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 25 phenotype, is a key mechanism for how animals adapt to environmental (including thermal) 26 variation. Vertebrate glucocorticoid hormones exert broad scale regulation of physiological, 27 behavioral and morphological traits that influence fitness under many life-history or 28 environmental contexts. Yet the capacity for vertebrates to demonstrate different types of 29 thermal plasticity, including rapid compensation or longer acclimation in glucocorticoid 30 hormone function, when subject to different environmental temperature regimes remains poorly 31 addressed. Here, we explored if patterns of urinary corticosterone metabolites responded (i.e. 32 evidence of acclimation) to repeated short- and sustained long-term temperature exposures in an 33 amphibian, the cane toad (Rhinella marina). In response to three repeated short (30 min) high 34 temperature (37°C) exposures (at 10 day intervals) toads produced sequentially greater 35 magnitude urinary corticosterone metabolite responses relative to controls. However, toads 36 subjected to 4-weeks of acclimation to either cool (18°C) or warm temperature (30°C) 37 environments did not differ significantly in their urinary corticosterone metabolite responses 38 during exposure to a thermal ramp (18 -36°C). Together these results indicated that adult toads 39 had different, including limited, capacities for their glucocorticoid responses to demonstrate 40 plasticity to different regimes of environmental temperature variation. We advocate further 41 research is necessary to identify plasticity, or lack thereof, in glucocorticoid physiology to better 42 understand how vertebrates can regulate organismal responses to environmental variation.