Читать книгу Adventures among Ants - Mark W. Moffett - Страница 14

5 group transport

It was late in 1983. For the final leg of my doctoral fieldwork, after traveling without a break for twenty-nine months, I had ended up in the Philippines. I had just arrived at the base of Mt. Apo, on the southern island of Mindanao, at almost 3,000 meters the highest mountain in the Philippines and cloaked with forests. On exiting the bus, I found José, a self-proclaimed guide who, an hour into our walk, broke his silence to speak of the need for revolution while patting what he claimed to be a gun in his waistband.

On my way to Mindanao, I had faced riots against the government outside the Manila airport. I now recalled a U.S. government advisory that Mindanao was the center of the Communist movement and unsafe for travelers. It occurred to me for the first time since I had left Massachusetts nearly two and a half years before that it might be pleasant to experience Christmas at home again—or even be reminded of what the holiday celebrated.

Mist and tree ferns gave each vista of the flanks of Mt. Apo a Jurassic Park flavor. While José talked, I located my old friend the marauder ant. Among their legions, fifteen minor workers—the chief food-delivery caste—carried a centimeter-long blue sphere through dense brush. When I picked it up, more ants poured from a crack in the sphere to expose the remains of a bird embryo.

Whether it’s political rebels or ants fetching home a bird egg, a successful social operation requires the coordination of individuals. The orchestration of a marauder ant raid is an obvious example of how organization emerges from collective masses within a superorganism. By moving in closer to watch the individuals as if they were tissues within organs, I could document equally compelling examples of social integration, involving smaller ant groups within a swarm.

Marauder ants are successful at “marauding” in part because workers can work together to haul provisions along their superhighways—or expel a rejected queen from the nest. Group transport is the carrying, dragging, lifting, rolling, or burying of a burden by multiple individuals.¹ We’ve all seen group transport, if only when a few ants pilfer crumbs at our picnics. But the average interloper at a picnic represents only a crude example of the sophisticated group transport accomplished by the marauder.

GROUP TRANSPORT AMONG ANIMALS

From watching the marauder ants for months while in Asia, I had become fascinated with group transport and began to investigate which other creatures could accomplish this basic task. I discovered that group transport is as scarce in the animal kingdom as using tools or hunting in a group. Such task-oriented cooperation is particularly rare in nonhuman primates, in part due to the overwhelming drive of each individual to keep food for himself and in part because they seldom deal with large objects in nature, although captive chimpanzees will reach food by carrying a branch together to use as a ladder or by group-dragging a box.

Even for species that cooperate to kill prey, jointly moving a cumbersome meal requires a delay in gratification that few can tolerate. Litter mates of rodents such as rats will sometimes jointly convey food they had initially fought over. Lions and also hyenas, wolves, jackals, African wild dogs, and other dog-family members typically feed in a free-for-all or along dominance-hierarchy lines, but occasionally they may jointly move meat to a shady spot or protected den. Even then, they tend to act like competitors who just happen to be pulling their dinner in the same direction.²

Nonhuman mammal societies usually contain one or two dozen individuals, with a few dozen at most. It’s often simpler for such a small group, for example a pack of wolves, to travel to the kill site than to take a large carcass to a more desirable setting. Similarly, in the few ants with small colonies that eat big prey—such as New Guinea’s Myopopone castanea, which feeds on blubbery insect larvae—the entire society may up and move to the food after it’s been killed.³

Outside of ants, the best examples of group transport of food are found among other arthropods. Certain spiders gang together to build a web and even capture prey and care for their young; in some species, the spiders jointly drag prey to the protected interior of their web, where all feed. Then there are mated couples that procure an item of food too massive for one of them to manage alone. Pairs of some dung beetles roll and bury a dung ball on which the female lays eggs. A male and a female carrion beetle join forces to bury the corpse of a small vertebrate such as a pigeon or squirrel, which becomes food for their larvae.

But ants reign supreme at this form of altruism. Why is this so? For one thing, there is little antagonism between nestmates over food, reflecting how well ants work together generally. Also, as central-place foragers, ants take food back to a busy nest where much of the fare is consumed by the growing larvae in their protected nurseries.

Beyond that, the availability of a large labor force and the use of chemical trails make it practical for large ant societies to assemble transport crews and thereafter coordinate the direction in which they move the food. At the same time, the scale of activities in these species puts a premium on workers handling heavy items efficiently. Compared to lions, who take large prey every few days, a marauder ant colony may bring down thousands of food items bigger than the workers in the course of a single day.

Aspects of ant anatomy also simplify group transport. Their forward-directed mandibles are more effective in lifting burdens cooperatively than are the jaws and limbs of most other social animals. (Humans are a significant exception: with our upright posture and opposable thumbs, we are experts at group transport.) An ant’s center of gravity is also low relative to that of large mammals, providing easier balance in group retrieval.

Portability is the minimum requirement for group transport of food. Ideal objects for transport, such as seeds or prey, come in solid packets just a few times heavier than their carrier—neither so small that lone individuals could carry them nor too large, soft, crumbly, or mushy to lift. It’s always possible to cut up an item that is too large, as long as the material can be carved into portions the right size for a group.⁴ Even marauders are unable to pick up rotten or soft fruit, which they ingest on the spot. (As a tool-based alternative to conveying liquidy meals, a slender New England ant sops them up with dried bits of plant material that the ants slice from nearby debris, 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. A worker carrying a burden on her own walks on all six legs, grasping it in her jaws. If she joins a group, however, she places her forelegs on the burden, then presses her head against its surface, jaws open, but she does not use her jaws to grip unless she can hold on to a projection such as a limb. She walks with her remaining legs as she and her nestmates transport their load.

With modest colonies of a few thousand, Daceton ants in Venezuela have developed only rudimentary cooperation in the transport of food. Here two workers have pulled so persistently in conflicting directions that the moisture has been wrung out of this caterpillar. Flies sneak in to drink from the oozing meat.

What about this technique makes marauder ants excel at gang retrieval? Picture several people hefting a box by thrusting upward, not only with the palms of their hands but with their foreheads as well. By pushing a load up, forward, and against each other, the clustered ants balance the weight effectively among themselves. Army ants use a different technique, lining up to straddle a burden under their bodies rather than encircling it. Either way, the groups cancel the rotational forces that solitary porters contend with when they lift a burden in front of them. Anyone who’s felt a heavy box twist out of his hands has experienced this force, a problem that disappears when another grasps the object on the other side.¹⁰

While participating in a lift-and-carry operation, each of the marauder ant “porters” performs a slightly different task. As when several people haul a piano, an ant’s movements depend on where she is located relative to the direction of motion. Workers at the forward margin walk backward, pulling the burden. Those on the trailing edge walk forward, apparently pushing it. Ants along the sides shuffle their legs sideways and slant their bodies in the direction of travel. The ants sort out their roles during a few minutes of turmoil, then whisk the item off with effortless grace. When a media worker joins in at the front or back ends of large booty, she appears to be adept at guiding the group around obstructions or through shifts in the trail course, performing another valuable role in the transport team.

BUT IS IT TEAMWORK?

Should we consider these groups teams? Dictionaries define the word team as a group organized to work together, which could apply to many social situations among ants. Although in many team sports there is a set roster for each game, with ants, under most circumstances, the participants change and are interchangeable.¹¹ We saw this for raids: marauder ants come and go while the quarry is being subdued, and similarly to and from the raid as a whole. By comparison, transport groups are more stable, though ants may leave or join a group when, for example, an object becomes snagged, at which point the participants must sort out their movements relative to each other afresh.

Often, members of human teams divide the labor, doing different things at once to get the job done. Although ant workers cannot recognize each other as individuals in the way human teammates do, many marauder ant activities—among them killing prey, attacking alien ants, and maintaining trails—probably conform to the American football model.¹² In some cases, different worker castes play specific “positions” and concentrate on distinct tasks, as when minors hold down prey while medias and majors shear its limbs. In other situations, all participants belong to the same worker caste and show flexibility in how they do their jobs, as when minor workers perform differently in the transport group depending on where they are located around the prey.

One species of wood ant shows the ultimate division of labor in a transport team, with a degree of leadership exceptional among ants. Among Formica incerta, common in New England fields, when a successful forager can’t move an item of food herself, she attracts ants in the vicinity or recruits some from the nest. Unable to assess the size of her find, she may not gather a suitable number of individuals. If not enough helpers arrive and she needs to leave to find more, those already on scene—even if they have already started carrying off her find—will wander away as if the food weren’t there. Only the original food finder can keep the team motivated, and only she can go for more help. She must be present to guide the transport team from start to finish. Outside her role with this particular meal, of course, there is nothing special about her. If she is later recruited by another scout, she goes to work as a regular worker, while the individual who located the food becomes the supervisor for its retrieval.¹³

Several years ago in El Salvador I watched workers of the army ant Eciton burchellii chop a scorpion to pieces. I could see that the workers fell into different positions as the transport groups came together, but they didn’t adopt behaviors specific to teamwork. This occurred because the media workers had trouble lifting an unwieldy hunk of the tail. Then one of the less common but bigger and stronger “submajor” workers arrived and was able to straddle it in the classic army ant manner and start it moving. Immediately one of the smaller medias crowding around was able to fit into the cramped space under the abdomen of the bigger ant, where she grabbed the scorpion’s stinger, which was trailing on the ground. Thereafter the two functioned, as they often do in this species, as a team, with the forward ant doing the power lifting and steering, while the little one kept the back of the prey from dragging. Meanwhile, the scorpion’s body was being carried by four ants: the same pairing of a submajor and a media handled the main axis of the corpse, with two more medias off to the side, helping lift the scorpion’s appendages.¹⁴

A “submajor” Eciton burchellii army ant hefting a chunk of centipede while a smaller media worker behind her lifts its dragging end. The minor worker lying below them in a pothole along the route serves as “living road fill.”

The workers of small ant societies seldom show such collaborations, even of the accidental kind typified by army ants. Being dependent on individual initiative to get things done, each worker is likely to do fine on her own, often aided by special tools, such as trap jaws. Marauder ants serve well as an example of a large society in that the workers are more likely to complete tasks by toiling together or by sharing information with other specialists by means of the language of complex ant societies—chemical communication.

Humans are in some ways similar. Anthropological studies have shown that small groups of hunter-gatherers tend to be labor generalists, with everyone having the ability to be self-sufficient or near to it and pulling his or her weight with a wide range of work (beyond some sexed-based differences). In larger human societies and with increasing urbanization, a complex division of labor in which individuals have limited employment skills becomes more prevalent—as it is for workers in many ant species with large colonies too. This pattern has been understood in humans since 1776, when a Scot, Adam Smith, founded modern economics with his book Wealth of Nations. Smith saw specialization as necessary to the growth and development of societies because of the productivity resulting from each laborer’s skill at his job and the reduction of time lost in switching between jobs.¹⁵ But Smith also saw in this specialization the tragic “mental mutilation” of laborers, a decline in intellect from the repetition of menial tasks that he claimed must be countered by management from the state.

This deficiency can be observed for large ant societies as well, in which specialized workers are incapable of accomplishing much without the cooperation of nestmates.¹⁶ A lone marauder ant is as hopeless as the urban sophisticate who, as in the movies City Slickers and Romancing the Stone, is dropped into a remote environment where he’s incapable of caring for himself. In contrast to the simple interactions between individuals in ant species with small colonies, however, marauder ants show synergy in spades—not only at the emergent level of entire raids, but also more intimately, in the coordination of smaller, local teams. Group transport of food may be the most vivid example.

Synergy and faithfulness to the whole, not independence, are integral to the functioning of the most well-integrated organisms, just as they are with their social counterpart, the superorganism. A sponge, for example, though clearly an organism, is so simple that its cells often survive for a day or two when forcibly separated from the whole and can reunite to form a new sponge, whereas the cells in spilled human blood or a severed finger will perish, and usually in fast order.

Other animals have learned to work around the marauder’s group transport finesse, as I saw for myself in the Philippines. Turning my guide’s animated political conversation to the wonders of animal behavior, I pointed to a blowfly on a leaf to one side of the exposed trail. Gray and black striped, big and stocky, the Bengalia fly twisted on its perch to follow with its big brown eyes the teams of marauder ants moving food on the trail a few inches below. The fly flew down to touch one of these objects, a seed, then flew back to the perch, leaving that vegetable matter alone. Its next choice was a beetle; this time the meat-eating fly wrestled the prey from the porters and soared away, lunch taken care of.¹⁷

The marauder ants, however, have a fly defense system. Minor workers will ride on any large item in transit, with jaws open. They don’t seem to be interested in eating the food. But when a fly descends, the riders, lifted with the booty into the air, rush in and bite hard. This forces the fly to let go of its meal and buzz loudly while hovering in midair, an action that, with luck, will knock off its tormentors. The fly has an additional tactic to safeguard it from this uncertain fate, though. After grabbing a food item, it immediately drops the piece several inches away from the trail before the riders can strike. Then it alights on a nearby plant and waits as the riding ants race off the food and back onto the ground, primed for action. The fly can now swoop down and depart with the unprotected morsel.

As José led me down the mountain late in the afternoon I decided that watching the interplay between ant and fly had been a fine note on which to end my travels in search of the marauder ant. The beauty of this performance, as elegant in its choreography as the peacock’s dance, was in the fact that both the ants and the fly had brains small enough to fit on the sharp end of a pin.