为了评估鸟类和哺乳动物对气候变化的脆弱性,最近的研究已将热中性的临界上限(TUC)用作耐热性的指标。但是,TUC和耐热性之间的关联并不简单,大多数研究仅基于代谢与基础水平的偏差来描述TUC,不考虑蒸发冷却的开始。最近有人争辩说,在极端高温事件中长时间暴露于高环境温度(即高耐热性)中幸存下来的某些使用torpor的蝙蝠物种是通过进入torpor并使用兼性异质热来热适应和节省体内水分来做到的。假设TUC表示耐热性,我们希望使用土壤的物种中的TUC高于专性同源的物种,尽管这种区别是基于在低温下使用Torpor的确认。为了测试这个预测,我们使用已发布的体温调节数据集,对已知使用Torpor的蝙蝠(n=48)和恒温动物(n=16)进行了系统发育知情比较,以比较与体温相关的热中性下限(TLC)和TUC.饮食的影响,生物地理区域,还考虑了体重和基础代谢率(BMR)。体重与BMR呈正相关,与TLC成反比,与TUC无关。正常体温与BMR呈正比例,TLC和TUC。饮食或区域与BMR之间没有关系,但两者都影响了热极限。使用Torpor的蝙蝠的体重和体温低于恒温蝙蝠,但是BMR没有区别,它们之间的TLC和TUC。在34种使用torpor的物种和8种恒温动物中观察到了生理灵活性的特殊例子,其中包括15种在环境温度高达40°C(以及相应的体温约39.2°C)时保持BMR水平代谢的蝙蝠。然而,我们认为,仅基于代谢的TUC并不是热耐受性的适当指标,因为它忽略了动物耐受更高水平高温的能力差异,水合状态和蒸发冷却能力的重要性。此外,基于饮食的TUC的变异性挑战了进化保守主义的观点,值得进一步考虑。
To assess the vulnerability of birds and mammals to climate change recent studies have used the upper critical limit of thermoneutrality (TUC) as an indicator of thermal tolerance. But, the association between TUC and thermal tolerance is not straightforward and most studies describe TUC based solely on a deviation in metabolism from basal levels, without also considering the onset of evaporative cooling. It was argued recently that certain
torpor-using bat species who survived prolonged exposure to high ambient temperatures (i.e. high thermal tolerance) experienced during extreme heat events did so by entering
torpor and using facultative heterothermy to thermoconform and save on body water. Assuming that TUC is indicative of thermal tolerance, we expect TUC in
torpor-using species to be higher than that of species which are obligate homeotherms, albeit that this distinction is based on confirmation of torpor use at low temperatures. To test this prediction, we performed a phylogenetically informed comparison of bat species known to use torpor (n = 48) and homeothermic (n = 16) bat species using published thermoregulatory datasets to compare the lower critical limit of thermoneutrality (TLC) and TUC in relation to body temperature. The influence of diet, biogeographical region, body mass and basal metabolic rate (BMR) was also considered. Body mass had a positive relationship with BMR, an inverse relationship with TLC and no relationship with TUC. Normothermic body temperature scaled positively with BMR, TLC and TUC. There was no relationship between diet or region and BMR, but both influenced thermal limits.
Torpor-using bats had lower body mass and body temperatures than homeothermic bats, but there was no difference in BMR, TLC and TUC between them. Exceptional examples of physiological flexibility were observed in 34
torpor-using species and eight homeothermic species, which included 15 species of bats maintaining BMR-level metabolism at ambient temperatures as high as 40 °C (and corresponding body temperatures ∼39.2 °C). However, we argue that TUC based on metabolism alone is not an appropriate indicator of thermal tolerance as it disregards differences in the ability of animals to tolerate higher levels of hyperthermia, importance of hydration status and capacity for evaporative cooling. Also, the variability in TUC based on diet challenges the idea of evolutionary conservatism and warrants further consideration.