Adventures Among Ants - Mark W. Moffett [17]
Army ants, and particularly swarm-raiding army ants, are exceptional for their ability to consistently trap difficult, even dangerous, prey. I now saw that this attribute applied to swarms of marauders as well. To uncover the secrets of the marauders’ predatory success, I began to study the moment-by-moment organization of their raids. By watching where the ants first advanced at the front and then doing a slow scan back to the base column, I discovered I could treat what I saw along the way as a chronological sequence. In practice, this wasn’t necessarily straightforward. While the workers might be fearless with prey, they are skittish when it comes to other interruptions. They will retreat from a simple breath of air. To interpret their behavior, therefore, I stood as far off as possible. At times I used binoculars, once confusing a group of birdwatchers with my concentration on what must have appeared to be barren earth.
The ants in the narrow swarm behind the raid front seem to move randomly, going backward, forward, and sideways with respect to the front. There obviously must be a net movement ahead to account for the raid’s progress, but it’s hard to detect ants following one another in that direction. Few trails are evident, and the ants appear to be moving through a diffuse cloud of orientation signals. The swarm advances to new ground every few minutes, and the land it formerly occupied is taken over by the forward part of the fan as more ants begin to form columns by running along specific tracks. The fan is differentiated from the swarm by the fact that it has these columns, and where that demarcation is made depends on the observer’s ability to pick them out. Farther back in the raid, the columns become fewer and busier, with an increasing proportion of ants on identifiable routes. Ultimately, at the back of the raid, the ants funnel onto one path, the base column.
Roughly speaking, each part of a raid has a different function, turning the ants collectively into a food-processing plant. Prey is located by the foragers at the front, subdued within the swarm, then torn up in the fan. From there, it is transported along the base column to the trunk trail and delivered to the nest, where most of it is ingested by the ants. Things aren’t always so clear-cut, of course; as when kids crisscross the same ground on an Easter egg hunt, it is possible for workers in a swarm to find something the lead ants have missed, or for those in the fan to contact prey that’s on the run from the other ants.
Mass foraging permits workers to flush prey and act in concert to catch it, like sportsmen engaged in a fox hunt but with the scale of operations increased a thousandfold. The downside of the ant stratagem is that the colony has to pack its greatest resource—its labor force—into an entity compact enough to cross a small area, instead of spreading those workers far and wide on individual search missions, as would a solitary-foraging species. The result is that the same number of workers finds less, but catches more.8 How? The deployment of these ants maximizes the capture of quarry too large for solitary species, yielding an intake of food that compensates for the slow encounter rate. All ant colonies stash a reserve of workers in the nest, to draw from as needed. Marauder ant swarms are made up of such assistants, transplanted from the nest to the site where food has been discovered.9 Keeping a reliable labor supply close at hand means that a raid can quickly respond to changing conditions—an essential component of success. Prompt conscription to the battlefront through explosive recruitment minimizes the time between the moment when workers first find prey and the arrival of reinforcements to pounce on it. No matter how fierce or capable the quarry may be, with no opportunity to make a getaway it will generally be overpowered by the rapidly escalating force of its assailants.
In his book on military theory, The Art of War, Sun Tzu recommended