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 like individual ants. But pack them tightly enough together, and they 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 a fluid.
“Aggregated, they can almost be considered a material, known as ‘active matter,'” said Hungtang Ko, now a postdoc at Princeton University, who began studying these fascinating creatures as a student. graduated from Georgia Tech in 2018. (And yes, he’s been stung many, many times.) He’s co-authored two recent papers on the physics of fire ant rafts. The first, published in the journal Bioinspiration and Biomimetics (B&B), investigated the behavior of fire ant rafts in running water relative 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. KB et al. were somewhat surprised to find that the main mechanism appears to be the so-called “Cheerios effect” – named for the tendency of the last remaining Cheerios floating in the milk to clump together in the bowl, either drifting to the center or at the outer edges.
A single ant has a certain hydrophobicity, that is, the ability to repel water. This property is intensified when they unite, weaving their bodies like an impermeable fabric. The ants collect all the eggs, rise to the surface via their tunnels in the nest, and as the flood waters rise, they gnaw at each other with their mandibles and claws until a flat, tooth-like structure is formed. raft forms. Each ant behaves like an individual molecule in a material, for example grains of sand in a pile of sand. Ants can accomplish this in less than 100 seconds. Plus, 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.
In 2019, Ko and colleagues reported that fire ants could actively sense changes in forces acting on their floating raft. The ants have recognized different fluid flow conditions and can adapt their behavior accordingly to preserve the stability of the raft. A paddle moving through the water of the river will create a series of swirling whirlpools (known as vortex shedding), causing the ant rafts to spin. These vortices can also exert additional forces on the raft, sufficient to break it. The changes in the centrifugal and shear forces acting on the raft are quite small – perhaps 2-3% of the normal force of gravity. Yet somehow ants can feel 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 fire ant rafts contract and form over time. As we reported at the time, the structures sometimes compressed into dense circles of ants. Other times the ants would begin to fan out to form bridge-like extensions (pseudopodia), sometimes using the extensions to escape from containers.
How did the ants make these changes? Rafts basically consist of two distinct layers. The ants on the bottom layer serve a structural purpose, forming the stable base of the raft. But the upper layer ants roam freely above the bound bodies of their lower layer brethren. Sometimes the ants move from the bottom to the top layer or from the top layer to the bottom layer in a cycle resembling a donut-shaped treadmill.
KB et al.The B&B study is somewhat related, except that the Boulder study looked at general collective dynamics rather than interactions between individual ants. “There are thousands and thousands of ants in the wild, but no one 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 rafts this large, repeatability can be an issue. Ko wanted to have a bit more control over his experiments and also study how ants adapted to different flow scenarios in water. He found that 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 cling to the vegetation than if it stayed true to the original larger pancake shape,” Ko said.
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