Patterns and consequences of behavioural responses to predators and parasites in Rana tadpoles

We examined the interactive effects of predators and trematodes on Rana syl vatica and Rana clamitans larvae. We hypothesized that exposure to predator s would increase tadpole susceptibility to trematode infection, by reducing tadpole activity and thereby increasing time spent on the bottom. We furth er hypothesized that the tadpoles would experience reduced rates of growth and development in the presence of either predators or parasites, and that predator presence would stimulate both species to develop larger tails and smaller bodies. Rana clamitans and R. sylvatica reduced their activity in t he presence of both predators and trematode cercariae. In the longer runnin g R. clamitans experiment, predator-exposed tadpoles developed significantl y shallower tails and wider bodies, while trematode infection had no effect on growth, development, or shape. Most significantly, we found that extend ed exposure to caged fish predators made R, clamitans tadpoles more suscept ible to trematode infection. A possible mechanism for this increased vulner ability is that reduced activity in the presence of predators increases tad poles' proximity to cercariae. Our study suggests that various factors that decrease tadpole activity-predator presence, trematode cercariae and certa in pesticides-may act synergistically to negatively impact tadpole populati ons. (C) 2000 The Linnean Society of London.