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

Early-life adversity influences adult fitness across vertebrate species. In polygynous systems with intense intrasexual competition, males may be more sensitive to conditions experienced during development. However, the importance of different aspects of the early-life environment and how their effects differ between the sexes remains poorly understood. Here, we used a long-term study of wild Soay sheep to characterise the early-life environment in terms of weather, infection risk, resource competition and maternal investment, and test the hypothesis that males are more vulnerable to early adversity. Birth weight, reflective of maternal investment and conditions during gestation, positively predicted lifetime breeding success in both sexes, suggesting a classic ‘silver spoon’ effect, though the effects were stronger in males. Males that experienced increased resource competition 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 was associated with stronger viability selection, 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, demonstrating distinct aspects of the early environment may shape fitness in different and sex-specific ways.