Adventures Among Ants - Mark W. Moffett [109]
Clearly, she couldn’t do her job if she were distracted by single foragers, because they were everywhere, and their occurrence would not necessarily indicate a colony nearby. It was only the presence of a nest that caused the scout to head home and recruit a raid party. I didn’t see it, but Howard tells me a scout may stick her head into an entrance to confirm that she has found an active colony. I imagine that Formica guarding the nest entrance will preemptively ensnare a scout if she is detected during her act of espionage.
In Arizona, Howard found that the scouts are few, typically the same two or three individuals every day, each going a different direction: if one didn’t succeed, another might. That seemed to be true at Sagehen Creek as well. On some afternoons, no raid departed, presumably because no scout had located a Formica nest. After waiting for a raid to begin its mad dash, I was often dismayed to see the workers continue to mill about the nest until the light declined and they trickled back to their abode.
Raiding army and marauder ants gamble on finding prey without sending out any scouts at all, a strategy that pays off because their strength in numbers increases the odds that they will catch any prey they do find. Amazons gamble in another way, by putting their options in the hands of a few. Why not engage more scouts?
Howard may have found one answer. When he removed scouts as they left a nest (returning them each night after the raiding period was over), within a few days their number increased to as many as thirty. Apparently there is a supply of ants that can take on the reconnaissance role as necessary. While a bigger party of these scouts would find the day’s target colony more quickly and dependably and perhaps closer to home, the fact that scouts have blinkers on with respect to Formica until they have traveled a certain distance suggests it is to the slavemakers’ advantage to avoid overharvesting the most easily available sources of slaves. Two or three scouts must be successful enough at finding the remote colonies that drawing from the reserve of scouts in the nest is a rare contingency.
MOBILIZING THE TROOPS
Preparations for the Amazon raid begin with a couple of intrepid scouts venturing into the wide beyond, while dozens of warriors rally near the nest entrance and thousands more wait inside. When a scout returns bearing news of a Formica nest, she mobilizes this widely spread workforce. As she passes one of her milling nestmates, she touches it vigorously. That worker in turn excites others. A chain reaction ensues.
That process begun, the scout enters her nest, where she continues to assemble the troops. Though she may contact just a few individuals in the minutes before she heads out again, by then hundreds have been alerted to follow her. (Some afternoons more than one scout locates a Formica nest, and multiple raids begin in conflicting directions. All but one of them usually quickly aborts. A reasonable hypothesis is that the outbound ants prefer the most “enthusiastic” scout—the one that conveys the strongest signals.) The raid advances, led by the former scout, who now serves as guide. The other ants diligently follow the scent she releases, very likely reinforcing her pheromone with some of their own to help keep the ensemble in formation.
I never did learn how to pick out the leader within an Amazon raid. There were always several identical ants spilling across a raid front as wide as my hand. How could I identify her? One time, I used a forceps to snatch a worker who was slightly out in front of the others and put her in a jar. The raid continued, unaffected. I took the next-foremost worker, then the next. Six ants later, the raiding