The physics of fire ant rafts could help engineers design swarming robots

Peer-Reviewed Publication


Protrusion on an ant raft

Noah rode out his flood in an ark. Winnie-the-Pooh had an upside-down umbrella. Fire ants (Solenopsis invicta), meanwhile, form floating rafts made up of thousands or even hundreds of thousands of individual insects. 

A new study by engineers at the University of Colorado Boulder lays out the simple physics-based rules that govern how these ant rafts morph over time: shrinking, expanding or growing long protrusions like an elephant’s trunk. The team’s findings could one day help researchers design robots that work together in swarms or next-generation materials in which molecules migrate to fix damaged spots.

The results appeared recently in the journal PLOS Computational Biology.

“The origins of such behaviors lie in fairly simple rules,” said Franck Vernerey, primary investigator on the new study and professor in the Paul M. Rady Department of Mechanical Engineering. “Single ants are not as smart as one may think, but, collectively, they become very intelligent and resilient communities.” 

Fire ants form these giant floating blobs of wriggling insects after storms in the southeastern United States to survive raging waters. 

In their latest study, Vernerey and lead author Robert Wagner drew on mathematical simulations, or models, to try to figure out the mechanics underlying these lifeboats. They discovered, for example, that the faster the ants in a raft move, the more those rafts will expand outward, often forming long protrusions.

“This behavior could, essentially, occur spontaneously,” said Wagner, a graduate student in mechanical engineering. “There doesn’t necessarily need to be any central decision-making by the ants.”

Treadmill time

Wagner and Vernerey discovered the secrets of ant rafts almost by accident. 

In a separate study published in 2021, the duo dropped thousands of fire ants into a bucket of water with a plastic rod in the middle—like a lone reed in the middle of stormy waters. Then they waited.

“We left them in there for up to 8 hours to observe the long-term evolution of these rafts,” Wagner said. “What we ended up seeing is that the rafts started forming these growths.”

Rather than stay the same shape over time, the structures would compress, drawing in to form dense circles of ants. At other points, the insects would fan out like pancake batter on a skillet, even building bridge-like extensions.

The group reported that the ants seemed to modulate these shape changes through a process of “treadmilling.” As Wagner explained, every ant raft is made up of two layers. On the bottom, you can find “structural” ants who cling tight to each other and make up the base. Above them are a second layer of ants who walk around freely on top of their fellow colony-members. 

Over a period of hours, ants from the bottom may crawl up to the top, while free-roaming ants will drop down to become part of the structural layer.

“The whole thing is like a doughnut-shaped treadmill,” Wagner said.

Bridge to safety

In the new study, he and Vernerey wanted to explore what makes that treadmill go round.

To do that, the team created a series of models that, essentially, turned an ant raft into a complicated game of checkers. The researchers programmed roughly 2,000 round particles, or “agents,” to stand in for the ants. These agents couldn’t make decisions for themselves, but they did follow a simple set of rules: The fake ants, for example, didn’t like bumping into their neighbors, and they tried to avoid falling into the water. 

When they let the game play out, Wagner and Vernerey found that their simulated ant rafts behaved a lot like the real things. 

In particular, the team was able to tune how active the agents in their simulations were: Were the individual ants slow and lazy, or did they walk around a lot? The more the ants walked, the more likely they were to form long extensions that stuck out from the raft—a bit like people funneling toward an exit in a crowded stadium.

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

Wagner suspects that fire ants use these extensions to feel around their environments, searching for logs or other bits of dry land.

The researchers still have a lot to learn about ant rafts: What makes ants in the real world, for example, opt to switch from sedate to lazy? But, for now, Vernerey says that engineers could learn a thing or two from fire 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,” he said.


PLoS Computational Biology




Computational exploration of treadmilling and protrusion growth observed in fire ant rafts



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March 6, 2022 6:50 pm

All I know is that I hate them. Bivouac where they are concentrated and your gonna want a hammock. And even then if your not lucky enough to have a jungle hammock then you better have some powerful repellent to put on the ropes and trees they’re tied to.

Curious George
March 6, 2022 6:51 pm

Any physics here?

Reply to  Curious George
March 6, 2022 7:25 pm

Not a lot, other than ants are buoyant, George.

I would like to know the name of the first genius ant who figured out that if they all stuck together, they wouldn’t have to start another colony from scratch.

And then that ant had to convince the others. Politics goes further back than one might think.

Paul Johnson
Reply to  H.R.
March 6, 2022 7:49 pm

Ants that banded together survived and reestablished their nests. Those that didn’t didn’t. Darwin strikes again.

Hokey Schtick
Reply to  Paul Johnson
March 6, 2022 10:37 pm

And that, children, is how the human brain was made. Just banging rocks together!

Reply to  Paul Johnson
March 7, 2022 4:32 am

It seems the dynamics of their raft succeeds because individual ants crawl over others nearby to keep from getting wet.

Reply to  H.R.
March 7, 2022 5:33 am

I’m thinking it’s not the buoyancy of the ants, but the surface tension of the water that figures larger in this case. A few ants at the bottom with their legs slightly spread could support a few more wandering around above.

Reply to  Spetzer86
March 7, 2022 9:07 am

I suspected the same.
Place a few drops of surfactant (detergent) to reduce the surface tension of the water and watch what happens.

Reply to  Spetzer86
March 7, 2022 12:03 pm

Spetzer86: I’m thinking it’s not the buoyancy of the ants, but the surface tension of the water that figures larger in this case.”

Curious George asked, “Any physics here?”

The answer is still not a lot whether it’s either, or, or both.

I know water spiders walk on top of the water using the surface tension. I wasn’t sure about ants. It’s my recollection that they swim around.

Anyhow, M Courtney gave a good explanation of why physics wasn’t much discussed, if at all.

M Courtney
Reply to  Curious George
March 7, 2022 12:39 am

It’s biology. A different science.

You can tell it’s science as they started by making real world observations and only then looked made models to see if their understanding fitted reality.

Hoyt Clagwell
March 6, 2022 8:17 pm

I think it would be time better spent to study fire ant rafts in order to figure out a better way to eradicate them.

Reply to  Hoyt Clagwell
March 7, 2022 2:37 am

I feel the same way, lots of fire ants here in Texas. I have also been studying them, while finding a better way to get rid of them..

What I observed while using dishwashing soap should be noteworthy. They hold their breath. The bigger they are the longer they can survive. My problem using soap was when the dirt absorbed the soapy water, they quit holding their breath and start moving around again. The secret was to keep applying soapy water to the bed without disturbing the nest which is up to a foot underground.

So after I apply the soap, I placed a small flat piece of wood above the nest. This allows them to kling onto something while drying out. Otherwise they will scatter and find a safe place to build another nest. It takes only a few minutes for them to make it up to the top so I check the wood and apply more soap. After doing the checking and applying several times, the smaller ones die off but the bigger ones can hold their breath longer and will work their way up to the top too.

But then after all the activity ends, you have to deal with the eggs hatching. So I leave the wood in place and place a flag next to the hole. After about three days the activity from the eggs hatching ends. But, because I hate them so much I continue to douse the nest for as long as it takes to satisfy my hate.

Soap is my way of studying the ants but The best poison is Acephate. It stinks like rotton cabbage but it will kill every ant below. But, again, the eggs will continue to hatch. They willl not hang out on the piece of wood because the ground is contaminated. The poison is absorbed by the dirt and it loses the kill power to take care of the hatchlings.

Recently after a long dry spell I started watering my grass. Fire ants do not like to be wet and come to the surface moving the eggs with them. After watering the dirt piles appear. So I apply the Acephate by sprinkling it over the piles of dirt which contains all the ants. then I immediately start the water sprinkler on again for a short while to flood the nest and the poison is absorbed. That kills all of them.

Another thing I have observed is another critter that looks like a silverfish or earwig living in the same nest. Must be cousins in the spare bedroom.

And I have also found in small areas which has access to ants smaller than the large eggers. They look like fire ants but dont think they are. Usually above the ground in things like pecan shells with a hole in them. Within this contained environment I found eggers but no eggs. Smaller ants were moving baby larvas into the space and feeding the bigger one. Several larvas i found with a big ant sucking away on it.

Curious George, it is the physics of survival.

Hope this helps to understand the fire ant. Wipe them out, please

Reply to  Hoyt Clagwell
March 7, 2022 6:05 am

Soapy water changes the surface tension of the water and then the ants drown.

J. R.
March 6, 2022 9:54 pm

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

Is that a tipping point?

Rich Davis
March 6, 2022 10:12 pm

What makes ants in the real world, for example, opt to switch from sedate to lazy?

Are these Democrat ants? The choices are sedate and lazy? I think the YouReekAlot! intern meant “switch from active to lazy”?

Mike McMillan
March 7, 2022 12:37 am

And they say socialism doesn’t work. 😉

Reply to  Mike McMillan
March 7, 2022 7:28 am

It does work, but only if all participants are unthinking automatons.

Andrew Lale
March 7, 2022 3:26 am

We have zero hope as a species. Our ingenuity is only matched by our malice and lack of foresight.

March 7, 2022 6:56 am

Not sure what is so groundbreaking about fire ant rafts. Any 7 year old boy who likes to play outdoors could have told us everything from this research… without using taxpayer dollars.

Meanwhile, people are dying from cancer and other diseases.

Bill Rocks
March 7, 2022 9:39 am

As I recall, the first thing that fire ants do is kill all of the native ants.

Fire ants are vicious and bite you and sting you at the same time. They get you with both ends. Some people are very allergic to their “bite”.

They will build a thin, hollow tube of dirt to create a connection from the soil up across the top of a concrete foundation and into the walls of a building. Need to check around the building once per week to keep them out.

March 7, 2022 2:45 pm

Because, after all kinds of robot and AI disaster movies, what we want is a whole lot of robots learning to swarm.

Grey goo anyone?

March 7, 2022 3:02 pm

You know, I am not totally sure why, but I am just not feeling excited about the prospect of swarm bots.

Reply to  Linnea
March 7, 2022 10:24 pm

“The physics of fire ant rafts could help engineers design swarming robots”
What could go wrong?

March 7, 2022 6:36 pm

I remember hearing about Funnel Web Spiders doing something similar during flooding in Australia a few years ago. They are definitely not social bugs.

March 7, 2022 7:50 pm

Unless the queen is with them, they’re just ants without their colony.
Nasty vicious, attack in hordes, ants.

Where they bite/sting a person, the small wound forms a pustule.

The best eradication method that I’ve seen is to pour molten tin or aluminum into their nest. Afterwards, one can dig up a neat cast of the nest.

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