Causes and consequences of immune variation in a wild mammal

Abstract

The immune system provides protection against parasites and is crucial for survival, but mounting, maintaining and controlling an immune response is expensive. Under limited resources these costs can lead to investment trade-offs between life history traits in order to maximize an individual's fitness. Understanding how these trade-offs relate to immunity can be important in understanding individual variation in fitness and the broader ecological implications that this may have in a population. In the wild there is evidence of trade-offs between life history traits and immunity, but there are relatively few studies which have measured specific aspects of the immune system under natural parasitic exposure. Using reagents developed in domestic sheep, I measured an unusually broad range of immune markers in a wild population of Soay sheep on the island of Hirta, St Kilda, Scotland. These include: T cell subsets (CD4+, CD8+, CD4+ & CD8+ naïve, gamma delta and Foxp3), anti-T.circumcincta (T. circ) antibody isotopes, (IgA, IgE, IgG), leukocyte subtypes (neutrophil:lymphocyte ratio & eosinophils), and leukocyte telomere length (LTL). I found that, in a year under high selection pressure for survival, anti-T. circ IgG positively predicted survival across all ages and for both sexes. Additionally, females had higher proportions of naïve T cells than males; a previously unreported sex difference in a wild mammal. In chapter 2, analysis of lambs in early life found higher growth rates associated with low antibody measures, while lower growth rates related to low antibody measures and high levels of inflammatory marker. I also found that male lambs with high anti-T. circ IgE and IgG were less likely to survive over-winter, contrary to the findings across all ages in chapter 1. In chapter 3, I detected an increase in LTL attrition with age in males >3 years, but this was not significant in females or in younger animals. In male lambs, high investment in horn growth was related to reduced LTL. Changes in LTL were independent of variation in leukocyte cell populations. The data in this thesis demonstrate the complexity of immune variation in the wild, and highlight the value of multiple ecologically relevant markers to understanding the evolutionary implications of resource trade-offs.