REPRODUCTIVE ROLES IN THE COOPERATIVELY BREEDING ACORN WOODPECKER - INCEST AVOIDANCE VERSUS REPRODUCTIVE COMPETITION

Incest is rare in the cooperatively breeding acorn woodpecker (Melaner pes formicivorus) despite a polygynandrous mating system in which near ly all group members are close relatives. Here we test the relative im portance of avoiding matings between close relatives (incest avoidance ) and within-sex competition for breeding opportunities (reproductive competition) in determining the mating system of acorn woodpeckers by examining how reproductive roles change following breeding vacancies. In 83% of cases in which helpers of the same sex were present in the g roup, reproductive vacancies were resolved when new unrelated immigran ts filled the vacancy to the exclusion of resident same-sex helpers, w ho generally emigrated or did not breed while they remained in the gro up. Helpers of the opposite sex, especially when male, were significan tly more likely to remain in their natal group and in about half the c ases inherited and bred following reproductive vacancies. This result was not explainable by reproductive competition, since the number of i mmigrants was often less than or equal to the number of same-sex helpe rs in the group. Apparent incest resulted in 5% of cases. The time req uired to resolve reproductive vacancies was significantly longer for g roups with helpers of the same sex as the vacancy. These results confi rm that both incest avoidance and reproductive competition are importa nt factors determining reproductive roles within groups of this highly social species.