Growth strategies of New Zealand fur seals in southern Australia

Post-weaning growth of male and female New Zealand fur seals Arctocephalus forsteri was examined using morphometric measurements from a cross-sectional sample of 326 females and 88 males captured between 2000 and 2003 on Kangaroo Island, South Australia. The age of each animal was estimated through examination of growth layer groups in the cementum of a postcanine tooth, which was removed from each seal. Females ranged in age from 1.5 to 23.4 years and males from 1.5 to 16.7 years. Gompertz and two-component logistic growth models were used to predict sex-specific growth patterns in body length and mass and to compare growth strategies. During the non-breeding season, physically mature adult males were on average 1.2 times longer and 2.2 times heavier than adult females. Sexual size dimorphism was most apparent after the age of 5–7 years, when differences in predicted growth rates were most evident. Post-weaning growth in females was predicted to be monophasic, characterized by high growth rates in length and mass during the juvenile growth stage, followed by a gradual decline in growth rates after reproductive maturity. In contrast, growth in males was predicted to be biphasic, with an apparent secondary growth spurt in both length and mass that coincided with sexual and social maturation, followed by a rapid decline in growth rates after social maturity. However, the number of males sampled between 9 and 10 years was low and is likely to have biased growth rates for sub-adults to some degree. Females were predicted to continue to gain body mass over a number of years following reproductive maturity, whereas adult males appeared to trade off further somatic growth with a marked seasonal accumulation and loss of mass in relation to the breeding season