Ecological and Evolutionary Context of the Stress Response in Larvcal Anurans

Abstract

Describing and understanding the interactions between environment and phenotype, and the resulting consequences for populations and communities, is a central goal of ecological and evolutionary research and theory. In vertebrates, the endocrine stress response provides a mechanistic connection between environmental variation and organism phenotype, with resulting effects on fitness. Using larval anurans as a model system, I combine field surveys, mesocosm experiments, and exogenous hormone manipulation to explore the complex interactions among the predator stress response, phenotype, and fitness, with respect to other contemporary stressors and species evolutionary history. First, I developed a comprehensive understanding of the predator stress response over time, mapping hormone production to changes in behavior and morphology, and demonstrating the effects on predation survival. I found that tadpoles modulate their predator stress response to maximize survival under predation risk, and additionally showed that the stress hormones produced during a predator response initiates morphological changes indistinguishable from predator-induced morphology. Next, I used a comparative approach to place the predator stress response into an evolutionary context, testing the hypothesis that species that predictably encounter high larval predator densities reduce the costs of chronic endocrine activation. Across four anuran species arrayed in conspecific pairs, I found support for this hypothesis, showing that species typically under high predation risk reduce the magnitude of their stress response to predator presence relative to conspecifics. I next considered the combined effects of inbreeding and predator presence, and showed that inbred tadpoles also elevate stress hormone production, although there is no additive effect in combination with predator presence. The resulting behavioral and morphological changes in inbred tadpoles may place them at a disadvantage under some circumstances. Finally, I showed that reduced food resource availability also increases stress hormone production independently of predator presence. Together, these results connect perceived predation risk, endocrine response, and phenotypic plasticity and place them in an ecological and evolutionary context. Furthermore, this research highlights the importance of using an integrative approach to investigate the impacts of developmental history on organism-environment interactions.