Early mortality and perinatal growth in the subantarctic fur seal (Arctocephalus tropicalis) on Amsterdam Island

Early mortality and Dug growth during the perinatal period were investigate d in pups of the subantarctic fur seal Arctocephalus tropicalis on Amsterda m Island. Mothers that were shorter in body length and expected to be young er, gave birth earlier in the pupping season, compared with longer/older mo thers. Pups burn early were often still-born, suggesting that shorter/young er mothers that gave birth early in the season were not able to carry their foetuses to term. Pregnant females arrived ashore 1.4 days before giving b irth, regardless of the date and their body condition. Then was a positive relationship between maternal body length and pup birth mass. Consistently, birth mass increased throughout the pupping period. After parturition, mot hers suckled their pups during an average 8.7-day postnatal period that was significantly shorter in mothers giving birth late in the season. However, the absolute gain in pup mass was 1.5 kg regardless of the birth date, sug gesting that mothers did not leave their pups before they had transferred a given amount of body reserves to them. We propose that pups born late grew faster because they were bigger at birth and because their mothers were li kely to be more experienced. Mothers in good condition, nursing male pups t ransferred more milk and therefore greater mass to their pup, whereas mothe rs in poor condition were unable to do so. Such differences did not occur i n smaller female pups, suggesting that pup growth was limited by maternal r esources in male pups but not in female pups. The lack of a relationship be tween birth mass and absolute gain during the perinatal period suggests tha t mass at birth determined pup body mass after the perinatal period. Body m ass is an important factor in growth rate and survival during the period of pup dependence.