Evolutionary decoupling of morphological and behavioural antipredator responses and their hormonal basis across anuran species

Adaptive plasticity allows organisms to adjust their phenotypes to environmental heterogeneity. Defensive responses to predation risk exemplify such plasticity, which, in the case of amphibian larvae, can involve changes in morphology, behaviour, physiology and life histories. These defences are hypothesized to be mediated by the glucocorticoid hormone corticosterone (CORT), but empirical support across species is inconsistent. We experimentally tested the generality of CORT involvement in predator-induced plasticity across larvae from 12 phylogenetically distant anuran genera, exposing them to either predation cues or different concentrations of exogenous CORT and comparing their responses. We measured changes in morphology, activity and life-history traits and quantified endogenous CORT levels. Species showed pronounced interspecific variation in morphological and behavioural plasticity. Perceived predation risk did not consistently elevate endogenous CORT concentrations across species. Moreover, predator-induced changes in body shape were generally distinct from, and even opposite to those elicited by exogenous CORT, indicating that CORT is not a direct regulator of anti-predator defences in amphibian larvae. Nonetheless, we found evidence for an indirect association between CORT and antipredator responses such that species with higher baseline CORT levels tend to have faster developmental rates, greater CORT response to predation cues but reduced morphological responses and vice versa.