Fire Ants Form Body Rafts to Stave Off Watery Doom

In the realm of the everyday, fire ants are better known for their stinging bite than for their contributions to science. However, with more and more studies exploring the fascinating lives of Solenopsis invicta, we are finding that the behaviors of ants have a surprising amount to offer the worlds of engineering, robotics, and physics.

Researchers from the Georgia Institute of Technology wanted to better understand the physics behind a particular fire ant behavior: forming rafts. These collective structures are formed by the interlocking legs and mandibles of the insects. In the event of flooding or some other form of watery disaster, entire colonies avoid drowning by forming rafts.

But what are the mechanics? How can a raft of ants remain undamaged when battered by waves, raindrops, and rocks?

In order to answer these questions, the researchers put a clot of fire ants—approximately 15 mL, or roughly 1,000 ants—through a series of experimental tests to examine how the clot would respond to stress. Both live and dead ants were used, in order to explore the difference between an active living response to stimuli versus the "purely geometric" alternative. Here's what it looked like:

To measure raft stiffness and viscosity, the researchers used a special device called a rheometer. According to David Hu, assistant professor of mechanical engineering and biology and one of the coauthors of this work, the rheometer in this case can be imagined as an Oreo cookie you'd never want to eat, where the totally unpleasant cream in the center is made of ants. As the cookie rotates, Hu said, resistance to that rotation is measured.

What Hu and his colleagues demonstrated, and presented at today's American Physical Society's Division of Fluid Dynamics meeting, was that fire ant rafts could best be understood as viscoelastic materials. In the words of the researchers, these rafts "drip, spread, and coagulate, demonstrating properties of an active material that can change state from liquid to solid."

They also showed that fire ant rafts are never static. As the squirming ants' bodies make and break bonds, the raft is both able to store energy in the bonds and release that very same energy when the bonds break. According to the researchers, "this type of behavior rarely happens" beyond the realm . Hu noted that the only other natural "structures" that come close to this type of behavior are bee curtains and biofilms.

Georgia Tech researchers have been studying fire ants for quite some time. Back in 2011, another paper from the school informed us that these rafts are waterproof, a characteristic that helps the structure itself and the ants within navigate fluid environments. And not just for short bursts, either—weeks can go by as a group of fire ants floats on water.

While the dynamics would be fascinating enough on their own, these new findings also hold some significance outside the insect class. In fact, roboticists and material scientists may discover something of valuable in these findings for their respective fields. Hu suggested that this work could be of interest to those working with modular robots, self-healing materials, intelligent materials, and self-repairing bridges, to name only a few.

Insights like these illustrate that there's far more to Solenopsis invicta than a bad attitude. Human society can unexpectedly learn a lot from fire ant dynamics, if we look close enough. Nevertheless, if I personally never come in contact with one again, I wouldn't be too bothered.

@heyiamlex