Video: Shape-shifting ants keep moving to hold together

How do fire ant rafts stay intact? (Image: Nathon Mlot)

Fire ant rafts hold together by constantly breaking apart. Swarms of the insects link up in vast groups to float across water, sometimes forming rafts the size of dinner plates. But how they withstand waves and other forces that threaten to sink them wasn’t known.

The ants evolved to form rafts to survive the tropical wet season in their native home, the Amazon rainforest. Entire colonies can escape flooding mounds in seconds, mobilising into buoyant heaps and floating safely to their next settlements. Invasive fire ants are now found across the southern US as well as in Australia, the Caribbean and parts of southern Asia.


Previous work revealed that the ants create their unusual groupings by grasping each other’s legs and jaws. In lab experiments, David Hu at the Georgia Institute of Technology and his colleagues found that the ant bundles can act as both a solid and a viscous liquid. Balls of ants will bounce back to their original form after being stretched or smashed, like rubber. The groupings will also flow slowly around obstacles.

“Biologists have known about these structures for a long time, but no one has really investigated how they resist outside forces,” says Hu. “We’re using tools from physics to study these rafts as a material.”

Fluid as fire ants

For their latest work, Hu and his team used micro-scale computer tomography and high-speed video to look more closely as they crushed and poked the rafts in the lab. They discovered that the structures are able to stay afloat and flow around objects because the ants are continually detaching from and reattaching to each other.

Most materials act like a solid on some scales and a fluid on others, says Hu. When you dip your hand into a swimming pool, the water gives easily and dissipates the force. But if you jump into the pool from a skyscraper, the water will resist the force of your impact, making for a painful landing.

A raft of live fire ants, on the other hand, resists and dissipates external forces equally well on all scales. The ants can act as tiny, resistive springs by flexing and extending their legs, and they break and reform connections with their neighbours to create a flow around external forces, like being prodded with sticks. Importantly, rafts of live ants are significantly more elastic than those made of flash-frozen dead ants.

“Imagine thousands of people linking their arms together, but everyone has six arms instead of two, and all of their limbs have tiny hooks and adhesive pads on them,” says Hu. “That’s why fire ants can do such dynamic restructuring.”

Presented this week at a meeting of the American Physical Society’s Division of Fluid Dynamics meeting in Pittsburgh, the work could help engineers to program self-assembling robots or develop materials that are more stretchy, waterproof and shock-resistant.

“The idea of conceptualising a swarm of ants as a smart material is quite imaginative,” says Scott Turner at Syracuse University in New York. “They were able to show how each of the units of the material are cognitive, aware of their surroundings and respond with a coordinated set of behaviours. This is opening the door to some really interesting questions.”