"The argument by the European group ran like this," Gordon said. "The reason ants in the Mediterranean don't fight is that, although they are genetically diverse, they have lost the diversity in the genes that are involved in recognition and thus don't recognize each other as different. This runs counter to the argument made by the UCSD lab - that California ants don't fight because they're related and therefore don't recognize each other as different."
The European and UCSD groups did agree on one point: The lack of aggression within the supercolony is the main reason Argentine ants have been more successful in the Mediterranean and California than in their native Argentina, where frequent territorial battles have kept the population in check.
But Gordon and her colleagues have found evidence contradicting that neat explanation. Heller, for example, has documented instances of both cooperation and combat among colonies in California. And in Argentina - where fighting prevails, according to the UCSD group - Heller observed widespread cooperation among densely populated colonies. Her findings raised questions about the hypothesis that aggressive ants have managed to keep Argentina's native population in check.
"So it's not simply a matter of native-versus-introduced or genetic diversity," Ingram said. "There are also ecological factors contributing to the ants' success."
Gordon agreed, pointing to an experiment by North Carolina State University in which researchers were able to turn nonaggressive nestmates into combatants simply by changing their diet.