Abstract 1. In many Coleoptera, iridescent colouration is generated by exoskeleton ultra-structure, within which multilayer interference reflects only certain wavelengths. Published work indicates that the colour polymorphism shown by some iridescent beetles is under genetic control. However, the present study suggests environmental involvement in the polymorphic southern African dung beetle, Gymnopleurus humanus Macleay.
2. At 24 study sites across a 1000-km latitudinal temperature gradient, population samples of G. humanus were dominated by blue individuals in the cooler south, by cupreous individuals in the warmer north, and by locally co-occurring blue, green and cupreous individuals in intermediate situations.
3. Using digital reflectance spectrophotometry, we measured wavelength intensity values across the visible spectrum (400–800 nm) and used the 70 measured specimens to estimate maximum reflectance from a further 3912 beetles. Differences in mean reflectance values between 24 populations were strongly correlated with average annual temperatures at study sites.
4. Much stronger correlations between mean reflectance values and average temperatures of the cool dry season months suggest that the cross-climatic patterns may be related to interaction between breeding seasonality and development under different cooler temperatures.
5. Published evidence suggests that inherent physical properties of cholesteric liquid crystals and their responses to different thermal conditions could, potentially, generate the different exocuticle ultra-structure responsible for different reflected colour wavelengths. Furthermore, colour polymorphism could be advantageous across a gradient from cooler to warmer climate as a result of the different thermal properties of different colours.
6. Given the correlation with temperature, it is predicted that the prevailing reflected colour balance in southern populations would shift in response to global climatic change.