Data from: Bergmann’s Rule rules body size in an ectotherm: heat conservation in a lizard along a 2200-meter elevational gradient

Bergmann's Rule predicts larger body sizes in colder habitats, increasing organisms’ ability to conserve heat. Originally formulated for endotherms, it is controversial whether Bergmann's Rule may be applicable to ectotherms, given that larger ectotherms show diminished capacity for heating up. We predict that Bergmann's Rule will be applicable to ectotherms when the benefits of a higher conservation of heat due to a larger body size overcompensate for decreased capacity to heating up. We test this hypothesis in the lizard Psammodromus algirus, which shows increased body size with elevation in Sierra Nevada (SE Spain). We measured heating and cooling rates of lizards from different elevations (from 300 to 2500 meters above sea level) under controlled conditions. We found no significant differences in the heating rate along an elevational gradient. However, the cooling rate diminished with elevation and body size: highland lizards, with larger masses, have a higher thermal inertia for cooling, which allows them to maintain heat for more time and keep a high body temperature despite the lower thermal availability. Consequently, the net gaining of heat increased with elevation and body size. This study highlights that the heat conservation mechanism for explaining Bergmann's Rule works and is applicable to ectotherms, depending on the thermal benefits and costs associated with larger body sizes.

贝格曼法则（Bergmann's Rule）预测，寒冷生境下的生物体型会更大，以此增强机体的储热能力。该法则最初针对恒温动物（endotherms）提出，而针对变温动物（ectotherms）是否适用仍存在争议——因体型更大的变温动物其加热能力会出现下降。我们提出假说：当更大体型带来的更高储热收益足以抵消其加热能力降低的代价时，贝格曼法则便可适用于变温动物。我们以西班牙东南部内华达山脉中随海拔升高体型增大的蜥蜴物种阿尔及尔麻蜥（Psammodromus algirus）为研究对象，对该假说进行检验。我们在受控环境下，测量了来自海拔300至2500米不同海拔区域的蜥蜴的加热与冷却速率。研究结果显示，沿海拔梯度的加热速率并无显著差异。但冷却速率随海拔升高与体型增大而降低：体型更大的高海拔蜥蜴拥有更强的冷却热惯性，使其能够更长时间维持体温，即便在热可利用性更低的环境中仍可保持较高体温。由此，净热获取量随海拔升高与体型增大而提升。本研究表明，用于解释贝格曼法则的储热机制是成立的，且可适用于变温动物，这一结论取决于更大体型所关联的热收益与成本。