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

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

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, they’ll chomp down on each other’s 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”: it’s 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.

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To recreate different fluid situations in the lab, Ko et al. mounted a large container of water on top of an old record player (spin table), with a small ant raft floating on the water’s surface. In one experiment, both the container and the raft were spinning. In another, the researchers created a vortex in the water with a magnetic stir bar, while the container remained stationary. A third control experiment placed the ant raft onto stationary water. In the first experiment, the primary force acting on the ant raft is centrifugal force, per Ko, while in the second, with the vortex, the raft experiences the shearing force.

They found that, in response to that shearing force, the area of the raft was much smaller than when the ants encountered just centrifugal force. Ants experience the latter regardless of where they are positioned in the ant raft, whereas only the ants at the boundary experience the strongest shearing force. Ko hypothesizes that the smaller rafts are the result of ants trying to avoid being at the boundaries, minimizing the surface area in the process.

Fire ants in a raft also explore more if the raft is stationary—usually spreading out horizontally, but also vertically, building temporary tower-like structures in hopes of finding a hanging branch to grab onto to get back to dry land. There will be a lot less exploratory behavior if the ant raft is spinning in response to centrifugal or shear forces.

“Our current hypothesis is that they explore less, because they need to form a stronger bond with their neighbors. We are still working on testing the hypothesis,” Ko said. “We think the independent response in individuals is enough in leading to the system-level deformation that we observe.”

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