Ecology and Biology of the Pacific Walrus, 

 Odohenm rosmarus divergens lUiger 



by 



Francis H. Fay 



Institute of Marine Science 

 University of Alaska 

 Fairbanks, Alaska 99701 



Abstract 



The distribution, physical development, pelage, dentition, feeding behavior, 

 reproduction, causes of mortality, and population structure of the Pacific walrus 

 (Odobenus rosmarus divergens) were studied intermittently from 1952 to 1979. In 

 winter, these animals tend to concentrate in north-central and southeastern Bering 

 Sea, where sea ice conditions are most favorable for them. In summer, they 

 concentrate mainly in northwestern and northeastern Chukchi Sea, along the edge 

 of the ice. Most of the northward migrants are females and young; a large 

 proportion of the adult males remains in the Bering Sea throughout the summer. 

 Pacific walruses show strong sexual dimorphism; adult males are about 18% 

 longer and 45% heavier and tend to have larger, more divergent tusks, as well as 

 thicker, lighter-colored, and less hairy skin than adult females. As in other sexually 

 dimorphic otarioid pinnipeds, males undergo secondary growth, beginning about 

 the time of puberty and ending in full physical maturity about 15 years of age. The 

 first pelage is a fine, white lanugo, which develops and is shed in utero. The 

 second (natal) pelage, which resembles that of the adult, is shed and replaced 2 to 

 3 months after birth, in synchrony with the molt of older animals. The full 

 primary and secondary dentitions include 38 and 30 teeth, respectively; 7 pairs of 

 primary and 6 pairs of secondary teeth occur in less than 50% of the animals. The 

 permanent first upper premolar is a secondary tooth, preceded by an uncalcified 

 primary tooth. The lower premolars appear to be homologues of P2-3.4, rather 

 than Pi.2.3. The cheek teeth grow in length very slowly, and the pattern of 

 decrement of their crowns indicates no contact with molluscan shells except in the 

 incisive area at the front of the mouth. The abrasion of the tusks indicates that 

 they are dragged through the bottom sediments, rather than used for digging or 

 raking. Food of the Pacific walrus consists of more than 60 genera of marine 

 organisms, most of which are situated on or just beneath the surface of the sedi- 

 ments. The walrus apparently locates these tactually with its sensitive mystacial 

 vibrissae and by "rooting" with its snout. Soft-bodied organisms are ingested 

 directly, without mastication; the soft parts (siphon, foot) of mollusks probably 

 are separated from the shells by suction. The intake of food is at least 5 to 7% of 

 the total body weight per day. Most females ovulate for the first time at 5 or 6 

 years; males become fertile at 8 to 10 years but probably do not participate in 

 mating until fully mature at 15 years. Walruses are polygynous; mating takes 

 place mainly in mid-winter. Implantation of the blastocyst takes place about 5 

 months later, and the calf is born in the following spring, after a pregnancy lasting 



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