Evolution of the army ant syndrome: the origin and long-term evolutionary stasis of a complex of behavioral and reproductive adaptations.

The army ant syndrome of behavioral and reproductive traits (obligate collective foraging, nomadism, and highly specialized queens) has allowed these organisms to become the premiere social hunters of the tropics, yet we know little about how or why these strategies evolved. The currently accepted view holds that army ants evolved multiple times on separate continents. I generated data from three nuclear genes, a mitochondrial gene, and morphology to test this hypothesis. Results strongly indicate that the suite of behavioral and reproductive adaptations found in army ants throughout the world is inherited from a unique common ancestor, and did not evolve convergently in the New World and Old World as previously thought. New Bayesian methodology for dating the antiquity of lineages by using a combination of fossil and molecular information places the origin of army ants in the mid-Cretaceous, consistent with a Gondwanan origin. Because no known army ant species lacks any component of the army ant syndrome, this group represents an extraordinary case of long-term evolutionary stasis in these adaptations.