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Curr Biol. 2014 Apr 14;24(8):880-4. doi: 10.1016/j.cub.2014.02.059. Epub 2014 Mar 20.

Divorce and infidelity are associated with skewed adult sex ratios in birds.

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Department of Animal and Plant Sciences, Alfred Denny Building, University of Sheffield, Western Bank, Sheffield S10 2TN, UK; Department of Limnology, University of Pannonia, PO Box 158, 8201 Veszprém, Hungary. Electronic address:
Department of Animal and Plant Sciences, Alfred Denny Building, University of Sheffield, Western Bank, Sheffield S10 2TN, UK.
Biodiversity Laboratory, Department of Biology and Biochemistry, University of Bath, Bath BA2 7AY, UK; Department of Animal Behaviour, University of Bielefeld, PO Box 10 01 31, 33501 Bielefeld, Germany; Department of Behavioral Ecology & Sociobiology/Anthropology, Johann-Friedrich-Blumenbach Institute for Zoology and Anthropology, University of Göttingen, Kellnerweg 6, 37077 Göttingen, Germany.


Adult sex ratio (ASR) is a fundamental concept in population demography, and recent theory suggests that ASR plays a central role in social behavior, mating systems, and parental care. Unbalanced ASRs are predicted to influence pair-bond and mating behavior, since the rarer sex in the population has more potential partners to mate with than the more common sex. Here we use phylogenetic comparative analyses to test whether ASR is related to three major aspects of mating behavior: divorce, social polygamy, and pair-bond infidelity. ASR is strongly correlated with long-term pair bonds, since the divorce rate is higher in species with a female-biased sex ratio, indicating that mate change by pair members and/or breaking of pair bonds by unmated individuals is more frequent when females outnumber males. Short-term pair bonds are also associated with unbalanced ASRs: males are more commonly polygamous when females outnumber males, and conversely, females are more polygamous when males outnumber females. Furthermore, infidelity increases with male-biased ASR in socially monogamous birds, suggesting that male coercion and/or female willingness to cheat the partner are facilitated by male-biased ASR. Our results provide the first comprehensive support for the proposition that ASR influences multiple aspects of pair-bond and mating behavior in wild populations.

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