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 P1-2-3. 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 sediments. 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 at least 15 months. Females tend to breed at 2-year intervals or less often and are most productive between the ages of 8 and 15 years. The principal causes of mortality appear to be predation, intraspecific trauma, and microbiological infections. The Pacific walrus population was severely depleted in the late 19th century and again in the mid-20th century by overharvests for commercial purposes. Over the past 20 years, it apparently has recovered rapidly, in response to reduced harvest. The sex ratio of breeding adults appears to be about 1 male:3 or 4 females. The crude birth rate in recent years is estimated at 17 ± 2%, and the survival of young to puberty appears to be very high.

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