Combat between large derived societies: A subterranean army ant established as a predator of mature leaf-cutting ant colonies

Abstract

Mature colonies of Atta leaf-cutting ants are dominant herbivores throughout the Neotropics. Although young colonies have natural enemies, mature colonies, which live in extensive nests containing millions of workers, currently have no recognised predators. New World army ants (Ecitoninae) are specialist social predators of other ants, and the army ant Nomamyrmex esenbeckii, a primarily subterranean species, is known to prey upon young Atta colonies. Here we present the results of the first long-term study of the predator-prey interaction between N. esenbeckii and Atta. Our study establishes the army ant N. esenbeckii as the only known predator capable of successfully attacking and killing mature as well as young colonies of Atta leaf-cutting ants. In natural raids, and experimental tests, Atta rapidly recruited their largest workers (majors) as a specific defensive response to N. esenbeckii raiders and both taxa used their largest individuals in the frontline of battles. The deployment and behaviour of these large workers demonstrates a size-related division of labour and agrees with the predictions of Lanchester’s Linear Law of Combat. Both taxa also used cooperative combat ‘teams’ to overwhelm large combatants from the other side. The success of N. esenbeckii raids varied greatly, such that they were prevented from entering Atta nests in the least successful raids, and completely overran Atta colonies in the most successful raids. The speed and magnitude of the defensive response of mature Atta colonies was key in determining the level of success of N. esenbeckii raids.