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By Root 454 0

back and forth within it to ascertain its volume and condition.15 Ants may choose living spaces in a similar manner. We don’t know exactly how they do it, but one plausible theory is that a scout assesses the area of a potential nest by laying a segment of pheromone trail and wandering on. The larger the space, the less often she will bump into her own trail again. The method driver ants use to gather intelligence on nest suitability is especially mysterious, given how voluminous their cavities must be to accommodate millions of ants linked together like curtains of chain mail, a great way to pack a huge population into a compact, but still substantial, volume. Sometimes they make errors, as Stefanie wrote me. “I once watched an E. burchellii emigration in Trinidad. They moved into a cavity under a rock which was obviously too small. In the middle of the emigration the ants appeared to ‘realize’ this and frantically started to search for new bivouac sites. They would start new bivouacs (i.e., attaching themselves and forming small balls of workers) almost everywhere—including under my boot!”

From studies of other ant species, we know that the more attractive a site, the more quickly and easily a scout will be able to recruit nestmates. She gauges the site’s desirability from her estimate of its size and from such other factors as the width of its entrance, its height versus its breadth, and the amount of light entering the space—a detailed evaluation that would seem to require some rudimentary cognition. With multiple workers gathering these kinds of data from different places, after a while more ants are recruited to superior locations, with each new arrival confirming the site’s virtues for herself, or following another worker to a different location if she’s unimpressed. The final choice comes about by quorum sensing, a kind of decision by voter turnout. When the ants recognize that enough nestmates have already accumulated at one particular site, the migration to the chosen real estate begins.16 Surprisingly, this voting behavior is not known to take place within organisms, or at least not healthy ones. It is thought that cancer cells may use it to regulate their interactions.17

THE SOLITARY ARMY ANT

There by the kitchen washbasin in Nigeria, what struck me about the subs was that they seemed to dine night and day, not on live prey, but on debris and human castoffs. Driver ants will devour some refuse, but only as a supplement to their regular diet, and only for a short time. At the field station, rubellus driver ants had consumed watermelon chunks in meals that lasted several hours. On another occasion, Caspar and I came upon a forest encampment; probably erected by poachers hunting bush meat, it was little more than sticks lashed with vines to form a primitive shelter. The poachers were off somewhere, but rubellus were massed thickly on their dishware and feasted on cooking-oil residue for a couple of hours.

By contrast, the subs at the research station fed at the washbasin for more than two weeks. They must have maintained stable underground trunk trails to reach the basin, much as did Stefanie’s Malay subs, but they emerged from a number of exit holes in frequently changing columns to access oily food scraps and drowned insects in the mud, and they went crazy over the neon-red fruit of palms I gave them from the nearby forest. Uncurbed by midnight chill or the direct midday sun, they continued to come and forage as long as the basin was moist from the cook’s discards.

This army ant species was common: Caspar collected them at baits of margarine that he buried at several sites in the forest. The swarm-raiding rubellus, in contrast, seldom showed up at the margarine baits. Perhaps they are poor competitors underground or, with their longer legs, are more circumscribed in their movements down below.

By the third day of observation the subs began to strike me as odd.18 While driver ants will feed opportunistically on fruit or corpses, they are overwhelmingly predatory. The popularity of the washbasin (which, with