In this article, we have focused on the developmental mechanisms underlying the evolutionary origin of horns and the subsequent diversification of horn forms. For space reasons, we have not elaborated on the mechanisms generating dimorphism in horn expression. However, it is already clear from the few species that have been studied to date that scarabs use a variety of means to shut off horn growth. Indeed, they appear to be remarkably good at it. Both the patterning and insulin pathways are required for horn growth, and a disruption or truncation in the activities of either pathway could halt the proliferation of horns and result in a hornless adult phenotype. Our studies measuring transcript abundances for the patterning gene wg and the InR gene in the species O. nigriventris suggest that both pathways may be involved. Both pathways showed reduced activities in the horn discs of small males and females (which grow only minimal horns) compared with same-stage horn discs from large males (which grow full horns). In addition, Moczek (2006b) showed that differential amounts of pupal remodeling also contribute to horn dimorphism in this same species: females and small males reabsorb greater amounts of horn tissue than large males. Thus, developmental studies from just this one species implicate three possible mechanistic routes to the suppression of horn growth and to the evolution of horn dimorphism. Other studies by Moczek and colleagues (Moczek and Nagy, 2005; Moczek, 2006a,b; Moczek et al., 2006b) have begun to relate domains of expression of patterning genes and relative amounts of pupal remodeling with horn dimorphism in additional Onthophagus species. These studies also reveal a variety of mechanisms for shutting off horn growth.
Even this preliminary examination of the mechanisms of beetle horn development reveals a great deal about their capacity for evolution. The conclusions from these studies of development are remarkably similar to the ones we get by mapping horns onto a phylogeny: it may not be hard to gain a horn. The outgrowth portion of the limb-patterning pathway is sufficiently autonomous that initiating this cascade may be all that is needed to get a fully formed horn. This might be possible in a single step or within a single beetle generation, as suggested by the sporadic appearance of mutant individuals that emerge with fully formed horns from species that are otherwise entirely hornless. It certainly could account for the numerous irregular appearances of horned taxa securely nested within clades of otherwise hornless species.
It also does not appear to be difficult to lose horns. To truncate horn growth, scarabs employ numerous mechanisms, any of which could lead