How ants avoid traffic jams

Why does insect congestion not lead to a hold-up, like it does for humans?
20 December 2019

Interview with 

Laure-Anne Poissonnier, University of Toulouse




The incredible thing about ants is that, despite running along congested roads and paths like we do, they never seem to get stuck in traffic jams. And, as she explains to Chris Smith, Laure-Anne Poissonnier, who’s at the University of Toulouse, wanted to know why…

Laure-Anne - I've observed social insects - bees and ants - since I was little. I always find it fascinating how they can be so organised because they don't have any signs, traffic lights or any police officers telling them what to do. When you see ants in the wild, you never observe a traffic jam. Yeah. We're wondering if that was really the case, and we wanted to test that.

Chris - And how much - before you did this very comprehensive study - how much did we know about ant traffic?

Laure-Anne - Actually, there was really little data. It's very time consuming to do these kinds of experiments and counting ants on a trail takes a lot of time.

Chris - Because it's fair to say that, apart from us humans, there are not many other species - maybe termites - that travel in both directions on roads?

Laure-Anne - Yeah, exactly. We share with ants and termites the fact that we live in one place and we get our food in another place and then we bring it back home.

Chris - But unlike us, where, given my experience of my commute, I don't know what yours is like in Toulouse, but mine's turning into a nightmare on a daily basis, ants don't seem to have this problem. You, you add more and more ants to the equation and they just seem to find a way and carry on moving.

Laure-Anne - Hmm. There are definitely way better than we are. They do get slowed down a little bit when there is a lot of ants, but that's a very limited effect compared to what we can observe with pedestrian traffic or, or cars. And one of the reason is because they have a hard body so they can bump into each other and it doesn't hurt them, contrary it to a car or us for example, we don't want to bump into each other. So we slow down and we avoid each other. So we lose time, and ants don't have to do that. And some other reason is that they can regain their maximum speed very quickly while for a car for example, you have to accelerate again and it takes some time to go back to the maximum speed and ants do that in one or two steps. They compensate for the time they have lost. When ants see another ant's coming from the opposite direction. They will stop and interact with this ant because they can, get information they can check if it's not an invader or another ant coming to attack them. So they do a, they touch antennae and they smell for example, if it does find some nice food there. And when there is a lot of ants, they shorten those interactions. So they do it way quicker and also accelerate between each interaction to regain the time they've lost. And the last thing when they see that there is a lot of ants already on the trail, they stop going there, which is what we should probably do. When we see that there's already a lot of cars, if we go, which is going to create a traffic jam which should just Wait rather than go there.

Chris - How did you observe this?

Laure-Anne - We used different colony sizes with really small density all the way across to very high densities. So just leaving the ants in their little nests in the lab, we linked the nests to a platform where we're giving them food, so we use the sugar water because those ants - we used Argentine ants and they really like sugar - they had to cross a bridge to go there and what we did is that we used colonies from 400 ants to the maximum group we had was 25,600 ants. And also we varied the width of the bridge to make them more crowded

Chris - And what you were counting them on and off the bridge so you knew roughly what the traffic flow was?

Laure-Anne - Exactly. I counted the ants crossing the bridge.

Chris - Did you have electronic help? Because that sounds like a nightmare when there's 25,000 ants. How on earth did you keep track of them?

Laure-Anne - That was a tedious work. The number of ants that are crossing the bridge I counted that by hand. So I had basically a help available software. So I was clicking every time an ant was crossing and it was recording the time and the direction of the ant. And then we we use image analysis to know how many ants were on the beach. So that was done by the computer.

Chris - Can you express any of these relationships mathematically? So are there now ways in which you can say you can model this effectively and show why it's good for ants and why it's not good for humans and therefore what we might be able to do to improve how humans get around?

Laure-Anne - There's a lot of difference between ants and humans, but we can compare things. For example, what is usually used to study the flow is to link the density. So the number of individuals or ants on a surface with the speed. So usually at the beginnings a flow increases with density because at the beginning there is not many individuals. So the flow is low, but they all, they can all go fast. And then after a while, when there is a lot of individuals, they all slow each other down. And so the flow decreases at very high density. So it gives a sort of a bell curve.

Chris - Given that it is Christmas time, am I allowed to try a cracker joke on you?

Laure-Anne - Please go ahead!

Chris - How do you tell the difference between a male and a female ant?

Laure-Anne - Oh they're all females. I just ruined the joke!

Chris - Well, not necessarily. The answer is you drop it in water. If it's male, it floats: "boy ant".

Laure-Anne - Ha ha! And do you know why ants never get sick?

Chris - It's gotta be something like "antibiotics" or "antibodies" or something. Is it?

Laure-Anne - Yeah - they have a lot of antibodies!


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