Genetic similarity between pair mates is not related to extrapair paternity in the socially monogamous coal tit

The benefits of extrapair mating behaviour for females of socially monogamous bird species are unclear despite substantial research effort. The genetic compatibility hypothesis proposes genetic benefits such that females paired to social mates of low genetic compatibility avoid or diminish negative fitness consequences by mating with a more compatible extrapair mate, resulting in offspring of higher genetic quality. Furthermore, within the context of inbreeding depression observed in natural populations, a high degree of overall genetic similarity between social pair mates may be regarded as a special case of genetic incompatibility. We tested the hypothesis that female extrapair matings represent an adaptive behavioural response to avoid negative consequences of being paired to a genetically similar social pair male in the coal tit, Parus ater, a socially monogamous passerine with high rates of extrapair paternity. In contrast to what was predicted, we found no evidence for a positive association between the genetic similarity of social pair mates (measured as band-sharing coefficients from multilocus DNA fingerprints) and the occurrence of extrapair paternity. Furthermore, the genetic similarity of the cuckolding female with its social mate was not higher when compared pairwise to that of its extrapair mate in 63 uniquely composed triplets. Finally, three parameters of reproductive success were not related to the genetic similarity of social pair mates. We conclude that avoiding the potentially negative fitness consequences of being paired to a genetically similar social pair mate did not select for and thus cannot maintain female extrapair mating behaviour in our study population.