Adventures Among Ants - Mark W. Moffett [110]
I treated my captive with respect, dropping her gently among her followers. Having ceased to receive her guidance, they were now acting the way the workers normally do when a raid reaches its destination, spreading out to search for a nest to attack. With her retinue dispersed, the leader wasn’t able to wrangle them any farther. Soon they all gave up and went home.
How does the guide find her way back to the Formica nest she located earlier, raiding squadron in tow? She did not leave herself any clues to her prior route—no pheromone breadcrumbs to follow. Howard discovered that she steers by the sun, using solar navigation not only on her initial hunting excursion but also on her return to mobilize an army, and again on her second outbound run, when she leads that army to the Formica booty. With no other information at her disposal, she takes a slightly different route each time.
The leader doesn’t need to see the sun itself. She can use polarized light, the scattering of sunlight through the sky. This allows the ants to determine their direction even if the sun is hidden behind a tree or a cloud. But trees and clouds hide polarization cues as well, so a patch of sky must be visible somewhere. I illustrated this with an experiment Howard suggested: I taped a piece of waxed paper between two sticks and held it taut above the foremost raiding ants. Immediately the workers scattered. Although the paper had let through plenty of light, the polarization signals were obscured, so the guide ant could no longer shepherd her nestmates. After some fruitless snooping around, the workers returned to their residence.
Amazon ants at the advancing end of a slave raid rush long distances at Sagehen Creek, California, led by a single worker.
Rain, like clouds, may force a raid to be canceled. In another experiment suggested by Howard, I was able to get a raid to turn back in a panic by showering the ants with a pinch of water. Fortunately, most afternoons during the raiding season are warm and sunny, the kind of weather the Amazon ants prefer.
Once the procession is under way, the guide simply halts when she reaches the site where she found the Formica nest, presumably ceasing to release her pheromone signal and thereby allowing the battalion to spread over the nearby terrain. At this juncture, her troops seem to switch from “follow the leader” mode to “search for nest” mode. (The change is remarkable and abrupt, mirroring the change in the scout’s behavior from “run ahead single-mindedly” to “search for nest” on her initial, outbound trek.) Insofar as searching for slaves is the closest the slavemaker workers come to foraging, foraging has begun.
As I watched the Amazons rooting around for a passage into the besieged colony, a simple explanation emerged for the wandering they did at their own nest prior to a raid. They were acting no differently than they were now. Perhaps the milling ants had been looking for Formica from the moment they emerged from their nest. The leader’s pheromones (I hypothesized) could have a hypnotic effect on her nestmates, diverting the overly eager raiding workers from their ferreting behavior long enough to take them to a location more sensible for their search.
A worker leading her companions with only the help of the sun is unlikely to arrive at the precise location of the Formica nest entrance she found earlier in the day. No matter: colonies have multiple entry holes. Brought to the outskirts of the targeted colony, her comrades need only spread over a few square