IRA FLATOW: This is Science Friday. I’m Ira Flatow.

Did you ever have one of those ant farms when you were growing up? You know what I mean. It’s one of those clear plastic colonies, where you can see the ants excavating dirt to build those intricate passageways in the tunnels, and the caverns, and all the stuff they do. It’s really captivating to watch, to see how industrious these insects are, and to witness the design they create, when they’re doing their little building. And how do they know what to build and where to do that?

Well, the ants’ feats of engineering are not limited to the underground, because my next guest finds fire ants– much to the bane of his university colleagues, I should add. And the reason he’s curious about studying fire ants is because the stinging insects build impressive sinking towers, which are constantly melting and being built up again. And I know they build these fluid-like structures with their bodies. How do they do it?

Here to tell us more is David Hu. He’s an associate professor of mechanical engineering and biology at Georgia Tech in Atlanta. Welcome back to Science Friday, David.

DAVID HU: Hi Ira. It’s great to be here.

IRA FLATOW: OK, since we’re on radio, and I know you’ve been here before, describe for us– paint us a picture of these towers that the ants build.

DAVID HU: Well, each ant is only a couple millimeters tall, but the towers themselves are several inches. So it’s the equivalent of a 30 story building. And they got to find a sort of stem or a twig to brace themselves against. But once they do, the tower on the very top looks tall and pointy, and then as it gets closer to the bottom, it starts billowing out, like the Eiffel Tower, so that it’s got quite a wide base.

IRA FLATOW: So they’re building like a human pyramid, like cheerleaders do?

DAVID HU: Yeah, that’s exactly the right analogy, because the ants, unlike bricks, they actually have limits to how much forces they can take. I was actually in one of these human cheerleading pyramids before, and it’s not so bad to be on the top. You’re just sort of like waving around. But if you’re at the bottom of one of these things, it’s just crushing.

IRA FLATOW: Let me just put out our Twitter. Our Twitter handle @scifi, if you’d like to get in on the conversation about these fire ants. Why do you think the ants do this?

DAVID HU: They only do this in an emergency, when they’re trying to basically escape some kind of container. Or they also do this to anchor onto vegetation. So they are actually evolved in the wetlands of Brazil, this place called the Patanal, which is great for most months of the year. But for the rainy season, it actually floods down to a meter deep.

And so these ants in their underground homes, they start getting wet, and they have to escape, so they build these rafts. Now the rafts would float out to ocean, unless they started anchoring onto things. And so if they find some nearby vegetation, the raft would actually extend a little pseudopod, kind of like an amoeba. And then it’ll grab onto a twig. And then it’s got to morph from this pancake shape into basically building a permanent structure to wait for a few months. And that’s what this ant tower is.

IRA FLATOW: And do all ant colonies do it or just the fire ants?

DAVID HU: There’s about 10,000 species of ants spread across all the continents. And only the army ants and the fire ants, which constitute about 100 species– so a very small fraction– have this ability to pile themselves up on top of each other, or link together, and build large aggregates. The other ants, they can build chemical trails and things, but they can’t really build these large structures, like these fire ants do.

IRA FLATOW: Did EO Wilson ever study this stuff?

DAVID HU: I think he mentioned in his book that he was one of the first to observe the fire ants coming in. And from Alabama, they came on maybe a boat of fruit. But one of the reasons they haven’t been studied before is because it takes them a really long time to build them. I mean, actually if you watch fire ants for hours and hours, it doesn’t seem like they’re doing anything. They’ve got to have the right conditions, and you’ve got to be really patient, and they’ve got to really want to escape.

IRA FLATOW: So who starts the building? Who says, hey, guys, let’s all build some pyramid here?

DAVID HU: Well, it’s like it emerges from all of them looking together. So for example, if they’re in a bin, they know to build these things at the corners of the bin. And I think what happens is that they try at different locations. Maybe one ant says, let’s start something here. And he’ll basically send out some pheromones and will gather two ants, which will gather four ants, and eventually get a couple thousand.

And they’ll try to build on the flat edges of the container, like the wall. But the problem is there’s just not enough places they can get support. So they really build and build, and it might get into something like 10 ants tall, by literally the ants piling themselves on top of each other. But then the whole thing will just–

[SLUMPING SOUND]

It just flops over. So it’s really just a process of trial and error, until they find the right location, which from our experience, turns out to be– they build around twigs that merge vertically or around the corners of a bin, where they can really brace themselves up against.

IRA FLATOW: Yeah, we have a little video on our website showing the fire ants building– I guess, from your lab– like a tube in the middle, and they climb up the tube and form a tower.

DAVID HU: There they’re building around a watering hole. So if they find a source of water, I think maybe they also try to protect it. So they’ll actually pile up, and actually surround, and cover up this pool of water, so that maybe no other insects can access it.

IRA FLATOW: Now I hear that studying these ants has made you the most hated professor in the Biology Department. Is that correct?

[DAVID LAUGHS]

DAVID HU: Yeah, I think there’s a consensus about that, that I’m the most hated professor in Georgia Tech School of Biology. And that’s because– we really do a good job of trying to keep the ants in containers. We’ve really built an ant Alcatraz.

We’ve taken big containers, and this goes beyond your– you mentioned these ant farms that you buy– we buy liquid Teflon– what’s called Fluon. And we coat the rims of all these containers. And so these containers are several inches tall. I mean, it would be impossible for an ant to escape with this Fluon, because they basically can’t grip. They just slide down the walls.

So imagine a container with just completely simply walls that are many, many times your height. You can’t even see the top. So we leave them in there, and things are going fine for one day. And we check on the ant bins every day, and sometimes they’ll build small walls and things like that. But every once in a while there’s a three day weekend, and that means they actually have enough time to assemble their troops.

And so there’s this one Memorial Day weekend. It was last year. And we didn’t check on them for three days. And so what happens is that they’ll eventually get wise, and they’ll find this corner of the bin, and they’ll pile up their entire colony. That’s 100,000 ants. That’s five times the student body of Georgia Tech. And they’ll pile them up, and they’ll build these Eiffel Tower shaped structures.

They do this by just such process of trial and error. If they built them straight up and down, they would crush the individual ants, and they wouldn’t go anywhere. But if they build them in this particular shape– they’re called towers of constant strength– then they can build them basically as high as they want, without the ants hurting themselves.

So they build this at the edge of the structure, and eventually they get to the top. And it looks like a siege tower, basically this pile of ants near the edge of the structure. And it looks like a ramp, and so they pull all their fellow ants out of the container. So it’s 100,000 ants, slowly, but they go out and march out of the container. And then they go down– we have a little shelf. They go down the shelf, and they go down the walls. And then they go down my floor.

And then we’ve got this little crack under the door where the entire 100,000 will march out of the crack, and there’s this unfortunate professor’s office that’s just right across from my lab. And it seems like a good place for them, so they’ll just go across the hall. And 100,000 ants will march under the door.

I think it was probably sometime Monday morning that they started building an ant tower under his desk. And so professors in biology, they wear Birkenstocks, and flip-flops, and things. So this guy comes in, and he goes. No one’s in. The office door is locked. So he goes and sits down at his desk, and his foot touches something soft.

And he looks down, and he sees there’s this huge pile of fire ants right under his desk. And of course he thinks the first thing is I played a terrible practical joke on him, that I basically found the key, and opened his door, and let in all these ants, because they’re just literally sitting there guarding their tower. They’ve got tons of eggs. They’ve got tons of food. They’re really there to settle down.

So he calls me and just curses me for about 15 minutes. And then I said, no, the ants did it themselves, and I’m trying to study it. So I think he finally understood, but I did have to pay for the exterminators to come and remove all the other ants from the rest of the building.

IRA FLATOW: Wow

DAVID HU: And they gave me a warning.

IRA FLATOW: What a story. Wow, I see where your reputation comes from. I have a tweet from Alex, who says “has specific communication chemicals associated with building been discovered?” I guess the pheromones you were talking about, have you discovered what they are?

IRA FLATOW: The ants have about 100 different types of pheromones, some of them for alarm, some of them for basically tracking food. The ones that tell them to gather, it might be similar to– they have little ant cemeteries, where ants actually keep all their bodies in one mass grave. But they don’t plan it like that. They actually put little cemeteries all over the place, and eventually the ants figure out how to gather them all into one place.

It might be a pheromone that attracts them, but it also might be the ants are also attracted to themselves. They like staying together. So it just might be once you get a few of them there, the other ones sort of get a process of what they call positive feedback, where as soon as one ant goes, then another ant goes, then another one, and all of them get there.

We haven’t identified the pheromone. It definitely doesn’t make the tower process happen any faster. It takes almost 20 minutes for them to build every single layer, and eventually it’s 20 stories tall. So it takes them a long amount of time.

IRA FLATOW: Ariel wants to know– Ariel Zych, our education director– is tweeting. She says, do all ants do this or just some? You said 100,000 ants. That must mean a whole colony is headed out the door there. And why some or not others?

DAVID HU: So the tower building is really a group effort. So out of those 100,000 ants, they’ll all be different sizes, too. I mean, the biggest ants will weigh almost 10 times as much the smallest one. And we haven’t seen any kind of coordination. For example, I mean, if ants were cheerleaders, I would want to put the big ones on the bottom.

And this is part of the beauty of the towers themselves. They’re not trying to build it. It’s something that just emerges. They don’t actually know they’re trying to build a tower. They just have these rules they follow, and when ants get there, this thing emerges just from their interactions.

IRA FLATOW: Wow.

DAVID HU: So they don’t know that they’re going to build a tower. They don’t know how tall it’s going to get, and they don’t even know if they’re ever going to stop. So because of all those things, they can’t coordinate– let’s say, the biggest ants go on the bottom, and the lightest smallest ants go on the top. Whatever happens to get there gets there first.

IRA FLATOW: Wow, so you wear two hats as a biologist and engineer. So this engineering feat must be fascinating to you. I mean, not even planning ahead for it and building these things.

DAVID HU: Yeah. What really struck me– I still remember the first time I saw this, these ant towers. And first, I was struck by how tall it was. I mean, I had never seen ants build anything that’s basically– I mean, 20 stories tall is very, very tall for an ant. And the next thing I thought, I was like, this shape, it looks a lot like the Eiffel Tower.

And it struck me. So there’s this old book written in the 1920s by Timoshenko. He was a contemporary of Eiffel. He wanted to basically break the world’s record for the tallest tower, and the ideas of the time were just to use very strong materials and just stack them as tall as possible. But you start to basically– no matter what material you use, you basically hit a limit. For example, if you just stack people on top of each other, eventually, a person just can’t hold. You just can’t support more than three– or for me, one– person on their shoulders for very long periods of time.

And so no matter what material you use, if you make the tower straight up and down, it’s going to be limiting. So what Eiffel thought and what Timoshenko thought, they thought about this ideas of what they called towers of constant strength. And basically, it’s a principle that you can use to build a tower as tall as you want. No matter what material you make it.

And the idea is that basically, for example, like I mentioned, if you’re at the top of the tower, you’re not supporting any load, because you’re not having anyone on top of you. If you’re in the middle of the tower, you’re supporting about half that tower’s weight, which could be very heavy. And if you’re the bottom of the tower, that’s the worst place to be, because you’re supporting the entire way to this thing.

And so instead of making it basically a constant cross section, where each cross section’s supporting more and more weight as you go away from the top, Timoshenko thought, let’s make each cross section bigger. Let’s make it trumpet shaped. So let’s make it wider correspondingly to how much weight we have to support.

IRA FLATOW: Let me just interrupt you there–

DAVID HU: And then so–

IRA FLATOW: I have one, because I have to rudely– and I get criticized soundly for having to interrupt up my cast. This is Science Friday from PRI, Public Radio International. Forgive me. That’s a great story.

DAVID HU: Oh, no problem.

IRA FLATOW: Keep going. You’re up to the trumpet shape.

DAVID HU: So they got this idea of this trumpet shape, to basically make each cross section wider to support the load that it needs to support. And if you do that, you can actually just make this tower as tall as you want, because as it gets heavier, it gets wider and wider.

And so the ants have basically figured this out, but they did it by basically just moving randomly and having this little trick up their sleeve, which is basically as soon as the tower starts assembling shapes that are just simply too heavy in one place, too straight up and down, they just decide to disintegrate it. They create little avalanches. I mean, I don’t hear any little explosions. It actually sounds little more like–

[SLUMPING SOUND]

Because the ants are really spongy. But basically, as they build this thing, you see avalanches here and there. And over a process of just accretion, you get parts of the tower breaking away, until what you have left emerges is this shape that Timoshenko predicted, this Eiffel Tower shape, that allows them to build it to much higher than they would ordinarily be able to build without following this shape rule.

IRA FLATOW: We have a tweet from Hester, who’s doing her master’s work in fire ants. She said, “my fire ants would escape to steal food from neighboring colonies. I lost two data points each time. Very frustrating work for master’s work.” You agree? You seem to–

DAVID HU: Yeah, I think they–

IRA FLATOW: Really enjoy it, though.

DAVID HU: Oh the fire ants escaping?

IRA FLATOW: Well, just the whole work with fire– well, we have another tweet that says– from [? Uday. ?] He says, “what’s being done for the ant accident not to happen at GA Tech?” If you built anything, maybe you put some gauze in there to close off the top.

DAVID HU: Well, here’s the thing. I mean, I also can’t help just rooting for the fire ants, because they’re doing almost impossible tasks. You give them three days, and they’re working 24 hours a day to build this thing. So it’s not that I want them to escape, but– well, I was at a conference. Someone suggested something very innovative that could prevent all escapes forever. They said, why don’t you just put a lid on it?

And I thought about it, but you know the ants needed this 12 hour light-dark cycle. They get unhappy, if they’re all in the dark. And we have to access them regularly, so we just like to have the top open.

IRA FLATOW: Give them a lot of air– sunlight and air. Fascinating, David. Thank you very much for sharing this with us. This is great. The tower of ants, David Hu is associate professor of mechanical engineering and biology. He’s a rambling wreck from Georgia Tech in Atlanta. Thanks again, David. Have a good weekend.

DAVID HU: Thanks a lot, Ira. Take care.

IRA FLATOW: We’re going to take a break, and after, we’re going to come back and talk about a new film that documents the devastation of bleaching coral in overheated ocean waters– why we should be motivated instead of depressed by all that coral dying off, especially in Australia. We’ll talk about it after the break. Stay with us. Don’t go away.

Copyright © 2017 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of ScienceFriday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/