It isn't always sexy when both are bright and shiny: considering alternatives to sexual selection in elaborate monomorphic species

Abstract

Since the dawn of abstract thinking, humans have wondered about the seemingly unnecessary elaborate ornamentations of birds. Gaudy colours, cumbersome tails, complex vocalizations and bizarre displays are found in bird species from all corners of the globe. Darwin (1871) provided an elegant explanation for the existence of these non-utilitarian traits: they increase mating success, and although they may impair survival, the costs of producing and bearing elaborate ornaments can be repaid in the currency of additional offspring. Darwin’s model still serves as the foundation for our concept of sexual selection but great strides have been made in our understanding of sexual selection processes since his time (e.g. Zahavi 1975, 1977, Lande 1980, Hamilton & Zuk 1982, Kirkpatrick 1982, Grafen 1990, Andersson 1994). The great majority of work to date has focused on species in which males alone are elaborately ornamented. Far less has been published on the function of ornaments that are expressed in both sexes (Kraaijeveld et al. 2007), a condition sometimes termed ‘elaborate monomorphism.’ As such, the question remains whether the strong generalizations that we make regarding male ornamentation also apply to species in which both sexes are ornamented. The relationship between female and male ornamentation can take several forms. In some species (for example, some fringillids and parulid warblers), females express relatively subdued vestiges of male ornaments. The presence of male-like characters in females of such species is consistent with the genetic correlation hypothesis (Lande 1980), which posits that females express non-adaptive traits as a by-product of sexual selection on male ornaments. Females bear the genes that give rise to the expression of ornaments in males, but because the costs to females of ornament expression are typically not repaid with increased fecundity, natural selection favours reduced expression of costly ornaments (Kokko & Johnstone 2002, LeBas et al. 2003, Chenoweth et al. 2006). In support of this hypothesis, some studies have detected sexual selection only on male traits but have failed to find a relationship between female ornamentation and reproductive success, performance, or a measure of female condition or quality (e.g. Muma & Weatherhead 1989, Cuervo et al. 1996, Wolf et al. 2004). However, in some species (for example, many parrots, motmots and relatives, and tropical songbirds), elaborately ornamented females and males are indistinguishable, or nearly so. The female ‘version’ of ornamentation in these species seems more elaborate than would be expected if these traits were only expressed as non-adaptive genetic by-products of sexual selection on males, or if they were shaped solely by natural selection. Indeed, compelling evidence from many species, including some that are sexually dimorphic, suggests that elaborate female traits may also function as signals, although female ornamentation may or may not stem from the same selective processes that lead to elaborate male ornamentation (Amundsen 2000, Lebas 2006, CluttonBrock 2007, 2009, Kraaijeveld et al. 2007, Dey et al. 2012). Some studies have supported the role of mutual sexual selection in maintaining ornamentation in both sexes by showing that males prefer more ornamented females (e.g. Amundsen et al. 1997, Griggio et al. 2005) or that more ornamented females are in better condition (Velando et al. 2001, Jawor et al. 2004, Siefferman & Hill 2005, Dakin 2011). Similarly, some studies have found that female–female competition for sexual or non-sexual resources can lead to the evolution of elaboration in females, wherein female ornaments convey information about fighting ability or dominance (e.g. Murphy et al. 2009a,b, Midamegbe et al. 2011, Cain & Ketterson 2012). In contrast, other studies have failed to find evidence of signal value associated with female ornamentation in elaborate monomorphic species (Murphy & Pham 2012). As we learn more about the potential costs and benefits of mate choice and competitive interactions, a richer understanding of how selection can favour sex-based signalling strategies in females is emerging. Such a perspective was rarely considered even 15 years ago (Amundsen 2000). Among many elaborate monomorphic species, members of a mated pair tend to be similarly ornamented, a phenomenon called assortative pairing (e.g. Moller 1993, Andersson et al. 1998, Regosin & Pruett-Jones 2001, Daunt et al. 2003, Masello & Quillfeldt 2003, Massaro et al. 2003, Boland et al. 2004, Kraaijeveld *Corresponding author. Email: keith.tarvin@oberlin.edu