Dataset for Body size impacts critical thermal maximum measurements in lizards

Understanding the mechanisms behind critical thermal maxima (CTmax, the high body temperature at which neuromuscular coordination is lost) of organisms is central to understanding ectotherm thermal tolerance. Body size is an often overlooked variable that may affect interpretation of CTmax, and consequently, how CTmax is used to evaluate mechanistic hypotheses of thermal tolerance. We tested the hypothesis that body size affects CTmax and its interpretation in two experimental contexts. First, in four Sceloporus species, we examined how inter- and intra-specific variation in body size affected CTmax at normoxic and experimentally-induced hypoxic conditions, and cloacal heating rate under normoxic conditions. Negative relationships between body size and CTmax were exaggerated in larger species, and hypoxia-related reductions in CTmax were unaffected by body size. Smaller individuals had faster cloacal heating rates and higher CTmax, and variation in cloacal heating rate affected CTmax in the largest species. Second, we examined how body size interacted with the location of body temperature measurements (i.e., cloaca versus brain) in Sceloporus occidentalis, then compared this in living and deceased lizards. Brain temperatures were consistently lower than cloacal temperatures. Smaller lizards had larger brain-cloacal temperature differences than larger lizards, due to a slower cloacal heating rate in large lizards. Both live and dead lizards had lower brain than cloacal temperatures, suggesting living lizards do not actively maintain lower brain temperatures when they cannot pant. Thermal inertia influences CTmax data in complex ways, and body size should therefore be considered in studies involving CTmax data on species with variable sizes.

阐明生物体临界热极限（critical thermal maxima，CTmax，即丧失神经肌肉协调能力时的最高体温）背后的机制，是理解变温动物热耐受特性的核心要务。体型是常被忽视的变量，其可能影响对CTmax的解读，进而影响CTmax用于评估热耐受机制假说的方式。我们针对「体型会影响CTmax及其解读」这一假说，在两种实验情境下开展了验证。其一，我们以四种棘鬣蜥属（Sceloporus）物种为研究对象，探究了常氧及实验诱导低氧条件下，种间和种内体型差异对CTmax的影响，以及常氧条件下的泄殖腔升温速率。研究发现，体型与CTmax之间的负相关关系在体型更大的物种中更为显著，而低氧诱导的CTmax降幅则不受体型影响；体型更小的个体泄殖腔升温速率更快、CTmax更高，且在体型最大的物种中，泄殖腔升温速率的差异会影响CTmax数值。其二，我们以西方棘鬣蜥（Sceloporus occidentalis）为对象，探究了体型与体温测量位点（即泄殖腔 vs 脑部）的交互效应，并对比了活体与死亡蜥蜴的相关差异。结果显示，脑部温度始终低于泄殖腔温度；由于体型较大的蜥蜴泄殖腔升温速率更慢，体型更小的个体其脑-泄殖腔温差要大于体型较大的个体。无论活体还是死亡蜥蜴，脑部温度均低于泄殖腔温度，这表明当无法通过喘息散热时，活体蜥蜴并不会主动维持更低的脑部温度。热惯性以复杂的方式影响CTmax相关数据，因此在针对体型存在差异的物种开展CTmax相关研究时，应将体型纳入考量范畴。