— 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."
"It's a combination of three traits," Brady says.
A Marching Syndrome
Most ant species send out scouts when they need food, but not army ants. The entire colony, numbering up to a million, sets off across the landscape, moving at about two to three feet per minute, Brady says, eating everything they catch.
And if you eat everything in sight, you've got to keep moving, so army ants are nomadic, literally eating themselves out of house and home and then moving on. No permanent nests for these guys.
And then there's the queen, blind, wingless, but very good at what she does for the colony. She produces eggs, up to four million in a month. That reproductive cycle forces the colony to hang out at one location for awhile, and they form huge nests by literally hanging on to each other.
And then they move on again, devouring around 50,000 insects in a single day, according to Bill Gotwald, professor of biology at Utica College in New York and author of Army Ants: The Biology of Social Predation. Gotwald says if you imagine yourself being attacked by 50,000 wolves, you'll have some idea of what it's like being an insect caught in the path of army ants.
There are reports of army ants killing animals the size of a horse, but Brady says he's not sure of the reliability of those reports. A calf, maybe, tied to a tree and unable to get out of the way.