Adventures Among Ants - Mark W. Moffett [58]
Army ants also employ trunk trails to access distant regions, and they can do so at breakneck speed. An Eciton burchellii colony in its stationary phase may develop several routes radiating in different directions. Rather than departing the nest in their usual swarm, the ants run in a line along one of their abandoned trails, retracing the trunk trail’s path for 50 to 100 meters. Only then does foraging begin in earnest, as ants pour off the trail in raid formation to explore promising new terrain.
It isn’t known yet how army ant workers mark their more productive paths to make them attractive for future reuse as trunk trails. In general, detecting an old route, and whether to follow or avoid it, should be easy for army ants, whose capacity to pick up scent is legendary. Some New World species will follow nearly anything, even a thin streak of water laid on the ground. This versatility suggests that their skills could even extend beyond recognizing the trails of their own species to tracking the trails of their prey. Workers at the front lines of a raid likely take whatever cues they find to lead them to a meal, scents left by other ants included. Caspar and I were focused on swarm raiders, but around the world most army ants mount column raids, which illustrate this ability well. Narrow columns don’t have the ant power to take down big prey, which will escape them easily. Instead, most column-raiding army ants depend on finding ant nests, with their hoards of brood. Even a weak raid can stage an effective attack on an ant nest if the workers, recruited from the column network, are quickly able to accumulate in numbers.
There is a logistical problem, however: the nest entrance can be difficult for a raid column to find. Picture the column as an elongating line, and the nest entrance as a point on the ground; the odds of them intersecting aren’t good. But if the column raiders avail themselves of their scent-tracking skills, their raid need only cross the trail of the other ant species. That’s an easier proposition, because everywhere in a rainforest there is a tapestry of the pheromonal guidance signals deposited by all kinds of ants.
I have devised an experiment to determine whether army ants are tracking prey by following their scent. When I find a column raid in progress, I scrape away the ground surface ahead of their front line. If the ants are still in search mode, they will continue across the upturned ground without hesitation. But if they are in pursuit of a prey species of ant, my action will have removed the pheromone signals they are tracking and disrupt their advance. Twice on my visits to Barro Colorado Island, a research station operated by the Smithsonian in Panama, I stopped the column raids of an Eciton hamatum colony cold—until they picked up the scent again and continued on their way.26
Whether reused on occasion or not, army ant trunk trails are generally less ubiquitous, obvious, and persistent than those of the marauder ant. Army ants focus on discovering virgin hunting grounds through migrations and shifting raids, because their diets are dominated by ant colonies and large invertebrates that are slow to replenish. Because marauder ants regularly eat fruit, seeds, and small and large prey, they can generally gather food even in the frequently reraided areas near their nests. There may be limits to the utility of their trunk trails, though: typically after a few weeks, a colony abandons one highway and starts another. I haven’t been able to prove it, but it may be they need a change of venue once they have depleted a region.
Conversely, the predilection among army ants for meals of large invertebrates and social insects may explain why their raids travel ten times faster and several times farther than those of marauder ants. With