Adventures Among Ants - Mark W. Moffett [33]
I went back to Orchard Road, depleting the grocery shelves of bird seed to continue my experiments. What mattered to the marauders seemed to be the relative abundance of food: when a raid was bringing in lots of other victuals, I needed more seeds to alter its course. The raids turned out to be smartly responsive to food in a variety of ways, branching or shifting in direction, width, and strength on the fly. Even though the absence of scouts made the raid blind to meals at a distance, the aggregate response of the workers to food at hand apparently enabled the raid as a whole to follow the food distribution in bountiful regions.
It’s a subject of endless fascination for scientists that each ant can only proceed locally on the limited information at hand, and yet their societies manage to act globally. Darwin was right when he wrote that for all ants do with their modest endowments, “the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more marvellous than the brain of man.”27 But the true power of the mind of an ant emerges at a superorganismic level, when those brains join to produce colony-level actions to accomplish a goal. Lewis Thomas, the author who first introduced me to the superorganism idea in my youth, described an ant society as “an intelligence, a kind of live computer, with crawling bits for its wits.”28
HOMEWARD BOUND
One afternoon it occurred to me that I could use the marauder ants’ ability to track seeds to unravel a mystery. Every trail has two directions. How do workers select the correct way home?29
In most situations, the ants have no problem choosing a direction. Because workers ordinarily find food at the raid front—the end of the trail—every returning ant has but one way to go. Along the route, though, are junctions with other trails. Some of these don’t present a problem: trails split at sharp angles, so nest-bound ants will make the right choice if they take the route that lies closest to straight ahead.30 Still, in the labyrinth of trails between raid and nest, I saw many situations in which the ants could have made directional mistakes but rarely did. Why?
I realized that by pouring seeds in an arc, connecting one point on a trunk trail to another point farther along the same trail, I could give the ants a choice of two equally good directions back to the nest. I watched in anticipation as the troops rushed from the trunk trail to track the line of seeds along each end of the arc. Every ant who picked up a seed from the advancing front of either column then turned around and carried it directly back to the trunk trail. When the advancing armies met, the ants now had the option of completing the full loop, and they often did so if they had’t picked up a seed. From their point of view, they were simply continuing as they had been going, away from the nest. A worker that picked up a seed after passing the site where the troops met would not turn around but rather would continue onward—a choice that, in any “normal” situation (not a loop), would have led her away from the nest.
The result was that all the seeds flowed away from where the armies converged. I called the trail segment within a centimeter or two to either side of this point the transition area because ants acquiring seeds in that stretch weren’t consistent in their choice of direction. The transition area was usually near