Parental investment in southern elephant seals, Mirounga leonina

The southern elephant seal is among the most sexually dimorphic and polygynous of all mammals: males may be more than 10 times the weight of reproducing females and only the largest 2–3% of males are likely to breed. Current optimization theories of sexual selection predict that evolution would favor greater parental investment in individual males than in females. Because southern elephant seals represent an extreme of polygyny and sexual dimorphism, they might be expected to show a dramatic difference in parental investment in male and female pups. However, in a study of parental investment in elephant seals at South Georgia, using several different methods, we found no such difference after parturition. Mother-pup pairs were immobilized and weighed early in lactation, recaptured near the end of lactation and reweighed. A further 30 pups were weighed an average of five times during lactation to establish the shape of the growth curve and to serve as partial controls for the previous set of animals. Initial post-partum weight in females ranged from 346 to 803 kg (=506, SD=111, n=26). Pup birth weight was related to mothers' post-partum weight in female pups but small females often gave birth to large male pups. Male pups were significantly heavier at birth than females. However, this size difference did not persist. Male and female pups were suckled for the same period, grew at the same rate and were not significantly different in weight at weaning. Mothers lost weight at the same rate regardless of their pup's sex.