Army Ants Have Defied Evolution For 100 Million Years

The common scientific belief has been that army ants originated separately on several continents over millions of years. Now it is found there was no evolution. Using fossil data and the tools of a genetics detective, a Cornell University entomologist has discovered that these ants come from the same point of origin, because since the reign of the dinosaurs, about 100 million years ago, army ants in essence have not changed a bit.

"Biologists have wondered why army ants, whose queens can't fly or get caught up by the wind, are yet so similar around the world. Army ants have evolved only once and that was in the mid-Cretaceous period," says Sean Brady, a Cornell postdoctoral researcher in entomology, whose study was conducted while he was doctoral candidate at the University of California-Davis.

Brady's paper, "Evolution of army ant syndrome: the unique origin and long-term evolutionary stasis of a novel complex of behavioral and reproductive adaptation," will be published on the Web by the Proceedings of the National Academy of Science (PNAS) Online Early Edition between May 5 and May 9 before being printed in PNAS.

Army ants are quite unlike the ants commonly found at family picnics. They have what scientists call the "army ant syndrome," comprising three characteristics: the ants are nomadic, they forage for prey without advance scouting, and their wingless queens can produce up to 4 million eggs in a month. While this syndrome is found in every army ant species around the world, scientific papers have postulated that army ants evolved these characteristics multiple times after the breakup of the supercontinent Gondwana about 100 million years ago.

In total, Brady studied the DNA of 30 army ant species and 20 possible ancestors within the army ant community, divided between the New World species in Ecitoninae and the Old World groups Aenictinae and Dorylinae. He specifically sought information from four different genes to uncover clues to their relationships. "Essentially I built a genetic family tree. Then I took that family tree and looked at its genetic tree rings to postulate what happened in the past," he said.

Brady combined the genetic data with the army ant fossil information and the ants' morphological (form and structure) information to establish ages for the different ant species. Combining this data, Brady found that all the species share some of the same genetic mutations. "If they share those mutations, we can infer they evolved from the same source," Brady said.

Instead of proving the common assumption that the Old World and the New World army ants developed their lineage independently on separate continents, the entomologist showed the ants evolved only once -- on Gondwana.

Brady examined the army ants' behavior on his trips to the Amazon jungle, Brazil's savanna region and the country's coastal rain forest near São Paulo. Periodically millions of army ants would march together through his camp, he says, like a flowing river of red. While the ants move silently, their presence is announced. "The other insects are scared, and they make noises as they flee the invading army," Brady says. "Ant birds follow the ants from the sky and feast on the remnants left behind by the ants. You will hear the high-pitched chirping of the other insects, and you'll hear them and other small animals scurrying in fear. They know what is next."