Published online 26 April 2011 | Nature | doi:10.1038/news.2011.257

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Fire ant colonies form water-repellent rafts to escape floods.

By linking together, ants can trap air bubbles and push back against the water surface. N. Mlot/PNAS

A team of engineers has discovered how colonies of ants survive floods by forming themselves into life rafts. The work shows how many simple components can interact to create complex structures and behaviours, a subject that touches on crowd control, urbanization and robotics.

An individual ant can float on water for a few minutes, but a clump of the insects is heavy enough to break the surface tension of the water and sink. Yet when a nest of fire ants (Solenopsis invicta) is flooded, the entire thousands-strong colony shapes itself into a raft that can stay afloat for months.

"Together they form this really complex material that water should be able to get through, but can't," says Nathan Mlot, a mechanical engineer at Georgia Institute of Technology in Atlanta. Mlot and his colleagues have shown how these rafts are built and why they float. Their findings are published this week in Proceedings of the National Academy of Sciences1.

Team building

When Mlot put thousands of ants in an empty beaker and swirled it, the insects clung to each other, gripping tightly to form a sphere that felt like a squashy ball, he says. Wearing gloves to avoid being bitten, Mlot could toss the clump in the air and catch it without any ants falling off.

When such a sphere is placed in water, it relaxes into a dome and, within minutes, flattens into a pancake.

The ants that hit the water first hook together with their jaws and legs, forming a tightly woven base. As the dome flattens and more ants reach the edge of the mass, their comrades at the bottom grip them tightly, weaving them into the base to help support the rest of the colony. At its flattest, the raft consists of two to three levels of ants, with the upper levels milling around on their interwoven nest mates.

A lone ant's exoskeleton can trap air bubbles and become slightly water repellent, but the surface of an interwoven group does so much more effectively, keeping the ants dryer.

Although the raft is porous and its base is below the water level, none of the ants are submerged, or even get wet. Instead, the ants at the base of the raft push against the water's surface and shape it around them into a bowl without breaking the surface tension.

Even when Mlot pushed the raft down with a twig or tweezers, it remained dry and afloat, so that the water's surface deformed like a trampoline flexing around a jumper. When Mlot pushed the raft under the water, it clumped up to form a ball, trapping air bubbles that allowed the ants to survive.

"When the cluster is underwater, we can see individual ants pushing against the air–water interface, actually deforming it around them," he says.

From rafts to robots?

"It's an example of how ants enhance their efficiency by being in a group," says Audrey Dussutour, a biologist at Paul Sabatier University in Toulouse, France, who studies collective ant behaviours such as foraging and path-finding. The raft phenomenon was previously known about, she adds, but has never been studied experimentally.

Mlot and his colleagues devised a mathematical model of how the rafts spread as they grow, allowing the engineers to predict how this would happen in larger groups of ants — such as mature colonies, which in the wild can reach hundreds of thousands of individuals.

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Ant rafts are an example of emergence, where many parts organize into a complex whole without any overall control. Emergent behaviour is seen in many large groups, be they crowds of people, flocks of animals, or networks of neurons.

The principles uncovered from studying ants could help us understand and harness such self-organizing activity, says Mlot, in the form of groups of simple, autonomous robots, for example.

But it might be a while before the ideas can be applied to other behaviours. Guy Theraulaz, who studies the collective behaviours of animals and robots at Paul Sabatier University, notes that Mlot's model "is not strictly devoted to understanding how the cooperative behaviours of ants give rise to the increased water-repellent surface, but only how this surface increases with time. It's a first step to understanding this complex phenomenon."