Social context and the lack of sexual dimorphism in song in an avian cooperative breeder

Elaborate male traits are typically used to attract mates and to compete with other males for mating opportunities. However, similarly ornate secondary sexual characteristics are also found in females in many species and may be used in competition in both sexual and nonsexual contexts. Trait elaboration in females may be particularly important in cooperatively breeding species where reproduction is monopolized by a few individuals in a group and where both sexes must compete for these limited mating opportunities. Previous work in African starlings has shown that females in cooperatively breeding species are larger and more ornamented than those in noncooperative species, resulting in reduced plumage and size dimorphism. To further examine patterns of sexual dimorphism in signalling traits and to better understand their role in mediating social competition, we investigated the form and function of song in cooperatively breeding superb starlings, Lamprotornis superbus. In addition to comparing song between sexes, we contrasted song in dominant breeders and subordinate helpers of both sexes and examined its use in various social contexts. We found that song was indistinguishable between males and females, with both sexes showing similar complexity, pitch, versatility and structure. However, the song of breeders was more versatile than that of helpers. Moreover, song structure differed when birds sang in chorus from when they sang alone. These findings suggest that, like male song, female song may be used in social competition, and that song may be an important signal in both sexes in species where intrasexual competition is high in males and females.