The role of kinship and parental manipulation in facultative eusociality in a tropical sweat bee (Megalopta genalis)

One of the key features of eusociality is the seemingly altruistic behavior of workers who forego their own reproduction to assist their mother in raising siblings. This behavior may be adaptive if gains in indirect fitness from rearing siblings outweigh the loss of direct fitness. If the presence of workers is sufficiently advantageous to mothers, however, worker fitness may not be the primary driver of eusocial evolution. This distinction is important, because indirect fitness benefits are often cited as prima facie evidence for the importance of kin selection in eusociality, but suitably large indirect fitness gains have rarely been demonstrated in natural populations. Here we compare the inclusive fitness of alternative social strategies in the tropical sweat bee, Megalopta genalis, for which eusocial nesting is optional. We show that inclusive fitness is similar among reproductive females with and without workers, but workers in eusocial nests have significantly lower inclusive fitness than would have been expected if they departed to found their own nests. In support for the role of kin selection in eusocial evolution, mathematical simulations based on M. genalis field data found eusociality cannot evolve with reduced intra-nest relatedness. In addition, the simulated distribution of alternative social strategies matched observed distributions of M. genalis social strategies when simulated as a maternal trait (i.e., manipulation), but not when helping behavior was coded as a worker trait (i.e., altruism). Thus, eusociality in M. genalis is best explained through kin selection, but the mechanism being selected is likely maternal manipulation.