Exposure to glucocorticoids alters life history strategies in a facultatively paedomorphic salamander

Polyphenisms, where two or more alternative, environmentally-cued phenotypes are produced from the same genotype, arise through variability in the developmental rate and timing of phenotypic traits. Many of these developmental processes are controlled or influenced by endogenous hormones, such as glucocorticoids, which are known to regulate a wide array of vertebrate ontogenetic transitions. Using the mole salamander, Ambystoma talpoideum, as a model, we investigated the role of glucocorticoids in regulating facultative paedomorphosis, an ontogenetic polyphenism where individuals may delay metamorphosis into terrestrial adults. Instead, individuals reproduce as aquatic paedomorphic adults. Paedomorphosis often occurs when aquatic conditions remain favorable, while metamorphosis typically occurs in response to deteriorating or "stressful" aquatic conditions. Since glucocorticoids are central to the vertebrate stress response and are known to play a central role in regulating obligate metamorphosis in amphibians, we hypothesized that they are key regulators of paedomorphic life history strategies. To test this hypothesis, we compared development of larvae in outdoor mesocosms exposed to Low, Medium, and High exogenous doses of corticosterone (CORT). Results revealed that body size and the proportion of paedomorphs were both inversely proportional to exogenous CORT doses and whole-body CORT content. Consistent with known effects of CORT on obligate metamorphosis in amphibians, our results link glucocorticoids to ontogenetic transitions in facultatively paedomorphic salamanders. We discuss our results in the context of theoretical models and the suite of environmental cues known to influence facultative paedomorphosis.