MASS CHANGES OF GREY SEAL HALICHOERUS-GRYPUS PUPS ON SABLE ISLAND - DIFFERENTIAL MATERNAL INVESTMENT RECONSIDERED

Theory predicts that where variance in reproductive success differs between sexes, as in many sexually dimorphic species, mothers should invest more in male than in female offspring. In pinnipeds, parental investment is the sole responsibility of the mother. Both size at birth and subsequent mass gain during lactation are measures of that investment. We studied mass gain of grey seal pups, a sexually dimorphic species, during lactation and mass loss during the first 13 days of the postweaning fast using longitudinal data obtained on Sable Island, Canada. A mixed-effects repeated measures model was used to account for the serial correlation in these longitudinal data. We found no significant difference in the birth mass of male (16.6 +/- 0.45 kg) and female (15.6 +/- 0.74 kg) pups. Rate of mass gain by males (3.0 +/- 0.13 kg/d) and females (2.7 +/- 0.12 kg/d) also did not differ significantly during the lactation period. Grey seal pups on Sable Island tended to be larger at birth and gain mass more rapidly (70%-100%) during lactation than those in the United Kingdom. Mass loss of weaned pups was curvilinear over time. Over the first 10 days of the fast, pups lost 22% of their weaning mass. In both males and females, large pups lost mass more slowly than small ones during the postweaning fast. Based on our results and a review of previous studies, we found little evidence of differential maternal investment in male and female grey seal pups.