After a four-hour drive in a sedan taxi crammed with five other people, including Caspar Schöning, I emerged barely able to stand. The driver had dropped us off in front of a low building, the headquarters of Nigeria’s best-known national park, Cross River. There a young woman showed us to the office of the assistant director, who informed us that we would have to wait for the director before seeking our ants. After an hour in his waiting room, he ushered us into an expansive office, through a door labeled “S.O. Abdulsalam, Director, Esq.”
Caspar and I explained to Mr. Abdulsalam, whose large frame was wedged behind an expansive desk, that we had just spent two weeks at Gashaka-Gumti, where we found just one species of swarm-raiding driver ant, Dorylus rubellus. After two days in long-distance taxis, we had arrived at Cross River with hopes of finding a greater diversity of army ants. Having listened to us intently, Mr. Abdulsalam declared that collecting ants at Cross River was a laudable and serious endeavor. Meanwhile, his underlings among the park staff arrived one by one and filled chairs around a long table. The director of tourism, the director of security, the director of the environment, the director of education, the director of research, the director of things-that-go-bump-in-the-night—in the end there was a baker’s dozen of them, each putting on a dazzling smile for the boss.
Over the next hour the director of the directors proceeded to invite each man to demonstrate his rhetorical skills. Every one expressed his sincere belief in the Importance of Ant Research at Cross River, his surprise that no Ant Scientist had discovered the park before, and his general gratitude for our visit. The director interrupted occasionally to embellish a point, and when each had finished, he would sum up the speech we’d just heard, adding flourishes of his own that left no doubt as to who wore the oratorical crown. Somewhere along the line I managed to interject that, while we appreciated Mr. Abdulsalam’s generosity, it was getting late and we had lamentably few hours in his fine park. No doubt, I explained, the primatologists from Gashaka (who planned to arrive tomorrow at noon, exactly when Caspar and I had to depart) could advise them after we left on unaddressed matters of every kind.
With this, Caspar and I were shown to the office of the director of security to pay the park entry fee of one dollar. There I learned one way to identify an official’s position in the Nigerian hierarchy. While the director had summoned his secretary with an intercom, and the assistant director had fumbled with an old hand-cranked bell, the lowly director of security had to scream over a shoulder—even though in each case the woman had been sitting close enough simply to talk to.
Bureaucracy loves a vacuum. It takes root and flourishes in places where the cogs and mechanisms of governance are rickety or dormant. I have been confounded by procedural excesses in offices and shops and checkpoints throughout the world, but sometimes the curlicue of red tape takes my breath away. Perhaps that’s why, as Aldous Huxley wrote, “However hard they try, men cannot create a social organism, they can only create an organization.”1 I left those offices thinking about the value we humans place in authorities and chains of command, despite their being so open to abuses of power and greed and so prone to failures of communication. Leaderless ant societies, by comparison, seem to be a universal success story, capable of mobilizing themselves as needed for any job.
By the time we were turned loose it was late afternoon, with a few low clouds rushing overhead. We hitched a ride from the headquarters into the park and, with no time to spare, hiked into the thickety forest. Immediately we found army ants pouring across the road. A little further on we encountered another species. Then another. Walking briskly, we discovered four species of army ants by sunset. Our most thrilling find was a Dorylus mayri driver ant raid, which resembled a swarm of Dorylus rubellus but scaled up a notch. There were millions of ants, and they ran blazingly fast, in a front that spread a full 20 meters wide and traveled high into the trees. Caspar assured me this species compared favorably in size and speed with Dorylus wilverthi of the Congo, which Albert Raignier and Jozef van Boven studied in the 1950s. Their Belgian jaws must have dropped when they documented the largest army ant colony yet recorded: twenty-two million workers, by their calculations. To sustain this army, they estimated, the queen had to lay three to four million eggs each month—about a quarter billion eggs in her lifetime.2 (Months later, on an expedition to Ghana with the army ant ecologist Bill Gotwald, I would come upon another of Africa’s most awe-inspiring driver ant species, Dorylus nigricans, raiding with a swarm front 32 meters across. Crying out from what felt like hundreds of tiny vises embedded in my legs, I ran through a stream of flowing bodies visible for as far as I could see, an experience almost up to the standard of the 1954 film The Naked Jungle, in which Christopher Leiningen, played by Charlton Heston, finds himself “up against a monster 20 miles long and 2 miles wide. . . . 40 square miles of agonizing death!”)
Caspar and I turned our attention to a batch of army ants pouring over a spot 3 meters in diameter. Their columns streamed under every object and reticulated through rotten logs. This “Dorylus species in the congolensis-kohli complex,” as Caspar described it, was intermediate, meaning it carries out its activities just out of view within the leaf litter. Other driver ant species, such as rubellus, mayri, wilverthi, and nigricans, are surface active, raiding on the exposed ground and occasionally in trees, whereas the subterranean army ants live and die mostly hidden from view.
The portioning off of foraging activities by layer may help avoid strife between colonies. While army ants seem willing to assault almost anything in their path, they rarely fight one another; instead, their raids shift out of the way with little squabbling and minimal loss of life.3 Given their cordial rules of engagement, the great myrmecologist Carl Rettenmeyer once proclaimed to me that army ants are the “civilized insect.” I suspect army ants hold back because they are not in competition with their neighbors over territory (unnecessary given the wide and ever-changing expanses they roam). Even in local clashes between army ant colonies, a détente is sensible: whereas the marauder ant uses its raid muscle to rout rival ants, army ants literally devour the competition, including other ant species. Applying lethal force against other army ant species could result in mutually assured destruction. I’m amazed that species raiding many meters horizontally can, as one solution to this problem, separate their actions over so few centimeters in depth, from the surface to the leaf litter to the soil below.
SLICERS AND DICERS
That night, Caspar and I set up our tents near the park entrance. I shared with him my most memorable moment of the day: watching a mass of Dorylus mayri ants in the midst of a huge swarm raid remove the eyes and limbs of a pair of 3-centimeter-wide freshwater crabs—small, but rock tough. This kind of activity would be difficult for most tropical American army ants, whose plier-like mandibles tear flesh but are incapable of slicing it. Only a few species can manage to consume even a frog; the others have to abandon their vertebrate carnage uneaten.
The mandibles of African driver ants, however, are like sharp scissors, built for severing. I had guessed that might explain why driver ant raids are eerily quiet to those who have witnessed the swarm raids of the New World Eciton burchellii, which are accompanied by the chirp of “ant birds” that snatch prey that has been missed by the ants, the buzz of parasitic “ant flies” that lay eggs on the escapees, and the flutter of “ant butterflies” that feed on the droppings of the birds. In the driver ant raids I witnessed in Africa, these attendants seemed to be missing—perhaps, I hypothesized, because the driver ants would make mincemeat of them. However, I later learned that birds are actually present, but of a species that is quiet and circumspect.4
What, then, of the claims of popular stories, such as the one about Leiningen, that army ants regularly kill and eat vertebrates? Caspar told me that for all his months in the field, the only examples he’d seen of live vertebrates being killed by driver ants were a frog and a litter of helpless “pinkies,” baby mice the size of the tip of a pinkie finger. In Cameroon several years before, I had helped tribal pygmies remove an antelope from a snare that had one flank partially carved away by driver ants. It must have been bleeding and restrained at the time of their arrival, making an easy target. Most records of vertebrate predation in equatorial Africa concern tethered specimens like that, Caspar said. It’s no coincidence that people living within range of driver ants keep their babies on their bodies and let their livestock roam free.
So, exactly how good were driver ants at carving meat? Two days earlier I had conducted an experiment: I gave a rubellus ant colony a fresh lizard kill, dropping its 6-centimeter carcass right on the ants’ trail. They showed no hesitation: in two minutes the body was packed with workers of all sizes trying to pry through its skin. Each stuck one mandible between the scales and squeezed hard; but again and again, the ant’s opposite jaw slid uselessly over the surface. Two hours later the lizard remained unscathed. During the third hour, the ants began to carve off strips of scales, and three hours after that, most of the skin was peeled away. Altogether, it took the ants ten hours to reduce the lizard to a spinal column. An impressive perseverance, though no doubt a thin-skinned frog or pinkie would have been an easier meal.
To a much greater degree than marauder ants, driver ants process food where it’s found—tearing up their prey to the extent that, to quote the coroner in The Wizard of Oz, “she is not only merely dead, she’s really most sincerely dead.” Their proficiency at butchering prey is essential to their livelihood, due to the fact that they heft burdens by slinging them under their bodies. Although this group transport technique is efficient in that it allows the transporting ants to easily pull together in a common direction, it means that all booty must be carved up until the pieces will fit between a worker’s legs. The legs of rubellus are long, but not long enough to handle large prey whole unless it’s skinny, such as earthworms and centipedes, which can be straddled by up to six ants—though in my experience, the workers beyond the first can be meddlesome assistants. As often as not, they climb partially or entirely atop the food, to gnaw at it rather than aid in its transport.
GENERALISTS AND SPECIALISTS
Our bedding prepared for the night, Caspar and I joined four rifle-bearing park guards dressed in green uniforms, berets, and canvas boots at a nearby concrete outpost for a dinner of spiced boiled yams, plantains, and chicken. We talked to the men about the diet of Americans and Europeans, and then about the African army ants and what they eat—something that probably differs by species. Most, such as the congolensis-kohli complex species, go after just about any prey: we had seen them retrieve caterpillars and grubs and even watched them storming the flank of a relatively vast grasshopper, which suffered them quietly for a minute before exuding a noxious snowy foam behind its head and leaping to safety. The swarm-raiding rubellus, which also feeds indiscriminately, is likewise a “generalist predator.” There are other army ants, however, that have been found carrying only earthworms and have been described as “earthworm specialists.”5
How can we tell if a species is a generalist or a specialist?6 After all, an individual’s diet depends on a cascade of contingencies. Once an ant finds prey, the choices she makes depend on her colony’s needs, the time and energy required to pursue the prey, the risk involved (whether it is likely to hurt her or to waste her time by escaping), and the nutrients and energy it contains.7 Many ants, for instance, give termite colonies a pass because termites are so well defended. Other species, such as Gashaka’s Pachycondyla, display behavioral and anatomical adaptations that specifically aid in catching termites, such as body armor and strategies for recruiting a regiment and disabling termite soldiers—though a cafeteria experiment, in which scientists provide the ants with a buffet, might show that they eat other things as well. In fact, some prey are so defenseless that almost any ant may consume them: a helpless caterpillar, for example, must be hard to pass up. Finally, familiarity with local sources of food can also be a factor for some ant workers, just as it is for the person who grew up in Chengdu, say, and craves Sichuan cuisine for the rest of her life.8
But whether a species is a specialist or generalist is determined not simply by what it will harvest and eat but also by its foraging behavior. A specialist not only has the skills to catch a specific food but has a search strategy that targets its food source. Consider, for example, the specialist Centromyrmex ants, which live and die inside a termite nest. The workers have to be blind, tough, and strong-limbed to invade termite galleries, where they are unlikely to contact anything to eat but termites.9 If, instead, Centromyrmex took their termites by foraging more widely, and if they readily transported such easy prey as caterpillars back to the nest, with their adaptations to capture termites they could be defined as “specialists” with a varied diet. We might liken them to early hominids, who made tools to hunt mastodon but, only occasionally having opportunities to use them, dined mostly on other things. In the end, whether a species is considered a food generalist or specialist depends on whether a researcher is interested in, among other things, its morphology, activities, habitat choice, ecological role, or everyday diet.
Driver ants concentrate their foraging on a specific stratum on or under the ground; yet within their preferred layer, most species are consummate generalists, scouring the raid front for any prey they can detect and catch, in whatever habitat they happen to be passing through, and with, as far as we know, little regard for costs and benefits. Their polymorphism, however, offers the possibility of specialization within a colony, because the workers can act almost like different species in their contributions to hunting and gathering. One way for this to occur is through size matching, in which, for example, small workers find or kill and harvest smaller prey than do larger workers. While there is no evidence that an army ant worker will pass up prey in favor of another more suited to her size, small workers are able to search tunnels too narrow for their larger sisters, which could open up unique opportunities for them.10 Meanwhile, bigger ants can carry bigger items to the nest, though rather than selecting their booty by size, it’s more likely each ant cuts a chunk appropriate for her to lift.
Back at the Gashaka field station there was a species of ant that didn’t put on anywhere near as obvious a show as rubellus. This ant, which belongs to one of the primarily subterranean groups of the genus Dorylus, was in an easily overlooked colony that I was lucky enough to locate and study. In the lingo of army ant biologists—of whom there are perhaps a dozen—these subterranean driver ants are simply “subs.” Compared to intermediate army ants like the congolensis-kohli complex species, with their moderately long legs, and the even more lissome surface-active driver ants such as rubellus, which use their spindly legs to run fast on open ground while holding large objects beneath them, subterranean army ants have narrow heads and short extremities.11 These are sensible adaptations. Long limbs get in the way in cramped quarters, whereas short, stocky legs are more suitable as digging tools, giving better leverage for moving soil. Compared to driver ants on the surface, which can be spaced several body lengths apart as they move along, the subs are often piled on top of each other within their narrow passages.
Subs live in terra incognita. Stefanie Berghoff, then a doctoral student at the University of Würzburg in Germany, has made the only attempt to date to study them in depth.12 She went after an Asian species, Dorylus laevigatus, one I have from time to time seen crossing trails and moving under logs, typically massed as thick as porridge. By placing bait in buckets full of holes, Stefanie showed that the ants employ stable underground trunk trails to continuously access the same foraging areas for two months or even longer—a pattern previously unheard of among the army ants.
As with marauder ant trails, the laevigatus trunk trails are an expanded base from which to hunt en masse. Shifting networks of raiding columns extend from the trunk trails to catch invertebrates. At the front of each column, Stefanie found, pioneer ants led the advance in classic army ant style, with each ant replacing and surpassing others while presumably laying a short exploratory trail. Columns stayed beneath the surface, but on occasion they would emerge aboveground. Usually the column raids retreated after ten or twenty minutes, but when the ants in the column contacted a termite mound or palm oil in one of Stefanie’s buckets, the route they were taking was reinforced by the workers exploiting the food bonanza until it transformed into a branch of the trunk trail, remaining active for twenty-seven days or longer. The workers harvested few termites at a time from the mounds, but they also attacked several ant species for their brood, along with worms and a wide spectrum of insect larvae.
Although laevigatus is essentially a column raider, Stefanie also recorded three full-bore swarm raids on the surface, all of them small for any of the army ants that swarm-raid full time, the widest being only 3.5 meters at the front. Each swarm advanced from multiple entrances, moving at the marauder ant’s slow crawl of 2 to 3 meters an hour for up to eight hours, in each case set off by a colony’s excited response to the baits of palm oil that Stefanie had placed nearby.
The workers of the subterranean army ant Dorylus laevigatus in peninsular Malaysia typically move in a dense mass.
The subs that I found myself observing at Gashaka, like the rubellus colony Caspar and I were following at the same time, lived on the grounds of the field station. This complex consisted of a couple of simple houses for the scientists and a concrete supply building with a veranda complete with two lounge chairs and a dining room table. Nearby was a dirt-floored, thatch-roofed kitchen. Most people would not have taken notice of a muddy basin a meter wide in front of this structure, where the cook discarded his dishwater, which was then slowly absorbed into the ground, leaving scraps of food. But for a biologist in a dry place, any source of nutrients and moisture is worth inspecting. And sure enough, here were the subs, smaller and a brighter orange-red than the rubellus workers. I spent hours watching them, pulling up a log for a comfortable seat. Each time I shifted a spaghetti strand to get a better view, the cook, keeping a safe distance, gave me an odd look—and who could blame him?
Watching the subs emerge from the earth made me wonder what it’s like down below for them. It must be one thing to excavate dirt incrementally, as most ants do while building a nest, and quite another to prowl through the soil for food day in and day out. Human beings rarely travel through the earth: cracks and crevices suited to our size and locomotion are scarce. But for creatures the size of an ant, the soil offers a number of travel options. Often, half of a soil matrix consists of pores, which result from the imperfect packing of different-sized particles, large to small, from sand to silt to clay. Larger organic matter can cement these particles into dirt clods, or peds, with crevices, or voids, between adjacent peds. All of these provide an infinite number of passageways for the subs, and if one of the pores or voids proves too cramped to pass through, they can expand it by raising and lowering their bodies.
Particularly beneficial to subterranean raiders are the cracks that form as soils dry, along with the macro pores that arise from the biological and physical impacts of dissolving minerals, tunneling worms, and decaying roots. Generally such conduits are more continuous than pores or voids, though the raiders still need to remove debris to keep them clear for transit. During such excavations, the soil particles are pushed from worker to worker under their bodies in a kind of bucket brigade.13 This was first observed in a South American army ant by the nineteenth-century naturalist Henry Walter Bates:
In digging the numerous mines to get at their prey, the little Ecitons seemed to be divided into parties, one set excavating, and another set carrying away the grains of earth. When the shafts became rather deep, the mining parties had to climb up the sides each time they wished to cast out a pellet of earth; but their work was lightened for them by comrades, who stationed themselves at the mouth of the shaft, and relieved them of their burdens, carrying the particles, with an appearance of foresight which quite staggered me, a sufficient distance from the edge of the hole to prevent them from rolling in again. All the work seemed thus to be performed by intelligent co-operation amongst the host of eager little creatures; but still there was not a rigid division of labour, for some of them, whose proceedings I watched, acted at one time as carriers of pellets, and at another as miners, and all shortly afterwards assumed the office of conveyors of the spoil.14
Sitting next to the washbasin, roasted by the midday heat, I pondered Bates’s observations. Stefanie’s Asian laevigatus nested in interconnected chambers underground, with one queen and more than three hundred thousand workers. I envisioned her ants racing out from these chambers along trunk trails that resemble subway tubes crammed with pedestrians, and wondered how they conducted their raids underground, an operation impossible to observe. In my imagination, a small worker pushed into a crack in the tube wall, to be followed by others. From this humble beginning, columns of workers soon spread through the soil matrix. Looking closely at one of the columns, I could see the ants shove aside or remove objects from their path, creating a route through the porous matter. As they reached a crack, they roused an earthworm and drove it ahead of them. (Although the surface-swarming driver ants earn their common name from this herding behavior, there is no reason to suppose the same thing doesn’t occur in species raiding underground.) The worm crawled into a labyrinth of abandoned root channels. The ants cornered it in a cul-de-sac, much as the rubellus forced the carpenter ants up grass stalks until they could go no farther. Their exertions focused thus far on advancing, the small workers had cleared a corridor just wide enough for themselves. While some of them restrained the earthworm, others went back over the raid path, enlarging it to make room for the larger workers who would be recruited to assist in the kill.
Barring cave-ins, such a passage will be available for later use. Because of the laevigatus ants’ constant raiding, commonly foraged areas will eventually fill with their tunnels “until they look like Swiss cheese,” Stefanie told me. By clearing their abandoned passageways during occasional raids, the army ant subs provide attractive living space—a limited resource belowground—for their future prey.
Everyday ants, not just army ants, build tunnels when constructing their nests, and these can yield “homegrown” food as well. As a child I had an ant farm, which the manufacturer provided with Formica “wood ant” workers and pupae. The earth I gave them for digging had prey in it. One day, I noticed a worm nose through the soil and enter one of the ant galleries. The ants normally came to the surface to eat bits of my dinner dropped on the area at the top of the farm, but they were able to grab the worm and consume it without leaving their chambers. I suspect this kind of thing happens all the time, even with species that search for food on the surface.
How does raiding underground compare to raiding on the surface? The capacity of subterranean army ants to swarm out in three dimensions vastly increases their prospects for locating prey, though the constant need to excavate raid pathways may slow the search. Yet because army ants often take prey larger than they are, the subs are unlikely to lose what they track. Their quarry, especially their larger quarry, is unlikely to find an escape route that the ants can’t follow.
Driver ants such as rubellus are conspicuous for raiding in swarms on the surface, but that doesn’t stop them from searching underground, too. Raiders investigate every cranny, occasionally dredging up earthworms, and as Caspar and I had discovered, they will also go underground to demolish immense termite nests embedded deep beneath their mounds. Even the trail leading from the raid to the nest may be replaced by alternative, subterranean routes. The first time I sat down to watch a rubellus trail, the ants had all disappeared by the time I looked up again from my notes. Then I noticed that they were avoiding me by going into and out of holes in the ground on either side of me. My meddling had set the ants to patrolling, and apparently they’d probed around for a preexisting tunnel that now served as a substitute route to bypass me. What a sneaky superorganism!
The difference in habitat between “sub” army ants and surface-foraging driver ants such as rubellus must therefore be one of degree. After all, driver ants nest underground. And since they migrate frequently to new sites, they must be skilled at exploring belowground to evaluate possible homesteads.
To determine if a cavity is suitable for a new nest, honeybees fly back and forth within it to ascertain its volume and condition.15 Ants may choose living spaces in a similar manner. We don’t know exactly how they do it, but one plausible theory is that a scout assesses the area of a potential nest by laying a segment of pheromone trail and wandering on. The larger the space, the less often she will bump into her own trail again. The method driver ants use to gather intelligence on nest suitability is especially mysterious, given how voluminous their cavities must be to accommodate millions of ants linked together like curtains of chain mail, a great way to pack a huge population into a compact, but still substantial, volume. Sometimes they make errors, as Stefanie wrote me. “I once watched an E. burchellii emigration in Trinidad. They moved into a cavity under a rock which was obviously too small. In the middle of the emigration the ants appeared to ‘realize’ this and frantically started to search for new bivouac sites. They would start new bivouacs (i.e., attaching themselves and forming small balls of workers) almost everywhere—including under my boot!”
From studies of other ant species, we know that the more attractive a site, the more quickly and easily a scout will be able to recruit nestmates. She gauges the site’s desirability from her estimate of its size and from such other factors as the width of its entrance, its height versus its breadth, and the amount of light entering the space—a detailed evaluation that would seem to require some rudimentary cognition. With multiple workers gathering these kinds of data from different places, after a while more ants are recruited to superior locations, with each new arrival confirming the site’s virtues for herself, or following another worker to a different location if she’s unimpressed. The final choice comes about by quorum sensing, a kind of decision by voter turnout. When the ants recognize that enough nestmates have already accumulated at one particular site, the migration to the chosen real estate begins.16 Surprisingly, this voting behavior is not known to take place within organisms, or at least not healthy ones. It is thought that cancer cells may use it to regulate their interactions.17
THE SOLITARY ARMY ANT
There by the kitchen washbasin in Nigeria, what struck me about the subs was that they seemed to dine night and day, not on live prey, but on debris and human castoffs. Driver ants will devour some refuse, but only as a supplement to their regular diet, and only for a short time. At the field station, rubellus driver ants had consumed watermelon chunks in meals that lasted several hours. On another occasion, Caspar and I came upon a forest encampment; probably erected by poachers hunting bush meat, it was little more than sticks lashed with vines to form a primitive shelter. The poachers were off somewhere, but rubellus were massed thickly on their dishware and feasted on cooking-oil residue for a couple of hours.
By contrast, the subs at the research station fed at the washbasin for more than two weeks. They must have maintained stable underground trunk trails to reach the basin, much as did Stefanie’s Malay subs, but they emerged from a number of exit holes in frequently changing columns to access oily food scraps and drowned insects in the mud, and they went crazy over the neon-red fruit of palms I gave them from the nearby forest. Uncurbed by midnight chill or the direct midday sun, they continued to come and forage as long as the basin was moist from the cook’s discards.
This army ant species was common: Caspar collected them at baits of margarine that he buried at several sites in the forest. The swarm-raiding rubellus, in contrast, seldom showed up at the margarine baits. Perhaps they are poor competitors underground or, with their longer legs, are more circumscribed in their movements down below.
By the third day of observation the subs began to strike me as odd.18 While driver ants will feed opportunistically on fruit or corpses, they are overwhelmingly predatory. The popularity of the washbasin (which, with its kitchen dregs, made for an interesting cafeteria experiment) among the subs suggested that they were indefatigable scavengers. Moreover, whenever they came upon live prey, even frail ones, they were ungainly in making the kill. The half-dead flies I slapped from my neck managed to crawl to freedom, with only one or two workers clinging ineffectually to them for a time. The subs were more successful when their numbers were greater: one healthy fly made the mistake of landing on a busy column of workers and was captured—an example of a trail serving by chance as a “trap.” The subs were also different from other army ants in that they seldom slung food, whether a rice grain or an insect, under their bodies. Instead, groups of up to six carried food between them by grabbing it at different angles in the manner of other species of ants. Perhaps the standard army ant protocol didn’t work well in the confined underground spaces where they normally foraged.
The real surprise was that the species didn’t seem to forage in a mass, at least not with the quick and cohesive advance of the trailblazers of other army ant groups, such as Stefanie’s tightly packed laevigatus. This had to be the reason the subs were ineffective at catching prey. Rather, the workers would slowly—over a couple of hours—spread apart from a hole in the ground or one of their trails until they had scattered across an expanse 30 centimeters or more from where they had started. As they searched during the ensuing hours, some went as far as 10 centimeters from their neighbors—a long way, relative to their size and leisurely movements. Mind-numbing to document, this activity didn’t involve a raid front or any other kind of regular progression.
The workers had no trouble, however, coordinating food harvesting. A strong column of them developed quickly after I dropped a dead cricket in front of a lone ant. But this behavior looked more like what I would expect from run-of-the-mill ants than from army ants, which recruit to food items during the advance of a tight raid. The pattern can be difficult to pick out from all the action, but to see recruitment in a swarming species like rubellus, it’s best to find a worker that has accidentally become separated from her neighbors, perhaps among the stragglers from a raid or on patrol near a trail after a disturbance. Drop an insect in front of this ant, and she runs in loops, releasing pheromones that set nearby ants to looping in the same manner until they converge on the prey.19 In a rush, a column of ants extends to the quarry, and they carve it apart.20 By strewing dead insects in the washbasin, singly or in numbers, I found not only that the sub recruited help far less explosively than her army ant relations, but that she wasn’t doing so during the advance of a raid.
The army ant “sub” species at the field station in Gashaka, Nigeria, would forage for kitchen refuse, such as this pebble slick with cooking oil that they are carrying.
If the Nigerian subs hadn’t belonged to one of the army ant subfamilies, I wouldn’t have given them a second glance. After all, many ants have foraging patterns in which many independent workers “diffuse” outward from trails or nest entrances, laying recruitment trails when they find new food.
That thought brought to mind Pheidologeton affinis, a similarly unprepossessing and partially subterranean ant that I’d studied a few years earlier at the Gombak Field Station in peninsular Malaysia.21 I had hoped to detect raids in affinis, since it is cousin to the marauder ant, my Pheidologeton diversus. At Gombak, in a low building in a clearing in a forest, I lived on instant noodles cooked in a room with no electricity. Bats swooped by my kerosene lamp each night as I wrote my notes. My clothes, washed in a bucket out back, never dried completely, which gave everything I owned a mildewed odor. One week, the kitchen tap yielded only a trickle of foultasting water. A brief investigation netted me the corpse of a snake that had plugged the pipe to the rooftop cistern. Feeling queasy, I sat down on my front stoop and watched the affinis ants, which were emerging from holes in the soil near the steps and crept along in several meandering columns extending into a patch of lawn.
I knew these columns typically lasted a day or two, with upward of five hundred ants passing by each minute, though most often their routes were much more ephemeral and lightly used. Sometimes a column would elongate by a meter or two over several hours. This happened in bursts after foragers came upon seeds from a crabapple tree, their most common food; the ants would then extend their route to the area with seeds. The surfeit of arriving ants would scout several centimeters in the vicinity, which often led to finding additional seeds.22 If that failed to happen, their activity would fade away until the column retreated.
Dependent as the workers were on finding food, their progress never transformed into anything that could be mistaken for a raid. Moreover, they acted cripplingly shy. Retreating from confrontation, they were usually hopeless with prey, avoiding the centimeter-long lawn-dwelling caterpillars even when I dropped one right on their column. If a caterpillar didn’t go far, however, ants would follow it for a few centimeters and give it a tug. Sometimes a media or major would arrive to crush it in her jaws, and minors and small medias would carry it off.
Day after day, my patience sorely tested, I spent hours putting little flags in front of roving individuals and groups, wishing I could figure out what affinis was up to so I could study the army ant–like Pheidologeton silenus in the forest nearby.23
Could what I saw with Nigeria’s army ant sub be some kind of spread-out, sluggish raiding? Perhaps the behavior of the sub and the Malaysian affinis looks different when the ants are constrained to narrow channels in their favored habitat, the soil; certainly packed together under these circumstances they must catch roving prey that blunder into their columns, as the ants had done in Nigeria with the hapless fly aboveground. But looks can be deceiving, and I knew that finding workers at high density wouldn’t be enough to establish that active mass hunting occurs. Army ant raids in particular don’t involve scouts and are not sustained by constant food discovery. I specifically wanted to determine if and when both the Nigerian subs and the affinis ants lay trails, not just to food but to new ground—exploratory trails. I pined for a way to make pheromones glow brightly—something like the spray used by CSI forensic detectives that makes blood stains visible—so that all these hidden details would be clear. Regardless, it seemed unlikely that the workers of either species were acting as a mass-foraging group.
Related to the marauder ant, Pheidologeton affinis collects only weak prey and tiny seeds (see the minor worker at bottom).
The slow-motion foraging of affinis and the Nigerian sub suggests possibilities for how army ant–style raids originated in the army ants, and among the marauder ants as well. Here’s what scientists call a thought experiment: Consider what incremental changes in behavior over time could transform a species that searches for food alone into a mass forager. It’s not as if scouts can begin recruiting nestmates only part of the way to food. A plausible option might be to start with a trunk trail that guides ants to places where they spread out to look for food individually. A trunk trail reduces the time required to obtain assistance: a successful forager need only draw from the ants on the trail, not the ones back at the nest, for help. The faster the trail system grows, and the less far the workers depart from it in a search for food, the shorter the lag time between finding a meal and getting help to harvest it. Reduce this lag time sufficiently, and the ants begin to become proficient at overpowering quick and dangerous prey. If these changes continue, the trail-making process begins to take on the form of an army ant raid.24
The full sequence, from individual recruitment trails to the emergence of trunk trails and continuing with the development of a coherent raiding army, offers for me the same sense of wonder that other biologists have experienced in unraveling the evolution of the human eye, in all its complexity, by looking at the light receptors in the skins of simpler creatures. Such are the pleasures of studying life at the scale of a superorganism.
Every evening the primatologists would return to the field station sweaty and aching, mostly without having seen a chimpanzee. As I sat by the washbasin, I reflected that with ants, discoveries don’t necessitate an arduous trek. Plop down anywhere, and wonders show up before your eyes. One good friend, Stefan Cover, found a new ant species floating, half drowned, in a swimming pool in Arizona. Bill Brown, a maverick of ant research, was renowned for collecting dozens of kinds of ants from a single rotten log.
When I was about to leave Gashaka for good, I poured some honey bait among the subs in the basin as a gesture of farewell. The cook, wearing a long, bright-colored shirt locally known as a buba, watched me from a shady corner of his kitchen hut. He looked relieved that I, who had sat in the heat-addling sun for days staring at offal, would be out of his way. As I departed, glancing over my shoulder at the station, I saw him dash out of the hut, get down on one knee, and stare into the basin before scratching his head and returning to the shade.