If you have ever observed ants marching in and out of a nest, you might have been reminded of a highway buzzing with traffic. To Iain Couzin, such a comparison is a cruel insult - to the ants.
Americans spend a 3.7 billion hours a year in congested traffic. But you will never see ants stuck in gridlock.
Army ants, which Couzin has spent much time observing in Panama, are particularly good at moving in swarms. If they have to travel over a depression in the ground, they erect bridges so that they can proceed as quickly as possible.
"They build the bridges with their living bodies," said Couzin, a mathematical biologist at Princeton University and the University of Oxford. "They build them up if they're required, and they dissolve if they're not being used."
The reason may be that the ants have had a lot more time to adapt to living in big groups. "We haven't evolved in the societies we currently live in," Couzin said.
By studying army ants - as well as birds, fish, locusts and other swarming animals - Couzin and his colleagues are starting to discover simple rules that allow swarms to work so well. Those rules allow thousands of relatively simple animals to form a collective brain able to make decisions and move like a single organism.
Deciphering those rules is a big challenge, however, because the behavior of swarms emerges unpredictably from the actions of thousands or millions of individuals.
"No matter how much you look at an individual army ant," Couzin said, "you will never get a sense that when you put 1.5 million of them together, they form these bridges and columns. You just cannot know that."
To get a sense of swarms, Couzin builds computer models of virtual swarms. Each model contains thousands of individual agents, which he can program to follow a few simple rules. To decide what those rules ought to be, he and his colleagues head out to jungles, deserts or oceans to observe animals in action.
What Couzin wanted to know was why army ants do not move to and from their colony in a mad, disorganized scramble. To find out, he built a computer model based on some basic ant biology. Each simulated ant laid down a chemical marker that attracted other ants while the marker was still fresh. Each ant could also sweep the air with its antennas; if it made contact with another ant, it turned away and slowed down to avoid a collision.
Couzin analyzed how the ants behaved when he tweaked their behavior. If the ants turned away too quickly from oncoming insects, they lost the scent of their trail. If they did not turn fast enough, they ground to a halt and forced ants behind them to slow down. Couzin found that a narrow range of behavior allowed ants to move as a group as quickly as possible.
It turned out that these optimal ants also spontaneously formed highways. If the ants going in one direction happened to become dense, their chemical trails attracted more ants headed the same way. This feedback caused the ants to form a single packed column. The ants going the other direction turned away from the oncoming traffic and formed flanking lanes.
To test this model, Couzin and Nigel Franks, an ant expert at the University of Bristol in England, filmed a trail of army ants in Panama. Back in England, they went through the film frame by frame, analyzing the movements of 226 ants. "Everything in the ant world is happening at such a high tempo it was very difficult to see," Couzin said.