Environmental drivers of structural colour in bees (Hymenoptera: Anthophila)

Aim: Environmental drivers frequently predict global patterns of colour diversity, but whether such patterns depend on the underlying colour mechanisms – pigments or microscopic structures – has scarcely been considered. Structural colour may have different functional properties that result in different associations with environmental variables. Here we test whether the presence of structural colour is linked with environmental variables that reflect potential thermoregulatory and water repellent properties. Location: Global. Major taxa studied: Bees (Hymenoptera: Anthophila). Methods: We scored the presence of structural colour and extracted climate data for 1,784 bee species. We used phylogenetic generalised linear mixed models to control for phylogenetic relatedness and tested whether environmental variables, related to temperature and humidity, explain the presence of structural colour. Results: A higher proportion of tropical species had structural colour, despite the higher species richness of bees at higher latitudes. Structural colouration was more likely to occur in cool, sunny environments, in which it is expected to provide the greatest thermal benefit. Structural colour was also more prevalent in environments with higher annual precipitation. These ecogeographic patterns were independent of body size. Main conclusions: Our study reveals global ecogeographic patterns of structural colouration in bees, consistent with thermoregulatory and potentially water repellence functions. In addition, our findings highlight that structural colours probably have multiple coexisting functions, including both visual and non-visual functions, that need to be disentangled to understand global patterns of colour diversity.