e and probably r e f l e c t e d general n u t r i t i o n a l as w e l l as endocrine l e v e l s . Figure 17 i n d i c a t e s that up u n t i l the middle of June the u t e r i n e horns of adult animals had developed more r a p i d l y than had those of sub-a d u l t s . Therein the data f o r the 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 those p o i n t s appearing w i t h i n the square. Those data appearing above the square represent u t e r i n e widths of preg-nant females and those to the 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 quiescence. I t i s s i g n i f i c a n t that among the younger animals l i t t l e development had taken place by the end of A p r i l but t h e r e -a f t e r the attainment 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 the females were c a r r y i n g young. During the s p r i n g s of 1948 and 1949 i t was p o s s i b l e to examine a t o t a l of 674 female muskrats and to a s c e r t a i n whether or not they had been bred. I n each i n s t a n c e shown i n Table 10 the animals examined had been k i l l e d w ith i n the two-weeks period p r i o r to 12th June, the end of l e g a l hunting i n the Mackenzie d e l t a . The 1 2 0 animals were a l l t a k e n i n the v i c i n i t y of A k l a v i k . TABLE 1 0 BREEDING CONDITION OE FEMALE MUSKRATS COLLECTED IN SPRING ANIMALS BRED NOT REMARKS Vag i n a l Implant- N e i t h e r BRED plug at ions 0 1 2 J 7 Animals shot 4 t o 7 June 1949 Boxer's Area 7 16 37 1 0 6 Animals shot 29 May t o 9 June 1 9 4 9 , Boxer's Area 14 1 1 4 4 1 3 8 Animals shot 29 May t o 1 2 June 1949, Lang's Area 0 0 26 27 Animals shot 1 t o 10 June, 1 9 4 8 Boxer's Area 0 0 13 3 3 Animals shot 2 2 - 2 7 May, 1 9 4 8 Wiedemann's Area 0 0 6 40 Animals shot 23 to 31 May, 1 9 4 8 Lang's Area 0 8 23 63 Animals shot 1 t o 8 June, 1 9 4 8 Lang's Area 0 9 4 18 Animals shot 9 June, 1 9 4 8 Lang's Area 21 45 176 432 Totals I t w i l l be noted t h a t animals which had been bred were d i v i d e d i n t o three c l a s s e s , those w i t h a v a g i n a l plug, those show-ing implanted f o e t u s e s , and those bred but shewing n e i t h e r . This l a s t category r e q u i r e s some c l a r i f i c a t i o n . I n the female muskrat, as i n many other rodents, a v a g i n a l plug i s formed f o l l o w i n g copulation by coagulation of the seminal 121 f l u i d . Because the plug i s p e r s i s t e n t and c l o s e l y a p p l i e d t o the.rugae of the vagina, i t locks seminal and other f l u i d s i n the u t e r u s . The r e s u l t i s that the accumulated f l u i d s s t r e t c h the w a l l s of the u t e r i n e horns so that even a f t e r the plug has d i s i n t e g r a t e d and t h e uterus drained, the general shape of t h e organ remains somewhat as i n i t s t u r g i d s t a t e . E s p e c i a l l y d i a g -n o s t i c was the apex of each u t e r i n e horn. I n the quiescent s t a t e the apex of each horn 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 with the 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 s t r e t c h e d the u t e r u s , a d e f i n i t e pouch or bursa was formed at the apex of each horn d i s t a l t o the entry of the t u b u l e s . This c h a r a c t e r i s t i c , together with the obvious t h i n n i n g o f the u t e r i n e w a l l s , could be seen w i t h f a c i l i t y and both were considered d i a g n o s t i c of s u c c e s s f u l breeding having occurred. During the l a s t two weeks of the hunting season i n the Mackenzie d e l t a (May 21 - June 7 ) , only 24-2 or 36 per cent of 674 female muskrats examined had been bred. 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 po p u l a t i o n , 64 per cent o f l i t t e r s could not be born p r i o r t o the p e r i o d 20th June t o 7th J u l y (assuming a g e s t a t i o n of 25 days). Of the number which had been bred, only 45 or about 19 per cent (6 per cent 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 im-p l a n t a t i o n s . The other 197 had been bred r e c e n t l y enough that no implantation s i t e s were evident from gross a n a l y s i s . Erom these data i t i s possible t o estimate t h a t breeding a c t i v i t y was be-ginning during the l a s t week i n May and became general during the f i r s t week i n June, assuming that the implanted ova would be e v i -dent w i t h i n ten t o f o u r t e e n days a f t e r breeding. This period 122 coincided very well with the usual dates of inundation of the delta by spring floods and the rapid disappearance of the ice cover from most delta lakes. The flooding of densites, the i n -crease in water temperature, and the general movement of animals into emergency shelter a l l helped to e l i c i t a general unrest among the population at a time which coincided with the commence-ment of sexual activity. If the data in Table 10 are representative, i t is suggest-ed that the females which showed early implantations were adults. They had arrived at breeding condition sooner than sub-adult fe-males and had been bred before extensive areas of open water appeared around t t o e lake edges. A review of available information indicates that the re-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 is a long breeding period but an ancestral state is encountered. Nearby i n New York the period of fecundity i s again shortened and so on westward and northward unt i l in 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 factors i n the environment which govern the onset of reproductive activity have not been demonstrated in f u l l though many investigators have given their 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 in addition three groups of proximate causes. The f i r s t of these was the internal reproductive rhythm of the species in question. The second was the external variant to which the cycle was attuned, and the third was that group of factors in the immediate neighbourhood of the individual (the breeding area, the mate, the social group, and so forth). It may be noted that this third group is largely of a psychological nature and often is necessary to trigger f u l l reproductive a c t i v i t y . In the Mac-kenzie delta this complex of factors reached i t s summation at the time of the spring floods. Light and temperature have been assigned major roles as factors regulating sexual cycles, and other have been investigated also. For arctic mammals, however, the influence of light and temperature i s relatively unknown, even though the variation in both is extreme. It probably can be stated a p r i o r i that temper-ature in an aquatic habitat, such as that frequented by muskrats, w i l l have less physiological effect than w i l l light because the variation 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 twilight or darkness of winter and i t s influence w i l l be considered hereafter. It was established by Rowan (1925) that light i s a causal 'factor in the sexual development and migration of birds. He as-cribed the sexual development of juncos exposed to an increasing light regimen to the opportunity for more exercise under those con-ditions. Bissonnette was able to eliminate exercise as a oausal factor and established that light was responsible per se. His work (1938b) with ferrets indicated that this effect was transmitted 12^ through the o p t i c nerve t o the a n t e r i o r lobe of the p i t u i t a r y . Hypophysectomy abolished sexual c y c l e s though blindness only r e -moved them from ph 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 experiments was a b l e t o a s s e r t t h a t "the a c c e l e r a t i o n of the oestrus c y c l e , g e n e r a l l y speaking, was 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 " . These r e s u l t s were i n accord w i t h Bissonnette (1938a) wherein he summarized a s e r i e s of experiments 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 the wave leng 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 sexual development. Research w i t h s t a r l i n g s showed th 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 low i n t e n s i t JBS, w h i l e green l i g h t and purple l i g h t were not s t i m u l a t i n g at a l l . 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 the v i s i b l e spectrum and a short con-tiguous part of the u l t r a - v i o l e t to 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 t h e 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 animals t o breeding 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 animals from r e g i o n s of extreme seasonal change i n l i g h t i n -t e n s i t y . Dempsey e t . a l . (1934) found t h a t guinea pigs would not respond t o v a r i a t i o n s i n day l e n g t h . And Moore and h i s cohorts (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 . I n the 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 temperatures was a necessary precursor 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 best of 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 of l i g h t on t h e s e x u a l c y c l e s o f muskrats has been con-ducted. 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 other 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 similar manner. In a l l of the investigations detailed above the method consisted i n a progressive increase i n illumination which event-ually brought about reproductive activity of varying degree. They shewed that up to a point an increase in the intensity of illumin-ation correspondingly accelerated sexual development. It was assumed that the increase in day length was the prime stimulus prompting gonadotropic activity of the pituitary. No attempt was made to measure c r i t i c a l l y the amount of light necessary to bring about this effect. For example, assuming the factors of intensity, wave length and so forth to be equal, would daylight of eighteen hours acting for one month be equivalent to 24 hours of light for two-thirds as long? Or stated in another way, is there a minimum and a maximum rate at which the threshold level of illumination can be achieved? There appears to be a need for further investigation of this question. The sequel to reproductive activity 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 periodicity, while conditioned by the environment, is regulated i n i t s success-ive phases by the combined integrative action of the nervous and endocrine systems". However, whether the animal i s monoestrus, like the ferret, or polyoestrus like the f i e l d mouse, this yearly internal periodicity usually ceases eventually and a refractory period follows. What prompts this cessation of sexual activity has not 126 been demonstrated satisfactorily with, any animal. Bissonnette (1938b) found that while decreasing length of day normally may help i n the induction of anoestrus, and may even accelerate i t , yet i t was not a necessary faotor in ferrets because blinded animals ceased breeding without functional light receptors. The same author (1938a) in referring to his studies with starlings 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 until 21 June and light intensity for a period thereafter. In animals with a gestation period extending over the major part of the year the fact of preg-nancy terminates oestrus. But i n polyoestrus animals with short gestation and recurrent heat periods other faotors in addition to light maybe important, for example, temperature, food, and the crowding of the habitat induced by the pressure of earlier-born young. In the section which i s to follow, the writer has used accumulative daylight as the measure of the lig h t factor in nature. This i s merely another way to measure an increase i n day length i n any particular area. It 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, total daylight w i l l be more or less the same in any locality. In the five 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 five general latitude bands were chosen at 30°N. (Texas and Louisiana), 40°N. (Maryland, Wisconsin 127 and Iowa approx.), 53°N. (The Pas, Manitoba), 60°N. (Athabasca-Peace River deltas), and 68°N. (Mackenzie River delta). The t o t a l accumulative light 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 for most races of muskrats repro-ductive activity had almost or entirely ceased by that date. The time of significant events in the reproductive activity of musk-rats in the several latitude bands was correlated with the accumu-lated hours of daylight for each date or period of time. In that way it 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 re-sults of this 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 cited in the table and in some cases to be more specific than the published data. The dates given, however, can be taken as averages for the phenomena observed. More accurate recording by these authors of specific events in the reproductive behaviour of the widely distributed raoes of muskrats would have allowed a better correlation of light and other factors in the en-vironment with this activity. However, some observations may be made concerning the information available. A study of these accumulated daylight totals, corresponding to events in the reproductive acti v i t y of muskrats i n various areas, w i l l reveal that the farther north the species i s found the more daylight is accumulated by the time the event occurs. The birth TABLE 11 DATES AND AC CUMULATED DAYLIGHT INDICES OF EVENTS IN THE REPRODUCTIVE CYCLE OF MUSKRATS IN NORTH AMERICA EVENT Aklavik Ft.. Smith LOCALITY The Pas Wiscons in Maryland Reproductive Organs (male) Recession primary organs to Primary organs dormant un t i l Adult male capable of breeding Mating occurs 15-92 to 20-10 01-3 until?20-1 20-4 (2100)^ 01-4 (1930) 01-6 (3030) 01-5 (2450) to u n t i l 01-4 (2050) 20-4 (2350) 10-11A 30-12 ±k 10-3 (1900) 01-4 (2200) 10-1 (1250) 20-3 (2050) Reproductive Organs (female ) Beginning of uterine enlarge] Females capable of breeding Few young born prior to 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) 15-4 (2400) Movement Movement of animals begins Movement well established 20-5 (2800) 20-4 (2300) 10-4 (2200) 20-2 (1650) 01-6 (3050) 10-5 (2650) 01-5 (2500) 20-3 (2050) Fighting Seme fighting Peak of fighting 20-5 (2800) 10-6 (3300) 10-5 (2650) 20-2 (1650) 01-5 (2600) Spring break-up 01-6 (3050) 10-5 (2650) 20-4 (2300) 15-3 (1950) A Date &ft Accumulated daylight i n hours 129 of l i t t e r s is a satisfactory and easily observed event. In Mary-land, Iowa and Wisconsin young muskrats seldom are born prior to the f i r s t of May, at which time there is 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 lastly, in the Mackenzie delta the ave-rage date was about 20 June and the accumulated daylight at that time 3500 hours. Some explanation for this variation in timing of births of the f i r s t l i t t e r s can be given. It was stated by Fuller (1951a) and noted by other investigators that "the time of mating seems to be determined by the time of break-up of the ice cover". In those continental areas under scrutiny the spring break-up varied from mid-March i n Iowa and Wisconsin to late April i n central Manitoba and northern Alberta and early June in the Mackenzie delta. There-by the mating activity of the muskrats was delayed until they were able to emerge from their winter habitat. There is some evidence that animals were capable of breed-ing prior to removal of the ice cover. In Wisconsin Beer and Meyer (195U pronounced adult males f e r t i l e by the tenth of March, though mating probably did 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 April (Fuller, op. cit.) and i n the Mackenzie delta by the end of A p r i l . 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 for a l l of these dates were 1900, 1950, 2300 and 2050 respectively. 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 less comparable in a l l locations. The figures for Maryland appear to be an exception because there spermatogenesis has been recorded early in January at a daylight index of only 1250. It is supposed that the lack of continuous ice cover there exposes the animals to greater amounts of day-light than otherwise would be the case. This explanation involving the amount of ice cover may be given further consideration. Female muskrats in the areas a l -ready mentioned reveal a greater disparity 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 total was the lowest and the Mackenzie delta total the highest. TABLE 12 GREATEST DIFFERENCE IN LENGTH OF DAY FOR FIVE AREAS OF NORTH AMERICA. Location Length of daylight in 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 Ft. Smith, NWT., 60°N. 22.0 7.5 14.5 Aklavik, NWT., 68°N. 24.0 5.0 19.0 131 No mention yet has been made of the effect 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 in those latitudes. This fact, coupled with the high mean annual temperature, may account for the fact that no anoestral period exists there. It is possible that there is not sufficient variation i n day length throughout the year to e l i c i t those changes in the physiology observed in muskrats farther north. In Louisiana Svihla and Svihla ( 1 9 3 D "found young muskrats or embryos every month of the year, yet the heaviest breeding appear-ed to take place frcm November to April inclusive". It has been notedthat there is some decrease of breeding a o t i v i t y during January. Inasmuch as this is 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 activity mainly to the cooler months of the year (see Appendix B) and, therefore, temperature and not light may have been the prime fac-tor governing reproductive capacity. If we examine the daylight indices for various events in the sexual a c t i v i t y of muskrats i n the Mackenzie delta, it 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 involves the extremely low angle of the sun at that latitude. 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 violet and ul t r a -violet end of the spectrum. The low angle of incidence certainly would decrease the amount of radiation available to wintering muskrats though the wave length of that radiation might be a less important consideration. In any case, i t i s assumed that the low intensity of radiation in the far 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 relatively greater thick-ness of ice interposed between the radiant energy of the sun and the animals beneath. It seems possible that the delta muskrats live i n a nearly dark environment during part of the winter and that the time of year at which activating radiation i s available to them w i l l vary with the thickness of the ice and the depth of the snow cover above i t . Temperature then becomes a factor in determining the light regime of muskrats because i t , in part, de-termines the amount and persistence of the ice and snow cover. Depth of snowfall also is an important consideration. If there i s a correlation between the physiological con-dition of muskrats and the amount of sunlight they receive, as there seems to be, then the factors of snow and ice depths be-come more important as one moves towards the higher (and colder) latitudes. These factors, and others, should be given consider-ation in assessing the relationship of light per se to the sexual activity of muskrats. On the basis of this study i t is not possible to state whether far northern muskrats are more or less susceptible to solar radiation than their counterparts in more temperate regions. 133 To answer t h i s question 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 than the w r i t e r was i n a p o s i t i o n t o undertake a t the t i m e . I t has been assumed t h a t they 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. I t f o l l o w s , t h e r e f o r e , that t h e i r apparent l a g i n reproductive development can be as c r i b e d to f a c t o r s i n t h e i r p h y s i c a l environ-ment which s h i e l d them from the r a p i d l y i n c r e a s i n g r a d i a t i o n 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 IN THE MACKENZIE DELTA 1947-50. Trapping Area Date Adult Sub adult Grassy Lake Nov. - Dec. 1947 11 56 Reindeer Depot Feb. 1948 4 8 A. J . Boxer Mar. 1948 7 33 Joe G u l l y Mar. 1949 10 59 Bob 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 T o t a l s 120 514 Pr o p o r t i o n 100 Adults 428 Subadults Age Ra t i o s I n the Mackenzie d e l t a data f o r age composition of musk-r a t s i n the sampled areas were obtained 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 during e a r l y w i n t e r , Table 13. A l l animals were d i f f e r e n t i a t e d as to age e i t h e r by post-mortem examination or by the f a c t that they bore a numbered ta g . From 634- animals g i v e n t h i s c r i t i c a l examination 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 of 428:100. S i m i l a r information from n o r t h e r n A l b e r t a gave an even higher 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-parison of age r a t i o s i n other, p a r t s of northern North America i s a v a i l a b l e from the published information of those i n v e s t i g a t o r s l i s t e d i n Table 14. I f we assume an approximately equal sex r a t i o f o r breeding a d u l t s then the number o f s u r v i v i n g young per adult female i s h i g h e r i n northern A l b e r t a and the Mackenzie D i s t r i c t than i n most other regions l i s t e d . Far the north c e n t r a l United S t a t e s the r a t i o s v a r y from 202 j u v e n i l e s (or sub-adults) per 100 a d u l t s i n Michigan, to 359: 100 i n Wisconsin, and 436:100 i n Iowa. The f i g u r e s f o r Iowa are higher than those f o r the Mackenzie d e l t a , but i f we consider t h a t the number of young born per female i n Iowa i s about t w i c e that f o r the A k l a v i k r e g i o n , then we can a s s e r t e i t h e r a superior s u r v i v a l of a l l ages of northern muskrats, or else a depressed s u r v i v a l of a d u l t animals. I t may be s i g n i f i c a n t t h a t only two geographical races are being compared h e r e i n , z i b e t h i c u s i n the south, and s p a t u l a t u s i n the north. A lower s u r v i v a l of j u v e n i l e animals may be a character-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 inherent char-a c t e r i s t i c s of t h e animals themselves, or by the pressure 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 the e f f e c t s of pre-dation on muskrats s t a t e d t h a t " i n the event of predator pressure being i n c o n s e q u e n t i a l , the greater p a r t of the balancing 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 P e r i o d Sample NUMBERS PROPORTION Si z e Subadult Adult S.ad.:100 Ad, McCann (1944) Minnesota F a l l 564 409 155 264 Baumgartner & B e l l r o s e (1943) I l l i n o i s 15 Nov. t o 15 Jan. 1147 784 318 243 Baumgartner & B e l l r o s e (1943) Michigan 1 Nov. to 31 Jan. 7511 4866 2645 202 Gashwiler (195 0b) Maine Nov. 951 681 270 252 Beer & Truax (195 0) Wiscons i n F a l l 24082 18860 5222 359 Alexander (1951) New York 1 Jan. t o 20 Mar. 541 392 149 263 E r r i n g t o n (1947) Iowa Nov. and Dec. 4652 3785 867 436 F u l l e r (1951a) A l b e r t a Dec. t o May 417 361 56 655 Stevens (unpubl.) Mackenzie R. Nov . to Apr. 634 514 120 428 136 later took the form of lethal f r i c t i o n among the muskrats them-selves". As indicated previously, the muskrat populations in the latitude of Iowa assume densities of several times those in the Mackenzie delta. 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 in northern Canada. The question of the survival of adult animals i s one which was mentioned previously as capable of altering age ratios. 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 relatively short, thereby re-quiring continual recruitment of young animals in order to maintain the population. If 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 possibility of di f f e r e n t i a l survival of adults and juveniles i t is the writer's opinion that the younger animals i n the Mackenzie delta reached adulthood i n greater numbers than those in most other areas because the population was always well dispersed in the habitat, thus avoiding "lethal 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 JUVENILE Male Fema le Male Female Number captured and tagged ft 100 96 246 180 Males per 100 females 104 100 137 100 Percentage of total tagged 16% 15% 40% 29% Number of animals recaptured ftft 17 24 75 51 Percentage of total recaptured 10% 14% 45f. 30% ft - Total number tagged 621 ftft - Total number recaptured 167 Sex Ratios Throughout this treatise i t has been necessary to compare the northwestern muskrat (spatulatus) with races of muskrat in other parts of North America. Having discussed the age ratios of muskrats in the Mackenzie delta and elsewhere as influencing the breeding population, i t is considered necessary also to deal briefly with the male-female ratios as affecting the same pheno-menon. Two factors w i l l be discussed, f i r s t the actual sex ratio observed and, second the probable effects of an unbalanced ratio. Many authors have published information concerning sex ratios of muskrats. One of the more recent of these was that of Beer and Truax ( 1 9 5 0 ) who summarized information available to that 138 date. I t was t h e i r contention t h a t a s h i f t i n sex r a t i o s t occurred p r o g r e s s i v e l y from b i r t h t o adulthood. Male-female r a t i o s were more n e a r l y balanced at b i r t h and d u r i n g the breeding season, w i t h males i n the ascendancy at a l l other p e r i o d s . These authors d i d not quote E r r i n g t o n (1947) who presented a d e t a i l e d summary of sex r a t i o i nformation i n Iowa, and i n other areas as w e l l . H i s data, however, i n d i c a t e d that 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 animals, r a t h e r than the females. I n the Mackenzie d e l t a the a d u l t animals tagged d u r i n g the breeding 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 to each 100 females, Table 15. This was v a s t l y d i f f e r e n t to the r a t i o s encountered i n the months ju s t p r e v i o u s . A sample of 5,054 muskrats trapped during the period 2 March t o 14 May was comprised of 2,732 males and 2,322 females - a r a t i o of 118 males per 100 females. Subsequently, a f t e r open water appeared, examination was made of 752 animals shot by three d i f f e r e n t hunters d u r i n g e a r l y June. This sample c o n s i s t e d of l 8 l males f o r every 100 females, evidence 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 the p e r i o d of water hunting. 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 the increased m o r t a l -i t y t o males j u s t p r i o r t o the breeding season. The f i r s t of these was the f a c t t h a t hunting c o i n c i d e d w i t h the a r r i v a l of open water which flooded most of the a v a i l a b l e h a b i t a t and ousted most a n i -mals from t h e i r w i n t e r dens. The males, already s e x u a l l y mature, at once occupied themselves w i t h mating a c t i v i t i e s . T h e i r s t a t e of unrest r e s u l t e d i n t h e i r becoming not only very 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 found, t h e r e f o r e , i n the more exposed s i t e s around lake edges and along 139 the river 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 . This sound was said to simulate the mating cries of the female. 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 ex-hibited quite the opposite qualities. She became wary and con-fined her acti 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-tive characteristics were found to be gravid. The hunter himself often was selective, in his hunting. Not only was i t easier to shoot males because of their vulnerability, but i f an animal was thought to 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 river channels 8 to 12 June showed seventeen to be males and fif t e e n females. This indicated that both sexes undertook these spring migrations i n quest of suitable breeding and rearing sites. The females showed lacerations from fighting but the males more often were wounded during this period, probably because they had to oompete not only for denning sites but for mates as well. There no doubt is evidence here of selective mortality acting against the male animals. The sex ratios of juvenile animals were in a l l cases found to be unbalanced in 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 DEN COMPLEMENT JUVENILE Male Female Male Female Grassy Lake. Aug. '47 Den. No. 1 1 1 2 0 2 1 1 4 0 4 1 2 4 1 6 1 1 2 0 7 1 1 4 3 8 1 0 3 3 9 1 1 1 4 10 1 0 1 1 Grassy Lake, J u l y '48 Den No. H 1 0 1 3 Grassy Lake. Aug. '49 Den No. G-l 1 0 1 4 G-2 0 1 1 3 G-3 1 1 4 1 G-3 1 1 3 2 G-6 1 1 3 3 G-7 1 1 3 3 Lake 2-W. Aug. '49 Den No. WS 1 1 4 1 1 1 2 3 4 3 1 2 2 4 Reindeer Depot, Jly.-Aug. '49 Den No. 1-1 1 1 0 p 2-1 1 0 4 1 2-2 2-3 1 1 0 1 3 3 0 ? 2-4 1 0 1 0 2-6 1 1 5 T JF - 1 1 2 s 5 X n JF - 2 1 1 1 u 0 3-4 1 1 1  3-7 1 1 6 4 3-8 1 1 1 2 3-10 1 1 3 4 3-11 1 1 1 3 l * f l were tagged f o r every 100 females. This 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 encountered an even greater imbal-ance. Dorney and Rusch (1953) i n Wisconsin noted 125 males t o each 100 female k i t s and E r r i n g t o n (1939a) recorded 120 males t o 100 female n e s t l i n g s . A l l authors agreed that even i n the very young animals more males are encountered than females. This would appear to be a c h a r a c t e r i s t i c of the species which ac t s t o ensure s u f f i c i e n t males i n the breeding pop u l a t i o n . The n e c e s s i t y f o r a roughly even sex r a t i o i n the breeding p o p u l a t i o n was demonstrated i n the course of the summer den t r a p -p i n g s t u d i e s , the r e s u l t s of which are d e t a i l e d i n Table 16. The 47 dens i n which l i v e t r a p s were placed, and i n which t r a p p i n g was continued u n t i l only tagged animals were taken, pre-sented i n f o r m a t i o n concerning den complement d u r i n g summer. I t may be noted t h a t 41 of these dens su p p l i e d more than one c a p t i v e muskrat. Of these, there were t h i r t y i n which there were e i t h e r an a d u l t male and young animals, or both a male and female plus young. There was only one den i n which a female but no male was captured. Moreover, i t i s s i g n i f i c a n t that a l l of the males were taken at only one den. The part the male played i n the r e a r i n g of the young was not demonstrated except that sometimes he gathered food and c a r r i e d i t to the den. About the time the f i r s t young of the year were being encountered den entrances 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 or h o r s e t a i l . Animals which c o l l e c t e d t h i s greenery and swam w i t h i t to 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 plant m a t e r i a l was not e n t i r e l y c l e a r though i t was employed t o stop holes i n 1^2 the runways and to seal off that part of the burrow system not in use. It may also have been eaten by the nursing female and the young, as well as by the male himself. The conclusion reached, by consideration of the available evidence, was that normally a male and a female were in 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 relationship the same male no doubt would sire the second l i t t e r i f one were produced. The result of the differential k i l l i n g of male muskrats may not have been to the advantage of the breeding population. Fuller ( 1 9 5 1 a ) stated the case succinctly 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 is probably necessary for maximum production. A l l schemes and arguments, therefore, aimed only at the protection of females as essential to reproductive success should be re-garded with suspicion". This necessity for an even sex ratio was more important i n the Mackenzie delta where muskrats were thinly 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 in the Mackenzie delta and i n northern Alberta would ensure that subsequently sufficient breeding males would be available for efficient reproduction. It is doubtful i f the number of males conceived is under the influence of any factor in the environment. More likely, itfc i s an attribute with a 1*3 genetic background just as is the larger number of young born at each partus. The demands of the habitat have, however, "selected" the genetic complex which ensures both sufficient young animals and sufficient males among them to provide for r a c i a l continuity. Litters Per Year In the Mackenzie delta a few muskrats produced l i t t e r s by mid-June, but most did not do so until early July. Assuming a post-partum oestrus, as Errington (1937b) demonstrated, it would be possible for second l i t t e r s to 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 in four years these were the latest births actually 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 births were considered quite exceptional. In spite of the short period in which breeding activity was possible there was an opportunity for some females to produce two l i t t e r s in one summer. Evidence that this had occurred was provided by the appearance of uterine scars or implantation sites of two ages in 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 pla-cental scars and six very early implantations as well. A similar instance was recorded on 21st July of the same year. The persist-ence of placental sites varied with the individual 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 five old and in-distinct scars and four which were larger 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 relative-ly low. Nulliparous animals usually matured sexually too late i n the 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 relatively early in the year. They evi-dently had mated before a l l the ice cover was removed from the lakes and produced young during the last half 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 until mid-June could not have a second l i t t e r before mid-August. Because very few late l i t t e r s were encountered, it was evident that most year-old females raised only one l i t t e r . Table 17 is a resume of information from several areas concerning the breeding season for muskrats in each. Therein the reproductive period was taken to be the time from the f i r s t matings to the birth of the last subsequent l i t t e r s . Average conditions are given, not exceptionally late or early occurrences. Information for the race r i v a l i c i u s is 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 history of any particular female, however, is lacking. The table demonstrates rather well the va r i a b i l i t y in reproductive activity which is exhibited within the range of the species. Therein it may be noted that the Mackenzie delta animals 1*5 TABLE 17 THE BREEDING ACTIVITIES OF FIVE RACES OF MUSKRATS AT SEVERAL. LOCATIONS IN NORTH AMERICA. . Race and Local i t y F i r s t M ating Last L i t t e r s Breeding Number Period of (wks.) L i t t e r s Average Young Tot a l Young z i b e t h i c u s Wisconsin (1) e a r l y Apr. mid-J i y . 14 2-3 & 7.4 15 Iowa (2) e a r l y Apr. e a r l y Aug. 15 2-3 6.8 14-20 Maine (3) e a r l y Apr. l a t e J l y . 15 2-3 ± 7.1 14 r i v a l i c i u s Texas (4) year-l o n g 52 4-5 4 .0 16-20 Lou i s i ana (5) year-long 52 4-5 3.8 14-19 macro den Maryland (6) l a t e Mar. mid-Aug. 18 3-4 4.4 13-18 albus Manitoba (7) l a t e Apr. l a t e J l y . 13 2-3 & - 6.8 14 spatulatus Athabaska D e l t a (8) l a t e Apr. l a t e J l y . 12 2 8.7 17 Ma eke nz ie D e l t a e a r l y June e a r l y Aug. 9 1-2 8.3 8-17 Alaska (9) mid-May e a r l y Aug. 11 2 7.7 15 & - Rare occurrence (1) Beer and Truax (1950) (5) S v i h l a & S v i h l a (1931) (2) E r r i n g t o n (194?) (6) Smith (1938) (3 Gashwiler (1950a) (7) McLeod (1950) (9) Buckley (1954 (4) Lav (1945) (8) F u l l e r (1951a) unpubl.) 1*6 have a far shorter breeding season than any other population l i s t e d . As stated previously, the breeding activity 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 is governed by the weather not only during the spring but also throughout the previous winter period. There-by breeding dates may vary from year to year, sometimes by as much as two weeks. In spite of that, the fact remains that ani-mals of some inland areas which experience a continuous ice cover may produce fewer young than they are biologically capable of doing. The decrement from their reproductive potential w i l l be a function of the lag between sexual maturity and spring break-up. In northern United States there does not appear to be any significant delay in breeding activity which may be ascribed to restriction by ice cover. But in central Manitoba McLecd (1950) found animals capable of breeding by late A p r i l , at a time co-incident with the disappearance of winter ice. 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 to July 28th with a time interval between l i t t e r s -of from twenty-five to thirty-four days". He later (1950) reported finding 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, is viewed as being exceptional for that area. His earlier estimate of two l i t t e r s no doubt comes nearer to the normal situation. The latitude of central Manitoba may be the dividing line beyond which ice cover is a significant factor i n r e s t r i c t i n g 1*7 productivity. Fuller (1951a) concluded that muskrats were capable of breeding by the f i r s t to the middle of April, but the ice cover was not'removed u n t i l the end of the month, or later. Therefore, the delay was of the order of about two weeks. He stated, however, that "an unusually early spring break-up may ... induce the onset of breeding at a sufficiently early date to allow some at least of the females to bear a third l i t t e r " . In this statement he re-cognized the fact that ice removal was the factor governing the onset of breeding activity and that the earlier i t was accomplished, the earlier breeding began. In the Mackenzie delta the delay in breeding activity was extended by ice conditions to as long as one month. This delay represented the time necessary for the production of one l i t t e r , which thereby was lost to the population. For northern races of muskrats, therefore, the climate be-comes a limiting factor not only in survival of animals but also in the rate at which they may be replaced in the population. As a result of this climatic pressure muskrats must maintain their numbers even though allowed to raise a lesser number of l i t t e r s . In the following section it w i l l be shown that they have compen-sated for 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 Litter It has been demonstrated that the yearly number of l i t t e r s produced by muskrats decreases from south to north. Therefore, it might be expected that seme compensating factors 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 in utero with extremes of one and six. Arthur (193U placed the average young per partus slightly higher at 3 . 8 , and Lay (1945) studying the same race in Texas found average l i t t e r sizes from 3.8 to 4.6 with four in-dividuals being very close to the average over a period of time. Muskrats in the tidewater marshes of Maryland and Delaware were reported by Smith (1938) to produce 4.4 young at each birth 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 inferior to the muskrats of the Texas coast. The inland races of muskrat appear to be just as p r o l i f i c as their coastal relatives in spite of a shorter breeding season. Average l i t t e r s from Iowa were 6 .8, from Wisconsin 7.4, and from Maine 7.1 young at a birth. These figures were comparable to those for central Manitoba. In the Athabaska-Peace delta of northern Alberta, Fuller (1951a) recorded 8.7 young as an average l i t t e r size, the largest authentic mean figure in the abundant literature concerning musk- . rats. The average of 8.3 for the Mackenzie delta was only slightly lower than the northern Alberta figure. In the Mackenzie delta the number of young per birth 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 total of 571 implantations were recorded for an average of 8.4 young, Table 18. This figure i s quite in agreement with Fuller's data. Uteri from eleven females which had produced only one l i t t e r shewed a total of 82 placental sites, an average of 7.5 per female, range 6 to 13. If 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 IN MUSKRATS OF THE MACKENZIE DELTA FROM PLACENTAL SCARS, EMBRYOS, AND CORPORA LUTEA OF PREGNANCY. Date Implantations P l a c e n t a l Scars Corpora Lutea B L R B L R B L R 13-6-47 10 5 5 10 5 5 16-6-47 8 2 6 8 2 6 6 3 3 8 3 5 f t 7 3 4 7 3 4 6 13- 7 6 7 21- 7 7 6 22- 7 6 0 6 6- 8 8 7- 8 7 14- 8 13 3- 9 13 4 9 7-11 12 7 5 (two ages) 4- 12 9 (two ages) 3-2-48 9 19-2-48 9 6 3 2-6 8 9-6 7 10 10 8 2 10 4 6 8 3 3 10-6 10 5 5 10 4 6 8 4 4 7 2 5 15- 6 9 25-3 to 8-6 9 6 3 10 7 3 9 4 5 10 4 6 7 4 3 10 6 4 8 1 7 11 6 5 21-6-49 12 8 11 8 7 8 11 10 9 (Table continued f o l l o w i n g page) ft - Two embryos being resorbed. TABLE 18 (CONTINUED) Date Implantations P l a c e n t a l Scars Corpora Lute a B L R B L R B L R 21-6-49 8 L O 6 9 8 7 9 I8-5-49 t o 8 L 9-6-49 O 2 8 9 9 4 10 9 8 11 24-8 9 27-3-50 9 8 4-6-50 9 2 7 11 3 9 2 7 11 3 8 7-6 9 3 6 11 3 8 9-6 8 6 2 9 7 2 9 2 7 9 2 7 6 0 6 8 11-6 8 4 4 4 4 8 5 3 8 5 3 6 2 4 9 2 ' 7 9 2 7 3 4 8 4 4 1 7 8 1 7 9 5 4 9 5 4 10 5 5 10 5 5 9 6 3 9 6 3 9 4 5 • ? 4 5 T o t a l implantations - 571, N = 68, Average - 8.4 (Range 2 - 12) T o t a l l e f t u t e r i n e horn - 132, N = 35, Average - 3.8 (Range 0 - 8 ) Total r i g h t u t e r i n e horn - 170, N = 35, Average - 4 . 9 , (Range 2 - 7 ) T o t a l p l a c e n t a l scars - 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 interesting side issue of the study of embryos in utero was the observation recorded in Table 16" that the right uterine horn bore more implantations than the l e f t . In 35 in-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-lar significance is attached to th i s differential placentation though it agrees with data for the house mouse (Mus mus cuius) presented by Laurie (1946). Table 17, which presents the breeding capacity of seyeral races of muskrats, is of interest at this point. It shows that, in spite of the wide disparity i n breeding season throughout the range of the muskrat, for each female the total productivity did not vary greatly. The compensating factor was the increase i n l i t t e r size as the number of l i t t e r s decreased. Excluding the Mackenzie delta the yearly productivity varied from 13 to 20 individuals with an average for a l l areas of about 16 animals. It would appear that the "environmental resistance" in each of the various habitats is roughly comparable because the "breeding potential" dees not vary appreciably from one to the next. In the Aklavik region the force of the environment may even be less than elsewhere because fewer young were produced there. As depicted in Table 17, the Mackenzie delta exhibited the lowest per capita productivity because only one l i t t e r was born to most of the females. 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 ratios of muskrats in the Mackenzie delta and 152 therein i t may be seen that there was about one adult female for every four sub-adult females in the breeding population. Inasmuch as the latter had only one l i t t e r and the former two, the sub-adults were producing two-thirds of the young and the adults one-third. Therefore, the average number of young produced by a breed-ing female in 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 in northern Alberta, then the total productivity would have increased by two-thirds * The late maturing of the f i r s t year females and the retarded break-up in the Mackenzie delta, therefore, reduced the breeding potential by forty per cent. FACTORS IN MORTALITY Attention already has been directed toward those physical factors acting to reduce the number of muskrats in the Mackenzie delta population. The following sections w i l l concern those agents of mortality that are biological in their origin. Intraspecifio Intolerance Beer and Meyer (195D wrote concerning the muskrats of Wisconsin that "the gonadotropic activity of the pituitary i s con-trolled by the season and that this activity controls the repro-ductive cycle and either directly or indireotly through the gona-dotropic hormones the social relationships of the animals". "Thus, when the muskrat is developing re productively, i t is also doing the maximum amount of travelling and f i l l i n g in vacant habitats". This general unrest associated with the breeding season is mani-fested clearly in the fighting which takes place in spring and early 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 flooding of a l l habitat so that few dens were habitable. Animals were forced to build nests of sedges on float-ing debris or i n the willow and alder brush ringing 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, lasting about a week during which time the ice in the main channels broke up and moved out. It was, however, attended by intolerance among muskrats concomitant with the f i r s t mating activity. 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 ice running in the East Channel. Animals were found subse-quently in the warehouses of the Reindeer Depot, in the swollen spring freshets from the Caribou H i l l s , and in 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 by just out of reach kept the sled dogs howling most of the night. The same performance was repeated the following night though there were fewer animals moving. By the next night the exodus of muskrats had ceased, the ice in the channels was gone and the water levels had receded by almost three feet. With this drop in water levels the bank dens once again became habitable. Mating activity had proceeded during the period of high water and even before dens were reclaimed a limited area 15* had been defended. From several instances observed i t was in-ferred that this territory was in the environs of the temporary nest occupied by the male and female. As the water subsided j there was a shrinking of the habi-tat and as a consequence animals were thrown into closer association. At this time the incidence of fighting increased as displaced ani-mals wandered from one burrow system to the next. Limited, though unequivocal observations, indicated that males fought for females not only in the water but also through the underbrush along the shore. But whether both members of a breeding pair shared in the defence of the denning site was not established. The pelted skins of females and males taken during this period showed bites and lacerations, and the view expressed by the local trappers was that both members of the pair cooperated to repulse interlopers. If this was the case, then the. den area was being defended most actively by the male, but also by the female, and any trespassers of either sex were driven away. No doubt the female had been bred by this time so that most contacts with other animals were made by the male. The incidence of mortality in animals attributable to spring fighting was d i f f i c u l t to ascertain. Fighting did not last for as long a period as that recorded by Beer and Meyer (op. cit.) and by Errington (1943) nor, evidently, was i t as violent. The shortness of the breeding season in the latitude of Aklavik, the sudden onset of breeding activity, and the relatively large amount of temporary summer habitat allowed the fighting among adult animals to 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. Field notes and post-mortem records revealed that the f i r s t fighting in spring was noted 25 April but that holes in muskrat pelts were not general u n t i l about a month later. During the last week in May and the f i r s t week i n June, at a time coin-cident with open water and mating activity, many pelts showed bites, some amounting to serious wounds. No dead animals were found though there oan be no doubt that animals could be and were kil l e d by their fighting activity. A barren female taken 22 June, 1947 had 54 bite wounds, one of which opened the abdomen and coelom. A male collected 28 May, 1948 was outwardly in good condition but was found to 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 fighting recorded during A p r i l , and three subsequently in 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 fighting was more l i k e l y for food and dens than for mates. As mentioned previously, there was an automatic adjustment among animals sealed beneath lake ice wherein a discrete group nor-mally used a common feeding ground. Until feeding zones had been established in f a l l , or after they had broken down from food scarcity in late winter, the possibility of s t r i f e remained. Adult -Young Association Compared with the situation mentioned by Errington (1943) 156 wherein "losses of young from attacks by older animals ... were sufficient practically to n u l l i f y continued reproductive effort", the young animals in the Mackenzie delta had a very secure exis-tence. During the whole period of the study there were almost no instances of adult attacks upon young animals. The only excep-tions noted occurred when an adult and a juvenile animal were con-fined over night in the same cage trap. Young animals, tagged i n one burrow system sometimes were caught in a nearby burrow. From this observation it can be assumed that there was some intermingling of the juvenile popu-lation, though probably not an interchange of young between fe-males. The whole picture was one of tolerance by adults of both sexes towards the young animals and of older juveniles for those newly weaned. We may assume, therefore, on the basis of Erring-ton's premises concerning "inter-compensation", that the habitat was adequate for the summer muskrat population and that no over-crowding occurred. .Cannibalism Cannibalism in muskrats i s usually a direct result of stress born of physical factors i n the environment. The term should be differentiated from st r i f e associated with the breeding season because i t refers only to purposeful k i l l i n g of other mem-bers of the same species for food. It operates in environments where animals are imprisoned by winter ice to a particular marsh or lake with no opportunity to escape therefrom. The phenomenon previously has been recorded by travellers in northern regions of North America. Hearne (1795) remarked 157 that " i t sometimes happens i n very cold winters that the holes in their houses freeze over, in spite of a l l their efforts to keep them open. When this i s the case, and they have no provision le f t in the house, the strongest prey upon the weakest, un t i l by degrees only one is l e f t in the whole lodge". Porsild (1945) stated for the Mackenzie delta that "the Eskimo, who are keen observers, deny that rats eat f i s h , but say that rats, when 'frozen-upT in their 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 in Manitoba and they are not unknown elsewhere. Henderson (1923) reported high winter death rates from the Peace River region and Euller (1951a) alludes to similar conditions in the Athabaska-Peace delta area. The muskrats of the Mackenzie delta have not been able to adjust to their environment sufficiently that they w i l l not freeze out under adverse conditions. Indeed, the lower third of the delta suffers yearly from animals freezing in shallow lakes and succumbing to starvation. Actual instances of cannibalism were rarely encountered in this area, either because i t was d i f f i -cult to find the remains of animals eaten by others, or the i n c i -dence of such predation was low. Most instances of cannibalism recorded were during the muskrat harvest when trapped animals were partially devoured by other muskrats. Because the number of animals freezing in push-ups is greater than the number of cases of reported oannibalism., i t is assumed that cannibalism often may be a scavenging of dead animals 158 by those s t i l l alive. One reason i t i s not more wide-spread is that by the time animals have begun to die of starvation or are at extremes of hunger sufficient to impel them to prey on weaker animals, their radius of activity i s circumscribed by the same ice and frozen s o i l that keeps them from their usual plant food. Furthermore, many of the dead animals have become sealed in frozen push-ups and are not available to surviving muskrats. True cannibalism more l i k e l y i s experienced where animals are sealed in bank dens by ice, in which case i t passes unnoticed. Mortal ity from Diseases and Parasites A hemorrhagic disease of unknown etiology has been de-scribed in some detail by Errington (1946b). This disease has since found favour with other authors to account for large-scale "die-of fs" of animals i n various parts of their range. McLeod (1950) asserted that "every major, decline in the past has been accompanied by an epizootic of Errington*s virus enteritis — or by some similar disease" and that "sufficient data are presently at hand to indicate strongly that i t recurs at regular intervals of 4 to 5 years regardless of population density". MacFarlane (1908 ) spoke of an epidemic liver disease that kil l e d thousands of muskrats. He reported that in one year the returns at Cumberland House, Saskatchewan, declined two-thirds as a result. No such disease was encountered in the Mackenzie muskrats though there were persistent opinions expressed by the trappers of the area that disease was a factor controlling muskrat numbers. The year 1933-34 often was quoted as one with a high rate of mortality among muskrats. Seme trappers who remembered 159 the situation at that time mentioned finding "white spots" in the livers of many animals. Therefore, during the whole course of the investigation a l l available animals were examined for this condition. It was found that a coenurus type cyst of an unidenti-fied cestode was not uncommon in the muskrats examined. These cysts appeared as blisters, usually at least one cm. in diameter, and though most often concentrated in the liver also were found in the spleen and intestinal mesentery. The number of cysts usually was less than seven though in a few cases they practi-cally 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 infestation was fifteen per cent for 97 males and seven per cent for 356 fe-males. In nearly a l l animals in this sample, and in those exam-ined subsequently, the parasite appeared to be doing no harm to it s host; the animals were in satisfactory condition and had pro-gressed as rapidly 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 tentatively placed i t in the genus Cladotaenla. He was of the opinion that the definitive host was one of the larger raptorial birds. The writer suggests that the horned owl (Bubo virginianus) was the most likely candidate among birds resident in the delta and able to prey on muskrats. It seems probable, therefore, that the "liver disease" of MacFarlane and the "white spots" seen by the local trappers 160 were the Cladotaenia larvae mentioned by Cowan. It is entirely possible that during years of high incidence of these tapeworm cysts the a f f l i c t e d animals may be especially vulnerable to some less evident condition which actually is the lethal factor. That disease and parasites were not important limiting factors in the delta muskrats may be asserted from the findings of this investigation. However, rumours of "rat sickness" persisted among the native trappers and there i s a possibility that it has existed and may appear again despite its absence during the per-iod covered by this study. Mortality from Predatory Animals Errington (1946a) has said that "regardless of the count-less individuals or the large percentages of populations that may annually be k i l l e d by predators, predation looks ineffectual as a limiting factor to the extent that intraspecific self-limiting mechanisms basically determine the population levels maintained by the prey". He stated further that most agencies of mortality may be looked upon as merely harvesting surplus animals which are insecurely established in a given habitat. We may infer, there-fore, that the self-limiting factors which determine population levels are density dependent and only at the upper levels of popu-lation numbers do they begin to exert their effect. If animals are amply provided for by the habitat which produced them, then un t i l the upper asymptote is reached wherein density, dependent factors begin to exert themselves, predation is not important and probably takes off a relatively constant proportion of animals, dependent upon the predator-prey ratio and the number of contacts possible between each. 161 I n the Mackenzie d e l t a the p o t e n t i a l f o r producing muskrats was r e s t r i c t e d by the short breeding season but not t o any extent by the h a b i t a t . There was a s u r f e i t o f .summer food and adequate denning s i t e s f o r the young animals born. Animals were spread throughout a l l h a b i t a t types that would sup-port them at t h a t period of the year. I f a l l of these muskrats had the s e c u r i t y of a good burrow system then the r e l a t i v e l y low d e n s i t y of predators i n t h e d e l t a to the end of summer had only minimal 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 marked-l y changed the general aspect of t h e d e l t a muskrat h a b i t a t . A n i -mals i n shallow sloughs suddenly found the Equisetum and Carex which once had been t h e i r c h i e f source of sustenance b l i g h t e d by f r o s t . 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 the l a k e s and sloughs, induced 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 pla n t s were a v a i l a b l e as food. A part of t h e p o p u l a t i o n thereby was made l e s s secure as i t moved i n t o areas occupied by w e l l e s t a b -l i s h e d animals. These disp l a c e d animals, t o quote F u l l e r (1951a) "may have t o f i g h t t h e i r way i n t o more favourable h a b i t a t s already f u l l y occupied". At t h i s time there was a " b i o l o g i c a l s u r p l u s " as animals searched f o r s a t i s f a c t o r y burrows and were harassed and wounded i n t h e process. U n t i l they found adequate dens they remained vulnerable and predators had increased o p p o r t u n i t y t o f i n d and overcome them. Furthermore, the population of predators by that time had been augmented by n a t u r a l increase and was i n a most favourable p o s i t i o n t o take advantage of the food source pre-sented by these excess prey animals. Their opportunity t o do so, 162 however, was sho r t - l i v e d . The movement of animals began i n late August and by early Oetober the lakes were frozen and r e -mained so u n t i l the following 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 predatory animals. That being the case, unless predation was swift and sure during the short period when i t was able to act, 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 reducing the number of muskrats. Some insight into the extent of mo r t a l i t y caused by the v i c i s s i t u d e s of autumn displacement accrued from the tagging of l i v e muskrats. As described e a r l i e r , juvenile muskrats were cap-tured f o r study i n two types of habitat; temporary or summer s i t e s and permanent or yearlong s i t e s . These marked animals were d i s -tributed therein as follows: 110 from the former and J l 6 from the l a t t e r . There is j u s t i f i c a t i o n for assuming an equal r e c o v e r a b i l -i t y of animals from each type of habitat. Thus the number of ani-mals recovered i n each case would supply an index of s u r v i v a l i n each class of s i t e . It was found that oraly nineteen (17 per cent) of the 110 . animals from temporary s i t e s were recovered by the writer or any of the trappers. On the other hand 107 ( 3 4 per cent) of the 316 animals from reasonably permanent s i t e s were again encountered. I t would appear, therefore, that those animals established i n permanent lakes had about twice the chance of su r v i v a l to y e a r l i n g status. The i n f e r i o r success of temporary habitats as rearing s i t e s may be ascribed not only to t h e i r progressive deterioration but also to the threat of ice and the dangers attending movement to more permanent quarters. 163 Hunting and Trapping The trapping effort of the human residents of the Mac-kenzie delta was the most evident, though not necessarily the most important biological factor limiting 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, and then jumped again to 333 thousand. The t o t a l for 1949-50 again was low (167 thousand), and since that date the harvests have increased (see Appendix D). Numerous authors have investigated the periodic abun-dance and scarcity of mammal populations in northern regions. The study of Elton and Nicholson (1942) dealt specifically with muskrats in 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. It is not the writer's intention to disagree with the 10-year cycle of Elton and Nicholson. Their findings must be accepted as factual u n t i l more information concerning animal cycles is made available. But something of the history of muskrat trapping in the Mackenzie delta might assist in illuminating the seemingly irregular fluc-tuations in the number of animals harvested there over a period of time. Toward the end of World War II the price paid for muskrat pelts was exceptionally high. Naturally, the large population of muskrats at the mouth of the Mackenzie River attracted trappers from a wide area. In the spring of 1946 they arrived by a i r , by boat, and by canoe from various areas of the Mackenzie District 164 to share in the lucrative spring hunt. As a result, the largest number of animals ever taken from the delta was shared among the greatest number of hunters ever assembled there. The following year the harvest was appreciably smaller. This was the result, 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 this, a satisfactory harvest was re-ported, especially during the period of open water when the ani-mals legally 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 for that year was only about half what i t had been two years previously. There were two local causes contributing to this decline. The f i r s t was a severe epidemic of influenza among the native trappers which so disrupted their early spring trapping activity that very few animals were taken prior to open water. And after the spring floods did arrive the temperature remained unusually low so that ice re-formed nightly on the lakes and travel by canoe was almost impossible. As a consequence, the hunting pressure was much lower than i t previously had been. After this respite the muskrats en-joyed a particularly favourable summer during 1948 and the follow-ing spring 333 thousand animals were harvested. It may be noted that in 1949-50 the harvest again was low. This was for almost the same reasons as those outlined earlier. 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 ice depths were unusually great. Coupled with this was a widespread epidemic of measles 165 which amounted t o a v i r t u a l catastrophe for many f a m i l i e s . The native t r a p p e r s were discouraged by t h i s bout w i t h disease and t r a p p i n g a g a i n was delayed. And as i n 1948, the i c e c o n d i t i o n s were severe up u n t i l the end of the l e g a l muskrat season i n mid-June . Since that time the number of animals harvested has not f l u c t u a t e d so g r e a t l y nor has there been any emergency to a f f e c t the extent of t r a p p i n g e f f o r t . Furthermore, 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 the i n s t i t u t i o n of a system of r e -g i s t e r e d t r a p p i n g areas, each reserved f o r one i n d i v i d u a l or a small group of i n d i v i d u a l s . This 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 pressure so that 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 the s i z e of the harvest d i d not always r e -f l e c t the numbers of muskrats i n the Mackenzie d e l t a , yet there was no doubt a general c o r r e l a t i o n between the f u r harvested and the animals a v a i l a b l e . Inasmuch as v a r i a t i o n s i n the harvest appear t o be i r r e g u l a r , r a t h e r than p e r i o d i c , i t i s reasonable to suppose t h a t i n t e n s i v e t r a p p i n g , as 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 important i n f l u e n c e governing population l e v e l s . P r i o r to 1941 the muskrat was not as important i n the f u r trade as i t now i s , and t r a p p i n g e f f o r t was d i r e c t e d toward more valuable " f i n e " f u r s p e c i e s . The d e l t a muskrat population as a consequence was l e f t much more to the i n f l u e n c e of n a t u r a l con-d i t i o n s . With the decrease i n the market value of other f u r s , the t r a p p i n g pressure d i r e c t e d toward muskrats i n c r e a s e d . 1&3S Thereby the biological system became increasingly a r t i f i c i a l . The muskrat population never was allowed to build up to a point where disease or other intrinsic factors were exerted. That situation has persisted to the -present time. If the Mackenzie delta muskrats ever did 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 yield of animals from their registered trapping areas. 167 DISCUSSION The ecological factors facing muskrats in the Mackenzie delta have been recounted in some detail. From that discussion i t i s apparent that no one factor of the environment i s particu-l a r l y unique but that a l l factors taken together comprise a habitat nearly marginal for the resident animals. It is evident that no organism can survive where any single factor necessary for i t s existence is lacking. But organisms usually can and do survive with only partial fitness to each ecological component in a complex environment. Such is 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 of habitat. Therein i t was pointed out that abandoning lodge building and resorting to burrows as a permanent abode is for 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 in the Slave River delta have abandoned lodges entirely even though it appears that the animals could construct and inhabit them successfully. It is li k e l y that the Mackenzie muskrats through long disuse have lost entirely the a b i l i t y to erect and occupy these structures. On the other hand, it is evident that they could not do so i f they tried because the vegetation is too sparse and the ice depths too great. Choice of habitat in summer may be a reflection of the 168 inherent tendency, exhibted by muskrats in more moderate climates, to choose to live in shallow marsh areas r i c h i n food and cover. The evidence of a depressed rate of survival i n such surroundings in the far north indicates that the animals have had to make the necessary shift towards areas of more permanent water or perish. This adjustment then, has been of value in the continuation of the population. The dispersed nature of the lake environment no doubt has formed certain ac t i v i t y patterns in the muskrats in question. For instance, there i s no assurance that food of -sufficient volume w i l l be contiguous to adequate denning sites. Therefore, it i s necessary for some animals normally to embark upon extensive forays in search of food, not only during summer, but also i n winter be-neath a heavy covering of ice. As a consequence the radius of activity of most animals is greater than that recorded for other areas of North America. During winter this movement is abetted by an extensive system of push-ups which replace the feeding lodges characteristic of house-dwelling animals. Yet there is a limit to the amount of area which can be u t i l i z e d by one animal during win-ter. The majority evidently use no more than two or three such push-ups in the v i c i n i t y of the heme den. It appears therefore that the muskrat is by nature a sedentary animal confined to a familiar heme range. The size of this range in 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 is divided rather sharply 169 between emergent species in summer and submerged species in winter. That they abandon one for 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 aerial portions of the food plants have been frozen they are no longer attractive as food and the muskrats then turn to other sources. This progressive substitution as food sources dwindle in winter or burgeon in summer is a reflec-tion of the adaptability of the species with respect to the sus-tenance it w i l l accept. Food per se is a limiting factor, especially when its availability is restricted. Submerged plants in a lake could be almost eliminated by muskrats grubbing out the roots and r h i -zomes because the pondweeds and related submerged plants repro-duce only by vegetative means in the latitude of Aklavik. Such an instance was not observed during t h i s study but information gleaned from the trappers was to the effect that a lake which had "frozen cut" took several years to regain i t s productivity. It i s probable that this delay is the result of a significant de-struction of the food plants. In several places in the previous discussion the question of a defended home range or territory was mentioned. Early in the breeding season, at a time coincident with flooding of lake-side habitat, there i s a zonation of lake perimeters into defended territory. But once the population has become sorted out into pairs and more dens become habitable as the water recedes, the incidence of fighting drops sharply. Thereafter the size of the territory shrinks to include only the home den as animals extend their radius of activity in search of food. 170 Food, however, i s not a b a s i s f o r defence of any area, during summer. Animals appear t o share a v a i l a b l e food resources w i t h no evidence of s t r i f e . In w i n t e r , on the other hand, there i s some s o r t of segregation of animals i n t o f a i r l y d i s c r e t e groups. D i r e c t observation of animals beneath the i c e cover i s almost impossible 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 zonation of l a k e areas f o r want of another e x p l a n a t i o n . Perhaps the most important adjustments t o the northern environment are the i n t r i n s i c or b i o l o g i c a l adjustments. There i s l i t t l e doubt t h a t some of these adjustments are of genetic o r i g i n though others may be merely a f u n c t i o n of the environment. One such phe nomenon - i n the l a t t e r category i s the s i z e of the i n -d i v i d u a l animals of the Mackenzie po p u l a t i o n . When compared wi t h muskrats elsewhere, even members of the same sub-species, they are smaller i n s i z e and weight. This reduced s i z e most l i k e l y i s the r e s u l t of t h e marginal h a b i t a t c o n d i t i o n s . But the p o s s i b i l i t y e x i s t s that 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 favoured small animals demanding l e s s sustenance from each u n i t area 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 genetic o r i g i n are 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 c o n d i t i o n s t e n d to delay reproductive a c t i v i t y i n s p r i n g and as a r e s u l t only one l i t t e r was the norm f o r a large percentage of the females. This s i t u a t i o n i s of environmental o r i g i n but the l a r g e r number of young produced at 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 the sex r a t i o which i s unbalanced i n favour of 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 the d e t r i -ment of males i t i s necessary t h a t t h e y be i n the ascendency so 171 that there is a roughly even sex ratio by the time breeding activity commerces. The observation was made that males occupy nest burrows •with females and young, that i s , that there is a f a i r l y s t r i c t monogamy. Inasmuch as monogamy is a method of ensuring repro-ductive efficiency, especially necessary where populations are sparse or discontinuous, it may have developed in the Mackenzie delta as a result of the dispersed nature of the habitat. The extent of a monogamous relationship 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, this i s an adaptation that w i l l ensure a male in attendance when a female is receptive for mating. The social relationship between a l l animals, both adult and young, was observed to be amicable during most of the year. The general fighting, characteristic of the breeding season, was of short duration and thereafter was of local occurrence only. It i s suggested that lack of intolerance is largely the result of space available to animals rather than any reticence on the part of individuals to engage in offensive activity. The writer does not dismiss the possibility that populations of muskrats may show inherent differences i n the i r tolerance to their own kind. Cer-tainly where the reproductive rate is high and the environment re-stricted, an increased i r r i t a b i l i t y on the part of individual ani-mals 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 to the detriment of r a c i a l survival. We have recounted some of the adjustments which the Mackenzie delta muskrats have made to their environment. In conclusion it i s proposed to mention some of the factors to which the animals have not been able to adjust, factors which may limit population growth. There i s no doubt that the long winter is the c r i t i c a l period for the muskrats in question though climate helps to 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. It is suggested that i t also tends to restrict the transmission of radiation which activates the endocrine system to promote sexual development. After belated sexual maturity is achieved, ice restricts breeding until the time i t i s l i f t e d by spring floods and the animals are able to escape from their depleted winter habitat. If those animals that do survive to breed are small in size and often in poor condition, then the severity of the climate is the chief con-tributing cause. On the other side of the ledger may be cited some com-ponents of the environment that are favourable to the Mackenzie delta muskrats. The level of predation from species other than human is only an incidental factor in summer and is almost absent in winter. The amount of disease and parasitism i s low, no doubt partially as a result of the cleansing action of spring floods and the low winter temperatures. Low population densities also act to prevent the spread of diseases and parasites. 173 I n other areas under review the f a c t o r s l i m i t i n g growth of muskrat populations u s u a l l y were other than c l i m a t i c . However, as E r r i n g t o n (1946a) explained, " l o s s types may s u b s t i t u t e natu-r a l l y f o r each other" and " v i c t i m s of one agency simply miss be-coming v i c t i m s of another". W i t h i n any p a r t i c u l a r r e g i o n the agencies of l o s s are apt t o be predominantly of one k i n d . But from one region to another we can expect them to change as the environmental complex changes. Thus i n Louisiana and Texas , favour able c o n d i t i o n s through-out the year allow muskrats t o b u i l d up such d e n s i t i e s of popu-l a t i o n t h a t they l i t e r a l l y consume t h e i r h a b i t a t . This one l o s s type i s so important t h a t O ' N e i l l (1949) was prompted to s t a t e "that a l l work thus f a r tends to i n d i c a t e t h a t food c o n d i t i o n s form the primary b a s i s for c o n t r o l of muskrat po p u l a t i o n s " . The climate e x e r t s a negative e f f e c t only i n times of drought or storms w i t h severe t i d a l a c t i o n . E s s e n t i a l l y the same f a c t o r s govern the muskrat numbers i n the c o a s t a l marshes of Maryland and Delaware, though these areas 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 f r o s t s . The i n l a n d w at erways and marshes of the northern United States and southern Canada support l e s s e r d e n s i t i e s of muskrats than do the c o a s t a l marshes. These animals, however, do not seem t o 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 the c o a s t a l races. E r r i n g t o n (op. c i t . ) has documented the o f t e n dramatic nature of t h i s l e t h a l f r i c t i o n wherein both adult and j u v e n i l e animals were el 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 increased. He found " i n t r a s p e c i f i c s t r i f e becoming dominantly o p e r a t i v e when preda-t i o n and the usual run of miscellaneous losses a l l happened t o 174 be low". If comparisons are made between the coastal races, in-cluding the Mackenzie delta population, on the other hand, and the animals of the prairie marshes and inland waterways on the other, one probable-reason for this r a c i a l s t r i f e can be pro-pounded. The former populations a l l occupy very extensive areas of continuous habitat while the latter are crowded into relatively small though often very favourable water areas. This discontinuous nature of the inland habitat with i t s sharp distinction between dry and wet areas would discourage emigration overland to new areas and thereby would foster population densities wherein lethal intolerance among resident animals would be achieved. Drought in the home range of the inland muskrats has an occasional influence but of late disease has been given more notoriety as a population depressant. Errington and his associates at Iowa State College have documented a hemorrhagic enteritis 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 that, "during lethal epi-zootics those places characteristically accommodating low den-si t i e s may indeed afford the safest refuges". As mentioned ea r l i e r , McLecd (1950) in Manitoba has accorded a prominent place to epizootic disease as a control of muskrat numbers. He, however, discounts the role of "intra-specific predation" and wrote that "no pronounced mortality from this cause has been noted locally". He considers winter food to be a limiting factor and holds the view (1948) that "in 175 the. presence of insufficient water and the lack of emergent vegetation to hold the snow, the frost may penetrate into the bottom and cut off the food supply". It may be said in summary that beginning with the Man! toba marshes and proceeding northward, climate exerts an in -creasing influence upon the various populations of muskrats as such other factors as disease, intolerance, and predation de-crease i n importance. 176 S U M M A R Y 1. Field studies f o r this investigation were conducted during the period June 194-7 to September 1950 in the delta of the Mac-Kenzie River in northwestern Canada. 2. The northwestern muskrat (Ondatra zibethicus spatulatus) was the object of this investigation and special attention was paid to the adjustments in i t s activities and l i f e processes occasion-ed by i t s location within the Arctic Circle. 3. Attention was accorded five other races of muskrats in North America, two of which are coastal and three of which occupy in-land regions. An attempt was made to outline how these races re-acted to factors in their particular habitats and to 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 into those which were extrinsic or physical and those which were i n t r i n s i c or biological. Generally these adjustments were in degree only. 5. The muskrats of the Mackenzie delta occupied bank dens ex-clusively. They could not build lodges because there was not sufficient vegetation and because ice depths in winter were too great. 6. Summer habitat was more abundant than winter habitat. The latter oomprised that portion of lakes between three and twelve feet deep wherein there was an abundant growth of submerged plant material. 7. Survival of young raised 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 their radius of a c t i v i t y to adequate supplies of food. Spatulatus was more dependent upon these push-ups for its winter survival than was any other race of muskrats under re-view. 9. The number of animals using each push-up varied from three to thirteen, with an average of six. Individual animals frequented only one to three push-ups. but in doing so they were forced to travel from 32 to 132 yards beneath the ice. 10. The distances travelled by muskrats both summer and winter were greater in the Mackenzie delta than i n other areas under consideration. This was because the bank dens were not always near to food plants and because there was a shifting of animals from summer to winter habitat. 11. In winter there appeared to be a definite zonation of lakes with each zone being used by a' discrete group of animals. This denoted either a limited t e r r i t o r i a l i t y or was the result of ani-mals from adjacent dens using a common food supply. 12. Food utilized by muskrats was divided sharply between emer-gent shore-line species in summer and submerged species in winter. Equisetum and Carex constituted the summer foods with Potamogeton and Myriophyllum replacing them in winter. 13.. The muskrats of the Mackenzie delta assumed only fractional densities of population when compared with their 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 of the unfavourable nature of the h a b i t a t t h e muskrats of the Mackenzie d e l t a were of smaller s i z e and took longer to reach t h e i r t e r m i n a l growth than d i d muskrats i n more optimum environments. 15. Tagging s t u d i e s l e d t o the conc l u s i o n t h a t the m a j o r i t y of muskrats i n the Mackenzie d e l t a d i d not s u r v i v e long beyond t h e i r second year of l i f e . 1 6 . Muskrats i n the Mackenzie p o p u l a t i o n had a very s h o r t breeding season. Females produced only one l i t t e r t h e i r f i r s t year but might have two the second year when they were s e x u a l l y mature at an e a r l i e r date. 1 7 . Breeding a c t i v i t y i n both sexes was delayed 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 considered t h a t t h i s delay might be as long as one month. Delayed breeding appears t o be a phenomenon p e c u l i a r t o northern r e g i o n s . 1 8 ; A review 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 of l a t i t u d e i n d i c a t e d that from south to north p r o g r e s s i v e l y more daylight was r e q u i r e d before animals became mature sex-u a l l y . I t was assumed t h a t p h o t i c c o n t r o l of sexual c y c l e s was i n h i b i t e d by greater 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 . 1 9 . For most places i n North America the number of young musk-r a t s produced per adult female v a r i e d from t h i r t e e n t o twenty w i t h an average of s i x t e e n . Where the number of l i t t e r s was r e -duced the compensating f a c t o r was an increase i n the number of young animals per l i t t e r . I n the Mackenzie d e l t a , however, there was an average of only t e n young per female. 179 20. The ratio of young to adult animals in winter and spring was as high in the Mackenzie delta as in most other areas notwithstand-ing the lower number of young produced per breeding female. This was ascribed either to superior survival of young animals or to a reduced survival of adults. 21. Adult sex ratios in delta muskrats did not differ 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 in spring for every 100 females. In spite of the fact that the animals exhibit-ed a f a i r l y s t r i c t monogamy, most females were bred and raised young during the summer months. 22. There was l i t t l e evidence of lethal f r i c t i o n between muskrats in the Mackenzie population. Fighting flared briefly at the time of spring break-up but was not common thereafter. This situation 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 sufficient to be a major check on population growth. 23. The incidence of disease and parasitism was low in the Ak-lavik region and these factors were considered to be minor ones in the control of population growth. 24. Predatory animals exerted a limited effect upon the delta muskrats which for eight months of the year were shielded from their enemies by the ice cover on the lakes and channels. The number of predatory animals was small because of intensive trap-ping within the area. 25. The fur industry yearly removed a significant portion of the muskrat population in the Mackenzie delta. It was surmised that this had the effect of suppressing those factors contributing to cyclic fluctuations in numbers reported from other northern regions. 180 26. Factors limiting population growth vary from one region to •the next. In northwestern Canada these factors were of a physical sect induced by the severity of the climate. Therefore, most of the adjustments made by muskrats were in response to this severe climatic regime. 181 LITERATURE CITED Aldous, Shaler E. 1947 Muskrat trapping on Sand Lake National Wildlife Refuge, South Dakota. Jour. Wildl. Mgt. 11 (1) :77-90. Alexander, M. M. 1951 The aging of muskrats on the Montezuma National Wildlife Refuge. Jour. Wildl. Mgt. 1 5 ( 2 ) : 1 7 5 - l 8 6 . Alexander, M. M., and M. Radway 1951 The distribution and production of muskrats on the Montezuma National Wildlife Refuge. Jour. Wildl. Mgt. 15(4) :360-367. Allee, W. C., et..al. 1949 Principles of animal ecology. Phila. and London, W. B. Saunders Co., 837 pp. Anderson, J. M. 1947 Sex ratio and weights of southwestern Lake Erie muskrats. Jour. Mamm. 28(4) :391-395. Anderson, R. M. 1946 Catalogue of Canadian recent mammals. Nat.-Mus. Canada Bui. 102, B i o l . Ser. 31 , 238 pp., maps. Anonymous 1951 A survey of West Virginia mammals. Cons. Comm. W. Virginia, P-R Project 22-R, 126 pp. mimeo. Arthur, S. C. 1931 The fur mammals of Louisiana. La. Dept. of Cons. Bui. 18 , pp. 9-439. Bailey, Vernon 1937 The Maryland muskrat marshes. Jour. Mamm. 18:350-354. Banfield, A. W. i ' . 1946 A preliminary investigation of the muskrat resources of the Mackenzie delta. Manuscript report, Canadian Wildlife Service, Ottawa. 22 pp., map. Barnes, H. T. 1906 Ice formation with special 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. Montreal, Renour Publ. Co. 364 pp., 5 p l . 182 Baumgartner, L. B., and F. C. B e l l r o s e , J r . 194-3 Determination of sex and age i n muskrats. Jour. W i l d l . Mgt. 7 U ) : 7 7 - 7 9 . Beer, J * R. 1950 The reproductive c y c l e of muskrats i n Wisconsin. Jour. W i l d l . Mgt. 14(2):151-156. Beer, J . R., and R. K. Meyer 1951 Seasonal changes i n the endocrine organs and behavior patterns of muskrats. Jour. Mamm. 32(2):173-191. Beer, J . R., and W. Truax 1930 Sex and age r a t i o s of Wisconsin muskrats. Jour. W i l d l . Mgt. 14(3):323-331. 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 populations t o v a r i o u s marsh and aquatic p l a n t s . Jour. W i l d l . Mgt. 14(3): 299-315. B e l l r o s e , F. C , and J . B. Low 1943 The in 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 muskrats. Jour. Mamm. 24(2):173-191. B l s s o n n e t t e , T. H. 1938a Experimental c o n t r o l 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. Jour. W i l d l . Mgt. 2 ( 3 ):104-ll8. 193ob Influence of l i g h t on the hypophysis. E f f e c t s of long-continued "night l i g h t i n g " on hypophysecto-mized female f e r r e t s and t h o s e - w i t h o p t i c nerves cut. Endocrinology 22(1):92-103. Buckley, J . L. 1954 P r e l i m i n a r y r e p o r t of muskrat i n v e s t i g a t i o n s i n Ala s k a . Alaska Coop. W i l d l . Res. Unit Report 18 pp. unpubl. Buss, I . 0. 1941 Sex r a t i o s and weights of muskrats (Ondatra z. z i -bethica) from Wisconsin. Jour. 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 study of muskrat food. Canad. F i e l d Nat. -54(3) :37-40. Camsell, Charles 1921 The Mackenzie R i v e r b a s i n . Geol. Surv. Canada, Memoir 108, 151 pp. 183 Cook, A. H. 1943 A technique f o r marking mammals. Jour. Mamm. 24(l ) :45-47. Cowan, I . McT. 1948 P r e l i m i n a r y survey of the Mackenzie D e l t a w i t h s p e c i a l reference t o the muskrat. Manuscript report t o Dept. Mines and Res., Ottawa - 79 pp. i l l u s t r . Dalquest, W. W. 1948 Mammals of Washington. Univ. Kans. P u b l . , Mus. Nat. H i s t . v o l . 2, 444 pp., 140 f i g s . D avis, W. M., and G. H. Lowery, J r . 1940 The systematic s t a t u s of the L o u i s i a n a muskrat. Jour. Mamm. 21(2):212. Dempsey, E. W., et. a l . Absence of l i g h t and the re p r o d u c t i v e cycle i n the guinea p i g . Amer. Jo u r . P h y s i o l . 109:307-311. Dorney, R. S., and A. J . Rusch 1953 Muskrat growth and l i t t e r p r o d uction. Wise. Cons. Dept. Tech. W i l d l . B u i . 8, 32 pp. Do 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 production. Nor. Amer. W i l d l . Conf. Trans. 12:398-420. 1948 E s t i m a t i o n of muskrat populations by house counts. Nor. Amer. W i l d l . Conf. Trans. 13:372-392. 1950 Muskrat trapping on the Montezuma N a t i o n a l W i l d l i f e Refuge, New York 1943-48. Jo u r . W i l d l . Mgt. 14(4): 403-412. 1953 Muskrat production and management. U. S. Dept. I n t . E i s h and W i l d l . Serv. C i r c u l a r 18, 42 pp., p i s . Dozi e r . H. L., M. H. Markley, 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 the Blackwater N a t i o n a l W i l d l i f e Refuge 1941-43, Jour. W i l d l . Mgt. 12(2):177-190. Eardley, A. J . 1938 Yukon channel s h i f t i n g . Geol. Soc. Amer. B u i . 49: 343-358. E l t o n , C , and M. Nicholson 1942 F l u c t u a t i o n s i n numbers of muskrats (Ondatra z i b e t h i c a ) i n Canada. Jour. An. Ecology 11:96-126. 184 E r r i n g t o n , P. L. 1 9 3 7 a H a b i t a t requirements of stream d w e l l i n g muskrats. Nor.. Amer. W i l d l . Conf. Trans. 2:411-416. 1 9 3 7 b The ^breeding season of the muskrat i n northwest Iowa. Jour. Mamm. 1 8 ( 3 ) : 3 3 3 - 3 3 7 . 1939 a Observations on young muskrats i n Iowa. Jour. Mamm. 20(4): 465-478. 1 9 3 9 b 1941 Reactions of muskrat populations to drought. Ecology 2012) : l 6 8 - l 8 6 . V e r s a t i l i t y i n feeding and p o p u l a t i o n maintenance of the muskrat. Jour. 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 muskrats i n north c e n t r a l United S t a t e s . Iowa Agr. Exp. Stn. 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 of tagged young muskrats. Jour. W i l d l . Mgt. 8(4) :300-306. 1946a P r e d a t i o n and ve r t e b r a t e p o p u l a t i o n s . Quart. Rev. 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 report on muskrat disease. Iowa Coop. W i l d l . Res. U n i t Quart. Rept. J u l y - S e p t . 1946:34-51. 1 9 4 7 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 muskrats. Iowa Coop. W i l d l . Res. U n i t . Quart. Rept. Oct.-Dec. 1 9 4 7 . I 9 4 8 Environmental c o n t r o l f o r i n c r e a s i n g muskrat pro-du c t i o n . Nor. Amer. W i l d l . Conf. Trans. 1 3 : 5 9 6 - 6 0 9 . Eorbes, T. R. 1942 The p e r i o d o f gonadal a c t i v i t y i n the Maryland 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 the muskrat i n the Athabaska-Peace d e l t a , Wood B u f f a l o Park. Canad. W i l d l . Serv. W i l d l i f e Mgt. B u i . Ser. 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 northern muskrats. Jour, Mamm. 32(3):360-362. Gashwiler, J . S. 1948 Maine muskrat i n v e s t i g a t i o n s . B u i . Maine Dept. I n -land F i s h and Game Augusta. 38 pp. 1950a A study of the r e p r o d u c t i v e c a p a c i t y of Maine musk-r a t s . Jour. Mamm. 31( 2) :l8o-185 . 1950b Sex r a t i o s and age c l a s s e s of Maine muskrats. Jour. W i l d l . Mgt. 14(4):384-389. Gould, H. N., and F. B. Kreeger 1948 The s k u l l of the 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. Roberts 195 0 Mammal survey of southwestern Pennsylvania. Penna. Game Comm. Ha r r i s b u r g , 99 pp., i l l u s t r . H a l l , E. R., and E. L. Cockrum 1953 A synopsis of the North American microt ine rodents. Univ. of Kans. P u b l . , Mus. Nat. H i s t . 5(27):373-498. Hearne, S. 1795 A journey from P r i n c e of Wales F o r t on Hudson Ts Bay t o the northern ocean i n the years 17&9, 1770, 1771, and 1772. London. Henderson, Ax. D. 1923 Cycies 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 . Jour. Mamm. 4:264-265. H o l l i s t e r , N. 1911 A systematic synopsis 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 Mackenzie, North-west T e r r i t o r i e s . Summary Rept. 1923 P t . B, Canada Geol. Surv., 15 pp. Jenness, J . L. 1949 Permafrost i n Canada. A r c t i c 2 (1)-.13-28. Johnson, C. E. 1925 The muskrat i n New York. Roosevelt W i l d l . B u i . 3:199-320. 186 Laur ie , E. M. 0. 1946 The reproduction of the house mouse (Mus mus cuius) l i v i in different environments. Proc. Roy. Soc. B, 133:248-281. Law, C. E. 1950 Ecology of the muskrat of the Slave River delta, N.W.T. Univ. of B. C., unpubl. B.A. thesis, 120 pp. Lay, D. W. 1945 Muskrat investigations in Texas, Jour. Wildl. Mgt. 9(l ) : 5 6 - 7 6 . Lay, D. W., and T. O'Neill 1942 Muskrats on the Texas coast. Jour. Wildl. Mgt. 6(4):301-311. Lynch, J. 1940 Origin and natural maintenance of some arctic water-fowl habitats. U.S. Fish and Wildl. Serv. unpubl. manuscript report, 34 pp. Lynch, J., T. O'Neill, and D. W. Lay 1947 Management significance of damage by geese and muskrats to gulf coast marshes. Jour. Wildl. Mgt. 11(1):50-76. McCann, L. 1944 Notes on growth , sex and age ratios, and suggested management of Minnesota muskrats. Jour. Mamm. 25(1):59-63. MacFarlane, R. 1908 Notes on mammals collected and observed in the northern Mackenzie River D i s t r i c t , Northwest Terri-tories of Canada. In Mair 1908, Through the Mac-kenzie 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., in Manitoba. Conv. Int. Assoc. Game, Fish and Cons. Comm. 39:64-71. 1950 A consideration of muskrat populations and population trends in Manitoba. Trans. Roy. Soc. Canada 44, Ser. Sect. 5:69-79. Marsh, D. B. 1948 Ratting i n the delta. Beaver 278:32-35. 187 M a r s h a l l , F. H. A. 1936 ' Sexual p e r i o d i c i t y and the causes which determine i t . Roy. Soc. London P h i l o s . Trans. Ser. B 226(339):423-456. 1940 The experimental m o d i f i c a t i o n of the oestrus c y c l e i n the f e r r e t by d i f f e r e n t i n t e n s i t i e s of l i g h t r a d i a t i o n and other methods. Jour. Exp. B i o l . 17 ( 2): 139-146. M a r s h a l l , F. H. A., and F. P. Bowden 1934 The e f f e c t of i r r a d i a t i o n w i t h d i f f e r e n t wave-lengths on the oestrus c y c l e of the f e r r e t , w i t h remarks an the f a c t o r s c o n t r o l l i n g sexual p e r i o d i c i t y . Jour. Exp. B i o l . 11:409-422. M a r s h a l l , W. H. 1937 Muskrat s e x - r a t i o s i n Utah. Jour. Mamm. 18(4) :5l8-519. Medawar, P. B. 1931 Problems of adaptation. New B i o l o g y 11:10-26. Moore, C. R., e t . a l . 1934 On the c o n t r o l of r e p r o d u c t i v e a c t i v i t y i n an annual-breeding mammal ( C i t e l l u s t r i d e c e m l i n e a t u s ) Anat. Record 60:279-289. M u l l e r , S. W. 1947 Permafrost or permanently f r o z e n ground and r e l a t e d engineering problems. Ann Arbor, Mich., J . W. Edwards Inc. 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 Orleans, La. 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 Div. , 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 reconnaissance of the Yukon R i v e r r e g i o n . U. S. Dept. Agr., B i o l . Surv., Nor. Amer. Fauna 19. Pancoast, J . M. 1937 Muskrat industry i n southern New J e r s e y . Nor. Amer. W i l d l . Conf . Trans. 2:527-530. Penfound, W. T., and J . D. Schneidau 1945 The r e l a t i o n of land reclamation to aquatic w i l d l i f e resources of southeastern L o u i s i a n a . Nor. Amer. W i l d l . Conf. Trans. 10:308-318. P o r s i l d , A. E. 1937 F l o r a of the Northwest T e r r i t o r i e s . I n Canada's Western Northland, Dept. Mines and Resources, Ottawa, pp. 130-141. 188 P o r s i l d , A. E. 1945 Mammals of the Mackenzie d e l t a . Canad. F i e l d - N a t . 5 9 ( D : 4 - 2 2 . 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 Athabasca Mackenzie r e g i o n . U. S. Dept. Agr. B i o l . Surv. , Nor. Amer. Fauna 27. Rae, John 1888 Notes on some of t h e b i r d s and mammals of the Hudson's Bay Company's t e r r i t o r y and the a r c t i c coast of America. Jour. L i n n . S o c , Zoology 20:136-145. Richardson, J . 1829 Fauna boreali-americana. P a r t F i r s t , Quadrupeds. London, I . Murray, 300 pp., 24 p i s . Robinson, J . L. 1946 Weather and climate of the Northwest T e r r i t o r i e s , Canad. Geog. Jour. 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 developmental changes. Nature 115:494-495. R u s s e l l , F. 1898 Explorations i n the f a r n o r t h . Iowa C i t y , Univ. Iowa, 290 pp., 21 p i s . , map. Smith, F. R. 1938 Muskrat i n v e s t i g a t i o n s i n Dorchester County, Maryland 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 . Iowa State C o l l e g e unpubl. r e p o r t . Sooter. C. A. 194b Muskrats of the Tule Lake Refuge, C a l i f o r n i a . J o u r . W i l d l . Mgt. 1 0(l ) : 6 8 - 7 0 . Soper, J . D. 1948 Mammal notes from the Grande P r a i r i e - P e a c e R i v e r region of A l b e r t a . Jour . Mamm. 2 9(l ) : 4 9 - 6 4 . S v i h l a , A., and Ruth D. S v i h l a 1931 The Lou i s i a n a muskrat. J o u r . Mamm. 1 2(l ) : 1 2 - 2 8 . Takos, M. J . 1944 Summer movements of banded muskrats. J o u r . W i l d l . Mgt. 8(4) : 3 0 7 - 3 H . 189 Takos, M. J. 1944 A semi-quantitative study of muskrat food habits. Jour. Wildl. Mgt. 11(4) : 331-3 39. Tut ton, A. E. H. 1927 A natural history of ice and snow. London. Kegan, Paul, Trench, Tubne r and Co., 319 pp. Warwick, T. 1934 The distribution of the muskrat (Fiber zibethicus) in the British Isles. Jour. An. Ecol. 3(2) :250-267. Yeager, L. E. 1945 Capacity of I l l i n o i s land types to produce furs. Nor. Amer. Wildl. 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 Loc a l i t y Month": J F M A M J J A S 0 N D Ave. » » * » « * » • * « « » Monthly and Year! Ly Averages of Daily Mean Temo. (K°) Aklavik N.W.T. -21 -15 -7 10 34 53 58 53 39 19 -7 •-16 16 Ft. Smiths N.W.T. -16 -9 3 26 44 54 60 56 45 30 10 -8 25 The Pas Man. -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 Ft. 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 0.5 2.2 12.6. New Orleans La. 0.1 0.2 Ti: 0 0 0 0 0 0 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 Total Hours Accumulated Daylight Date 3o°m 40°Ni 53PN? 6o°m 68GN. Sep. 30 130 130 132 135 140 Oct. 10 256 256 257 261 267 20 379 377.' 375 378 382 30 499 493 487 485 482 Nov. 10 617 605 592 583 570 20 733 714 692 675 648 30 846 820 788 762 717 Dec. 10 958 923 880 843 776 20 1067 1024 969 920 827 30 1177 1126 1058 997 879 Jan; 10 1288 1231 1149 1077 937 20 1401 1337 1243 1162 1003 30 1516 1445 1341 1253 1078 Feb. 10 1632 1556 1444 1350 1163 20 1751 1671 1554 1455 1261 28 1873 1791 1670 1569 1370 Mar. 10 1998 1915 1792 1691 1491 20 2126 2044 1921 1822 1625 30 2257 2178 2058 1963 1772 Apr. 10 2391 2316 2203 2114 1937 20 2528 2459 2356 2276 2119 30 2668 2608 2516 2448 2323 May 10 2811 2761 2683 2631 2552 20 2956 2918 2857 2825 2792 30 3104 3078 3036 3030 3032 Jian. 10 3253 3241 3220 3247 3272 20 3403 3405 3407 3478 3512 Gulf Maryland The Pas Peace R. Aklavik Coast Iowa Man. Delta 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 Year Kind of Fur Fox Coloured Mink' Muskrat Weasell 1930-31 537 686 93,963 1271 1931-32 1277 22111 200,992 3641 1932-33 1294 4465 170,139 2691 1933-34 1420 4673 60,680 755 1934-35 2390 2417 82,339 963 1935-36 2582 908 54,370 566 1936-37 3216 629 64,798 1551 1937-38 2144 112 117,802 1658 1938-39 2027 216 190,740 3076 1939-40 1431 813 216,115 2010 1940-41 2148 1345 256,195 2133 1941-42 3468 2668 237,708 5455 1942-43 2893 1213 298,787 2343 1943-44 3552 792 192,123 558 1944-45 1114 1094 229,753 486 1945-46 1656 2282 344,393 2022 1946-47 1112 1907 234,475 1672 1947-48 462 1245 178,956 1069 1948-49 321 10165 332,619 1578 1949-50 380 1000 167,318 2318 1950-51 323 1495 245,300 2762 1951-52 649 1270 303,831 3190 1952-53 168 1343 193,392 1978 1953-54 58 1590 206,603 2170 Totals 36622 37395 4,673,931 47916 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 wishes to p o i n t out tha t the data presented i n the preceding pages were gathered w h i l e was employed as a b i o l o g i s t w i t h the Canadian W i l d l i f e S e r v i c e . Some o f the i n f o r -mation h e r e i n may be found i n r e p o r t s to tha t agency. There are many persons who c o n t r i b u t e d i n one way or another to t h i s study. Dr. Ian 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 ashore i n A k l a v i k from the f l o a t - e q u i p p -ed a i r c r a f t which had c a r r i e d us n o r t h from Norman Wells. A f t e r our a r r i v a l many r e s i d e n t s of the 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 of the trappers of the area c o n t r i b u t e d advice and a s s i s t a n c e but s p e c i a l thanks should be expressed to Khud Lang, Bert 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 of the f i e l d work was done and they always were w i l l i n g to give f r e e l y o f t h e i r time, and o f t e n t h e i r r equipment as well!. The personnel a t the Government Reindeer Range S t a t i o n deserve mention not only f o r p r o v i d i n g l i v i n g quarters but 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 the 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 to Mr. and Mrs. Lee Post, Mr. and Mrs. F i n l e y Mclnnes, and Andrew P i l o n . D.R. Flook as a student a s s i s t a n t made the summer o f 1949 much more p r o f i t a b l e by h i s help and c h e e r f u l companionship. Dur-i n g the succeeding summer C.E. Law, who served i n the same c a p a c i t y , provided the b e n e f i t of h i s experience w i t h muskrats i n the d e l t a 196 of .the Slave River. It is a pleasure to acknowledge the help of both these men. Plant specimens collected during the course of the i n -vestigation were identified by A. E . Porsild of the National Herbarium of Canada and by Dr. T. M. C. Taylor of the University of British Columbia. In conclusion the writer wishes to express his appreciation to Dr. W. A. Clemens, past head of the Depart-ment of Zoology at the University of B r i t i s h Columbia, for his help in outlining the objectives of this study. 

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