Adventures among Ants. Mark W. Moffett
to 12,000—and that the marauder and the army ant are no more closely related than the hawk and the dove.
Convergence is the process by which living things independently evolve to become alike, as a result of like responses to similar conditions or challenges. The wings of bats, birds, and bugs are convergent because they are limbs that have been independently modified to function in flight; the jaws of humans and the mandibles of insects are convergent because both can be used to hold objects and chew food. If the marauder ant and army ants proved to be alike in how they hunt and capture prey, it would be a similarly marvelous example of evolutionary convergence. That day in Sullia as I watched the ants dispatch that unfortunate frog, I made a decision that would affect the first years of my budding professional life: I would study the kill strategy of the marauder ant. I would make that my quest.
FEEDING THE SUPERORGANISM
Standing in a Sullia field on a tepid afternoon, with Raja’s guitar providing an incongruous musical accompaniment to the massacre at my feet, I felt like a general observing his troops from a hilltop and trying to make sense of the skirmishes below. My brain was whirling: one moment, trying to picture what it’s like inside one of those tiny, chitinous heads; the next, envisioning all the ants at once, forming a kind of arm flung over the ground with fingers that were rummaging through the soil and low plants.
The nineteenth-century philosopher Herbert Spencer was the first to treat in detail the simultaneous existence of these two levels, individual and society, and in 1911 the ant expert William Morton Wheeler came up with the term superorganism to describe ant societies specifically. Both men saw an ant colony not merely as an individual entity, as one might think of a bank or a school, but more specifically as the exact equivalent of an organism.4 They could readily make this point because others had already described the human body as a society of cells.5 The superorganism concept took on real meaning for me as I watched marauder ants. Before coming to India I had read an essay by the physician and ant enthusiast Lewis Thomas, who took Wheeler’s writings to heart:
A solitary ant, afield, cannot be considered to have much of anything on his mind; indeed, with only a few neurons strung together by fibers, he can’t be imagined to have a mind at all, much less a thought. He is more like a ganglion on legs. Four ants together, or ten, encircling a dead moth on a path, begin to look more like an idea. They fumble and shove, gradually moving the food toward the Hill, but as though by blind chance. It is only when you watch the dense mass of thousands of ants, crowded together around the Hill, blackening the ground, that you begin to see the whole beast, and now you observe it thinking, planning, calculating.6
Like a more traditional organism, a superorganism is most successful when its activities are carried out with maximum productivity at the group level. Consider the cells of a human body, an assembly of trillions. Although these cells may be doing rather little as individuals, collectively they can yield results as intricate and choreographed as a dancer’s in a corps de ballet. I developed a feeling for a marauder colony as an organism. I watched as the ants worked together like the organs in a body to keep the ensemble healthy and stable, with their trails serving as a nervous system used by the whole to gather knowledge and calculate its choices. With mindless brilliance, this colony-being established itself, procured meals and grew fat on the excess, engineered its environment to suit its needs, and fought—and on occasion reproduced—with its neighbors. I imagined that, given enough time, I could watch each superorganism mature, spin off successors that bred true through the generations, and die.
How do the members of an ant superorganism supply food for the whole? Unlike the body of an ordinary organism, a colony can send off pieces of itself—the workers—to find a meal. Regardless of species, once an ant detects food, her searching behavior stops and is replaced by a series of very different harvesting activities: tracking, killing, dissecting, carrying, and defending. In the majority of species, an ant can mobilize others to assist her. This communication practice is known as recruitment and usually involves chemical signals called pheromones. Often, a wayfaring ant releases a scent from one of a battery of glands on her body, a mixture that serves to stimulate or guide her nestmates. The mobbing of marauders at prey reflects the speed and effectiveness of their recruitment.
I’d known about recruitment, without having a name for it, since I was a child. At family picnics, I would drop a crumb in front of a lone worker. Within minutes, a hundred ants would be pouring along a column to the bread. Had I been able to inspect the successful hunter who first found the crumb, I would have seen her glide the tip of her abdomen on the ground on her return to the nest, depositing a pheromone that diffused in the air—a common, though not universal, ant practice. When ants form a line or travel in a column, they are tracking such a plume with their sensitive antennae, which they sweep left and right before them, in many cases while running faster for their size than any baying foxhound.
Each ant adds pheromone to a trail offering a good payoff, so the scent builds over time. Then, when the food supply runs low and the ants begin returning unrewarded, the pheromone is no longer replenished and the scent dissipates, attracting fewer ants. (Pharaoh ants have an even more efficient way to flag a route that has ceased to be profitable, signaling “don’t bother” by depositing a different pheromone at the start of the trail.)7 The chemicals required to convey a message can be minuscule. With one species of leafcutter ant, a thousandth of a gram of recruitment pheromone—a minute fraction of one droplet—would be enough to lead a column of workers around the world sixty times.8
Since traffic depends on pheromone strength, it is modulated by the ants’ overall assessment of a trail’s offerings—what we call mass communication. This technique can lead to what appear to be deliberate choices by the colony, despite the ignorance of the individual ants of such matters as the size of the food item they are visiting and the number of workers needed to harvest it. For instance, a colony will more quickly exploit a nearby food source than one farther away, simply because it takes less time for the ants to walk the shorter distance. This results in the quicker accumulation of the trail pheromone, which in turn attracts more ants to the meal.9
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