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 fire ants far apart behave like single ants. But pack enough of these tightly together and they appear more like a single entity, exhibiting both solid and liquid properties. You can form rafts to survive flash floods, form towers, and you can even pour them like liquid from a teapot.
“Aggregated, they can almost be viewed as material known as ‘active matter,'” he said I’m waiting, now a postdoc at Princeton University, who began studying these fascinating creatures in 2018 as a graduate student at Georgia Tech. (And yes, he’s been stung many, many times.) He is a 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 investigated in Physical Review Fluids the mechanism by which fire ants come together to form the rafts in the first place. knockout et al. were somewhat surprised when we found that the primary mechanism was the so-called “Cheerios Effect“—named in honor of the tendency of the last Cheerios swimming in milk to clump together in the bowl, either drifting towards the center or towards the outer edges.
A single ant has some degree of hydrophobicity, that is, the ability to repel water. This ownership is intensified When they connect, they weave their bodies together like a waterproof fabric. The ants collect all the eggs, make their way to the surface through their tunnels in the nest, and when the high tide rises they eat each other’s bodies with their mandibles and claws until a flat, raft-like structure is formed. Each ant behaves like a single molecule in a material – say, grains of sand in a pile of sand. The ants can do it 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 for months.
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 raft’s stability. A paddle moving through river water creates a series of swirling vortices (known as vortex shedding), causing the ant rafts to rotate. These vortices can also exert additional forces on the raft sufficient to break it apart. The changes in both the centrifugal and shear forces acting on the raft are quite small – perhaps 2 to 3 percent of normal gravity. But somehow the ants can feel these small shifts with their bodies.
Earlier this year, researchers at the University of Colorado, Boulder, identified some simple rules which seem to determine how floating rafts of fire ants contract and expand their shape over time. As we have reported At this time, the structures sometimes condensed into dense circles of ants. In other cases, the ants began fanning out to form bridge-like appendages (pseudopods) and sometimes used the appendages to escape the containers.
How did the ants achieve these changes? The rafts essentially comprise two distinct layers. Ants on the bottom layer serve a structural purpose and provide the raft’s stable base. But the ants in the upper tier move freely on the conjoined bodies of their brothers in the lower tier. Sometimes ants move from the bottom to the top tier or from the top to the bottom tier in a cycle that resembles a donut-shaped treadmill.
is et al.‘s B&B study has a similar focus, except that the Boulder study looked at broad collective dynamics rather than interactions between individual ants. “There are thousands and thousands of ants in the wild, 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 adapt 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. “Maybe it takes less force or less metabolic cost to hold vegetation down than if they stuck with the original larger pancake shape,” Ko said.