Summary
Individual life-history traits, such as age and sex, influence vital rates in most large vertebrate species. Developmental differences are especially profound in polygamous mating systems. One of the most polygamous of all mammals, the southern elephant seal (Mirounga leonina) is a sexually dimorphic, capital breeding marine predator. Only one of its four major breeding populations, Macquarie Island, continues to slowly decline. We use a demographic model, supported by long-term capture-mark-recapture records to investigate the influence of sex and age on survival in this population. This study revealed a clear difference between female and male age-dependent survival rates. Overall, juvenile survival showed consistent parallel trends for both sexes, initially increasing for two years, then tipping towards a slow decline. Male survival estimates were consistently 5-10% lower than females in the same age classes until 8 years of age, at which point male survival decreased more rapidly than female survival (males aged 10-years: 50% ± 10% vs. 80% ± 5% for female aged 10-years). Female survival estimates peaked at age 3 (85%) slowly decreasing to 75% at 12 years of age. Different energetic requirements could be underpinning intersex differences in adult foraging behaviour. The marked sexual dimorphism and polygamous mating system may lead to males pursuing riskier foraging strategies than females to prioritise growth. Specifically, adult males are known to frequent energy rich shelf habitats where encounters with killer whales or sleeper sharks are likely. Maximising growth is especially advantageous for males, with size being a major determinant of breeding success.