Army Ants Obey Traffic Plan to Avoid Jams, Study Says

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One of the rules obeyed by ants is that each blind forager instinctively turns away from other ants approaching it in the opposite direction.

In order to determine how a phalanx of foragers leaving the nest chooses a direction in which to raid, the pair looked to another behavioral pattern known as a circular mill. When ants are separated off from the main colony—in the laboratory or under exceptionally heavy rainfall in the wild—they often form a milling circle, trailing around in the same direction.

The computer model revealed that the rotation direction is mostly determined by chance. As more and more ants move in one random direction, it becomes increasingly difficult for other ants to go against the flow, as they collide with ants moving in the opposite direction, and are forced to turn around. This behavior#151;whereby ants eventually are forced to move in the same direction—may explain how the raiding party decides on a direction in which to hunt, said Couzin.

The Big Picture

"Many complex and seemingly organized group behaviors…have been shown to emerge from the collective action of individuals that do not have an understanding of the big picture," commented Martin Burd who studies evolution and behavior at Monash University in Melbourne Australia.

Another simple rule is that ants follow a trail of smelly chemicals, laid down by other ants. Like painted stripes on a road, these chemicals tell the poorly-sighted foragers which way to go.

When raiders set out, they move along the chemical trails at high speed in one direction. However, as they encounter prey, they must return along the freeway to the nest. This task is initially very difficult with an onslaught of speeding traffic coming in the opposite direction.

Couzin and Franks found that a simple difference in the rate at which returning ants are prepared to turn away to avoid head on collisions is enough to order ant-freeways into three efficiently organized traffic lanes.

All the ants instinctively prefer to be at the center of the trail, where the strongest marker fragrance can be found. However, as returning ants—burdened with invertebrate cargo—are less likely to turn to avoid a collision, a stream of these foragers end up forming the central lane of the freeway.

Outbound ants, which more rapidly dodge to avoid collisions, end up forming two lanes on either side of the homeward-bound trail.

This research "shows how simple responses to local information, allows organized traffic lanes to form, instead of a helter-skelter mob of aimless ants…no traffic cops, no road maps, no ministry of transportation," said Burd.

The emergence of "self-organized" patterns has been shown to be a general pattern in many other species of social insects, such as termites and bees, agreed David Sumpter a mathematician at Oxford University in England.

The ability to form congestion-minimizing traffic lanes in E.burchelli has probably evolved due to the great time constraints imposed on raiding parties, said Couzin. Raiding parties leave at dawn and must return by dusk, when the colony emigrates.

The findings were recently detailed in the journal Proceedings of the Royal Society B.

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