It has been hypothesized that females mate multiply to increase the heterozygosity of their progeny because heterozygous individuals are assumed to have a fitness advantage. Females can maximize heterozygosity of their offspring by mating with genetically unrelated and/or heterozygous males. We tested these predictions in a socially monogamous passerine, the reed bunting (Emberiza schoeniclus), where extrapair paternity occurs frequently. The results based on genotypes at nine microsatellite loci revealed that females were no less genetically related to the extrapair male(s) (EPMs) than to their pair male (i.e., breeding partner) and that EPMs were no more heterozygous than the males they cuckolded. In addition, a direct comparison of maternal half-siblings naturally raised in the same brood showed that extrapair young were no more heterozygous than within-pair young. Thus, female reed buntings do not seem to mate with EPMs to increase offspring heterozygosity. It is not yet known whether extrapair mating involves any benefits at all to females in this species.
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