| Summary: | The ability of parasites to manipulate their hosts is the major empirical theme of this thesis, in particular the problem of cooperation among parasites that host manipulation entails. Contributing to host manipulation is likely to bear a cost to the individual, while the benefits will be felt by every conspecific parasite (and others) within the host. I develop in Chapter Two a game-theoretical model of parasite-induced host manipulation, which focuses on co-operative interactions between individuals and a larger group. In Chapters Three and Four I consider two macroparasitic case-studies. Chapter Five marks the beginning of the macroparasitic half of the thesis, with a study of the cestode <I>Ligula intestinalis</I> in its intermediate host, the Roach (R<I>utilus rutilus</I>). Chapter six uses a similar mix of statistical and analytic approaches to investigate the interaction between the manipulative trematode <I>Microphallus papillorobustus</I> and its intermediate gammarid host, with a focus on the potential costs of host manipulation. Chapters Five and Six introduce aggregation as respectively a side-effect and a cost of manipulation. In Chapter Seven, I add that, should aggregation be costly to established worms, the probability of subsequent recruits may itself fall under the influence of established parasites. Using the case study of schistosome worms in their definitive mammalian hosts, I develop a stochastic-simulation model to explore the potential for adaptive population regulation in parasites. In Chapter Eight, I review the progress made in this thesis, with an emphasis on developing a synthesis between models of virulence and of host manipulation.
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