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Fire Ants Surf Floods on Rafts of Their Own Bodies

Fire ants are depressingly resilient. Colonies of the aggressive, stinging insects can withstand floods, fires, and pesticides. Now, researchers have revealed the mechanisms behind one of the bug's neatest survival tricks: rafts composed of thousands of ants linked together, which repel water and sometimes float for months. The findings could eventually help engineers design self-sealing life boats and new waterproof materials.

Fire ants are native to South America, but they have invaded large sections of the American South. The ants swarm over other insects, worms, and rodents. They can strip small animal carcasses to the bone in a few hours. If flood waters deluge a colony, a common occurrence in the rainforests of Brazil where the insect evolved, individual ants join their bodies to form a raft that carries them safely to new areas.

Engineers at the Georgia Institute of Technology in Atlanta wanted to figure out how these ant rafts work. So they collected colonies of fire ants from nearby roadsides and took them back to the lab for a battery of tests. The researchers dropped spoonfuls of ants into water, counted the number of insects in rafts of a fixed size, and measured the walking speed of free ants that sauntered atop their interlinked colleagues. They also froze ant rafts in liquid nitrogen to capture their structure and recorded movies to visualize how the rafts changed over time (see videos). They even glued a live ant onto the bottom of a glass slide, then lassoed the waist of a second ant with a small loop of elastic. The first ant gripped the dangling second ant with its claws, and the researchers tested the strength of the ants' grip by pulling on the elastic waistband.

Contents may settle. A time-lapse video shows a raft with approximately 3000 ants spreading out on the surface of water. (The video is sped up 10 times.)
Credit: Nathan Mlot

Working with the ants was a challenge, says team member Nathan Mlot, a mechanical engineering Ph.D. student who suffered more than a few ant bites for his troubles. "I never really feel too bad when a colony doesn't make it," he says. Still, he adds, it was a privilege to "see first-hand these amazing structures [the ants] can form with their bodies."

The researchers found that fire ants dumped into water quickly spread out into pancake-like rafts as the insects crawl over each other and grip their fellow ants' legs with claws or jaws, forming interweaving patterns similar to a waterproof fabric. The grip is so strong it would be approximately equivalent to a human being dangling six full-grown elephants from the top of a building.

From the top now. An 8000-ant raft forms as the ants fan out. (The video is sped up 15 times).
Credit: Nathan Mlot

The researchers also found that ants underneath the water support those above them, but they do not drown because of trapped air pockets that surround their bodies and buoy the raft. "Watching the ant clusters trap large air bubbles under the water at depths much greater than the scale of a single ant was incredible," says Mlot. The team, whose results appear today in the Proceedings of the National Academy of Sciences, also used its observations and measurements to construct a model for the rate of raft construction based on simple parameters, such as raft height and average ant speed atop the raft.

The model is important, says Jeffrey Aristoff, an applied mathematician at Princeton University, because it allows researchers to predict what would happen to a structure—say a million-member ant raft--that would be daunting to produce in the lab. The model can also be used to determine how varying packing tightness affects the water repellent properties of the raft, he says.

"This research was an experimental tour de force," says Scott Turner, a biologist at the State University of New York College of Environmental Science and Forestry in Syracuse. The results may guide engineers who want to design self-assembling boats, he says, but such applications could be decades in the future. As for the study's immediate impact? "I think people may be terrified at how easily fire ants can float," he says.