The tiny Argentine ants that are a household pest throughout California are evolving to be more aggressive towards ants from other colonies, according to researchers at the University of California, Davis, and UC Berkeley.

Argentine ants (Linepithema humile) have spread from South America around the world, displacing native ants and other insects. At home, they usually form colonies hundreds of yards across. But recently, U.S. and European researchers found giant colonies spanning all of California, and Spain, France and Italy. Ants within these supercolonies recognize and cooperate with each other, even though their home nests may be hundreds of miles apart.

Superior numbers allow the Argentine ants to gang up on bigger insects and also to out-compete them for food and resources.

The ants left behind pests, predators and competitors from their native range when they entered their new environment, allowing them to gain a foothold, said UC Davis researcher Neil Tsutsui. But they have also undergone social and behavioral changes since then.

Tsutsui, with Andrew Suarez at UC Berkeley and Rick Grosberg, professor of evolution and ecology at UC Davis, found that compared to ants from colonies with high diversity, ants from the supercolony were more likely to attack and were more likely to survive the encounter, Tsutsui said.

"They have an advantage if they attack first, like throwing the first punch," he said.

Ants recognize each other by smell, with each colony having its own genetically determined mix of odors. Ants from the California supercolony possess less genetic diversity than other ants and are more likely to identify other ants as "foreigners."

When the ants arrived in California, the small size of the founding population restricted genetic diversity, Tsutsui said. Because increased aggression gives low-diversity colonies an edge, natural selection is promoting aggressive behavior and forcing genetic diversity even lower.

For now, the invading Argentine ants are doing fine. But eventually, without an infusion of new genes, their genetic diversity will get so low that the population will become unsustainable, Tsutsui said.

The work is published in the Jan. 21 issue of Proceedings of the National Academy of Sciences of the USA.