Morphological and behavioural responses of frog tadpoles to perceived predation risk: A possible role for corticosterone mediation?

Predators can have an important influence on prey survival and fitness, and many prey species exhibit morphological or behavioural responses to perceived predation risk. Although basic characteristics of anti-predator responses have been well documented, physiological pathways underlying such responses are poorly understood. We sought evidence for a role of corticosterone, a major stress hormone in amphibians, in the behavioural and morphological anti-predator responses of leopard frog tadpoles (Rana pipiens) exposed to caged dragonfly nymphs (Aeshna spp.). By superimposing a metyrapone treatment (corticosteroid synthesis inhibitor) over chronic predator exposure in a 2 x 2 factorial design, we evaluated if tadpole anti-predator responses were mediated by corticosterone. Tadpoles were less active and more likely to exhibit a startle response when exposed to perceived predation risk, but direct and interactive effects of the metyrapone treatment on behaviour were negligible. Predator-exposed tadpoles grew larger and had deeper tail fins, whereas the metyrapone treatment resulted in smaller tadpoles with shallower tail fins. Tadpoles simultaneously exposed to metyrapone treatment and predation risk had reduced tail-fin depth and increased body: tail ratio compared to steroid-normal animals. Because both traits are implicated in tadpole vulnerability to predation, these results suggest that the corticosteroid pathway may mediate tadpole morphological response to perceived predation risk. We provide evidence supporting a possible role for corticosterone in anti-predator responses of amphibians specifically in terms of morphological responses. Our results suggest that corticosteroid adjustment may impact prey survival through phenotypic change upon exposure to predation risk and thereby suggest a possible functional role of this hormonal pathway in amphibian physiological ecology.