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.

El cambio climático está aumentando tanto las temperaturas ambientales como las sequías. Muchos ectotermos responden conductualmente al calor, evitando así los daños por temperaturas extremas. Dentro de las especies, la tolerancia térmica varía con factores como la hidratación y la ontogenia. Muchos anuros tropicales depositan huevos terrestres que dependen de la humedad ambiental para el desarrollo embrionario. Estos huevos son vulnerables a la deshidratación y los embriones de algunas especies pueden eclosionar prematuramente para escapar de la desecación de los huevos. Las temperaturas más cálidas pueden acelerar el desarrollo y, por lo tanto, la eclosión, pero el exceso de calor puede matar a los embriones. Por lo tanto, planteamos la hipótesis de que los embriones pueden mostrar un límite de tolerancia térmica conductual, eclosionando prematuramente para evitar un calentamiento potencialmente letal. Si es así, debido a que el calentamiento y la desecación a menudo están asociados, planteamos la hipótesis de que este límite, medible como una tolerancia térmica voluntaria, puede depender de la hidratación. Manipulamos la hidratación de los huevos terrestres de Agalychnis callidryas, en posturas intactas y grupos de huevos aislados de la gelatina de la postura. Luego, los calentamos para evaluar si los embriones eclosionan temprano como respuesta conductual a las altas temperaturas y si su tolerancia térmica varía con la hidratación o la estructura circundante. Descubrimos que el calentamiento induce la eclosión. Estos embriones muestran una respuesta conductual de eclosión de escape que les permite evitar un calentamiento potencialmente letal. Los huevos y las posturas hidratadas perdieron más agua y se calentaron más lentamente que los deshidratados, lo que indica que la hidratación protege a los embriones del calentamiento ambiental a través del enfriamiento por evaporación. Los embriones en posturas hidratadas toleraron un mayor calentamiento antes de la eclosión y sufrieron una mayor mortalidad, lo que sugiere que su margen de seguridad térmica conductual es pequeño. En contraste, la tolerancia térmica más baja protegió a los embriones deshidratados, y a los aislados de la gelatina, del calentamiento letal. La eclosión inducida por calor ofrece un ensayo de comportamiento conveniente para la tolerancia térmica de los embriones terrestres de anuros y los efectos integrados del calentamiento y la deshidratación en una etapa temprana de la vida. Este estudio amplía el conjunto de amenazas contra las cuales los embriones usan la eclosión en defensa propia, creando nuevas oportunidades para estudios comparativos de tolerancia térmica, así como estudios integradores de mecanismos de autodefensa en etapa de huevo.

As mudanças climáticas estão aumentando as temperaturas ambientais e as secas. Muitos ectotermos respondem comportamentalmente ao calor, evitando danos devido a temperaturas extremas. Dentro das espécies, a tolerância térmica varia com fatores como a hidratação e a ontogenia. Muitos anuros tropicais depositam ovos terrestres que dependem da umidade ambiental para o desenvolvimento embrionário. Esses ovos são vulneráveis à desidratação, e os embriões de algumas espécies podem eclodir prematuramente para escapar da dessecação dos ovos. Temperaturas mais altas podem acelerar o desenvolvimento e, portanto, a eclosão, mas o excesso de calor pode matar os embriões. Portanto, propomos a hipótese de que os embriões podem mostrar um limite de tolerância térmica comportamental, eclodindo prematuramente para evitar o aquecimento potencialmente letal. Nesse caso, como o aquecimento e a dessecação são frequentemente associados, propomos a hipótese de que esse limite, mensurável como uma tolerância térmica voluntária, pode depender da hidratação. Manipulamos a hidratação dos ovos terrestres de Agalychnis callidryas, em desovas completas e grupos de ovos isolados da geleia da desova. Depois, aquecemos os ovos para avaliar se os embriões eclodem cedo como uma resposta comportamental a altas temperaturas, e se sua tolerância térmica varia com a hidratação ou a estrutura circundante. Descobrimos que o aquecimento induz a eclosão. Esses embriões mostram uma resposta comportamental de eclosão que lhes permite evitar o aquecimento potencialmente letal. Ovos e desovas hidratadas perderam mais água e se aqueceram mais lentamente que os desidratados, indicando que a hidratação protege os embriões do aquecimento ambiental através do resfriamento por evaporação. Os embriões em desovas hidratadas toleraram maior aquecimento antes da eclosão e sofreram maior mortalidade, sugerindo que sua margem de segurança térmica comportamental é menor. Por outro lado, a menor tolerância térmica protegeu os embriões desidratados e os isolados da geleia, do aquecimento letal. A eclosão induzida pelo calor oferece um teste comportamental conveniente para a estimativa da tolerância térmica de embriões de anuros terrestres e dos efeitos integrados do aquecimento e da desidratação em um estágio inicial da vida. Este estudo expande o conjunto de ameaças contra as quais os embriões usam a eclosão como defesa, criando novas oportunidades para estudos comparativos de tolerância térmica, além de estudos integrativos dos mecanismos de autodefesa no estágio de ovo.

Correction: Does behavioral thermal tolerance predict distribution pattern and habitat use in two sympatric Neotropical frogs?

[This corrects the article DOI: 10.1371/journal.pone.0239485.].

Effects of dehydration on thermoregulatory behavior and thermal tolerance limits of Rana catesbeiana ().

Predicting the effects of high environmental temperatures and drought on populations requires understanding how these conditions will influence the thermoregulatory behavior and thermal tolerance of organisms. Ectotherms show proportional (fine-tuned) and all-or-none (abrupt) responses to avoid overheating. Scattered evidence suggests that dehydration alters these behavioral responses and thermal tolerance, but these effects have not been evaluated in an integrative manner. We examined the effects of hydration level on the behavioral thermoregulation and behavioral and physiological thermal limits of the "bullfrog" (Rana catesbeiana), a well-studied and important invasive species. To examine the effects of dehydration on proportional responses, we compared the Preferred Body Temperatures (PBT) of frogs with restricted and unrestricted access to water. To assess the effect of dehydration on all-or-none responses, we measured and compared the Voluntary Thermal Maximum (VTMax) at different hydration levels (100%, 90%, 80% of body weight at complete hydration). Finally, to understand the effect of dehydration on physiological thermal tolerance, we measured the Critical Thermal Maximum (CTMax) of frogs at matched hydration levels. PBT, VTMax, and CTMax all decreased in response to higher dehydration levels. However, bullfrogs changed their PBT more than their VTMax or CTMax in response to dehydration. Moreover, some severely dehydrated individuals did not exhibit a VTMax response. We discuss the implications of our results in the context of plasticity of thermoregulatory responses and thermal limits, and its potential application to mechanistic modeling.

Does behavioral thermal tolerance predict distribution pattern and habitat use in two sympatric Neotropical frogs?

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, VTMax) on anuran local (habitat) and regional distribution patterns and associated behavioral responses. We tested for differences in Voluntary Thermal Maximum (VTMax) of two sympatric frog species of the genus Physalaemus in the Cerrado. We mapped the difference between VTMax and maximum daily temperature (VTMax-ETMax) and compared the abundance in open and non-open habitats for both species. Physalaemus nattereri had a significantly higher VTMax than P. cuvieri. For P. nattereri, the model including only period of day was chosen as the best to explain variation in the VTMax while for P. cuvieri, the null model was the best model. At the regional scale, VTMax-ETMax values were significantly different between species, with P. nattereri mostly found in localities with maximum temperatures below its VTMax 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 VTMax, whereas P. nattereri tends to be abundant in both open and non-open (and cooler) areas and occurs mostly in cooler areas regarding its VTMax. 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.