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Ferry Tale: Fire Ants Aggregate into Living Rafts to Escape Floods

Colonies of fire ants numbering in the thousands can morph into sturdy, waterproof rafts that can remain afloat for months. A new study finds out how they do it



Nathan Mlot

If you're ever pursued by a colony of fire ants, it might occur to you to try to escape their painful, itchy stings by diving into the nearest body of water. Wrong move. It turns out that thousands of fire ants can easily self-assemble into a waterproof raft, which can float on water for months.

A new study by researchers at the Georgia Institute of Technology, published April 25 in Proceedings of the National Academy of Sciences, provides a mechanical analysis of the fire ant's considerable raft-building talents.

When their habitat floods, be that their native Brazilian rainforests or the roadsides of Georgia—where accidentally imported fire ants are now plentiful—the ants will evacuate their nests and come out of the ground. "The colony will stay together as a unit and float," says Nathan Mlot, a graduate student and lead author of the study. "In the South, during rainy season, you can sometimes see them down a river."

In less than two minutes the entire colony will coalesce into a watertight, pancake-shaped raft by grasping onto one another using their mandibles, claws and adhesive pads on the ends of their legs. The largest such raft created in the lab, Mlot says, was about 20 centimeters in diameter and consisted of about 12,000 ants. Even larger ones are likely in nature because colonies can number in the hundreds of thousands. The rafts allow the colony to migrate to drier land intact. "If each ant ran off, there would be no guarantee they could find each other," Mlot says.

The key to the ant raft's success, of course, is that it floats. Whereas a single ant can float, thanks to its small size, hydrophobic exoskeleton, and water surface tension, it is not immediately clear how a large, dense mass of ants can stay afloat. By observing the beading of water on individual ants and on ant rafts, the researchers found that ants in raft formation are more water-repellent. The increase in water-repellency is consistent with the Cassie–Baxter Law of wetting, which says that increasing the roughness of a surface can increase its water-repellency. Ants on the bottom of a raft create a solid textured surface with their hairy entangled bodies, trapping a thin layer of air. The trapped air prevents water from sticking, adds buoyancy to the craft, and allows the bottom ants to breathe. "The ant raft represents a clear example of a surface rendered water repellent by cooperative behavior," the researchers wrote.

If an ant raft is disturbed by an outside force—such as a curious researcher poking it with a twig—it tightens up, becoming more rigid and watertight. Even when the entire raft is submerged several centimeters underwater, it is surrounded by a bubble of air that will allow it to bob back to the surface.

When a clump of ants is dropped into water, the ants on the bottom grip onto one another to form a stable base whereas ants on top slowly expand the edges of the raft. This seemingly coordinated teamwork is actually just random walking on the level of individual ants, Mlot says. When an ant arrives at the edge of the raft, it either turns around or is forced to the bottom by other ants pushing behind it and then walking on top of it. "Central to the construction process is the trapping of ants at the raft edge by their neighbors, suggesting that some 'cooperative' behaviors may rely upon coercion," the researchers wrote.

"It's a wonderful example of how looking at the diversity in nature can lead us to think about inspiration for the design of novel devices," says Robert Full, who directs a lab at the University of California, Berkeley, studying the biomechanics of a variety of animals. The new study suggests that robots capable of mimicking the ant rafts would need to be on the millimeter scale and capable of reversibly attaching and climbing over one another.

Scientists are already developing micro-robots that can build self-assembling and self-healing structures, although the smallest such robots are still quarter-size, according to Full. "As our human technologies take on the characteristics of nature, then nature becomes a better teacher," Full says.

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