Heat tolerance in ectotherms scales predictably with body size

This study analyses how heat stress varies with body size in ectothermic organisms. The analytical approach is based on thermal death time (TDT) curves, which take into consideration both the intensity and the duration of a thermal stress, and result in a linear relationship between temperature and the logarithm of time. We analyzed two separate heat tolerance datasets measured in mollusk, arthropod, fish, amphibian and reptile species, covering nearly 9 orders of magnitude in size. Datasets ware intrinsically different but ultimately convey the same type of information on thermal tolerance. The first dataset comprised individual measurements of heat stress (i.e., data points expressed in temperature and time coordinates) whereas the second dataset described the intercept and slope of TDT curves. In both cases, smaller organisms exhibited a higher tolerance to an acute heat stress than larger ones, but their tolerance declined faster in time.We then combined the results from our scaling analyses with geographic information to quantify thermal safety margins standardized by mass and time. 

Methods
All the data employed in this study was compiled from the literature. Dataset 1 includes estimates of critical thermal limits, complemented with information on the duration of each assay obtainedfrom the original sources (n= 328 measurements), whereas Dataset 2 describes slopes and intercepts of individual TDT curves in a different and more restricted sample (n= 62 curves). We combined these datasets with information on the collection sites and wet body mass. Collection sites were then employed to retrieve maximum mean monthly air and sea surface temperatures for terrestrial and marine organisms, respectively. The detailed sources are provided in the attached Readme.txt and in the original paper.

本研究分析了变温动物（ectothermic organisms）的热胁迫随体型大小的变化规律。本研究的分析方法基于热致死时间（thermal death time, TDT）曲线，该曲线同时考量热胁迫的强度与持续时长，可建立温度与时间对数之间的线性关系。我们分析了两套独立的耐热性数据集，涵盖软体动物（mollusk）、节肢动物（arthropod）、鱼类、两栖类与爬行类物种，体型跨度接近9个数量级。两套数据集本质上存在差异，但均传递了关于热耐受性的同类信息。第一套数据集包含热胁迫的个体测量值（即以温度和时间为坐标的数据点），第二套数据集则描述了TDT曲线的截距与斜率。两类分析均显示，体型更小的生物对急性热胁迫的耐受性强于体型更大的生物，但其耐受性随时间的衰减速率更快。随后，我们将尺度分析结果与地理信息相结合，量化了以体重和时间为标准化基准的热安全裕度（thermal safety margins）。

研究方法
本研究所用全部数据均来自已发表文献。数据集1包含临界热极限（critical thermal limits）的估算值，辅以从原始文献中获取的各试验持续时长信息（共328组测量数据）；数据集2则针对另一组更为受限的样本，描述了各TDT曲线的斜率与截距（共62条曲线）。我们将这两套数据集与采样点及湿体重（wet body mass）信息相结合。随后根据采样点，分别获取陆生与海洋生物的最大月平均气温与海表温度。详细数据来源见附件Readme.txt及原文。