Reproductive skew in the Australian allodapine bee Exoneura robusta

Uneven sharing of reproduction (reproductive skew) among members of a cooperative animal society is one of the key features associated with advanced sociality and task specialization. Allocation of reproduction varies greatly among taxa, and reproductive skew models are evolutionary models that aim at explaining this variation. We tested reproductive skew theory using the Australian social allodapine bee Exoneura robusta. Reproductive skew was negatively correlated with relatedness, as predicted by the so-called tug-of-war model and the restraint model. Both models assume that a dominant breeder in a society does not have full control over the allocation of reproduction among group members, in contrast to classical concession models. Overall, the tug-of-war model seems to account better for the evolution of reproductive sharing in E. robusta than the restraint model. However, neither model can be ruled out, and a tug-of-war as well as a restraint model mechanism could apply in parallel in newly founded and overwintered colonies, respectively. This raises the possibility that skew models are not mutually exclusive and points to a more dynamic view on reproductive partitioning among breeders of an animal society.