Data from: Colour lightness of dragonfly assemblages across North America and Europe

Dark-coloured ectotherms absorb energy from the environment at higher rates than light-coloured ectotherms. The thermal melanism hypothesis (TMH) states that this physical mechanism links the colour lightness of the body surfaces of ectotherms to their thermal environment and hence to their geographical distribution. Studies on different insect taxa in Europe found support for this prediction of the TMH. However, whether these results hold also for other biogeographical regions remains unclear. Here, we quantify and map the colour lightness of dragonfly species in North America and directly compare our results to previously published findings for Europe. We estimated the colour lightness of 152 North American dragonfly species from published illustrations, compiled their distribution data from the literature and combined all these data with six biologically relevant environmental variables. We evaluated the importance of phylogenetic autocorrelation for the spatial variation of mean colour lightness of dragonfly assemblages (grid cells of approximately 50 km × 50 km size) by repeating all analyses also for the phylogenetically predicted component of the colour lightness of species and the species-specific deviation from this prediction. We also accounted for spatial autocorrelation with autoregressive error models. All statistical approaches showed that dragonfly assemblages from both continents consistently tended to be darker coloured in regions with cold climates and lighter coloured in regions with warm climates. Regression slopes, however, were significantly less steep, and the amount of variance explained by environmental variables was lower for North America than for Europe. Our results highlight the importance of colour lightness for the distribution of dragonfly species, but they also indicate that idiosyncrasies of the continents modify the general pattern.深色外温动物从环境中吸收能量的速率高于浅色外温动物。热黑化假说（Thermal melanism hypothesis, TMH）提出，这一物理机制将外温动物体表的色彩明度与其热环境乃至地理分布联系起来。针对欧洲不同昆虫类群的研究为该假说的这一预测提供了支持，但目前尚不明确这些结论是否同样适用于其他生物地理区域。本研究对北美蜻蜓物种的色彩明度进行量化并绘制其空间分布图谱，同时将所得结果与欧洲地区已发表的相关研究结论进行直接对比。研究团队从已发表的插图中估算了152种北美蜻蜓的色彩明度，从文献中整理了它们的分布数据，并将所有此类数据与六项具有生物学相关性的环境变量进行整合。通过针对物种色彩明度的系统发育预测组分以及物种相对于该预测的特异性偏差重复所有分析，本研究评估了系统发育自相关对蜻蜓群落（约50 km × 50 km大小的网格单元）平均色彩明度空间变异的重要性；同时采用自回归误差模型控制空间自相关效应。所有统计分析方法均显示，两大洲的蜻蜓群落均呈现出一致的趋势：气候寒冷区域的群落色彩更深，而气候温暖区域的群落色彩更浅。不过，北美地区的回归斜率显著更平缓，且环境变量所能解释的方差量也低于欧洲地区。本研究结果既凸显了色彩明度对蜻蜓物种分布的重要性，同时也表明各大陆的独特属性会对这一通用模式产生修饰作用。