The emergence of division of labour in forced associations of normally solitary ant queens

We tested the hypothesis that division of labour is an inescapable property of social groups, rather than an evolutionary event separate from sociality. We propose a variance-based emergent property model in which division of labour can spontaneously emerge when (1) individuals in a group vary in their intrinsic sensitivity to stimuli for a given task, and (2) the performance of a task by individuals with higher task sensitivities reduces performance of the task by the other group members. Under these conditions, the individuals with higher sensitivity become the task specialists. To determine if division of labour can occur in the absence of direct selection, we created artificial foundress associations (groups of queens during nest establishment) of the ant Pogonomyrmex barbatus, which has no history of cooperative colony founding. We compared the incidence of task specialization in this species to that in Pogonomyrmex californicus, in which foundress associations commonly occur. Levels of task specialization for nest excavation were high in both species, arguing that division of labour emerges in groups regardless of evolutionary history. Consistent with the variance-based model the role of nest excavation specialist in P. barbatus foundress associations could be predicted by both (1) differences in excavation roles in prior pairs and (2) variation in excavation activity while solitary. Furthermore, the assumption of the excavator role by one foundress dramatically reduced the performance of that task by the other foundress of the pair. We also found a strong negative relationship between excavation activity and foundress survival in P. barbatus, suggesting that division of labour in this context may act as a constraint on social evolution.