Adventures Among Ants - Mark W. Moffett [108]
For a distance of up to 140 meters, the procession glides at speeds amounting to nearly 200 meters an hour—or a foot every few seconds. That’s ten times faster than an army ant raid. Though I’ve become pretty good at stalking animals, on two afternoons I staked out a colony and missed the whole raid: both times I looked away from the nest for a moment, and all the ants were gone. Even the ants’ orange color and the open landscape didn’t help me find the raid once it slipped out of sight.
What triggers the ants to leave the nest, and how are they able to travel so quickly and efficiently? Most of the details of the subjugation of Formica by Polyergus breviceps have been gathered by Howard Topoff. Working in Arizona, Howard discovered that, unlike raiding army ants, Amazon ants target a single location and are brought there by a leader. Earlier in the day, in fact while her nestmates are ambling haphazardly around their nest, this motivated individual has been scouting for victims, and when the raid begins, she is in charge of the group.4
Researchers studying other populations and species of Polyergus, however, have denied the existence of scouts.5 Watching the Amazons at Sagehen Creek, I understood their doubts. I had been unable to pin down the leader of the raid or to discover her earlier in the day, while she was seeking a target colony. But animal behavior can vary. Even the species of ant enslaved by the Amazon ant differs between locations. Perhaps the California and Arizona Amazons had different scouting behaviors as well. I took it as a personal challenge to confirm the existence of scouts at Sagehen Creek.
And I failed. Hour after hour, day after day, I could not identify a single scout.
Not only was I unable to pick out any leader within a raid, but I was puzzled by certain things about the raids themselves. I could scrape away the soil in front of the column and the ants would continue, which told me that, like foraging army or marauder ants, the Amazons were not tracking a scent. But if the foremost individuals weren’t following an obvious leader, they also didn’t replace each other the way those at the front line of an army or marauder ant raid do, by advancing incrementally before retreating. Instead, all the Amazon workers kept pace with each other while moving forward, with only occasional meanders and a few abrupt shifts in course. They had to be acting on some kind of guidance. That would explain why Amazon raids advance more quickly than army ant raids, despite the fact that the army ant workers are much faster runners.
More than anything else, the Amazon ant raids brought to mind the termite-hungry gangs of Pachycondyla I’d seen while studying driver ants in Nigeria. Instead of taking victims en route, as army ants do, the Pachycondyla raids target a clump of termites, make their kills, and then retreat. Compared to the Amazon raids, though, the Pachycondyla groups were slow and even more tightly compact—one or two ants wide—making it easy to pick out the group leader who had scouted out the termites earlier in the day.6
It seemed, then, that my best hope was to spot the Amazon trailblazer before the raid started, as she departed the nest to scout for Formica. But what chance did I have to spot one ant, when a whole raid could so easily escape my notice? I decided the problem was manpower.
So a year later, I hired two eagle-eyed assistants and returned to Sagehen Creek. Starting at 4 P.M. each day, we walked in wide circles around three different Amazon nests, beyond the reach of the milling ants. I figured any Amazon traveling that far from home, that early in the day, would have to be a scout. Dizzy from circumambulating, we finally located a loner moving in a beeline away from one of the nests—a scout for sure! We watched her progress closely, beguiled by the possibility of untangling a mystery. She was unswerving during her outbound run, undistracted by any Formica worker or nest she came across. When I dropped a Formica pupa in front of her, she didn’t take notice, even though capturing