The largest insect colony in the world has been discovered in southern Europe. From the Atlantic coast of Spain to Mediterranean Italy, invading Argentine ants recognize and treat each other as if they were from the same nest, despite being unrelated. The new study sharpens a debate over how such cooperation arose.
Since humans introduced Argentine ants to Europe at the beginning of the 20th century, the critters have expanded to a range of 6000 kilometers. Like their cousins in California, the European invaders form an unusual supercolony, with workers moving freely between nests and gathering food for unrelated queens. Genetic studies of Argentine ants in California suggested that cooperation arose by accident, when the introduced population passed through a genetic bottleneck: a loss of half its genetic variation, including the variation in genes that help ants recognize friend and foe.
To measure the size of the European colony and test the bottleneck theory, ecologist Laurent Keller and colleagues at the University of Lausanne in Switzerland captured ants from 33 places across coastal southern Europe and performed behavioral and genetic tests. First, they put pairs of ants from different locations into a small vial and checked for signs of aggression. Ants from 30 locations across Europe showed no aggression (although they fought with ants from a splinter supercolony in eastern Spain).
Second, the researchers compared 22 genetic markers known as microsatellites with a population from Argentina and found that the European populations had lost less than 30% of their genetic variation. That loss is too small to explain such a large supercolony, they suggest in the 16 April Proceedings of the National Academy of Sciences; more likely, cooperation evolved in Europe because it was beneficial to all Argentine ants, at least while they were expanding into a new environment.
Kenneth Ross, an entomologist at the University of Georgia, calls the data "unassailable" but the interpretation still uncertain. "What we need now is actually to know more about the genes involved in recognition, and that is a huge black box," he says.