(2011) noted that use of the neck in foraging did not exclude a possible role in sexual selection. An improved understanding of the biology of display structures and sociosexual behaviour in extant animals, coupled with the realization that fossil animals must have been subject to the same selection

pressures as extant ones, means that many workers favour sociosexual selection as the primary mechanism driving the evolution of these structures in non-avialan dinosaurs (e.g. Farlow & Dodson, 1975; Hopson, 1975; Hone et al., 2012; Knell et al., 2013). These features could be used intraspecifically in advertising fitness (e.g. Spassov, 1979; Hayashi, Carpenter & Suzuki, 2009; Tomkins et al., 2010), as advertisers of social status, and in intraspecific control of resources IWR-1 solubility dmso (Hieronymus www.selleckchem.com/products/DAPT-GSI-IX.html et al., 2009). A lack of convincing dimorphism across many such exaggerated structures has led some authors to reject sexual selection as an explanation for their evolution (Padian & Horner, 2011a), despite concern about the small sample sizes involved. Even if sexual dimorphism is demonstrably absent, a rejection of sexual selection ignores the possible presence of mutual sexual selection, the phenomenon – well studied and well established in extant animals

– in which both genders are ornamented (see Hone et al., 2012 and references therein). The assumption that an absence of sexual dimorphism is incongruous with sexual selection also ignores the possibility that exaggerated structures could function as other kinds of social dominance signals relevant to both genders. Instead, it has repeatedly been suggested that exaggerated structures in non-avialan dinosaurs may have functioned as species recognition devices (e.g. Main et al., 2005; Hieronymus et al., 2009; Padian & Horner, 2011a; Allain et al., 2012; Schott & Evans, 2012; Taylor & Wedel, 2012). This is despite the fact that similar structures in extant vertebrates have roles in sexual selection (Knell et al., 2013) and the lack of evidence for species recognition. The term ‘species recognition’

has been applied to different concepts by different authors and there is little consistency in its use (Mendelson & Shaw, 2012). Padian & Horner (2011a) noted that ‘functions of species recognition encompass interactions both between Mannose-binding protein-associated serine protease species (discourage association of non-conspecifics) and within species (“encourage association of conspecifics”)’. However, while these two aspects of behaviour are linked (the second is generally termed ‘social selection’; West-Eberhard, 1983), the former is closer to interspecific signalling and need not have any effect on conspecifics. Moreover, ‘species recognition’ may refer to the behaviour whereby individuals identify and keep track of conspecifics for herd coherence, or identify a suitable sexual partner. These two behaviours need not be mutually exclusive; however, they may be associated with different selective pressures.