Growth in pups of the subantarctic fur seal (Arctocephalus tropicalis) on Amsterdam Island

Growth in body mass in pups of the subantarctic fur seal Arctocephalus trop icalis was studied at Amsterdam Island, southern Indian Ocean during the re aring period between December 1995 and August 1996. We examined the change in the rates of mass gain and mass loss with pup age to explain the overall change in mass throughout the rearing period, when mothers alternate forag ing trips at sea and attendance periods ashore. In the absence of their mot hers, pups fast and lose mass. During the maternal attendance period ashore pups suck and gain mass. There was no significant difference in the rate o f growth in mass between male and female pups. The rate of loss in mass dur ing a fasting event was positively related to the initial body mass but neg atively related to the age and duration of the fast of the pup. The rate of mass loss was best related to pup age by a quadratic relationship. The rat e of mass loss increased until pups were 120 days old and then decreased un til the end of the study period. The rate of gain in mass following a mater nal foraging trip was positively related to the initial pup mass but negati vely related to the foraging trip duration, and we found that the rate of m ass gain decreased linearly with the age of the pup. The concomitant decrea se in the rate of mass loss and mass gain for pups > 120 days old allowed p ups to maintain a positive growth until they were 220 days old. After 220 d ays of age the rate of mass loss exceeded the rate of mass gain and the pup s start to lose mass until weaning.