The ants go marching -- and manage to avoid traffic jams

By Steven Schultz

Princeton biologist Iain Couzin has found a society in which hundreds of thousands of travelers speed along densely packed roadways, transporting huge amounts of materials, all without a bit of congestion.

Iain Couzin
 
  
If they don't sound like the kind of drivers you're used to dealing with, they aren't. Couzin, a postdoctoral researcher in ecology and evolutionary biology, found this paragon of efficiency in colonies of army ants in Panama.

In a recently published study, Couzin and Nigel Franks of the University of Bristol in the United Kingdom, showed that these ants have evolved a three-lane traffic system that puts snarled human highways to shame. The traffic system achieves the maximum possible flow as 200,000 ants pour out of their nest on giant hunting raids and return with more than 30,000 pieces of slain prey in a day.

The study, a unique combination of mathematical analysis and field observations, is part of a broad effort to understand the highly coordinated movements of all sorts of organisms, whether flocks of birds, schools of fish or even groups of single cells. "The collective properties of these groups are really just stunning," said Couzin.

"I am very interested in self-organization," said Couzin, "how relatively local interactions can scale up to large-scale and often complex collective behaviors."

A sensible pattern

In the case of the army ants, scientists had previously observed that ants returning to the nest, with their booty of slain grasshoppers and other insects, form a center lane, while outgoing ants split into lanes on either side. The practical benefits of this arrangement and the behaviors that give rise to it, however, were completely unknown.

   Panamanian army ants travel in a three-lane system in which ants returning to the nest occupy the center lane (shown in black) and outbound ants (gray) form lanes on either side. Iain Couzin, a postdoctoral researcher in ecology and evolutionary biology, used computer simulations to show that this pattern minimizes congestion and maximizes the flow of slain prey back to the nest. The image shows the movements of real ants in a videotape along a 12-centimeter length of ant highway. The dots represent ant positions at intervals of a 50th of a second.
 
In their paper, published in the Jan. 22 issue of the Proceedings of the Royal Society of London, Series B, Couzin and Franks found that a key factor is the angle at which the ants turn aside when they collide with oncoming nestmates. Outbound ants tend to turn relatively sharply, while returning ants stay on a straighter line, thereby cutting a center path between two outgoing lanes. This arrangement seems to make sense, since incoming ants are likely to be less maneuverable due to their heavy cargoes, said Couzin.

Ants stick to this pattern, however, even when the incoming ones have nothing to carry. The reason, concluded Couzin, is that it is simply an efficient way to travel. He devised a computer simulation of an ant highway and watched how traffic went faster or slower as he adjusted the turning angles of his virtual ants. He achieved the swiftest flow at almost exactly the turning angles observed in real ants, suggesting that ants have naturally evolved to near-optimum efficiency. He also noted that the three-lane structure emerged only when the turning angles were in their optimum range.

"This study is outstanding in the level of detail with which they make the linkage between the individual and the group," said Thomas Seeley, professor of neurobiology and behavior at Cornell University. Up until now, said Seeley, scientists had written equations to try to describe an average behavior for many animals. In Couzin's computer simulation, individual virtual ants are assigned just a few simple behavioral rules, from which emerge the complex behaviors seen in real ants.

"It's one thing to say that in words, but it's another thing to demonstrate it in a rigorous way in a simulation. That is, in a word, elegant," said Seeley.

Meticulous field studies

These results, said Couzin, would be impossible without computers that perform the vast number of calculations necessary to simulate many organisms at once. The study of animal behavior "is like physics a century ago," he said. "We're just beginning to get the tools and the techniques that we need to tease apart how these systems interact."

The work also depends on meticulous field studies. Couzin developed computer vision software that tracks individual ants in his videotapes. In some cases, he stuck tiny letters, printed on a standard laser printer, to the backs of ants and used character recognition software to track them. The observations tell him how to program his computer simulations to accurately reflect the real world.

In addition to answering the broader questions about self-organization, understanding the ants themselves is important because they play a major role in their local ecology, said Couzin. Living in million-member colonies, the ants swarm at the base of a tree and send out raiding parties that systematically scour the jungle within 100 meters of the base tree. Many species of birds follow the raiding parties, feasting on insects flushed out by the advancing front. Parasitic flies lay eggs in escaping insects.

"They really are a keystone species and contribute enormously to the diversity of the rain forests," Couzin said.

At an entirely different level, the research also is of great interest to robotics engineers. Couzin recently began working with Naomi Leonard, Princeton associate professor of mechanical and aerospace engineering, who is developing schools of autonomous underwater vehicles that coordinate their actions with no central leadership. These vehicles could be used to gather data on ocean currents and ecology or to monitor or clean up pollution.

"We're hoping to build on this link between departments," said Leonard, who also has a student with interests in biocomplexity. "We get so much inspiration about what they learn from biology."

Applications for other systems, but not human traffic

Couzin also believes the work could apply to other transportation systems, including vascular systems of humans and plants, which form branched structures that are remarkably similar to the tree-shaped path charted by raiding ants. One area where it might not apply very well, though, is human traffic.

The ants -- which are sterile and have evolved to promote the group's, rather than their own, productivity -- are very consistent in all their behaviors. Humans, who have lived in dense populations only recently, did not evolve finely tuned ways to coordinate their movements en masse.

"Unlike the ants, it's extremely difficult for people to come to a consensus about how they are going to drive and stick to that," said Couzin, "because someone will always be able to cheat the system. As soon as the system can be infiltrated by cheaters, then it's not an evolutionarily stable strategy."

Couzin, who received his Ph.D. from the University of Bath in the United Kingdom, came to Princeton in 2002. For the next three years, he will have a joint appointment at Princeton and Oxford and plans to conduct studies of swarming locusts in Morocco. *

Panamanian army ants travel in a three-lane system in which ants returning to the nest occupy the center lane (shown in black) and outbound ants (gray) form lanes on either side. Iain Couzin, a postdoctoral researcher in ecology and evolutionary biology, used computer simulations to show that this pattern minimizes congestion and maximizes the flow of slain prey back to the nest. The image shows the movements of real ants in a videotape along a 12-centimeter length of ant highway. The dots represent ant positions at intervals of a 50th of a second.

 
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March 31, 2003
Vol. 92, No. 21
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Contents

Page one
Greenstein: Bush a 'less sure-footed' leader in Iraq war, despite his post-Sept. 11 growth
Princeton scientists have high hopes for hydrogen

Inside
United Way campaign conclusion celebrated
The ants go marching -- and manage to avoid traffic jams
Study in Chile whets appetite for return visit under Dale award

People
Spotlight
LeMenager moves to campus life
Briefs

Sections
Calendar of events
Nassau Notes
By the numbers: The Papers of Thos Jefferson


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