When Fire Ants Build Rafts, There Are No Free Loaders

When fire ants form floating balls, not a single leg or antennae goes to waste

Photo: junglecat

Researchers have long been fascinated with ant rafts. These floating mats form during rain storms and floods and are composed of thousands of individual insects. Scientists have found that the living rafts possess their own unique material properties, displaying buoyancy and behaving, alternately, like a solid and like a liquid. How the ants manage to create such engineering masterpieces, however, has remained largely unknown. 

Now, researchers have discovered one architectural secret behind the ant rafts. The ants, it turns out, cling to one another using all six of their legs—a single ant can have up to 20 of its comrades' legs grabbing its body. The Georgia Institute of Technology researchers found that 99 percent of ant legs are gripping another ant, meaning "there's no free loaders" when it comes to hitching a ride on the rafts, they said in a statement.

Scientists didn't discover this trick earlier because it's exceedingly difficult to look inside those dense balls of insects. To get around this problem, the team first created a number of ant rafts by swirling 110 insects in a beaker full of water. After the rafts formed, the researchers froze them with liquid nitrogen and used super glue to ensure the ants stayed in place. CT scans allowed the researchers to examine how the rafts' individual components were related. 

Ed Yong elaborates on the findings for National Geographic

They don’t just stick their pads to the nearest thing they can find; they typically attach to their neighbours legs and feet, rather than their bodies. These connections allow the ants to change the shape of their structures by bending or stretching their legs. That explains why the structures are so elastic, and why they can absorb incoming forces more effectively.

The foot-to-foot connections also suggest that the ants actively control the nature of their balls. The team found other such clues. For example, a ball of living ants is less densely packed than a ball of dead ones, implying that they are actively pushing their neighbours away. This presumably helps to create the air pockets that keep the rafts afloat.

While constructing the rafts does not involve intelligence, the team told Yong, the nature of those balls does turn out to be much more complex than scientists expected. 

Get the latest stories in your inbox every weekday.