The evolution of sexual dimorphism in relation to mating patterns, cavity nesting, insularity and sympatry in the Anseriformes

1. In addition to genetic drift, both natural and sexual selection may be responsible for interspecific differences in male and female size and coloration. Comparative methods were used to analyse the patterns of dimorphism in the Anseriformes (wildfowl) in relation to mating patterns, nest placement, insularity and number of closely related, sympatric species. 2. The following predictions were tested about the evolution of dimorphism: (a) that evolutionary changes are more common in male than in female coloration; (b) that the intensity of sexual selection is correlated with the evolution of bright coloration and sexual dimorphism; (c) that the nature of nest sites influences the evolution of bright coloration in females via its effect on predation risk, and the extent of size dimorphism via the limits to the size of females using cavities; (d) that insular species show more size dimorphism and less colour dimorphism than mainland species; and (e) that sexual dichromatism is more common in species living in sympatry with a larger number of similar species. 3. Evolutionary changes in plumage brightness were found to be more frequent in males than in females. Changes in mating patterns were significantly correlated with changes in plumage brightness in both males and females and in plumage dichromatism, but not with changes in size dimorphism. Evolutionary transitions from open to hole nesting did not affect plumage dichromatism or size dimorphism. A significant association was detected between insularity and the probability of changes in male brightness, but there was no relationship between insularity and size dimorphism. Sexual dichromatism and bright male (but not female) plumage tend to occur in wildfowl sympatric with a high number of species from the same tribe. 4. In conclusion, mating patterns, insularity and sympatry appear to have evolved in correlation with changes in sexual dichromatism and plumage brightness in the Anseriformes, but not with changes in sexual size dimorphism.