Pre-natal investment in reproduction by female Antarctic fur seals
Maternal investment, in terms of pup birth mass, in gestation by Antarctic fur seals (Arctocephalus gazella) was related to the date of birth in two consecutive years. There were significant differences in birth mass between years and between the sexes within years. Birth mass was used to calculate the mean energetic cost of producing a pup to parturition. The cost for a male pup in 1986 was 173 MJ while it was 191 MJ in 1987. For female pups the cost was 152 and 166 MJ in 1986 and 1987 respectively. Given the probable pattern of foetal growth, this constitutes a minimum of 5–15% of the maternal energy budget in the final months of gestation. Birth mass varied inversely with date of birth, but more strongly for male than for female pups. The sex ratio at birth was unity in both years and this did not vary with time through the birth season. In a subsample of mothers (n=79) which were captured on the day of birth, there was a decline in the body mass and standard length with date of birth. Male birth mass was directly related to maternal mass and maternal condition (mass/length) but there was no significant relationship for females. These results suggest that the growth of male foetuses is limited by maternal resources while female foetuses do not exploit fully maternal resources.