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.