Unrelated males in societies of a facultatively social bee

Societies of social Hymenoptera usually consist of groups of closely related females in which the dominant female(s) is specialized for reproduction and subordinate females care for immature offspring. Studying simple societies allows the identification of factors responsible for early stages of evolution of sociality. Here, we examined facultative social nesting in the species <i>Ceratina chalybea</i> using nest dissection, observation of nests, and evaluation of relatedness using microsatellite loci. Social nests were composed of an old female and on average 3.38 young adults and 2.05 newly provisioned brood cells. Social nests had a smaller number of brood cells, and empty cells made up a smaller proportion of the nest in comparison with solitary nests. Moreover, social nesting was strongly associated with nest reuse. Almost all of the young adults present were males, and about half of them were unrelated to the old female. The old female was the only member of the society who performed regular foraging. Feeding of mature offspring by the mother is typical for <i>Ceratina</i> bees. We suppose that social nests emerge when the mother begins provisioning new brood cells in the same nest where young adults are still present. As young adults do not perform physically demanding or risky activities, they incur little or no cost, but they can benefit from the food they obtain from the old female. Our results suggest that relatedness may be unimportant for eusociality maintenance when costs of helping behaviour are small.