The evolution of body coloration in dung beetles: diel activity and sexual dimorphism

Animal colouration may evolve due to its effects on predator avoidance, reproduction and thermoregulation, and is likely influenced by the organism’s diel activity. The dung beetle species of the Phanaeini tribe (Coleoptera: Scarabaeinae) show a wide variety of times of activity and pre-copulatory behaviours. Here we tested the effect of time of activity on body coloration and sexual dichromatism of Phanaeini beetles using a quantitative spectrophotometric method in a phylogenetic framework. Considering the potential functions of dung beetle colouration, we expected to find brighter, more colourful animals with a higher degree of sexual dichromatism in diurnal than in non-diurnal species. We measured the reflectance spectrum (300–700 nm) and calculated the brightness and chroma in four body areas for each individual (588 individuals of 51 species). We analysed the data in a Bayesian phylogenetic approach. The analyses indicated that diurnal species have higher chroma and brightness than non-diurnal ones. Furthermore, we found an evident sexual dimorphism in the clypeus of diurnal species: males have a colourful clypeus, whereas females have a dark non-colourful one. The condition of low light and variable light in nocturnal and crepuscular species may not favour the bright and colourful patterns or may have caused its loss. The presence of dichromatism in clypeus may be related to communication between conspecifics and different reproductive behaviours in Phanaeini. Thus, we demonstrate that diurnal species evolved more colourful bodies than non-diurnal species and that diurnal species exhibit sexual dichromatism in the clypeus, suggesting a role for sexual selection in the evolution of clypeal colouration.