Within the context of this specific argument, note that multiple signals are known to correlate with sexual selection (Omland, 1996). In any case, it is not clear that species recognition requires the existence of exaggerated structures at all. Among extant taxa, sympatric members of species complexes, including tyrant flycatchers (Birdsley,

2002), bushbabies (Zimmerman, 1990), anoles (Jenssen & Gladson, 1984), frogs (Heyer, García-Lopez & Cardoso, 1996) and numerous insects (Wells & Henry, 1998) GPCR Compound Library purchase have no apparent trouble in recognizing conspecifics or potential mates, and there is no reason to think sympatric dinosaur groups would have been different (as suggested by, for example, those sympatric iguanodontians that lack the crests present in hadrosaurs). In short, in addition to the issues of mutual sexual selection and social dominance characteristics, it seems plausible that there may be several sympatric, closely related species with exaggerated structures where those structures are too similar to be easily separated on osteological morphology alone. The benefits of such structures would thus be profoundly limited while

the costs would be potentially high. Several additional questions present problems for the species recognition hypothesis in non-avialan dinosaurs. As noted by Knell & Sampson (2011), there has yet to be any documented case in any extant species where a crest or similar Poziotinib cell line structure functions primarily in species recognition. 上海皓元医药股份有限公司 Padian & Horner (2011b) countered that species recognition has been little studied, and indeed Mendelson and Shaw (2012) noted that, to date, study has been both limited (in terms of documenting increased selection for correct identification of mates) and

problematic. It is true that studies of species recognition in extant taxa are uncommon; however, some ethological studies have specifically tested the species recognition hypothesis with respect to the presence of exaggerated structures and found it wanting (e.g. Harrison & Poe, 2012). That no extant species, including the thousands of extant dinosaurs, has yet been demonstrated to use exaggerated morphological structures for the purposes of ‘species recognition’ argues against the idea that we should assume such a role among Mesozoic taxa. Similarly, if such structures were so important for non-avialan dinosaurs, their absence, reduction or loss in various lineages is incongruous. Critically, it is not clear how such a structure would evolve to separate putative species through mate identification. Given that extant taxa do not appear to be using these structures for species recognition, a plausible mechanism is required to explain their origin, retention and propagation, and to our knowledge none has been proposed. If speciation occurred allopatrically, an exaggerated structure would be unnecessary (see Alatalo et al., 1994) because the populations would not be at risk of interbreeding.