Larger colony sizes favour greater division of labour between queens and workers in ants.

Explaining variation in the extent of division of labour remains a major problem for our understanding of how complex life evolved. Ants show remarkable variation in their extent of reproductive division of labour, from workers who can reproduce sexually and are approximately the same size as queens, to workers that are completely sterile and 300x smaller than their queens. Examining data from 546 species of ant, we found that: (i) the ancestral ant worker likely had full reproductive potential, though was effectively sterile in the presence of a queen; (ii) the loss of worker reproductive potential generally followed a sequential step-by-step process, via reduced capacity for sexual reproduction, then the production of males only, and finally complete sterility; (iii) the independent evolution of complete sterility has occurred approximately 17 times, with only 42% of ant species having sterile workers; (iv) reproductive size dimorphism has increased to higher levels around 9 times. Exploring potential causality, we found support for the size-complexity hypothesis, that increased colony size has favoured increased division of labour between queens and workers, examining both queen-worker size dimorphism and the loss of reproductive capacity in workers.