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

Zoom / Fire ants form a bump from an ant raft.

Vernerey Research Group / CU Boulder

Fire ants are a biblical example of group behavior, they are able to act as individuals, and also combine together to form floating rafts in response to floods. Now a pair of mechanical engineers from the University of Colorado, Boulder, have identified some simple rules that seem to govern how floating fire ant rafts contract and expand their shape over time, according to new paper Published in the journal PLOS Computational Biology. The hope is that by gaining a better understanding of the simple rules behind fire ant behaviour, they can develop better algorithms that control how swarms of robots interact.

It is not a matter of mental strength or careful planning. “This behavior can occur, essentially, spontaneously,” co-author Robert Wagner said. “There is no need for any central decision making by the ants necessarily.” In fact, “single ants are not as smart as one might think, but collectively, they become very intelligent and resilient communities,” Co-author Frank Fernery said:.

as we are I mentioned earlier, a few well-spaced ants behave as individual ants. But pack enough of them together closely, and they’ll behave like a single unit, exhibiting solid and liquid properties. It can form rafts or towers, and you can even pour it from the teapot as a liquid. Fire ants also excel at organizing their ants Flow of traffic.

Any ant on its own has a certain amount of hydrophobia – the ability to repel water – and this Property has been condensed When bonded together, they weave their bodies like a waterproof fabric. They collect any eggs, make their way to the surface through their nest tunnels, and as the flood waters rise, they will nibble each other’s bodies with their mandibles and claws, until a flat raft-like structure forms, with each ant acting like an individual molecule in a substance—say, grains of sand in a heap sand.

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Ants can achieve this in less than 100 seconds. In addition, the ant raft is “self-healing”: it is strong enough that if an ant is lost here and there, the overall structure can remain stable and intact, even for months at a time. In short, the ant raft is a superorganism.

In 2019, researchers at Georgia Tech Prove That Fire ants can actively sense changes in the forces acting on the raft under different fluid conditions and adapt their behavior accordingly to keep the raft stable. For example, with the shear force, the area of ​​the raft was much smaller than when the ants encountered only the centrifugal force. The latter ants experience no matter where they are placed in the ant raft, while only ants at the boundary experience the strongest shear force. Scientists hypothesized that the small rafts are the result of ants trying to avoid being at the boundary, reducing surface area in the process.

The spinning fire ant raft in the David Hoe Biokinetics Laboratory at Georgia Tech is an example of collective behavior.
Zoom / The spinning fire ant raft in the David Hoe Biokinetics Laboratory at Georgia Tech is an example of collective behavior.

Hangtang Koh

The Georgia Tech team also noticed that fire ants in a raft are exploring further whether or not the raft is stationary, usually spreading horizontally, but also vertically, to build temporary tower-like structures in hopes of finding a hanging branch to catch on to dry. Earth. There would be much less exploratory behavior if the ant raft rotated in response to centrifugal or shear forces.

Vernerey and Wagner’s new research is based on study They published last year. They conducted experiments by dropping hordes of fire ants into a bucket of water with a vertical plastic rod in the middle, and then observing the raft-building behavior of the ants over the next eight hours. The idea was to observe how the rafts evolved over time. Note that the pontoons did not maintain their shape. Sometimes the structures are compressed into dense circles of ants. Other times, ants begin to spread out to form bridge-like extensions, sometimes using them to escape enclosures, suggesting that the behavior may serve an evolutionary advantage.

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The duo were fascinated by how the ants achieve these changes in shape through a process they dubbed “mill.” Floats mainly consist of two distinct layers. The ants in the bottom layer serve a structural purpose, as they form the stable base of the raft. But the ants in the upper layer move freely over the bodies attached to their brethren in the lower layer. Ants sometimes move from the bottom layer to the top layer, or from the top layer to the bottom layer in a cycle that Wagner calls a “pie-shaped vicious circle”.

Agent-based model schematic.
Zoom / Agent-based model schematic.

Wagner & Verneri, 2022

Vernerey and Wagner wanted to determine if this treadmill behavior was a deliberate decision of the ants, or if it appeared spontaneously. So they devised a series of factor-based models composed of 2,000 particles (“factors) representing each individual ant, confined to a network of water nodes. One group of worker ants (shown in cyan) made up the structural core network; the other worker ants were ( shown in red) are free to move over them.

The ants are programmed to follow a simple set of rules, such as avoiding collisions with other ants, and not falling into the water (“edge deposition rule”). Then they let the simulation play. And the simulated ants behaved very much like their real-world counterparts.

For example, when active worker ants reach the edge of the raft and come into contact with water, they avoid moving into the water unless forced to do so by nearby active worker ants—and then only if there are enough ants supporting the structure. to seize it. The simulations also showed bridge-like protrusions forming spontaneously, and the researchers were able to correlate these formations with the relative activity of the ants. The more active the ants are, the more likely the bumps will begin to form.

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“The ants at the tips of these spurs are almost pushed away from the edge into the water, creating a fast-loosing effect,” Wagner said. These outcrops are likely a means used by fire ants on a raft to explore their environment, possibly searching for tree trunks or dry land.

The authors concluded, “Although cue factors such as pheromones are not excluded and should be tested in future experimental studies, this model generally poses local mechanisms by which fire ants can achieve gait and grouse growth without central control or purposeful intent.” However, they acknowledge that this is a homogeneous model, and that it is likely that there is more than one set of rules governing the behavior of treadmills and the appearance of spurs – another future focus of their research.

DOI: PLOS Computational Biology, 2022. 10.1371 / journal.pcbi.1009869 (About DOIs).

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