Growth of southern elephant seals, Mirounga leonina, during their first foraging trip

A longitudinal study of growth of southern elephant seals, Mirounga leonina, during their first foraging trip was undertaken at Macquarie Island. On average, body mass increased by 75% while foraging at sea, with individuals growing at 0.34 +/- 0.12 (s.d.) kg day(-1) (n = 64), and spending 182 +/- 51 days (n = 64) at sea. Relatively smaller changes in body length were recorded during the same period, suggesting that growth was composed primarily of adjustments to body composition, rather than increases in gross body size. This may be in response to the functional demands of pelagic life. Body size established early in life (birth mass and departure mass) positively influenced body mass upon return from the first foraging trip. Growth rate, however, was negatively related to departure mass for females, and this is hypothesised to be related to sex differences in body composition, as well as intrasex differences in foraging skills, diving ability and food-conversion efficiency. Despite this, there was no detectable age-specific sexual dimorphism in the first year of life. Animals that were at sea longer tended to return in better body condition. Interspecific comparison suggests that southern elephant seals grow more than do northern elephant seals, Mirounga angustirostris, and this difference may be related to prey abundance and distribution.