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. A pair of mechanical engineers from the University of Colorado, Boulder, have identified some simple rules that seem to govern how floating rafts of fire ants contract and expand their shape over time.

It is not a matter of planning or brain power.

We reported previously that a few ants behaved like individual ants. Pack enough of them together and they will behave like a single unit. They can form rafts or towers, and you can even pour them from a teapot like a fluid.

The ability to repel water is intensified when they link together, weaving their bodies like a waterproof fabric.

The ants can do this in less than a minute. The structure of the ant-raft can stay stable and intact even for months at a time if it loses an ant.

Fire ants can sense changes in forces acting on the raft under different fluid conditions and adapt their behavior accordingly to preserve the raft's stability, according to researchers at Georgia Tech. The ants at the boundary experience the strongest shearing force, regardless of where they are positioned in the ant raft. The scientists theorize that the smaller rafts are the result of ants trying to avoid being at the boundaries.

The Georgia Tech team noted that fire ants in a raft explore more if the raft is stationary, but also vertically, building temporary tower-like structures in hopes of finding a hanging branch to grab onto to get back to dry land. If the ant raft is spinning in response to shear forces, there will be less exploratory behavior.

Last year, Vernerey and Wagner published a study. They dropped fire ants into a bucket of water with a plastic rod in the middle and monitored their raft-building behavior over the next eight hours. The idea was to observe how the rafts changed over time. The rafts did not stay the same shape. Sometimes the structures would be too big for the ants. Sometimes the ants would start to fan out to form bridge-like extensions and use them to escape the containers, suggesting that the behavior might serve an evolutionary advantage.

The rafts are comprised of two distinct layers and the duo was fascinated by how the ants achieved those changes in shape. The stable base of the raft is made up of ants on the bottom layer. The ants on the upper layer are free to move on top of their brethren on the bottom layer. Sometimes ants move from the bottom to the upper layer, or from the upper to the bottom layer in a cycle that is called a doughnut-shaped treadmill.

Vernerey and Wagner wanted to know if the treadmilling behavior was a deliberate decision by the ants or if it emerged spontaneously. They created a series of agent-based models consisting of 2000 particles representing each individual ant, confined to a lattice of water nodes. The other agent-ants were free to move on top of the one population that made up the structural base network.

The ants were programmed to follow a set of rules, such as avoiding collision with other ants and not falling into water. They let the simulations play out. The ants behaved like their real-world counterparts in the simulation.

If there were enough structural-supporting ants to grab onto, active agent-ants wouldn't move into the water unless forced to do so by their neighbors. The researchers were able to link bridge-like formations with the relative activity of the ants, thanks to the simulations. The more active the ants were, the more likely they would start to form.

The ants at the tips of these protrusions almost get pushed off of the edge into the water, which leads to a runaway effect. It is possible that the fire ants on a raft are looking for a log or dry land.

The model poses local mechanisms through which fire ants may achieve treadmilling and protrusion growth without centralized control or intent. They acknowledge that the model is a homogenized one, and that there is likely to be more than one set of rules governing the treadmilling behavior.

The journal Computational Biology was published in 2022. The journal has an About DOIs section.