Genetic similarity of social mates predicts the level of extrapair paternity in splendid fairy-wrens

Why females mate with multiple males, particularly in socially monogamous species, is one of the central unanswered questions in sexual selection and behavioural ecology. Recent theory suggests that socially monogamous females may improve the genetic quality of their offspring by mating with extrapair males with whom they are more genetically dissimilar (relative to their social mates), because decreased genetic similarity between mates may lead to decreased inbreeding depression of offspring and other benefits. We found that levels of extrapair paternity in splendid fairy-wren, Malurus splendens, broods were predicted by genetic similarity between social mates. Females whose entire broods were sired by extrapair males were more genetically similar to their social mates than they were to their extrapair mates, but females were no less similar to their extrapair mates than they were to the average male from the population. Within-pair young (WPY) from mixed-paternity broods showed higher levels of inbreeding and lower heterozygosity than did their extrapair nestmates or WPY from broods without extrapair paternity. Several processes could account for these patterns, including female choice of genetically dissimilar mates, differential sperm investment by males or differential survival of embryos as a function of genetic dissimilarity between mates, or postcopulatory processes such as sperm competition, cryptic female choice, or interactions between sperm and ova. Regardless of the mechanism, our results suggest that female splendid fairy-wrens may indirectly benefit from extrapair mating because it leads to lower levels of inbreeding and increased heterozygosity in their offspring.