Fire ants may be the scourge of southern states like Georgia and Texas, but scientifically they are endlessly fascinating as an example of collective behavior. A few well spaced fire ants behave as individual ants. But pack enough of them close together and they act more like a single entity, exhibiting both solid and liquid properties. They can form rafts to survive storm floods, arrange themselves in towers, and you can even pour them from a teapot like a liquid.
“Aggregated, they can almost be considered a material, known as ‘active matter,'” said Hungtang Ko, now a postdoctoral fellow at Princeton University, who began studying these fascinating creatures as an undergraduate at Georgia Tech in 2018. (And yes, he has been stung many, many times.) He is the co-author of two recent papers examining the physics of fire ant rafts. The first, published in the journal Bioinspiration and Biomimetics (B&B), examined how fire ant rafts behave in flowing water compared to static water conditions.
The second, accepted for publication in Physical Review Fluids, explored the mechanism by which fire ants come together to form the rafts in the first place. Queue et al. was somewhat surprised to find that the primary mechanism appears to be the so-called “Cheerios effect”—named for the tendency for the last remaining Cheerios floating in milk to clump together in the bowl, whether they drift to the center or the edges.
A single ant has a certain amount of hydrophobicity, i.e. the ability to repel water. This trait is enhanced when they are linked together, weaving their bodies much like a waterproof fabric. The ants collect any eggs, make their way to the surface via their tunnels in the nest, and when the flood waters rise, they chop down each other’s bodies with their mandibles and claws until a flat raft-like structure is formed. Each ant behaves as an individual molecule in a material – for example, a grain of sand in a pile of sand. The ants can do this in less than 100 seconds. In addition, the ant raft is “self-healing”: it is robust enough that if it loses an ant here and there, the overall structure can remain stable and intact, even for months at a time.
In 2019, Ko and colleagues reported that fire ants could actively sense changes in forces acting on their floating raft. The ants recognized different fluid flow conditions and can adapt their behavior accordingly to preserve the stability of the raft. An oar moving through river water will create a series of swirling eddies (known as vortex shedding), which cause the ant rafts to spin. These eddies can also exert additional forces on the raft, sufficient to break it apart. The changes in both centrifugal and shear forces acting on the raft are quite small—perhaps 2 percent to 3 percent the force of normal gravity. Nevertheless, the ants can somehow feel these small displacements with their bodies.
Earlier this year, researchers at the University of Colorado, Boulder, identified a few simple rules that appear to govern how floating rafts of fire ants contract and expand their shape over time. As we reported at the time, the structures sometimes coalesced into dense circles of ants. At other times, the ants began fanning out to form bridge-like extensions (pseudopods), sometimes using the extensions to escape the containers.
How did the ants achieve these changes? The fleets mainly consist of two distinct teams. Ants on the bottom layer serve a structural purpose, forming the stable bottom of the raft. But the ants on the upper layer move freely on top of the interconnected bodies of their brethren on the lower layer. Sometimes ants move from the bottom to the top layer or from the top to the bottom layer in a cycle that resembles a donut-shaped treadmill.
Queue et al.‘s B&B study is somewhat related in focus, except that the Boulder study looked at the broad collective dynamics rather than interactions between individual ants. “There are thousands and thousands of ants in nature, but no one really knows how a pair of ants will interact with each other and how that affects the stability of the fleet,” Ko told Ars.
With such large fleets, repeatability can be an issue. Ko wanted to gain a little more control over his experiments and also study how the ants adapted to different flow scenarios in water. He found that the ants use an active streamlining strategy, changing the shape of the raft to reduce drag. “So maybe it takes less force, or less metabolic cost, to hold on to the vegetation than if they stuck to the original larger pancake shape,” Ko said.