Adventures Among Ants - Mark W. Moffett [63]
Generally, the exodus of migrating driver ants continues round the clock for two to four days. The number of ants participating can reach into the millions. But this seemed to be a small colony, in the hundreds of thousands, and its migration was over by the second morning. Caspar and I located its new nest, 67 meters from the old site, and we pried up a rock for a view of the massive company beneath. The workers were piling termites in a larder 15 centimeters wide in a preexisting cavern.
This was the first time a driver ant colony had been found to stash its food. If their raids have been crafted by evolution to take advantage of the rare windfall, the ants should be masters at stockpiling an excess catch in this way.7 However, whereas the Pachy ants sting their prey to keep it incapacitated but alive, the driver ant kills it, and corpses rot fast in tropical heat. Driver ants and other army ants also lack repletes and don’t take the seeds that marauder ants can horde.8 Napoleon observed that an army travels on its stomach—anticipating the idea of a superorganism by seeing the ensemble as an individual—and the same is true of an ant army: unable to keep a fresh larder, this colony was forced by its stomach to stay on the move.
PREDATION VERSUS DEFENSE
Two weeks later, my arms were blotchy from bites. I had stared at ants so long, I saw their flowing columns even when I closed my eyes. I felt like an obsessed FBI profiler investigating the habits of a serial killer. By now, the daily activities of rubellus ants had fallen into a predictable pattern. Raids began early in the evening and continued into the morning hours. After a raid ended, the flow of returning workers on the trail could proceed for hours, even on into a second night.
From my first day at Gashaka watching the open-jawed guards, I’d documented how protective the driver ants were of the commerce on their trails. The rubellus reacted even more hysterically to my presence than did the marauder ants. When I so much as breathed on them, the food-transporting ants retreated, while the other workers scurried off to patrol up to 30 centimeters from the trail. I used my mask, constructed earlier from a disintegrating T-shirt, to keep from creating a ruckus.
I had thought that standing guard over a procession and patrolling near it after a commotion must be part of the colony’s defense, not part of its foraging behavior. And to some degree this must be true. Except for a few long-term trunk trails, all the trails of driver ants and other army ants are created during raids that have recently cleared the surroundings of food. For this reason, the numbers of guards or patrollers are out of proportion to the likelihood that those ants will find a meal; ipso facto, they will more likely serve to protect a column than locate prey.
This is unambiguously true for the majors of the New World Eciton army ants, which have fishhook-shaped mandibles suited only for suicidal defense against vertebrates—their jaws have to be pulled out of the skin with tweezers.9 Except for these specialist saber-bearers, which never catch prey, there’s no evidence that any army ants distinguish enemies from meals—a driver ant’s actions don’t differ whether she bites an entomologist or an aphid.
But no matter what the ant species, the line between defense and foraging can be blurry, because any concentration of ant workers has the capacity to serve as a snare for food. As an example, an insect might flee from the army ant raid front into the raid fan, where ants stationed as guards along the network of trails can participate in dispatching it. Dorylus rubellus on trails far from the raid reacted to grasshoppers, crickets, foreign ants, and striped mice in the same way they did to my clumsy presence: by patrolling and attempting to seize them. They caught two of the crickets and a carpenter ant, and in another case a grub that had caused no disturbance, and cut them up and carried