Synchrony between parasite development and host behaviour change

Abstract

Many parasites rely on the predation of one host by another to be transmitted. Such parasites are expected to have evolved strategies of host manipulation that alter the susceptibility of current hosts to predation by suitable predators, and many examples of altered behaviour exist. For many parasites, however, a period of growth and development in each host is essential before they are ready to be transmitted. If changes in host behaviour are truly parasite adaptations, rather than side‐effects of infection, then host behaviour change should only be predicted when the parasite is infective to the next host. This study documents the behaviour of individual three spined sticklebacks Gasterosteus aculeatus over a 16‐week period following experimental infection with the common cestode parasite Schistocephalus solidus, which requires infected fish to be ingested by birds before it can attain sexual maturity. In addition to regularly monitoring shelter use and escape response behaviour of infected fish, we also used non‐invasive morphometric techniques to estimate parasite size, and thus were able to link behaviour changes to the growth of the parasite in individual fish. The behaviour of experimentally infected sticklebacks deviated significantly from that of controls (sham‐exposed) fish when the parasites they harboured approached an estimated weight of 100 mg. The results are discussed in the light of a further study, examining the relationship between parasite size and ultimate fecundity in the bird host.