Casteless sociality in an allodapine bee and evolutionary losses of social hierarchies

Communal behaviour is a form of social behaviour where two or more females nest together and have no reproductive hierarchies. Communal behaviour has often been regarded as an evolutionary 'stepping stone' to more complex forms of sociality involving castes, as well as a social form derived from solitary behaviour with no further evolution towards eusociality. However, recent phylogenetic studies on halictine bees suggest that some instances of communal behaviour are derived from eusociality. Here, we describe social nesting in an allodapine bee, Braunsapis puangensis, which has been introduced to Fiji from southern Asia. We show that this bee has a casteless form of sociality similar to communal organization, but which has been derived from an ancestrally hierarchical social system. This is likely due to a combination of small benefits for social nesting that rapidly saturate as colonies become larger, along with low costs for dispersal. We suggest that casteless forms of sociality have frequently evolved from hierarchical societies across many insect groups, but the analyses required for recognizing such societies are often undeveloped and hampers comparative approaches. Transitions from hierarchical to casteless societies challenge the notion that eusociality is an evolutionary 'end point' and we argue that eusociality can, in some cases, be regarded as an evolutionary step towards egalitarian societies. We also suggest that evolutionary periods involving reproductive hierarchies could select for traits that allow individuals to better assess their social environment and subsequently enable lower reproductive skew.