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the Pilgrims who came here in 1620,” says expert Neil Tsutsui.19 Each supercolony therefore contains only a sampling of the genes that existed within the same colony at its home base in Argentina. This is known as the founder effect, which commonly occurs when a population is established in isolation from the rest of its species. It is widely thought that the founder effect explains how Argentine ants form super-colonies overseas: a pilgrim colony might be missing some of the genes that encode labels for colony identity or that are involved in the ants’ ability to discriminate between labels and thereby distinguish colonymate from outsider. By simplifying the factors involved in colony identity, the loss of these genes might reduce the misidentifications that lead to civil unrest within a large population, allowing the invading colony to expand into a supercolony.20

This hypothesis is based on the premise that the complete allegiance of the overseas ants to their huge colonies is a result of genetic differences between supercolonies and their less impressive counterparts in Argentina. But do their much denser populations, vast territories, and capacity to wipe out other ant species indicate a change in the Argentine ants’ behavioral abilities in other parts of the world?

I think not. While the colonies in Argentina are smaller than most supercolonies abroad, close inspection shows that they usually contain numerous queens and nests spread over hundreds of meters, a phenomenal area by any standard. Any of these colonies has the ability to grow to a mighty size, requiring only favorable conditions with no equally matched competitors.21 The Argentine ant’s flexible approach to food and shelter evidently trumps any limitations to its colonies caused by a dearth of genes. We saw that biological success emerges with little species diversity in weaver ants (of which there are only two kinds); Argentine ants go a step further and do surprisingly well with little genetic diversity.

How, then, do independent Argentine ant colonies with their own identity originate? In Argentina as in California, no airborne queen has been recorded. In the absence of mating flights, the intriguing possibility arises that there are no truly new colonies. When Argentine ants bud a nest, it remains part of the original society because all its workers and queens mix freely with residents of the nests from which they emigrated. The only way for another colony to appear at a location is for a fragment of a different colony, complete with queens and workers, to arrive at that spot by jump dispersal. Before people introduced more reliable forms of long-distance transportation, this was possible only by rafting on river debris to new locations, which yielded the intricate patchwork of colonies in Argentina.

All Argentine ants, both in Argentina and abroad, therefore must identify with a limited number of colonies that continue indefinitely and are largely inbred.22 Each of California’s four supercolonies, for example, originated from a different colony in Argentina, with its own social identity. Each is able to associate only with the populations it spins off and its mother colony, and not with the populations derived from any of the other supercolonies. The main reason the Very Large Colony is so very large is that it was first to arrive in California.

SOCIETIES AS SPECIES

It is reasonable, then, to think of California’s four supercolonies as nothing less than the very same societies that invaded the state a century ago. Whereas most ant colonies go through a life cycle similar to that of an organism—being born when a queen rears her first brood and dying when the queen dies—Argentine ant societies are different. They have achieved a kind of immortality. Of course, both the queens and workers in them today are distant descendants of the original founders, much as the cells in our body are replaced many times in a lifetime.23 But unlike the cells in a human being, the lines of descent within a supercolony constitute an ever-expanding body—a superorganism