Developmental variation in resistance and tolerance in a multi‐host–parasite system

Summary 1. Organisms can protect themselves against parasitism by reducing either parasite burden (resistance) or damage caused by parasites at a given burden (tolerance), but little is known about resistance and tolerance to multiple parasites among wild animal populations and species. The fitness effects of parasitism can be broken down into two components: (i) cost of parasite exposure, the difference in fitness between organisms that were not exposed to parasites and those that were exposed but not infected, and (ii) tolerance, the change in host fitness as a function of parasite burden. For amphibians, a taxon that is being decimated by disease, knowing which species and ontogenetic stages are least resistant or least tolerant to which parasites would help target research and management efforts. 2. We reared American toad (Bufo americanus) and green frog (Rana clamitans) tadpoles in a common garden environment and quantified survival and trematode burdens after exposure to 20 Echinostoma trivolvis, Ribeiroia ondatrae or plagiorchid trematode cercariae. 3. All three trematodes caused significant B. americanus mortality, but R. ondatrae was the only species to induce significant mortality of R. clamitans and was the most deadly of the parasites. Young tadpoles had greater parasite‐induced mortality than older tadpoles. 4. Mortality patterns were driven by B. americanus having lower resistance and a higher cost of parasite exposure than R. clamitans, older tadpoles having higher tolerance than younger tadpoles, B. americanus and R. clamitans exhibiting lower resistance and a higher cost of parasite exposure, respectively, to R. ondatrae relative to the other tested trematodes, and skin‐penetrating trematodes inducing a higher cost of parasite exposure than non‐skin‐penetrators. Host size was not predictive of resistance but was a positive predictor of the cost of resistance and tolerance; however, size alone could not fully account for host differences in tolerance. 5. Interactions among developmental stage and host and trematode species emphasize the plasticity and context‐dependency of defence strategies and the importance of considering resistance and tolerance to multiple parasites in wild host species.