Adventures Among Ants - Mark W. Moffett [32]
From springtails and seeds to frogs and large fruit, marauders harvest a cornucopia. They are reminiscent of humans, who apply the dictum “because it was there” not only to climbing mountains but also to adding tasty morsels to our diets. Marauders and people are exceptions to the general rule that in the tropics, where so many different organisms live together, most species, like the springtail-hunting trapjaw ants, become specialists in a narrow niche to survive the intense rivalry for resources.23 Marauder ants, in contrast, by interfering with all contenders for each meal and taking prey where others fail, exceed expectations by being geniuses at the competition game.
TRACKING FOOD FROM A TRUNK TRAIL
In Singapore’s Botanic Gardens one day, I placed a meter-wide plywood board in front of a raid. The ants crossed it in swarm formation, which confirmed my suspicion that their raids don’t depend on workers finding food or retracing old routes. Even so, I knew the ants were no fools—their raids slowed in areas with little to offer, the number of workers in them declining as the ants drained back to the nest until, if the dearth continued, the whole army would retreat. I decided to find out how the plenitude or distribution of booty changed an army’s strength and direction.
The marauder ant’s vegetarian proclivities made the job easy: it’s more difficult to manipulate caterpillars and crickets than to move fruit and seeds.24 Loaded with supplies from the grocery store on Orchard Road, I headed back to the Botanic Gardens and spread canary seed in a line extending from a trunk trail. It didn’t take long for the marauder workers to leave their highway and flow along this line. They tracked the seeds precisely, continuing outward in a column even after they had passed the last seeds. I had launched my own raid!
Did the distribution of food affect how the raid progressed? I poured a seed pyramid ahead of a swarm. The ants continued forward for several minutes after contacting this jackpot and then drained back to the food, where they rapidly built up in numbers. The swarm raid now over, the excess arriving ants radiated from the seed pile in a network of branching column raids spread over several square meters (a process called recruitment overrun, described in chapter 2). I had seen marauder ants generate similar trail networks under trees dropping fruit, which they thus track down quickly. While column raids are ineffective for catching fast prey, these bifurcating formations shine when it comes to fanning a foraging populace out over large areas. Each time one of the weak raids in a network encounters a bonanza, any number of workers can be summoned within minutes from the trunk trail to seize and consume it.25
What if the enticements are less concentrated? My next approach was to scatter a few seeds in a meter-wide swath off to one side of a swarm raid that was crossing a field with little in the way of food. The raid turned and followed my swath its entire 15-meter length, even though I laid few seeds—one every 20 square centimeters or so, which would put three of them in an area the size of my palm. Somehow, raids track subtle changes in food density, even though the workers coming upon each seed are ignorant of the food distribution as a whole.
How does that happen? While the ants follow exploratory trails at the raid front, they are more attracted to any recruitment trails they come across, which lead to food. When there are more seeds on one side of a raid, ants must be drawn to them by the buildup of recruitment pheromones left by the successful foragers from that direction. New arrivals tend to follow