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.