ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



211 



and the presence of a placental scar in association with a corpus albicans. Using 

 those criteria, I ranked my specimens according to their age at the time of their 

 latest ovulation and determined from them their rates of success in conception 

 (implantation) and completed gestations. The results (Table 35) indicate that the 

 lowest rates of success in conception and gestation (40 to 60 % ) tend to occur in 

 the youngest and the oldest age classes of fertile females; the highest rates 

 (>80%) occur between the ages of 8 and 15 years. The rates of reproductive 

 failure in Pacific walruses appear to be high, relative to other pinnipeds (see 

 review by Harrison 1969), although comparatively lower than in domestic 

 mammals and man (Hafez 1967; Hertig 1967). 



Age-related capability for success and failure in reproduction seems to be the 

 rule in mammals (cf. Montagu 1957; Andersen et al. 1962; Belling 1963; Parsons 

 1964), although most free-living individuals do not survive long enough to reveal 

 a decline in productivity in old age. The walrus appears to be unusual in that 

 respect; some of the oldest females appear to be altogether incapable of 

 reproduction (Krylov 1966Z?). 



Because the duration of pregnancy in the walrus is more than 1 year, the 

 individual female tends to ovulate biennially or less often (excluding the 

 postpartum estrus). Hence, less than half of the females in a given year are newly 

 pregnant, and less than half give birth to a calf. On the basis of the sample of 206 

 potentially estrus females taken in February to August (Table 31), the propor- 

 tion of such females that are impregnated per year apparently is about 68 % ; 

 32% fail to conceive. Of the successful conceptions, about 95% apparently result 

 in completed gestations (Table 35). These findings suggest that, in any given 

 year, about 40% of the females of breeding age will have conceived, 38% will 

 have produced a calf, and 21 % will be neither pregnant nor productive. These 

 pregnancy and birth rates are within the range (but somewhat higher than the 

 means) of some derived from another, larger set of data from both Soviet and 

 American sources (see Population). Because all such estimates have been based 

 on samples drawn from a rapidly growing population, they may be typical only 

 of the growth phase. The population at its upper limit, K, the carrying capacit}^ 

 of the environment, probably will have a lower birth rate due to later matura- 

 tion of females and reduced fecundity of adults (Siniff et al. 1978). 



Morbidity and Mortality 



The walrus has the lowest known reproductive rate for a pinniped, is slow to 

 mature, and has a long life-span. For those reasons, its rate of natural mortality 

 probably is very low. As in most other pinnipeds, however, the causes and 

 amounts of natural mortality in walruses are little known and difficult to deter- 

 mine. Few biologists have had opportunities to examine sick, moribund, or dead 

 walruses in sufficient detail to determine the cause of those conditions. Where 

 natural deaths have been identified, their importance to the population as a 

 whole has been impossible to assess reliably. Although generally acknowledged 

 as a factor to contend with in population dynamics of walruses, natural 

 mortality is so poorly understood that it usually is dismissed as interesting but 

 unimportant, relative to that from the more visible, more accountable harvests 



