Prenatal investment in the subantarctic fur seal, Arctocephalus tropicalis

We investigated prenatal investment in a large sexually dimorphic mammal, t he subantarctic fur seal, Arctocephalus tropicalis, on Amsterdam Island in the Indian Ocean. Pups' sex ratio and body mass, body length, and body cond ition at birth were studied in relation to timing of birth and maternal cha racteristics (body length and body condition) during three consecutive bree ding seasons. Pups' sex ratio did not differ from unity throughout the pupp ing period. The sex of the pup was related to neither maternal body length (i.e., maternal age) nor maternal body condition when mating occurred or at parturition (1 year later), which suggests that the sex ratio was not bias ed toward one sex during gestation. Newborn male pups were heavier and long er than female pups in all years. Longer mothers tended to arrive later in the season regardless of their body condition, and gave birth to heavier pu ps whatever the sex of the pup. Mothers in good condition gave birth to hea vier male pups than mothers in poor condition, but no significant differenc es were found for female pups, suggesting that the costs of carrying male f oetuses is higher than that of carrying female foetuses. Differences in all ocation of maternal resources between male and female pups may be due to se x-related differences in body composition, since male pups were heavier tha n female pups for a given body length at birth. Thus, male and female foetu ses may use maternal resources differently, with males growing in length wh ereas females appear to grow in body mass. The mothers we monitored over 2 consecutive years gave birth to pups that were similar in quality (in terms of birth mass) over years regardless of the sex of the previous pup and th e mother's body length, suggesting that individual reproductive value is in dependent of maternal age. Furthermore, maternal body condition was not aff ected by the sex of the foetus, suggesting that there is no differential re productive cost in carrying a male or a female foetus. Interannual differen ces in pup body size at birth suggest that environmental conditions such as prey availability during the last stages of gestation, and consequent mate rnal body condition, are important components of maternal investment in fur seals.