Data from: Size dependency in colour patterns of Western Palearctic carabids

Body colouration is of high evolutionary relevance for most animals. Several competing hypotheses exist regarding the evolutionary reasons for animal colouration ranging from predator avoidance and sexual advertisement to neutral selection. Among these hypotheses, biophysical principles suggest the thermoregulatory importance of dark colouration which in turn strongly depends on species body size. This body size – darkness trade-off is based on sound theoretical background conceptualized in the thermal melanism hypothesis and is confirmed by numerous case studies for individual species. However, evidence for the general relevance of this trade-off on large spatial and taxonomic scale is still missing. Here we specifically focus on this body size – colouration trade-off for a hyper-diverse and cosmopolitan group of insects, namely ground beetles. We combined colour information with trait data and distributional as well as bioclimatic attributes for more than 1,000 carabid species from the entire Western Palearctic. We quantified species-specific body colouration from high-quality, standardised digital photographs using the Munsell colour system. We detect a strong increase of colour darkness with body size from small to medium-sized carabids up to a body size threshold of 15 mm which is consistent with the thermal melanism hypothesis. However, body size showed no effect above this threshold and colour darkness remained constantly high which is in accordance with previous ideas about the size-dependency of thermoregulative control mechanisms (size dependence hypothesis). By demonstrating a strong tendency towards darkness with increasing body size, we illustrate the inter-specific relevance of body colouration for this cosmopolitan group of ectotherms on a continental scale. The putative thermoregulative trade-off between body size and melanism seems to be of significant importance for carabids on a broad spatial scale and may be a general but still underestimated phenomenon for ectotherms in general, although other mechanistic drivers cannot be completely neglected.体色对于绝大多数动物而言具有极高的进化意义。关于动物体色的进化成因，目前存在多种相互竞争的假说，涵盖反捕食防御、性信号展示以及中性选择等多个方向。在这些假说中，生物物理原理表明深色体色具备体温调节的重要性，而该重要性又强烈依赖于物种的体型大小。这种“体型-体色深浅权衡”基于热黑化假说（thermal melanism hypothesis）所构建的坚实理论背景，且已被众多单个物种的案例研究所证实。然而，该权衡在大空间尺度与宽泛分类学尺度上的普遍相关性，至今仍缺乏相关证据支持。 本研究针对一类超多样化且广布的昆虫类群——步甲科（carabid）昆虫，聚焦于其体型与体色之间的权衡关系。我们整合了来自整个西古北区（Western Palearctic）的1000余种步甲科物种的体色信息、功能性状数据、分布数据以及生物气候属性数据。研究采用孟塞尔颜色系统（Munsell colour system），通过高质量标准化数码照片量化了各物种的特异性体色。 我们发现，在体型介于小型至中型的步甲中，体色深浅随体型增大而显著加深，直至15毫米的体型阈值，这一结果与热黑化假说相符。但当体型超过该阈值后，体型不再对体色深浅产生影响，体色始终保持较高水平，这与此前关于体温调节控制机制的体型依赖性的观点（体型依赖假说，size dependence hypothesis）一致。本研究通过证实体色深浅随体型增大而显著加深的趋势，阐明了体色对于这类广布外温动物（ectotherms）类群的种间相关性，且相关结论基于大陆尺度的分析。尽管不能完全忽视其他潜在的机制驱动因素，但体型与黑化体色之间假定的体温调节权衡，在大空间尺度上对步甲科昆虫而言似乎具有重要意义，且该现象可能是外温动物中普遍存在但仍被低估的一般性规律。