Adventures Among Ants - Mark W. Moffett [134]
The quiet suburb we were visiting, it turned out, was just one front in a vast war between gigantic Argentine ant empires. Around Escondido and to the west lay the holdings of the Lake Hodges Colony, a kingdom spreading over almost 50 square kilometers. To the north was the dominion of the Very Large Colony, a single society whose territory, stretching almost 1,000 kilometers from the Mexican border to California’s Central Valley and on past San Francisco, boggles the mind. Given that the average Escondido backyard can sustain a million ants, the total population of the Very Large Colony could approach a trillion individuals, with a cumulative weight approximating that of the human residents of Carmel, one of the California cities it occupies.
Little wonder, then, that these Argentine ant republics are called supercolonies.
Argentine ants are unicolonial, which means individuals mix freely among nests, or rather, untold millions of nests. Ants from other colonies or other species, though, are attacked. In disputed areas, combat rages. As the losses build on either side, the boundaries shift. This movement of borders can be as slow as the creep of a glacier.1 The most extreme change recorded was 70 meters over the course of a month, when troops from the Very Large Colony overran the Lake Hodges Colony and seized a portion of scrubby Escondido land. In time, the Lake Hodges army took it back again.
Stepping away from the front lines, Melissa kicked a chunk of wood to reveal a mass of ants and brood. Three tea-colored queens ran to hide below a leaf. As a fourth queen rushed into view, David explained that the supercolonies owe their enormous populations in part to a myrmecological twist: they produce multiple queens that grow wings but travel only on foot, staying in their colony to give birth to still more ants. The California supercolonies contain queens by the millions. Nothing can stop a colony from thriving, growing, and expanding—except clashes with other supercolonies.
COLLIDING KINGDOMS
Until 1997, fighting among Argentine ants was unknown, and many thought the species was in effect one big happy family. That summer, UC San Diego undergraduate Jill Shanahand accidentally incited an Argentine ant skirmish—and so drew back the curtain on their internecine warfare.
Jill was assisting with a project on Argentine ants spearheaded by UCSD graduate student Andrew Suarez. The lab maintained a stock of the ants collected on campus. One day, Jill decided to augment their supply by gathering ants in her parents’ yard in Escondido, where Argentine ants were plentiful. When she put the new ants into a plastic tub containing the ants from campus, she was shocked to see the two groups rip into each other. Were the fights a function of the ants’ captivity, or were the workers clashing for some other reason, most likely because they originated from independent, hostile colonies?
The answer to this question came only in 2004. David had recently become a professor at UCSD, and another Melissa, Melissa Thomas, was his first postdoctoral student. For three months, day after day, she drove around San Diego County looking for a colony perimeter. She had batches of live ants stashed in the back seat, which she planned to mix with others from different sites like a chemist looking for a reaction. If her captive ants from UCSD were attacked by the local ants, it signified they were from a different colony; if they were not molested, it indicated they were from the same colony. Her goal: to use this test for aggression to find where the two colonies came into contact.
Melissa’s task proved more laborious than she had anticipated, given how huge their territories turned out to be. Starting in Jill Shanahand’s parents’ neighborhood, she expanded her search street by street until she eventually found two different supercolonies, each occupying one side of a road—an asphalt no-ant’s land that must greatly reduce the death toll, at least from combat.