Reproductive senescence in female Soay sheep: Variation across traits and contributions of individual ageing and selective disappearance

Although senescence in reproductive success has been observed in a number of wild animal populations, longitudinal analyses determining the actual processes responsible for population-level patterns of ageing, and in particular, the relative contributions of within-individual senescence vs. selective mortality, are rare. Furthermore, many studies also only consider single traits in isolation, despite evidence that different aspects of physiology and fitness may show varying patterns of senescence. We used data from an unmanaged population of Soay sheep on the islands of St Kilda, NW Scotland to analyse age-related change in four aspects of female reproductive performance: annual fecundity, twinning rate and the maternally influenced offspring traits of lamb birth weight and early survival. We present the results of three analytical techniques, which attempt to disentangle the contributions of within-individual ageing, selective mortality and terminal effects to the observed population-level patterns of age-specific change. All four traits showed considerable age-related change, but population-level trajectories varied markedly across them. Population-level patterns were underpinned by considerable within-individual senescence including within-individual terminal declines in performance occurring only in the final year of life. However, at the population level, these processes were also masked to some extent by selective disappearance (mortality) of poor breeders. The contributions of individual-level ageing and selective mortality to population-level ageing patterns varied considerably between traits, as did the nature of individual-level senescence. We did not observe detectable significant late-life declines in all four traits: in particular, there was no evidence of senescence in twinning rate. The results indicate that senescence may not be ubiquitous across all aspects of reproductive performance within a population.