Adventures Among Ants - Mark W. Moffett [34]
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
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