| Summary: | This thesis focuses on the early development of parasitism in a wild population of Soay sheep on the Scottish island group of St. Kilda. The research used statistical analysis and modelling of a detailed data set collected over a year of field based study. The outcome is a comprehensive description of the epidemiology of nematode parasitism in young animals from a wild ungulate population. An immuno-epidemiological model is used to explore the dynamics of acquired protective immunity against parasites in early infection. The model predicts early male/female differences in immunocompetence. The model also allows estimation of the relationship between weight, age, and infection rate for males and females. It shows that as lambs get older, net infection increases in males, as immunity becomes more important in females. The predictions of the model suggest that differences in immunity between hosts drive the patterns of aggregation observed in the system. Significant intermediate-scale spatial heterogeneity in early parasitism on St. Kilda is demonstrated. Lambs from three hefts, which frequent three distinct parts of the study area, are shown to have significantly different mean faecal egg counts (p<0.001). An attempt is made to tease out the causes of the observed heterogeneities, but further empirical research is required before firm conclusions may be drawn. It is established that such intermediate -scale spatial heterogeneity is not the cause of the high levels of parasite aggregation observed in the system. The implications of parasitism for over-winter survival in lambs are explored.
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