Heat-Induced Hatching of Red-Eyed Treefrog Embryos: Hydration and Clutch Structure Increase Behavioral Thermal Tolerance.

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 the 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 the 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.