Adventures Among Ants - Mark W. Moffett [10]
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