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Environmental temperatures are a major constraint on ectotherm abundance, influencing their distribution and natural history. Comparing thermal tolerances with environmental temperatures is a simple way to estimate thermal constraints on species distributions. We investigate the potential effects of behavioral thermal tolerance (i. e. Voluntary Thermal Maximum, VT Max) on anuran local (habitat) and regional distribution patterns and associated behavioral responses. We tested for differences in Voluntary Thermal Maximum (VT Max) of two sympatric frog species of the genus Physalaemus in the Cerrado. We mapped the difference between VT Max and maximum daily temperature (VT Max-ET Max) and compared the abundance in open and non-open habitats for both species. Physalaemus nattereri had a significantly higher VT Max than P. cuvieri. For P. nattereri, the model including only period of day was chosen as the best to explain variation in the VT Max while for P. cuvieri, the null model was the best model. At the regional scale, VTMax-ET Max values were significantly different between species, with P. nattereri mostly found in localities with maximum temperatures below its VT Max and P. cuvieri showing the reverse pattern. Regarding habitat use, P. cuvieri was in general more abundant in open than in non-open habitats, whereas P. nattereri was similarly abundant in these habitats. This difference seems to reflect their distribution patterns: P. cuvieri is more abundant in open and warmer habitats and occurs mostly in warmer areas in relation to its VT Max , whereas P. nattereri tends to be abundant in both open and non-open (and cooler) areas and occurs mostly in cooler areas regarding its VT Max. Our study indicates that differences in behavioral thermal tolerance may be important in shaping local and regional distribution patterns. Furthermore, small-scale habitat use might reveal a link between behavioral thermal tolerance and natural history strategies.

Climate change is increasing both environmental temperatures and droughts. Many ectotherms respond behaviorally to heat, thereby avoiding damage from extreme temperatures. Within species, thermal tolerance varies with factors such as hydration as well as ontogenetic stage. Many tropical anurans lay terrestrial eggs, relying on environmental moisture for embryonic development. These eggs are vulnerable to dehydration, and embryos of some species can hatch prematurely to escape from drying eggs. Warmer temperatures can accelerate development, and thus hatching, but excess heat can kill embryos. Thus, we hypothesize that embryos may show a behavioral thermal tolerance limit, hatching prematurely to avoid potentially lethal warming. If so, because warming and drying are often associated, we hypothesize this limit, measurable as a voluntary thermal maximum, may depend on hydration. We manipulated hydration of the terrestrial eggs of Agalychnis callidryas, in intact clutches and egg-groups isolated from clutch jelly, then warmed them to assess if embryos hatch early as a behavioral response to high temperatures and whether their thermal tolerance varies with hydration or surrounding structure. We discovered that heating induces hatching; these embryos show a behavioral escape-hatching response that enables them to avoid potentially lethal warming. Hydrated eggs and clutches lost more water and warmed more slowly than dehydrated ones, indicating that hydration buffers embryos from environmental warming via evaporative cooling. Embryos in hydrated clutches tolerated greater warming before hatching and suffered higher mortality, suggesting their behavioral thermal safety margin is small. In contrast lower thermal tolerance protected dry embryos, and those isolated from clutch jelly, from lethal warming. Heat-induced hatching offers a convenient behavioral assay for thermal tolerance of terrestrial anuran embryos and the interactive effects of warming and dehydration at an early life stage. This work expands the set of threats against which embryos use hatching in self-defense, creating new opportunities for comparative studies of thermal tolerance as well as integrative studies of self-defense mechanisms at the egg stage.

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