Reproductive promiscuity in the splendid fairy-wren: effects of group size and auxiliary reproduction

Extrapair fertilizations complicate our understanding of cooperative breeding in a number of ways. For example, auxiliaries may reduce the costs of seeking extrapair fertilizations for breeding males or females, and auxiliary males may themselves seek copulations with the breeding female in their own group. We employed microsatellite markers to examine patterns of parentage in the cooperatively breeding splendid fairy-wren (Malurus splendens melanotus). Our study population exhibited a relatively high level of extrapair paternity (42% of 386 offspring) with considerable annual variation (range = 24-52%). Across years the proportion of offspring sired by extrapair males was significantly correlated with the average number of auxiliaries per group. Furthermore, the proportion of extrapair young within a brood was related to group composition; groups with multiple auxiliaries were twice as likely as groups with zero or one auxiliary to contain extrapair young. Most offspring were sired by dominant breeding males, but auxiliary males sired approximately 25% of all extrapair young (10% of all offspring), and about half of these were cases in which the auxiliary male sired offspring in his own group. Within-group sirings by auxiliary males were most common after replacement of the breeding female, and they also appeared to be more likely when the auxiliary was not related to the breeding male. Thus, the presence of auxiliary males increased the likelihood that females would produce extrapair young, and although incest avoidance mechanisms usually prevent within-group copulations by auxiliary males, a conflict of interest among group males arises when a new female joins the group.