Sex-specific effects of early-life adversity on adult fitness in a wild mammal

Early-life adversity influences adult fitness across a range of vertebrates. In polygynous systems with intense intrasexual competition, males may be more sensitive to conditions experienced during growth and development. However, the relative importance of different aspects of the early environment and how their effects differ between the sexes remain poorly understood. Here, we used a long-term study of wild Soay sheep to characterise the early-life environment in terms of weather, infection, resource competition and maternal investment, and test the hypothesis that males are more vulnerable to early adversity than females. Birth weight positively predicted lifetime breeding success in both sexes, suggesting a classic ‘silver spoon’ effect, though the effects were stronger in males. Males experiencing high population densities in their first year had lower lifetime breeding success suggesting lasting negative consequences of nutritional stress, but there was no association in females. In contrast, challenging weather in the first winter of life appeared to act as a selective ‘filter’, with males surviving these harsh conditions having higher adult fitness. Our findings further evidence the important long-term fitness consequences of early-life adversity in wild vertebrates, highlighting that different aspects of the early environment may shape later fitness in different and sex-specific ways.