Читать книгу Adventures among Ants - Mark W. Moffett - Страница 12

3 division of labor

In the short grass of the Singapore Botanic Gardens, I dropped to my knees, then lowered myself to my elbows and, at last, to my stomach, eye pressed to soil, camera extended in front of me. My perspective standing up had been abstract, like that of a general assessing the movements of troops from a hilltop, where they were more pawns in a game than people engaged in a life-and-death struggle. Now, seen close-up through my camera lens, a marauder minor worker stood tall and solid before me, antennae moving as if to sniff me out. Her forebody was raised, forelimbs almost lifted from the ground, mandibles open. She was ready to pounce. Suddenly I saw the silvery blur of some creature, through my lens the size and shape of a tank, and the worker was yanked from her spot. I recognized the beast as a roly-poly, or pill bug, a quarter-inch multilegged crustacean presumably flushed at the raid’s front lines.

My worker had seized one of the pill bug’s furiously moving legs. Though knocked about violently, she managed to hold on. Two other minors, and then three more, grabbed the pill bug by other legs or the edge of its carapace. One whose head somehow got smashed released her grip and fell away. The others were strong enough to bring the pill bug to a halt. It tried to roll into a ball—a ploy that gives the bug its common name—but the tightly anchored workers prevented it from protecting itself. From the left, a media worker lumbered into view. She used her antennae to survey the scrimmage. Then she opened her club-shaped mandibles wide and struck. The pill bug’s pale underbody went limp. Watching this skirmish conclude, I couldn’t help but think about how groups of early humans brought down woolly mammoths using nothing but guts and some simple stone tools.

When I left Boston for Asia in 1981, I had a premonition that I would discover amazing things about the marauder ant—so amazing that my thesis committee might suspect I had concocted stories while smoking an illegal substance with an Indian guru. Knowing I had to come home with indisputable documentation, before I left for Asia I bought a how-to book on photographing supermodels, Cosmopolitan-style. With $230 in equipment that included a used Canon SLR, a macro lens, and three $15 flash attachments that gave me electric shocks, I miniaturized the glamor studio the book described by affixing the flashes to the front of the lens with a pipe clamp. By adjusting the strength of my lights, I adopted the concepts of “fill” and “hair light” to accentuate the gleaming exoskeletons of my minuscule models, defining each limb and chiseling every fiber on film.

During my travels in Asia, I used my camera to observe ants, triggering it whenever something happened that I wanted to examine later. In India, trying my equipment for the first time outside, I was stunned to see that through my lens, ants towered. Soon I was stalking them through the viewfinder with all the thrill nineteenth-century hunters must have felt tracking lions. With both quarries, the trick is to go unnoticed, to catch everyday behavior without being bitten—admittedly a more high-stakes proposition with a lion. Still, when tracking an ant in this way, I would forget her size, and she gained all the grandeur of the king of the jungle.

A minor worker stands a couple of millimeters tall. Photographing such a tiny insect requires concentrated effort and lots of illumination. When I focused the camera on my leg, my cheap flashes gave such an intense pulse of heat and light that smoke rose from my jeans. Fortunately, reducing the setting to one-quarter power solved the problem while providing sufficient exposure, but even then, the part of the picture in focus was often only a fraction of a millimeter deep—the length of a paramecium. With the flashes toned down, most ants ignored my “light cannon,” especially when struggling with prey. Like a lion, an ant is easiest to approach and photograph when it is preoccupied.

In my six months in India, my photography budget was tight, but I took an occasional picture of marauders swarming, collecting seeds, and being harassed by hairy Meranoplus workers. Before I flew to Singapore to continue my work in Southeast Asia, I wrote the Committee of Research and Exploration at the National Geographic Society, which had given me a grant, to ask if they could develop my film. The committee’s chairman, Barry Bishop—a member of the first American team to climb Mount Everest—kindly agreed. I put six rolls of Kodachrome 64 film in an express package and sent it off to him. Two weeks later, I was surprised by a Telex announcing that a writer from National Geographic was flying to India to meet me—about what, it didn’t say.

A few weeks later, I left Sullia and traveled to Bangalore, where I was to meet the writer, Rick Gore, for breakfast at his hotel, Bengaluru, the finest in the city. By then, I had been living in rural villages so long that the hotel gave me culture shock. The corn flakes and coffee, though everyday American foods, were pricy by Indian standards, costing more than I spent in a week in Sullia.

Rick told me my photographs had gone to Mary Smith at “the magazine,” who wanted to support my efforts, maybe even have me write a story for the magazine. I didn’t know it at the time, but Mary is legendary for her work with such iconic scientists as the paleontologists Louis and Mary Leakey, the undersea explorer Jacques Cousteau, and the ape experts Dian Fossey and Jane Goodall. Why did she want to work with me? “She likes what you are discovering,” Rick told me. “She also has no idea how you are making the ants look so glamorous.”

I had no idea either. Up to that time the only photos of mine I’d seen were test shots I had taken of dead specimens back in Massachusetts, and they weren’t anything to crow about. So a month later, when I arrived in Singapore, where Mary had sent the developed slide images, I was stunned. The ants that had been half visible to me through my camera in dim light were clear and crisp on film. Here were marauders confronting furry Meranoplus, sleek Leptogenys hunting termites, eagle-eyed Harpegnathos seizing crickets in mid-jump. Two years later, after my return to the States, when I met Mary, she compared my images to the visuals in the film The Terminator. “For you ants are huge, so they become huge for the rest of us,” she told me. The photographs became part of my first article for National Geographic magazine.¹

THE PLAN OF ATTACK

In Singapore, I splurged on flash attachments that did not shock me. To take in the mass-foraging pattern, I stepped back each day to observe the raids as a whole. But like a physiologist who examines muscle fibers to find out how humans move their fingers, I also came in close with my camera “microscope” to record the individual ants in action and learn the details of how they made their kills and harvested the victims.

These observations came as a welcome relief after months at Harvard measuring ants in museum drawers and categorizing them as minor, media, or major based on their frequency and size.² What I discovered in the field was that the slender minor workers form 98 to 99 percent of the population. Tiny, with heads about 0.6 millimeter wide, they are distinct. There are no intermediates between them and the other ants, which range widely and continuously in size. Within this continuum, there is a distinct peak in the numbers of ants at just over 2 millimeters’ head width, and so these I called “media workers,” and another peak at just over 3 millimeters’ head width, for the majors. A few of the majors are substantially larger, with heads 5 millimeters wide or more—the size category I informally called the giants. The queen, who ordinarily stayed in the nest, had a smaller head than a giant, but a much larger body: she could be about 2 centimeters long.

Among different kinds of ants, I learned, work is divided up in two ways. In some species the workers are similar in appearance but flexible in their job skills, temporarily taking on any tasks as they arise, but the colonies of other species can also develop workers of different sizes to do different jobs on a more permanent basis. The former method allows colonies to adjust more rapidly to changing conditions, but it has its limitations: since the workers are identical and interchangeable, duties that require a specialized skill set may be poorly executed. Polymorphism—variation in size and shape, along with physiology and brain development—is an indicator of a more permanent specialization, and is the primary determinant of division of labor in the marauder colony. Because the workers of differing size are suited to a narrower set of tasks, they expand their activities, if at all, only under stress; in some ant species, for example, soldiers who ordinarily do little except fight will help tend the brood if other workers are taken away by a meddling researcher.³

From this, it has been determined that an extremely polymorphic species like the marauder ant is likely to have predictable labor needs, because the number of members in each physical caste, or size group, changes slowly, if it can be changed at all, based on the colony’s requirements.⁴ In fact, the size frequency distribution reveals something about how many ants of each caste a society requires, somewhat equivalent to the distribution of people in different job descriptions in a city.⁵

To pursue again the earlier metaphor, a colony can be seen as a “superorganism” that functions like the body of an organism, with the number of castes and the frequency of each being analogous to the number of types of cells and tissues and the size of organs. Ant species with small colonies are like the cells in simple organisms in that they have few labor specialists, but marauder ants are intricately specialized. Add the arrangement of the workers in space and their interactions with each other to the numbers and frequencies of the various workers, and one has the “scaffolding” of the superorganism, much as a body is built upon the number, location, and interactions of cells. The parallels are all the more remarkable since both the ant workers in a colony and the cells in a body communicate largely by chemical cues (hormones being a prime example for cells), the biggest difference being that workers are mobile and accumulate dynamically when and where they are needed, while most cells are fixed in place within the body.

Essentially all the participants in the raid front are the little minors. With my photographs, I was able to disentangle the blur of action as these ants brought down a nightcrawler or grasshopper thousands of times their weight. A single minor worker has no more chance of catching such a behemoth on her own than would an equally small worker of a solitary-foraging ant species. But she shares the front with other minors that contact prey at about the same time, and they pile on like tacklers in a game of American football. With this strategy, the chances of capture improve markedly: as in Swift’s tale of Gulliver toppled by the Lilliputians, strength in numbers can’t trump size.

It makes sense for a colony to produce a lot of minor workers and concentrate them at the front. If the prey were confronted by a single media ant instead, even one weighing as much as all those smaller tacklers combined, the larger worker would be less effective at subduing the worm or grasshopper. Though individually weak, minors working together simultaneously grab their quarry at different places and angles, making it hard for a victim to move. The prey is also more likely to slip by a single big worker than by a barricade of spread-out small ones.

Countless times I’ve watched a nightcrawler inching over the ground or a grasshopper resting on its green blade, minding its own business, as a swarm moves toward it with a whisper like a snake in the grass. If it doesn’t respond by reflex, death is certain. At the touch of the first worker, the worm flips back and forth; the grasshopper makes its leap. But out of view in the vegetation, more ants are swarming in. About half the directions the flipping worm or leaping grasshopper could choose will land it deeper among the ants, while the other half will allow it to evade the ants by getting ahead of the raid. Blundering deeper is like colliding with a dragnet with a mesh of the width and strength approximated by the closeness and size of the ants; the more the worm or grasshopper struggles, the more the masses converge on it, as other ants are alerted and drawn into the fray. Soon all the little ant jaws hold their prey taut.

Avoiding the ants by moving ahead of the raid provides a temporary respite. The best hope for any creature is to dash to freedom to the left or right of the raid, and so carry itself out of the ants’ path; but the distribution of ants must be difficult for prey to determine down among all the litter and plants on the ground, so taking this course may be a matter of chance. If the prey fails to chose the right direction, the army will advance to its new location and strike again. And if it escapes once more, a swarm may try a third time, or more. Because of their width, swarm raids are most likely to repeatedly contact the flipping worm or leaping grasshopper. (A narrow column raid is different; its net is too narrow and weak, and most victims break free. The ants in column raids therefore reap mostly seeds and frail prey, though the raid may burgeon into a swarm if they find bigger spoils.)

Minor workers at the front of a marauder ant raid in Singapore being cut to pieces while subduing a termite soldier.

A major worker crushing the termite after the minors pinned it down.

Even escapees may not survive. I once saw a cricket rocket from its hiding place beneath a leaf. In a series of zigzag moves it ended up far from the raid, but a few ants still clung to it stubbornly. Their gnawing slowed it down, until at last its body convulsed. However, the ants that subdued it were now so far from their colony that they would die before ever finding their nest again.

Participants in a marauder raid seem to be forever in battle mode. They fight with a dogged precision that is chilling, and in large raids there certainly seem to be troops to spare. The minors show by far the highest casualties. The bounding cricket managed to chew a couple of the minors on its leg to a pulp before succumbing to the rest. On my way back to the raid, I saw minor workers puncturing a plump caterpillar, and one drowned in the jelly that oozed from it. Later on in that raid I saw a termite soldier with a burnished red head that dwarfed the minor workers surrounding her like a grizzly bear cornered by dogs. The termite’s black jaws were sharp as knives, and each minor that came near was sliced apart as cleanly as if by a guillotine, until a dozen ants stormed her hindquarters and brought her down.

Like a war correspondent inured to tragedy, I watched hundreds of minors being sundered and smashed in struggles with prey, the horror of the slaughter magnified through my camera lens. By never straying from the task to save themselves, they displayed breathtaking devotion to their duty. It made me wonder about the advantage of psychological numbness in combat even among sentient humans. As one author wrote of the Civil War, “Soldiers perhaps found it a relief to think of themselves not as men but as machines.”⁶

Such thoughts reflect how caught up I was in the drama of the moment, pressing the button of my camera each time a surprising event happened. I saw that the minor workers were able to stretch the legs of the termite soldier until she was spread-eagled (click). By this time, the raid front had advanced beyond the victim, who was now deep within the swarm. Here the media and major workers roamed in numbers (click). The large ants were as plucky as the minors, and they had the size and mandibular power to be worthy of the designation “soldier”; but by dint of their location, most of them joined the fray at the termite only after the prey was felled (click).

My images transferred onto a storyboard that showed that inside the raid, after the minor workers immobilized the body, the medias and majors were a strike force that moved in to inflict what carnage they could. Small media workers fit into tighter nooks and crannies than the majors can reach, perhaps yielding a kind of division of labor in destruction.

The allocation of effort between the minors, which restrain prey, and the medias and majors, which smash it, is related to their respective locations within the phalanx. It’s unlikely that special communications are used to get ants to these positions; instead, the minors reach the front lines first because they walk more nimbly than their larger sisters, while the larger ants are waylaid by their duty to crush prey farther back in the raid. Regardless, the role of minors at the front lines is clear. Only they and a few small medias secrete trail pheromones, testament to their importance in moving the raid ahead and summoning others to prey.

To a military historian, the marauder ant strategy evinces a classic use of personnel. Placing large numbers of abundant and expendable weak individuals in jeopardy at the front lines not only increases the catch but also minimizes the loss to the society overall. The Romans used a similar strategy at their battlefronts: instead of drawing from highly trained city dwellers, they largely conscripted farmers, who were available in droves and could be replaced at little social cost—a practice that continued at least into medieval times, when poorly trained men were, literally, used as cannon fodder.⁷

The minors’ bold actions assure few large warriors being sacrificed, a sensible outcome given the expense of raising majors that can weigh hundreds of times as much as one minor. In a sense, the medias and majors are equivalent to the human warrior elite—physically stronger, superior fighters, often positioned behind the relatively inefficient front-line rabble. The human elite are provided with better weapons and training and protected by the most expensive armor, as tough as a soldier ant’s exoskeleton.

The large workers are attracted to a prey’s flailing extremities and dutifully hack off every moving leg and antenna. With the prey rendered powerless, unless its shape is awkward (like that of a praying mantis, which the ants will tear apart), the minors heft its body back in one piece. I once saw the ants retrieving a limbless gecko, which clued me in that they had taken it alive.

Dismemberment immobilizes but doesn’t necessarily kill. Moving animal prey to the safety of the nest before the coup de grâce may reduce the chance of its being stolen by competitors or washed away in a storm. By keeping prey alive, the ants may also be able to preserve their meat (something that ants with stingers do by paralyzing their victims).⁸ I learned of this strategy one day at the Botanic Gardens when I snatched a limbless katydid from marauders on the way to their nest. I put it in a jar and forgot about it until, two days later, I noticed its leg stubs still writhing. That night I dreamed I was that katydid, being helplessly transported to the bowels of the nest, to be digested at the ants’ convenience by the protein-hungry larvae.

SPRINGTAILS

Marauder ants conduct raids to catch tough prey, but mass foraging helps them obtain other kinds of meals as well. The poorly armored minors, though not intimidating, are agile and have good vision. I’ve watched hundreds of them retrieve speck-sized jumpers called springtails.

Springtails are the rabbits of the insect world—fast breeding, abundant, and prodigiously jumpy. As the name implies, they use their tails as a spring. If one senses a threat, its tail, or furcula, normally folded under the body, snaps downward, launching the insect through the air.

Before exploring the marauder ant’s tactics for capturing these motile creatures, let’s first look at a very different approach. A speck herself, a burnished red Acanthognathus teledectus ant moves stealthily through the forest litter in Costa Rica, her long, pitchfork-shaped mandibles held straight to each side. Coming on a springtail, she slows to a glacial creep until two long hairs extending from her mandibles touch the quarry, indicating that her distance is perfect. Her jaws snap forward; their prong tips puncture the springtail and hold it tight. Quickly now, the ant slings her hind end under her body and incapacitates the prey with an injection of toxins through her sting, after which she hefts it overhead and carries it home.⁹

With blows from her mandibles, an Acanthognathus trapjaw worker in Costa Rica repels a pseudoscorpion from the tiny hollow twig occupied by her colony. Behind her, a larva feeds on a springtail.

Acanthognathus displays the special skills required for solitary-foraging species to snare these speed demons. Success among springtail-hunting virtuosos depends on stealth and the use of mandibles as an unusual tool. Devices such as trap jaws and stingers are especially common among species with small colonies with only one kind of worker, such as Acanthognathus, whose workers so often need to act alone. Unlike with the antlers of moose or the tusks of elephants, their function is not to impress but to kill and butcher.

“Trapjaw ants” like Acanthognathus have evolved repeatedly among lone-foraging species. Typically, their mandibles are long, with pitchfork-like teeth only at their far ends, and they can open 180 degrees or more. In many cases, the jaws come equipped with trigger hairs. While the ants can be slow, their “bear-trap” jaws are not: the fastest muscular-driven action for any animal is achieved by the jaws of one group of these ants, Odontomachus.¹⁰ These speed-biters nab insects and also ply their mandibles as defensive tools, striking them against the ground when harassed; the resulting recoil sends them flying head over heels to safety. In Surinam, I’ve seen schoolchildren, betting over candy, make a game of encouraging the Odontomachus ants’ bouncing behavior while trying to avoid their searing stings.

Long jaws are great for catching prey but impractical at mealtime. Asian Myrmoteras, another group of creeping trapjaw ants that nest in any dark corner of the leaf litter, chew their prey from afar using the spiked tips of absurdly thin mandible blades that they can open an extraordinary 280 degrees. After chewing, they walk forward to place their mouths on the victim and feed at the oozing wound, then circle back to chew some more—the most awkward and labor-intensive approach to dining I have witnessed in all my travels.¹¹

Acanthognathus have a partial solution to this logistical problem. While they use their long jaws to seize skittish springtails, they avoid the arduous dining experience of Myrmoteras by having a face like a Swiss army knife, with an entire arsenal of utensils at their disposal. To eat, they open their jaws wide, revealing a pair of what look like normal mandibles but are actually curved teeth, sprouting near the base of the longer bear-trap blades. The workers masticate their springtail meals to a pulp with these minijaws. As the small jaws are of a piece with the rest of the mandible, chewing with them sets the bear-trap blades waving to such a degree that feeding ants often knock over their neighbors.

Marauder ants have no elaborate built-in tools with which to seize springtails. Instead they must rely on commonplace, workaday mandibles (which have several small teeth along their forward margins, as do those of most ants). Furthermore, the marauder’s massive, frenetic societies are at the opposite extreme from those of the slow, stealthy Myrmoteras and Acanthognathus. The tempo of an ant species tends to relate to its colony size.¹² Workers in small societies tend to be slow and cautious—a sensible way to approach elusive prey like the springtail on a low-energy budget. (Is the per capita energy quota of a small colony indeed likely to be smaller than that of a large one? Picturing a colony as a superorganism, a physiologist might predict that this would be the case. Since larger creatures are relatively efficient, burning fewer calories per unit of weight—or when measured microscopically, per cell—this gives them energy to spare.¹³ We can extrapolate that the same would be true for superorganisms, resulting, for example, in decreased labor demands for each individual in a large nest.¹⁴ If so, life must be precarious for Acanthognathus colonies—which, in my experience, are very rare, with no more than eighteen workers nesting in the rotted-out core of a single twig on the rainforest floor.)

How does the marauder ant, with its numbers and seeming chaos, nab the wily springtail? Lots of the ants seem to be doing the same thing at once, with sloppy overlap in their activities. But the effectiveness of large societies often has to do with redundancy rather than precision: although an individual ant may not be reliable, the density and overlapping actions of multiple ants ensure success for the raid. As each point on the ground is probed exhaustively, every critter, no matter how small, is rooted out.¹⁵ Once flushed, a springtail leaps about as one ant after another frightens it. Sooner or later, one of the minor workers will snare the springtail and make the kill. The raid, in its entirety, becomes the colony’s bear trap.

The effectiveness of this form of predation lies in exhausting the victim. Lions and wild dogs accomplish much the same thing. Although a solitary cheetah may have the edge on them in terms of speed, working as a pack the group predators can kill a gazelle that easily outruns them, wearing it down by chasing it sequentially, like relay runners, or by driving the animal toward an individual lying in wait. Marauder attacks aren’t as subtle or as calculated, but given the ants’ massive numbers, they may not need to be.

THOSE VORACIOUS OMNIVORES

The marauder ants’ predatory skills are only part of the picture. “The voraciousness of these ants is very great,” wrote a Vietnamese phytopathologist named Pham-tu-Thien in 1924. “We are dealing with a species whose greediness has fully developed its capacity for work.” Pham recorded marauder ants consuming insects, seeds, and fruit.¹⁶ What they take varies widely according to availability—they nibble on such oddities as leaves, flowers, bird droppings, and fungi when few other resources are available. But even when foods are bounteous, marauder ants tend to be wide-ranging gourmands.

Swarm-raiding army ants, often said to have among the Earth’s broadest diets, don’t compete with marauders in this regard. In particular, army ants are poor vegetarians, while marauders collect equal amounts of plant and animal material. Vegetable matter contains cellulose that many carnivores find indigestible. The only army ant approaching the marauder’s omnivorous diet is south Asia’s Dorylus orientalis, which, like the marauder, is considered an occasional agricultural pest—though it eats tubers such as potatoes, rather than the rice and other grains fancied by the marauder.¹⁷

The marauder ant species—Pheidologeton diversus—shows a proficiency at seed harvesting equal to that of many of its seed-harvesting relations in the group to which Pheidologeton belongs, the Myrmicinae, and I imagine the ancestor of Pheidologeton was like many of these relatives in eating seeds while scrounging for dead insects and perchance killing the occasional live one.¹⁸ On my Indian palm plantation, instead of taking their seeds straight to the nest as they did prey, the workers established caches along trails, carrying grain down holes or under leaves, where it was stored or milled to an edible flour by medias and majors. The ants also harvested an herb called goatweed by dropping its seeds to the ground, where workers of all sizes congregated to chop them up for immediate consumption.

Marauders are even more organized when they harvest grasses, one of their pastimes in the Singapore Botanic Gardens. When a raid passes a fruiting grass plant, only the minor workers and small medias can climb the slim stalk. The first minors gnaw the attached seeds ineffectually, but productivity skyrockets when a media arrives. The ants now set up a little assembly line, in which the media extracts one seed after another and then appears to hand it to a minor to haul away. What is really happening, however, is that the minor, who is too weak to pull a seed free from the stalk on her own, snatches the seed from the media before the larger ant can depart with it. The media dutifully plucks another seed, which another minor grabs. With minor workers so numerous, a media seldom has an opportunity to exit with her find.

Windfall fruit and vertebrate carcasses draw much larger crowds that defend and often consume them where they lie. Tens of thousands of workers will dismantle a mango or a dead bird. When I spilled a bag of canary food next to a trail, the ants arrived by the thousands to carry off 300 grams of seeds in eight hours, ten minutes. Under ordinary circumstances, the workers never seemed to become finicky or grow tired of a food, but this overfed colony refused over the next several days to touch any more of the seed.

An assembly line of marauder ants on a grass stalk in Singapore. A media worker extracts grass seeds, which the minor workers carry away.

The one food source that marauders forgo is another kind of bonanza, the populous nests of social insects. Tackling well-fortified bees, wasps, termites, and other ant species requires a convergence of forces to break through the foe’s weak points—a military tactic that marauders lack, though army ants display it in abundance.¹⁹ Indeed, almost all army ants gang-raid social insects routinely; many species especially relish the eggs, larvae, and pupae seized from colonies of their ant relations.

Marauder ants don’t just steer clear of social insect nests; they actively avoid making meals of them. When marauders kill another ant species in a skirmish, they cover the bodies with soil and abandon them. Despite this odd and unexplained aversion to cannibalism, the marauders evolved mass foraging in part for the same reason army ants did, as an aid in battle. They might not eat other ants, but they do compete with them for meals. The swarming multitudes in the raids that the workers at the front lines draw upon to subdue prey can also be used to overpower any rival that gets in their way.

Among combative ant species, known as extirpators, trumping competitors is generally a matter of preemptive control of resources. Arriving at the contested area “first with the most,” as General Nathan Bedford Forrest said of battle strategy in the U.S. Civil War, these species succeed by assembling quickly and in abundance. After driving off more timid species, the ant troops can block other belligerent ants from building up at the site in sufficient numbers to fight back.

Because the marauder ant doesn’t employ wide-ranging scouts, this species is seldom first to show up at a feast. But this doesn’t present a problem: the raiding deluge overruns any competitor and keeps rivals at bay—even other extirpators, army ants among them.²⁰ Their tactics bring to mind the “rapid dominance” military doctrine proposed in 1996 by American military theorists. For humans, being on the offensive puts the enemy in a vulnerable position, giving the invaders a sense of invincibility even when it isn’t justified.

The key objective of rapid dominance is to impose this overwhelming level of Shock and Awe against an adversary on an immediate or sufficiently timely basis to paralyze its will to carry on. In crude terms, Rapid Dominance would seize control of the environment and paralyze or so overload an adversary’s perceptions and understanding of events that the enemy would be incapable of resistance at the tactical and strategic levels.²¹

Marauders similarly take the offensive from the moment they contact alien ants, whether the foreigners number in the thousands or are just two carrying a seed. Often the minor workers blast forward in such abundance that other species fall back with hardly a fight. Even when clashes occur, the marauders triumph by using their first-strike capability. By mowing down enemies a few at a time as the raid advances, the minor workers suffer far fewer casualties than they would if they faced the opposition all at once, a similar outcome to that of the divide-andconquer strategy of large-scale human military actions.²² With the other side routed and unable to recruit assistance, the marauder ants’ control of the booty is likely to remain absolute and uninterrupted from the moment of first contact. In Singapore I watched marauders steer hostile weaver ants up the tree in which this canopy species was nesting, and then the marauders gathered by the hundreds for a meal: they tore off the tree’s bark, rotating bits of it between their mouthparts and forelimbs while sucking out the sap. This food ordinarily draws the marauder ants only in times of scarcity, and indeed at the time there had been no rain for a week.

From springtails and seeds to frogs and large fruit, marauders harvest a cornucopia. They are reminiscent of humans, who apply the dictum “because it was there” not only to climbing mountains but also to adding tasty morsels to our diets. Marauders and people are exceptions to the general rule that in the tropics, where so many different organisms live together, most species, like the springtail-hunting trapjaw ants, become specialists in a narrow niche to survive the intense rivalry for resources.²³ Marauder ants, in contrast, by interfering with all contenders for each meal and taking prey where others fail, exceed expectations by being geniuses at the competition game.

TRACKING FOOD FROM A TRUNK TRAIL

In Singapore’s Botanic Gardens one day, I placed a meter-wide plywood board in front of a raid. The ants crossed it in swarm formation, which confirmed my suspicion that their raids don’t depend on workers finding food or retracing old routes. Even so, I knew the ants were no fools—their raids slowed in areas with little to offer, the number of workers in them declining as the ants drained back to the nest until, if the dearth continued, the whole army would retreat. I decided to find out how the plenitude or distribution of booty changed an army’s strength and direction.

The marauder ant’s vegetarian proclivities made the job easy: it’s more difficult to manipulate caterpillars and crickets than to move fruit and seeds.²⁴ Loaded with supplies from the grocery store on Orchard Road, I headed back to the Botanic Gardens and spread canary seed in a line extending from a trunk trail. It didn’t take long for the marauder workers to leave their highway and flow along this line. They tracked the seeds precisely, continuing outward in a column even after they had passed the last seeds. I had launched my own raid!

Did the distribution of food affect how the raid progressed? I poured a seed pyramid ahead of a swarm. The ants continued forward for several minutes after contacting this jackpot and then drained back to the food, where they rapidly built up in numbers. The swarm raid now over, the excess arriving ants radiated from the seed pile in a network of branching column raids spread over several square meters (a process called recruitment overrun, described in chapter 2). I had seen marauder ants generate similar trail networks under trees dropping fruit, which they thus track down quickly. While column raids are ineffective for catching fast prey, these bifurcating formations shine when it comes to fanning a foraging populace out over large areas. Each time one of the weak raids in a network encounters a bonanza, any number of workers can be summoned within minutes from the trunk trail to seize and consume it.²⁵

What if the enticements are less concentrated? My next approach was to scatter a few seeds in a meter-wide swath off to one side of a swarm raid that was crossing a field with little in the way of food. The raid turned and followed my swath its entire 15-meter length, even though I laid few seeds—one every 20 square centimeters or so, which would put three of them in an area the size of my palm. Somehow, raids track subtle changes in food density, even though the workers coming upon each seed are ignorant of the food distribution as a whole.

How does that happen? While the ants follow exploratory trails at the raid front, they are more attracted to any recruitment trails they come across, which lead to food. When there are more seeds on one side of a raid, ants must be drawn to them by the buildup of recruitment pheromones left by the successful foragers from that direction. New arrivals tend to follow the strengthened routes leading to the food-rich region, causing the raid to turn and track the seeds without any of the ants comprehending what is happening—a fine example of what artificial intelligence experts call collective or swarm intelligence, in which the raid viewed as a whole deals effectively with problems by adapting to changes in the environment. A.I. experts would describe the raid as “robust.” Indeed, from computers to the natural world, scientists have found that seemingly thoughtful processes often emerge spontaneously from the integrated actions of simple-minded agents, like ants, with no need for leaders or any kind of management or centralized control.²⁶

I went back to Orchard Road, depleting the grocery shelves of bird seed to continue my experiments. What mattered to the marauders seemed to be the relative abundance of food: when a raid was bringing in lots of other victuals, I needed more seeds to alter its course. The raids turned out to be smartly responsive to food in a variety of ways, branching or shifting in direction, width, and strength on the fly. Even though the absence of scouts made the raid blind to meals at a distance, the aggregate response of the workers to food at hand apparently enabled the raid as a whole to follow the food distribution in bountiful regions.

It’s a subject of endless fascination for scientists that each ant can only proceed locally on the limited information at hand, and yet their societies manage to act globally. Darwin was right when he wrote that for all ants do with their modest endowments, “the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more marvellous than the brain of man.”²⁷ But the true power of the mind of an ant emerges at a superorganismic level, when those brains join to produce colony-level actions to accomplish a goal. Lewis Thomas, the author who first introduced me to the superorganism idea in my youth, described an ant society as “an intelligence, a kind of live computer, with crawling bits for its wits.”²⁸

HOMEWARD BOUND

One afternoon it occurred to me that I could use the marauder ants’ ability to track seeds to unravel a mystery. Every trail has two directions. How do workers select the correct way home?²⁹

In most situations, the ants have no problem choosing a direction. Because workers ordinarily find food at the raid front—the end of the trail—every returning ant has but one way to go. Along the route, though, are junctions with other trails. Some of these don’t present a problem: trails split at sharp angles, so nest-bound ants will make the right choice if they take the route that lies closest to straight ahead.³⁰ Still, in the labyrinth of trails between raid and nest, I saw many situations in which the ants could have made directional mistakes but rarely did. Why?

I realized that by pouring seeds in an arc, connecting one point on a trunk trail to another point farther along the same trail, I could give the ants a choice of two equally good directions back to the nest. I watched in anticipation as the troops rushed from the trunk trail to track the line of seeds along each end of the arc. Every ant who picked up a seed from the advancing front of either column then turned around and carried it directly back to the trunk trail. When the advancing armies met, the ants now had the option of completing the full loop, and they often did so if they had’t picked up a seed. From their point of view, they were simply continuing as they had been going, away from the nest. A worker that picked up a seed after passing the site where the troops met would not turn around but rather would continue onward—a choice that, in any “normal” situation (not a loop), would have led her away from the nest.

The result was that all the seeds flowed away from where the armies converged. I called the trail segment within a centimeter or two to either side of this point the transition area because ants acquiring seeds in that stretch weren’t consistent in their choice of direction. The transition area was usually near the middle of the arc, but I could change its location by laying down the seeds earlier at one side of the loop, causing the ants who found that end of the loop to travel farther than they did on the other side before the armies merged.

At first, I guessed that the ants had marked the trail with some kind of “arrow,” as invisible to our eyes as the pheromone trail itself, which told their colleagues, “Go this way!” But that hypothesis crumbled when I waited until the seeds were nearly gone and the ants still moved around the arc with nothing to carry. I poured a new heap of seeds along the arc away from the transition area. If the trail contained a directional cue, all the ants taking seeds from the new pile should have gone in the same direction the workers had taken earlier when they took seeds from that spot. Instead, the ants proceeded to haul the seeds in both directions. While the workers were still retrieving seeds from the new pile, I poured yet another pile elsewhere along the arc. All the ants taking seeds from the first pile passed the second one and continued in the same direction they’d been going. But when ants began to pick up seeds from the second, newest pile, all of them followed the lead of the ants going past them with booty from the first pile.

Other experiments confirmed this behavior: ants picking up seeds took the direction of any passersby with food (and if there were none, they could go either direction). Were they being physically forced to go the same way, bystanders compelled to join a mob? No—the seeds weren’t bulky enough, and the carriers weren’t numerous enough, to inhibit ants from going whichever way they wanted.³¹ Instead, it appeared the food-bearing ants were taking notice of each other’s choices and deciding accordingly.³²

As it turns out, this “go with the flow” approach is essential to the marauders’ response to bedlam. Crush a marauder ant underfoot, and some workers, detecting alarm pheromones released by the body, rush off the trail on patrols in which they attack whatever they find. While the patrollers are in defense mode, the food-bearing ants do an about-face, clearing the disturbed area by rushing outbound along the trail instead of continuing to the nest. As laden ants farther along the trail confront this backflow, they turn and join the exodus, in this case propelled away from the nest by the urgent multitudes.

If the laden backtrackers reach the trail’s end, they mill about before starting back to the nest. Usually they don’t get that far: as the fleeing ants spread out more and more along the trail, their frantic pace slows to a normal gait, and they gradually start to turn around again under the influence of all the workers carrying food in the “correct,” nest-bound direction. In either case, by the time the ants return to the point on the trail where the fracas took place—anywhere from five to twenty minutes later—the problem is long gone and the patrolling has all but ceased. It’s now safe to go home.

Except in such emergency situations, traffic on busy marauder ant trails is well organized so as to avoid congestion. The scheme isn’t to stay to the right or left, as on human thoroughfares. Rather, nest-bound ants tend to use the trail center, while the outbound ants stay to the sides. The center is easiest to travel, being concave from use, with few obstructions and the most concentrated pheromone. The inbound ants with their unwieldy loads end up there because they have difficulty maneuvering. Carrying nothing, outbound ants can quickly move to the sides of a trail to avoid their encumbered sisters. Similarly efficient patterns emerge among people, too. Think of how pedestrians will be diverted to the gutter as they try to circle around someone hefting a big package on a crowded sidewalk.³³ And during rush hour, without anyone thinking it out, clusters of pedestrians will move in alternative directions through bottlenecks—a pattern I have seen in marauder ants as well, where their routes head through a bottleneck underground.³⁴