Floating Fire Ant Rafts Form Mesmerizing Amoeba-Like Shapes

Researchers say the morphing colonies help ants feel for solid land in a flooded environment—and might inspire swarming robots one day

A fire ant swarm protrusion
Fire ant swarms form tentacles when they float on water. Vernerey research group, University of Colorado, Boulder

Birds of feather flock together. Fish of a school also follow this rule. In nature, animals that flock, school, huddle, hive, pack or aggregate tend to form tight-knit groups to protect the members nestled within. This tendency also minimizes the groups’ collective exposure to the external elements, such as the cold or predators.

Fire ants also swarm, but with a slightly different form. They will clump into a floating raft when they encounter a body of water, but they don’t always stick together into a tight glob. Sometimes, a handful of capricious ants will wander out from the middle safe zone and willingly throw themselves off the edge of their cluster, forming finger-like protrusions over the water. This previously undescribed, projection-producing behavior can persist for hours, as reported in a study published today in Journal of the Royal Society Interface.

Animal groups are tight balls for a reason. “Anything that’s sticking out could get cut off,” says David Hu, a mechanical engineer at Georgia Institute of Technology who wasn’t involved in the study. He was surprised by the article's finding, as ants also usually stay close to their queen. Those severed from their raft won’t survive, “so it does seem like a risk to the ants,” says Hu. Yet the ants seem to be making these moving fingers—up to eight inches long—without fail, researchers found.

Fire ant swarm evolution
Fire ant swarms can extend and retract their arms over the course of several hours. Vernerey Research Group, University of Colorado, Boulder

Many insects are veritable engineers, working together to build mighty structures. But few species assemble like the fire ant; they use their bodies as construction material simply by gripping each other with their jaws, claws and sticky legs—the ant equivalent of holding hands. Alone, a fire ant is nothing spectacular. But lump them together, and the insects behave with what is called swarm intelligence; individuals work as a team, obeying simple rules to give rise to far more complex collective behavior.

“[The swarm] is almost like a smart system,” says Franck Vernerey, a soft matter physicist at the University of Colorado, Boulder, and the senior author of the fire ant study. “It's flowing by itself, producing those large, long protrusions and retracting them.”

To study protrusion formation in fire ants, the researchers collected ants from the wild, then placed them by the thousands into a tank of water. In the water was a rod poking out of the surface, so that the ants would attach to it and their raft wouldn’t drift to the walls. As the researchers expected, the ants would initially huddle together around the rod in a circular pancake. Over the next few hours, the ant boat would randomly form tentacle-like features, making the raft look less like a pancake and more like an amoeba—an evolving shape with a mind of its own.

Protrusion formation probably helps fire ants search their environment for new ground in a flooded environment, akin to casting a wide net and hoping something catches, says Linda Hooper-Bui, an environmental scientist at Louisiana State University. “They've never been described the way [the researchers] described them, that was very novel,” she says. From an environmental perspective, she says that the study is important to help scientists better understand how these ants get around and establish new footholds in the wild, especially after a flood. “The more we know about them, the better,” she adds.

Originally from South America, fire ants have invaded U.S. soil, and now they’re taking over east Asia. They have powerful stings and can endanger humans, livestock and crops, as well as damage human-made equipment. Climate change has only hastened their spread. ”They can solve these remarkable problems,” says Hu. “This whole world is going to be covered in fire ants.”

Lone fire ant
Fire ants can live in colonies with up to 500,000 members. Sean McCann photo / Thompson Rivers University via Flickr under CC BY-NC-SA 2.0

When Vernerey’s team looked closely at the raft, they noticed the structure’s innards were churning. The raft seemed to be made of two distinct layers: the ants buoying the raft underneath and the topside ants tromping over their supporting comrades. Ants on the top of the raft would roam off the edge, to start or contribute to growing protrusions. The ants in the submerged half would eventually crawl out from beneath and through the middle of the raft to replenish the topside ants. The circulation of ants within the raft is what the researchers call “treadmilling.”

“The whole thing is kind of like a doughnut-shaped conveyor belt,” says Robert Wagner, a materials scientist at the University of Colorado, Boulder and the lead author of the study. He calls the ant swarm “a fluid over really long timescales.”

Protrusion close up
Some fire ants wander off the raft to add to the growing protrusion. Vernerey Research Group, University of Colorado, Boulder

Individual ants can collectively give rise to an emergent behavior—such as flowing like a fluid—despite the lack of a command center. Wagner says that congregating fire ants can be thought of as a shape-changing material. As a swarm, these ants can take on any shape that best suits the colony’s purpose by stiffening into a solid or oozing like goo—disintegrating or regrouping to adapt to hostilities in their environment.

Humankind has yet to produce such kind of material, but nature is full of these kinds of systems, from animals that gather in groups to microbial swarms that pulsate with purpose. The researchers hope to study fire ants to inch toward recreating an artificial yet “living” material that mimics this company of critters. Wagner also says that the way these fire ants organize could also boost humankind’s efforts to build intelligent swarm robots. “You have this model system for inspiration,” he says.

Despite the ants’ fiery sting, researchers in the field are willing to risk a few jabs to study these insects’ extraordinary teamwork skills. Hooper-Bui is taking on an even bigger occupational hazard: She’s extremely allergic to fire ant pheromones. Yet she has studied them for over a decade because she says she finds them so fascinating.

Fire ants are also crafty escape artists, partly thanks to the swarm’s ability to probe with protrusions. After an overnight experiment, Wagner returned to the lab the next morning, only to find the water tank on his table ant-free. His video footage showed that the amoeba-shaped colony had extended one of its arms far beyond the camera frame, probably finding a promising escape route along one of the tank’s side walls. He watched as the blob of ants seeped across the video screen along the protrusion, eventually disappearing from the frame entirely. The next thing he saw was the blurry silhouette of ants in the foreground, marching outside of the container. The stowaway swarm had snaked its way across the water to freedom—only to roost in one corner of the table in a docile clump.

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