M. Andersson C. Kupper I. C. Cuthill J. Kis Richard B. Lanctot Brett K. Sandercock T. Szekely J. Wallander Bart Kempenaers Donald Blomqvist 2002 <p><span>Matings between close relatives often reduce the fitness of offspring, probably because homozygosity leads to the expression of recessive deleterious alleles</span>^{<a id="ref-link-section-d242128358e533" title="van Noordwijk, A. J. &amp; Scharloo, W. Inbreeding in an island population of the great tit. Evolution 35, 674–688 (1981)" href="https://www.nature.com/articles/nature01104#ref-CR1" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 1" data-mce-href="https://www.nature.com/articles/nature01104#ref-CR1">1</a>,<a id="ref-link-section-d242128358e536" title="Thornhill, N. W. (ed.) The Natural History of Inbreeding and Outbreeding (Univ. 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Microsatellite diversity predicts recruitment of sibling great reed warblers. Proc. R. Soc. Lond. B 268, 1287–1291 (2001)" href="https://www.nature.com/articles/nature01104#ref-CR11" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 11" data-mce-href="https://www.nature.com/articles/nature01104#ref-CR11">11</a>}<span>. These studies indicate that natural selection may favour the avoidance of matings with genetically similar individuals. But constraints on social mate choice, such as a lack of alternatives, can lead to pairing with genetically similar mates. In such cases, it has been suggested that females may seek extra-pair copulations with less related males</span>^{<a id="ref-link-section-d242128358e575" title="Bensch, S., Hasselquist, D. &amp; von Schantz, T. Genetic similarity between parents predicts hatching failure: non-incestuous inbreeding in the great reed warbler? Evolution 48, 317–326 (1994)" href="https://www.nature.com/articles/nature01104#ref-CR4" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 4" data-mce-href="https://www.nature.com/articles/nature01104#ref-CR4">4</a>}<span>, but the evidence is weak or lacking</span>^{<a id="ref-link-section-d242128358e579" title="Bensch, S., Hasselquist, D. &amp; von Schantz, T. Genetic similarity between parents predicts hatching failure: non-incestuous inbreeding in the great reed warbler? Evolution 48, 317–326 (1994)" href="https://www.nature.com/articles/nature01104#ref-CR4" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 4" data-mce-href="https://www.nature.com/articles/nature01104#ref-CR4">4</a>,<a id="ref-link-section-d242128358e582" title="Kempenaers, B., Adriaensen, F., van Noordwijk, A. J. &amp; Dhondt, A. A. Genetic similarity, inbreeding and hatching failure in blue tits: are unhatched eggs infertile? Proc. R. Soc. Lond. B 263, 179–185 (1996)" href="https://www.nature.com/articles/nature01104#ref-CR5" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 5" data-mce-href="https://www.nature.com/articles/nature01104#ref-CR5">5</a>}<span>. Here we report a strong positive relationship between the genetic similarity of social pair members and the occurrence of extra-pair paternity and maternity (‘quasi-parasitism’) in three species of shorebirds. We propose that extra-pair parentage may represent adaptive behavioural strategies to avoid the negative effects of pairing with a genetically similar mate.</span></p> application/pdf 10.1038/nature01104 en Nature Publications Genetic similarity between mates and extra-pair parentage in three species of shorebirds article