Bates, with their more important and characteristic differences; thus within the same natural section of the genus Onthophagus, there are species which have either a single cephalic horn, or two distinct horns. Emlen et al. (2005) have recently verified Bates’ observations using a phylogeny of 48 species of Onthophagus; Emlen counted at least 25 gains and losses of horns within this clade, with no indication of any directional trend in horn

morphology. There Protein Tyrosine Kinase inhibitor is little question that, when present, these horns have an adaptive function, allowing males to increase their fitness by increasing their number of matings, so the lack of directional change likely results from the gains and losses occurring too rapidly for any directional change to be evident. Horn losses appear to be causally linked to changes to ecological variables such as population density or sex ratios favouring hornless males (Moczek, 2003; Pomfret & Knell, 2008). Studies of introduced populations of horned beetles have shown measurable changes in horn size and frequency after <40 years, apparently linked to densities of the introduced populations selleck inhibitor (Moczek, 2003). Although exaggerated

structures in dinosaurs (e.g. horns, frills, crests and domes) would have evolved more slowly than beetle horns due to longer generation times, it selleck is nonetheless possible that they showed a similar amount of evolutionary lability, particularly over macroevolutionary timescales. If so, especially given the relatively low temporal resolution characteristic of the Mesozoic vertebrate record, we should not be surprised to find a lack of evidence for directional morphological change in exaggerated characters evolving under sexual selection. The second test of the species recognition hypothesis proposed by Padian & Horner is that species with exaggerated traits should occur in sympatry with others bearing similar features at some point during the evolution of these

traits. This contention is founded on the idea that traits used in species recognition should be more divergent when species occur in sympatry. Thus, the songs of closely related sympatric pairs of antbird (Thamnophilidae) differ from each other more than the songs of closely related allopatric pairs (Seddon, 2005). Similarly, island-dwelling species of wildfowl (Anseriformes) that live in sympatry with few congeners are than less brightly coloured than anseriforms sharing the same habitat with more congeners (Figuerola & Green, 2000). This prediction has several problems as applied to Mesozoic dinosaurs. The first is that the proposed correlation does not seem to be universal among extant animals, weakening any inferences based upon the fossil record.