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By Root 572 0

of labor, and the redundancy afforded by large ant societies helps make specialization pay off, as it does for large human groups and complex organisms. With humans in, say, a small military squadron, the loss of the one person who knows how to radio headquarters could be devastating; in small groups it therefore pays for everyone to be a generalist, with overlapping knowledge and skills. Larger divisions can afford to include more specialists, among them helicopter pilots, tank drivers, and snipers. Large ant societies can similarly produce more specialists. Consider the outrageous polymorphism of the marauder ant: by having workers that span the size distribution of many of their competitors combined, they may be able to outperform them all the more effectively.

We don’t know why, given a certain outlay of resources, one ant species produces only large workers, another produces only small ones, and a third—the weaver ant—produces a mixed population, biased toward the majors. Yet so omnipresent are the major workers, so complete their concerted action when they sense a person, that I have often felt, on walks in Africa or Asia, as if some predator were spying on me from the trees—only then to hear on all sides a muted drumming of alarm, similar to the sound of peas dropping onto a plate, as one study puts it.42 That is the collective sound of a superorganism, generated by the crowds of agitated weaver ants striking their leaf roosts.

Amazon ants returning to their home nest at the conclusion of a slave raid near Lake Tahoe, California. One of their gray slaves aids them by carrying a pupa.

12 slaves of sagehen creek

A thousand orange, pumpkin seed–sized ants raced across the earth so quickly they seemed to fly over the stones. They were arranged in a phalanx 4 to 5 meters long and 25 centimeters wide, exhibiting the orderliness of a military parade conducted at a full run. I followed the raid’s almost arrow-straight course up an embankment and onto sandy ground dotted with rock and scraggly patches of sage, huckleberry oak, and powdery-leaved mule’s ears. There, the column began to disintegrate. My companion, Alex Wild, then a graduate student at the University of California at Davis, warned me to watch closely. Ceasing their coordinated advance, the workers spread out over a couple of square meters in an area occupied by the ant Formica argentea and started scouring every hole and crevice for the Formica nest. After five minutes of frenetic searching, they began to emerge from alongside a stone, hefting silky objects—pupae stolen from the Formica.

In their staging of concerted attacks on other ant colonies, the orange ants resemble a species unrelated to it, the army ants, but with a critical difference: this species, Polyergus breviceps, does not eat the plundered ant brood; rather, it keeps the young alive to raise as slaves.

It isn’t necessary to mount expeditions to the remote tropics to see dramatic behavior in ants. Slavery, curiously enough, is known only for species living in the temperate zones. Polyergus breviceps is one of five Polyergus species that jointly range across North America, Europe, Russia, and Japan.1 All of them enslave ants belonging to the related genus Formica. The slavemaking ants of this group are commonly known as Amazon ants, after the mythical female warriors who were said to steal children and make them their own.

Orange-colored Amazons continued to pour in and out of the black fissure next to the stone. A few centimeters away, soil flew as a pugnacious pack of them used their forelegs to dig at another entryway. In the face of the onslaught, the Formica had blockaded this backup passage with soil transplanted from deeper in the nest—an approach to survival I have witnessed also among the recalcitrant victims of army ant sieges.

All Alex and I could see on the surface was the coming and going of Amazon workers and the occasional Formica walking by without putting up a fight. Curious, I overturned the stone, exposing the nest galleries. The only sign of struggle was a