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.