Mass change in Antarctic fur seal (Arctocephalus gazella) pups in relationto maternal characteristics at the Kerguelen Islands

Maternal allocation to growth of the pup was measured in Antarctic fur seal s (Arctocephalus gazella) at the Kerguelen Islands during the 1997 austral summer. Absolute mass gain of pups following a maternal foraging trip was i ndependent of the sex of the pup but was positively related to foraging-tri p duration and maternal length. However, daily mass gain (i.e., absolute ma ss gain of the pup divided by foraging-trip duration) decreased with increa sing foraging-trip duration but increased with maternal length. While the p up were fasting, their daily mass loss was related to their sex and initial body mass: both heavier pups and female pups lost more mass per day than l ighter pups and male pups. The mass-specific rate of mass loss was signific antly higher in female than in male pups. Over the study period, the mean g rowth rate was zero, with no difference between female and male pups. The g rowth rate in mass of the pup was positively related to maternal length but not to maternal condition, and negatively related to the foraging-trip dur ation of the mother and the initial mass of the pup. This indicates that du ring the study period, heavier pups grew more slowly because of their highe r rate of daily mass loss during periods of fasting. Interestingly, for a g iven maternal length, the mean mass of the pup during the study period was higher for male than for female pups, even though the rate of daily mass ga in was the same. Such differences are likely to result from sex differences in the mass-specific rate of mass loss. As female pups lose a greater prop ortion of their mass per day, a zero growth rate (i.e., mass gain only comp ensating for mass loss) is reached at a lower mass in female pups than in m ale pups. Our results indicate that maternal allocation does not differ acc ording to the sex of the pup, but suggest that the two sexes follow differe nt growth strategies.