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The physics behind how fire ants band together into robust floating “rafts”

Enlarge / A spinning fire ant raft in David Hu's biolocomotion lab at Georgia Tech is an example of collective behavior.Hungtang Ko

Fire ants can survive floods by linking their bodies together to form large floating rafts. Now researchers at Georgia Tech have demonstrated that fire ants can actively sense changes in forces acting upon the raft under different fluid conditions and adapt their behavior accordingly to preserve the raft's stability. Hungtang Ko described their work at a meeting of the American Physical Society's Division of Fluid Dynamics, held in Seattle just before the Thanksgiving holiday.

Fire ants (and ants in general) provide a textbook example of collective behavior. A few ants spaced well apart behave like individual ants. But pack enough of them closely together, and they behave more like a single unit, exhibiting both solid and liquid properties. You can pour them from a teapot like a fluid, or they can link together to build towers or floating rafts—a handy survival skill when, say, a hurricane floods Houston. They also excel at regulating their own traffic flow.

Any single ant has a certain amount of hydrophobia—the ability to repel water—and this property is intensified when they link together, weaving their bodies much like a waterproof fabric. They gather up any eggs, make their way to the surface via their tunnels in the nest, and as the flood waters rise, theyll chomp down on each others bodies with their mandibles and claws, until a flat raft-like structure forms, with each ant behaving like an individual molecule in a material—say, grains of sand in a sand pile. And they can do this in less than 100 seconds. Plus, the ant-raft is “self-healing”: its robust enough that if it loses an ant here and there, the overall structure can stay stable and intact, even for months at a time. In short, the ant raft is a super-organism.

Ko works in David Hu's biolocomotion lab at Georgia Tech, which investigates not just the collective behavior of fire ants, but also water striders, snakes, various climbing insects, mosquitos, the unique properties of cat tongues, and animal bodily functions like urination and defecation. (One of his students, Patricia Yang, won a 2019 Ig Nobel Prize for her study of why wombats produce cubed poo.) Ko and his colleagues thought that fire ants might be able to sense changes in the forces acting upon the rafts under different conditions of fluid flow and decided to test that hypothesis.

A paddle moving through river water will create a series of swirling vortices (known as vortex shedding), causing the ant rafts to spin. These vortices can also exert extra forces on a floating ant raft, sufficient to break it apart. The changes in force acting on the raft are still quite small—maybe 2 percent to 3 percent the force of normal gravity.

Ko hypothesizes that the ants' sensitivity to such small shifts might have something to do with how ants perceive their surroundings. Human beings react to visual information—for instance, bracing themselves while riding a roller coaster because they can see a big drop is ahead on the track and know that they will experience a sharp increase in acceleration. Insects like ants, however, have very poor eyesight and sense forces with their bodies.

To recreate different fluid situations in the lab,Read More – Source

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