Credit: HEATHER ANGEL

The parasitic fluke Microphallus piriformes has a problem. To complete its life cycle it needs to travel between two hosts: the rough periwinkle (Littorina saxatilis, pictured, a seashore mollusc) and the herring gull. In the normal run of things, these species have little to do with each other. But the cunning parasite has a way of making introductions.

By comparing the behaviour of infested and healthy periwinkles, Helen McCarthy and colleagues have discovered that M. piriformes seems to bend the periwinkles' behaviour to its own ends. In both the laboratory and field experiments on Muck Island, Scotland, parasitized periwinkles showed a greater tendency to crawl upwards — into positions where they are more visible to gulls, and presumably more likely to be eaten by them — than their healthier counterparts (Anim. Behav. 59, 1161–1166; 2000).

To reach their elevated positions, the suicidal periwinkles reduce their amounts of horizontal and downwards travel. It is the direction, rather than the amount, of movement that changes. Infected animals also alter their responses to the tide, moving upwards as it rises — healthy periwinkles do the opposite.

This change in behaviour happens only when the infection is mature and the fluke is ready to switch hosts. In the early stages, infected periwinkles behave normally; after all, the parasite doesn't want its home to perish from desiccation or predation too early. The parasite's timing also seems designed to bring periwinkles and gulls into contact during the summer, when gulls are gathered at their breeding colonies. The proportion of parasitized periwinkles is much greater near gull breeding colonies than at their foraging sites, although McCarthy et al. have yet to show whether infested periwinkles are actually more likely to find their way into a gull's stomach.

Nor are vertebrates immune to this form of parasitic trickery. A paper by Manuel Berdoy and colleagues, published earlier this month (Proc R. Soc. Lond. B 267, 1591–1594; 2000), shows that rats infected with the protozoon Toxoplasma gondii, which they catch from eating cat faeces, become more curious and less fearful. This makes them easier prey for cats — enabling Toxoplasma to get back to its preferred host.