Adventures Among Ants - Mark W. Moffett [11]
Among the Sullia oil palms, however, such subtleties of individual reaction and mass response were hidden to me. Instead I recorded seemingly spontaneous eruptions of ant multitudes followed by sudden mass retreats, like an arm that was extended, pulled back, and then extended somewhere else. What was going on?
I thought back to a similar eruption involving army ants that I had seen as an undergraduate studying butterflies in Costa Rica. I was awakened one morning to a rustling sound in the room of the hacienda where I was a guest. Eciton burchellii army ants were everywhere, moving in waves over the floor, flowing through cracks in the wall, falling from furniture while clinging to the backs of beetles and silverfish. I heard a plopping sound as an inch-long body landed on the carpet next to my bed: a scorpion cloaked in ants had dropped from the ceiling. The only reason no ants had swarmed my body was that each leg of the bed had been set in a dish of oil by the owner’s wife. Thank heavens—I doubt Señora Perez would have appreciated my dashing naked, draped only in ants, into her parlor. I put on my robe and ran to the ant-free hallway, then waited out the ant raid over toast and scrambled eggs.
What had those ants been doing? In a word, they were foraging.10 For all ant colonies, this search for food is carried out by multiple workers at once. But while the foragers of most ant species operate independently of each other, army ants forage together, much like a pack of wolves looking for elk.11 Unlike a wolf pack, however, army ant hunting groups do not have a circumscribed membership. Thousands may be present in a raid, but different workers come and go en route to the nest, a search strategy called group foraging or (my preference, because there is no set “group”) mass foraging.12
Many fierce predators dispatch difficult prey without searching for it in a group. In certain ant species, workers acting alone can both find and kill small vertebrates. Workers of one Brazilian ant dispatch tadpoles larger than themselves.13 But most predatory ants cannot overpower such prey without help. Most commonly, a successful forager—called a scout when a few scattered individuals are doing the reconnaissance—recruits a raiding party, often guiding it for many meters to the specific site where she discovered the prey.14
Elsewhere in the Western Ghats of India, I saw this system used by Leptogenys. The tight pack of slim, glossy ants was moving through the dry litter at the reckless speed of an Indian bus driver. I followed and watched as they entered the mud galleries of a termite colony. The ants soon emerged, each with a stack of termites in her jaws. This regimented form of group predation was a joy to observe, as long as I stayed back far enough to avoid the needlelike stingers that Leptogenys use to immobilize their prey in one-on-one combat. Later I determined that this species employs scouts. These individuals then return by themselves to the nest and recruit a few dozen nestmates who together do the potentially dangerous work of mining and transporting the unwieldy termites.
Army ants employ a completely different foraging technique. Rather than proceeding with guidance to food already found, the workers sweep ahead blindly in a mass, the absence of a single target turning the whole raiding business into a gamble.
Some army ants regularly invade homes, and in the underdeveloped world their arrival is welcome (even though they force everyone out for an hour or two), for they clear out vermin such as roaches and mice. Marauder ants perform a similar service—though they also make a nuisance of themselves by absconding with grain and other human foods, as Captain Bingham recorded.
Indeed, it was impressive to watch marauder ant mobs take on centipedes and frogs in India. From those clashes I saw that, like army ants,