256 NORTH AMERICAN FAUNA 74 



Table 46. Comparative percentage compositions inferred for the 1960 and 

 1972 populations of Pacific walruses.^ 



age (years) 1960 1972 



Males 







0-9 



21 



16 



10-14 



15 



12 



>14 



14 



10 



Total 



50 



38 



Females 







0-5 



12 



16 



>5 



38 



46 



Total 



50 



62 



^From Tables 43 and 45. 



consistent with known events. The catches and losses in the harvests after 1958 

 were made up of about 75% males and 20% females, which probably could 

 have created an imbalance in the overall sex ratio tending to favor the females. 



The sex ratio of the breeding adults has been a point of contention between 

 Soviet and American biologists in recent years. Whereas Chapskii (1936) 

 estimated it as 1 male:3 females, his successors in the Soviet Union (Fedoseev 

 1962; Krylov 1968; Fedoseev and Gol'tsev 1969; Gol'tsev 1975) and some North 

 American workers (Mansfield 1958fl; Burns 1965) have assumed that the ratio of 

 adults was 1:1, like that of the newborn calves. In the estimated compositions 

 derived herein, the implied ratio of breeding adults is about 1 male:3 females to 

 1:5, which is consistent with those of other polygynous pinnipeds {Arctocephalus 

 1:5, Rand 1972; Zalophus 1:4, Peterson and Bartholomew 1967; Mirounga 

 1:2-1:3, Le Boeuf 1974; Halichoerus 1:4-1:5, Anderson et al. 1975). Since male 

 walruses require about 15 years to reach full maturity, whereas most females 

 mature by age 6 or 7, it is possible that the number of females of breeding age 

 could be about double the number of mature males, even if their survival rates 

 were equal. Because the mortality of males of other polygynous species tends to 

 be greater than that of the females (Chapman 1964; Johnson 1968; Le Boeuf 

 1974), this probably is true also of walruses. If so, the overall sex ratio in an 

 unexploited population would be expected to include more females than males, 

 and the sex ratio of breeding adults would be highly disparate. 



Production 



The gross productivity of walruses is substantially lower than that of other 

 pinnipeds, because the females ordinarily reproduce at intervals of 2 years rather 

 than 1 year. Hence one would expect the birth rate to be about half that of 

 species which reproduce annually. 



Several recent harvest samples from the Pacific walrus population in which 

 there was no selection for females with calves showed pregnancy rates of 35 to 

 44 % and birth rates of 41 to 51 % in females of reproductive age (Table 47). The 

 fact that the birth rates were mostly higher than the pregnancy rates indicates 



