Almost without exception, exaggerated ornaments and weapons of sexual selection are confined to the male sex. This is certainly true of beetles: the majority of 'horned' beetle species show male (and not female) horn expression, and males use these horns in contests over reproductive access to females.

Rare exceptions to this rule do occur, and these provide ideal systems for testing fundamental tenets of sexual selection theory. Several species in the genus Onthophagus produce female horns. In some cases (e.g. O. ferox) females produce weapons similar in form and size to those expressed by males; in other cases (e.g. O. sagittarius), females produce novel horns not present in males.

In the field, and in collaboration with Leigh Simmons (University of Western Australia), we are exploring the reproductive and parental behavior of species with female horns, including O. sagittarius. In the laboratory, Cerise Allen has completed a half-sib breeding experiment with this same species, and we are beginning to explore developmental mechanisms regulating female horn expression.

 

The behavior of species with male horns (e.g. Emlen 1997; Moczek & Emlen 2000; see also publications of David Cook, John Hunt and Leigh Simmons) provides an informative backdrop for comparison. In O. acuminatus and O. taurus, for example, male beetles fight to guard entrances to tunnels containing females. Smaller males are poor competitors, and adopt a 'sneaking' alternative tactic: they slip into guarded tunnels on the sly. Sometimes, small males dig side-tunnels that intercept guarded tunnels beneath the guarding male, bypassing the guarding male entirely.

Both O. acuminatus and O. taurus are 'male-dimorphic' for horn expression (large males produce horns, smaller males do not), and horn morphology matches well with male reproductive behavior (large, horned males guard tunnels; small, hornless males sneak). Female horns are not dimorphic (i.e. females of all body sizes produce horns), and these animals may use their horns in very different ways.