Study: Army Ant's Bloody Rampage Is in its Genes

ByABC News
July 9, 2003, 9:46 AM

July 10 -- It looked like a red river, Sean Brady says as he recalls the spectacular sight of millions of army ants advancing through the dense Amazon jungle, devouring everything in their path.

"The whole forest was percolating with insects trying to get out of there," he says, and even small reptiles, or other animals that couldn't flee, were ripped to pieces by the ferocious ants.

It's a very strange life style, and it sets army ants apart from all other ants. And that's only part of the story.

Brady, an entomologist at Cornell University, has found that army ants have been acting that way for at least 100 million years. The "army ant syndrome," essentially how they live their lives, hasn't changed much in all that time.

Marching Genes Go Way Back

That evolutionary stability is quite remarkable, and it all began when India, Australia, Antarctica, Africa and South America formed a single super-continent called Gondwana. Evolutionary biologists had assumed that army ants developed their peculiar lifestyle in different areas of the globe after the continents separated, but Brady's research shows that's not the case.

"Once the original army ant evolved this syndrome, no army ant has ever lost it," Brady says. Brady began his research as a doctoral candidate at the University of California, Davis, before moving on to Cornell. His findings were published in a recent issue of the Proceedings of the National Academy of Sciences.

He reached his conclusions after studying the DNA from dozens of army ant species, and combined that with the fossil record to reconstruct the evolutionary history of army ants. Specifically, he found that all the species share some of the same genetic mutations, and thus came from the same ancestors who evolved back when the dinosaurs ruled the planet.

They don't all look the same any more. Most have no eyes and thus cannot see, although some now have a single, large eye. Some have evolved other new physical features, and even some have new behavioral patterns. But all of them, whether in Africa, Asia or the Americas, are still ruled by the "army ant syndrome."