Plasticity and flexibility in the anti-predator responses of treefrog tadpoles

Tadpoles can respond to perceived predation risk by adjusting their life history, morphology and behavior in an adaptive way. To evolve, adaptive plasticity must be (or have been) heritable. And heritability requires that different genotypes follow different plastic rules. Using an incomplete factorial design, we analysed the environmental and parental components of variation in predator-induced life history (age and size at metamorphosis), morphology (tail depth) and behavior of Italian treefrog tadpoles (Hyla intermedia). Results provided strong evidence for an environmental effect on all three sets of characters. Tadpoles raised with caged predators (dragonfly larvae, gen. Aeshna) metamorphosed earlier (but at a similar body size) and developed deeper tails than their fullsib brothers raised without predators. When tested in experimental arenas either with or without a caged predator, all tadpoles, independent of their experience, flexibly changed their activity and position, depending on whether the cage was empty or contained the predator. Tadpoles of the two experimental groups, however, followed different flexible rules: those raised with predators were always less active than their predator-naive brothers and differences slightly increased in the presence of predators. Besides this strong environmental component of phenotypic variation, results provided evidence also for parental and parental-by-environment effects, which were strong on life-history and morphology, but weak on behavior. Interestingly, additive parental effects were explained only by dams. This supports the hypothesis that phenotypic plasticity might depend on epigenetic factors and that it might be the expression of condition-dependent mechanisms.