Postcopulatory mechanisms of inbreeding avoidance in the island endemic hihi (Notiomystis cincta)

Avoiding genetic incompatibility resulting from inbreeding is thought to be one of the main drivers of mate choice, promiscuity, and sexual conflict. Inbreeding avoidance has been found across a wide range of taxa and is predicted to be adaptive when the costs of inbreeding outweigh the benefits. This study tests the inbreeding avoidance hypothesis at the precopulatory and postcopulatory stages in a natural population of the promiscuous endemic bird, the hihi. This species has high costs associated with inbreeding as it depresses offspring survival. We generate alternative predictions to explain the observed fertilization patterns based on the existence or absence of precopulatory and/or postcopulatory mechanisms of inbreeding avoidance. Nonrandom mating with respect to relatedness is found mainly at the postcopulatory stage. Interestingly, mating patterns appear opposed. There is a trend for females choosing more closely related social males than random, but postcopulatory patterns are biased toward less related extrapair males. This strategy suggests that at the precopulatory stage females may tolerate inbreeding as the costs of developing inbreeding avoidance may be high, especially in light of forced copulations, if natal dispersal is limited or if they gain inclusive fitness. However, as postcopulatory patterns are biased toward less-related individuals inclusive fitness explanations are unlikely. Postcopulatory patterns may arise if there are mechanisms such as sperm ejection or gametic compatibility such as sperm selection or biased fertility/mortality of offspring by related males. The observed patterns are likely to be an optimal compromise between the divergent selection pressures on each sex.