Cooperation is the key to the evolution of life, even though natural selection favors genetic selfishness. Since all complex life, whether it’s a tree or a whale, reproduces through sex, genes have only a 50 percent chance of getting into an egg or a sperm. This means that even genes have to cooperate, because any gene that cheated and found itself a greater than 50 percent chance of success would prosper — and sex and complex life wouldn’t work.

Without such genetic cooperation, the only life on earth would be simple, single-celled bacteria.

Animals in groups must also work together to build and run the societies in which they live. In an ant nest, for example, there may be millions of workers, foraging, building and maintaining the nest and defending it against predators and other ants. An ant colony is often referred to as a superorganism because it is composed of a single individual who reproduces, the queen, and thousands or even millions of other ants, all sterile, working to raise the queen’s offspring. An ant colony is, in effect, a single cooperative unit, albeit one of huge size.

But cooperation can be more successfull still.

In southern Europe there is a “supercolony” of ants covering 6,000 km, from Italy to France and on to the Spanish Atlantic coast. It comprises millions of nests and billions of ants. It forms the largest cooperative unit ever recorded.

The species is the Argentine ant, inadvertently introduced into Europe from South America. It has displaced native species, led to the destruction of plants and has even invaded houses.

When most ants encounter ants from a different nest, they immediately engage them in combat, even if that ant is a member of the same species. Soldier ants have lethal jaws for biting off the heads and legs of their opponents and are equipped with tanks of toxic chemicals that they spray on the enemy. But the Argentine ant freely mixes with other Argentine ants from different nests. They fight other ant species, but work with members of the same species, even helping to raise ants in foreign nests. This worried researchers because it seemed to contradict kin selection theory.

Kin selection explains, from a gene point of view, why some individuals, such as ant workers, help others to reproduce instead of doing so themselves. Since the ant queen is the mother of all the ants in her nest, by helping their mother reproduce they are helping to pass on their own genes. Argentine ants wandering into another nest altruistically help raise another queen’s offspring.

Now researchers led by Laurent Keller from the University of Lausanne, Switzerland, think they have found out why. Their paper was published in Tuesday’s Proceedings of the National Academy of Sciences.

In its native habitats in South America, the Argentine ant behaves more like regular ants — it fights ants from other nests when it encounters them. But when its invasion force landed in Europe it was able — like many pest species — to quickly spread over vast areas. It established nests at far higher densities than in its native habitats.

“They create a lot of problems,” said Keller, in an e-mail interview. “They eradicate a large proportion of the arthropods and over 90 percent of the native ant fauna. They really disturb the habitat and lead to a major decrease in biodiversity.”

At such high densities, it was better to cooperate than to fight, and a “supercolony” of nonaggressive ants began to emerge. The genes enabling the ants to recognize ants from other colonies were gradually lost — there was “genetic cleansing” of this ability.

Although they continued to fight European ants, they didn’t fight among themselves, enabling them to overrun most of the Mediterranean and achieve incredible ecological dominance.

The scientists took Argentine ants from nests thousands of kilometers apart and put them together in an arena in the lab. They staged the scientific equivalent of dog fights. The ants, however, didn’t recognize each other as enemies and showed no aggression.

“We knew for quite a while that there was no aggression on a relatively large scale,” said Keller, “but it was only when we collected ants over all Europe that we realized that the supercolony spread for thousands of kilometers.”

They did find a smaller supercolony, however, in Catalonia, in Spain. Keller and colleagues put ants from the two supercolonies together in the arena and found that they quickly fought, often to the death. Argentine ants in Europe, in fact, form two supercolonies — and only ants from different supercolonies recognize the other as an enemy.

But despite the almost complete dominance of Europe, the supercolony system is thought to be unstable. Since workers are raising offspring that they are not related to, they are vulnerable to replacement by a selfish mutant worker that regained the ability to recognize its kin. If such mutants could get a foothold, aggression would increase and the supercolony might even collapse. This would be good news for native European species that have been decimated by the Argentine ants.

“But density is now so high that the mutant would have to fight the whole supercolony to get established,” said Keller. “New mutants are thus unlikely to to spread and become established.”

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