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Aggression studies reveal the existence of ant supercolonies


Scientists have identified two distinct supercolonies of Argentine ants in Europe. The main supercolony includes millions of nests comprising billions of workers; it extends more than 6,000 kilometers along the coastline of Italy, France, Spain, and Portugal and is the largest cooperative unit ever recorded in nature, according to the researchers.

Argentine ant (Linepithema humile) workers from the two European supercolonies invariably fight with each other. Death of one of the protagonists usually occurs within one minute after the onset of the fight.

The two supercolonies—known as the Catalonian and the main supercolony—were identified based on aggression tests with ants in the laboratory. When workers from different nests were placed in glass jars, individuals of the same supercolony were never aggressive to each other, while the level of aggression between ants of the two different supercolonies was always high.

The fact that members of the same supercolony from distant nests were friendly "is particularly striking because workers came from populations up to 6,000 km apart, which encompass a wide range of environmental conditions," the researchers write in Proceedings of the National Academy of Sciences.

Given the severe aggression typically seen in ants from different nests, the researchers have developed theories to explain the workers' different responses to ants of the same or the different supercolony.

The genetic background of worker ants is a primary factor in determining how aggressive they are to other ants, according to Laurent Keller, of the University of Lausanne, Switzerland, who led the study.

Keller's team has not yet identified specific genes involved in recognizing members of the same supercolony, but that will be a focus of future research. The idea is that differences in the genome may lead to different responses and recognition among related and unrelated ants. The researchers will also evaluate the role of the environment in ant behavior.

The importance of ants to the economy and their unusual social organization make the species a good candidate to be sequenced, says Keller. Since its accidental introduction to Europe, the Argentine ant Linepithema humile has been a successful and pesky invader. It displaces local fauna, protects insects that destroy plants and fruits, and invades homes.

"Although progress in ant genomics is being made, little is still known about the ant genome," says Keller.

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Giraud, T. et al. Evolution of supercolonies: The Argentine ants of southern Europe. PNAS Early Edition. Published online April 16, 2002.

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