Heat-Induced Hatching: Clarifying Effects of Hydration and Heating Rate on Behavioral Thermal Tolerance of Red-Eyed...

Anurans are one of the most diverse groups of vertebrates but also most threatened by current climate change effects such as increasing environmental temperatures and more frequent and prolonged periods without rain. Many tropical anurans lay terrestrial eggs that are particularly vulnerable to drying and warming. In some such species, embryos hatch prematurely to escape from drying eggs. In red-eyed treefrogs, Agalychnis callidryas, embryos hatch early to escape both drying and excessive warming, expressing a behavioral thermal tolerance (i.e., VT_Max). Prior research suggested that drying reduces the VT_Max of embryos. However, because hydrated clutches warmed more slowly, the effect of drying on VT_Max was confounded with that of faster warming. To disentangle these dynamics, we designed a novel apparatus to warm terrestrial frog egg-clutches at controlled rates and minimize evaporative cooling. We independently manipulated clutch hydration and heat input to assess their individual and combined effects on embryo VT_Max. Proportional egg-volume loss was similar across hydration × heat input categories. High heat input resulted in higher clutch warming rates and shorter trial durations, across hydration levels. Within clutches, warming rates differed between back and surface thermocouple positions, generating thermal gradients as warming progressed. Clutch dehydration reduced embryo VT_Max, with no main or interacting effect of heat input. This evidence that egg drying reduces thermal tolerance across testing conditions supports a direct role for hydration in the behavioral decisions of warming embryos, rather than an indirect effect mediated by changes in evaporative cooling. It suggests that embryos assessing risk integrate information about hydration, and perhaps changes in hydration, with information about current and changing temperature. These findings highlight the value of methods to independently manipulate hydration and heating rate, showing the complexity of thermal ecology in embryonic ectotherms. We encourage further research on temperature and hydration effects on embryo hatching to better understand tropical anurans' adaptive strategies under climate change.