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Army Ants Obey Traffic Plan to Avoid Jams, Study Says

John Pickrell
for National Geographic News
February 24, 2003
 
Just as a city relies on an efficient transportation network, research shows that vast army ant colonies also employ simple mechanisms to organize traffic flow and mimimize congestion.

According to a new study, some carnivorous ants use just a few simple rules of thumb to determine the direction taken by prey-seeking raiding parties and to organize potentially chaotic forest-floor freeways, packed with up to 200,000 fast-moving workers.

"It's clear that the functioning and success of modern cities is dependent on an efficient transportation system," said Iain D. Couzin, a biologist at Princeton University, New Jersey, and co-author of the new study. "[Therefore] the effective management of traffic is likely to be essential to insect societies," he said.


"When one studies the organization of insect societies, the similarities [to human societies] are often striking, whether we like it or not," commented Madeleine Beekman, who studies bees at the University of Sydney in Australia.

Attack of the Clones

Effective congestion management is especially important for the jungle-living army ants of central and South America, Eciton burchelli, said Couzin. Colonies of E.burchelli can be made up of up to half a million workers or more, he said.

E.Burchelli ants stage colossal raids in search of invertebrate prey. During these raids, up to 200,000 near-blind ants stream out of their nest and form multiple freeway-like trails that are up to 20 meters (65 feet) wide and 100 meters (330 feet) long. In a raid the ants can attack and kill as many as 30,000 prey items.

"These ants can sweep over an area of more than 1,500 square meters [1,800 square yards] in a single day, and devastate the invertebrate fauna to such a degree that the colony has to be nomadic," said Couzin.

Army ants are also unique in constructing bivouac-like nests entirely from their own bodies. A nest is made up of sheets of ants connected by special claws, said Couzin. The strategy is necessary when, during nomadic phases, the colony may move every day for up to twenty days at a time.

Intrigued at the ants' ability to form separate traffic lanes within the foraging freeways, Couzin and his colleague, Nigel R. Franks at Bristol University's Centre for Behavioural Biology in England, designed a computer model to mimic the individual interactions and movements of ants and shed light on their foraging behavior.

The pair then compared the computer data with the real behavior of ant colonies filmed in Panama's Soberania National Park.

The scientists found that simple movement rules, obeyed by each ant, collectively add up to the large-scale movement of the entire raiding party. "Local interactions can have a very large influence on large-scale patterns and behavior," said Couzin.

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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