Many parasites apparently change the behavior of their hosts in a way that seemingly increase the probability of successful reproduction and transmission, suggesting that parasites somehow are able to manipulate the behavior of hosts to their own advantage. Such adaptive manipulation implies that [1] different roles are played by manipulated and manipulator individuals; [2] manipulation reduces the fitness of the manipulated individual; [3] the manipulator gains a fitness advantage; and [4] this order of events should hold up when analyzed in a phylogenetic context. While some examples of parasite-host interactions are consistent with some of these criteria, there is little strict evidence consistent with all four criteria. Parasite manipulation of vertebrate hosts may differ from that of invertebrates because of differences in cognitive ability, and complexity of the parasite community. Literature on avian brood parasites and their hosts suggests that hosts may be fully aware of their parasitism status. Using studies of the great spotted cuckoo and its magpie host I argue that parasitized hosts probably are doing the best they can, given their status, and that their fitness pay-offs would be even worse if they produced higher levels of resistance. Next, I argue that hosts in general may be aware of their infection status, and that each host individual interacts with so many different parasites, each with their ‘own’ evolutionary interests, that hosts are unlikely to behave only in response to any single parasite. Rather, host behavior could be considered to reflect a compromise between the evolutionary interests of all the inhabitants of a given host individual. Therefore, it might be difficult to argue that hosts are manipulated by parasites, and I suggest that we may learn more about parasitehost interactions by quantifying the evolutionary interests of hosts and their multitude of parasites, amensals and commensals, and that host behavior may more readily be understood from the point of view of the participants involved in these different interspecific interactions.
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