How fire ants form a raft that can float on water

US researchers have used modelling to reveal the processes behind floating ‘rafts’ that help fire ants survive floods.

From quieter turbines based on owls’ wings to a self-cleaning bioplastic modelled on the surface of a lotus leaf, drawing inspiration from nature is a fruitful avenue for engineering research. By harnessing the systems and designs already refined by evolution, scientists can get a head start on designing new materials and structures for myriad applications.

In the recent study, two researchers from the Mechanical Engineering Department at the University of Colorado Boulder, US, worked to investigate how groups of fire ants (Solenopsis invicta) form raft-like structures that allow them to float on water.

The fire ant rafts are dynamic, changing shape over time, and composed of two layers of ants: one that forms the base of the raft, and a second of free-moving ants walking around on top of their friends.

In this timelapse footage, researchers capture a fire ant raft morphing from a spread-out state with long protrusions to a more compact form. Credit: Warner and Vernerey, PLOS Computational Biology, 2022.

Over time, ants switch places from the top layer to the base and vice versa.

“The whole thing is like a doughnut-shaped treadmill,” says lead author Robert Wagner.

After a previous study where they described the behaviour of raft-forming ants in real life, the pair set out to understand the mathematical rules that underlie this behaviour.

To do so, they designed mathematical models that would attempt to replicate the ant behaviour.

In place of real ants, the simulations used about 2,000 ‘agents’ programmed to follow some simple rules. For example, agents on the top layer could only occupy the space defined by agents on the bottom layer – otherwise, the ants would be falling into the water.

The simulations were able to capture a phenomenon that particularly interested the scientists: the formation of long ‘protrusions’ from the main fire ant raft.

The results suggested that such protrusions are formed when the ants are more active.

Engineers at CU Boulder developed computer simulations to try to recreate the dynamics of fire ant rafts. Blue dots represent ants at the bottom of the raft, and red dots are ants walking around freely on top of the raft. Credit: Warner and Vernerey, PLOS Computational Biology, 2022.

“The ants at the tips of these protrusions almost get pushed off of the edge into the water, which leads to a runaway effect,” Wagner says.

He thinks the protrusions may help the floating ants find debris or dry land to settle on.

Importantly, the ants don’t need to be consciously searching for land for the protrusions to form – they just need to follow the mathematical rules.

“This behaviour could essentially occur spontaneously,” Wagner says. “There doesn’t necessarily need to be any central decision-making by the ants.”

“Single ants are not as smart as one may think, but, collectively, they become very intelligent and resilient communities,” says senior author Franck Vernerey.

The researchers hope their findings could aid the design of robots that work in swarms that follow similar rules to the ants.

“Our work on fire ants will, hopefully, help us understand how simple rules can be programmed, such as through algorithms dictating how robots interact with others, to achieve a well-targeted and intelligent swarm response,” says Vernerey.