Department of Biological Sciences, University of Cape Town, Cape Town, 7701, South Africa; Department of Zoology, Nelson Mandela University, Gqeberha, 6031, South Africa.
School of Biology and Ecology, University of Maine, 04469 Orono, ME, USA; School of Life Sciences, University of Nevada Las Vegas, 89154, Las Vegas, NV, USA.
J Therm Biol. 2024 Aug;124:103933. doi: 10.1016/j.jtherbio.2024.103933. Epub 2024 Aug 24.
To assess the vulnerability of birds and mammals to climate change recent studies have used the upper critical limit of thermoneutrality (T) as an indicator of thermal tolerance. But, the association between T and thermal tolerance is not straightforward and most studies describe T 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 T is indicative of thermal tolerance, we expect T 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 (T) and T 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 T and no relationship with T. Normothermic body temperature scaled positively with BMR, T and T. 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, T and T 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 T 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 T based on diet challenges the idea of evolutionary conservatism and warrants further consideration.
为了评估鸟类和哺乳动物对气候变化的脆弱性，最近的研究使用了热中性区的上限临界值（T）作为耐热性的指标。但是，T 与耐热性之间的关系并不简单，大多数研究仅基于代谢偏离基础水平来描述 T，而没有考虑蒸发冷却的开始。最近有人认为，某些使用冬眠的蝙蝠物种在极端高温事件中经历了长时间的暴露，从而幸存下来（即具有较高的耐热性），它们通过进入冬眠并利用适应性异温来热适应并节省身体水分。假设 T 表示耐热性，我们预计使用冬眠的物种的 T 高于必须恒温的物种的 T，尽管这种区别基于低温下确认使用冬眠。为了验证这一预测，我们使用已发表的体温调节数据集，对已知使用冬眠（n=48）和恒温（n=16）蝙蝠物种的蝙蝠物种进行了系统发育信息丰富的比较，以比较热中性区的下限临界值（T）和 T 与体温的关系。还考虑了饮食、生物地理区域、体重和基础代谢率（BMR）的影响。体重与 BMR 呈正相关，与 T 呈负相关，与 T 无关。正常体温与 BMR、T 和 T 呈正相关。饮食或区域与 BMR 之间没有关系，但两者都影响热极限。使用冬眠的蝙蝠的体重和体温低于恒温蝙蝠，但它们的 BMR、T 和 T 之间没有差异。在 34 种使用冬眠的物种和 8 种恒温物种中观察到了异常的生理灵活性例子，其中包括 15 种蝙蝠在环境温度高达 40°C（相应的体温约为 39.2°C）时保持代谢水平的代谢。然而，我们认为仅基于代谢的 T 不是耐热性的合适指标，因为它忽略了动物耐受更高水平的体温过高、水合状态的重要性和蒸发冷却能力的差异。此外，基于饮食的 T 的可变性挑战了进化保守主义的观点，值得进一步考虑。
