Biology of a weakly social bee, Exoneura (Exoneurella) setosa (Hymenoptera: Apidae) and implications for social evolution in Australian allodapine bees

Australian allodapine bees provide excellent material for comparative approaches to understanding social evolution. The subgenus Exoneurella occupies a cladistically basal position in the Australian Exoneura group and comprises only four species. We describe sociality in one Exoneurella species, E. setosa, and combine this with other data to infer some patterns of social evolution in allodapines. E. setosa rears a first brood solitarily, although staggered brood production and the production of a second brood in some nests leads to a situation where older, recently emerged brood have the ability to help rear their younger siblings and this overlaps with opportunities to lay eggs. This is similar to the situation for two other phylogenetically distal species of Exoneurella, as well as for members of the genus Braunsapis, which is used as an outgroup for Exoneura. When combined with other studies, our results suggest that the opportunity for sib-rearing is a plesiomorphic trait for Australian allodapines and this has been largely lost in a distal subgenus, Exoneura sensu stricto. Instead, multifemale brood-rearing colonies in this latter group mostly comprise individuals of the same generation, and species exhibit large group size, univoltinism and kin cofounding. This suggests that evolution can favour semisociality and quasisociality, even when eusociality has already arisen.