Adventures Among Ants - Mark W. Moffett [74]
I knew these columns typically lasted a day or two, with upward of five hundred ants passing by each minute, though most often their routes were much more ephemeral and lightly used. Sometimes a column would elongate by a meter or two over several hours. This happened in bursts after foragers came upon seeds from a crabapple tree, their most common food; the ants would then extend their route to the area with seeds. The surfeit of arriving ants would scout several centimeters in the vicinity, which often led to finding additional seeds.22 If that failed to happen, their activity would fade away until the column retreated.
Dependent as the workers were on finding food, their progress never transformed into anything that could be mistaken for a raid. Moreover, they acted cripplingly shy. Retreating from confrontation, they were usually hopeless with prey, avoiding the centimeter-long lawn-dwelling caterpillars even when I dropped one right on their column. If a caterpillar didn’t go far, however, ants would follow it for a few centimeters and give it a tug. Sometimes a media or major would arrive to crush it in her jaws, and minors and small medias would carry it off.
Day after day, my patience sorely tested, I spent hours putting little flags in front of roving individuals and groups, wishing I could figure out what affinis was up to so I could study the army ant–like Pheidologeton silenus in the forest nearby.23
Could what I saw with Nigeria’s army ant sub be some kind of spread-out, sluggish raiding? Perhaps the behavior of the sub and the Malaysian affinis looks different when the ants are constrained to narrow channels in their favored habitat, the soil; certainly packed together under these circumstances they must catch roving prey that blunder into their columns, as the ants had done in Nigeria with the hapless fly aboveground. But looks can be deceiving, and I knew that finding workers at high density wouldn’t be enough to establish that active mass hunting occurs. Army ant raids in particular don’t involve scouts and are not sustained by constant food discovery. I specifically wanted to determine if and when both the Nigerian subs and the affinis ants lay trails, not just to food but to new ground—exploratory trails. I pined for a way to make pheromones glow brightly—something like the spray used by CSI forensic detectives that makes blood stains visible—so that all these hidden details would be clear. Regardless, it seemed unlikely that the workers of either species were acting as a mass-foraging group.
Related to the marauder ant, Pheidologeton affinis collects only weak prey and tiny seeds (see the minor worker at bottom).
The slow-motion foraging of affinis and the Nigerian sub suggests possibilities for how army ant–style raids originated in the army ants, and among the marauder ants as well. Here’s what scientists call a thought experiment: Consider what incremental changes in behavior over time could transform a species that searches for food alone into a mass forager. It’s not as if scouts can begin recruiting nestmates only part of the way to food. A plausible option might be to start with a trunk trail that guides ants to places where they spread out to look for food individually. A trunk trail reduces the time required to obtain assistance: a successful forager need only draw from the ants on the trail, not the ones back at the nest, for help. The faster the trail system grows, and the less far the workers depart from it in a search for food, the shorter the lag time between finding a meal and getting help to harvest it. Reduce this lag time sufficiently, and the ants begin to become proficient at overpowering quick and dangerous prey. If these changes continue, the trail-making process begins to take on the form of an army ant raid.24
The full sequence, from individual recruitment trails to the emergence of