Similar preferences for ornamentation in opposite- and same-sex choice experiments

Selection due to social interactions comprises competition over matings (sexual selection stricto sensu) plus other forms of social competition and cooperation. Sexual selection explains sex differences in ornamentation and in various other phenotypes, but does not easily explain cases where those phenotypes are similar in males and females. Understanding such similarities requires knowing how phenotypes influence nonsexual social interactions as well, which can be very important in gregarious animals, but whose role for phenotypic evolution has been overlooked. For example, 'mate choice' experiments often found preferences for ornamentation, but have not assessed whether those are strictly sexual or are general social preferences. Using choice experiments with a gregarious and mutually ornamented finch, the common waxbill (Estrilda astrild), we show that preferences for ornamentation in the opposite-sex also extend to same-sex interactions. Waxbills discriminated between opposite- and same-sex individuals, but most preferences for colour traits were similar when interacting with either sex. Similar preferences in sexual and nonsexual associations may be widespread in nature, either as social adaptations or as by-product of mate preferences. In either case, such preferences may set the stage for the evolution of mutual ornamentation and of various other similarities between the sexes.