Mate choice in the dung beetle Onthophagus sagittarius: are female horns ornaments?

Sexual selection typically operates via male contest competition and female choice, favoring the evolution of secondary sexual traits in males. However, there are numerous taxa in which females possess secondary sexual traits and the selective pressures operating on female ornamentation are not well understood. We examined sexual selection operating via mate choice in the dung beetle Onthophagus sagittarius, a species exhibiting sex-specific ornamentation. Precopulatory mate preferences and their subsequent effects on breeding success were examined. Female preferences favored large males and when paired with small males, males with high courtship rates. Large females were choosier than small females. No overt male preferences for female size or ornamentation were found; thus, we conclude that female horns in this species are unlikely to have evolved as ornaments via precopulatory male mate choice. Relative horn length in females determined brood ball productivity, whereas female body size and an interaction between male courtship rate and male body size determined brood ball weight. Our results provide evidence for female but not male mate choice. They suggest that attractive males increase reproductive performance, but it is unclear whether the effect of male phenotype is mediated via differences in paternal investment or female differential allocation toward attractive males. Copyright 2010, Oxford University Press.