Emergence of division of labour in halictine bees: contributions of social interactions and behavioural variance

A central question in the study of insect societies concerns the emergence of division of labour. Among the different models of division of labour, the threshold model postulates that it can emerge from initial interindividual differences in the propensity to perform a task. The requirements of this variance-based model are likely to be present at the origins of sociality and may thus contribute to the emergence of division of labour in early social evolution. However, social interactions among members of the incipient groups are also likely to shape the genesis of division of labour. In this study, we compared task performance and social interactions during nest construction in forced associations of the solitary halictine bee, Lasioglossum (Ctenonomia) NDA-1, and of the communal Lasioglossum (Chilalictus) hemichalceum. We recorded and compared individual behaviours during nest construction for pairs of solitary or communal bees. These data were compared, in turn, to the excavation behaviours of the same bees while alone to determine how the presence of a social group influenced task performance. We additionally measured social interactions of pairs outside the context of nest construction. Pairs of solitary bees displayed higher levels of division of labour than communal pairs. A numerical model implementing behavioural rules derived from experiments suggests that in communal bees, division of labour can be primarily explained by differences in task propensity. In solitary bees, both behavioural asymmetry and social interactions contribute significantly to division of labour. These results are discussed in the framework of social transitions in halictine bees.