The timing of thermal acclimation and its adaptive role to face heat impacts. An examination in tropical tadpoles

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.