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

Abstract

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.

Fig. 2.

Bayesian divergence dating analysis. NW, New World; OW, Old World. Divergence dates were estimated on the ML phylogeny derived from COI, 18S rDNA, 28S rDNA, and wingless genes (–ln L = 26603.88301). Clades marked with asterisks had a posterior probability of >95% after independent Bayesian phylogenetic analysis. Lowercase letters at nodes indicate minimum age constraints obtained from the fossil record: a–c, 20 Mya (34, 35); d, 25 Mya (36, 37); e–f, 42 Mya (38); g, 50 Mya (39); h, 65 Mya (40); i, 92 Mya (32). Army ant taxa are shown in thick type. Branch lengths are drawn scaled to estimated mean values of absolute time. The origin of army ants is estimated at 105 Mya (±11 SD).

Fig. 1.

Phylogenetic relationships among army ants and cerapachyine relatives. Al., Alaopone; An., Anomma; Ch., Cheliomyrmex; D., Dorylus; Di., Dichthadia; N., Neivamyrmex; No., Nomamyrmex; and Rh., Rhogmus. The phylogeny is the strict consensus of the two most parsimonious trees (length = 5,592, consistency index = 0.313, retention index = 0.536) resulting from equal weights analysis of 3,654 characters from COI, 18S rDNA, 28S rDNA, wingless, and morphological data. Numbers above branches indicate clades with >50% bootstrap support. Army ant taxa are shown in thick type.