At peace with itself, an ant triumphs.

Argentine ants are marching relentlessly around the globe in part because they have given up scrapping among themselves, suggest California researchers.

These ants, Linepithema humile, reap big benefits from a lets-just-get-along style, say David A. Holway and his colleagues at the University of California, San Diego. In the Oct. 30 SCIENCE, they report that peaceful laboratory colonies of Argentine ants spent at least twice as much time foraging and produced almost three times as many offspring as warlike colonies.

That peace dividend by itself may not entirely explain why the species has burst out of South America during the past century and spread as far as Australia. However, "it's a contributing factor," argues Holway.

At first glance, Argentine ants seem unlikely world conquerers. "They're 2 millimeters long, they're brownish, and they're not very exciting-looking for an ant," Holway admits. Yet in many countries, they crowd out native ants and disrupt the lives of other creatures (SN: 8/23/97, p. 116).

In studying the Argentines' success, Holway and his colleagues tested aggression between colonies. When the researchers paired individuals from different colonies, ants collected in their original South American range often fought. "They lunge at each other; they form a ball, and in a few minutes, they're both dead," Holway explains. However, most Argentine ants collected in California remained peaceable when confronting a stranger from a distant nest.

The researchers then used California ants to create 22 pairs of colonies of identical size. Most colony pairs came from nests that tolerate each other, but some came from the rare nests known to fight each other. After 70 days, ants without same-species aggression had bigger colonies. The researchers believe that those superior numbers are the key to the ants' expansion, Holway says.

He's not the first to note the ants' neighborly manners. For article on altruism, TRENDS IN ECOLOGY AND EVOLUTION ran a cover in March 1997 showing a French Argentine ant queen posing amicably beside a Swiss one, with their nation's flags painted on their bodies.

Behavioral ecologist Deborah M. Gordon at Stanford University says the beneficial effect of low aggression is "an idea that should be pursued, but there are other ideas that should be pursued, too." For example, in California, whichever ants find food usually keep it, and latecomers just back away. "Generally, the Argentine ants tend to get there first," she says.

Phil Ward, an ant biologist at the University of California, Davis, finds decreased aggression a plausible factor in takeover success. Once they have high numbers, Argentine ants can "both find food well and defend it well," he says. Also, the United States offers the ants freedom from Argentine parasitic flies.

Being neighborly is hardly the only path to world domination, Walter R. Tschinkel of Florida State University in Tallahassee points out. Many fire ant colonies rip apart strangers within minutes, he says, yet their spread through the southern United States isn't going badly at all.