Behaviour 2019
Colony expansions underlie the evolution of army ant mass raiding
Vikram Chandra1, 2, Asaf Gal1, Daniel Kronauer1. 1The Rockefeller University, New York, NY, United States; 2Harvard University, Cambridge, MA, United States

The mass raids of army ants are an iconic collective phenomenon, in which many thousands of ants spontaneously leave their nest to hunt for prey. How such complex coordination evolved is not understood. Here, we show that army ant mass raiding has evolved from a different form of cooperative hunting called group raiding, in which a scout directs a small group of ants to a specific target through chemical communication. We describe the structure of group raids in the clonal raider ant, a close relative of army ants in the subfamily Dorylinae. We find that the coarse structure of group raids and mass raids is highly conserved, and that all doryline ants likely follow similar behavioral rules for raiding. We also find that the evolution of army ant mass raiding occurred concurrently with expansions in colony size. By experimentally increasing colony size in the clonal raider ant, we show that mass raiding gradually emerges from group raiding without altering individual behavioral rules. This suggests that ancient expansions in colony size, rather than changes in individual behavioral rules for foraging, led to the evolution of mass raids in the first army ants.