European Union for Ants: supercolony reigns from Italy to Portugal. (This Week).

Researchers have documented the largest ant supercolony yet, a network of Argentine ant nests stretching at least 6,000 kilometers across Europe.

It's "the largest cooperative unit ever recorded," says Laurent Keller of the University of Lausanne in Switzerland. He and his colleagues describe the supercolony and a smaller, separate supercolony of the ants in Spain in an upcoming Proceedings of the National Academy of Sciences.

Just how a supercolony forms has long puzzled biologists, Keller says. Back in the ant's native range in and around Argentina, the species, Linepithema humile, doesn't seem cooperative at all. Colonies stay small, and workers clash violently with neighbors of the same species.

However, along much of the California coast, Argentine ants treat each other like family, according to a 2000 study by Nell Tsutsui of the University of California, Davis. "You can take ants from San Diego and drive them to a colony in San Francisco, and they'll act like they've known each other all their lives," says Tsutsui.

He and his coworkers compared genetic diversity in the California ants with diversity in their South American cohorts. At specific test points in the genomes, the California ants showed about half as much diversity in their DNA sequences.

The ants' adoption of a laid-back lifestyle in California could have come from passing through a so-called genetic bottleneck, the researchers proposed. The supercolony may have emerged from just a few far-flung Argentine ants, resulting in enormous populations of ants so genetically similar that even widely separated members respond as if everybody's from the same nest.

To see how Argentine ants fared when invading Europe, Keller and his colleagues collected ants from 33 spots along the coast from northern Italy to northwestern Spain. Out of more than 1,000 staged encounters between ants from various places, the majority ended peacefully--the hallmark of a supercolony. However, certain pairings resulted in lethal fights. The researchers concluded that southern Europe actually hosts two supercolonies of Argentine ants.

"The study confirms that a lot of what we know from North America is going on in Europe," says Tsutsui.

The European study diverges from the North American in its genetic findings. Keller and his colleagues report that European ants are only 28 percent less diverse genetically than those in South America. This reduction in diversity could have helped--but would not have been sufficient to--pave the way for a supercolony, Keller says.

That's why he and his colleagues propose that supercolonies developed through a process they call genetic cleansing. In a new territory away from old enemies, Argentine ants prospered and formed dense clusters of colonies. Genetically similar colonies, which could cooperate, wasted less time fighting and were more successful than the others, until a genetically more compatible supercolony emerged.

"It's an interesting and plausible hypothesis that needs to be tested," Tsutsui says.

Population biologist David Queller from Rice University in Houston says he has some questions about whether genetic cleansing would work. "If it's so advantageous to cooperate, why don't we see it more often?" asks Queller.