EQUAL INVESTMENT IN MALE AND FEMALE OFFSPRING IN SOUTHERN ELEPHANT SEALS

Sex ratio theory predictions concerning differential parental investment in offspring by sex were tested on southern elephant seals, Mirounga leonina, breeding at Peninsula Valdes, Argentina. Females invested equally in sons and daughters, as reflected by the similar mass at birth (mean +/- 1 S.D.) of 14 males (44.1 +/- 6.5 kg) and 14 females (43.4 +/- 3.8 kg), and similar mass at weaning of 52 males (131.5 +/- 22.4 kg) and 38 females (131.4 +/- 18.3 kg). There were also no sex differences in the rate of mass gain during nursing (males = 4.0 +/- 0.9 kg/day; females = 3.9 +/- 0.8 kg/day), rate of mass loss during the first month of post-weaning fast (males=0.85 +/- 0.19 kg/day; females = 0.92 +/- 0.15 kg/day), mean age at weaning (males = 22.3 +/- 1.6 days; females = 22.7 +/- 1.7 days), and female nursing behaviour. Mother's size accounted for most of the variation in mass of pups at weaning. Mothers ranked as small, medium and large, weaned pups with a mean mass of 102, 130 and 145 kg, respectively. The sex ratio of weanlings did not differ from unity. These data are consistent with Fisher's (1930) sex ratio theory.