The Science of Ant’s Underground Cities

Credit: Pixabay/CC0 public domain

Imagine an anthill. What do you see? A small pile of sand and crumbly dirt poking through the lawn? A small hole that disappears into the ground? A few ants are scrambling around. Not very impressive, right?

But slide below the surface and the above-ground simplicity gives way to underground complexity. Tunnels plunge down, branch out and lead to specialized chambers that serve as a home for the colony’s queen, as a nursery for her young, as farms for fungi grown for food, and as dumping grounds for its waste. These are not just caves. They are underground cities, some of which are home to millions of individuals, reaching up to 25 feet underground and often lasting for decades.

This kind of construction would be an impressive undertaking for most creatures, but when performed by animals that don’t grow much bigger than your fingernail, it’s especially remarkable.

Now, driven by a desire to improve our own ability to dig underground — be it for mining, subways, or underground farming — a team of researchers at Caltech has unraveled one of the secrets behind how ants build these amazingly complex and stable structures. to build.

Led by the lab of Jose Andrade, the George W. Housner Professor of Civil and Mechanical Engineering, the team studied the digging habits of ants and discovered the mechanisms that guided them. The research is described in a paper published in the journal Proceedings of the National Academy of Sciences.

The Science of Ant's Underground Cities

Grain forces (black lines) in the same place in the soil before (left) and after (right) ant tunneling. Credit: Jose E. Andrade and David R. Miller (California Institute of Technology, Pasadena, CA).

What do ants think (if anything)?

Before starting this research, Andrade, who is also the Cecil and Sally Drinkward Leadership Chair and Executive Officer for Mechanical and Civil Engineering, had a big question he wanted to answer: “Do ants know how to dig tunnels, or are they? just blindly digging?

“I was inspired by these excavated ant nests that they pour plastic or molten metal into and you see these huge tunnel systems that are incredibly impressive,” says Andrade. “I saw a photo of one of these next to a person and I thought, ‘My goodness, what a fantastic structure.’ And I started to wonder if ants ‘know’ how to dig.”

“We didn’t interview ants to ask if they know what they’re doing, but we did start with the hypothesis that they dig intentionally,” Andrade says. “We assumed maybe ants were playing Jenga.”

What he means by “playing Jenga” is that the team suspected that the ants made their way through the dirt, looking for loose grains of soil to remove, much the same way someone playing Jenga searches for loose blocks that are safe. to delete. remove from the pile. The blocks that cannot be removed — the blocks that carry the load of the stack — would be part of the structure’s “force chains,” the collection of pieces clamped together by the forces applied to them.

“We assumed the ants could sense these power chains and avoided digging there,” Andrade says. “We thought maybe they were tapping earth grains, and that way they could assess the mechanical forces on them.”

Ants do what they want

To learn about ants, the team needed to have ants to study. But Andrade is an engineer, not an entomologist (one who studies insects), so he enlisted the help of Joe Parker, assistant professor of biology and biological engineering, whose research focuses on ants and their ecological relationships with other species.

“What Jose and his team needed was someone who works with ants and understands the adaptive, collective behavior of these social insects to give them context for what they were doing,” Parker says.

With Parker on board, the team began breeding and learning to work with ants. It was a process that took almost a year, Andrade says. Not only did they have to breed enough ants to work with, there was a lot of trial and error to get the ants to dig into small cups of soil that they could load into an X-ray camera. Through that work, they determined an optimal cup size to use and an ideal number of ants to put in each cup. Yet the ants did not always cooperate with the researchers’ own priorities.

“They’re a bit erratic,” says Andrade. “They dig whenever they want. We put these ants in a tank, and some of them started digging right away, and they would make this amazing progress. But others would take hours and they wouldn’t dig at all. And some would dig for a while and then would stop and take a break.”

But once the ants got going, the researchers took the tiny cups and X-rayed them using a technique that created a 3D scan of all the tunnels inside. By taking a series of these scans and letting the ants work a bit between each, the researchers were able to create simulations that showed the progress the ants made as they expanded their tunnels further and further below the surface.

Understanding Ant Physics

Then Andrade’s team analyzed what the ants were actually doing while they were at work, and a few patterns emerged. First, Andrade says, the ants tried to be as efficient as possible. That meant they dug their tunnels along the inner rims of the cups, because the cup itself would act as part of the structures of their tunnels, which would mean less work for them. They also dug their tunnels as straight as possible.

“That makes sense, because a straight line is the shortest path between two points,” says Andrade. “And because they take advantage of the sides of the container, it shows that the ants are very efficient at what they’re doing.”

The ants also dug their tunnels as steeply as they could, up to what is known as the angle of repose. That angle represents the steepest angle a granular material — a material made of individual grains — can pile up before collapsing. To understand the angle of rest, imagine a child building a sand castle on the beach. If the child uses dry sand, each shovel of sand they add will slide down the sides of the pile they have already made. More sand makes the heap bigger, but also wider and steeper. On the other hand, if the child uses wet sand, he will be able to pile up the sand steeply enough to build walls and towers, and all the other things a sand castle might have. Wet sand has a larger angle of repose than dry sand and each granular material has a unique angle. The ants, Andrade says, can see how steep that angle is for whatever they’re digging in, and they don’t cross it. That also makes sense, he says.

“If I’m a digger and I’m going to survive, my digging technique will be in accordance with the laws of physics, otherwise my tunnels will collapse and I’ll die,” he says.

Finally, the team discovered something about the physics of ant tunnels that could one day be useful to humans.

When ants remove soil grains, they subtly reshuffle the power chains around the tunnel. Those chains, somewhat randomly before the ants start digging, rearrange around the outside of the tunnel, kind of like a cocoon or liner. As they do this, two things happen: 1.) the power chains reinforce the existing walls of the tunnel and 2.) the power chains relieve the pressure of the grains at the end of the tunnel where the ants are working, causing the for the ants to remove them safely.

“It’s been a mystery in both engineering and ant ecology how ants build these structures that last for decades,” Parker says. “It turns out that by removing grains in this pattern we observed, the ants take advantage of these circumferential chains as they dig.”

But what about the central question of the team hypothesis? Are ants aware of what they do when they dig?

What ants do and don’t know

“What we found was that they didn’t seem to ‘know’ what they were doing,” Andrade says. “They didn’t systematically look for soft spots in the sand, but evolved to dig according to the laws of physics.”

Parker calls this a behavioral algorithm.

“That algorithm doesn’t exist within a single ant,” he says. “It’s this emerging colony behavior of all these workers acting like a superorganism. How that behavioral program is spread across the cerebellum of all these ants is a wonder of the natural world that we have no explanation for.”

Andrade says he hopes to work on an artificial intelligence approach that can emulate that behavioral algorithm so he can simulate how ants dig on a computer. Some of that emulation, Andrade says, will determine how to scale ant physics for human-sized tunnels.

“Grainated materials scale in different ways than other materials like liquids or solids,” he says. “You can go from experiments at the grain scale, in this case a few millimeters, to the meter scale by scaling the intergranular friction coefficient.”

The next step after that? Robotic ants that can dig tunnels for humans.

“Moving granular material is very energy intensive and very expensive and you always need an operator to operate the machines,” he says. “This would be the last frontier.”

The article describing the research, titled “Unearthing real-time 3D ant tunneling mechanics,” appears in the Aug. 23 issue of the magazine Proceedings of the National Academy of Sciences.

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More information:
Excavating real-time 3D ant tunneling mechanics, Proceedings of the National Academy of Sciences (2021).

Provided by the California Institute of Technology

Quote: The Science of Ants’ Underground Cities (2021, Aug. 23) retrieved Aug. 25, 2022 from

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