Evidence for multiple functions in a sexually selected ornament

In gregarious animals, social interactions frequently take the form of dominance hierarchies that maintain stable relationships between individuals, and settle disputes without extra costs. Traits that function as signals of status can play an important role in mediating interactions among individuals, both in social and in sexual contexts. Carotenoid ornaments are more generally assumed to be sexually selected and not so relevant to general social contexts. However, it is possible for them to function in social contexts if they signal socially relevant aspects. Here we experimentally analysed social dominance and resource control in male groups of a gregarious species, the European serin, Serinus serinus, in relation to a sexual ornament. We tested whether yellow carotenoid-based plumage coloration, age, body size and testosterone were predictors of social dominance over a nonsexual resource (i.e. feeding context). We showed that dominance hierarchies were steep and were related to testosterone levels and ornamental coloration, particularly the male yellow carotenoid-based crown patch. Our results suggest that carotenoid-based colour and testosterone levels can be reliable predictors of social status in agonistic encounters in groups of male serins. Moreover, together with previous work on the sexual function of male coloration, this study provides evidence that male serin yellow coloration has a dual function in both sexual and nonsexual contexts. These results raise the possibility that this ornament may have evolved and be maintained via social selection over social competition/cooperation for reproductive opportunities and ecological resources.