Reproductive skew and selection on female ornamentation in social species.

Many birds are distinguished by colourful males and drab females, referred to by bird-watchers as LBJs ('little brown jobs'). This is true for species in which males court many females and the females are choosy about mates — necessarily so, because they make most of the investment in raising young. But what about species that are co-operative breeders, where males and females share the load? In such species the sexes tend to look more alike. The conventional wisdom is that the males become less ornamented, but this is not necessarily the case, as shown by a study of ornamentation levels in male and female African starlings. In this group-living species, females compete with one another for the attention of males, and become more ornamented, matching the males. In species that do not live with relatives, males are more ornamented than females. Male animals are typically more elaborately ornamented than females, probably because females make more of an energetic investment in raising young. However, this generality may not apply in cooperatively breeding vertebrates, where the energetic load is similar in males and females. The socially diverse African starlings are now used to study this issue, revealing that where intrasexual competition among females may be intense, female trait elaboration is selected for. Male animals are typically more elaborately ornamented than females1. Classic sexual selection theory notes that because sperm are cheaper to produce than eggs2, and because males generally compete more intensely for reproductive opportunities and invest less in parental care than females3, males can obtain greater fitness benefits from mating multiply2,4. Therefore, sexual selection typically results in male-biased sex differences in secondary sexual characters1,4. This generality has recently been questioned, because in cooperatively breeding vertebrates, the strength of selection on traits used in intrasexual competition for access to mates (sexual selection1,4) or other resources linked to reproduction (social selection5,6) is similar in males and females7,8. Because selection is acting with comparable intensity in both sexes in cooperatively breeding species, the degree of sexual dimorphism in traits used in intrasexual competition should be reduced in cooperative breeders6. Here we use the socially diverse African starlings (Sturnidae) to demonstrate that the degree of sexual dimorphism in plumage and body size is reduced in cooperatively breeding species as a result of increased selection on females for traits that increase access to reproductive opportunities, other resources, or higher social status. In cooperative breeders such as these, where there is unequal sharing of reproduction (reproductive skew) among females, and where female dominance rank influences access to mates and other resources, intrasexual competition among females may be intense7 and ultimately select for female trait elaboration9. Selection is thereby acting with different intensities on males and females in cooperatively versus non-cooperatively breeding species, and female–female interactions in group-living vertebrates will have important consequences for the evolution of female morphological, physiological and behavioural traits.