Extrapair fertilization and genetic similarity of social mates in the Mexican jay

Inbreeding depression should favor the ability of females to avoid inbreeding or minimize its effects. We tested for a relationship between genetic similarity of social pairs and the occurrence of extrapair fertilization (EPF) in the Mexican jay (Aphelocoma ultramarina). Multilocus minisatellite and microsatellite DNA fingerprinting was used to detect extrapair young and measure genetic similarity between social parents. We found that 12 of 31 (39%) nests had at least one EPF and 15 of 93 (16%) young were the result of EPF. The mean DNA fingerprinting band sharing score between social mates who had at least one EPF was significantly higher than the mean band sharing score between mates who did not (0.35 versus 0.25). The mean band sharing score for non-EPF dyads (0.25) was similar to the background band sharing among nonrelatives (0.23). The mean band sharing score for mates that had an EPF was significantly higher than that of nonrelatives (background) and was significantly lower than that of half-siblings (0.52). Our results showed a highly significant relationship between genetic similarity of social mates and incidence of EPF.