Raiding swarms with few rules avoid gridlock. (Ant Traffic Flow).

A novel analysis shows how individual ants' behavior keeps the traffic flowing as 200,000 virtually blind army ants use a single trail to swarm out to a raid and return home with the booty.

The South American army ant Eciton burchelli avoids epic gridlock by forming traffic lanes on its trail, explains Iain D. Couzin of Princeton University. The ants don't follow people's simple stay-to-the-right (or left) paradigm. Instead, they create three lanes--the outer two carrying raiders to the job and the middle one returning them to the nest. This pattern can develop from just the basic behavioral tendencies of individual ants, say Couzin and Nigel Franks of the University of Bristol in England in an upcoming Proceedings of the Royal Society of London B.

This work is the first to examine individual ants' traffic rules, Couzin says. He finds that the system is innately different from the human-traffic patterns that he has modeled. The crucial difference: "Ants are not selfish," he says.

Couzin began his army ant analysis by formulating a mathematical model to describe an individual rushing along a chemically marked trail until it detects a possible obstacle and chooses whether to turn aside. Next, he tuned the model by observing the behavior of real raiders.

In the jungle at Soberania National Park at the Smithsonian Tropical Research Institute in Panama, Couzin and Franks filmed ant raids. During its 10-hour workday, a. ant colony flows across the forest floor catching some 3,000 invertebrates each hour. The swarms flow so densely that the ants' feet make an audible rustle, Couzin says. "I think it's one of the wonders of the natural world," he says.

Plugging measurements of ant movement into the computer model, the researchers found that ants have optimized their tendency to turn aside when encountering a possible obstacle, such as another ant. More sensitivity to collisions would have made the ants cringe and defer so much that they'd never get anywhere, but too little sensitivity would have created hundred-ant pileups.

The scientists also discovered that army ants carrying home their prey don't turn aside as much as ants on the way to work do. The computer model showed that this factor could enable ants to initiate a return lane by pushing into and deflecting the arriving ants.

The ants' three-lane system probably works better for them than a two-lane system would, speculates Couzin. A two-lane system would require a tendency to turn one way more often than another, he says. Such a bias could easily have undesirable effects, such as reducing the raiding party's tendency to forage in certain directions from its nest.

Another ant biologist, Neil Tsutsui of the University of California, Davis, speculates that a three-lane trail provides better defense for the booty lane than two lanes would. Tsutsui praises Couzin and Franks for their "unique and valuable" insights in showing how simple individual behavior can add up to a complex pattern for a whole group.