Hierarchical responses of tadpoles to multiple predators

Abstract

The impact of multiple factors on the expression of phenotypic plasticity has been poorly studied. The simultaneous presence of factors inducing diverging responses may result either in a trade-off between the responses or in a hierarchy of responses. Inducible defenses offer a suitable model to investigate these alternatives. Inducible defenses evolve in response to variability in predation risk. Here, we investigated the impact of the nonlethal presence of both pursuing (fish, Gasterosteus aculeatus) and sit-and-wait (dragonfly larvae, Aeshna cyanea) predators on tadpole morphology in two frogs (Rana dalmatina and R. ridibunda). Predation tests showed that Aeshna were the more dangerous of the two predators for the tadpoles of both species. In both species, induced responses differed according to predator type. In the presence of fish, tadpoles invested in both tail muscle depth and tail length. In the presence of dragonfly larvae, the investment was made in tail fin depth. When faced with the two types of predators simultaneously, the response was similar to that expressed in the presence of Aeshna alone, suggesting a hierarchy of response according to predation risk. Such a hierarchy of response could result from selection against the phenotype induced by the other predator.