A manipulative parasite increasing an antipredator response decreases its vulnerability to a nonhost predator

Trophically transmitted parasites have to deal with the antipredator adaptations of their intermediate hosts. Some of these parasites induce behavioural changes in their intermediate hosts that make them more vulnerable to predation by definitive hosts. However, the adaptiveness of behavioural manipulation also depends on the probability of being eaten by a nonhost predator. Parasites might therefore try to use specific antipredator responses of intermediate hosts to avoid this dead end. We tested this hypothesis using the acanthocephalan Polymorphus minutus and its intermediate amphipod host, Gammarus roeseli. In their natural habitat, uninfected G. roeseli shelter near the river bottom while infected gammarids tend to cling onto floating materials. In microcosm experiments, infected amphipods were preyed upon by sticklebacks (nonhost predators) less than uninfected individuals when refuges were provided near the water surface. Regardless of their infection status, G. roeseli reacted to olfactory cues of predation by spending more time on refuges and near the surface. However, these behaviours were increased in infected gammarids. These results suggest that, in addition to specific induced behaviours, parasites may also enhance an existing antipredator behaviour to increase their own survival.