Seasonal activity and sexual selection in an urban dung beetle

Onthophagus orpheus Panzer (Coleoptera: Scarabaeidae) is the most abundant dung beetle in urban forests in central Massachusetts, but little is known about its behavioral ecology. We measured individuals throughout the breeding season to characterize male horn allometry and track seasonal variation in population abundance, adult sex ratio, and male morph ratio. Large major males have a forked thoracic horn that they use in male-male fights over ownership of breeding tunnels; small minor males have rudimentary horns; and females are hornless. Unlike many onthophagine dung beetles that exhibit sigmoidal horn allometries with distinct horned and hornless male morphs, O. orpheus exhibits a segmented horn allometry in which many males have intermediate-sized horns. Abundance peaked in July, and average densities were comparable to those observed for other horned Onthophagus species that experience intense male-male competition. The season-wide adult sex ratio was significantly female-biased, and males exhibited size-dependent seasonal activity. These ecological conditions are likely to promote a highly competitive, defense-based mating system that favors the development of sexually selected horns. Because of its prevalence in urban forests across the eastern United States and the expected increases in resource availability (i.e., dog dung) in urban environments, we highlight O. orpheus as an emerging model species for investigating the effects of urbanization on mating dynamics.