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.
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This research was supported by grants from NSF to I.J.L. (DEB-0515981) and a postdoctoral fellowship from the Miller Institute for Basic Research in Science at the University of California, Berkeley to D.R.R. We thank E. Lacey, B. Lyon, B. McCleery, D. I. Rubenstein, R. Safran, A. L. Talaba and the Lacey laboratory for advice and assistance. P. Hohohnle helped conduct and interpret the MIPoD analyses. We acknowledge S. Edwards, J. Trimble (Harvard Museum of Comparative Zoology), R. Prum, K. Zyskowski (Yale Peabody Museum), J. Cracraft, P. Sweet (American Museum of Natural History), N. Rice (Academy of Natural Sciences, Philadelphia), M. Braun, D. James (National Museum of Natural History) and R. Prys-Jones (British Museum) for access to their collections.
Author Contributions D.R.R. conceived the project and analysed the data. D.R.R. and I.J.L. jointly collected the data and prepared the manuscript.
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Rubenstein, D., Lovette, I. Reproductive skew and selection on female ornamentation in social species. Nature 462, 786–789 (2009). https://doi.org/10.1038/nature08614
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