Group size and labour demands determine division of labour as a consequence of demographic stochasticity.

Division of labour (DoL) is most prominently observed in eusocial insects but also occurs in much smaller cooperative groups where all individuals could potentially perform any task. In such groups, previous experience and learning are the most important mechanisms underlying specialization. Using behavioural simulations, we investigate the dynamics of task specialization in groups of various sizes and with different constraints on the choice of task. We assume that individuals choose tasks by weighing their own competence to perform a task against the group requirement of how much that task needs to be performed. We find that task specialization occurs even if individuals choose tasks based solely on the group's needs rather than their own competence. As large groups are less affected by demographic stochasticity, they can more accurately distribute labour across tasks, and individuals become more effective due to a reduced need to switch between tasks. This effect is enhanced if groups must perform a larger number of tasks. However, from an evolutionary point of view, individuals in larger groups develop a greater responsiveness to group requirements than those in small groups when labour variation carries a fitness penalty and thus will more readily switch between tasks. Small groups thus seem less able to distribute labour optimally over tasks through increased switching, and therefore evolve to ignore task imbalances up to a higher level before the threshold to switch between tasks is crossed. Further, we find that selection on learning ability is stronger in small than in large groups. We conclude that the reason why DoL may emerge more readily in large groups might not be due to a group-size effect on optimal decision-making, but rather because of a lower degree of variation of the labour distribution as a consequence of demographic stochasticity.This article is part of the theme issue 'Division of labour as key driver of social evolution'.