Humidity induces structural colour change and contributes to biogeographic colour variation in sweat bees

Dynamic coloration is one of the most striking visual displays in the animal kingdom. While reversible colour changes are well characterized in animal communication, more passive effects of climate on baseline coloration remain poorly understood. Here, we present a novel experimental demonstration of reversible, humidity-induced colour change in bees. In controlled lab experiments, we show that relative humidity affects cuticle colour of the sweat bee Agapostemon subtilior, changing dramatically within 24 h from a deep blue-green at low humidity to a pale, coppery green at high humidity. Older specimens experienced greater magnitude colour shifts, suggesting that cuticular degradation may increase water permeability and amplify moisture effects. To understand whether these effects shape colour variation in the wild, we extracted colour data from a large dataset of crowd-sourced field images. We found that ambient humidity weakly predicts colour variation across A. subtilior's western range, in a manner consistent with the direction of colour change established in lab experiments. While the structural basis for this colour change is still unknown, these shifts are directionally consistent with moisture-induced swelling of multilayer structures that causes reflection of longer wavelengths, a mechanism described in other insects and cephalopods. Together, these results demonstrate that climate modulates structural coloration in bees, emphasizing the role of abiotic conditions in shaping dynamic visual traits.