ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



213 



by man. I regard this poor understanding as one of the major gaps in present 

 knowledge of the walrus. 



The following account is a review of the information available from published 

 sources up to 1978 and unpublished data to 1979. It includes information on 

 morbidity and mortality not only from "natural" (nonhuman) causes but from 

 the harvests by man, as well. From a biological aspect, the killing of walruses by 

 man is a form of predation, and it appears to have been the most important 

 single cause of death over the past 120 years or more. 



The consequences of this mortality, in terms of its effects on the population, 

 are discussed further in the next chapter. My objective here is principally to 

 document the nature and causes to the extent that they are known. That the 

 information is meager is testimony to the fact that much more work is needed on 

 this subject. 



Fetal and Calf Mortality 



As considered here, mortality begins in utero. Before conception, some of the 

 fertilized ova fail to develop properly and are rejected; some blastocysts fail to 

 implant. My data (Table 35) suggest that preconception failures may be greater 

 than 10% per year; after conception, about 5% of the implanted embryos fail to 

 result in live births. 



In the first weeks after birth, some calves die, presumably from neglect, 

 trauma, predation, and infectious diseases. Belopol'skii's (1939), Loughrey's 

 (1959), and my Eskimo informants also expressed belief that some calves die 

 from hypothermia in cold, stormy weather. This seems plausible, for the balance 

 between energy intake and heat loss in these young animals may be easily upset. 

 The surface area of their extremities is relatively large (Iverson and Krog 1973), 

 their metabolic rate is about twice that of terrestrial mammals of comparable 

 size (Iverson and Krog 1973), and their surface temperature (indicative of heat 

 output) tends to be very high (Ray and Fay 1968). The calves appear to require 

 protection by the mother from excessive heat loss, at least during the first few 

 days or weeks after birth, probably until their blubber layer is well developed 

 (Fay and Ray 1968). During very cold or stormy weather, the adults tend to stay 

 in the water which could be thermally disadvantageous for the calf. 

 Unfortunately, there are no reliable indices of death due to hypothermia (Moritz 

 1953); hence mortality from this cause will be difficult to identify with certainty. 



The fate of calves that are abandoned or become separated from the mother by 

 death or accident has not yet been fully determined. Chapskii (1936) and Kenyon 

 (1960Z?) expressed the opinion that all such orphans starve to death, but several 

 investigators have observed that those orphans usually are carried away by 

 another adult when the mother is killed. Burns (1965 and unpublished data), 

 Eley (1978), and I obtained eight unequivocal records in May and June of cows 

 with calves that were not their own, which suggest that fosterage is common. 

 Popov (1960fl), Krylov (1962), and Gol'tsev (1975) found that 4 to 5% of cows 

 taken in July to September were nursing a calf of the year while pregnant with a 

 new fetus. Because the probability of annual breeding in walruses appears to be 

 very slight, I suspect that those also were calves that had been adopted by foster 

 mothers. 



