FIGURE 14.2 Evolution of horn location. Head and thorax shown for 11 species of Onthophagus (Coleoptera: Scarabaeinae). Horns can extend from at least five body regions—the dorsal surface of the front, middle, or base of the head, corresponding to the clypeus, frons, and vertex head segments, respectively, and the center or sides of the thoracic pronotum. Top row: O. sagittarius (female); O. lanista; O. pentacanthus; O. raffrayi. Middle row: O. taurus; O. nigriventris; O. brooksi; Bottom row: O. demarzi; O. sharpi; O. praecellens; O. andersoni. Bottom: within-species variation in horn shape. Head horns of five males sampled from a single population of an unidentified Onthophagus species from Africa, illustrating a transition between a single horn at the center of the head and a pair of horns at the sides of the head.
male dimorphism and sexual dimorphism. In most species with male dimorphism, males smaller than a critical, or threshold, body size dispense with horn production, resulting in horn lengths that scale according to a very different relationship than in large males. Females also often dispense with horn production, sometimes entirely, as in species where females never produce horns. In other cases females do produce the horn but the relative sizes of female horns differs from that of the males. Horn dimorphism also appears to have been gained and lost repeatedly in the