Data from: Disentangling environmental drivers of metabolic flexibility in birds: the importance of temperature extremes versus temperature variability

Examining physiological traits across large spatial scales can shed light on the environmental factors driving physiological variation. For endotherms, flexibility in aerobic metabolism is especially important for coping with thermally challenging environments and recent research has shown that aerobic metabolic scope [the difference between maximum thermogenic capacity (Msum) and basal metabolic rate (BMR)] increases with latitude in mammals. One explanation for this pattern is the climatic variability hypothesis, which predicts that flexibility in aerobic metabolism should increase as a function of local temperature variability. An alternative explanation is the cold adaptation hypothesis, which predicts that cold temperature extremes may also be an important driver of variation in metabolic scope. To determine the thermal drivers of aerobic metabolic flexibility in birds, we combined data on metabolic scope from 40 bird species sampled across a range of environments with several indices of local ambient temperature. Using phylogenetically-informed analyses, we found that minimum winter temperature was the best predictor of variation in avian metabolic scope, outperforming all other thermal variables. Additionally, Msum was a better predictor of latitudinal patterns of metabolic scope than BMR, with species inhabiting colder environments exhibiting increased Msum over their counterparts in warmer environments. Taken together, these results suggest that cold temperature extremes drive latitudinal patterns of metabolic scope via selection for enhanced thermogenic performance in cold environments, supporting the cold adaptation hypothesis. Temperature extremes may therefore be an important selective pressure driving macrophysiological trends of aerobic performance in endotherms.在大空间尺度上探究生理性状，可揭示驱动生理变异的环境调控因子。对于恒温动物（endotherms）而言，有氧代谢（aerobic metabolism）的灵活性对于应对热胁迫环境尤为关键。近期研究显示，哺乳动物的有氧代谢范围（metabolic scope，即最大产热能力（maximum thermogenic capacity, Msum）与基础代谢率（basal metabolic rate, BMR）的差值）随纬度升高而增加。针对这一规律，存在两种解释假说：其一为气候变异性假说（climatic variability hypothesis），该假说预测有氧代谢灵活性应随局地温度变异性的提升而增强；其二为冷适应假说（cold adaptation hypothesis），该假说提出极端低温或许同样是代谢范围变异的重要驱动因素。为明确鸟类有氧代谢灵活性的热驱动因子，本研究整合了覆盖多种生境的40种鸟类的代谢范围数据，并结合多项局地环境温度指标。通过系统发育信息分析（phylogenetically-informed analyses），本研究发现冬季最低温是鸟类代谢范围变异的最佳预测因子，其表现优于所有其他温度变量。此外，相较于BMR，Msum能更好地解释代谢范围的纬度分布格局：栖息于寒冷环境的物种，其Msum显著高于温暖环境中的同类物种。综合上述结果，本研究表明极端低温通过对寒冷环境下增强型产热性能的定向选择，驱动了代谢范围的纬度分布格局，从而支持冷适应假说。因此，极端低温或为驱动恒温动物有氧表现宏观生理趋势的重要选择压力。