Extrapair copulations reduce inbreeding for female red-backed fairy-wrens, Malurus melanocephalus

In many socially monogamous species, females copulate with and produce offspring sired by males other than their social mates, yet it remains controversial whether or how females benefit from these ‘extrapair’ copulations. Recently, it has been suggested that females might benefit if they are able to copulate with extrapair males that are genetically dissimilar to themselves, thereby potentially increasing the heterozygosity and/or reducing the level of inbreeding of the resulting offspring. However, empirical tests of this hypothesis have been criticized because a low number of molecular markers can lead to biased estimates of relatedness among individuals, and because all studies to date have been correlational and therefore unable to rule out potentially confounding factors. The red-backed fairy-wren is a bird with very limited dispersal, and hence the risks of inbreeding are high. We used a panel of microsatellite markers to examine paternity and relatedness between mates in this species, and also conducted an experiment that manipulated relatedness between a female and her social mate. Results from both approaches showed that females paired to genetically similar males were more likely to produce young sired by extrapair males, and that those offspring were less inbred (more heterozygous) than within-pair offspring. Thus, female fairy-wrens are able to avoid the potential costs of close inbreeding through extrapair copulations.