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




We examined the interactive effects of predators and trematodes on Rana sybatica and Rana clamitans larvae. We hypothesized that exposure to predators would increase tadpole susceptibility to trematode infection, by reducing tadpole activity and thereby increasing time spent on the bottom. We further 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 clamilans and R. sybatica reduced their activity in the presence of both predators and trematode cercariae. In the longer running R. clamitans experiment, predator-exposed tadpoles developed significantly shallower tails and wider bodies, while trematode infection had no effect on growth, development, or shape. Most significantly, we found that extended exposure to caged fish predators made R. clamitans tadpoles more susceptible to trematode infection. A possible mechanism for this increased vulnerability is that reduced activity in the presence of predators increases tadpoles' proximity to cercariae. Our study suggests that various factors that decrease tadpole activity–predator presence, trematode cercariae and certain pesticides–may act synergistically to negatively impact tadpole populations.