Adventures among Ants. Mark W. Moffett
drop on the food, and then take to their nest. Feeding from these items must be like squeezing soup from a sponge.)5
There are occasional examples among animals of altruistic transport of objects other than meals—most often other members of one’s own species. Dolphins and gray whales will hold a weakened or injured companion at the surface to breathe. Elephants will work together to lift a fallen comrade to its feet. These mammals will also crowd around to help a disabled individual walk or swim from place to place, though the stricken beast often moves in part under its own power. This is a behavior I have never seen among worker ants; even if she is one of the relatively valuable soldiers, a wounded worker is left to hobble on as best she can.
Although workers of the Arizona honeypot ant, Myrmecocystus mimicus, group-transport other workers, they do so with a less friendly intent: in this species certain individuals are repletes, which have abdomens swollen to the size of a small grape with honey that they regurgitate like living spigots to nestmates. After a battle, groups of ants from the victorious colony will drag the vanquished repletes to their nest, where, hanging from the ceilings of the chambers, they are condemned to a life of slavery.6
During nest emergencies such as floods, ant workers rescue the brood, which can’t escape danger on their own; they group together to move the cumbersome ones. Workers also help the queen get around, especially if she’s wounded, as is true for the queens of another group of social insects, the termites. In South Africa, attending a conference of entomologists who spent their days rummaging through elephant excrement for dung beetles, I shattered a half-metertall, rock-hard nest mound of Macrotermes termites to expose a chamber containing their grotesquely rotund queen, who was over 5 centimeters long. A rescue party of workers immediately surrounded her and pulled her vast bulk out of view.
SHARING THE LOAD
On Mt. Apo, José and I settled down on the trail for a moment to watch the marauder ants at work. Although, as Frank Sinatra laments in the song “High Hopes,” “an ant can’t move that rubber tree plant,” all ant species are celebrated for the loads they can bear—even as singletons. A marauder ant minor worker is no exception, carrying up to five times her mass. It’s not that she is particularly muscular. Rather, it’s a question of proportion. Total strength is determined by muscle thickness, which is proportional to the animal’s height squared, while weight is proportional to the cube of its height. This means that unwieldy vertebrates end up with too much body weight for too little muscle. Galileo worked this out in 1636, writing that “a small dog could probably carry on his back two or three dogs of his own size, but I believe that a horse could not carry even one of his own size.”7 This formula explains how ants have the power to carry striking weights.
Through group transport, marauders take this excess-weight capacity to unparalleled levels. Not only do they haul food together, but they also gang-transport brood during a migration or in emergencies, the corpses of enemy ants that they dump near their trails, queens endangered during pandemonium at the nest, hunks of refuse bound for the colony dump, and, on occasion, obstructions on a trail or chunks of soil for making their arcades. In each case, they work in groups with greater effectiveness than any other living thing.
The capacity of a minor worker to carry five times her weight on her own sounds impressive, but in Singapore I had figured out that a 10-centimeter-long earthworm, like the largest ones I saw being heaved whole balanced between a hundred ants on the ant trail on Mt. Apo, would require a thousand ants if it had been cut in pieces and carried off one ant at a time—yet the ants gang-transporting the burdens on Mt. Apo were slowed to only about half the speed at which they hauled items by themselves. Even when the worm was five thousand times the weight of a minor worker and ten thousand times as voluminous, a gap was usually visible between the cargo and the ground: marauder ants lift rather than drag burdens. In human terms, that would be equal to getting friends together to run at breakneck speed while lifting overhead 250 tons, which would likely amount to far more than the contents of all their houses combined—an utter impossibility for a human.
Before coming to the Philippines, I had conducted an experiment. Although my breakfast ritual in Singapore was to have roti prata at an Indian food nook, for days when I needed to get going before 7 A.M. I kept a supply of a cheap Australian cereal called Grainut (which, being virtually inedible, has since gone off the market). One morning I was sitting in the Botanic Gardens next to a marauder ant trail eating said cereal, when I decided to crush some chunks among the ants and document the outcome.
As it turned out, the bigger the chunk, the more efficient they were, and the more food each ant was able to move along the trail in a unit of time. Beyond a certain size chunk, however, efficiency declined. It was apparently a matter of geometry: with increasing size, the weight of the chunks increased faster than their circumference, until there wasn’t enough space to accommodate the number of ants needed to lift the food. For the cereal, this occurred with chunks requiring more than fourteen ants.
Marauder ants carrying an earthworm at the Singapore Botanic Gardens. Two minors ride on the prey as “guards.”
Looking closely, I could see how the heavier items would cause the ants problems. The porters tend to space themselves evenly, but the bigger a burden is, the more tightly packed they become, until they are barely far enough apart to avoid treading on each other. An earthworm, which is lighter per unit volume than the Grainut chunks and, being long and slender, offers more space for porters, could weigh more overall than a cereal chunk and still be transportable, because more ants could gather around it.
Thanks to the marauder’s skills, few foods need to be diced for carriage—this labor-intensive step can be delayed until the catch is in the protected confines of the nest. Once its flailing limbs are removed, dinner is sped away posthaste. Among marauder ants, more than 80 percent of the colony food supply is brought to the nest by groups of ants. The rest consists of small items carried by individuals.
AMATEURS AND EXPERTS
My surveys of the animal kingdom had shown that group food-carrying exists in only 40 of the 283 known genera of ants. Of the remaining species, some restrict themselves to small prey that do not require this skill (such as Acanthognathus trapjaw ants that prey on tiny springtails). Even among ants with a well-suited diet and adequate means for communicating the location of meals, there are species that fail at group transport because of poor coordination: they end up engaging in a tug-of-war, though clumsy retrieval can occur if perchance the workers pull in the same direction. Otherwise, they eat the food where it’s found or divide it into single servings and cart those away. Even that requires some cooperation, since all but one worker has to let go of each piece before it can be moved. In species adept at group transport, the workers are able to postpone dissecting and consuming the food while they coordinate as a group to move it.
In 1960, John Sudd of the University of Hull studied a British big-headed ant that performed badly in a freight-hauling group, often working at cross-purposes and dragging prey rather than carrying it. But given time, Sudd observed, the workers modified their behavior in such a way that the force they exerted on the food generally increased until they got the job done.8
It turned out the adjustments they made, such as changing the angle at which they applied force or shifting from pushing to pulling, were identical to the changes they made when hauling food alone, and these changes led, as they did for the solitary ant, to the food being moved. In other words, the British workers succeeded in group transport by behaving as if none of the others were there.
Programmed to replicate this kind of coordination, a group of simple robots was able to move a large object. European scientists even used these so-called swarm-bots to stage a mock “rescue” of a child by dragging her across the floor. Another team used tiny swarm-bots scented of cockroach to influence the roaches’ collective decision about where they would gather to hide from the light.9
In contrast to Sudd’s ants and the simple swarm-bots, marauder ants are unambiguously cooperative. Moreover, the behavior they display when moving food as a group is seen only during gang retrievals.