I am waiting
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. Some fire ants that are far apart behave like individual ants. But they pack enough of them closely together and act more like a single unit, exhibiting both solid and liquid properties. They can form rafts to survive flash floods, organize themselves into towers and you can even pour them from a teapot like liquid.
“Collectively, they can almost be thought of as one material, known as ‘active matter,’” said Hungtang Ko, now a postdoc at Princeton University who began studying these fascinating creatures as a Georgia Tech graduate student in 2018. (And yes , he’s been stung many, many times.) He is a co-author of two recent papers investigating the physics of fire ant designs. The first, published in the journal Bioinspiration and Biomimetics (B&B), investigated how fire ant rafts behaved in flowing water compared to static water conditions.
The second, accepted for publication in Physical Review Fluids, investigated the mechanism by which fire ants come together to form the rafts in the first place. Ko et al. were somewhat surprised to find that the primary mechanism appears to be the so-called “Cheerios effect”—named after the tendency of leftover Cheerios floating in milk to clump together in the bowl, either drifting toward the center or the outer edges.
A single ant has a certain amount of hydrophobicity, that is, the ability to repel water. This quality is intensified when they bond with each other, weaving their bodies together like a waterproof fabric. The ants collect any eggs, emerge through their tunnels into the nest, and as the flood waters rise, clamp each other’s bodies together with their mandibles and claws until a flat, raft-like structure forms. Each ant behaves like an individual particle in a material—say, grains of sand in a pile of sand. Ants can accomplish this in less than 100 seconds. What’s more, the ant raft is “self-healing”: it’s sturdy enough that if it loses an ant here and there, the overall structure can remain stable and intact, even for months at a time.
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In 2019, Ko and colleagues reported that fire ants could actively sense changes in the forces acting on their floating raft. The ants recognized different fluid flow conditions and can adjust their behavior accordingly to maintain the stability of the raft. A paddle moving through river water will create a series of swirling eddies (known as eddies), causing the ants’ rafts to spin. These eddies can also exert additional forces on the raft, sufficient to break it up. The changes in both the centrifugal and shear forces acting on the raft are very small—perhaps 2 percent to 3 percent of the force of normal gravity. Yet somehow, ants can sense these small changes with their bodies.
Earlier this year, researchers at the University of Colorado, Boulder, identified a few simple rules that appear to govern how floating ant rafts contract and expand in shape over time. As we reported at the time, structures were sometimes compressed into dense ant circles. At other times, the ants began to deflate to form bridge-like extensions (pseudopods), sometimes using the extensions to escape the containers.
How did the ants achieve these changes? Rafts essentially consist of two distinct layers. The ants in the bottom layer serve a structural purpose, forming the stable base of the raft. But the ants in the upper layer move freely over the connected bodies of their brethren in the lower layer. Sometimes the ants move from the bottom to the top layer or from the top to the bottom layer in a circle that looks like a donut-shaped corridor.
The B&B study by Ko et al. somewhat related in focus, except that the Boulder study examined broad collective dynamics rather than interactions between individual ants. “There are thousands and thousands of ants in nature, but nobody really knows how a pair of ants would interact with each other and how that affects the stability of the raft,” Ko told Ars.
With such large rafts, 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 the 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 vegetation than if it stuck with the original larger pancake shape,” Ko said.
