Parasites of all kinds alter the behaviour of their hosts. In many systems, these behavioural modifications appear adaptive for the parasite, by facilitating the completion of its life cycle. However, not all parasitized hosts are under the influence of parasites. This may be due to the timing of the onset of behavioural manipulation by the parasites: changes in behaviour may coincide with completed parasite development and only appear late in the infection. In addition, certain hosts may oppose the parasite's attempts at manipulating their behaviour. Hosts with high expected future reproductive success, i.e. young hosts or hosts that have not yet reproduced, are more likely to benefit by opposing the influence of parasites, as their expected gains would outweigh the costs of opposition. Such conditional opposition would be a better alternative to resistance to infection itself if resistance is costly, and would explain the considerable variability often observed in the behavioural responses of parasitized hosts. Future tests of these ideas should give insights into the coevolution of hosts and parasites and the parasites' struggle for the control of host behaviour.
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