A few simple rules determine how floating fire ant rafts change shape over time

Fire ants are a textbook example of collective behavior, capable of behaving as individuals and also banding together to form floating rafts in response to flooding. Now, a pair of mechanical engineers from the University of Colorado Boulder has identified some simple rules that seem to govern how floating rafts of fire ants contract and expand their shape over time, according to a new paper published in the journal PLOS Computational Biology. The hope is that, by gaining a better understanding of the simple rules underlying fire ant behavior, the engineers can develop better algorithms controlling how swarms of robots interact.

Fire ant rafts are not a matter of brain power or careful planning. "This behavior could, essentially, occur spontaneously," said co-author Robert Wagner. "There doesn't necessarily need to be any central decision-making by the ants."

Indeed, "single ants are not as smart as one may think, but, collectively, they become very intelligent and resilient communities," said co-author Franck Vernerey.

As we've reported previously, 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. They can form rafts or towers, and you can even pour them from a teapot like a fluid. Fire ants also excel at regulating their own traffic flow.

Any single ant has a certain amount of hydrophobia, i.e., the ability to repel water. This property is intensified when they link together, weaving their bodies much like a waterproof fabric. The ants gather up any eggs, make their way to the surface via their tunnels in the nest, and as the flood waters rise, they chomp down on each other's bodies with their mandibles and claws until a flat raft-like structure forms. Each ant behaves like an individual molecule in a material—say, grains of sand in a sand pile.

The ants can accomplish 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 superorganism.

In 2019, researchers at Georgia Tech 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. For instance, with a 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. The scientists hypothesized that the smaller rafts are the result of ants trying to avoid being at the boundaries, minimizing the surface area in the process.

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

Credit: Hungtang Ko

The Georgia Tech team also noted that fire ants in a raft 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.

Vernerey and Wagner's new research builds on a study they published last year. They conducted experiments by dropping hordes of fire ants into a bucket of water with a plastic vertical rod in the middle, and then they monitored the ants' raft-building behavior over the next eight hours. The idea was to observe how the rafts evolved over time. The engineers noticed that the rafts didn't stay the same shape. Sometimes the structures would compress into dense circles of ants. Other times, the ants would start to fan out to form bridge-like extensions, sometimes using the extensions to escape the containers. This suggested that the behavior serves an evolutionary advantage.

The two engineers were fascinated by how the ants achieved those changes in shape through a process they dubbed "treadmilling." The rafts essentially comprise two distinct layers. Ants on the bottom layer serve a structural purpose, making up the stable base of the raft. But the ants on the upper layer move freely on top of the linked bodies of their bottom-layer brethren. Sometimes ants move from the bottom to the upper layer or from the upper to the bottom layer in a cycle that Wagner calls "a doughnut-shaped treadmill."