The size–complexity hypothesis is a leading explanation for the evolution of complex life on earth. It predicts that in lineages that have undergone a major transition in organismality, larger numbers of lower-level subunits select for increased division of labour. Current data from multicellular organisms and social insects support a positive correlation between the number of cells and number of cell types and between colony size and the number of castes. However, the implication of these results is unclear, because colony size and number of cells are correlated with other variables which may also influence selection for division of labour, and causality could be in either direction. Here, to resolve this problem, we tested multiple causal hypotheses using data from 794 ant species. We found that larger colony sizes favoured the evolution of increased division of labour, resulting in more worker castes and greater variation in worker size. By contrast, our results did not provide consistent support for alternative hypotheses regarding either queen mating frequency or number of queens per colony explaining variation in division of labour. Overall, our results provide strong support for the size–complexity hypothesis.

大小-复杂性假说是地球上复杂生命进化的主要解释。它预测，在有机体发生重大转变的谱系中，更多的低级亚基选择增加劳动分工。来自多细胞生物和社会性昆虫的当前数据支持细胞数量和细胞类型数量之间以及菌落大小与种姓数量之间的正相关。然而，这些结果的含义尚不清楚，因为菌落大小和细胞数量与其他变量相关，这些变量也可能影响分工的选择，并且因果关系可以是任一方向的。在这里，为了解决这个问题，我们使用来自 794 种蚂蚁的数据测试了多个因果假设。我们发现，较大的菌落规模有利于劳动分工增加的进化，导致更多的工蚁种姓和更大的工蚁体型变化。相比之下，我们的结果并没有为关于蚁后交配频率或每个菌落的蚁后数量解释劳动分工变化的替代假设提供一致的支持。总的来说，我们的结果为大小-复杂性假设提供了强有力的支持。