Reproductive skew in an avian cooperative breeder: an empirical test for theoretical models

Reproductive skew models predict how animal groups containing multiple males maintain stable through partitioning reproduction. However, these models have rarely been tested in vertebrates, especially birds where tests have been made for less than 10 of more than 800 species that regularly exhibit cooperative breeding behavior. In the ground tit Parus humilis, a cooperative group consists of primary pair members and secondary individuals that are mainly failed male breeders. A previous study showed that primary males of low quality and thus facing high cuckoldry risks preferred to accept relatives as secondary members and yielded paternity to them. After showing no probability for secondary males to leave the group and live solitarily, the current study revealed that paternity share by a secondary male was positively correlated with his relatedness to the primary male and group productivity but independent of his body mass ratio to the primary male as a measure of relative competitive ability. These results are consistent with restraint transactional models, which predict that when a subordinate can claim unsanctioned reproduction for itself even though the dominant controls membership, both relatedness and group productivity favor the dominant to tolerate increased reproduction share by the subordinate.