Anuran survival is strongly affected by exposure to high environmental temperatures. However, their upper thermal tolerances vary between species and within developmental stages. The aims of this research were to measure the median lethal temperature (LT50) of three anuran developmental stages (Gosner stages 10, 20, and 25) at a constant thermal regime, and of developing embryos (stage 10) until they became tadpoles (stage 25) exposed to daily peaks of temperatures between 1000 and 1600. Four Colombian species (Emerald-eyed Treefrog, Hypsiboas crepitans (Wied-Neuwied, 1824); Tungara Frog, Engystomops pustulosus (Cope, 1864); Rivero’s Toad, Rhinella humboldti (Gallardo, 1965); Emerald Glassfrog, Espadarana prosoblepon (Boettger, 1892)) were used in these experiments. An ontogenetic increase was observed in the upper thermal tolerance from embryos to tadpoles for all species studied. In addition, developing embryos exposed to peak temperatures showed a LT50 fairly close to the mean of the maximum habitat temperatures, particularly in H. crepitans and E. pustulosus that lay egg masses exposed directly to the sun. Environmental temperatures in the microhabitat of species studied showed values remarkably higher than their experimental LT50. Therefore, we postulate that rapid increases in environmental temperatures, as result of global or local changes, might be a critical factor for anuran survival, mainly during the embryonic stages when they are more sensitive to temperature.

Current climate change is generating accelerated increase in extreme heat events and organismal plastic adjustments in upper thermal tolerances, (critical thermal maximum ‐CTmax) are recognized as the quicker mitigating mechanisms. However, current research casts doubt on the actual mitigating role of thermal acclimation to face heat impacts, due to its low magnitude and weak environmental signal. Here, we examined these drawbacks by first estimating maximum extent of thermal acclimation by examining known sources of variation affecting CTmax expression, such as daily thermal fluctuation and heating rates. Second, we examined whether the magnitude and pattern of CTmax plasticity is dependent of the thermal environment by comparing the acclimation responses of six species of tropical amphibian tadpoles inhabiting thermally contrasting open and shade habitats and, finally, estimating their warming tolerances (WT = CTmax – maximum temperatures) as estimator of heating risk. We found that plastic CTmax responses are improved in tadpoles exposed to fluctuating daily regimens. Slow heating rates implying longer duration assays determined a contrasting pattern in CTmax plastic expression, depending on species environment. Shade habitat species suffer a decline in CTmax whereas open habitat tadpoles greatly increase it, suggesting an adaptive differential ability of hot exposed species to quick hardening adjustments. Open habitat tadpoles although overall acclimate more than shade habitat species, cannot capitalize this beneficial increase in CTmax, because the maximum ambient temperatures are very close to their critical limits, and this increase may not be large enough to reduce acute heat stress under the ongoing global warming.

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Phenotypic plasticity of the upper critical thermal limits (CTmax) may be crucial for ectotherms when it enables them to respond rapidly to extreme and novel thermal conditions. Although current studies have widely reported on the effect of increasing temperature on the magnitude of the plastic response of ectotherms, little is known about timing of upper thermal acclimation. These temporal components may be adaptive and of major environmental concern, especially under the increasing frequency of episodic heatwaves, predicted by climate change models together with quick habitat conversion. We experimentally studied the temporal acquisition of a greater thermal tolerance by acclimation effect in four species of tropical tadpoles, adjusting the daily variation in the CTmax to an asymptotic function and analyzing its main parameters: asymptotic CTmax (CTmax∞) and acclimation rate of the CTmax (K), under two realistic daily thermal fluctuations: mean daily fluctuation (MF) and extreme hot fluctuation (HF), and under the corresponding constant temperatures, mean constant (MC) and hot constant (HC). The rate of acclimation was higher for constant and hotter conditions, with which the CTmax∞ was reached in a shorter time under these conditions. The time to achieve the CTmax∞ was between one and three days depending on the treatment of acclimation and species. Plastic responses are species-specific and appear to be adaptive to the level of thermal heterogeneity of their breeding environment. Engystomops pustulosus tadpoles, that develop in hot and thermally variable temporary pond had the greatest acclimation. This suggests that species exposed naturally to extreme heat events may exhibit the highest plastic response in acclimation to upper thermal tolerances.