Data from: Female ground tits prefer relatives as extra-pair partners: driven by kin-selection?

Socially monogamous female birds routinely mate with males outside the
pair bond. Three alternative hypotheses consider genetic benefits as the
major driver behind the female strategy. The inbreeding avoidance
hypothesis predicts that females paired with closely related males should
seek copulations with distantly related extra-pair partners to avoid
fitness loss from inbreeding depression; the outbreeding avoidance
hypothesis predicts the opposite; the kin-selection hypothesis suggests
that regardless of social mate relatedness, females should give related
males extra-pair fertilization opportunities to gain inclusive fitness if
the costs from inbreeding are minor. We test these hypotheses with a
facultative cooperative breeder, the ground tit (Parus humilis). Social
pairs of ground tits formed randomly with respect to genetic relatedness.
In both bi-parental and cooperative groups, a female’s engaging in
extra-pair mating was independent of relatedness to her social mate;
however, females preferred extra-pair sires to which they were more
related than to their social mates. Moreover, females had higher
relatedness with either their extra-group extra-pair sires in both
bi-parental and cooperative groups, or within-group helper sires in
cooperative groups, than expected by chance. When more than one potential
extra-pair partner was available around a female’s nest, she tended to
select a relative. There was no indication of fitness reduction from
extra-pair mating, which occurred at an intermediate level of inbreeding.
These data support the kin-selection hypothesis, although there might be
alternative non-genetic reasons associated with the extra-pair mating
preference. Our finding offers a new explanation for why female birds
pursue extra-pair mating. It also may broaden our understanding of the
role of kin-selection in the evolution of cooperative society.