Survival, natural selection and foraging efficiency in Soay sheep on St. Kilda

The feral Soay sheep of St. Kilda exhibit dramatic population fluctuations and recurrent crashes, the survivors of which are favoured by natural selection. In this thesis, I establish which factors influence over-winter survival. I investigate phenotypic selection of body size traits, and demonstrate their heritability, so establishing the occurrence of natural selection. Finally I explore the relationships between body size and survival in terms of foraging behaviour and diet selection. Over-winter survival of lambs was influenced by both density and density-independent climatic fluctuations. Adults were less sensitive to both factors, especially females which showed no evidence of density-dependence. Generalised linear mixed modelling demonstrated that once between-year random variation was taken into consideration, body weight was the most significant determinant of survival in all age, sex classes. Evidence was found of repeated directional selection of the three traits, body weight, hindleg length and incisor arcade breadth. Whilst selection differentials showed strong positive selection for all these traits in high mortality years, selection gradients demonstrated that body weight experienced direct selection whereas selection of other traits was indirect due to their correlation with body weight. No evidence of opposing selection was found. All the morphometric traits were significantly heritable. An evolutionary response would therefore be expected, but heritabilities were low, and because of environmental noise, no significant increase in body size was detectable over the 12 year period of data. Observed grazing behaviour in relation to vegetation structure was not influenced by individual variation in body size during winter, although female body weight affected diet selectivity in summer. Seasonal patterns were very marked, with summer grazing behaviour influencing survival the following winter. Seasonal differences in faecal nitrogen content and a decline in faecal nitrogen with increasing body size were also apparent. These observations were interpreted using diet quality and body size arguments