Data from: Bergmann's body size rule operates in facultatively endothermic insects: evidence from a complex of cryptic bumblebee species

According to Bergmann's rule we expect species with larger body size to inhabit locations with a cooler climate, where they may be well adapted to conserve heat and resist starvation. This rule is generally applied to endotherms. In contrast, body size in ectothermic invertebrates has been suggested to follow the reverse ecogeographic trend: these converse Bergmann's patterns may be driven by the ecological constraints of shorter season length and lower food availability in cooler high latitude locations. Such patterns are particularly common in large insects due to their longer development times. As large and facultatively endothermic insects, bumblebees could thus be expected to follow either trend. In this investigation, we studied body size of three bumblebee species over a large spatial area and investigated whether interspecific trends in body size correspond to differences in their distribution consistent with either Bergmann's or a converse Bergmann's rule. We examined the body size of queens, males and workers of the Bombus lucorum complex of cryptic bumblebee species from across the whole of Great Britain. We found interspecific differences in body size corresponding to Bergmann's rule: queens and males of the more northerly distributed, cool-adapted, species were largest. In contrast, the mean body size of the worker caste did not vary between the three species. These differences in body size may have evolved under selection pressures for thermoregulation or starvation resistance. We suggest that this case study in facultatively endothermic insects may help clarify the selection pressures governing Bergmann rule trends more generally.

根据伯格曼法则（Bergmann's rule），体型更大的物种通常栖息于气候更凉爽的区域，这类物种往往能够良好适应低温环境以保存热量、抵御饥饿。该法则最初主要应用于恒温动物（endotherms）。与之相反，现有研究提示变温无脊椎动物（ectothermic invertebrates）的体型变化遵循反向的生态地理趋势：这种反向伯格曼模式的成因，可能是高纬度凉爽地区生长季更短、食物可获得性更低所带来的生态约束。由于昆虫发育周期较长，这类体型模式在大型昆虫中尤为普遍。

作为兼性恒温昆虫（facultatively endothermic insects），熊蜂（bumblebees）理论上可能符合上述任意一种体型趋势。本研究针对分布于广阔空间范围内的3种熊蜂开展体型分析，旨在探讨其种间体型趋势是否符合伯格曼法则或反向伯格曼法则所对应的分布差异。我们对覆盖全英国全境的卢氏熊蜂复合群（Bombus lucorum complex，一类由多种隐蔽种熊蜂组成的类群）的蜂后、雄蜂及工蜂的体型进行了检测。结果显示，种间体型差异符合伯格曼法则：分布更靠北、适应凉爽环境的物种，其蜂后与雄蜂体型更大；而工蜂品级的平均体型在三个物种间并无差异。这种体型差异可能是在体温调节或抗饥饿相关的选择压力下演化形成的。我们认为，这项针对兼性恒温昆虫的案例研究，或有助于更普适地阐明调控伯格曼法则趋势的选择压力机制。