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For Argentine Ants in Europe, Life Is a Picnic

Biology: Mediterranean supercolony has grown massive by getting along. In their native land, the insects fight.


The massive supercolony of pesky Argentine ants that stretches across California has been trumped: A newly discovered supercolony in Europe is nearly three times as long, spanning the Mediterranean coast from Portugal to Italy.

Like the California supercolony, the European counterpart appears to be so successful because the ants behave so strangely: Residents from different nests cooperate and share instead of fighting one another to the death as Argentine ants do back home.

Scientists on both continents are having a difficult time explaining just why these "tramp ants" are so nice to each other. The amiable behavior among distant strangers contradicts a basic idea of evolutionary biology called kin selection theory--the notion that gallant behavior should persist only among related individuals because helping your kin helps perpetuate the genes you share.

In Argentina, typical ant behavior--defending boundaries, ripping off the legs and heads of enemies and spurting out toxic chemicals--leaves piles of wartime casualties and colonies about the size of suburban lots.

The mellower Californian and European transplants by contrast share what amounts to a peace dividend. Instead of using resources for war, the peaceniks spend time looking for food and nurturing teeming numbers of young. The result: massive supercolonies of ants that don't fight even though they are unrelated.

"It's clear that in Europe the same thing is happening that's happening here," said Neil D. Tsutsui, a postdoctoral researcher at UC San Diego who published a paper with UCSD colleagues two years ago showing that a huge empire of Argentine ants stretches across California from Mexico to Oregon.

Unwilling to give up the mantle of largest supercolony just yet, Tsutsui said it is possible that the California supercolony is larger than reported and stretches outside his study area, deep into Mexico and north into the Oregon woods.

Argentine ants in Hawaii may also be members of the California colony that hitched rides to the islands in plants or the camping equipment of tourists, he said. In this era of globalization, it is possible that European and California ants are members of the same globe-spanning supercolony.

Researchers determine if ants from different areas are part of the same supercolony by pairing them in miniature gladiatorial bouts in the lab. Members of the same supercolony occasionally tap each other with their antennae but tend to peacefully coexist. Members from different colonies, however, rise up on their rear legs, spray venom and bite each other to shreds.

The huge numbers of Argentine ants in California--too many even for scientists to estimate--swamp the armies of larger native ants, kill rival queens, take over nest sites and munch on all available food. Their success has meant trouble for coastal horned lizards, which are starving as their accustomed food--native ant species--disappears. The lizards won't make a meal of the bitter-tasting Argentines.

In his paper two years ago, Tsutsui, who calls the success of the invading ants "testimony to the power of cooperation," had proposed that the amiability of local ants was due to their lack of genetic diversity. Some of their genes are so similar, he suggests, that they treat each other like kin even though they are not related.

Such "genetic bottlenecks" often result when a small number of individuals establishes a new population far from home. It is thought that Argentine ants reached the U.S. in small waves in the late 1800s aboard coffee and sugar ships from South America. But the new research, led by biologist Laurent Keller of the University of Lausanne in Switzerland, suggests a lack of genetic diversity is not enough to explain the ants' communal behavior.

The European ants still have a fair amount of genetic diversity. Keller's research suggests that the unusual amount of cooperation among them comes about because natural selection has favored those less likely to fight. Over time, he suggests in a paper published in today's Proceedings of the National Academy of Sciences, this resulted in a kind of "cleansing" of the recognition cues ants normally use to tell friend from foe.

But the normally peaceful ants will still fight if confronted with ants from different colonies. That, researchers say, shows that genetic changes have not hampered the insects' natural aggressiveness or ability to fight. Instead, evolution has altered their ability to recognize enemies.

"What's new is the suggestion that selection acts to winnow down variation to minimize aggression between colonies," said Ken Ross, an expert on genetics and ant social systems at the University of Georgia.

Keller's research provides strong evidence that the aggressive behavior of ants is genetically and not environmentally determined. Lab-born ants behave the same way as those from their home nests, he said, suggesting that the aggressive behaviors are inherited.

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