Global studies of organismal distribution and climatic vulnerability rely on the mostly untested assumption that heat tolerance restricts the maximum temperatures estimated at the warm edges of their geographic distribution (Tmax). Herein we test this assumption across the animal kingdom and examine whether the strength of restrictions depends on how challenging heat becomes for their tolerance. Thermal limits restrict species’ warm distributional edges across the animal kingdom, being restrictions less consistent for reptiles and describing a striking non-linear relationship for marine fish that contrast with terrestrial groups. Besides, heat tolerance restricts the geographic warm edges more strongly for species exposed to more defying temperatures at ranges’ warm edges.

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.

Critical thermal limits (CTmax and CTmin) are predicted to decrease with elevation, with greater change in CTmin, and the risk to suffer heat and cold stress increasing at the gradient ends. A central prediction is that populations will adapt to the prevailing climatic conditions. Yet, reliable support for such expectation is scant because of the complexity of integrating phenotypic and molecular divergence. We propose that phenotypic plasticity and breeding phenology may hinder local adaptation cancelling the appearance of adaptive patterns. We examined intraspecific variation of CTmax/CTmin in 11 populations of an amphibian across an elevational gradient, and assessed (1) the existence of local adaptation through a PST-FST comparison, (2) the acclimation scope in both thermal limits, and (3) the vulnerability to suffer acute heat (CTmax–tmax) and cold (tmin–CTmin) thermal stress, measured at both macro- and microclimatic scales. Our study revealed significant microgeographic variation in CTmax/CTmin, and unexpected elevation gradients in pond temperatures. However, variation in CTmax/CTmin could not be attributed to selection because critical thermal limits were not correlated to elevation or temperatures. Differences in breeding phenology among populations resulted in exposure to higher and more variable temperatures at mid and high elevations. Accordingly, mid- and high-elevation populations had higher CTmax and CTmin plasticities than lowland populations, but not more extreme CTmax/CTmin. Thus, we confirm our prediction that plasticity and phenological shifts may hinder local adaptation, promoting thermal niche conservatism and a higher vulnerability to climate change. This contradicts some of the existing predictions on adaptive thermal clines.