Adventures among Ants. Mark W. Moffett

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 the middle of the arc, but I could change its location by laying down the seeds earlier at one side of the loop, causing the ants who found that end of the loop to travel farther than they did on the other side before the armies merged.

      At first, I guessed that the ants had marked the trail with some kind of “arrow,” as invisible to our eyes as the pheromone trail itself, which told their colleagues, “Go this way!” But that hypothesis crumbled when I waited until the seeds were nearly gone and the ants still moved around the arc with nothing to carry. I poured a new heap of seeds along the arc away from the transition area. If the trail contained a directional cue, all the ants taking seeds from the new pile should have gone in the same direction the workers had taken earlier when they took seeds from that spot. Instead, the ants proceeded to haul the seeds in both directions. While the workers were still retrieving seeds from the new pile, I poured yet another pile elsewhere along the arc. All the ants taking seeds from the first pile passed the second one and continued in the same direction they’d been going. But when ants began to pick up seeds from the second, newest pile, all of them followed the lead of the ants going past them with booty from the first pile.

      Other experiments confirmed this behavior: ants picking up seeds took the direction of any passersby with food (and if there were none, they could go either direction). Were they being physically forced to go the same way, bystanders compelled to join a mob? No—the seeds weren’t bulky enough, and the carriers weren’t numerous enough, to inhibit ants from going whichever way they wanted.³¹ Instead, it appeared the food-bearing ants were taking notice of each other’s choices and deciding accordingly.³²

      As it turns out, this “go with the flow” approach is essential to the marauders’ response to bedlam. Crush a marauder ant underfoot, and some workers, detecting alarm pheromones released by the body, rush off the trail on patrols in which they attack whatever they find. While the patrollers are in defense mode, the food-bearing ants do an about-face, clearing the disturbed area by rushing outbound along the trail instead of continuing to the nest. As laden ants farther along the trail confront this backflow, they turn and join the exodus, in this case propelled away from the nest by the urgent multitudes.

      If the laden backtrackers reach the trail’s end, they mill about before starting back to the nest. Usually they don’t get that far: as the fleeing ants spread out more and more along the trail, their frantic pace slows to a normal gait, and they gradually start to turn around again under the influence of all the workers carrying food in the “correct,” nest-bound direction. In either case, by the time the ants return to the point on the trail where the fracas took place—anywhere from five to twenty minutes later—the problem is long gone and the patrolling has all but ceased. It’s now safe to go home.

      Except in such emergency situations, traffic on busy marauder ant trails is well organized so as to avoid congestion. The scheme isn’t to stay to the right or left, as on human thoroughfares. Rather, nest-bound ants tend to use the trail center, while the outbound ants stay to the sides. The center is easiest to travel, being concave from use, with few obstructions and the most concentrated pheromone. The inbound ants with their unwieldy loads end up there because they have difficulty maneuvering. Carrying nothing, outbound ants can quickly move to the sides of a trail to avoid their encumbered sisters. Similarly efficient patterns emerge among people, too. Think of how pedestrians will be diverted to the gutter as they try to circle around someone hefting a big package on a crowded sidewalk.³³ And during rush hour, without anyone thinking it out, clusters of pedestrians will move in alternative directions through bottlenecks—a pattern I have seen in marauder ants as well, where their routes head through a bottleneck underground.³⁴

      4 infrastructure

      Through my camera lens, I closed in on a gray Diacamma worker with an elegant silver sheen striding along with what appeared to be a sense of purpose. I tracked her ascent of an embankment of soft soil. She went over the top and landed squarely among marauder ants following a trunk trail on the other side. Six minor workers pinned her in place as workers laden with food retreated; then a major arrived and executed her with a crushing blow, discarding the corpse just off the trail, where several minors buried her in the dirt as their food delivery operations resumed.

      Marauder colonies maintain a fast, steady, well-protected flow of food and labor on their trails. Whereas small ant colonies, like people in small societies, are able to access and distribute the supplies they need without roads, larger groups depend on an infrastructure so complex that in the marauder ant it rivals human highway systems. The idea of a superorganism applies here, of course: whereas a microscopic organism like a microbe can rely on simple diffusion to distribute nutrients through its body, a large one, such as a human being, needs a circulatory system.¹

       A marauder ant major worker hefting a Diacamma ant killed after intruding on the colony’s trunk trail. The discarded corpse was buried by the minor workers.

      ROADS

      Marauder colonies avoid both gridlock and species confrontations, like that with the Diacamma worker, when trails are in good shape. Highway construction efforts are part of the society’s logistics, providing supply lines for fresh combatants on the front lines as well as streamlined routes for bringing home the plunder.² Trunk trails are well looked after—that’s how they can be distinguished from the fleeting paths created by raids.

      Each worker size class participates in the creation of the roadways. All the castes eliminate surface irregularities along a trunk: while the medias and majors chisel out embedded roots and pebbles, minor workers extract grains of soil, establishing the road’s slightly concave shape in cross section. The dross is discarded along the edges of the trail, where it accumulates in embankments like the one the Diacamma walked over. When the ground is moist, the minor and media workers build up the ramparts into a complete soil cover, or thin-roofed arcade, fabricated from soil extracted from the trail surface or from mining shafts—blind-ended tunnels near the trail used specifically as quarries.

      Members of the construction crews expend their efforts foraging for building material rather than food. It is likely that no communiqués pass among them.³ Rather, like compulsive bricklayers unable to go by an unfinished wall, passing ants respond to the ongoing building project, and the structures emerge without any active collaboration. The portions of the walls that are suitably positioned and shaped along a trail attract the most attention from passersby bearing soil bits. As a result, the arcades rise to completion where they are most needed, without a blueprint, and damage to them later is repaired without fuss.

      Accomplishing large projects without communications is called stigmergy. The marauders’ approach