Beyond Gloger's rule: multiple biogeographic drivers of dark and red colouration in ants

Colour is a key trait involved in camouflage, physiological protection and thermoregulation. Yet environmental drivers of colour variation remain poorly understood at large spatial scales. Glogers rule predicts animals should be darker in warmer and wetter climates, and in a complex version, redder in warmer and drier climates. Here, we present the first test of the complex Glogers rule in insects using 34,331 images of 10,400 ant species across 586 assemblages worldwide. We decompose species mean colouration into two orthogonal axes, linked to darkness and redness. Assemblages were darker under high UV-B radiation and low dry-season precipitation, consistent with UV protection and desiccation resistance via melanisation. Canopy height increased both axes, suggesting camouflage. In contrast, higher mean temperature of the warmest quarter increased redness, as predicted by the complex Glogers rule. Ant colouration cannot be explained by one macroecological rule but reflects environmental drivers acting independently on darkness and redness.