Facultatively social insects are an optimal model group for the study of the emergence of cooperation between individuals. Factors influencing the fitness benefits of social nesting are still debated. Nonreproductive subordinates can benefit from indirect fitness benefits due to increasing reproductive success of related individuals or direct fitness benefits due to direct future reproduction. Here, we studied the costs and benefits of social nesting in the small carpenter bee Ceratina albosticta. From demographic data and within-nest relatedness, we obtained key parameters for assessing the fitness of solitary females, social primaries, and social secondaries. C. albosticta were found to usually mate with one male and multiple mating is not common although exists at low frequencies. Social nests usually contain two females, which were found to be related (full sisters), but also some females were unrelated to each other. Patterns of parentage from microsatellite loci revealed that only one female reproduces in social nests. Our results show that relatedness, per capita brood productivity, and offspring survival strongly increase the fitness benefits of social nesting strategies. Social secondaries, when related to the social primaries, have higher inclusive fitness than solitary females, but unrelated social nesting females had no indirect fitness and much reduced inclusive fitness compared to solitary females. Interestingly, average fitness benefits of the social secondary were higher than solitary females. This study provides important empirical data on the costs and benefits of sociality in a facultatively social bee and sets the stage for future comparative studies.
