Background/Question/Methods Organisms can protect themselves against parasitism by either reducing parasite burden (resistance) or the damage caused by parasites at a given burden (tolerance).  Despite these two strategies having different implications for host-parasite co-evolution, little is known about resistance and tolerance among wild animal populations and species challenged with multiple parasites.  Restif and Koella (2004) hypothesized that high rates of parasite exposure but low parasite virulence should favor tolerance-biased genotypes, whereas the opposite conditions should select for resistance-biased genotypes, but this hypothesis has yet to be tested empirically.  We reared American toad (Bufo americanus) and green frog (Rana clamitans) tadpoles in a common garden environment, and quantified their survival and trematode loads after a constant exposure to one of three trematode species – Echinostoma trivolvis, Ribeiroia ondatrae, and a plagiorchid sp. 
Results/Conclusions Parasite-induced mortality was greater for B. americanus than R. clamitans, for younger than older tadpoles, and for tadpoles exposed to R. ondatrae cercariae than to E. trivolvis or plagiorchid cercariae.  These mortality patterns were caused by B. americanus having lower resistance and tolerance than R. clamitans, older tadpoles being more tolerant than younger tadpoles, and B. americanus and R. clamitans exhibiting relatively low resistance and low tolerance, respectively, to R. ondatrae.  Resistance and tolerance often depended on interactions among developmental stage and host and trematode species, emphasizing the plasticity of the traits and the importance of considering responses to multiple parasites, but there was no evidence for a trade-off between resistance and tolerance.  The results generally supported the Restif and Koella hypothesis, especially for locally abundant trematodes and the host species with higher overall parasite exposure.  This hypothesis might therefore provide a useful general framework for predicting resistance-tolerance investments.