The role of atmospheric humidity in the evolution of endotherms' thermoregulatory performance remains largely unexplored, despite the fact that elevated humidity is known to impede evaporative cooling capacity. Using a phylogenetically informed comparative framework, we tested the hypothesis that pronounced hyperthermia tolerance among birds occupying humid lowlands evolved to reduce the impact of humidity‐impeded scope for evaporative heat dissipation by comparing heat tolerance limits (HTLs; maximum tolerable air temperature), maximum body temperatures (Tbmax), and associated thermoregulatory variables in humid (19.2 g H2O m−3) versus dry (1.1 g H2O m−3) air among 30 species from three climatically distinct sites (arid, mesic montane, and humid lowland). Humidity‐associated decreases in evaporative water loss and resting metabolic rate were 27%–38% and 21%–27%, respectively, and did not differ significantly between sites. Decreases in HTLs were significantly larger among arid‐zone (mean ± SD = 3.13 ± 1.12°C) and montane species (2.44 ± 1.0°C) compared to lowland species (1.23 ± 1.34°C), with more pronounced hyperthermia among lowland (Tbmax = 46.26 ± 0.48°C) and montane birds (Tbmax = 46.19 ± 0.92°C) compared to arid‐zone species (45.23 ± 0.24°C). Our findings reveal a functional link between facultative hyperthermia and humidity‐related constraints on evaporative cooling, providing novel insights into how hygric and thermal environments interact to constrain avian performance during hot weather. Moreover, the macrophysiological patterns we report provide further support for the concept of a continuum from thermal specialization to thermal generalization among endotherms, with adaptive variation in body temperature correlated with prevailing climatic conditions.

中文翻译：

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鸟类耐热性的进化：大气湿度的作用

尽管已知湿度升高会阻碍蒸发冷却能力，但大气湿度在吸热体温度调节性能演变中的作用在很大程度上仍未被探索。使用系统发育学比较框架，我们通过比较耐热极限（HTLs；最大可忍受空气温度）、最大身体温度，测试了以下假设：居住在潮湿低地的鸟类进化出明显的高温耐受性，以减少湿度阻碍范围对蒸发散热的影响。温度（时间乙max），以及潮湿条件下的相关体温调节变量（19.2 g H2奥米−3）与干燥（1.1 g H2奥米−3）来自三个气候不同地点（干旱、中等山地和潮湿低地）的 30 个物种的空气。与湿度相关的蒸发水损失和静息代谢率下降分别为 27%–38% 和 21%–27%，并且不同地点之间没有显着差异。与低地物种 (1.23 ± 1.34°C) 相比，干旱地区 (平均值 ± SD = 3.13 ± 1.12°C) 和山地物种 (2.44 ± 1.0°C) 的 HTL 下降幅度显着更大，低地物种中的高温更为明显 (时间乙最大值 = 46.26 ± 0.48°C) 和山地鸟类 (时间乙与干旱区物种 (45.23 ± 0.24°C) 相比，最大值 = 46.19 ± 0.92°C)。我们的研究结果揭示了兼性高温与蒸发冷却的湿度相关限制之间的功能联系，为了解湿热环境如何相互作用以限制鸟类在炎热天气下的表现提供了新的见解。此外，我们报告的宏观生理模式进一步支持了吸热动物从热专门化到热普遍化的连续体概念，体温的适应性变化与当时的气候条件相关。