Abstract
Labeled water methodology was used to quantify maternal energy investment in northern elephant seal (Mirounga angustirostris) pups. Pup milk intake, field metabolic rate, mass change, and body composition were monitored during the nursing period and throughout the postweaning fast. Maternal investment did not vary significantly as a function of pup sex. There was no evidence for sex differences in growth rate or milk intake rate, and mean metabolic rate of suckling pups did not differ significantly between the sexes. Males and females appeared to be in similar condition at the end of the postweaning fast, just prior to their first trip to sea. Some male pups may attain greater size relative to females without additional cost to their mothers. These data do not provide evidence to reject R. A. Fisher's theory of equal investment in the sexes.
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