Reproductive skew is highly variable and correlated with genetic relatedness in a social apoid wasp

Our knowledge of primitively eusocial societies is focused particularly on cooperatively breeding vertebrates and vespid wasps, whereas numerous taxa representing independent origins of social behavior have been largely overlooked. The lineage of apoid wasps including the genus Microstigmus represents a relatively neglected independent origin of eusociality. We present the first use of modern hypervariable genetic markers, in combination with behavioral observations, to investigate reproductive division of labor and cooperative brood care in an apoid wasp, the Brazilian M. nigrophthalmus. Microstigmus nigrophthalmus is unusual because, although there is cooperative brood care, reproductively dominant females carry out at least as much risky foraging as their subordinate nest mates. Empirical studies of reproductive skew are often hampered by a lack of variation in skew. We find that reproductive skew is highly variable between nests in M. nigrophthalmus. There was no correlation between skew and either body size or group size. The absence of an effect of body size is typical of studies of skew in insects and may indicate that body size is a poor measure of an individual's ability to control a group. However, skew was positively correlated with genetic relatedness. This provides rare support for concession models based on "social contracts" between dominant individuals and their subordinates.