Testing for Short- and Long-Term Thermal Plasticity in Corticosterone Responses of an Ectothermic Vertebrate

Phenotypic plasticity, broadly defined as the capacity of one genotype to produce more than one phenotype, is a key mechanism for how animals adapt to environmental (including thermal) variation. Vertebrate glucocorticoid hormones exert broad-scale regulation of physiological, behavioral, and morphological traits that influence fitness under many life-history or environmental contexts. Yet the capacity for vertebrates to demonstrate different types of thermal plasticity, including rapid compensation or longer acclimation in glucocorticoid hormone function, when subject to different environmental temperature regimes remains poorly addressed. Here, we explore whether patterns of urinary corticosterone metabolites respond (i.e., evidence of acclimation) to repeated short-term and sustained long-term temperature exposures in an amphibian, the cane toad (Rhinella marina). In response to three repeated short (30-min) high-temperature (37°C) exposures (at 10-d intervals), toads produced urinary corticosterone metabolite responses of sequentially greater magnitude, relative to controls. However, toads subjected to 4 wk of acclimation to either cool (18°C)- or warm (30°C)-temperature environments did not differ significantly in their urinary corticosterone metabolite responses during exposure to a thermal ramp (18°-36°C). Together, these results indicate that adult toads had different, including limited, capacities for their glucocorticoid responses to demonstrate plasticity to different regimes of environmental temperature variation. We advocate further research as necessary to identify plasticity, or lack thereof, in glucocorticoid physiology, to better understand how vertebrates can regulate organismal responses to environmental variation.