The evolutionary ecology of ageing in a wild Soay sheep population

Abstract

Senescence, declining physiological function resulting in reduced survival and fertility with increasing age is a widely observed natural phenomena. Many studies of senescence in natural populations have found actuarial and reproductive senescence in wild vertebrates. More recently, evidence of ageing in diverse phenotypic traits has been reported. This has revealed the complex nature of senescence with huge variation in ageing patterns across vital rates and phenotypic traits within a population. Understanding how variation in ageing patterns is generated and maintained is important to understand the evolution of senescence in the wild. Many factors are likely to contribute to variation in ageing patterns observed in the wild and recent studies have highlighted the role of some processes such as – a) tradeoffs between life-history traits underpinning ageing and how molecular markers may behave as a mediator of these tradeoffs; b) the influence of parental age on offspring performance and ageing; c) the role of potential declines in selection with increasing age driving the ageing process across phenotypes in the wild. In this thesis, I set out to investigate some of the proximate and ultimate drivers of variation in senescence in a natural Soay sheep population. In Chapters 2 and 3, I examined the role of a molecular marker, telomere length in mediating tradeoffs between different life-history traits by investigating whether costs of maternal reproductive investment, infection and immunity influence TL. In Chapter 4, I investigated the short- and long-term impact of parental age effects on offspring traits examining the effect of both maternal and paternal age on four offspring traits – a) first-year survival, b) offspring adult lifespan, c) offspring lifetime breeding success and d) offspring lifetime recruitment success. In Chapter 5, I jointly developed evolutionary theory to explain variation in ageing patterns across phenotypic traits within a population (also known as asynchronous ageing) and proposed a methodological framework to estimate age-specific phenotypic selection in natural populations. In Chapter 6, I implemented the methods outlined in Chapter 5 to estimate age-dependent phenotypic selection across six traits in female Soay sheep