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.

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