No evidence sex-specific senescence varies with resource competition in Soay sheep

Demographic senescence, the age-related decline in survival and reproduction, is widespread. Older individuals, and particularly males in polygynous systems, are expected to be disproportionately vulnerable to environmental stressors due to physiological decline and high reproductive costs. However, studies testing if harsh conditions exacerbate senescence, and whether this effect is sex-dependent, remain limited in wild populations. Using 40 years of data from wild Soay sheep (Ovis aries) on St Kilda, Scotland, we tested whether increased resource competition (indexed by population density) accelerates demographic senescence, and if late-life environmental sensitivity is male-biased. High densities consistently suppressed overall adult performance, making individuals less likely to survive, breed, or rear high-quality offspring under more intense competition. Although survival and reproductive traits exhibited clear senescence, increased population density did not accelerate late-life declines in any trait. Rates of decline with age were unaffected by density and did not differ between the sexes. Our results challenge the assumption that older individuals and males are inherently more vulnerable to harsh conditions, suggesting physiological or behavioural buffering may maintain late-life resilience against resource limitation in some systems.