The study of host manipulation by parasites presents a special complication for behavioural ecologists: the trait of interest is the product of the interaction between the genotypes of two different organisms. The behaviour of a parasitized host is the simultaneous expression of host and parasite genes, and investigating the function, evolution and proximate basis of a change in host behaviour can be very challenging. In this chapter, I have considered only cases in which the modified host behaviour appeared to be mainly the product of parasite genes and therefore of benefit to the parasite. However, without proper study of both the immediate causes of the change in host behaviour and of its net fitness consequences for the parasite, it is premature to consider it as a parasite adaptation. Behavioural ecologists have been accused in the past of adopting a Panglossian view of nature, in which every feature of an organism is always beneficial (Gould and Lewontin, 1979). Although changes in host behaviour are often detrimental to the host, they are not necessarily always advantageous for the parasite. And, even if they are advantageous for the parasite, they may not be true adaptations. Given the tools now available, the next step in this field of research should be to integrate investigations into the functional, causal and historical aspects of host manipulation. First, clear, quantitative predictions derived from optimality theory or game theory need to be tested in experimental systems or in comparative analyses to provide a deeper understanding of when the ability to manipulate hosts evolves and how strongly it manifests itself. Secondly, the proximal cause of behavioural changes needs to be elucidated in more than a handful of systems. Thirdly, information on both the magnitude and type of manipulation induced by parasites and on the physiological mechanisms used to induce them needs to be mapped on parasite phylogenies to provide insight into their evolutionary history. There have been recent developments in the broader implications of host manipulation by parasites, such as its influence on the development of parasite communities, on the evolution of other sympatric parasites and on the evolution of hosts themselves (see Poulin and Thomas, 1999; Lafferty et al., 2000). To better understand the phenomenon itself, however, the three steps outlined above will be necessary if we are to go beyond the mere natural-history perspective.
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