VARIATION IN WEANING MASS OF PUPS IN RELATION TO MATERNAL MASS, POSTWEANING FAST DURATION, AND WEANED PUP BEHAVIOR IN SOUTHERN ELEPHANT SEALS (MIROUNGA-LEONINA) AT SOUTH-GEORGIA

Female southern elephant seals, Mirounga leonina, assemble in large groups and each gives birth to a single pup which is nursed for some 3 weeks. Weaning mass is highly variable; some pups are three times as heavy as others at weaning. After weaning, the pup fasts for several weeks before departing to sea. The function of this fast is unknown. We examined the relationships between maternal mass, pup weaning mass, and pup behaviour during the postweaning fast in 377 pups and 128 adult females over four breeding seasons at South Georgia. Pup weaning mass was positively related to maternal post-partum mass, which accounted for 55% of the variation in weaning mass. Over all 4 years male pups were significantly heavier at weaning than female pups (130 vs. 123 kg) but this difference disappeared after maternal mass was controlled for. After fasting for 21 - 66 days, weaned pups went to sea at an average of 68% of weaning mass. Heavier pups remained on the beach longer after weaning than lighter pups. There was no evidence that pups synchronized their departure to sea. Only male pups were observed to take part in mock fights. With increasing age, weaned pups spent more time in the water. Mortality during the postweaning fast was negligible (0.1%). The timing of departure of weaned pups may involve a trade-off between an early departure with greater fat (energy) stores but poorer foraging ability and a late departure with increased swimming, diving, and social skills but reduced fat stores.