FACTORS AFFECTING THE GROWTH-RATE AND MASS AT WEANING OF ANTARCTIC FUR SEALS AT BIRD-ISLAND, SOUTH GEORGIA

We studied the influence of sex of pup, maternal age, birth date of pu p, number of foraging trips, and the mean duration of foraging trips a t sea and nursing visits ashore on the growth and mass at weaning of p ups of Antarctic fur seals (Arctocephalus gazella) during austral summ ers of 1988-1990. Although growth and mass at weaning were highly corr elated, they were not related to maternal characteristics in 1988 or 1 989. However, in 1990 there was a negative relationship between growth of pups and mean duration of foraging trips. Growth rates of males an d females varied considerably between 1972 and 1991 and appeared to de cline from 1984 through 1990. Methods used to collect and weigh the pu ps influenced the nature and magnitude of sex differences in estimated growth rates. Growth rates of male and female pups did not differ whe n weighed serially (same individuals weighed throughout lactation), bu t males grew faster than females when weighed cross-sectionally (diffe rent individuals weighed throughout lactation). Based on our results o f pairs of mothers and pups followed over the lactation period, matern al investment was greater in males than females because males were hea vier at birth and older at weaning than females and not because of any differential growth between the sexes. Mothers appear to have to work longer, but not harder, to wean males than females. Under the favorab le feeding conditions that usually exist, individual differences in th e growth of pups are most likely influenced by variation in foraging e fficiency of mothers.