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维度。在这些假说中，生物物理原理指出深色体色具备体温调节的关键意义，而该意义的强弱直接取决于物种的体型大小。这种体型与体色深浅的权衡关系，以热黑化假说（thermal melanism hypothesis）所构建的扎实理论为基础，已在多个单物种类群的个案研究中得到验证。然而，该权衡关系在大空间尺度与宽分类学尺度上具备普遍适用性的证据仍然缺失。本研究专门针对一类高度多样且广布全球的昆虫类群——步甲，探讨其体型与体色的权衡关系。我们整合了来自整个西古北区的1000余种步甲物种的体色信息、形态性状数据、分布数据以及生物气候属性数据。我们借助孟塞尔颜色系统（Munsell colour system），基于高质量的标准化数码照片对各物种的特异性体色进行了量化分析。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呈显著上升趋势，直至15毫米的体型阈值，这一结果与热黑化假说相符。然而当体型超过该阈值后，体型对体色深浅不再产生影响，体色深浅始终维持在较高水平，这与此前关于体温调节控制机制的体型依赖性的观点（体型依赖性假说，size dependence hypothesis）一致。本研究通过证实体型增大伴随体色加深的显著趋势，阐明了体色在大陆尺度上对这类广布外温动物类群的种间相关性。体型与黑化体色之间推测的体温调节权衡关系，在大空间尺度上对步甲类群似乎具有重要意义；该现象可能是外温动物普遍存在但仍被低估的一般性规律，但其他驱动机制也不可完全忽视。