The discovery of a supercolony of ants that extends over more than 3,700 miles of Europe represents the largest cooperative unit ever recorded, scientists say. A report describing the find appears in the current issue of the Proceedings of the National Academy of Sciences.

Since its accidental introduction into Europe, the Argentine ant (see image) has invaded vast areas and become a major pest species. Tatiana Giraud of the University of Lausanne in Switzerland and colleagues collected thousands of workers from 33 ant populations located along the Mediterranean and Atlantic coasts of southern Europe and compared them both to each other and to native South American populations. In addition to the 3,700-mile supercolony, the team also found evidence of a smaller supercolony in Spain's Catalonia region. Whereas ants from within the same supercolony did not fight each other, despite being from different nests, ants from different supercolonies invariably fought, often to death. According to the report, the lack of ecological constraints in the novel European environment led to a high density of Argentine ant nests. This in turn set the stage for the emergence of cooperative--instead of aggressive--behavior between colonies with similar recognition cues. As a result, supercolonies formed.

This unique social structure probably contributed to the ants' success at invading new territory, the authors suggest. But the advantage is fleeting, they conclude, because workers are required to help raise even unrelated offspring. Because selection will most likely eventually reduce this altruistic behavior, the social system may well be doomed to failure.