Pathways to elaboration of sexual dimorphism in bird plumage patterns

Abstract

Patterns, such as bars and spots, are common in birds. Some patterns can function in camouflage and/or communication and can benefit both males and females, paving the way for elaboration in sexual dimorphism. Historically, sexual dichromatism was predominantly considered to be a consequence of mating systems. However, the distribution of traits between the sexes is not always indicative of function; genetic correlation may cause traits to evolve in both sexes and traits may serve a social function in males and/or females. In addition, sexual dichromatism in bird plumage patterns can be composed of multiple types of patterns within and/or between the sexes. Therefore, there can be more than one type of dimorphism and some are more elaborate than others. Under classical models of genetic correlation, patterns evolve in both sexes followed by a loss of patterning in one sex. Elaborate types of sexual dimorphism in plumage patterns may be due to selection acting on existing patterns and are perhaps derived. Waterfowl (Anseriformes) and gamebirds (Galliformes) arguably have the most striking plumage patterns. Using 288 species from these orders I reconstructed the evolutionary history of plumage pattern dimorphism. There was little support for genetic correlation but elaborate types of dimorphism are probably derived. Backward and forward evolutionary transitions between different types of dimorphism can occur by loss or elaboration. These results demonstrate that plumage patterns are evolutionary labile and current forms may represent shifting adaptations to a changing environment.