'Monogamous' female birds often produce young with another partner. This was presumed to yield offspring fitter than those produced with the paired partner, but a study of song sparrows suggests that 'cheating' comes with no evolutionary reproductive benefit.
Jane Reid at the University of Aberdeen, UK, and her team analysed 17 years' worth of genetic parentage data from a small population of song sparrows (Melospiza melodia; nestlings pictured) on Canada's Mandarte Island. They compared the lifetime reproductive success of half siblings with the same mother and found that young sired outside of monogamy were less reproductively fit than their half-siblings, producing on average 40% fewer offspring and 30% fewer grand-offspring.
The researchers suggest that there may be indirect selection against, not for, cheating in song sparrows.
Am. Nat. http://dx.doi.org/10.1086/665665 (2012)
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Cheating cuts offspring fitness. Nature 485, 151 (2012). https://doi.org/10.1038/485151d
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DOI: https://doi.org/10.1038/485151d
