Convergent evolution, superefficient teams and tempo in Old and New World army ants

Abstract

Swarm raiding army ants, with hundreds of thousands or millions of workers per colony, have evolved convergently in the Old World and New World tropics. Here we demonstrate for the first time, to our knowledge, superefficient foraging teams in Old World army ants and we compare them quantitatively with such teams in New World army ants. Colonies of Dorylus wilverthi in the Old World and Eciton burchelli in the New World retrieve almost identical sizes of prey item and the overall size range of their workers is very similar. However, 98% of D. wilverthi workers are within the size range of the smallest 25% of E. burchelli workers. In E. burchelli larger workers specialize in prey retrieval, whereas in D. wilverthi workers form many more teams than in E. burchelli. Such teams compensate for the relative rarity of larger workers in Dorylus. The proportions of prey items retrieved by teams in Dorylus and Eciton are 39% and 5%, respectively. The percentages of all prey biomass retrieved by teams in Dorylus and Eciton are 64% and 13%, respectively. Working either as single porters or teams, Dorylus carry more per unit ant weight than do Eciton, but Eciton are swifter. However, these different ergonomic factors counterbalance one another, so that performance at the colony level is remarkably, although by no means completely, similar between the Old and New World species. The remaining differences are attributable to adaptations in worker and colony tempo associated with the recovery dynamics of their prey populations. Our comparative analysis provides a unique perspective on worker-level and colony-level adaptations and is a special test of the theory of worker caste distributions.