Data for: Adaptive variation in the upper limits of avian body temperature

Physiological performance declines precipitously at high body temperature (Tb), but little attention has been paid to adaptive variation in upper Tb limits among endotherms. We hypothesized that avian maximum tolerable body temperature (Tbmax) has evolved in response to climate, with higher Tbmax in species exposed to high environmental heat loads or humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we compared Tbmax and related variables among 53 bird species at multiple sites in South Africa with differing maximum air temperatures (Tair) and humidity using a phylogenetically-informed comparative framework. Birds in humid, lowland habitats had comparatively high Tbmax (mean ± SD = 45.60 ± 0.58°C) and low normothermic Tb (Tbnorm), with a significantly greater capacity for hyperthermia (Tbmax -Tbnorm gradient = 5.84 ± 0.77 °C) compared to birds occupying cool montane (4.97 ± 0.99 °C) or hot arid (4.11 ± 0.84 °C) climates. Unexpectedly, Tbmax was significantly lower among desert birds (44.65 ± 0.60°C), a surprising result in light of the functional importance of hyperthermia for water conservation. Our data reveal a macrophysiological pattern and support recent arguments that endotherms have evolved thermal generalization versus specialization analogous to the continuum among ectothermic animals. Specifically, a combination of modest hyperthermia tolerance and efficient evaporative cooling in desert birds is indicative of thermal specialization, whereas greater hyperthermia tolerance and less efficient evaporative cooling among species in humid lowland habitats suggests thermal generalization.

当体温（body temperature，简称Tb）升高时，生物体的生理机能（physiological performance）会急剧下降，但学界对恒温动物（endotherms）的体温上限适应性变异的关注却十分有限。我们提出假说：鸟类的最大耐受体温（maximum tolerable body temperature，简称Tbmax）是随气候演化而来的，暴露于高环境热负荷或受湿度制约蒸发散热（evaporative heat dissipation）的物种，其Tbmax更高。为验证该假说，我们采用基于系统发育信息的比较分析框架（phylogenetically-informed comparative framework），对南非多个不同最高气温（maximum air temperature，简称Tair）和湿度条件的研究地点的53种鸟类的Tbmax及相关变量进行了对比分析。
栖息于湿润低地生境的鸟类，其Tbmax相对较高（平均值±标准差=45.60±0.58℃），常态体温（normothermic Tb，简称Tbnorm）更低；与栖息于凉爽山地（Tbmax-Tbnorm梯度为4.97±0.99℃）或炎热干旱（梯度为4.11±0.84℃）气候的鸟类相比，其高热耐受（hyperthermia）能力显著更强，该梯度达5.84±0.77℃。
出乎意料的是，沙漠鸟类的Tbmax显著更低（44.65±0.60℃）——考虑到高热耐受对水分保存的重要生理意义，这一结果颇令人意外。我们的研究数据揭示了一种宏观生理学模式（macrophysiological pattern），并支持学界近期提出的观点：恒温动物的热适应性演化存在热泛化与热特化的分化，这一模式与变温动物（ectothermic animals）的适应性连续谱类似。
具体而言，沙漠鸟类兼具适度的高热耐受能力与高效的蒸发散热能力，这是热特化的典型特征；而湿润低地生境的鸟类拥有更强的高热耐受能力，蒸发散热效率却更低，这则体现了热泛化的适应性策略。