Parasites and the Costs of Reproduction in Soay Sheep

Abstract

Over the lifetime of an individual, decisions are constantly being made. These arise as a result of trade-offs between life-history traits (e.g. lifespan and fecundity). Tradeoffs may occur when two traits are limited by the same resource, such that one trait can be increased only at the expense of another. The cost of reproduction is the tradeoff between current and future reproduction. An intermediate trade-off that can affect reproduction, is that between parasite resistance and reproduction. This thesis attempts to explore the interaction between parasites and the costs of reproduction in a wild population of Soay sheep on the Scottish island group of St Kilda (Chapter 2). In domestic sheep, costs associated with lambing incur a temporary increase in parasite burden, termed the peri-parturient rise. Soay sheep that successfully rear a lamb also suffer from this increase in parasitism (Chapter 3), whereas non-lambing adult ewes do not. In contrast, non-lambing yearling ewes do suffer from an increase in parasite burden at this time, despite incurring only minimal costs of reproduction, probably as a consequence of poor body condition and a lack of acquired immunity to parasites. Male Soays also suffer an increase in parasitism during spring, even though they do not reproduce at this time, suggesting that the spring rise in parasitism is partly associated with poor body condition. The effects of parasites on the lambing success of Soay ewes were explored using observational (Chapter 4) and experimental methods (Chapters 5 and 6). Ewes treated with an anthelmintic to temporarily remove their parasites over the winter were in better condition in the following sprmg. This did not affect their probability of lambing, but there was a trend for treated ewes to have a higher incidence of twinning than control ewes, and for their lightest lambs to have a higher probability of survival than those of control ewes. Early growth and development is not improved by treatment of the mother, despite these ewes being in a better condition. It is suggested that this may reflect a possible trade-off between current and future reproductive effort: ewes may not exceed the investment necessary to ensure survival of offspring, in anticipation of future breeding events.