Reproductive skew and selection on female ornamentation in social species

Male animals are typically more elaborately ornamented than females(1). Classic sexual selection theory notes that because sperm are cheaper to produce than eggs(2), and because males generally compete more intensely for reproductive opportunities and invest less in parental care than females(3), males can obtain greater fitness benefits from mating multiply(2,4). Therefore, sexual selection typically results in male-biased sex differences in secondary sexual characters(1,4). This generality has recently been questioned, because in cooperatively breeding vertebrates, the strength of selection on traits used in intra-sexual competition for access to mates (sexual selection(1,4)) or other resources linked to reproduction (social selection(5,6)) is similar in males and females(7,8). 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 breeders(6). 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 intense(7) and ultimately select for female trait elaboration(9). 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.