Figure 2
Four trajectories of beetle horn evolution. (a) Species differ in the location of horns; side-views of nine species of Onthophagus (Scarabaeinae) shown. (b) Species differ in horn shape. Head horns shown for 10 Onthophagus species; arrows indicate relative frequencies of changes as reconstructed from a phylogeny (from Emlen et al, 2005b). (c) Species differ in horn allometry, the slopes, intercepts, and even the shapes of the scaling relationships between horn length and body size. Data for thoracic horns of seven Onthophagus species shown. Finally, (d) species differ in the presence and nature of dimorphism in horn expression (males=closed circles; females=open circles). Top to bottom: sexual dimorphism (O. pentacanthus); male dimorphism and sexual dimorphism (O. nigriventris); reversed male & sexual dimorphism (O. sloanei); male dimorphism and sexual dimorphism (Enema pan (Dynastinae); unpublished data, JM Rowland).
