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The Pacific walrus (Odobenus rosmarus divergens) : spatial ecology, life history, and population 1955

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THE PACIFIC WALRUS (ODOBENUS ROSMARUS DIVERGENS): SPATIAL ECOLOGY, LIFE HISTORY, AND POPULATION by Francis Hollis Fay A Thesis submitted i n 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 de/scree of Doctor of Philosonhv Members of the Department of Zoology THE UNIVERSITY OF BRITISH COLUMBIA July 1955 ABSTRACT The Pacific Walrus (Odobenus rosmarus divergens): Spatial Ecology , Life History, and Population by Francis H. Fay The Pacific walrus inhabits the Bering Sea during winter and the Chukchee Sea in summer, generally in close association with sea ice. The year-round northern limit to this range i s marked by the southern edge of the relatively unbroken pack ice which, though not impenetrable, i s usually avoided. The southern limit appears to be set by a i r tem- o peratures, regions with monthly means of 50 F or more being unoccupied. Between these two "barriers," the animals frequent waters of less than 50 fathoms depth in which their preferred food, the pelecypods Mya. Saxicava. Astarte. Ma coma, and Clinocar'dium. occur. Seasonal migrations between the Bering and Chukchee Seas appear to be partly in response to changing physical conditions and partly due to an innate or learned behaviour pattern. Females are the most regular migrants; males are more subject to the inconsistencies of ice d r i f t . The bull Pacific walrus reaches sexual maturity at six to eight years of age, the cow at four.to five years of age. Breeding takes place mostly from April to June as the animals are migrating northward , and there i s no evidence of any organized polygamy or "harem breeding." Gestation i s one f u l l year, and twinning i s unknown. An individual cow rarely conceives in successive years, the f i r s t three preg- nancies generally being at 2-year intervals and later ones three or more years apart. Males become senile at about fifteen years of age. years of age by both sexes, though growth continues slowly thereafter. The tusks and other teeth grow at a relatively high rate throughout the l i f e span, and analyses of their structure and si&e have yielded good technique^for age determination. The population, upwards of 40,000 animals at present, has declined slightly in the past fifteen years, but i t has reached or i s approaching equilibrium. The birth rate and death rate are about equal, human predation aoxounting for most of the latter. Since the population i s currently too small to satisfy the Alaskan Eskimos' needs, i t i s recommended that i t be permitted to increase by eliminating some of the wasteful hunting practices which are now in effect. F u l l adult body size i s achieved at four to six THE UNIVERSITY OF BRITISH COLUMBIA F a c u l t y of Graduate Studies PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of FRANCIS HOLLIS FAY B.S. (New Hampshire) 1950 M.S. (Massachusetts) 1952 WEDNESDAY, J u l y 27, 1955 at 2:30 p.m. In ROOM 187A, BIOLOGICAL SCIENCES BUILDING COMMITTEE IN CHARGE: H.F. Angus, Chairman I.McT. Cowan S.E. Read P. Ford E.S. Carpenter P.A. L a r k i n V.J. O k u l i t c h G.J. Spencer W.D. K i t t s W.M. Cameron E x t e r n a l Examiner - M.J. Dunbar M c G i l l U n i v e r s i t y LIST OP PUBLICATIONS Q u a n t i t a t i v e Experiments on the Food Consumption of Parascalops b r e w e r i , Journal of Mammalogy 3 £ , 1 0 7 - 1 0 9 , 195k> The Geographical and E c o l o g i c a l D i s t r i b u t i o n of C o t t o n t a i l Rabbits i n Massachusetts, Journal of Mammalogy 3 _ 6 ( i n p r e s s ) , 1 9 5 5 . THESIS THE PACIFIC WALRUS, ODOBENUS ROSMARUS DIVERGENS: SPATIAL ECOLOGY, LIFE HISTORY, AND POPULATION The h o l a r c t i c d i s t r i b u t i o n of walruses i s defined by three primary f a c t o r s : sea i c e , a i r temperatures, and d i s t r i b u t i o n of c e r t a i n pelecypod genera upon which they feed. Pronounced migrations occur, p a r t l y i n response to seasonal v a r i a t i o n s i n these f a c t o r s . * Q u a n t i t a t i v e analyses of reproduction, growth, and m o r t a l i t y have been founded upon the development of an age determination technique i n v o l v i n g dental morphometry and h i s t o l o g y . Tusk growth Is the key to aging, since i t proceeds at a r e l a t i v e l y h igh r a t e throughout the l i f e span. There has been a s l i g h t decrease i n the number of P a c i f i c walruses w i t h i n the past f i f t e e n years, but at present the pop u l a t i o n has reached or i s approaching s t a b i l i t y . C u r r e n t l y there are too few to supply the maintenance needs of some Eskimo v i l l a g e s , and since human pr e d a t i o n Is the primary f a c t o r c o n t r o l l i n g p o p u l a t i o n s i z e , recommendations have been given f o r reducing the k i l l without a f f e c t i n g the harvest. GRADUATE STUDIES F i e l d of Study: Zoology H i s t o r y and General P r i n c i p l e s B i o l o g i c a l Methods and Procedures Forest and Wilderness Game Population Dynamics W.A. Clemens Zoology Department I.MoT. Cowan P.A. L a r k i n Other Studies: Synoptic Oceanography Forest A s s o c i a t i o n s Biometrics People of the A r c t i c W.M. Cameron V. K r a j i n a V.C. B r i n k G.'H. Marsh ACKNOWLEDGEMENTS This study was made possible by grants-in-aid from the Arctic Institute of North America under contractual arrangements with the United States Office of Naval Research. The writer i s particularly grateful for the financial support thus provided. To the United States Coast Guard (C.G.C. Storis and C.G.C. Northwind). Navy (U.S.S. Burton Island). Air Force, Fish and Wildlife Service, Weather Bureau, and the Alaska Native Service I am indebted for transportation and other services rendered. Dr. I. McT. Cowan, University of British Columbia, i n i t i a l l y suggested the project and, together with Mr. R.F. Scott, U.S. Fish and Wildlife Service, Anchorage, Alaska, has provided much of the stimulus and constructive criticism leading to i t s successful completion. Many of the data and specimens u t i l i z e d were donated by James W. Brooks, now of the Alaska Fisheries Commission, whose sincere cooperation has been an invaluable asset. Dr. Robert Rausch, Arctic Health Research Center, Anchorage, contributed f i e l d equip- ment and technical assistance. To a large degree, success in the f i e l d has been directly attributable to the kind hospitality of Mr. and Mrs. Paul E. Tovey, Mr. William Caldwell, and Mr. and Mrs, Donald G. McLean, recently of Gambell, Alaska. The Eskimos of St. Lawrence Island, most particularly Messrs. Charles - i - and Vernon Slwooko, patiently and unselfishly aided the writer, though the techniques and objectives of the study were largely incomprehensible to them. Without their help many of the data could never have been obtained. Constant assistance and moral support has been given by my wife, Barbara, both in the f i e l d and in the preparation of this thesis. To the numerous other friends, acquaintances, and organizations who contributed information and services, the writer i s deeply grateful. - i i - TABLE OF CONTENTS Page INTRODUCTION 1 I. SPATIAL ECOLOGY 6 Distribution 7 January-February 8 March 11 April 11 May 12 June 14 July-August 15 September 19 October 20 November-December 21 Discussion 22 General Features of Distribution 22 Migrations 26 Range Reduction 30 II. LIFE HISTORY 32 Reproduction 34 Sexual Maturity 34 Breeding Season and Location 41 Breeding Behaviour 46 Gestation 51 Birth 52 Breeding Frequency 57 The Young 65 Sex Ratio 65 Morphology 65 Precocity 67 Parental Care 67 Nutrition 71 Quality 71 Quantity :.... 71 Predation on Seals 72 Feeding Behaviour . 74 - i i i - TABLE OF CONTENTS—Continued Page Growth 77 Age Determination 77 Body Growth 101 Baculum Growth 108 Skin and Pelage I l l Skull Growth 115 Dentition 115 Death 135 Density Independent Factors 137 Density Dependent Factors 141 III. THE POPULATION 148 APPENDICES 155 LITERATURE CITED 166 - i v - LIST OF ILLUSTRATIONS Figure Page 1. A Map of Alaska and S i b e r i a Showing , L o c a l i t i e s Mentioned i n the Text 4 2. Walrus D i s t r i b u t i o n i n Winter 9 3. Walrus D i s t r i b u t i o n i n E a r l y Spring 9 4. Walrus D i s t r i b u t i o n i n Late Spring 9 5. Walrus D i s t r i b u t i o n i n Summer 17 6. Walrus D i s t r i b u t i o n i n E a r l y F a l l 17 7. Walrus D i s t r i b u t i o n i n Late F a l l and E a r l y Winter 17 8. General P h y s i c a l Boundaries of H o l a r c t i c Walrus D i s t r i b u t i o n i n Summer 24 9. Surface Currents: B e r i n g , Chukchee, Beaufort, and East S i b e r i a n Seas. 28 10. Average T e s t i s Weight Growth of P a c i f i c Walruses 38 11. P a r t i a l Diagram of the Female Reproductive Organs..... 58 12. S a g i t t a l S e c t i o n of the Skin and S u p e r f i c i a l Musculature of a 2-Iear-01d B u l l Walrus 76 13. Root Ridges on Male Tusks 79 14. Frequency Histograms of Observed E x t e r n a l Lengths of Male and Female Tusks 84 15. Tusk Lineaments U t i l i z e d i n Age and Growth Analyses • 85 16. Tusk Root Ridge Curves 90 17. Average L i n e a r Growth and Wear of Male P a c i f i c Walrus Tusks 92 - v - LIST OF ILLUSTRATIONS—Continued Figure Page 18, Relationship Between Female Tusk Size and Reproductive History 95 19• Average Linear Growth and Wear of Female Tusks 100 20. Body Weight Growth of Five Young A t l a n t i c Walruses i n Capti v i t y . . . . 103 21. Average Body Length (nose-tail) Growth of P a c i f i c Walruses 105 22. Approximate Average Body Weight Growth of P a c i f i c Walruses 106 23. Average Hind Foot Length Growth of P a c i f i c Walruses 106 2V. Relative Pinniped Body Growth, I l l u s t r a t i n g the "Double Sigmoid" Curve of Harem Breeders 107 25. Average Baculum Length Growth of P a c i f i c Walruses 109 26. Average Baculum Weight Growth of P a c i f i c Walruses 109 27. Comparison of Calf and Adult Bacula 110 28. Age-Relative Increase of Male P a c i f i c Walrus S k u l l Dimensions 116 29. Age-Relative Increase of Female P a c i f i c Walrus S k u l l Dimensions 117 30. S a g i t t a l Sections of the Second Upper and Lower Premolars Showing Structural Growth.. 120 31. Relationship Between Tusk 0EL and Number of Cementum Layers i n the Cheek Teeth of Male P a c i f i c Walruses...... 125 32. S a g i t t a l Sections of Male Tusks Showing Structural Growth 127 - v i - LIST OF ILLUSTRATIONS—Continued Figure Page 33. Relationship Between Tusk Length and Cross-Sectional Shape at the Gum Line...... 131 34. Relationship Between Observed External Length and Circumference of the Tusks 132 35. View of Old Female and Old Male Tusks Showing Heavy Medial Wear 134 36. Catch Curve of Male Walruses Taken at Gambell and Savoonga in the Spring of 1953 145 - v i i - LIST OF TABLES Tables Page 1 . A Histological Comparison of Testis Development With Age and Season..... 3 5 2 . Testis Lengths and Weights 39 3 . Comparative Ovarian Histology by Age and Season . 4 3 4 * Calculated Conception Dates of Atlantic and Pacific Walrus Embryos. • 4 5 5 . Comparative Sexual Dimorphism Among Pinnipeds . 4 8 6. Parturition Dates 5 4 7 . Post-Partum Regression of the Uterine Horn 5 9 8 . Idealized Schedule of Conceptions from Three Possible Ovulations Experienced by Pacific Walrus.. 63 9 . Calf Body Measurements 6 6 1 0 . Proportion of Feeding Walruses by Season and Location 7 3 1 1 . Root Ridge Intervals (inches) on Male Tusk Roots 8 8 1 2 . Female Tusk Lengths at Successive Stages in Reproduction 9 6 13. Os Cl i t o r i d i s Weights 1 1 2 1 4 . Intervals Between Major Medial Wear Loci on Female Tusks 1 3 6 1 5 . Estimated Annual Pacific Walrus Mortality from Human Predation 1 5 0 16. Number of Reports on Population Trends.... 1 5 2 - v i i i - SUMMARY The holarctic distribution of walruses i s defined by three primary factors: sea ice, a i r temperatures, and distribution of certain pelecypod genera upon which they feed. Pronounced migrations occur, partly in response to seasonal variations in these factors. Quantitative analyses of reproduction, growth, and mortality have been founded upon the development of an age determination technique involving dental morphometry and histology. Tusk growth i s the key to ageing, since i t proceeds at a relatively high rate throughout the l i f e span. There has been a slight decrease in the number of Pacific walruses within the past fifteen years, but the population (about A-0,000) has reached or i s approaching s t a b i l i t y . At present there are too few to supply the maintenance needs of some Eskimo villages, and since human predation i s the primary factor controlling population size, i t has been recommended that current wasteful hunt- ing practices be abolished. - ix - INTRODUCTION Throughout the pages of history i t has been a general rule that European man's expansion of his environment has been accompanied by thoughtless exploitation of natural re- sources for economic gain, some to the point of complete or near extinction. The animal resources of the new fringe areas have usually been the f i r s t to suffer because of their accessibility and per capita values, but due to their powers of reproduction, many exploited populations have recovered fu l l y when with a changing economy or restrictive l e g i s l a - tion they have been l e f t alone once more. Such has been the fate, notably, of the fur seal and sea otter in North America, though some others have not fared as well. Mod- erately tapped as early as the 8th and 9th centuries A.D., the world walrus population f e l t i t s f i r s t real blow from commercial enterprise in the early lSOO's, and by 1850 walrus hunting in northern seas had become an extremely profitable business. Harvests from the North Pacific region between i860 and 1880 averaged about ten thousand animals annually, the f r u i t s of the hunt being mainly o i l and ivory for sale in the ports of western North America and eastern Asia. A serious decline in numbers became very evident towards the end of the 19th century, and Clark (1884) pre- dicted that the population would soon be so thoroughly dec- imated that i t would no longer be profitable to seek them. -2- This p r e d i c t i o n became r e a l i t y w i t h i n the next f o r t y years, though land-based t r a d e r s c a r r i e d on a l i v e l y business with Eskimo hunters i n some areas u n t i l h a l t e d by the "Walrus Act" of 1941. Since t h a t time, walruses have been harvested only by Eskimos, presumably only f o r t h e i r own use. The general opinions of Eskimo and White observers i n Alaska from 1935 to the present i n d i c a t e no major changes i n walrus population d e n s i t y ; indeed, some observers consider t h a t there has been a s l i g h t increase over an a l l - t i m e low experienced about 1920. What, then, i s the s i z e of the pres- ent population? What are i t s c h a r a c t e r i s t i c s and p o t e n t i a l - i t i e s f o r growth? How l a r g e an annual harvest can be s a f e l y taken without endangering i t s f u t u r e status? I f t h i s resource i s t o be maintained f o r the b e n e f i t of the Alaskan n a t i v e s who are dependent upon i t , then these questions must be an- swered and a s u s t a i n e d - y i e l d management program devised. These are the problems which the w r i t e r set out t o solve i n 1951. F o r t u n a t e l y , at the same time and independently, Mr. James W. Brooks of the U n i v e r s i t y of Alaska was making prepar- a t i o n s f o r a s i m i l a r i n v e s t i g a t i o n , and i t has been l a r g e l y through our cooperative e f f o r t s i n the f i e l d t hat many of the conclusions i n the f o l l o w i n g pages can be reached. Our e m p i r i c a l data have been pooled where d e s i r a b l e , but the analyses and conclusions have been a r r i v e d at independently. The w r i t e r ' s f i e l d work was centered about the reg i o n of S t . Lawrence I s l a n d , A l a s k a , s i t u a t e d i n the n o r t h - c e n t r a l -3- portion of the Bering Sea (Fig. 1). Of the three most suit- able l o c a l i t i e s (St. Lawrence, King, and L i t t l e Diomede Islands) i t was i n i t i a l l y chosen because of i t s accessibility by air during the spring months. This choice later proved to be a very good one from many other aspects. The spring hunt- ing seasons of 1952, 53, and 54 were spent at the village of Gambell (Sevuokuk) on the northwest extremity of the island where i t was possible to take an active part in the annual hunt and to gather much first-hand information and specimens. Occasional v i s i t s to Savoonga, a village about 35 miles ESE, also yielded many data and specimens, a l l of which were col- lected by local people at the writer's request. In 1952 a survey of other Eskimo villages and coastal waters north to Barrow was made possible by the cooperation of the U.S. Coast Guard (C.G.C. Storis), and much supplementary information was later obtained by questionnaire and vocal communication with teachers and other local observers from nearly a l l of the coastal villages north of the Alaska Peninsula. Brooks* work covered the villages of Wales, Diomede, and Barrow and the vi c i n i t y of the Walrus Islands in Bristol Bay. Thus, between us, we have directly or indirectly investigated nearly every locality where the Pacific walrus occurs east of the International Date Line, and Soviet reports of con- ditions west of the Line have completed the overall picture. Most of the f i e l d investigations were centered upon problems of general ecology, productivity, growth, and mor- Fig. 1.—A map of Alaska and Siberia showing l o c a l i t i e s mentioned in the text. t a l i t y , for these are the foundations upon which a population study must be built . Analyses of these data form the bulk of this thesis. PART I SPATIAL ECOLOGY - 7 - DISTRIBUTION The general range of the P a c i f i c walrus has been b r i e f l y o u t l i n e d by many w r i t e r s , v a r y i n g somewhat w i t h t h e i r experience, t h e i r survey of previous l i t e r a t u r e , and t h e i r personal i n t e r p r e t a t i o n of the data and i t s i m p l i c a t i o n s . Of these, the most complete and a u t h o r i t a t i v e are the d i s c u s s i o n s by Belopolsky (1939), Brooks (1954), C o l l i n s (1940), H e i n r i c h (1947), and N i k u l i n (1940), each of which has helped t o r e - solve some of the s a l i e n t f e a t u r e s of the d i s t r i b u t i o n , t h e i r causes and e f f e c t s . G e n e r a l i z i n g from these and other recent data, i t i s apparent t h a t i n w i n t e r the bulk of the popula- t i o n occurs about the Bering Sea between l a t . 57°N and 64°N, while i n summer i t occupies the Chukchee Sea north t o l a t . 72°N, east to l o n g . 155°W, and west to long. 175 < SE. To t h i s p a t t e r n there have been many exceptions, but most concerned only small groups of wanderers. The l i t e r a t u r e concerning t h i s region p r i o r to 1900 suggests t h a t the general f e a t u r e s of d i s t r i b u t i o n have been about the same f o r the past 150-200 years, the p r i n c i p a l exceptions being t h a t the outer f r i n g e s of the range extended f a r t h e r and t h a t the p o p u l a t i o n was more dense throughout i t s range than i t i s now. The o v e r a l l shrinkage appears to have occurred mostly i n the l a s t h a l f of the 19th century, and i t has continued i n t o the 20th. -3- I n order t o f u l l y appreciate the design of modern e x p l o i t a t i o n and p o p u l a t i o n , a b r i e f examination of seasonal movements i s necessary. This i s presented i n the f o l l o w i n g pages. I t i s i n part a summary and i n part a supplement to the e a r l i e r works. January-February P r i o r to t h i s w r i t i n g there has been i n s u f f i c i e n t m a t e r i a l w i t h which t o judge the b a s i c character of w i n t e r d i s t r i b u t i o n . The e a r l i e r records c o n s i s t e d e n t i r e l y of a few l o c a l i z e d observations from the v i c i n i t y of s e v e r a l Eskimo v i l l a g e s and negative r e p o r t s from oth e r s . In the w i n t e r s of 1953 and 1954, however, the w r i t e r , through the kindness of the U.S. Navy E l e c t r o n i c s Laboratory, S p e c i a l Studies Branch, was able to o b t a i n a s e r i e s of s i g n i f i c a n t observations which were secured and recorded by personnel of the i c e b r e a k e r s U.S.S. Burton I s l a n d and C.G.C. Northwind i n the Bering Sea (Ryder, 1953; R.E. M o r e l l , i n l i t t . ) . These c o n s t i t u t e the bulk of the data shown i n F i g . 2. From e a r l y January t o mid-March the population appears t o be l o o s e l y aggregated i n a broad und u l a t i n g band s t r e t c h i n g , roughly, from B r i s t o l Bay, A l a s k a , t o the Gulf of Anadyr, S i b e r i a . I n January the width of t h i s band probably averages about 150-200 m i l e s , but by mid-February i t may be as much as 300 m i l e s , due t o the gradual southward d i s p e r s a l of i c e i n response to strong n o r t h e r l y winds. Within t h i s loose aggregate appears a w e l l defined p a t t e r n of l o c a l concentra- -9- Fig. 2.—Distribution in winter Fig. 3.—Distribution in early spring ^ 1 • > j£LJr * • / May — June Fig. 4 . — D i s t r i b u t i o n in late spring. Dots are recorded observations; crossmatching represents probable range occupied by the bulk of the popula- tion. - l o - tions in two general environments: (a) on the southside of large islands and peninsulas, and (b) along the southern edge of the pack ice. Because of the strong north wind dur- ing the winter months and i t s effect of pushing the pack in a general southerly direction, i t would be expected that immediately south of the islands and peninsulas there would be more open leads amongst the ice than elsewhere. This has been verified by the St. Lawrence Island men, who state that the "south-side" i s usually quite open in winter, and seals and walruses are more abundant there than they are in the more tightly packed ice to the west and north. The second area of concentration, along the edge of the pack, i s probably occupied for the same reason, i.e. ample open water between the floes. Observers on the C.G.C. Northwind in the winter of 1954 reported that the walruses seen appeared to have been exclusively "in areas of l i t t l e or no ice pressure" (R.E. Morell, in l i t t . ) . North of St. Lawrence Island the pack ice i s essen- t i a l l y entire, for although leads-are frequently formed, they quickly freeze over again. In spite of such unfavor- able conditions, there i s a small(?) proportion of the pop- ulation which remains there in winter. These "winter wal- ruses" (Heinrich, 1947) are usually lone bulls, and they have been reported occasionally north of St. Lawrence Island and Nunivak Island, near Nome, King Island, Wales, Diomede, and in the vicinity of Point Barrow. Possibly they are sparsely distributed over the rest of the north Bering Sea -11- and Chukchee at that time, as well. In Muller's account (Golder, 1914) of travels in northeast Siberia there i s mention of the natives hunting walruses near Wrangell Island in winter. March By March, though the overall pattern remains about the same, there i s a definite tendency for a few small herds to begin moving northward, at least through the Strait of Anadyr, between St. Lawrence Island and Cape Chaplino. Nikulin (1940) mentions some which were seen near the latter point at this time, and his distribution maps (oj>. c i t . , Figs. 1,9) indicate their presence in the strait and slightly north of i t . Airline pilots flying the Nome-Gambell mail route have also reported that the f i r s t walruses observed north of the island are generally sighted in the f i r s t week of March with a gradual increase in numbers thereafter. These were believed to be mostly males, and the few taken in that month by Gambell hunters are predominantly of that sex. April As the southern edge of the pack begins to melt and break up, and the leads south of Bering Strait remain open longer, the northward migration commences. In April herds have been observed in Etolin Strait and on the north- side of Nunivak Island (Lantis, 1946; R.B. Gibson, in l i t t . ) , near Cape Chaplino and Arakamchechen Island (Nikulin, 1940), north, east, and west of St. Lawrence Island, and even as far -12 north as the west side of Bering Strait (Nikulin, op_. c i t . ) . This "advance guard" i s composed almost wholly of males. The herds are small (2 to 10 individuals) and for the most part are widely scattered, for there are s t i l l large unbroken expanses of ice, particularly in the northeast sector of the Bering Sea. In recent years the most southern extremes of range have been recorded at this time (Ryder, 1954). These records, shown in Figure 3, probably represent individuals whose current and wind-propelled ice pan disintegrated, leaving them stranded and separated from the rest of the population. Nikulin also attributes his southernmost records to this phenomenon. May The beginning of the major northward surge of females and young i s observed in May. By this time the ice north and south of Bering Strait breaks up and begins to melt under the influence of wind, surface currents, and warmer air tem- peratures. The north wind decreases in force and south, east, and west winds are frequent, serving to more nearly coordinate a i r and water (and thus ice) movements into a common vector towards the north. The last of the passing herds are seen at Nunivak and Nelson Island early in the month (Lantis, 1946; R.B. Gibson, in l i t t . ) , and by the end of the month the movement through Bering Strait i s well under way (Fig. 4). The animals passing north between St. Lawrence Island and Siberia throughout this period are -13- principally females accompanied by juveniles and newborn young, though a few adult bulls are usually seen amongst them. Those found within 1$ miles of the north side of the island, however, are predominantly bulls (98 per cent) which appear to be the bulk of the former advance guard now linger- ing behind the rest of the population. As noted above, the passage through Bering Strait commences earliest on the west side, and i s not evident on the east side u n t i l sometime after the middle of May. This i s to be expected, for there i s an earlier breakup on the Siberian side (Goodman, et a l , 1942). "Bimodal" migration through the eastern half (Brooks, 1954, p. 7) i s undoubtedly a result of ice conditions between St. Lawrence Island and the Alaskan and Siberian mainlands, for the Strait of Anadyr breaks up earliest,and the distance that herds must travel from there to Bering Strait i s much less. Thus the f i r s t migrants are animals which have come through the Strait of Anadyr, and the second wave those moving up from Bristol Bay and the St. Matthew-Pribilof region, along the Alaskan coast. At King Island, Diomede, and Wales the f i r s t animals observed are females with juveniles and young, followed by a mixture of adults and immatures of both sexes, and f i n a l l y by a few old bulls (Bailey and Hendee, 1926; Brooks, 1954; Heinrich, 1947; W.E. Rasmussen, in l i t t . ; A. Nagozruk, Sr., voc. com.). By the end of the month some may be as far north -14- as Point Hope (J.D. O'Donahue, in l i t t , ) or Icy Cape, in the case of an early breakup such as was experienced in 1954 (Ryder, 1954; R.F• Gray, in l i t t . ) . Two southern concentrations of bulls are recorded during May, one in the vic i n i t y of Hagemeister and the Walrus Islands in Bristol Bay (Brooks, 1954), and the other deep in the Gulf of Anadyr near Kresta Bay (Belopolsky, 1939), The f o«rmer was estimated to contain about a thousand animals in 1953, and apparently remains in the same region throughout the summer, June By the f i r s t week of June the sea south and east of St. Lawrence Island i s ice free, but much s t i l l remains along the Siberian coast in St. Lawrence Bay, the Gulf of Anadyr, and in patches as far south as Karaginskoi Island (Jackson, 1896; Nikulin, 1940; Transehe, 1925), probably due to the colder coastal waters found in that region (Good- man, et a l , 1942). The last of the migrating herds are observed passing the east and west ends of St. Lawrence Island at this time, those to the west being females and those to the north males. Within the second week they have a l l passed by King Island (W.E. Rasmussen, in l i t t . ) , and by the third week most of the f i n a l migrants have swum through the east side of Bering Strait (Brooks, 1954; Heinrich, 1947). On the west side the migration i s com- pleted by the end of the month when the ice i s f i n a l l y gone -15- (Nikulin, 1940). Big and L i t t l e Diomede Islanders state that the greatest proportion of the herds pass through the west side, and a few late stragglers are sometimes seen in the Strait (both sides) as late as July (Heinrich, 1947; Nikulin, op. c i t . ) . Eskimos at St. Lawrence Island state that many females haul out on the beaches north of Gape Chaplino in June. To the north, large herds are reported near Wrangell Island and Long Strait by the end of the month, while at Cape Serdtse Kamen and Point Hope the animals are seen for the f i r s t time in large quantities near shore (Nikulin, 1940; Rainey, 1947; J.D. O'Donahue, in l i t t . ) . This lag between occurrence at sea and observation near land appears to be the rule throughout the entire migration and may be due to less favorable ice conditions inshore. In the southern extremities, in addition to the Bristol and Kresta Bay herds noted above, small groups are sometimes found near Karaginskoi Island, and other larger herds have been reported south of Cape Navarin and along the north side of the Gulf of Anadyr near Cape Bering (Nikulin, op. c i t . ) . Apparently these are a l l bull herds, though Belopolsky (1939) states that females and young appear with the bulls at Kresta Bay in June and July. July-August During July the northward movement i s completed in the Chukchee Sea. Most of the migrants (particularly females -16- and young) have progressed northwestward from Bering Strait and are abundantly distributed (Fig. 5) off the northeast coast of Siberia as far west as Long Strait and amongst the ice south of Wrangell Island (Nikulin, 1940; Transehe, 1925). In the days of heavy commercial exploitation by American ships, this was the preferred hunting area, according to Collins (1940) and Nikulin. As the ice deteriorates and retreats northward in August, the herds advance slightly to the region about Wrangell and Herald Islands, and some have been observed by Nikulin near the mouth of the Kolyma late in the month. On the Alaskan coast, inshore records are common near Icy Cape, Waihwright, and Barrow in July and early August (Bailey, 1948; Bailey and Hendee, 1926; Brooks, 1954; Collins, 1940; Rainey, 1947; Ryder, 1953a; R.F. Gray, in l i t t . ) . At Wainright the bulls appear f i r s t , followed a few days later by the cows (R.F. Gray, in l i t t . ) , while at Barrow the animals observed are nearly a l l bulls (Brooks, 1954). By.,mid-August the coast i s generally ice-free at least to Point Franklin, and walruses are no longer seen near shore. Although herds and individuals were formerly encountered as far east as Banks Land and the B a i l l i e Islands (Anderson, 1937; Clarke, 1944; MacFarlane, 1905; Porsild, 1945), none was observed by vessels operating east of Barrow in 1952 (McAllister, 1953), 1953 (Ryder, 1953a), or 1954 (F. Neave, in l i t t . ) . -17- F i g . 5 * — D i s t r i b u t i o n i n summer Fi g . 6 . — D i s t r i b u t i o n in early f a l l F i g . 7»—Distribution i n late f a l l and early winter. Dots are re- corded observations; crosshatching represents probable range occupied by the bulk of the population. -18- To the south, ice remains along the Siberian Coast from Kresta Bay to St. Lawrence Bay u n t i l late July or August (Jackson, 1896; Nikulin, 1940; Transehe, 1925), and with i t walrus herds also remain. When this ice f i n a l l y melts, the Kresta Bay group moves slightly eastward to the . beaches at Meyecheken Island and Achchen where they haul out in locations regularly occupied at that time each year. Other regular hauling grounds for semi-stationary summer herds are located at Arkamchechen Island, Cape Inchoun (near C. Deznev), and Cape Serdtse Kamen. There i s apparently some intercommunication between a l l of these in the late summer and early f a l l , while throughout the summer period additional herds and individuals of irregular occurence are observed along the entire coast from Cape Navarin to Cape Bill i n g s , according to Nikulin (1940) and Siberian Eskimos now li v i n g at Gambell. On the Alaskan side of the International Date Line a few individual strays are also reported along the coast and about the islands, but the abundance of these, compared to those on the Siberian shores, appears to be relatively insignificant. At present only one regular semi-stationary summering herd (Walrus Islands) i s known to occur in Alaskan waters, and like i t s Siberian counterparts, i t i s composed wholly of bulls (R. Mahaffey, in l i t t . ) . Herds have apparently occupied this same local i t y throughout historic times though they have varied, quantitatively, dur- ing this period. -19- "Resident" herds such as this formerly occupied regular hauling grounds on the Pribilof and St. Matthew Islands ( E l l i o t t , 1882; Hanna, 1920, 1923; Preble, 1923) but were apparently wiped out by the commercial ivory hunters many years ago. None but an occasional lone individual has been seen in either locality in recent summers (Wilke, 1942; CA. Barnes, voc. com.; E.B. E l l i o t , voc. com.; R. Rausch, in l i t t . ) . Small summering herds (either or both sexes) are said to occur about the Punuk Islands (lat. 63 PN, long. 169°W), but none was observed there in 1952 or 1953. At Capes Thompson and Lisbourne, both near Point Hope, herds of varying sizes and either sex formerly remained in the same vi c i n i t y during July, August, and September, according to local Eskimos, and transient herds often stopped there in the f a l l to rest on the beaches. Within the past four years, however, the summer residents have not been seen, and transients are apparently very infrequent. Local opinion attributes this to an increase in aircraft t r a f f i c along the coast. September The distribution during September (Fig. 6) resembles that which i s observed in August, the main differences being the records of "farthest north" and the beginning, late in the month, of southward migration. The farthest north observation was recorded on September 5, 1951, when two walruses were seen hauled out on the edge of the Arctic pack at l a t . 75°52'N, - 2 0 - long. 164° 15* W (CA. Barnes, voc. com.). These were probably a cow and c a l f or yearling, f o r one was about twice the size of the other. Two other loose aggregations of 30 and 200 animals have been recorded at about l a t . 73* N, long. 161° W by Barnes i n 1951 and by observers on the H.M.C.S. Labrador i n 1954 (A. Loughrey, i n l i t t . ) . respectively. In addition to these, a few have been observed between Barrow and Icy Cape by Ryder (1953a) and R.W. Niesz ( i n l i t t . ) . and N i k u l i n (1940) has indicated "thousands personally observed" off the southeast coast of Wrangell Island. In the l a s t h a l f of the month the vanguard of southgoing herds i s sometimes seen near, Point Hope (J.D. O'Donahue, i n l i t t . ) and even as f a r south as Bering S t r a i t (Heinrich, 1947). October There are few records f o r t h i s month, except those from the Siberian coast given by N i k u l i n . 0'Donahue indicates that some are occasionally seen near Point Hope i n the f i r s t h a l f of October, but none was observed anywhere along the A r c t i c coast of Alaska by personnel of the U.S. Coast Guard icebreaker Northwind. returning from the Beaufort Sea i n 1954 (F. Neave, i n l i t t . ) . New ice begins to form i n the regions north of 70°N l a t . about the middle of the month, and strong northerly winds begin to blow i n the Chukchee, dr i v i n g the new and old ice southward. The general pattern of walrus movements i s rather problematical with such a dearth of data, but Nikulin*s - 2 1 - suggestion that they proceed southward well off shore, with and ahead of the ice, seems most logical, for they are seldom seen u n t i l they reach Bering Strait. When the f a l l i s warm, and the "freezeup" i s late, the herds are said to mostly swim south at least as far as Bering Strait, where they some- times haul out in great numbers at Cape Deznev (East Cape), Big Diomede Island, and other places along the Siberian coast in October and November (Heihrich, 1947; Nikulin, 1940). November-December The f a l l migrations observed from L i t t l e Diomede Island by Heinrich (1947) seemed to lack the purposiveness of the spring movement, for herds were seen passing either north or south from September to December, inclusive, and those which went ashore at Big Diomede sometimes basked there for as long as a week. However, a strong southward movement through the Strait was noted in November, during which time the animals were very noisy and mostly in the water, swimming. The herds usually arrive in the vic i n i t y of St. Law- rence Island late in November (Fig. 7) . Bulls are the f i r s t to appear (swimming) and they are followed in early December by a mixture of herds, most of which are cows, according to Eskimo informants. At this time the animals are known occasionally to haul out in certain places along the north side and regularly on the Punuk Islands, sometimes in suf- ficient quantities to nearly cover the north island which 22- i s about 30 acres in area. The latter are said to consist of both sexes, the bulls being particularly vociferous and aggressive. This was interpreted by two informants as breed- ing behaviour, though i t i s highly improbable that any breed- ing occurs in the f a l l months. By late December very few are seen passing Gambell, and the winter pack ice i s well formed. It i s presumed that the regular wintering distribution pattern i s achieved by mid-January. DISCUSSION General Features of Distribution The spatial ecology of walruses, judging from the preceding account and what l i t t l e i s known of their occur- rence in other parts of the world, can be resolved into four major components, a l l of which must be present at the same time. These are food, air, haul-outs, and suitable a ir temperatures. It i s quite obvious that food and a i r must be available at a l l times for mere existence. Thus, of. necessity, the majority of the "Pacific" population i s limited to regions south of the relatively unbroken Polar pack ice in summer and south of the Chukchee and North Bering Sea pack in winter, for there are few l o c a l i t i e s in the northern regions where ice i s sufficiently loose to permit access to both sea and air at w i l l . Walruses are capable of making and maintaining breathing holes under -23- certain conditions (Vibe, 1950) but generally retire to regions of less ice stress when possible. The need for haul-outs further restricts the range by necessitating close juxtaposition to either land or ice. In the Western Nearctic ice appears to be preferred — at least i t i s occupied most frequently. In the Eastern and Palearctic, land seems to play a more important role as a summer hauling ground, but even there Johansen (1910), Neale (1882), and Zalkin (1937) have suggested that ice i s preferred when conditions are not unfavorable for i t s use. These conditions involve the distribution of food. Through- out the holarctic range of walruses, the foods most often u t i l i z e d consist of six genera of pelecypods (My a, Saxicava, Cardium, Clinocardium, Astarte, and Macoma) which inhabit Arctic, Subarctic, and certain North Temperate seas at depths ranging from the l i t t o r a l zone to about 50 fathoms (Allen, 1880; Brooks, 1954; Johansen, 1910; Keen and F r i z z e l l , 1939; Nikulin, 1940; Pratt, 1935; Soot-Ryen, 1939; Vibe, 1950; Zalkin, 1937). This means a drastic reduction in the poten- t i a l range (Fig. 8 ) , especially in the eastern and Palearctic region, much of which i s characterized by precipitous coast- lines and deep fjords. Unless ice i s in close proximity to the few shallowcareas where food species occur, land must be used for haul-outs. Exceptions to this rule occur, but when the animals are located over deeper water, they either starve or feed upon some alternative prey such as the larger - 2 4 - Fig. &.—The general physical boundaries of holarctic walrus distribution in summer. The potential range l i e s within waters of 50 fathoms or less, north of the 5 0 °F monthly mean isotherm, and south of the polar pack edge. -25- plankters and/or other marine mammals (Chapsky, 1936; Vibe, 1950; Zalkin, 1937). The type of ice used as a haul-out seems to be of l i t t l e importance except that i t s upper surface should be close enough (2 feet or less) to the water line for ease of access, and i t must be strong enough and large enough to hold i t s potential occupants. Of the physical properties of land haul-outs, beach materials appear to be of no conse- quence, at least in the Bering-Chukchee and Kara Sea (Chapsky, 1936) regions. Anything from boulders to sand or turf i s suitable. So far as the writer i s aware, however, regular land haul-outs are generally located close (0-5 miles) to 10-fathom water and near or adjacent to high promontories. The latter probably serve as landmarks for navigation, since most of the Alaskan west coast north of l a t . 60°N i s elevated only slightly above sea level and i s quite invisible from a few miles offshore. A swimming walrus, whose horizontal line of vision i s highly restricted, would, i f he were in need of a rest, be expected to swim towards any visible i n d i - cation of land, viz. a geographical feature jutting above the horizon. Similarly, the consistently close relationship between the hauling grounds and 10-fathom water i s probably a matter of convenience, since i t i s more l i k e l y that the animals would find their way back to the same beach i f the feeding area was not too far distant from i t . The southern limit to distribution appears to be - 2 6 - defined by temperature, though i t i s not possible at present to decide whether t h i s i s cause or e f f e c t . On the basis of food d i s t r i b u t i o n , walruses would be expected to occur south- ward along the coasts to C a l i f o r n i a , China, France, etc., yet within h i s t o r i c times they have been very exceptionally re- corded south of 65°N l a t . i n Europe (Mohr, 1940), 52°N l a t . i n the North P a c i f i c ( A l l e n , 1880; Turner 1886; Golder, 1922) and 43°N l a t . i n eastern North America ( A l l e n , 1380; Dunbar, 1954)* An examination of the recent climatologieal data f o r Alaska (U.S. Weather Bureau, 1951-54) reveals that the monthly mean temperatures experienced by walruses near the coastal and i n s u l a r stations are between 0°and 50° F, the mode being about 20°F. Daily extremes of up to 30°F may occur i n the southern fringes of t h e i r present range ( i . e . B r i s t o l Bay), but the majority probably never experience d a i l y temperatures warmer than 50o-60°F. South of the Alaska Peninsula, p a r t i c - u l a r l y along the "Panhandle" and northern B r i t i s h Columbia, monthly mean winter temperatures (15°-40°F) appear to be we l l within the range of walrus tolerance, but i n summer they are s l i g h t l y higher (50°-60°F). I f temperature i s the l i m i t i n g factor as i t seems to be, then the southern l i m i t of d i s t r i b u t i o n appears to be at about the l e v e l of the 50° July isotherm (Fig. 8). Certainly a l l of the present world populations of walruses occur well within t h i s boundary. Migrations The r e g u l a r i t y and apparent purposiveness with which -27- the seasonal migrations occur indicate that there i s an innate and/or learned behaviour pattern directing the process. Al- though ice movements, influenced by winds and surface currents (Fig. 9 ) , tend to parallel the peregrinations of walruses, they evidently are not directly involved in the causative mechanism, for i t has frequently been observed that animals w i l l swim towards their goal when ice i s in apposition. Ice i s often u t i l i z e d as a haul-out during migration regard- less of the direction in which i t i s oriented, but the writer's impression from observations at Gambell is that floes proceed- ing northward in spring are more often occupied than those which are drifting southward. As Brooks (1954) and Nikulin (1940) have noted pre- viously, females are the most actively migratory, males in general being more apathetic and subject to irregularities of ice d r i f t . Undoubtedly this accounts, in part at least, for the occurrence of bull herds in the southern extremes of summer range, for these lingering animals are found in regions where sea ice persists long after the rest of Bering Sea i s icerfree. At St. Lawrence Island, for example, a few bulls can usually be found amongst the brash and floes caught in certain north-side bays two or three weeks after the last migrants and d r i f t ice have passed Gambell. This, most cer- tainly, i s the explanation for bull aggregations on the Sibe- rian coast in summer, judging from the literature, and though there are no specific data on this phenomenon for the Bristol -28- Fig. 9.—Surface currents: Bering, Chukchee, Beaufort, and East Siberian Seas. Data from Goodman, et al (1942), LaFortd (1954), Mineyev (1945), and Sverdrup (1928). -29- Bay herd, i t i s presumed to be the case there as well. Per- haps the choice of certain land haul-outs after the ice has melted i s partly due to learning as well as the other factors mentioned above. East-west limits of the summer range appear to be governed by the Polar pack, the direction of ice d r i f t , and food. Except in late summer and early autumn, the passages along the Siberian coast at Cape Shelagsky and the Alaskan coast at Point Barrow are largely blocked by the pack. There are open leads and channels through which the animals could pass to the west and east, but in the latter instance off- shore currents and food conditions appear to be relatively adverse. Recent oceanographic investigations (LaFond, 1954; W. Cameron, voc. com.) indicate that along the Arctic coast of Alaska there may be a weak coastal current towards the east, but offshore the tendency i s for clockwise rotation in the Beaufort Sea with a convergence of easterly and westerly currents at about the region of Point Barrow (Fig. 9 ) . The latter, together with general clockwise d r i f t of the Polar pack (Crary, et a l , 1952; Koenig, 1952) would be expected to direct walrus movements towards the north and west, rather than east, once they had reached Barrow. According to the Eskimos of Wainwright and Barrow (R. Gray, in l i t t . ) , this i s the observed pattern of their movements. Several observers (Anderson, 1913; Bailey, 194&; McAllister, 1953; F. Neave, in l i t t . ) have noted that birds -30- and seals are scarce along the Alaskan coast east of Barrow, and surveys in 1950-51 (M.W. Johnson, in l i t t . ) have revealed many more larvae of benthic invertebrates in the Chukchee than in the Beaufort Sea, though this apparent difference is not necessarily a constant one (F. Neave, in l i t t . ) . . A l l of these indicate a much lower productivity east of the "Barrow convergence," presumably due to physico-chemical properties of the water (the eastern Chukchee i s warm and saline; the Beaufort cold and brackish, according to Tully, 1952). In the west there appear to be similar current com- plications (Fig. 9 ) , though Sverdrup (1928) concluded that ice d r i f t along the North Siberian shelf was due entirely to wind and that no permanent currents existed. Walruses, in small numbers at least, do penetrate the East Siberian Sea, according to Nikulin (1940) and Ognev (1935). The pelecypod fauna, however, seems to be rather poor in comparison to the Chukchee (Soot-Ryen, 1939), and may be another factor controlling walrus abundance in that region. Range Reduction There has been much speculation upon a cause for the more restricted distribution of the present population as compared to the past. There are three general theories: (a) overall decimation of the population, (b) decimation of the more accessible elements of i t , i.e. the southern herds, and (c) climatic changes. In a l l probability i t has been -31- brought about by a l l of these and possibly some other more subtle ones. The "shrinkage" i s evident in a l l of the former fringe areas. Some animals are occasionally seen in most of them but they are less abundant than they were 100 years ago, indeed 50 years ago. This of course, points to overall destruc- tion. The disappearance of the Pribilof summering herd was diagnosed by E l l i o t t (1882) as the work of commercial ivory hunters, and no doubt this was the fate of the St. Matthew herd as well. If i t were not, they would have been expected merely to decrease in quantity, rather than disappear completely. Finally, climatic changes would bring about alterations in the physical and biological properties of the environment, the effects of which would be most noticeable on the fringes of the range. There are some indications of climatic ameliora- tion during the past 2-300 years (Dunbar, 1954) which may have resulted in a slight northward shift of the southern temperature tolerance limit of walruses. PART II LIFE HISTORY e -33- The basic features of l i f e history are fundamental to a study of animal population, for they are the mechanisms of productivity and reflections of environmental resistance. The most significant recent contributions to Pacific walrus l i f e history are those of Brooks (1954), Freimann (1940), and Nikulin (1940), to which might be added the papers of Collins (1940) and Heinrich (1947), whose observations offer many helpful clues to the unsolved problems. Chapsky (1936), Vibe (1950) and others have contributed basic information on Atlantic walruses. These, plus many lesser contributions, are largely supplementary to one another, each adding struc- tural members to the framework of knowledge; yet, among them there i s a degree of disunion which renders the structure unstable. In an effort to rebuild and strengthen this frame- work, the writer expended most of his f i e l d time and effort in the pursuit of empirical proof of earlier theories and points of dissension. Much of the information on breeding and birth has been gained from examining reproductive organs, the bulk of which were collected by Eskimo cooperators prompted by monetary compensation. These specimens have also contributed much to the analysis of growth, supplemented by a large series of other morphometric data secured by the writer and James W. Brooks. Except in the following paragraph, a l l age -34- designations are based upon the writer's interpretation which i s elaborated in the section on "GROWTH" (p. 77 et seq.). REPRODUCTION Sexual Maturity- Males.—The Eskimo hunters at Gambell stated that bulls are sexually mature at four years of age, and Vibe (1950) reports a similar opinion among Greenlanders in the Thule d i s t r i c t . Chapsky (1936) suggests maturity at five, while Belopolsky (1939), Freimann (1940), Freuchen (1935), and Murie (in Asdell, 1946) say five or six. Brooks (1954) i s non-committal, but doubts that puberty i s reached in the f i f t h year. Perhaps each of these authorities i s correct within his own realm, since the basic difference between them l i e s not in the determination of which bulls are mature but in assigning an age to the animals examined. - The materials u t i l i z e d in the present approach to this problem consisted of 106 testis pairs, each of which was accompanied by tusk measurements or tooth specimens for age reference. These were prepared in the f i e l d by removing the tunica vaginalis and epididyrays after which the main testicular body was measured to the nearest millimeter and weighed to the nearest gram. Tissue samples from these, excepting the majority of calves', were fixed in a formol- acetic solution and later sectioned for histological exam- ination. Table 1 l i s t s the resulting data on spermatogenesis. - 3 5 - TABLE 1 A HISTOLOGICAL COMPARISON OF TESTIS DEVELOPMENT WITH AGE AND SEASON Spec. No. Age Date Spermatogenic Cell Types Sptg P.Spc S.Spc Sptd Spz Sert Giant A- 73 0 May X X X A- 72 1 June 2 X X X A- 23 2 May 23 X X X X A- 26 2 May 24 X X X X A- 32 2 May 25 X X X X A - 4 5 8 3 May 11 X X X X X X X A - 3 0 3 3 May 28 X X X X A - 3 2 4 3 June 2 X X X X X X X A-106 3 June 8 X X X X X A - 2 7 7 4 May 25 X X X X X X X A - 2 4 8 5 April 2 6 X X X X X X X X X X X X X X X A -272 5 May 15 X X X X X X X X X X X X X X A - 2 8 0 5 May 25 X X X X X X X X X X X X A -318 5 June 2 X X X X X X X X X X X X X X A-266 6 May 12 X X X X X X X X X X X A- 1 4 6 May 21 X X X X X X X X X X X X A- 28 6 May 24 X X X X X X X X X X X X X X A -276 6 May 25 X X X X X X X X X X A- 6 4 6 June 2 X X X X X X X X X X X X X X A-322 6 June 2 X X X X X X X X X X X X A - 2 4 4 7 April 20 X X X X X X X X X X X X X X X A- 1 0 7 May 2 1 X X X X X X X X X X X X X X A - 2 7 8 8 May 25 X X X X X X X X X X X X X X A - 2 7 9 8 May 25 X X X X X X X X X X X X X X A - 2 4 3 9 April 2 0 X X X X X X X X X X X A- 3 1 9 May 25 X X X X X X X X X X X X X X A - 2 8 2 10 May 24 X X X X X X X X X X X X X X A-246 >10 April 22 X X X X X X X X X A -245 >10 April 23 X X X X - X X X A - 2 5 4 >10 April 24 X X X X X X X X X A -253 >10 April 26 X X X X X X X X X X X A - 2 6 0 >10 May 12 X X X X X X X X X X A- 29 >10 May 2 4 X X X X X X X X X X A -275 >10 May 25 X X X X X X X X X A - 2 8 3 >10 May 25 X X X X X X X X X X X X X X A - 2 8 4 >10 May 25 X X X X X X X X X X X X X A - 2 8 5 >10 May 25 X X X X X X X X X X A- 7 1 >10 June 2 X X X X X X X X X X X A -122 >10 June 6 X X X X X X X X X X X X *Sptg - spermatogonia; P.Spc - primary spermatocytes; S.Spc - secondary spermatocytes; Sptd - spermatids; Spz - spermatozoa; Sert - Sertoli cel l s ; Giant - giant cells x r few; xx - common; xxx - abundant -36- While the presence of sperms in the seminiferous tubules i s not necessarily an indication of breeding a b i l i t y , i t at least establishes a limit below which no breeding can occur. The youngest animals with sperms in their testes were the 5-year-olds, though i t may be argued that the single 4-year-old specimen i s not a sufficient representation of his age class. Ninety-four per cent of the specimens from five to ten years, inclusive.,were fecund (fecundity i s used here in reference to the presence of spermatozoa, hence the potential a b i l i t y to produce mature gametes), while those over ten years were only 16.5 per cent fecund — a highly significant difference in proportions. The seminiferous tubules of the older animals were mainly characterized by empty lumina and abundant Sertoli c e l l s , which are associated with senility and cryptorchidism in humans (Maximow and Bloom, 1948,.p. 521). From these data i t has been concluded that the potentially effective population of breeding males prob- ably consists of animals under fifteen years of age. Possi- bly the apathy in migration exhibited by old bulls i s corre- lated with their relatively degenerate gonads. The Bristol Bay summering herd, for example, appears to be mainly old males. F u l l maturity appears to occur between six and eight years, judging by Freimann's (1940, Table 5) analysis of progressive development of secondary characters (thickened and "warty" skin), and i t i s further indicated by the attain- -37- ment of f u l l adult t e s t i s dimensions by age s i x or seven (Fi g . 10 and Table 2). There are no indications of seasonal fluctuations i n t e s t i s s i z e , though t h i s p o s s i b i l i t y has not yet been thoroughly investigated. Females.--The l i t e r a t u r e i s i n e s s e n t i a l l y the same state of disagreement about sexual maturity of the cows as i t i s of the b u l l s , but again i t i s l a r g e l y a matter of age determinations. The writer's material consisted of twenty- eight reproductive t r a c t s from animals one year old or older, plus a large sample (51) from newborn calves for comparison* In the f i e l d the u t e r i were measured and examined for evidences of placental scars or recent implantations, a f t e r which they were discarded. The ovaries were preserved i n formaldehyde and were l a t e r sectioned i n the laboratory into s e r i a l 3mm.- thick s l i c e s f o r macroscopic inspection. Brooks supplied twenty-two additional pairs for t h i s phase of the work. Of a l l the c a l f ovaries, only two bore f o l l i c l e s of macroscopic dimensions, those being about 0.5 to 1 mm. i n diameter and very sparsely d i s t r i b u t e d . Two yearling pairs were comparable to the c a l v e s 1 , only a few f o l l i c l e s of 1 to 3 mm. diameter being v i s i b l e . In the four pairs of 2- year-old ovaries considerable advancement was apparent, with large numbers of f o l l i c l e s up to 6 mm. i n diameter crowded into the cortex and appearing externally through the tunica as t small translucent bubbles. A t r e s i a , indicated by cloudy l i q u o r f o l l i c u l i ( A l l e n , et a l , 1930), was very prevalent, Fig. 10.—Average testis weight growth of Pacific walruses. Open circles are mean values given in Table 2 . - 3 9 - TABLE 2 TESTIS LENGTHS AND WEIGHTS Age Length I mm.) Weight (grams) (years) No. Range Mean ± S.E. No. Range Mean ± S.E. 0 64 60 - 88 73.2 ± 0 . 3 64 13 - 2 6 18.4 ± 0.1 1 3 73 - 91 82.5 ± 5.3 2 20 - 29 24.5 ± 4.5 2 5 1 0 5 - 112 109.2 ± 1 . 2 5 44 - 58 50.4 ± 3.0 3 4 113 - 135 127.0 =b 5.2 4 64 - 98 39.0 ± 8 . 4 4 1 130 135 - 170 1 130 115 - 275 5 4 155.0 ± 8 . 4 4 197.5 ± 1 8 . 2 6 6 150 - 197 176.5 ± 7 . 7 6 191 - 322 265.2 ±19.6 7 2 178 - 195 186.5 ± 8 . 5 2 250 - 415 332.5 ± 8 2 . 7 8 2 203 - 220 212.5 ± 8 . 5 2 325 - 3 3 0 327.5 =*= 2.5 9 2 2 0 5 - 206 205.5 ± 0 . 7 2 408 - 433 420.5 ±12.6 10 1 2 2 5 153 - 227 1 417 2 5 6 - 565 >10 12 190.3 ± 7 . 0 12 399.4 ±24.5 -40- about one-third of the visible f o l l i c l e s being in various stages of degeneration. The only pair of 3-year-old ovaries examined by the writer was identical in appearance to the 2-year-olds 1, the largest f o l l i c l e measuring 5 mm. in diameter (as opposed to the 10-30 mm. diameter of Graffian f o l l i c l e s in adults). Four others of this age recorded by Freimann (1940, Table 3) bore no embryos in the uterus, and thus had probably not ovu- lated, though he does not specify this. Within the 4-5 year group a l l the animals examined by Brooks and the writer were either newly impregnated or had borne their f i r s t calf in the current season. The young- est pregnant animals recorded by Freimann also f a l l within this group. Thus i t i s probable that most of the females are sexually mature at 4 years of age, though some may not mature u n t i l later. Lowered productivity in later years has been briefly demonstrated in tabular form by Freimann (1940, Tables 2 and 3) and exemplified by Eskimo reports that "very old" females are occasionally barren. Four-animals which f i t t e d the l a t - t e r f s description (i.e., tusks heavy and worn short, yellowed, and cracked) were examined by the writer, two of which had borne calves in the current season, the third had borne one the previous year and was not oestrus, and the fourth dis- played a fresh corpus luteum verum in one ovary though no implantation was macroscopically evident. The abundance of -41- developing f o l l i c l e s in the ovaries of the f i r s t was compar- able to younger adults; in the second and third they were sparse; and in the last only three could be found, each of which was atretic and measured less than 1 mm. in diameter. These specimens are mentioned only as examples of relative ovarian histology, for the proportion of f e r t i l e to i n f e r t i l e animals suggested by them i s not representative of old animals in general (see below, p. 57 et seq.). Their non-representa- tion i s due to Gambell hunting policy, which stresses the capture of only those cows accompanied by newborn young. Breeding Season and Location The only direct observations of copulation are a few recorded by Brooks (1954) for May and June in Bering Strait, and July at Barrow. Russian literature contains no mention of breeding observations. Information solicited by questionnaire from Nunivak Island, Hooper Bay, Shaktoolik, King Island, Point Hope, and Wainwright yielded negative reports, and personal interviews with natives at St. Lawrence Island, Wales, Diomede, Point Hope, and Wainwright also were negative. Near a herd of cows and young on May 21, 1954, (lat. 67 °56»N, long. 16S°43'W) Ryder (1954) observed an " adult bull which was decidedly more sluggish than others in the v i c i n i t y , and which lay on i t s back on an ice floe u n t i l the ship approached to within f i f t y feet. "Since the penis of this bull was partly unsheathed, ... he may have been spent from breeding efforts, although no females were noted -42- alongside him." The i n d i r e c t evidences of breeding season a r e , f o r - t u n a t e l y , more numerous. These may be c l a s s i f i e d i n t o f o u r groups: (a) t e s t i c u l a r development, (b) o v a r i a n c y c l e s , (c) f o e t a l growth, and (d) b i r t h dates. T e s t i c u l a r development.—During the p e r i o d A p r i l 20 t o June 6 there was no s i g n i f i c a n t change i n the p r o p o r t i o n of fecund t o non-fecund males (2:1). Nevertheless, the production of "giant c e l l s " (Maximow and Bloom, 1948, p. 521) i n l a t e May and June by a few specimens (Table 1) i s b e l i e v e d to be an i n d i c a t i o n of seasonal degeneracy, a c o n d i t i o n f u r - t h e r i n d i c a t e d by Brooks' (1954) and Chapsky's (1936) s p e c i - mens c o l l e c t e d l a t e r i n the summer (June through September). I t seems probable that the t e s t e s undergo an annual c y c l e of spermatogenic a c t i v i t y w i t h peak production i n the s p r i n g months. Ovarian c y c l e s . — S i x t e e n p a i r s of o v a r i e s from oestrus and e a r l y post-oestrus females were examined. These (Table 3) show a d e f i n i t e t r e n d of o v u l a t i o n date v a r i a b i l i t y w i t h age. By June 1, plus or minus a week, the youngest (under ten years) and o l d e s t (over twenty years) animals had already ovulated at l e a s t once; a l l but one had formed a corpus luteum verum; and three had implanted embryos of mac- ros c o p i c s i z e . Conversely, none of the "middle-aged" females had ovulated i n the l a s t week of May. A f u l l d i s c u s s i o n of f a c t o r s a f f e c t i n g age-ovulation sequence i s given below (p. 64 )• In general these data suggest t h a t most of the -43- TABLE 3 COMPARATIVE OVARIAN HISTOLOGY BY AGE AND SEASON Specimen Number Age Date Ovarian Structures Vi si bl e Em br yo  Ri pe ni ng  Fo ll ic le s Co rp us  Lu te um  Sp ur iu m Co rp us  Lu te um  Ve ru m Vi si bl e Em br yo  B-201 4 May 24 X X B-15A 4 June 2 X X B-214 5 May 30 X B-35A 5 June 7 X B-34A 5 June 7 X X B-123 5-6 May 22 X B-12Q 5-6 June 2 X X B-117 7 May 22 X X X B-17A 7-8 June 2 X X B-25A 7-8 June 5 X A-42 >10 May 26 X B-114 13 May 22 X B-113 17-18 May 22 X B-200 20 May 24 X B-102 >20 May 22 X X A-304 >20 May 28 X -44- breeding occurs from April to June, inclusive. Foetal growth.—Belopolsky (1939), Brooks (1954), and Chapsky (1936) recorded the crown-rump lengths of six- teen separate embryos and foetuses taken in summer and f a l l ' and estimated the conception dates for a few. Ut i l i z i n g the measurements given by these authors, the writer has calculated the probable dates of conception (Table 4) by comparison with equivalent stages of human pre-partum growth (Appendix I ) . The results indicate that most were conceived prior to mid- May. In calculating the mean conception date and length of gestation period, the data from Brooks' specimens have been omitted. His were taken during the breeding season and apparently are representative of only the earliest breeders, for several other f e r t i l i z e d females taken at the same time had no macroscopic embryos. While there may be serious objections to this tech- nique on the basis of the animals compared (i.e., humans and walruses), i t s apparent correlation with observed ovarian development suggests that i t is at least a close approxima- tion of the truth. Birth dates.—Though the length of the gestation period has not been precisely established, i t has been cal- culated to be about 367 days (Table 4 ) . On this basis, four newborn young and two f u l l term foetuses examined by the writer could have been conceived about May 8, 11, 18, 19| 21, and 22. But the value of birth dates i s mainly found in their chronological frequency which i s an indica- -45- TABLB 4 CALCULATED CONCEPTION DATES OF ATLANTIC AND PACIFIC WALRUS EMBRYOS Source CD •p CO o c o •rH •P o CD o o a s 6 o C 43 S bO o c CD 3 fa cd J 3 43 I W3 C <D •4 +3 C I CD O o S u u CD CD fa E-« IX) CO X) 43 CD CO +3 CD CD O rH a +3 6 c o CD O O c J H O CD - H fa +3 O X! 43 I u rH - H O 0Q O >•> O CO CD Q rH CO CO 43 >, O CO T3 CD • 43 43 CO CO rH CD SO o rH CO O CD c co o >»>iH CO 43 CO O r H 3 CD O O rH G CO «H O CO CD 43 CO O G o • r l 43 a. CD a c o o Brooks(in l i t t . ) Brooks(1^541 Belopolsky(1939) Chapsky(1936) May 22 June 15 June 16 July 7 July 7 July 8 July 13 Aug. 19 Aug. 23 Aug. 24 Aug. 28 Aug. 29 Aug. 30 Sept. 6 Sept. 6 Oct. 12 Oct. 12 0.6 2.3 1.7 2.3 2 .7 2.8 5.2 11.0 22.0 26.5 18.0 17.5 16.0 18.0 18.0 30.0 40.0 0.59 2.25 1.67 2.25 2.65 2.74 5.1 10.8 21.6 26.0 17.6 17.5 15.7 17.6 17.6 29.4 39.2 9.2 14.1 13.0 14.1 14.9 15.0 18.2 24.0 32.2 35.2 29.4 29.3 28.0 29.4 29.4 37.5 44.5 374 333 332 309 309 310 300 268 264 263 259 258 257 250 250 214 214 412 388 382 360 363 365 368 353 390 406 367 365 357 354 354 342 386 35 52 48 52 55 55 67 88 118 129 108 107 103 109 109 137 162 April 17 April 24 April 30 May 16 May May May May May May May May May May Mean of 14 (Belopolsky, Chapsky)f 366.6 13 14 12 23 April 27 April 17 May 12 14 19 20 20 28 3 May 13 fj F u l l term crown-rump length = about 102 cm. (average of 6) D Extrapolated from curve of equivalent foetal growth (App. I) c Average birth date = May 14 d Days to birth x Q̂O Per cent gestation to be completed e (Mean of d) x P e r c e n t o f gestation completed See text p. 44 -46- tion of the time range during which breeding occurs. Since this subject i s discussed in detail below (p. 52 ) i t i s sufficient to state here that the majority of births occur during May, but occasionally a few may be as early as Feb- ruary or March and as late as July or August. The mode appears to be slightly before the middle of May. In summary, the few direct observations suggest that the breeding season extends from May to July, but indirect techniques indicate that most of the breeding occurs from April to June. During the latter period the animals are engaged in their annual northward migration and may be found anywhere between latitudes 60°and 70°N in the Bering and Chukchee Seas. Breeding Behaviour The social organization of walrus breeding activity has been speculated upon for a very long time. In most instances i t was presumed to resemble that of i t s otariid cousins, which are characterized by a form of organized polygamy usually referred to as "harem breeding." The principal bases for this assumption were: (a) the apparent disparity in size between sexes, and (b) the occurrence of large on-shore herds at certain l o c a l i t i e s . Nikulin (1940) notes that "/"Among mammals not a single systematic group i s known where polygamy does not go hand in hand with sharply defined sexual d i m o r p h i s m , b u t he failed to recognize that in a relative sense, the dimorphism displayed -47- by walruses i s i n s i g n i f i c a n t when compared to the o t a r i i d s or to the harem-breeding phocids. The walrus i s , i n f a c t , a sort of intermediate between the r e l a t i v e monomorphism of most phocids and the strong dimorphism of o t a r i i d s and elephant s e a l s . This i s demonstrated i n Table 5 and i s p a r t i c u l a r l y evident i n the c o n t r a s t i n g forms of t h e i r r e s p e c t i v e growth curves ( F i g . 24, p.107). According to N u t t i n g (1891, p. 103), ...polygamy...can p r o p e r l y only apply to those species i n which a s i n g l e male h a b i t u a l l y copulates w i t h s e v e r a l females, and j e a l o u s l y and p e r s i s t e n t l y defends them from the approach of other males. Bertram (1940) has observed that t h i s polygyny could only e x i s t where the animals are hauled out on la n d , since i t would be impossible to defend a harem from i n t r u d e r s when i n the water. P a c i f i c walruses do not h a u l out on land i n the s p r i n g when they breed, but are i n the process of a c t i v e south-north m i g r a t i o n , spending much of t h e i r time swimming and the remainder on the i c e . I t i s obvious t h a t such a schedule does not lend i t s e l f to harem maintenance. F u r t h e r - more, an i c e f l o e l a c k s long-term s t a b i l i t y and would be d i f f i c u l t to defend unless i t was of very broad dimensions. The w r i t e r ' s observations suggest t h a t , i n the s e l e c t i o n of haul-outs, f l o e s i z e i s of no importance except t h a t i t be l a r g e enough to support the animals. Without exception, persons who have been f a m i l i a r w i t h the P a c i f i c walrus have observed the general tendency f o r monosexual aggregations, but "harems...have not been -48- TABLE 5 COMPARATIVE SEXUAL DIMORPHISM AMONG PINNIPEDS Species A u t h o r i t y a Male Body Size as Per cent of Female Length Weight Phoca v i t u l i n a Phoca hispida Phoca groenlandica, Halichoerus grypus 6 Erignathus barbatus Cystophora c r i s t a t a Mirounga a n g u s t i r o s t r i s " Mirounga l e o n i n a 0 1,8,10 1,10 1,10 1,10 1,10 1,10 12 9,11 100 - 130 100 - 120 110 130 - 135 120 - 145 120 - 125 150 - 190 165 1 0 5 - 110 155 ? ?, 9 about 400 Odobenus rosmarus Odobenus divergens 1,3 1,2,4,5 120 120 ? 150 Otaria byronia1-* Eumatopias jubata 1 3 Zalophus c a l i f o r n i a n u s 0 Callorhinus ursinus , Arctocephalus a u s t r a l i s 6 1,7,11 8 11 1 130 140 135 150 135 1 380 - 400 5 0 0 480 ? a 1-Bertram (1940), 2-Brooks (1954), 3-Chapsky (1936), 4-Fay, 5-Freimann (1940), 6-Hamilton (1934,1939), 7-Kenyon (1952), 8-Kenyon, et a l (1954), 9-Laws (1953), 10-Mohr (1952), 11-Scheffer and Wilke"Tl953), 12-Townsend (1912). Known to be polygynous -49- noticed even by those people who report seeing copulating walrus" (Brooks, 1954, p. 52). Both sexes are highly gre- garious throughout the year, the cows forming up into groups whether b u l l s are present or not. The fact that b u l l s may encounter and join such groups does not constitute harem formation, f o r more than one b u l l may accompany a small cow herd. During the cruise of the U.S.S. Burton Island i n May, 1954, Ryder (1954) recorded 57 separate groups of walruses between the Alaska Peninsula and Point Barrow, of which at least 16 were male herds, 18 were female herds, and 12 were bisexual (considering only the a d u l t s ) . The mean r a t i o of males to females i n the l a t t e r was about 1:1, except i n herds of f i f t y or more ind i v i d u a l s where cows predominated. In the few bisexual groups observed by the writer near Gambell, the adult sex r a t i o was also estimated as 1:1. Considering the circumstances under which breeding probably occurs, i t seems most l i k e l y that males encounter females p a r t l y by chance and that no harem organization can e x i s t . The so-called "breeding rookeries" referred to by Hanna (1923) were a l l - b u l l herds, while Eskimo and popu- l a r reports of breeding at Cape Lisbourne and the Punuk Islands seem untenable, since they occur i n l a t e summer and f a l l during anoestrus. Under the conditions noted above, a single b u l l may copulate with several cows, but i t i s also conceivable that - 5 0 - a cow could be serviced by more than one b u l l . Thus, unless future investigations of walrus social behaviour prove other- wise, i t must be concluded that the Pacific walrus i s pro- miscuous, bordering on polygamous — an intermediate between phocids and otariids. This social pattern may stem from an ancestral stock of on-shore harem breeders which were more sexually dimorphic than the modern genus. Certainly there i s a strong potentiality for that form of behaviour to develop again should walruses be restricted to land haul-outs during the breeding season (e.g. by a warmer world climate). Copulation has been observed very infrequently, and there are no descriptions of i t . Brooks (1954) records one occurrence on the ice and states that the Diomede Eskimos occasionally witness i t in the water. It may seem incredible that the breeding act would be so seldom encountered i f i t was a common phenomenon during spring migration when the herds are so much in view, but to anyone familiar with these animals, i t i s no surprise. When they are on the ice, they l i e upon, beneath, and between one another in such a mixture of bodies and appendages that i t i s exceedingly d i f f i c u l t for the observer to distinguish entire individuals, let alone sexes u n t i l they are alarmed by shots or by the nearness of hunters. In the melee that follows, the hunter's mind i s generally occupied with thoughts of procuring meat and ivory rather than with satisfying his curiosity. In addition, i t i s probable that copulation i s a -51- very brief process, the chances being exceedingly slight that i t would be observed whether on ice or in the water. In his studies of the northern elephant seal, Mirounga an- gustirostris. Bartholemew (1952) concluded that the main advantage of a large baculum or os priapi l i e s i n the speed with which intercourse can commence. Because of the struc- tural support offered by the baculum, no preliminary erection i s necessary, and sexual contact may be accomplished as soon as a receptive female i s found. Coitus i s completed in three to seven minutes by the elephant seal (Bartholemew, op. cit.) and about six minutes by the Alaska fur seal, Callorhinus ursinus, (Bartholemew and Hoel, 1953). Since about two-thirds the bulk of a walrus penis i s baculum, i t s copulatory activity may be equally as brief and incon- spicuous. Gestation Estimates ranging from ten months (Vibe, 1950) to one year (Brooks, 1954, and others) have been suggested as the probable length of the gestation period. The writer's interpretations tend to support the latter. U t i l i z i n g data from the sixteen embryos mentioned above (Table 4 ) , the mean calculated gestation i s about 367 days. Although delayed implantation seems to be the rule with other pin- niped groups (Fisher, 1954), there i s no evidence of i t s occurrence among walruses. Nevertheless, i t cannot be ruled out u n t i l further data have been obtained to demonstrate i t s -52- absence. The corpus luteum verum remains large and f u l l y luteinized throughout the gestation period. Degeneration and connective tissue invasion appears to begin immediately after parturition. Birth Season.—Opinions expressed by previous investigators are in general agreement that the young are mostly born between April and June, with occasional reports for other seasons. These have been mainly based upon implication and Eskimo statements. The earliest birth recorded by the writer was inferred by the condition of a calf taken at Gambell on May 13, 1953. In color i t was dark brown; the umbilicus was completely healed; and the points of the tusks were exerting pressure on the gums. Comparing i t to Chapsky's (1936) and Nikulin*s (1940) descriptions of young observed in mid and late summer, the animal was considered to be not less than two months old, placing i t s birth date in late February or early March. Such early births are apparently not common, for i t was exhibited as a sort of curiosity by the hunter who had k i l l e d i t . Although several full-term foetuses were recorded in April (¥. Caldwell, in l i t t . ) , no newborn balves were seen near Gambell from 1952 to 1955. Charles Slwooko, a local resident, stated that he heard one barking on April 23, -53- 1953, and Nikulin also reports that a Cape Chaplino Eskimo found evidences of parturition in April. At least two out of twenty calves examined by the writer could have been born in that month. Newborn young and fresh afterbirths were observed by Ryder (1954) on May 2 and again on May 21 amongst large herds of females, and the bulk of those observed and examined near Gambell appeared to have been born in the f i r s t three weeks of May. No full-term foetuses were observed at Gambell after May 26, and they are rare in the experience of Wales and Diomede hunters (late May, early June). Calculated parturition dates of nineteen females taken in May and June range from April 13 to June 1 with a mean of May 14 (Table 6 ) . Births in seasons other than spring are occasional, but probably cannot be considered common. Vibe (1950) states that the West Greenlanders talk of small calves being seen in any season, though he saw none except in May and June. Diomede Eskimos speak of " f a l l babies" (Heinrich, 1947), and R.F. Gray (in l i t t . ) reports that "every year there are cases of calving in the summer and f a l l " noted by the people of Wainwright, Alaska. Heinrich also has "seen one in autumn that was too small to have been born the previous spring," while Zalkin (1937) observed a 96 cm. foetus in July. One taken at Gambell in February, 1953, was judged to be no more than six months old. The probable explanation for early and late births appears to be closely connected with breeding frequency -54- TABLE 6 PARTURITION DATES Sp ec im en  Nu mb er  CO Co ll ec ti on  Da te  Es ti ma te d Da ys  S in ce  Pa rt ur it io n*  Parturition Date Sp ec im en  Nu mb er  Co ll ec ti on  Da te  Es ti ma te d Da ys  S in ce  Pa rt ur it io n*  Range Median A-271 5 May 15 5 - 10 May 5 10 May 8 A- 15 5 May 22 10 - 20 May 12 - 17 May 15 A- 24 5 May 24 (Foetus) May 24 - 26 May 25 A-289 7 May 28 30 - 60 March 28 - April 28 April 13 A-258 7 May 12 2 - 5 May 7 - 10 May 9 A-261 7 May 13 0 - 2 May 11 - 13 May 12 A-305 7 May 28 5 - 10 May 18 - 23 May 21 A- 58 7 May 26 2 - 5 May 21 - 24 May 22 A- 59 7 May 26 0 - 2 May 24 - 26 May 25 A- 65 7 June 2 0 - 2 May 31 - June 2 June 1 A-267 8-9 May 15 0 - 2 May 13 - 15 May 14 A-93 9 June 7 10 - 20 May 18 - 28 May '23 A-256 10 May 12 0 - 2 May 10 - 12 May 11 A- 30 11 May 25 (Foetus) May 25 - 27 May 26 A-102 >20 June 8 30 - 60 April 9 - May 9 April 24 A-273 >20 May 12 0 - 2 May 10 - 12 May 11 A-263 >20 May 13 0 - 2 May 11 - 13 May 12 A-101 >20 June 8 20 - 30 May 9 - 19 May 14 A- 2 >20 May 19 2 - 5 May 14 17 May 16 Mean May 11 - 17 May 14 *Based on post-partum regression of uterine horn plus general condition of the young. -55- phenomena, discussed below (p. 57 .et seq.). Behaviour at parturition.—Reports from the l i t e r - ature and Eskimo observers lead to the conclusion that birth occurs on the ice, rather than, as Freuchen (1935) suggests, in the water. So far as the writer i s aware, the process has been witnessed only once by white observers and very infrequently by Eskimos. This scarcity of records i s prob- ably due to the rapidity with which the act i s accomplished, the chances for observation being very slight. Matthews (1952, p. 61) describes the birth of a southern elephant seal thus: I was looking at a pupless cow when suddenly, she gave a convulsive heave and the baby shot out like a torpedoe being launched, the mother at the same time lashing her hind end about and swinging the body around so that the umbilical cord was broken leaving about a foot of i t attached to the baby. The placenta was born about ten minutes later... Such brief parturitions are also reported for other pinnipeds by Mohr (1952) and Bertram (1940), while Ryder (1954) describes a walrus birth on the ice as follows: She sat more or less upright on her front flippers with her body at right angles to the ship... The pos- terior portion of her body was turned somewhat later- al l y with the ventral side away from the /~observer_7 ... When f i r s t seen, the calf was partly visible, steaming and wet-appearing. As the ship continued to approach, the female squirmed occasionally and looked back at the newborn calf which gradually be- came more vi s i b l e . It lay motionless for a minute or so u n t i l the mother saw the ship and s l i d into the brash. The parent promptly surfaced close by the floe and looked at the calf /~ which__7.. .slowly wiggled to the edge of the floe and f e l l into the brash and water... No sounds were heard from the cow or calf and no umbilical cord attachment or actual severing of same by the female was observed. -56- The posture and general features of this account are similar to another description as told to the writer by Ph i l l i p Campbell, a Gambell native, who had heard i t second or third-hand. His description was approximately as follows: The female sat upright with her head bent down. When the calf emerged with umbilical cord s t i l l attached, the mother cut the cord by rubbing the point of her tusk across i t . She then took the baby into the water for two or three minutes to wash i t off, after which she placed i t on the ice and swam off to feed. The afterbirth of the walrus i s apparently ignored by the mother, as i s that of other seals. Ryder (1954) observed that glaucous gulls (Larus hyperboreus) and k i t t i - wakes (Rissa tridactyla) were actively feeding upon such remanents as well as mucous and feces. Possibly Arctic foxes (Alopex lagopus), which are constantly roaming the ice fields, may also forage upon this material. Twinning.—Among pinnipeds the birth of twins seems to be very rare, in fact Hamilton's (1939) record for the southern sea lion, Otaria byronia, seems to be the only one in existence. Eskimos at L i t t l e Diomede (Brooks, 1954) and at Gambell rarely observe two calves attended by a single cow, and they consider such combinations to be the result of "adoption" by the female of an orphaned calf. Belopolsky (1939) states that "/"some Eskimo hunters claim to have seen two embryos in one u t e r u s 7 " hut none of the Alaskan natives contacted by Brooks or the writer had knowledge of this phenomenon. -57- Uterine dimensions and post-parturn regression.—Uteri examined at Gambell were measured in order to obtain a rough index of growth and post-partum regression. Lengths of the cornua were measured along the anterior curved surfaces from the apex of their external division to the ostea (Fig. 11) . Diameter was measured at the widest point along the length. From these i t was apparent that the uterus reaches f u l l adult size at age three or four, and that complete regression f o l - lowing parturition probably requires 2-3 months (Table 7 ) . The southern sea lion i s considered to accomplish this in about four months (Hamilton, 1939) and the Alaska fur seal in less than three (Enders, et a l , 1946). Average breadths of the zonal placenta scars are also recorded in Table 7. The scars may remain evident for more than a year, by which time they become much reduced and take on an orange hue. After two years they cannot be discerned with certainty, i f at a l l . Breeding Frequency That walruses are basically biennial breeders has been well established by several earlier investigators, notably Brooks (1954), Ghapsky (1936), and Freimann (1940), and their conclusions are upheld by the writer's observations. Bertram (1940) considers that a possible explanation for this phenomenon may be found in the extraordinary length of the gestation period, and that the bulls may become seasonally i n f e r t i l e before the parous cows have time to recover from -5a- Fig. 11.—A partial diagram of the female reproduc- tive organs showing general internal structure (broken lines) and gross measurements u t i l i z e d . Lengths and diameters of both uterine horns were recorded. -59- TABLE 7 POST-PARTUM REGRESSION OF THE UTERINE HORN Estimated Time Breadth of Specimen Since Horn Horn Placental Number P a r t u r i t i o n Length Diameter Scar (days) (mm) (mm) (mm) A-263 0-2 • • • • • • • 150 A-273 0-2 1028 254 100 A-261 0-2 889 203 130 A-256 0-2 864 230 100 A- 59 0-2 870 200 90 A- 58 2-5 840 214 80 A- 2 2-5 813 230 89 A-258 2-5 750 173 110 A-305 5-10 750 170 70 A-271 5-10 698 273 70 A- 15 10-20 610 159 60 A- 98 10-20 • » • • • • • 60 A-101 20-30 • • • • • • • 40 A-102 30-60 318 • • • 35 A-289 30-60 315 95 35 A-249 one year 230 60 25 -60- p a r t u r i t i o n and develop a new G r a f f l a n f o l l i c l e . R.M. Laws a l s o suggests that t h i s may be the cause of "missed pregnan- c i e s " i n the southern elephant s e a l , Mirounga l e o n i n a (Lockley, 1954). According to Dr. A.J. Wood, Animal N u t r i t i o n Labora- t o r y , U n i v e r s i t y of B r i t i s h Columbia, most mammals which breed s h o r t l y a f t e r p a r t u r i t i o n ovulate e i t h e r w i t h i n a few days post-partum or a f t e r about tw o - t h i r d s or more of the l a c t a t i o n p e r i o d i s past. Those species which experience the f i r s t ("post-partum") u s u a l l y are capable of o v u l a t i n g again at the second ( " p o s t - l a c t i s " ) i f f e r t i l i z a t i o n i s u n s u c c e s s f u l . Pinnipeds i n general seem to f i t t h i s p a t t e r n r a t h e r w e l l , the harem-breeders being f e r t i l i z e d at the equivalent of the f i r s t o e s t r u s , pupping and c o p u l a t i n g w i t h i n a week or ten days, and the intermediates and non-harem-breeders at the second. Examination of o v a r i e s from f o r t y a d u l t female P a c i f i c walruses revealed t h a t these animals have three p o t e n t i a l oestrus pe r i o d s : (1) post-partum, a week more or l e s s a f t e r p a r t u r i t i o n , (2) p o s t - l a c t i s I , about one year a f t e r p a r t u r i t i o n ( two-thirds the way through the lc>-20 month l a c t a t i o n ) , and (3) p o s t - l a c t i s I I , about one month a f t e r the l a t t e r . The f i r s t or post-partum oestrus i s very seldom achieved; i t had occurred i n only two (9 per cent) of the twenty-three p a r t u r i e n t specimens examined. Hence, though walruses appear t o be p o t e n t i a l l y annual breeders, there i s some r e t a r d i n g i n f l u e n c e , p o s s i b l y n u t r i t i o n , which -61- does not often permit i t . Perhaps the frequency of annual ovulations i s related to calf survival, for i t has been ob- served (Br'ody, 1945, p. 434) that individual productivity in some mammals varies directly with infant mortality. The only estimate of natal and early post-natal calf mortality which the writer has been able to obtain suggests a rate of about 3-5 per cent per year, similar to the rate of annual ovulations (3-9 per cent, the former from Ffeimann, 1940). The second ovulation (post-lactis I) had been or was being experienced by sixteen (94%) of the seventeen non- parturient specimens (Table 3, p» 43). The calendar date at which this ovulation occurs varies with the age and previous history of the animal. Young females in their f i r s t oestrus (actually not "post-lactis," but equivalent in time and manner) apparently ovulate earliest in the season (March?), so early in fact that they may not be'fertilized, due, perhaps, to a dearth of potent males at that time. Females having one or two previous pregnancies to their credit are next in succes- sion (April-May), and f e r t i l i z a t i o n success appears to be very high (90-100 % ) . A few of the oldest animals (twenty years more or less) also f a l l within this group. Last in succession (late May-June) are the "middle- aged" females, about 10-15 years old, which have experienced three or more previous pregnancies. The proportion of these successfully f e r t i l i z e d i s not known, but there are several indications that i t i s small — perhaps 25 per cent or less. F i r s t , i t has been noted above that there are signs of a - 6 2 - decline in male gametogenesis after late May; hence the chances of meeting a f e r t i l e bull may decrease rather sharply in June. Second, in the proportions of breeding females recorded by Freimann (1940, Tables 2 and 3 ) , none of the 9 to 13-year- olds was pregnant. Third, in the writer's and Brooks' sample of parturient females there i s a notable scarcity of animals between ten and fifteen years of age. If, for some reason, the post-lactis I ovum i s not f e r t i l i z e d , a second post-lactis ovulation takes place about a month later. What proportion of the female population reaches this stage i s not known, but the data suggest that i t involves mainly the youngest and middle-aged adults, the former because post-lactis I was too early and the latter because i t was too late. For the former, post-lactis II occurs at the optimum time in April or May and undoubtedly achieves a high degree of f e r t i l i z a t i o n success, but for the middle-aged females, i f i t occurs at a l l , i t i s probably seldom successful. An idealized schedule of conceptions resulting from a l l three ovulations i s shown in Table 8. From the above material i t i s apparent that Bertram's proposed scheme i s operative, but i t s effect i s triennial rather than biennial breeding. That i s , the middle-aged females, because of their late oestrus, generally "miss" one or two pregnancies and are not f e r t i l i z e d u n t i l at least two years after their last (third) parturition. The delay in ovulation i s probably associated with the long lactation period, for Espe notes (1941, p. 161) that " . . . i n certain -63- TABLE 8 IDEALIZED SCHEDULE OF CONCEPTIONS FROM THREE POSSIBLE OVULATIONS EXPERIENCED BY PACIFIC WALRUSES Age (years) Oestrus Date Relative Quantity Fe r t i l i z e d 4-5 Post-lactis l S a March-April Few Post-lactis I I a April-May Most 5-10 Post-lactis I April-May Most Post-lactis II May -June Few 10-15 Post-lactis l b b June Few Post-lactis I I b July -Aug. Very few 20i Post-lactis l b b April-May Most Post-lactis I I D May -June Few any Post-partum April-June Most (? a The i n i t i a l ovulations, hence not s t r i c t l y "post- l a c t i s . " May actually be 2-5 years post l a c t i s . -64- species...lactation /~may_7 prolong the l i f e of the corpus luteum, and ovulation ceases." It i s apparent that i t s effect i s cumulative (cf. the progressive stages and ages of post- l a c t i s I) and does not result in a "missed pregnancy" u n t i l two or three young have been born in successive biennial cycles. With advancing age and approaching senility, other factors may enter the picture, resulting in lowered ovulation frequency. An example of this has been seen (p. 40) in which an apparently healthy elderly female had borne a calf one year previously but showed no signs of ovulating in the cur- rent season. Prodf of the above breeding frequency hypothesis i s found in the sequence of birth dates (p. 52 et seq.), in the calculated conception dates (Table 4, p. 45) , and in the crude birth rate. If breeding was s t r i c t l y biennial or was partly biennial and partly annual, the crude birth rate should be about 0.5 young per adult female per year. According to Freimann1s (1940) data, however, i t i s between 0.37 (Table 3; 71 animals) and 0.43 (Table 2; 35 animals), while in a series of twenty-one specimens loaned by Brooks i t i s 0.33. These differ significantly from the expected rate, attesting to the strong tendency for other than biennial breeding. Unfortunately, because the writer's and Brooks' (1954) data are so sparse and Freimann's are so ill-organized, i t i s not possible to construct a table of age specific birth rates of the female population. -65- THE YOUNG Sex Ratio Of 139 calves known to have been captured by Gambell hunters in 1952, 53, and 54, the proportion of males to fe- males did not d i f f e r significantly from the expected 1:1 ratio. Nevertheless, males slightly outnumbered females in this and other recorded samples (Brooks, 1954; Nikulin, 1940) about 1.2:1. Morphology At birth the calf i s pale slate gray, and the soles of the fore feet are often blotched with flesh color. The pelage i s sparse and fine, the vibrissae long (up to 3 inches) and slender, and the claws pointed, slender, and soft. Gen- erally no teeth are visible, though small bumps indicate their presence beneath the gums in some instances. Five out of sixteen examined by the writer had one or both of the lower canines partially exposed. Except for external geni- t a l openings, the sexes are identical in size and general appearance. Body measurements are shown in Table 9. Within a few days the young walrus changes to a very dark, almost black color. Gradually, as the weeks pass, this i s transformed into a rich chocolate brown and i s , according to Nikulin (1940), accompanied by a post-natal molt which takes place partly in June and July. These color changes involve skin as well as pelage pigmentation. TABLE 9 CALF BODY MEASUREMENTS* Age Body Length (inches) Hind Foot Length (inches) (months) No. of Speci- mens Range Mean ± S.E. No. of Speci- mens Range Mean ± S.E. Newborn 8 40 - 52 1/4 45.8 ± 1.4 5 11 - 13 3/4 12.8 ± 0.5 0 - 1 25 45 - 52 1/4 48.6 ± 0.4 13 12 1/2 - 14 1/2 13.4 =•= 0.2 1 - 2 21 49 1/4 - 59 1/4 49 - 72 1/2 54 .0 ± 0.5 58.3 =-= 0.7 2 - 3 40 *Data from Brooks (1954), Fay, and Nikulin (1940). -67- Remanents of the umbilical cord remain attached u n t i l July or August (Nikulin, op_. c i t . ; Chapsky, 1936), and by late summer or f a l l the tusks may begin to erupt. For the f i r s t three months body length increases at the rate of about four or five inches per month (Table 9). Precocity The young are probably capable of swimming immediately after birth, though they are usually carried by the mother. Calves no more than one or two weeks old have been observed to swim very well indeed when pursued, often with a leaping and diving motion similar to the otariids. Independently they appear to be unable or unwilling to dive more than a few feet below the surface or to remain submerged for more than one or two minutes. Much of their swimming and diving a b i l i t y in later l i f e may, therefore, be a result of learn- ing. Parental Care The strong maternal bond between cow and calf has been elaborated in great detail by many earlier writers, notably Allen (1880). Aside from i t s remarkable anthro- pomorphic implications, this inseparability of parent and offspring i s of particular significance as a survival mechanism. Food and protection appear to be the primary effects. -68- Food relationship.—The young feed exclusively upon milk for at least the f i r s t eighteen months, and intermit- tently up to two years, according to stomach analyses per- formed by Brooks (1954), Chapsky (1936), Freimann (1940), Nikulin (1940), Zalkin (1937), and the writer. Among pinnipeds this i s an exceptionally long duration, though some sea lions may nurse as much as one year (Otaria: Hamilton, 1934; Eumatopias: Kenyon, in l i t t . ) . In view of the number of walrus calves that have been maintained in captivity on a variety of other diets (Mohr, 1952; Murie, 1872; Nikulin, 0£». c i t . ) , i t i s apparent that they N are not dependent upon milk as such, so long as their nutritional requirements are met. This suggests that the situation as i t occurs in Nature is due, rather, to physical or psychological i n a b i l i t y of the young to obtain solid foods. Mitchell (1909) observed that young captives "showed no knowledge of how to extract.../"mollusks from the shells_7, although they would bring.../~them_7 from the bottom of their pool." An orphaned yearling reported by Brooks (oj>. c i t . , p. 59) and another by Heinrich (1947) appeared to be in good physical condition, and the former's alimentary tract contained "stones and animal matter" demonstrating bottom- feeding a b i l i t y . A third specimen examined by the writer displayed severe gastric inflamation, and except for a few stones and one brachiopod shell fragment, i t s intestine -69- contained only mucous and large quantities of b i l e . This animal was in i l l health. Its blubber was only about one centimeter thick, as opposed to 3.5 cm. recorded by Chapsky for yearlings, and i t s ga l l bladder was about six times normal size, indicating that no food had been ingested within, perhaps, two weeks or more. Two-year-old walruses appear to be capable of f u l l nutritional independence, though they mostly remain in close association with female herds even at that age. Nikulin (1940) and others have suggested that this i s primarily due to their dependence upon adults to s t i r up food from the bottom, their own tusks being too short for efficient grub- bing. This, however, could be either cause or effect, and i t seems more probable, as Brooks has noted, that such a relationship i s purely incidental to the social pattern, for tuskless adults reported by him and the Gambell men were in good health. Protection.—Being bora during the spring migra- tion, the calves are faced with a 500 to 1000-mile swim. Whether or not they could or would successfully accomplish this alone is not known, for they are carried most of the way by their mother, either on her back or clasped to her breast. When threatened from above (e.g. by hunters), the cow dives with the calf in her "arms"; when threatened from below (e.g. by k i l l e r whales), the youngster climbs high on her shoulders. The latter escape behaviour of the young - 7 0 - i s evident up to at least two years of age, though body size probably renders i t s goal unattainable, hence relatively i n - effective, after the f i r s t eight or ten months. The social t i e between mother and young i s practically unbreakable — where one goes the other must also go. If one i s k i l l e d , the other stays with i t or carries i t away, depending upon which one of the pair survives. If the cow i s k i l l e d , an- other cow or even juveniles may "rescue" the calf, and occasion- ally , orphaned calves are "adopted" by other cows. As the off- spring advance in age, the bond weakens and i s generally broken at the end of the second year. When a calf i s injured or by some means separated from the herd, i t barks incessantly. This penetrating sound has been heard at distances up to two or three miles under certain conditions and appears to be a highly effective device for attracting the attention of older animals. It efficiency was demonstrated to the writer on May 22, 1952, when a calf was permitted to bark briefly before being dispatched. Prior to this time only two other walruses had been seen in an area of perhaps twenty square miles west of Gambell, yet within five minutes a bull and two cows appeared near the ice floe on which we stood. No others could be found that day in a s t i l l larger area of exploration. The value of these social features i s obvious. Each contributes to a high rate of calf survival — a necessity where the reproductive potential i s low. -71- NUTRITION Quality The basic features of nutrition as they affect ecological and maternal relationships have been outlined above, but before proceeding with the discussion of growth, some additional review i s desirable. In general, the pelecypods (Mya. Clinocardium. Astarte, and Macoma make up the bulk of the Pacific walrus' spring and summer diet, other genera being largely incidental (Brooks, 1954). There are no data concerning seasonal preferences, except that a large tunicate forms an important part of the f a l l diet aiear St. Lawrence Island. Quantity Observations suggest that there i s no regular daily feeding schedule. Probably food i s taken when and where available, regardless of the hour, for daylight rhythms would have l i t t l e effect upon either the food species or the walrus' a b i l i t y to find them in the 10-30 fathom dark- ness. The abundance of empty stomachs, even in regions where a rich bottom fauna exists, suggests that feeding i s infrequent, though Murie (1872, p. 461) notes that the great glandular superficies and correlated large lymphatics point to means of speedy and fre- quent digestion; and in the walrus these apparatus are extraordinarily developed. Daily rations for a 6-month-old captive included twenty pounds of mackerel and 3-4 pounds of liquid nutrients -72- (R. McClung, in l i t t . ) , while Chapsky (1936) reports that captives in Hagenbeck's Zoo, Hamburg, consumed up to 15 kg. (33 pounds) of chopped fish per day. A f u l l stomach from an adult male examined by Brooks contained about one hundred pounds of invertebrates. There i s no conclusive evidence of seasonal fasting on the part of either sex (Table 10), except in l o c a l i t i e s where suitable benthos i s absent. Brooks (1954) believes that the bulls feed very l i t t l e in summer, but growth stud- ies, contrarily, indicate (p.102) that the greatest annual growth increments are achieved in summer and f a l l ; hence feeding cannot be less frequent then than at other times of the year. On the basis of tusk structure, breeding cycles, and comparison with other pinnipeds, fasting, i f i t occurred at a l l , would be expected to take place in late winter or spring. Predation on Seals Predatory walruses or "rogues" are well-known in Eskimo lore and have been observed and/or reported by many earlier investigators. In the Bering-Chukchee region they occur rarely, perhaps one in a thousand or even less fre- quently. The universal theory among Alaskan natives i s that these animals were orphaned when very young and never learned to bottom-feed; consequently they fed upon whatever animal matter they could get (e.g. seals). The credulity of this tale seems rather doubtful, since a l l rogues are apparently -73- TABLE 10 PROPORTION OF FEEDING WALRUSES BY SEASON AND LOCATION Authority- No. and Date Location Per Food cent with in Stomach Sex of Speci- mens Male Fe- male Both Fay 12 M 15 F April-May Gambell 42 20 30 Brooks(in l i t t . ) 40 MF June Bering Str. . — — 12 Brooks(1954) 71 M July-Aug. Barrow 16 — — Zalkin(1937) 56 M* July-Sept. Franz Jos. L. 43 — — Chapsky(1936) 46 MF* Aug.-Oct. Kara Sea — — 26 *0.r. rosmarus -74- adult bulls, no females or sub-adults being reported. A more probable solution (viz. that flesh-eating i s induced when benthos i s unavailable because of too-deep water or other conditions) has been offered by Chapsky (1936) and Southampton Island Eskimos (A. Loughrey, in l i t t . ) . Under such conditions Vibe (1950) found seal remains in two out of about one hundred stomachs, the remainder being empty. Males should be most prone to this, for they occur more frequently in the "fringe" areas than females do. Once begun, this type of feeding i s evidently continued indef- in i t e l y by some individuals whether or not bottom forms can be obtained thereafter. Seal eaters are generally solitary, according to Heinrich (1947), and are readily distinguishable by their grease-stained tusks and body. In the Bering and Chukchee Seas their importance i s questionable, but i t i s said that they may be detrimental at times by frightening seals away from certain l o c a l i t i e s (Nikulin, 1940). Their flesh i s unpalatable (Freuchen, 1935), and Alaskan natives claim that i t causes serious illness when eaten. The described symptoms of the latter correspond to those of hypervitamin- osis-A (Rodahl, 1949, 1950), as might be expected. Feeding Behaviour The feeding behaviour has been deduced by several writers, but there i s l i t t l e general agreement among their theories. This problem i s an intriguing one, since bivalve -75- flesh in the stomachs consists only of non-masticated feet and siphons; rarely are mantle or shell fragments present. Mollusks are apparently rooted from the sea bottom by means of the muscular mistachial pads and the tusks. The latter may be used in either a raking, digging, or brush- ing motion, as observed by Johansen (1910) and indicated by the worn antero-lateral surfaces. Vision probably plays no part in selecting food items, for the eyes are small and appear to be ill-adapted to low light intensities. Thus food i s probably selected by touch, using the l i p s and the vibrissae which are amply supplied with nerve endings of the trigeminal at their bases (Mohr, 1952). Once a mollusk has been located, one of three possi- ble methods i s used to devour i t : (a) the exposed siphon or foot i s bitten or torn off; (b) the soft parts are sucked out of the shell; or (c) the prey i s taken into the mouth and the shell crushed and ejected after certain soft parts have been selected by tongue and l i p action. Materials from various sources (Allen, 1880; Brooks, 1954; Cobb, 1933; Mohr, 1952; Vibe, 1950; and others) have given evidence for each of these methods, but the f i r s t appears to be by far the most common. The unusual a b i l i t y of walruses to bite and tear off chunks of flesh without a dental battery adapted for this type of feeding becomes more credulous when the fa c i a l structure i s c r i t i c a l l y examined (Figure 12). - 7 6 - Fig. 12.-—Sagittal section of the skin and super- f i c i a l musculature of a 2-year-old bull walrus (from a photograph). When the teeth are f u l l y occluded, the bony epiphyses of the lower jaw are about one-half inch away from the upper. Tough gingival tissue on both surfaces f i l l s this gap. Pelecypod feet and siphons could be bitten ("squeezed") off, either at that point or by the muscular l i p s . -77- GROWTH To a study such as this one, which has as i t s goal the assessment of population trends, an empirical system for dividing the animals into groups of like age i s indis- pensable. This must be based upon certain morphological characters whose rate of change can be charted with s u f f i - cient accuracy to yield reliable ageing c r i t e r i a . The search for such characters has led far afield and has un- expectedly revealed many interesting and useful facts in addition to those for which i t was originally designed. In the following pages these are presented in their entirety for their value in the present analysis and for the sake of future investigators who may wish to direct studies along similar channels. Age Determination The most effective technique for determining age cr i t e r i a of wild animals i s to observe and record the mor- phological features of known-aged individuals. This may be done by either of two methods: (a) keeping the animals in captivity or (b) marking wild stock at birth and recapturing them at frequent intervals thereafter. Several captive walruses have been raised in the zoological parks of Europe and North America, but few records of their development have been kept. These few (Mohr, 1952; B. Benchley, in l i t t . ; G. Crosby, in l i t t . ; R. McClung, in l i t t . ) , though helpful, - 7 8 - are not adequate as a basis for age classification. Eleven wild calves were tagged and released in 1953 by Brooks, but none has been recaptured as yet. Direct methods.--Scheffer (1950) and Laws (1952) suggested that encircling ridges on the roots of walrus tusks might represent annuli which could be readily counted and yield an accurate age appraisal. Upon investigating this feature i t was found that (1) ridges are evident only on the roots of bull tusks eighteen inches or more overall length; (2) the larger, hence older the tusk, the smaller the interval between ridges (Fig. 13); (3) those on any given root represent only a small portion of that tusk's growth history; and (4) each ridge i s the outward equivalent of an internal dentin layer pattern. It has not been conclusively established that the ridges or layers are produced at the rate of one per year, but tusks from bulls k i l l e d at Gambell, Wales, and Barrow in winter, spring, and summer, indicate that one ridge i s formed during late winter and early spring. None is formed before puberty; thus they appear to be asso- ciated with an annual physiological rhythm related to repro- ductive cycles. Twenty-one juvenile and young adult tusks were longitudinally sectioned in a search for structural clues to pre-pubertal development. Traces of interrupted dentino- genesis were found, but in several instances tusks of equal size and shape did not seem to bear the same or even similar -79- F i g , 13.--Root ridges ( l e f t ) on male tusks, showing narrower in t e r v a l s with increasing tusk si z e . -80- layer patterns, and some had no visible layers whatever. This line of assault was therefore abandoned. A similar technique involving the molariform teeth was tested after a preliminary examination revealed regular laminations in the cementum. This promising feature was also independently discovered by Mohr (1952, p. 33) and Brooks (1954) and pursued by the latter. Preliminary con- clusions from Brooks' and the writer's studies were similar: (1) the laminae were well defined only in male teeth, and (2) their number seemed to be significant only as a rough indication of relative age. Later, when more specimens and comparative data had been accumulated, the latter relation- ship was re-examined. The results, outlined below (p , 1 2 4 ) » indicate that the number of cementum layers i s a relatively precise indication of age in males. Another method briefly investigated was suggested by Plehanov's (1933) report of annuli in the claws of harp seals (Phoca groenlandica). Walrus claws, however, are comparatively fragile, and though layering was found to be prominent, no more than three annuli were ever found in one claw. Counts of corpora lutea and corpora albicantia in the ovaries were tried in view of their usefulness for interpreting ages of cetaceans (Wheeler, 1930; Laurie, 1937) and their probable value in ageing pinnipeds (Bertram, 1940). The conditions for success in using this technique depend -81- upon the reproductive cycles of the animal; a monoestrus annual breeder which retains a l l corpora permanently i s most ideal. The irregular ovulations of walruses, coupled with b i - and triennial conceptions, render interpretation d i f f i c u l t , and the writer's f i r s t t r i a l was unsuccessful due to smallness of sample and what appeared to be a total lack of correlation between corpus numbers and body measure- ments. It was not un t i l more specimens had been acquired that the design of ovulation became evident, and the results f i n a l l y obtained were of considerable value in the construc- tion of an age classification for females (see below). At best, however, corpus counts are probably very crude indices of age. Indirect methods.—Since the bulk of the calves are born within one month in spring, i t would be expected that at a given instant a l l individuals of the same age would have approximately the same body dimensions. The frequency diagram of a given measurement from a l l the animals in a population sample would, then, be expected to have a poly- modal form, each mode representing a separate, normally distributed age class. Using this technique, age c r i t e r i a have been independently derived for the Pacific walrus by Belopolsky (1939), Brooks (1954), Freimann (1940), and the writer, but each analysis differs slightly from the others. The reasons behind this disagreement seem to be that f i r s t , Belopolsky had too few data, and he did not consider sexual -82- dimorphism. Second, Freimann apparently overlooked the fact that the sample should be recruited within a short space of time, for growth can obliterate instantaneous modes within a few months (his collecting period covered about four months). Partly because of this, he also failed to recognize that the tusks grow:throughout the lifetime at a relatively greater rate than body length and are therefore a better measure of age. Third, Brooks was a victim of limited time and inade- quate library f a c i l i t i e s , for he had potentially at his disposal the same data u t i l i z e d in the following pages. To the extent that he was able to pursue the problem, his conclusions nearly coincide with those which were tentatively reached by the writer at an equivalent stage in the analysis. The data u t i l i z e d below were from specimens taken between April 20 and mid-June at Gambell, Savoonga, Wales, and L i t t l e Diomede Island. The majority was recruited during the last half of this period. It was not possible to obtain body measurements in sufficient quantity for analysis as Freimann had done, so the most available features, the tusks, were used instead. Tusks from freshly k i l l e d animals were measured to the nearest l/8-inch along the anterior surface from the gum line to the distal t i p . This measurement was recorded as Observed External Length or "OEL" to distinguish i t from other tusk lineaments under consideration. Accom- panying circumferences at the gum line were measured to the nearest millimeter or sixteenth of an inch. -$3- Histograms of OEL frequencies are shown by sexes in Figure 14. Males w i l l be considered f i r s t : (1) The male OEL's exhibit a series of reasonably prominent modes which are apparent both in spring and summer samples. If these are significant trends, i t should be expected that the Eskimos, who are most familiar with the animals, would have some knowledge of at least the most obvious ones, i.e. those at the lower end of the age scale. Indeed, the Gambell men readily classify specimens up to four years of age on the basis of OEL, and their concepts of these are approximately as indicated in Figure 14. Cor- responding body lengths also agree with Freimann's (1940, Fig. 1) body length-age class modes; hence i t i s highly probable that these represent true age classes. Beyond the fourth OEL mode there are three others well defined which probably represent ages 5, 6, and 7, respectively, but before their validity can be judged, some other tusk lineaments must be considered. These are shown in Figure 15. (2) The tusk can be grossly divided into three basic parts, (a) the Root Length. "RL", which i s enclosed in the alveolus, (b) the Observed External Length, "OEL", which has already been considered above, and (c) the part, "W", which has been Worn or broken from the di s t a l t i p mostly as a result of the feeding process. Together, these consti- tute the Absolute Total Length or "ATL." In the f i e l d , the f i r s t two are readily measured when the tusks have been re- -84- AGE (yeors) TUSK LENGTH (inches) 10 ac al m 5 U 3 Z AGE (years) 4 5 6 8 10 IS 20 3 0 -p i — I I | i in F E M A L E S 8 10 12 W 16 TUSK LENGTH (inches) 2 0 22 Fig. 14.—Frequency histograms of Observed Exter-nal Lengths (OEL's) of male and female tusks measured in late April, May, and early June. Age scales are from data in Appendix IIA,B. -85- Fig. 15.—Tusk lineaments utilized in age and growth analyses: RL - root length; OEL - observed external length; W - linear or distal wear; OTL - observed total length; ATL - absolute total length. -86- moved from the skull, but the third i s most d i f f i c u l t to evaluate unless a large series of tusks of various ages i s available for visual and metric comparison. At St. Law- rence Island the writer was able to obtain the f i r s t two paired measurements (RL and OEL) from sixty-two individual animals. Together, these were designated as Observed Total Lengths ("OTL"), and a frequency histogram of their values revealed a series of modes equivalent to those in the OEL frequency. That i s , modal values of the OTL classes, less the average root lengths (RL fs) per class, were equal to the apparent OEL modes up to 5 years, beyond which the sample was too small to be of value. Since OTL i s a better measure of tusk growth than OEL (i.e., i t i s closer to the true value, ATL), the OTL modes should be more reliable as indices of age classes. Consequently the OEL modes must also be reason- ably valid representations of age classes 1 to 5. (3) With the above data, the regressions RL and OTL on age were plotted, and both were tentatively projected to 7 years by visual estimate and the use of OEL modes 6 and 7 plus projected RL values. Using a series of seventeen male tusks of various ages from birth to 6 years (by OEL), the linear wear, W, was estimated for each age class. This, when added to the respective OTL values, yielded a possible curve of Absolute Total Length-growth (OTL + W = ATL) up to 6 years which could be projected with reasonable accuracy to 9 years. -87- (4) If the root ridges are annual formations, the interval between any two succeeding ridges must be equal to the linear increment of ATL growth for one year. Assum- ing this to be true, the root ridge intervals on tusks from twenty-three individual animals were measured. These tusks ranged in size from 19 1/2 to 38 inches, OTL, the youngest being six years old (by OEL) and the oldest near maximum old age (by Eskimo standards). These were tabulated and their position in the table carefully adjusted such that the values of each incremental series were of comparable magnitude to the next (Table 11). The resulting mean i n - crements were then smoothed graphically and the ultimate ATL-growth curve plotted as a projection of that which was derived in step 3 above. The two curves coincided i n the region of overlap (5-9 years). (5) In order to test the accuracy with which root ridge intervals had been matched in Table 11, the empirically defined (root ridges) portion of each specimen's ATL growth- history was plotted graphically. This gave a series of short curves (Fig. 16) which, by visual matching and combination . of slopes, yielded a second possible ATL-growth curve. The form of the latter was nearly identical to that obtained in step 4, the only difference being a slight divergence at the extreme upper end of the age scale. It was concluded that the combined ATL/age curves of steps 3, 4, and 5 are probably close to the true curve of average linear tusk -88- TABLE 11 ROOT RIDGE INTERVALS (inches) ON MALE TUSK ROOTS Specimen Number Age (years) A- 19 9 c? LA <: A -5 01  S 1 < ir\ O ^ I A- 20 1 A- 92 L « OS i A- 19 3 IT\ OS H 1 <J A -1 92  o o 1 <; sO Os " f «! 32 1/16 31 1/16 1/8 30 1/8 3/16 29 1/8 28 3/16 27 3/16 3/8 26 3/8 3/8 25 3/8 3/8 7/16 24 7/16 7/16 7/16 23 1/2 7/16 1/2 1/2 1/2 5/8 5/8 22 1/2 7/16 1/2 5/8 5/8 5/8 5/8 21 9/16 7/16 9/16 5/8 5/8 3/4 5/8 5/8 5/8 20 11/16 7/16 5/8 5/8 3/4 3/4 3/4 5/8 3/4 19 7/16 5/8 5/8 3/4 3/4 3/4 7/8 3/4 13/16 18 7/16 3/4. 3/4 5/8 7/8 7/8 5/8 5/8 17 7/16 3/4 3/4 3/4 7/8 7/8 3/4 3/4 16 3/4 3/4 3/4 3/4 7/8 13/16 15 3/4 7/8 1 13/16 14 3/4 1 15/16 13 7/8 1 1 1/16 12 11 10 9 8 7 6 5 t » Os I 3/4 7/8 15/16 -89- TABLE 11--Continued Age (years) Specimen Number o f to i CM O U A I o CM I vO o I •=£! vO O rH I < O H I CM I 13/16 1 1/16 7/8 1 3/16 5/8 5/8 15/16 15/16 1 1/8 1 1/4 1 15/16 15/16 L L 1/8 15/16 1 1/16 7/8 1 1 1 1 3/8 1 1/8 1 1/4 1 3/8 1 1/2 1 7/8 1 1/2 1 1/2 3/4 1 1/2 1 7/8 1 3/4 2 2 1/4 Fig. 16.—Tusk root ridge curves (lower). Each curve i s a graphic representation of Absolute Total Length growth as depicted by root ridge intervals (Table 11) of individual specimens. By matching shapes and superimposing the equivalent ones, a complete ATL growth curve (upper) was derived. -91- growth up to 30 years. The latter i s near the expected maximum longevity (p.143 )• (6) Assuming that the tusks used in the root ridge analysis were of ages corresponding to their position (by slope comparison) on the ATL-growth curve, their OTL/age and RL/age values were plotted. With the aid of these points, the RL curve of step 3 was projected to 30 years and an approximate projection of the OTL curve to 30 was sketched in. (7) As noted above (step 3)> the rate of wear from 0 to 6 years was known. Beyond 6 i t could be estimated from the difference between ATL and the OTL sketch curve. By making slight adjustments (smoothing) in the resulting "negative increment" values, a f i n a l OTL curve was construc- ted which f i t t e d the observed data well. A table of tusk lineaments in which (ATL - W = OTL) - RL = OEL i s given in Appendix IIA, and a graphic representation i s shown in Figure 17. It i s f a i r l y evident that the above technique has some weaknesses and that the f i n a l synthesis has necessar- i l y been rather subjective. Nevertheless, having checked, cross-checked, calculated and re-calculated the same data by several other methods too numerous and confusing to men- tion here, the writer has concluded that the result shown in Figure 17 i s a true and accurate representation of aver- -92- T 1 1 1 1 1 1 1 1 I I I I I ' I 1 R AGE (years) Fig. 17.—Average linear growth and wear Pacific walrus tusks, based upon Appendix IIA -93- age male tusk growth. The pattern of female tusk growth was more d i f f i c u l t to interpret, for the OEL distribution (Fig. 14) has no modal regularity comparable to the males', and there were few com- plementary measurements of other physical features which could be u t i l i z e d for evaluating the possible modes, A reasonable plan has, however, been deduced as follows: (1) In general, the teeth of female pinnipeds are > slightly smaller than the males'. This i s probably a func- tion of relative growth in most instances, though i t i s even evident in some species which do not display prominent sex- ual dimorphism of body size (e.g. Phoca hispida). The molar- iform teeth of female walruses are only about three-fourths the size of the males', the tusk circumferences for equal lengths about 7 5 - 8 5 per cent, and the OEL maxima recorded by various investigators, 7 5 - 8 5 per cent. Thus i t was expected that the female ATL-growth curve would have the same or similar slope as the male's, but that i t s asymptote would be a somewhat smaller value. In view of the smaller cross-sectional area, i t was also expected that the wear rate would be greater and, as a result, the OTL values proportionately smaller. This should become more pronounced with advancing age, though the relative difference between male and female wear rates should diminish due to changes in size and shape (p. 130) of the surface being abraded. (2) The St. Lawrence Island men recognize females -94- up to at least 4 years of age on the basis of general mor- phology, including tusk OEL. Their concepts of the latter agreed with the writer's f i e l d conclusions and were similar to the males' (Fig. 14), for although the OTL's were actually smaller, the root lengths were also smaller. Body lengths of these presumed age classes agreed closely with those depicted by Freimann's (1940, Fig. 1) large sample. (3) Beyond 4 years, separation of age classes was less certain, but the ovaries contributed supporting evidence to some probabilities of the OEL frequency. According to the Gambell men, animals experiencing their f i r s t oestrus would be mostly 4-year-olds — an idea upheld by the OEL range and median for such specimens (Fig. 18 and Table 12). On this basis, the next reproductive class (Parturition I) should be mostly 5-year-olds, the third (Conception II) mostly 6-year-olds, etc. Since medians of f i r s t parturients, second conceptions, and second parturients (Table 12) corre- spond to possible OEL frequency modes at 10 1/4, 11 1/4, and 12 inches, respectively, the latter have been regarded as the most probable OEL modes for ages 5, 6, and 7. (4) The ranges and medians of tusks from animals in later pregnancies did not follow any regular progression (Table 12). This apparent non-conformity could be due to smallness of sample, but another factor, namely variations in wear rates, could also have had considerable effect. Experiments with rats and farm animals have demonstrated -AAA I 1 I 1 I * 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 ' I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I * ' 1 I 1 ' 1 I 1 y\[11 11 1 1 * i 1 1 1 i 1 6 - UJ o z U J 5 tr UJ or o CO - i - T - r Z 6 \ • o BO @ (3 • m O Ovulating or conceived • Parturient Non-breeding i i i i i i i I i i i i i i . i i i i i • i i 1 • i i i i i i i • i • i • i • I • i i i • i • i • i • i • i • i • i i I • i i l i i i l i i i I i i i [ 'OVV 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 TUSK OEL (inches) F i g . I S . — R e l a t i o n s h i p between female tusk s i z e and reproductive h i s t o r y . Numbers w i t h i n the symbols represent t o t a l pregnancies, i n c l u d i n g current con- ceptions and o v u l a t i o n s , experienced by each i n d i v i d u a l . Symbols without num- bers are from Brooks (1954, Table 6, and i n l i t t . ) or specimens from which only one ovary was secured. « I -96- TABLE 12 FEMALE TUSK LENGTHS AT SUCCESSIVE STAGES IN REPRODUCTION Reproductive No. of Tusk Length in Inches (OEL) Expected Mean Class Specimens Range Median Age (years) Conception I 3 8 1/4 - 9 1/2 8 7/8 4 Parturition I 3 9 - 10 1/2 9 3/4 5 Conception II 3 10 - 12 1/2 11 1/4 6 Parturition II 5 10 - 14 12 7 Conception III 2 12 1/8" - 12 1/4 12 1/4 8 Parturition III 3* 11 - 13 1/4 12 1/8 9 •Excluding the specimen in Fig. 18 with 9 3/4" OEL, whose tusks had been broken. -97- that the food intake during pregnancy i s slightly higher than usual and that "lactating animals...consume two to three times as much food as non-lactating animals" (Brody, 1945, p. 821)• Assuming this to be basically true of a l l mammals, i t would be expected that walrus tusk wear would increase proportionately during pregnancy and lactation, since i t i s largely an effect of feeding. Also, i t follows that, taking any two females of like age, the more productive one would have the/shortest tusks. In Brooks' and the writers f i e l d data there are numerous evidences which uphold these hypotheses. In Fig- ure 18, for example, the animal with the longest tusks (21 inches) had experienced only two pregnancies, while the specimen with the most pregnancies (six) had very short tusks (13 3/4 inches). The difference between these two i s partly a result of breakage, but breakage i s also a result of wear which tends to weaken as well as shorten the tusk (p. 130). Individual differences in tusk shape and circumference also appear to be important factors in this process. Consider an animal which has experienced three pregnancies in biennial succession with 18-20-month lac- tations following each one. The resulting overlap of pregnancies and lactations should affect a wear rate at least double that of an equivalent aged female who had one or no pregnancies during the same period. Since the former -98- appears to be the mode in early years of maturity, a random sample of OEL's from the female population should reveal a major peak of overlapping age classes between 4 and 10 years. Following the third (or second) pregnancy there i s a period of "reproductive quiescence" resulting either from missed pregnancies, a reduced ovulation rate, or both (p. During this period the wear rate should be lower than pre- viously, and the OEL's should therefore increase at a rela- tively greater rate. In the OEL frequency, this period should be represented by a slight trough. Finally, with further intermittent pregnancies and a gradual reduction in tusk growth rate there should be a less prominent but broader peak of OEL's extending from the trough above to the asymptote of average OEL "growth." Each of these major peaks and troughs does exist (cf. Figs. 14, 18; Freimann, 1940, Table 1); thus i t i s apparent that the average curve of OTL (hence OEL) growth cannot be a smooth progression but must be punctuated at intervals by minor undulations, each the result of increased wear during a pregnancy-lactation period. (5) Using the data derived in steps 2, 3, and 4, preliminary OTL/age and RL/age curves were plotted to 9 years. Wear estimates for the f i r s t five years were obtained by visual comparisons of tusks in the same manner as was done for the males. With the aid of these, an approximate ATL-growth curve was derived and tentatively extended to -99- 7 years. Since there are no definite root ridges on female tusks, i t was not possible to project the ATL curve further by that technique, but knowing the approximate value for i t s upper end (i.e., at 25-30 years) from specimen measure- ments and wear estimates, a probable approach to the asymptote was plotted. This was accomplished by using a slope equiva- lent to the males'• From the same specimens upon which the upper ATL limits were based, average OTL and RL values at 25-30 years were also established. (6) An adjusted table of hypothetical tusk growth was then constructed (Appendix IIB), using values of the above ATL curve as the bases and derived OTL values to 9 years as the most probable ones for the lower end of the OTL curve. Again this was constructed by the method (ATL - W - OTL) - RL Z OEL. Allowance was made for three consecutive biennial preg- nancies, beginning with the f i r s t oestrus at age 4, and three additional pregnancies in later l i f e — a total of six, the maximum observed. The latter three were spaced, temporally, according to the relationship observed in Fig- ure 18, The f i n a l synthesis (Fig. 19 and Appendix IIB) sat- i s f i e s every requirement for average female growth and rep- roductive history; indeed there seems to be no alternative solution. Its goodness of f i t to the observed conditions of OEL frequency was tested by sampling a theoretical pop- ulation having tusk lengths normally distributed about the -100- 10 12 14 16 AGE (years) Fig. 19.—Average linear growth and wear of female tusks, based upon Appendix IIB. Irregularities in the OTL-growth curve are due to increased wear rates during pregnancy and lactation. -101- hypothetical OEL/age means of Appendix IIB. The male "catch- curve" (p. 145 ) was used as a model for size and age d i s t r i - bution of the sample. This yielded an OEL frequency whose major peaks and troughs were s t a t i s t i c a l l y identical to Figure 14 and Freimann's (1940) data. It must be stressed once more that this plan of tusk growth i s designed to f i t average conditions. For this rea- son i t has a m£jor weakness in that i t cannot be applied directly to animals having more or fewer pregnancies per unit of time than have been allowed for. Such deviates, however, can usually be diagnosed individually when tusk morphology and reproductive history are known. Age designations for specimens of both sexes u t i l i z e d throughout the present study have been primarily based upon the above.hypothetical tusk growth patterns. When no tusk measurements were available, ages were approximated for females from evidence (ovaries) of their reproductive history and for males by the number of cementum layers in their teeth. The latter technique i s discussed below (p. 124). Body Growth In the f a l l of 1951 two young walruses were acquired by North American zoological parks, and by request, serial body weights were recorded during their brief l i f e spans. The writer i s indebted to Dr. Robert McClung, of the New - York Zoological Society, and to Mrs. Belle J. Benchley and -102- Dr. Glen G. Crosbie, of the Zoological Society of San Diego, for their kind assistance in collecting these data, shown in Figure 20. Some presented by Mohr (1952, p. 246) have been included for comparison. A l l are Atlantic walruses, but their weights do not differ from Pacific specimens of equiv- alent age recorded by Brooks (1954). Thus, i t i s assumed that the young of both populations have similar growth histories in the f i r s t two years of l i f e . There are two features of particular interest i l l u s - trated by Figure 20. The f i r s t is that, although each animal's growth during the f i r s t year was rather different from the others, a l l tended to approach the same value and slope in the second year. Prior to their arrival at the zoos, growth had been retarded (probably due to inadequate nutrition), but upon receiving a regular nutritive diet, each compensated for i t s losses u n t i l the intrinsic growth rate (Brody, 1945, p. 544) was attained. Teeth and tusks from two of these specimens ("Bosco" and "Herbert") were examined by the writer, and i t was found that there had been no compensatory accretion there. The second feature to be noted i s a unanimous, though slight, indication of seasonal growth rhythm, with maximum increases from late spring to early winter and minimums in late winter and early spring. As would be expected, the maxima and min- ima of this rhythm antecede the periodic increases and de- creases of blubber thickness reported by many Eskimo and White observers, including Brooks (1954) and Vibe (1950). -103- i—i—r i — i — i — i — i i i r p - * 500-O 400- Thoro . Herbert Bosco I I L M J J A S O N D J F M A M J J A S O N O J F M MONTH Fig. 20.—Body weight growth of five young Atlantic walruses in captivity. I n i t i a l weights were presumably recorded when the animals f i r s t arrived at the zoos, except for "Bosco" who was received two months earlier. Each was under one year of age when captured. -104- The animals are fattest in f a l l and winter, thinnest in spring and summer. Generalized body length, weight, and hind foot length- growth of the Pacific walrus from birth to old age are shown in Figures 21, 22, and 23, as derived from the records of Brooks (1954), Nikulin (1940), Perfilyevsky (in Nikulin, op; c i t . ) , and the writer, As was noted earlier (Table 5, p. 48) the degree of sexual dimorphism i s not excessive and i s by no means comparable to the differences recorded for harem breeders. Males of the latter (Otaria: Hamilton, 1934; Callorhinus: Scheffer and Wilke, 1953; Mirounga; Laws, 1953) exhibit a sort of "double" sigmoid growth pattern (Fig. 24) in which the added sigmoid curve i s apparently a secondary sex characteristic associated with dominance order. It i s initiated prior to puberty and culminated at "sociological maturity" (Scheffer and Wilke, 1953, p. 131) when the animals have reached a sufficiently large size for effective competition with older harem masters. Adaptive double-sigmoid growth i s probably in the genetic potential of a l l pinnipeds, but the mechanisms for i t s selection are not generally activated except when the conditions for harem maintenance are particularly suitable. As might be expected, the males of wholly promiscuous species (Phoca vitulina and P. groenlandica: H.D. Fisher, in l i t t . j Leptonychotes Weddelli: Bertram, 1940) appear to reach f u l l adult size in a single geometrically-decelerating curve - 1 0 5 - Fig. 21.—Average body length (nose-tail) growth of Pacific walruses. -106- 3 5 0 0 r 3000- ~i r T r "i 1 1 r - i — / / - O ft p' Males * Females © Mean-both sexes 5 6 7 8 9 10 It AGE (years) 12 13 14 15 >I5 Fig. 22.—Approximate average body weight growth of Pacific walruses. 38r 36- 34- "i i i i i i i i i 1 1 r p Males Females ® Mean-both sexes J _ I I L -J 1_ -y/r I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 > 15 AGE (years) Fig. 23.—Average hind foot length (heel to tip of longest toe) growth of Pacific walruses. -107- ~ l I I I I I I I 1 I 1 1 1 1 1 1 1 1 1 1 r moles J I I I L O a. ui u AGE UNITS o o Fig. 24.—Relative pinniped body growth, illu s t r a t i n g the "double sigmoid" curve of harem breeders. -108- similar or identical to the females'. There are a few species such as walruses and gray seals, Halichoerus grypus. which appear to be intermediate in their breeding behaviour and would be expected to have correspondingly intermediate growth patterns. Neither Figure 21 nor 22, however, yields any definite indication of i t for the Pacific walrus. Baculum Growth Baculum size i s a crude guide to age, since length and weight increase rapidly during the f i r s t five or six years. The approximate average growth history of this bone i s shown in Figures 25 and 26. At birth i t i s about 20 per cent of adult length and 0.5 per cent of adult weight. Thereafter i t s growth i s strongly sigmoid, with weight increases lagging slightly behind length. The bone, i t - self, i s generally S-shaped (Fig. 27), though as Murie (1936) observed, the form may be rather variable. Sagittal sec- tions have revealed layering in the internal structure, but i t does not appear to be of any value for ageing. A peculiar phenomenon associated with this structure i s the occasional breakage which occurs under natural con- ditions. Murie (1936) illustrates several specimens of this kind and states that fifteen out of twenty-five sent to him from St. Lawrence Island by Otto Geist had healed fractures. This proportion, however, does not appear to be the usual one, for Gambell natives consider broken bones to be rather uncommon. In the writer's sample of 217 specimens, only -109- >I5 Fig. 25.—Average baculum length growth of Pacific walruses. -1 1 1 1 r — — 1 1 1 1 1 r 1 n r lOOOt- >I5 Fig. 26.—Average baculum weight growth of Pacific walruses. Arith-log scale. -110- F i g . 27.—Comparison of c a l f and adult bacula, the l a t t e r showing t y p i c a l s-shape. - I l l - three had obviously been broken, and three others displayed slight irregularities which might have been sites of earlier injuries. Two of the broken bones, one a new fracture and the other partially healed , were from calves. The third was a f u l l y healed young adult baculum. It i s d i f f i c u l t to imagine how these loosely-supported structures could be broken, especially in the calves, though Murie (1936) con- siders that, due to the great bulk of the adults, such minor injuries might easily result from mishaps which occur while climbing about on uneven ice floes. A female counterpart of the baculum, the os c l i t o r i - dis, was found in most specimens examined by the writer (Table 13). The proportion of non-osseus structures in the oldest animals was significantly higher than that of the calves, suggesting that ossification decreases with age. In walruses these bones are generally somewhat question-mark shaped, though a few exhibit a totally unrelated form. Skin and Pelage Chapsky (1936) gives the following for skin thick- ness of Kara Sea walruses: young 8 mm.; yearlings 13.5 mm.; sub-adults 20 mm.; adult males 26 mm.; and adult females 22 mm. These agree closely with Brooks' (1954) data, and he notes that the bulls' neck skin may be as much as 7 cm. thick. Nikulin (1940) reports 3-4 cm. as an average for Pacifies. It was believed by several earlier writers (e.g. Brown, 1868; Brehm, 1926) that the characteristically -112- TABLE 13 OS CLITORIDIS WEIGHTS Age (years) Number of Specimens Weight (grams) Range Mean ± S.E. 0 7 cartilage 0 34 0.026 - 0.242 0.1182 ± .0005 1 - 2 4 0.049 - 0.237 0.1397 =*= .0426 6 - 9 4 0.023 - 0.202 0.0827 ± .0402 >20 3 cartilage >20 1 0.028 -113- thick, scarred skin of adult bulls was the result of tusk wounds received during breeding season forays, for at that time walruses were thought to be aggressive harem masters. This theory of origin has been rejected by Brooks (1954), Chapsky (1936), Freimann (1940), and Nikulin (1940), and the writer's experience upholds their views. Thick, lumpy skin on the neck and shoulders i s merely a secondary male sexual character, and i t s surface scars seem to be mostly the result of injuries caused by sharp-edged ice on the projecting tubercles, though a few are undoubtedly tusk- inf l i c t e d wounds. Freimann (op., c i t . , Tables 4 and 5) has demonstrated that this characteristic tuberculation appears at about age 6 to 8 in most bulls. General changes in blubber thickness and pelage color with age have been adequately discussed by Brooks (1954) and others. The question of pelage molts has not as yet been f u l l y answered, largely, i t seems, because i t has not been f u l l y investigated. Seals in general display three stages of hair covering: foetal or lanugo, juvenile, and adult (Mohr, 1952). The lanugo may be molted in utero or at some time within three or four weeks post-partura, depend- ing upon the species; the juvenile-adult molt usually occurs in the succeeding autumn. Information concerning an equiva- lent lanugo stage in walruses i s not complete, but apparently the slate colored natal coat i s shed one or two months after birth (Nikulin, 1940). A molt (juvenile to adult?) in the -114- f i r s t autumn seems to be w e l l e s t a b l i s h e d by the r e p o r t of Sokolovsky (Mohr, 1952) and c o r r o b o r a t i n g observations by R. McClung ( i n l i t t . ) of another young c a p t i v e . These began shedding between October and December, and the new h a i r was f u l l y grown by s p r i n g . Mohr (1952) presents convincing evidence of an annual adu l t molt which occurs between May and J u l y i n A t l a n t i c walruses, both w i l d and c a p t i v e . T h i s corresponds to the general seasonal rhythm of other pinnipeds described by her, Gambell n a t i v e s have not recognized such a phenomenon i n t h e i r l o c a l i t y , and i t was never n o t i c e d by the w r i t e r , but herds of females photographed near P o i n t Hope i n l a t e May (Ryder, 1954) show a patchy coat which i s suggestive of the i r r e g u l a r sheetmolt p a t t e r n d i s p l a y e d by some other pinnipeds ( c f . Laws, 1953, p i . Vb). I t has been noted (Mohr, 1952) t h a t the whiskers or v i b r i s s a e are shed annually and are not, as A l l e n (1880), Brooks (1954), and others have suggested, permanent f i x t u r e s which grow c o n t i n u a l l y throughout l i f e . T h i s may account f o r some of the wide v a r i e t y of measurements recorded i n the l i t e r a t u r e , since they must be very short (worn) j u s t p r i o r to m o l t i n g and very long when f u l l y grown anew. The maximum lengths reported are 10 1/2 inches f o r A t l a n t i c s (Mohr, op., c i t . ) and 10 inches f o r P a c i f i e s ( N i k u l i n , 1940) — the s h o r t e s t , about 1/8-inch. Replacement seems t o be i n t e r m i t t e n t over a p e r i o d of s e v e r a l months i n summer and f a l l and a l l of the v i b r i s s a e may not be shed each year -115- (Mohr, op., c i t . ) . Possibly there i s a regular increase in the basal diameters of these structures with advancing age. Skull Growth General skull growth i s depicted in Figures 28 and 29, in which a few of the primary measurements have been plotted by age. These were extracted from the work of Allen (1880), from specimens collected by the writer, and from those in various North American Museums. The writer i s indebted to J.W. Brooks, University of Alaska; CO. Handley, Jr., U.S. National Museum; Barbara Lawrence, Museum of Com- parative Zoology; and W.C. Pelzer, Milwaukee Public Museum, for the latte r . It i s apparent that adult skull dimensions are at- tained at about five years of age by both sexes, but that slow growth persists throughout the l i f e span, especially in the rostral and mastoid regions. The latter i s most evident in males and i s believed to be directly correlated with tusk development. Lacking other means, good c r i t e r i a for identifying the sexes of adult Pacific walrus skulls are the rostral and mastoid breadths, those of the males being characteristically larger than the females', as indicated. Dentition Succession.--Typically, pinnipeds are born with their adult dentition preformed, though i t may not erupt for several days or weeks thereafter. A minute, non-functional set of milk teeth i s resorbed either in utero (phocids) or shortly - 1 1 6 - 40 55 30 25 CO or U J 20 r-2 UJ o 15 10 Condylobasal length „X' ' Greatest mastoid breadth Zygomatic breadth Greatest rostral breadth " 5- -J L 1 1 1 1 | I I I I I J 1 L y/-0 I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 >15 AGE (years) Fig. 28.—Age-relative increase of male Pacific walrus skull dimensions. -117- Condylobasal length Greatest mastoid breadth Zygomatic breadth A Greatest rostral breadth- -I 1 U -1 1 L. 0 I 2 3 4 5 6 7 8 9 10 II 12 13 14 I5//>I5 AGE (years) 1 _ Fig* 29.—Age-relative increase of female Pacific walrus skull dimensions. -118- after birth (otariids). The progressive stages of walrus dental development are most closely a l l i e d to the otariids, but they have some peculiarities which distinguish them from both of the other pinniped groups. The f u l l deciduous dentition of walruses had not yet been investigated, since i t apparently i s present only dur- ing some prenatal stage. It i s presumed to be I C i P rik ~ 28 123 I 12] but the maximum observed formula at birth seldom exceeds 1 ^ C 1 ^ 123 " 1 8 due to prenatal resorption. Upper milk premolar no. 4 has been found in only one natal skull, a specimen collected at Gambell. Contrary to Cobb's (1933) conclusions, i t seems very improbable that any of the milk set i s actually shed or retained to adulthood. Instead, the entire battery seems to be resorbed before one year of age without ever breaking the surface of the gums. In twenty-one calves and foetuses examined at Gambell, only one had an erupted milk tooth, and resorption of the remaining few in Cobb's and the writer's natal skulls was already well under way. The f u l l set of successional teeth i s present at birth, but except for an occasional lower canine, none i s erupted. The maximum observed formula i s 1 H 0 1 p ilr 1 4 u = 3 2 2 1 though, again, i t i s seldom realized, for I ^ j , P 4, and M -± -119- are vestigial and are often absent, especially after adult- hood i s reached. Structure and growth of the cheek teeth.—Cobb (1933) has given a detailed account of dental development as he saw i t in a series of museum skulls, but due to the limitations of his specimen material and information on natural history, there are some features which he seems to have evaluated incorrectly or not at a l l . Since these may be relatively important as a background for future investigations, they are treated briefly below. The hollow, conical natal tooth i s composed of dense primary dentin with a thin cap of enamel (Fig. 30a), and by one year of age, more dentin has been added internally and a single layer of cementum externally. Dentin deposition ceases after the third or fourth year. In succeeding years only cementum i s added, "sealing off" the dentinal portion and gradually increasing the cross-sectional area (Fig. 30d,e). Each tooth i s continually erupted as i t s exposed surfaces are worn away, and the lingual and buccal surfaces remain about level with the gum line at the-gingival-dental junction at a l l times (Fig. 30d,e). Within the dentin some more or less prominent lam- inations occur, the most constant and sharply defined of which i s marked by the "neonatal line" (Massler, et a l , 1941) between foetal and post-natal dentin (Fig. 30b). In human teeth the major incremental lines result from "constitutional adjustments such as birth,...weaning,...dietary disturbances, a . b c d e Fig. 30.—Sagittal sections of the second upper and lower premolars in situ show- ing structural growth: a - newborn; b - yearling; c - two years; d - four or five years; e - about ten years. Scale about three-fourths actual size. Detail slightly exaggerated. -121- ,../"~and_7 diseases that affect calcium metabolism directly..." (Noyes, et a l , 1938, p. 125), and in teeth of the elephant seal Laws (1952) notes that they are a result of regular fasts during breeding and molting periods. Externally, canines of the latter and of the Alaska fur seal (Scheffer, 1950) re- veal prominent annular ridges, each of which marks the end of one year's growth. Some Pacific walrus molariforms dis- play similar laminae and ridges, but for reasons not yet understood neither of these phenomena appears to have any definite annual rhythm, individuals of the same age (by OEL) often having different patterns of lamination. Possibly this i s due to the fact that the dentinal portion of the tooth i s formed entirely before puberty, and during that time there are no regular, sharply defined physiological adjustments excepting birth. Definite annular ridges on the tusk roots do not form u n t i l after puberty. In the center of each tooth there is generally a slender core of translucent material which i s most prominent in males. This i s not dentin, as usually defined, but con- sists instead of a mass of imperfectly calcified "pulp stones" or "denticles" (Kronfeld, 1937, p. 59) partially fused to- gether and usually with numerous interglobular "spaces" f i l l e d with organic material;.. This core i s a distinctive characteristic of walrus dentition (Cammann, 1954; Penniman, 1952) and i s particularly well developed in the tusks (p. 129). The degree of hypercementosis displayed by walruses -122- appears to be unexcelled among mammalia, except perhaps for the hooded seal, Cystophora cristata. Cobb (1933) considers this to be a result of stresses involved in crushing clam shells — a logical conclusion under the circumstances but one which the writer believes to be incorrect. Almost with- out exception, the stomach contents of walruses examined by the writer and reported in the literature were composed of non-masticated pelecypod feet and siphons which, in l i f e , are expressed beyond the margins of the shells and cannot be withdrawn between the valves. As seen in the walrus stomachs they appeared to have been bitten or squeezed off at their bases by pressure exerted either with the muscular lip s or the toothless incisive portion of the jaws — prob- ably the former. Occasional marginal fragments of the shells also attest to this process. I f , on the other hand, the shells had f i r s t been crushed in the mouth and the soft parts later extracted by tongue and l i p action, we should expect to find at least some of the feet and siphons masticated and a larger proportion of mantles, g i l l s , visceral masses, and shells present in the stomach. There would appear to be no advan- tage gained by crushing the shells i f only the exposed t i s - sues are eaten. A second point of disagreement with Cobb's conclusion was found in the teeth, themselves. Histological examinations of occluded, non-occluded, and non-erupted teeth from two 2-year-old specimens revealed that every tooth from a given animal has the same proportion of cementum, regardless of -123- i t s functional status. This suggests that cementogenesis i s governed by overall physiological controls which may not be related to the feeding process at a l l . Indeed, i t appears to be correlated with tusk growth (see p. 128 )• This also negates Cobb's theories of tooth wear, for i f shells are not taken into the mouth, they cannot be respon- sible for the pattern observed. Wear occurs on three planes: lingual, apical, and buccal. The f i r s t usually results in a highly polished, convex surface, the second a rough, con- cave surface, and the third mostly a polished, convex sur- face (Fig. 30d,e). Occlusal abrasion accounts for a l l apical and part of the buccal wear, and from i t s character- i s t i c s i t i s evident that particles of a harder substance than either dentin or cementum are partly involved — prob- ably sand and/or gravel. Gravel i s a common part of the stomach contents, as might be expected, and in winter i t i s frequently observed about walrus breathing holes where i t has been spewed out upon the ice in large quantities. Clam shells, mostly unbroken, are occasionally found with i t or near i t . Thus i t i s believed that lingual wear, both upper and lower, i s due to tongue action plus abrasion by acciden- t a l l y mouthed sand and gravel, while the smooth portion of buccal wear i s also due to sand and gravel plus f r i c t i o n of the gingival tissue below the opposing tooth (Fig. 30e). Since cementum layering in human teeth follows no seasonal rhythm, the number of layers i s only a crude i n - -124- dic a t o r of r e l a t i v e age (Noyes, et a l , 1938). But i n the teeth of hooded and elephant seals examined by Laws (1952) they appeared to be deposited i n an annual cycle s i m i l a r to the dentinal laminae, and the number of layers was therefore equal to the in d i v i d u a l ' s age. P a c i f i c walruses also have an annual rhythm of cementogenesis, though i t did not become apparent u n t i l the problems of tusk growth had f i r s t been solved, for the rate of deposition i s i r r e g u l a r i n the f i r s t 6 to 7 years. I t was suspected e a r l i e r that i n adulthood one cementum layer was deposited per year, for those which are formed at that time are much more c l e a r l y defined than the pubertal and prepubertal lamina, and i n the tusks there i s one layer per root ridge. This r e l a t i o n s h i p was tested by (1) p l o t t i n g the regression OEL on number of cementum layers, (2) constructing a y-axis age scale on the basis of Appendix IIA and an x-axis age scale on the above as- sumption, and (3) drawing a sketch curve which s a t i s f i e d both the empirical data and the hypothetical age scales. The res u l t (Fig. 31) i s a sa t i s f a c t o r y demonstration that cementum layers are as good indices of age as tusk lengths; i n f a c t , they are probably better i n the l a t e r years. The only specimen for which tooth, OEL, and root ridge data were available was used as a p a r t i a l t est of conformity amongst the three ageing methods. The values obtained were 21, 20, and 22 years, respectively. This could be mere coincidence, of course, f o r i t i s evident that the OEL range per layer number i s rather wide. I n s u f f i c i e n t -125- Fig. 31.--Relationship between tusk OEL and num- ber of cementum layers in the cheek teeth of male Pacific walruses. The sketch curve i s based upon OEL age classes (Appendix IIA) and the x-axis age scale, which assumes that one layer i s added per year after the f i r s t seven years. -126- data are available for the females, but preliminarily their OEL/cementum layer relationship appears to be radically d i f - ferent from the males' and may have l i t t l e or no practical application. Structure and growth of the tusks*—At birth the upper canines are completely enclosed in the alveoli and do not erupt u n t i l four to six months later (Chapsky, 1936; Nikulin, 1940). Judging from records of captive specimens, this may be delayed s t i l l further by nutritional deficien- cies, some appearing as late as December (B. Benchley, in l i t t . ; R. McClung, in l i t t . ) . The natal canine has the same general structure as a molariform tooth (viz. a delicate, hollow cone of primary dentin capped with enamel), and, sim- i l a r l y , by one year of age more dentin has been added inter- nally and one layer of cementum externally (Fig. 32a,b). Un- like the molariforms, dentinogenesis continues thereafter at a relatively high rate throughout the entire l i f e span. Thus the canine becomes a tusk, for as dentin i s added to the root, the distal portion i s expressed further from the alveolus. The structural pattern involved may be likened to a series of hollow cones, each nesting within the one below and one or more being added at the top (root apex) each year. As new ones are added, the older ones are pushed farther away from their point of origin. Histologically, this results in a series of diagonal contours in a sagittal section (Fig. 32d), and concentric rings in a transverse -127- pulp stones Fig. 32.—Sagittal sections of male tusks in situ, showing structural growth: a - newborn; b - yearling; c - two years; d - about ten years; e -'"very old." Scale about one-half actual size. Detail slightly exaggerated. -128- section (see Penniman, 1952, p i . VIII). The significance of the indistinct contours formed in the early years of l i f e i s unknown (except for the neonatal line), animals of the same apparent age having different patterns and some having none whatever. Sometime after sexual maturity i s attained the laminations become very regular and pronounced, the upper extremity of each lamina being visible externally as a "root ridge" (p. 30). These ridges and contours, as noted above, appear to be formed during late winter and early spring and are believed to be the result of poor nutrition or reproductive cycles (or both). They may be present though less distinct in female tusks. Cementum layers are added to the root surface in the same chronological order as was observed in the molariforms, each of the 1, 2, and 3-year-old animals examined having the same number in both cheek teeth and tusks. Though their relationship to cheek tooth function seems dubious, they have a definite functional relation to tusk growth. Cementum acts primarily as a place of attachment for fibers of the periodontal membrane, a connective tissue occupying the space between tooth root and alveolar bone (Kronfeld, 1937). This serves to hold the tooth in place and to cushion i t when in use. As dentin i s added within the pulp cavity, the whole tusk must move downward in order to make room for the new material, and when this happens, the periodontal fibers are broken from their point of attachment in the -129- cementum. They must then be anchored into a new cementum matrix at the new point of contact. This process i s prob- ably a gradual one, though the cementum laminae suggest that i t occurs spasmodically. Obviously, when the tusks are extruded most rapidly, more cementum layers must be added per unit time; hence the rate of deposition would be greatest in the early years of l i f e . Later, when root ridges begin to form, only one layer per ridge i s added in male tusks. It has been concluded that the rate of cementogenesis i s prob- ably governed by tusk growth rate and that i t i s expressed in a l l of the other teeth at the same time by a universal physiological mechanism. Quantitatively, cementum comprises about the same proportion of tooth bulk in females as in the males, but there seems to be no definite pattern of lamina- tion. In the center of each tusk i s a thick core of "pulp stones" fused into a matrix of irregular dentin (Fig. 32, and see Penniman, 1952, p i . VIII). This structure i s r e l - atively larger in male tusks, comprising one-third to one-half their bulk, while in female tusks i t generally constitutes less than one-fourth. As in the molariforms, formation commences sometime shortly after birth, but in the tusks i t continues thereafter to old age, when i t f i n a l l y ceases. The core i s then sealed over by dentin (Fig. 32e). In transverse section the tusks of both sexes have the form of a slightly flattened elipse. This shape i s most -130- elongate in young tusks (Fig. 33), becoming gradually rounder with age as circumference increases (Fig. 34). The ratio of root-apex-circumference:gum-line-circumference i s generally a positive value to about 7 or S years, after which i t gener- a l l y becomes negative. This i s primarily due to reduced linear increments in late years and consequent greater bulk of overlapping cementum layers at the gum line (Fig. 32d,e). As a general rule, male tusks are parallel or slightly divergent (see Brooks, 1954, Fig. 1), while females' range from nearly parallel to strongly convergent. The former i s mainly due to postero-laterad curvature and a twist (clock- wise, right; counterclockwise, left) of about one-fourth turn per 35 inches. Some male tusks, however, have neither curve nor twist. A l l female tusks observed by the writer had postero-mediad curvature, and twist seldom exceeded one- eighth turn per 35 inches. Tusk wear may be divided into three types: antero- lateral, d i s t a l , and medial. The f i r s t i s apparently caused by grubbing for mollusks on the sea bottom, and i t s location suggests that i t i s brought about by a side-to-side sweeping motion. The loss of tusk material in this manner i s far greater than either of the other two., and in old animals i t often exposes the core for two-thirds the OEL (Fig. 32d). By making the tusk more slender, i t also increases the poten- t i a l i t y for distal wear, the second type being considered. The latter includes breakage and has already been quantita- -131- (mediol) Cross section of tusk Z.Or < ~i 1 1 1 1 i i i i i r n i i i I r - FEMALES T 1 r • - . • • : — — * • - i.oL • ' i I 1 I I I I I I 1 1 1 1 1 L . J I I I t_ 8 10 12 14 T U S K OEL (inches) 16 18 20 22 24 Fig. 33.—Relationship between tusk length (OEL) and cross-sectional shape at the gum line. (Arith-log scale) -132- TUSK LENGTH (inches) Fig. 34.—Relationship between observed external length (OEL) and circumference (at gum line) of the tusks. (Log-log scale) -133- tively discussed above (p. #5 et seq.) in connection with linear tusk growth* The third type, medial wear, occurs immediately below the gum line, creating a series of shallow depressions which are scarcely visible except when sighting along the medial surface. On male tusks the distances be- tween troughs appear to be equal to equivalent root ridge intervals, and on old tusks which have practically ceased growing, a broad, relatively deep trough occurs near the gum li n e . In addition to the usual shallow ones, middle-aged and elderly females often display a series of the deeper troughs widely spaced along the medial surface (Fig. 35). Because of the i n i t i a l location next to the gum, the causal factor of medial tusk wear i s unquestionably associated with some activity of the mouth, probably while feeding. It appears to be a result of either lower jaw action (with sand and gravel between i t and the tusk) or forcible expulsion of materials from the mouth. Eskimo theories favor the latter. Its appearance as seria l depres- sions seems to be an effect of varying tusk growth rates (seasonally) and a more or less constant rate of feeding; that i s , when tusk growth i s slowest (winter and spring), wear i s concentrated over a relatively smaller area per unit time than when growth i s rapid (summer and f a l l ) . This is verified by young, rapidly-growing tusks on which wear extends evenly over the whole medial surface and there are no notice- able depressions. The deep troughs (Fig. 35) hold greater -134- F i g . 35•—Posterior view of old female (upper) and old male (lower) tusks showing heavy medial wear. The "deep" trough on the male tusk i s imme- di a t e l y below the gum l i n e ; on the female tusk several are v i s i b l e along the medial surface. -135- significance, for they must be caused by either (a) drastic reduction in tusk growth rate or (b) an enormous increase in feeding activity. The f i r s t applies to old animals, specif- i c a l l y , and to occasional aberrants whose growth rates have been reduced by disease or injury. Several of the latter were examined by the writer. The second applies generally to females, and the increased food intake i s believed to be associated with pregnancy and lactation (p. 97), for the depressions usually occur in pairs, the f i r s t more shallow than the second. The distance between the deepest troughs i s comparable to the total ATL increments between pregnan- cies postulated in Appendix IIB. Expected and observed data are shown in Table 14. Again, the probable explana- tion for their absence or shallowness on tusks of young parturients i s that tusk growth i s more rapid then, and wear i s therefore distributed over a wider area. On this premise i t would be expected that the latest formed depres- sions on old tusks would be the deepest (tusk growth de- creases with age), which i s definitely true. The possibil- i t i e s of applying this information as a f i e l d technique for evaluating recent reproductive history i s self-evident, DEATH To the natural historian, one of the most elusive features of wild populations i s length of l i f e . Whole gen- erations are born and die, leaving scarcely a trace of the -136- TABLE 14 INTERVALS BETWEEN MAJOR MEDIAL WEAR LOCI ON FEMALE TUSKS P a r t u r i t i o n s ATL Increment (inches) Expected* Observed A-139 A-79 A-198 3rd to 4th 4th to 5th 5th to 6th 4 3 3/4 4 1/8 5 1/2 2 3/4 5 1/4 3 3/8 3 1/4 •Values extracted from Appendix IIB. -137- time, place, and manner in which they ceased to exist. Among the least understood are the marine mammals, since they are seldom observed for more than an instant, and their dead are often lost to the sea. Much of the information on walrus mortality has been gained from the dramatic accounts of Eskimos, sea captains, and explorers, to whom objective and quantitative interpretations were unknown. To evaluate these in terms of their relative 1 importance i s practically impossible, but a brief discussion of the various categories i s presented below for the sake of completeness. This i s followed by a crude evaluation of mortality rates and longevity. Density Independent Factors Weather.—As noted above (p. 52), some calves are bom very early in the spring (March-April) when a i r temper- atures are s t i l l rather low (0°=fc 10°F). According to the writer's and Belopolsky's (1939) Eskimo informants, these occasionally freeze to death shortly after birth. While this may be possible, i t seems more l i k e l y that freezing i s a post-mortem effect, for dead calves are about as frequently observed in May. No detailed autopsies have been performed on any of these young animals. Winter storms may1be a f a i r l y frequent cause of juvenile and some adult mortality. About St. Matthew Island, Hanna (1920) observed carcasses of five young animals which had apparently been crushed by shifting ice, and a dead juve- nile was also reported from there in 1954 by R. Rausch -138- (in l i t t . ) . Numerous others (mostly sub-adults) have been seen at various times about Nunivak Island (R.B. Gibson, in l i t t . ) , the Pribilofs (Preble, 1923; E.B. E l l i o t , voc. com.; V.B. Scheffer, voc. com.), and the Commander Islands (Barabash- Nikiforov, 1938). These records are sufficiently abundant and consistent to indicate a significant juvenile mortality along the edge of the winter pack, and i t is possible that the cause of death i s , as Hanna has suggested, accidental crushing between ice cakes during severe storms. Again, however, this could be post-mortem effect, and until autop- sies can be performed on the fresh specimens, no definite conclusions can be reached. S t i l l b i r t h s and orphans.—Stillbirths and miscarriages had been observed infrequently by the Gambell men. One of the former was reported in 1954, and another potential one was observed by the writer in 1952. The latter specimen's amnionic sac was f i l l e d with semi-putrid blood from a recent hemorrhage, and i t would undoubtedly have been born dead. Since the calf harvest at Gambell from 1952 to 54 was about 175-200 animals, these records suggest that about 1 per cent of the calves are dead at birth. One emaciated orphan was observed near Gambell in 1954 by W. Caldwell (voc. com.), but, generally, separation of mother and offspring probably does not happen very often except as a result of human interference. At L i t t l e Diomede and King Island large numbers of adult females are k i l l e d -139- annually for ivory, and their calves are set free. Unless these young are adopted by foster mothers, there is l i t t l e likelihood that they survive, for they appear to be unable to feed themselves. Natives believe that most of these are adopted by other cows (Brooks, 1954) — perhaps those which have lost their own young by s t i l l b i r t h or accident. One female taken at Gambell in 1953 had borne a calf one or two days previous to being captured, yet she was accompanied by a 2-month-old calf which could not have been her own. Orphaned yearlings may be able to sustain themselves in most instances, judging from Brooks' (1954) and Heinrich's (1947) observations, though the writer's specimen (p. 68) indicates that their survival i s not assured. It probably depends upon the amount of bottom feeding experience acquired prior to separation. Parasites and diseases.—The literature and the writer's observations indicate that parasites and diseases play a very insignificant role in Pacific walrus mortality. Ectoparasites are common but appear to be only i r r i t a n t s ; endoparasites are rare, and no pathological effects have been observed. Except for Heinrich's (1947) report of a tuberculosis-like infection in one specimen, no major dis- ease symptoms have been recorded. Occasional uterine tumors ( = "cysts" of Brooks, 1954, p. 62) seemed to have had no i l l effects upon the bearers, and animals with gangrenous tusk roots were otherwise in good health, judging by gross -140- appearance. The latter i s probably the cause of occasional one-tusked and tuskless individuals, but tusk loss seems to have no serious consequences. It i s conceivable that in a more dense population disease would be relatively more common, but there are no indications that i t ever has been. Non-human predation.—From the literature and native reports i t is clear that k i l l e r whales (Grampus rectipinna) prey upon walruses, but i t seems probable that they account for only a very small fraction of the annual mortality, for they do not usually frequent the pack ice region. At present they are uncommon in the Bering Sea (Kenyon, et a l , 1954). Polar bears (Thalarctos maritimus) have often been cited as possible walrus predators, but judging from Brooks' (1954), Nikulin's (1940), and Sverdrup's (Mohr, 1952) con- clusions, they probably do not have much effect upon the walrus population. Their predation appears to be limited mainly to play (Brooks, 1954) and to situations in which no regular foods (i.e. seals) are available. Human predation.—Unquestionably, human predation has been the primary factor controlling walrus numbers with- in the past 150 years in both eastern and western hemispheres. Its effect has been a marked reduction in population size and maintenance of low densities thereafter. White hunters in quest of o i l and ivory were responsible for the major decima- tion of Pacific walruses prior to 1930, but Natives armed -141- w i t h high-powered repeating r i f l e s have played an important part i n keeping the herds i n check since then. The l a t t e r has been due p a r t l y to modern demands f o r i v o r y and p a r t l y t o poor techniques f o r securing dead and wounded animals. So f a r as the w r i t e r i s aware, i n t e n s i v e i v o r y hunt- i n g i s p r a c t i c e d only i n Alaska at present and i n only two l o c a l i t i e s there — L i t t l e Diomede and King I s l a n d . I t accounts f o r at l e a s t o n e - t h i r d of the t o t a l annual k i l l i n Alaskan waters. The problem of securing the animals a f t e r they have been shot i s a u n i v e r s a l one, f o r i n the seasons when they are most v u l n e r a b l e to hunting, they are a l s o most l i k e l y t o s i n k when k i l l e d i n the water. In Alaska about 50 per cent of the dead and wounded are l o s t , and i t i s estimated t h a t at l e a s t h a l f of these e v e n t u a l l y d i e . There are no data a v a i l a b l e on Soviet l o s s e s , but they are probably not as gr e a t , f o r i v o r y hunting i s not p r a c t i c e d t h e r e , and the e n t i r e carcasses are u t i l i z e d . At Gambell, where the same p o l i c i e s apply, l o s s e s are 30-40 per cent. The average annual harvest i n Alskan waters i s c u r r e n t l y about 1200 per year (Appendix I I I A ) — the a c t u a l k i l l about 17-1800. Judging by e a r l i e r r e p o r t s (Brooks, 1954; N i k u l i n , 1940; Taracouzio, 1938), the annual S i b e r i a n k i l l i s between 2-3000. Density Dependent Factors Under p r i m i t i v e c o n d i t i o n s , walrus populations were -142- unquestionably much larger than they are at present, and factors other than human predation must have controlled population size. What these factors may have been can only be reasoned from present conditions and the few bits of i n - formation written by early explorers. Two possibilities are suggested below. Food.—Pelecypods are very abundant in certain north- ern l o c a l i t i e s , and the extensive shallows of the Bering and Chukchee Seas provide broad expanses of potential habitat. But walruses consume enormous numbers of these animals (e.g. Zalkin, 1937, found more than 2000 feet and siphons in one bull's stomach), and i t i s conceivable that a very dense walrus population could effectively control the pelecypod population, hence control i t s e l f . Possibly this was one of the causes for broader range of walruses in early times, for they would be expected to disperse more widely in search of better feeding grounds. Social aggregates.—An exotic factor intrinsic of the social behaviour i s death by crushing and suffocation. When large herds haul out on land, a few individuals on the bottom of the pile succumb from the tremendous weight of their companions on top. In a dense population there would be relatively more large herds (i.e., the chances of indi- viduals meeting other individuals would be greater) and death by crushing should, therefore, be relatively more frequent. Extensive deposits of "beach ivory" at many old -143- and new hauling grounds (cf. Wilke, 1942) are probably a result of this phenomenon. Calves and juveniles are occasionally crushed in this manner, but in general they tend to stay on top of the " p i l e . " One crushed calf was taken at Gambell in 1954. The Effect Maximum longevity.—Tooth and tusk analyses above indicate that walruses have a potential physiological lon- gevity of about thirty years. This agrees with the general "rule" that the l i f e span of mammals in roughly five times the period of growth (Brody, 1945, p. 681). Similar figures have been reported for some other pinnipeds (e.g. Phoca groenlandica-28 yrs; Fisher, 1952; Halichoerus-26 and 42 yrs: Matheson, 1950), but the majority range about twenty years. Ecological longevity.—"Ecological longevity i s the empirical average longevity of the individuals of a popula- tion under given conditions" (Bodenheimer, 1938, p. 19). This s t a t i s t i c can be readily calculated from the age struc- ture of the population, but age structure in i t s e l f i s more useful than the average figure, for i t can be used as a basis for calculations of mortality rates, survivorship, and productivity. The relative abundance of age classes in the present Pacific walrus population has been approximated as follows: -144- (1) In the spring of 1953, one tooth was collected from each of 155 bulls (exclusive of calves) k i l l e d at Gambell and Savoonga. Using cementum layers as indices of age, a "catch curve" of these specimens was plotted and smoothed with a running average of five. The result (Fig. 36) bears a close resemblance to some presented by Ricker (1948) for fish populations, and i t illustrates the St. Lawrence Island policy of selecting adult animals, (2) For purposes of discussion, Ricker (o£. cit.) divides catch curves into three parts: (a) the ascending l e f t limb, which i s a result of incomplete sampling of the youngest age classes, (b) the dome-shaped middle portion, and (c) the descending right limb. The latter, in the f i s h populations considered, i s a random sample of the older age classes and can, therefore, be used as a measure of survivor- ship and mortality rates within that portion of the popula- tion. The above walrus catch curve i s believed to be a random sample of the male population over 7 or 8 years of age, for selection i s inoperative beyond that point. Though there are many irregularities, probably due to random error of sampling, the right limb from about 8 years on can be considered as a straight line with a slope of about 0.12 (= it the instantaneous mortality rate). (3) Several estimates of proportions of the popula- tion represented by younger age classes (under 8 years) have been obtained by other methods. F i r s t , i t was observed above -145- Fig. 36.—Catch curve of male walruses taken at Gambell and Savoonga in the spring of 1953 (arith-log scale). Approximate age structure and survivorship of the male pop- ulation are indicated by the right limb of the curve plus relative proportions of young and juveniles, derived from photo analyses. See text (p. 144 et seq.) for explanation of techniques. -146- that the crude birth rate i s about 0.39 calves" per adult cow (over 3 years of age) per year. Assuming that the pop- ulations of both sexes are approximately equal, the propor- tion of male calves per bull over 3 years of age should be about 0.20:1. Applying this to the catch curve sample (137 animals over 3 years), a value very close to the y-intercept of the adult survivorship curve (step 2, above) was obtained. (4)' Several recent photographs of large herds were supplied by the U.S. Navy (U.S.S. Burton Island) in conjunc- tion with Ryder's (1953a; 1954) reports on walrus observa- tions. In these photos i t was possible to distinguish an- imals of ages 1 to 3, inclusive, by means of general morphol- ogy and tusk size. With the exception of yearlings, only those animals whose tusks were visible were counted. These yielded ratios (number of males in age class t to total males 4 and over) of 0.20:1 for yearlings, 0.17:1 for 2-year- olds, and 0.19:1 for 3-year-olds. Applying these to the catch curve sample as in step 2, three additional points very close to the adult survivorship curve were obtained (Fig. 36). Since a l l of the data ut i l i z e d above represent rather small samples, only the broadest generalities can be con- cluded from them. As a whole, the age structure (of males, at least) indicates a stable, highly productive population in which the birth rate and death rate are approximately uniform and are of about equal magnitude. The total annual -147- mortality (crude death-rate) i s about 12 per cent of the population, most of which i s due to human predation. This i s roughly comparable to Brooks' (1954) productivity estimate (13 per cent per year) and to Chapsky's (1936) conclusion that the stable Kara Sea population has a 10-15 per cent annual turnover. Ecological longevity at present i s approximately nine years, according to the above calculations. This checks favorably with an estimate derived from "corpus counts" in the ovaries of forty-two adults. The average number of pregnancies per female, 4 years and over, was 2.4, which i s equivalent to;alongevity of about eight or nine years. PART III THE POPULATION -149- The problem i s not entirely solved, but the solution i s near at hand. Within the limited time that i t has been pursued, a number of significant facts and hypotheses have been derived, and these, with some further exploration, should yield a l l of the necessary biological bases for sound population management. Tentatively, a few conclusions can be reached regarding the present population's numerical status and i t s probable past and future trends. From the harvest figures (Appendix IIIA) i t i s appar- ent that there has been a slight decrease in the number taken in Alaskan waters over the past fifteen years. This applies specifically to the coastal points, some of which have dropped more than 50 per cent. According to Brooks (1954), R.F. Gray (in l i t t . ) , and personal interviews, the change has been a result of reduced hunting effort as well as a notable scar- city of walruses in some l o c a l i t i e s . The latter i s also indicated by other data (see below) and has probably resulted in an equivalent decrease in current Siberian harvests. Very roughly, the total annual k i l l (Alaskan-Siberian) in the late 30's must have been about 5,000, while at present i t i s approx- imately 4,500 (Table 15). Since human predation appears to be responsible for the bulk (more than three-fourths) of annual mortality, and the death rate i s roughly 12 per cent per year, i t follows that the total population i s probably upwards of 40,000 -150- TABLE 15 ESTIMATED ANNUAL PACIFIC WALRUS MORTALITY FROM HUMAN PREDATION Late 1930's Location Harvest Per Year a Losses** Total K i l l Per Year Alaska Siberia Totals... 1300 2300 650 800 1950 3100 3600 1450 5050 Early 1950's Alaska Siberia Totals... 1170 2070° 590 720 1760 2790 3240 1310 4550 aSee Appendix IIIA ^Alaskan - 50 per cent; Siberian - about 35 per cent cAssuming a decrease equivalent to the Alaskan (10 per cent) -151- individuals. This figure compares favorably with Brooks' (1954) conclusion that 15,000 migrants passing through the eastern half of Bering Strait represented less than 50 per cent of the population. It also agrees, roughly, with another estimate derived from Ryder's (1954) data, which were u t i l i z e d in a crude census of the migrant population (Appendix IIIB). The resulting figure (38,000 animals) approaches the others in magnitude. Overall hunting pressure seems to have remained f a i r l y constant except at the most northerly Alaskan villages, many inhabitants of which emmigrated to Barrow in recent years for employment at the Naval Petroleum Reserve. In view of this, the Alaskan harvest data indicate a slight decline in the walrus population since the late 30's — probably less than 10 per cent. This amount would be imper- ceptibly small to the average observer except over a period of several decades, for the number of animals encountered near any Alaskan village fluctuates widely from year to year, and local opinions of population status vary directly with these fluctuations. If opinions were recorded over a long enough duration, however, some definite trend should be evident. Table 16 presents a compilation of these as extracted from various sources. Unfortunately they are not of equal, weight, for some represent reports from individuals, and others are averages from several, but in both groups of data there i s a consistent trend pointing toward a slight -152- TABLE 16 NUMBER OF REPORTS ON POPULATION TRENDS Population Status* Years Increasing Stable Decreasing 1939-45 1 3 3 1951-54 2 4 4 •Based upon Collins (1940), Brooks (1953, 1954), personal questionnaire replies, and various reports in U.S. Fish and Wildlife Service f i l e s . -153- po p u l a t i o n d e c l i n e . Another comparison of the population of the 30*s w i t h the present one was accomplished by matching Freimann*s (1940, F i g . 2 and Table 1) data against Brooks* (1954, F i g . 6) and w r i t e r ' s ( F i g . 14). Freimann's tusk OEL freque n c i e s (both sexes) e x h i b i t a higher p r o p o r t i o n of o l d a d u l t s than the recent ones do, suggesting a d i f f e r e n c e i n age s t r u c t u r e corresponding to a d e c l i n i n g p o p u l a t i o n ( c f . A l l e e , et a l . 1949 p. 281). There i s no apparent danger of e x t i n c t i o n under present c o n d i t i o n s ; indeed, a s t a t e of s e m i - e q u i l i b r i u m has been or i s being achieved. Nonetheless, i n terms o f maximum b e n e f i t s f o r a l l , the population should be permitted to i n c r e a s e , f o r the current harvest r a t e per n a t i v e v i l l a g e i s too e r r a t i c . The people of Gambell, f o r example, need at l e a s t 200 walruses per year to s a t i s f y t h e i r maintenance requirements ( e x c l u s i v e o f i v o r y ) , yet t h e i r harvests from 1952 to 1955 ranged between 70 and 275 (Appendix I I I A ) . This v a r i a t i o n was independent of hunting e f f o r t (average 1 hunting day per 2.4 calendar days each s p r i n g ) . I t appeared to be e n t i r e l y a r e s u l t of i c e c o n d i t i o n s which were, i n t u r n , a product of the weather. Should walruses become more abundant, the chances of these v a r i a t i o n s o c c u r r i n g would be very much reduced by reason of the greater d e n s i t y of animals per u n i t a rea. Should the pop u l a t i o n remain unchanged or become l e s s abundant, the few v i l l a g e s dependent upon i t must e i t h e r d i m i n i s h i n s i z e o r -154- become burdens to the Government. The remedy is simple: eliminate ivory hunting and hunting losses. These wasteful practices can be tolerated no longer, for they serve no social or economic purpose, and at present they are responsible for more than one-fourth of the total annual mortality. Admittedly, unless every animal i s harpooned before being shot (which i s feasible but not necessary), there w i l l continue to be losses from wound- ing and sinking, but a planned, long-term educational program could easily reduce losses by one-half. The consequent decrease in mortality (losses halved and ivory hunting elim- inated) would permit the walrus population to double in less than thirty years. Application of the remedy would be f a i r l y complex, for as Brooks (1954) and Scott (1951) have noted, there are many socio-economic problems involved. Since these have already been adequately dealt with by the above authors, no further comment i s necessary, except to say that they could be solved within a few years and the time to begin i s now. Biological investigations should be continued, and, u l t i - mately, international (Soviet-U.S.) cooperation must be achieved. APPENDICES -156- APPENDIX I PRENATAL CROWN-RUMP LENGTH GROWTH OF HUMANS* No. of Days Per cent of Crown-rump Per cent of Since Total Length Natal C-R Conception Gestation (mm.) Length Achieved 14 5.2 0.1 0.30 21 7.8 1.5 0.45 28 10.4 2.5 0.74 35 13.1 5.5 1.64 42 15.7 11 3.27 49 18.3 17 5.06 56 20.9 25 7.44 84 31.3 63 20.2 112 41.8 121 36.0 140 52.3 167 47.9 168 62.7 210 62.5 196 73.2 245 72.9 224 83.6 284 84.5 252 94.0 316 94.1 268 (Birth) 100.0 336 100.0 *Data from Noyes et al (1938, p. 52). -157- APPENDIX I--Continued GESTATION COMPLETED (%) Curve of equivalent foetal crown-rump length growth, based upon Noyes, et al's (1938) data for humans• -158- APPENDIX IIA LINEAR GROWTH, WEAR, AND OBSERVED LENGTHS OF MALE TUSKS IN INCHES Age ATL Wear OTL RL OEL Increment Total Rate Total 0 1 2 3 2 1/8 2 3/a 4 7/8 4 3/8 3 1/4 3 2 1/4 1 7/8 1 5/8 1 1/2 1 3/8 1 1/8 1 1/8 2 4 9 13 1/8 1/2 3/8 3/4 0 0 3/3 1/2 5/8 1/2 3/8 1/2 3/8 3/8 1/2 3/8 3/8 3/8 3/8 3/8 3/8 1/4 3/3 0 0 3/8 7/8 2 4 9 12 1/8 1/2 7/8 2 3 5 6 i / a 1/2 1/4 1/4 0 1 3 3/4 6 5/8 4 5 6 17 20 22 1/4 1 2 2 1/2 3/3 15 18 19 1/2 7/8 6 6 6 5/8 3/4 3/4 8 7/8 11 1/4 13 1/8 7 24 1/8 2 7/8 21 1/4 6 7/8 14 3/8 3 25 3/4 3 1/4 22 1/2 6 7/3 15 5/3 9 10 27 28 1/4 5/8 3 4 5/8 1/8 23 24 5/8 1/2 .7 7 16 5/3 17 1/2 11 29 3/4 4 1/2 25 1/4 7 1/8 18 1/8 12 30 7/8 4 7/8 26 7 i / a 18 7/8 13 X 7/8 7/8 3/4 3/4 3/4. 31 7/8 5 1/4 26 5/8 7 1/4 19 3/3 14 32 3/4 5 5/8 27 i / a 7 1/4 19 7/8 15 33 5/8 6 27 5/8 7 1/4 20 3/3 16 34 3/8 6 3/8 28 1/8 7 3/8 20 3/4 17 35 1/8 6 5/8 28 1/2 7 3/8 21 1/8 18 35 7/8 7 28 7/8 7 3/8 21 1/2 -159- APPENDIX IIA—Continued Age ATL Wear OTL RL OEL Increment Total Rate Total 19 5/3 5/3 5/8 1/2 1/2 1/2 1/2 1/2 3/8 3/8 3/8 1/4 1/4 1/8 36 1/2 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 7 1/4 29 1/4 7 1/2 21 3/4 20 37 i / a 7 1/2 29 5/3 7 1/2 22 1/8 21 22 37 38 3/4 1/4 7 8 3/4 30 30 1/4 7 7 1/2 1/2 22 1/2 22 3/4 23 33 3/4 8 1/4 30 1/2 7 1/2 23 24 39 1/4 8 1/2 30 3/4 7 1/2 23 1/4 25 39 3/4 8 3/4 31 7 1/2 23 1/2 26 40 1/4 9 31 1/4 7 1/2 23 3/4 27 40 5/3 9 1/4 31 3/8 7 5/3 23 3/4 28 29 30 31 41 41 41 41 3/8 5/3 7/8 9 9 10 10 1/2 3/4 1/4 31 31 31 31 1/2 5/8 5/3 5/3 7 7 7 7 5/8 5/3 5/3 5/3 23 7/8 24 24 24 32 42 10 1/2 31 1/2 7 3/4 23 3/4 ATL r Absolute Total Length OTL = Observed Total Length or ATL-Total wear RL - Root Length OEL = Observed External Length or OTL-RL -160- APPENDIX IIB LINEAR GROWTH, WEAR, AND OBSERVED LENGTHS OF FEMALE TUSKS IN INCHES Age ATL Increment 0 1 2 3 4° 5? 6° 7 P a ° 9 P 10 n 12° 13 p 14 15 16 17° 1 8 P 1 3/4 1 7/8 4 1/8 4 5/8 3 1/4 2 3/8 1 7/8 1 1/2 1 3/8 1 1/4 1 1/8 1 1 7/8 7/8 3/4 3/4 3/4 5/8 Total 1 3/4 3 5/8 7 3/4 12 3/8 15 5/8 18 19 7/3 21 3/8 22 3/4 24 25 1/8 26 1/8 27 1/8 28 28 7/8 29 5/8 30 3/8 31 1/8 31 3/4 Wear Rate 0 0 3/3 5/8 1/2 5/3 7/8 5/3 1 1/2 3/4 3/8 3/8 3/8 3/4 1/4 1/4 1/4 3/8 Total 0 0 3/8 1 1 1/2 2 1/8 3 3 5/8 4 5/8 5 1/8 5 7/8 6 1/4 6 5/8 7 7 3/4 8 8 1/4 8 1/2 8 7/3 OTL RL OEL 1 3/4 3 5/8 7 3/8 11 3/8 14 1/8 15 7/3 16 7/8 17 3/4 18 1/8 18 7/8 19 1/4 19 7/8 20 1/2 21 21 1/3 21 5/8 22 1/8 22 5/8 22 7/8 2 2 3/4 4 1/8 5 1/3 5 1/2 5 5/8 5 5/8 5 5/8 5 3/4 5 3/4 5 3/4 5 3/4 5 3/4 5 7/8 5 7/3 5 7/8 6 6 6 0 7/8 3 1/4 6 1/4 8 5/8 10 1/4 11 1/4 12 1/8 12 3/8 13 1/8 13 1/2 14 1/8 14 3/4 15 1/8 15 1/4 15 3/4 16 1/8 16 5/8 16 7/8 -161- APPENDIX IIB—Continued Age ATL Wear OTL RL OEL Increment Total Rate Total 5/8 3/8 1/2 3/3 19 32 9 23 6 17 5/8 1/4 3/8 17 3/8 20 33 9 5/8 23 6 1/2 1/2 1/4 21 33 9 7/8 23 5/8 6 17 5/8 1/2 1/4 22 34 10 1/8 23 7/8 6 1/8 17 3/4 1/2 1/2 1/4 1/8 23 34 10 3/8 2 4 6 1/8 18 1/2 1/4 18 1/4 24 l /? 35 i 10 5/8 2 4 3/8 6 1/8 25° i/<i 35 1/2 i/4 10 7/8 2 4 5/3 6 1/8 18 1/2 26p 3/3 3/8 35 7/8 1/2 11 1/4 24 5/8 6 1/4 18 3/8 3/8 27 1 /A 36 1/4 11 3/4 24 1/2 6 1/4 18 1/4 28 I/O 36 5/8 1/4 12 24 5/8 6 1/4 18 3/8 1/4 1/4 1/4 29 i /A 36 7/8 l /i. 12 24 5/8 6 1/4 18 3/8 30 1/5 37 1/4 12 1/2 24 1/2 6 1/4 18 1/4 1/8 1/8 1/4 31 37 12 3/4 24 3/3 6 3/8 18 1/8 1/4 32 37 1/4 13 24 1/4 6 3/8 17 7/8 ATL s Absolute Total Length OTL = Observed Total Length or ATL-Total wear RL = Root Length OEL = Observed External Length or OTL-RL ° 0 e s t r u s ^Parturition APPENDIX IIIA THE ALASKAN WALRUS HARVEST* Harvest Per Year Location Average: Late 1930's 1939 1940 1944 1945 1947 1950 1951 1952 1953 1.954 1955 Average 1945-55 Barrow 75 0 105 4 1 + w mm mm 35 Diomede 250 — —- 400* — — 350 507 120 350 Gambell 200 313 275 200 70 120 170 Hooper Bay- 25 25 King I. 300 147 300 50 250 Kipnuk 4 5 Mekoryuk 25 — 20 20 5 —_ 15 Nome 25 12 10 Pt. Hope 75 .m. 5 — —- 5 Pt. Lay 75 16 2* 0 —- 5 Savoonga 100 175 120 150 Shaktoolik — 19 — 0 — 5 St. Michael — 11 —- —— —- 5 Tananuk 6 —- 5 Togiak 75 —- —- 75 Unalakleet 10 — —- 5 Wainwright 100 76 — — 0 4 — —- 5 Wales 50 42 70 ——— —— 55 Totals 1300 1175 *Data and averages from: Brooks (1954), Collins (1940), Hughes (1953), Lantis (1946), "Mukluk Telegraph" (Dec. 1951), W. Caldwell (in l i t t . ) . R.B. Gibson (in l i t t . ) t C.K. Ray (voc. com.), U.S. Fish and Wildlife Service f i l e s , questionnaire replies, and personal observation. -163- APPENDIX IIIA—Continued THE SIBERIAN WALRUS HARVEST Taracouzio (1938) gives the following figures for walrus harvests of the early 1930's in Siberian waters: Year Total Harvest by Hunting Vessels 1930 1560 1931 3244 1932 1428 1933 2723 Average. 2240 Nikulin 1s (1940) data, while less specific or com' plete, suggests that the harvests from 1935-38 were about the same as those above and that less than 200 additional animals were taken annually at coastal native villages. Hence a conservative estimate of the average late 30's harvest i s about 23-2400. - 1 6 4 - APPENDIX IIIB CALCULATION OF TOTAL POPULATION BY THE STRIP SAMPLING METHOD 1 . Ryder's ( 1 9 5 4 ) data consist of 5 7 separate obser vations within the spring pack of the North Bering Sea and Chukchee Sea between latitudes 6 1 ° N and 7 2 ° N in May, 1 9 5 4 . 2 . These can be divided into four "strips," two one-way trips from the region of St. Lawrence Island to Point Barrow, and two return trips between the same points. 3 . The approximate linear miles traveled in each strip were measured and multiplied by 3 , the estimated strip width in miles. Serious error could be involved here, since Ryder states (in l i t t . ) that strip width varied widely and at times was n i l , due to fog and darkness. The proposed 3-mile width i s an attempt to average these conditions. 4 . The mean number of walruses per square mile thus derived was applied to the probable total walrus-occupied area shown in Figure 4 (p. 9 ) — about 1 3 0 , 0 0 0 square miles. The results are tabulated below. -165- APPENDIX I I I B — G o n t i n u e d I S t r i p No. D i r e c t i o n Traveled L i n e a r M i l e s Traveled Square M i l e s Censused Walruses Seen Walruses Per Square M i l e I North 770 2310 841 + 0.36 I I South 575 1725 81 0.05 I I I North 580 1740 129/ 0.75 IV South 630 1890 35 0.02 T o t a l s . . . 2555 7665 2254+ mm mm M M Averages*.. 639 1916 563+ 0.29 T o t a l p o p u l a t i o n = ( t o t a l area) x (avg. walruses per sq. mile) 130,000 x 0.29 = 33,0001"animals -166- LITERATURE CITED Allee, W.C., A.E. Emerson, 0 , Park, T. Park, and K.P. Schmidt. 1949. Principles of animal ecology. Philadelphia. Saunders. 837pp. Allen, E., J.P. Pratt, Q.U. Newell, and L.J. Bland. 1930. Human ova from large f o l l i c l e s ; including a search for maturation divisions and observations on atresia. Amer. Jour. Anat. 46 :1-54. Allen, J.A. 1880. History of the North American pinnipeds. 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