Genetic similarity between mates predicts extrapair paternity—a meta-analysis of bird studies

Extrapair mating has been recorded in approximately 90% of investigated avian monogamous species. However, factors triggering female mating decisions and potential fitness benefits from extrapair matings still remain poorly understood. Some studies suggest that females mate socially with low-quality males but seek extrapair mates offering superior genes for their progeny. This mating strategy may also help in mitigating the potential negative effects of pairing with a genetically similar mate. Here, we investigate whether genetic similarity within a social pair may predict the occurrence of extrapair paternity (EPP) in birds. Using a meta-analytical approach to a number of studies performed on birds, we found a positive relationship between the occurrence of EPP and the relatedness of social mates. Moreover, we found that the type of molecular markers used to estimate relatedness significantly affected the observed effect size. Specifically, we showed that only microsatellite markers were associated with significantly positive effect sizes. Thus, failure of some of the previous studies to detect the relationship between occurrence of EPP and the relatedness of social mates may at least partly arise due to methodological reasons.