Factors affecting the growth rate and mass at weaning of Antarctic fur seal pups at Bird Island, South Georgia

We studied the influence of sex of pup, maternal age, birth date of pup, number of foraging trips, and the mean duration of both foraging trips at sea and nursing visits ashore on the growth and mass at weaning of pups of Antarctic fur seals (Arctocephalus gazella) during the austral summers from 1988 to 1990. Although growth and mass at weaning were highly correlated, they were not related to maternal characteristics in 1988 or 1989. However in 1990 there was a negative relationship between growth of pup and mean duration of foraging trips. Growth rates of male and female pups varied considerably between 1972 and 1991 and appeared to decline from 1984 through 1990. Methods used to collect and weigh the pups influenced the nature and magnitude of sex differences in estimated growth rates. Growth rates of male and female pups did not differ when weighed serially (same individuals weighed throughout lactation), but males grew faster than females when weighed cross-sectionally (different individuals weighed throughout lactation). Based on our results of pairs of mothers and pups followed over the lactation period, maternal investment was greater in sons than daughters because males were heavier 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 male pups than female pups. Under the favorable feeding conditions that normally exist, individual differences in the growth of pups are most likely influenced by variation in foraging efficiency of mothers.