Biological Journal of the Linnean Society (2000), 71: 513–528. With 4 figures doi:10.1006/bijl.2000.0459, available online at http://www.idealibrary.com on Patterns and consequences of behavioural responses to predators and parasites in Rana tadpoles GREGORY W. THIEMANN Department of Biology, Dalhousie University, Halifax, NS, B3H 4J1, Canada RICHARD J. WASSERSUG* Department of Anatomy and Neurobiology, Dalhousie University, Halifax, NS, B3H 4H7, Canada Received 1 December 1999; accepted for publication 15 May 2000 We examined the interactive effects of predators and trematodes on Rana sylvatica 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 clamitans and R. sylvatica 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.  2000 The Linnean Society of London ADDITIONAL KEY WORDS:—trematode – Echinostoma – cercaria – metacercaria – Rana sylvatica – Rana clamitans. CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . 514 Material and methods . . . . . . . . . . . . . . . . . . . 515 Animal subjects . . . . . . . . . . . . . . . . . . . . 515 Experimental design . . . . . . . . . . . . . . . . . . 516 Response variables . . . . . . . . . . . . . . . . . . . 517 * Corresponding author. Email: tadpole@is.dal.ca 513 0024–4066/00/110513+16 $35.00/0  2000 The Linnean Society of London