Cross River National Park, with its rich yield of driver ants, was the highlight of my visit to Nigeria with Caspar Schöning. But the highlight of that highlight was a dramatic Mission: Impossible–style assault I saw occur between driver ants and weaver ants.
It was our only night inside the park. Caspar and I had pitched our tents at the edge of the rainforest, near the guard post. All was silent. I awoke early, my back aching from the hard ground, and got up to find a line of bright orange weaver ants hanging by their long legs from nearby tree foliage. Marching swiftly below them was a column of the darkly pigmented driver ant Dorylus sjöstedti, returning from a night of raiding.
Lowering herself carefully, suspended by her hind legs, one of the weaver ant workers reached into the mass of driver ants and, without drawing the attention of the other ants in the surging column, grabbed a worker by the waist. Then, with the assistance of other weaver ants on the same leaf, she lifted the driver ant from the column and pulled it into the tree, where the group tore it to pieces. A minute later another weaver ant did the same thing.
I called out to Caspar and reached for my camera: this was a moment worth recording. Oecophylla longinoda weaver ants dominate Africa’s treetops, creating territorial empires sharply different from the army ant’s roving bands on the ground. (The weavers have an equally rapacious cousin in tropical Asia, Oecophylla smaragdina, a species that tends toward green in the easternmost extreme of its range in Australia and the Solomons.)1 Their initiative in executing driver ants is well documented.2 The move I saw seemed a kind of Russian roulette, though: surely the heavily carapaced driver ants would demolish the slim weaver ants the instant they noticed their depredations.
A weaver ant nest of bound leaves at Ta Prohm temple, Angkor Wat, in Cambodia, where the ants are a local delicacy.
I saw a way to test this. Up in the tree above the driver ant column was a leafy sphere held together with white silk: a weaver ant nest. I clipped off the lightweight bundle and dropped it onto the driver ant column, expecting to see a full-bore retaliation resulting in the death of all the weaver ants. Instead, the normally unflappable Dorylus driver ants withdrew entirely, with every one of the workers doing an abrupt about-face.
So many tens of thousands were in retreat that the traffic artery swelled to the width of my clasped hands; it then grew arms as ancillary columns shot out from it at almost the full ant-running speed of 3 meters per minute. When I scratched the dust in front of one arm, the workers pooled there, confused. That told me the panicked ants were making their escape by backtracking along the base trail of that day’s raid as well as all its abandoned subsidiary trails that the ants could still detect by scent. Thick ribbons of ants flowed on a dozen routes spread over an area the size of a one-bedroom apartment. The army ants were obeying their life’s simple rule “follow a trail.” In applying this rule now, there was a danger: when the path to their nest is cut off, backtracking on old pheromone trails can eventually lead army ants in a circle many meters across. The throngs starve to death as they follow each other round and round, stuck in the social equivalent of the endless loop of a defective computer program or unbalanced brain. This colony was fortunate: by chance one ribbon extended on a path that linked back to the driver ants’ original trail on the other side of the weaver ant nest, enabling the driver ants to retreat to their own nest while giving the weaver ants a wide berth. Soon, half a million ants had drained from the system of emergency trails.
It’s common for ants to react explosively to serious threats. Marauder ants carrying food flee when their trail is interrupted, and carpenter ants rush up grass blades during a driver ant raid. But the scale of this driver ant retreat made “overkill” an understatement, especially for an army ant, whose mode of life is to eat other ants.3 Strangely, though, not a single worker on either side had come into conflict, let alone been killed. Clearly the withdrawal began with the detection not of the weaver ants themselves but of their nest. What about Oecophylla merited such an extreme response?
Each weaver ant nest can hold thousands of workers. A colony can occupy hundreds of these nests in one or two dozen full-grown trees spread over 1,600 square meters of ground, with a total population of half a million, the same size as many driver ant colonies. Weaver ants don’t have stingers; like driver ants, they overwhelm their enemies with their superior numbers and sheer mandible power.
And quite a bite they have, too. Javanese children are warned that they will be tied to trees, to be overrun by biting weavers, if they misbehave. People around the world have learned from experience to back off when they notice the spastic, open-jawed motions of alert weaver ants on leaf and twig outposts in their nest trees. Though driver ants are more powerful, weaver ant bites are intensified by acidic secretions, which feel like lemon juice rubbed into abraded skin. (That said, some cultures have learned how to benefit from these ants’ defensive chemistry. The best chicken dish I ever ate was served in Cambodia with a tangy weaver tapenade. And in Australia I joined an Aborigine suffering from nasal congestion in sniffing a cake of fresh-crushed weaver ants; the fumes had the sinus-clearing effect of the mentholated gel Vicks VapoRub.)
I retrieved the nest I had dropped on the trail. It tore like paper and revealed a mere fifty workers. From the driver ants’ reaction to this little nest, I predict that if there ever were a battle between weaver and driver ants, the ordinarily unstoppable driver ants, normally the terrors of the jungle, would be unlikely to come out on top.4
One basic difference between army ants and weaver ants is this: army ants strip away the workers’ autonomy to the point where few signals are needed to coordinate their troops, whereas weaver ants move with more freedom—even getting lost can be an asset if a stray ant blunders into an overlooked meal.5 But this freedom comes at the cost of added logistics, because a worker must persuade others to come together to help her perform tasks as varied as making a leaf nest and killing driver ants. Perhaps the endless looping that can happen to an army ant column shows that their colonies can at times be too much like an organism for their own good—no such catastrophic failures are known to befall the weaver ant.
Weaver ant nests are most common in the outer, often uppermost, sunlit branches of trees. The site of energy influx and photosynthesis, this shell of greenery is where most biological action in forests takes place and thus where the majority of resources sought by the ants accumulate. There the ants bind adjacent living leaves into a kind of arboreal tent. Ranging from the size of a baseball to the size of a volleyball but weighing not much more than an inflated balloon, the nests look frail, but they shelter the ants from wind, rain, and rivals. Transpiration from their leaf walls creates a built-in HVAC system, providing relatively stable temperature and humidity.
To begin building a nest, a worker pulls at the edge of a leaf, and if she’s successful in bending it, nearby ants join her. The workers may stand side by side while gripping the leaf margins, but if the leaves are too far apart, they climb on top of one another and, seizing each other by the waist, form leaf-to-leaf chains that are strong enough to drag the foliage together.6 Within hours, the nearby leaves are drawn tight and aligned in a nest configuration.
The name weaver ant comes from the next step, which involves a kind of child labor. In many ant species, the larvae spin silk cocoons in which they transform into adults. But a weaver ant larva does not make a cocoon. Instead, it produces silk at a young age, when still small enough to be held and manipulated by an adult worker. After bearing the larva to the construction site, the worker locates a leaf edge through palpations of her antennae, then lowers the larva’s head to it. The larva attaches a silk line to the edge, and the worker then shuttles it back and forth between the leaves, like a weaver working a loom, until the foliage is bound by woven sheets. As a finishing touch, the nest is detailed with tidy entries and internal walls and galleries.7 The nests, I suspect, can last for years: when the leaves wither, workers bind fresh ones into the structure to replace them.
Weaver ants avoid the inconveniences endured by most ants, which, as central-place foragers, spend considerable time commuting from one central nest. This is evident in driver ant raids, where hundreds of thousands of ants regularly travel dozens of meters or more. Weaver ants minimize the amount of moving around they do by spacing leaf nests throughout their territory, erecting them wherever their workers are needed and foliage is available for construction. This also makes it easy for them to handle unforeseen events quickly: a worker seeking assistance need only communicate with the ant reserves in the nearest nest.8
Inside the tent, among the brood piles, are smaller workers with shorter limbs. In most polymorphic ants, the major workers are scarce and specialized, but with Oecophylla the opposite is true, with the majors doing the foraging and nest construction, serving as the workaday ants rather than “soldiers,” in the sense of a specialized defensive caste. The minors are less numerous and tend the eggs and small larvae. The physical differences between minors and majors are more modest than in the marauder and driver ants, but the two are relatively distinct, with only occasional intermediates. Typically, the queen is in a nest toward the center of the territory near the top of a crown, though she moves from time to time.9 Her eggs are distributed among the nests by her workers.
Because weaving a nest requires an assembly of workers and larvae, one wonders how weaver ant colonies get started. What does the first nest look like? Once, in the Australian outback, I peeled apart two small leaves sewn together at chest level to find four queens and forty workers, the latter each the size of a small major worker in a mature colony, cohabiting in a space the size of a change purse. Making such a tiny nest need not be difficult. Before their first workers are old enough to do the job, the neophyte queens are likely to join forces to hold larvae and weave the nest together.10
A weaver ant grasping a larva that is dispensing a silk thread to bind leaves for a nest in Queensland, Australia.
MAPPING AND DEFENDING AN EMPIRE
Weaver ants are excellent nest builders, but they excel equally in transportation and communication. Their colonies employ a flexible network of routes between population centers and valued resources, much as human civilizations have done since ancient times.11 The ants travel via trunk trails, marked by pheromones produced by the rectal gland, an organ unique to these ants. Trails are made more durable by droplets of worker excrement, which they deposit in a wide swath along the thoroughfares. The droplets harden like shellac when dry, creating a sort of “blacktop” that renders the path extremely persistent.12 The workers can rediscover lost routes even after months of torrential rain or drought.
The workers use different forms of communication to convey different messages.13 Before a trail is reinforced through time and usage to become a highway, it is likely to be ignored unless the ants producing it provide additional signals. As a worker deposits trail markings from her rectal gland, she employs a combination of signals—jerky moves, gentle touches, regurgitation—to identify whether her chemical path goes to some distant food, a leaf construction site, a newly finished nest, fresh territorial space, or an enemy confrontation. An ant returning from an unexplored part of the hinterlands, for example, lays a trail while shaking her body at passersby. Recruited ants, following the trail, enter the unoccupied area, then do what any self-respecting dog would do: they mark the space by relieving themselves. In this situation the fecal droplets are the ant version of urine and serve to identify the society, rather than the individual (as is the case with a dog), and claim the area in the colony’s name. Given that the marks persist for months, their use along trails and at borders most likely serves a long-term strategic function: the colony that first marks a site has the edge in later conflicts.
When an ant contacts an enemy worker, she rushes back to familiar territory, encouraging others to follow her by conducting mock fights in which she stands tall and jabs at her fellow workers as if to bite them. Like a dog’s bared teeth, this serves a warning function, in this case communicating with nestmates about the battle to come or already afoot. Her movements are one of several symbolic ant behaviors, actions normally associated with a practical activity, such as fighting, that have been ritualized into communication signals about that activity.14
Among weaver ants in Australia, combatants are usually recruited from “barrack” nests at the territory perimeter near the outer crowns of trees or at the base of their trunks. These guard posts look like normal nests but rarely hold brood, being occupied instead by expendable elderly ants. “It can be said,” write Edward O. Wilson and Bert Hölldobler, “that while human societies send their young men to war, weaver-ant societies send their old ladies.”15
Once the battle is joined, the ensuing conflict involves thousands of workers circling one another, high on their legs, with raised abdomen and open jaws. When a worker seizes one of the foe, she releases a substance from a second gland unique to weaver ants. This “short-range” pheromone secreted from the sternal gland diffuses and dissipates quickly, like an emergency flare, inciting nearby ants to rush to her aid. (She uses the gland to similar effect when seizing prey.) Mortal conflicts do not continue indefinitely; eventually, a “no-ant’s land” emerges between colonies.16 Defined by the fecal droplets, these perimeters can remain in place for a long time. Similar buffer zones have been common among both humans and aggressive species such as chimpanzees in hotly contested areas with large populations. Other social species don’t defend a specific property. The honeypot ants of the American Southwest fight not so much over land as over shifting patches of food.17 This lower-cost defense stratagem is not unlike the one adopted by small bands of human hunter-gatherers.
While most major human civilizations have undertaken full-time defense of large tracts of land, until modern times borderlands were often fluid, reflecting not wars but decisions about current utility. For example, in fallow months the Mongols would abandon the pastures on which they grazed livestock during fruitful seasons.18 Our modern fixed territories are similar to the “absolute territories” of weaver ants, where space is defended all the time, and reflect the close packing of their populous societies. In a rainforest filled with ants, no colony can afford to relinquish its territory. “Free space is the enemy of true warfare,” writes the military theorist Robert O’Connell, and the stranglehold weaver ants maintain over their crowded canopy dominions gives credence to this view.19 The only territorial changes typical of a weaver ant society are shifts in battle lines with a neighbor or changing levels of worker activity on the forest floor—which weaver ants treat as a less essential, and often seasonal, part of their home range. They abandon it, much as the Mongols did their marginal grazing lands, when conditions are too wet or too dry. Their territoriality reflects this: when certain competing ant species move on the ground beneath weaver ant–occupied trees, for example, the weaver ants merely avoid them, but when the same ants dare to ascend the weavers’ tree, the invasion elicits a massive fighting response.20
The versatility of weaver ant communication systems is without parallel among the ants, but that’s no surprise given the nature of their operations. There are parallels here between the size of a superorganism and the size of an organism. To handle logistical issues within their bodies, big creatures often require more-elaborate organs, including brains and hormone-secreting endocrine glands, than do tiny ones, which sometimes get by with no neurons or hormones at all. Large body size can also mean a capacity for behavioral innovations, which are most common in vertebrates with big brains, such as chimpanzees, who use sticks to catch ants.21 The massed workers in a large ant weaver society may be similarly adept at solving problems or achieving goals, including keeping track of territorial space, enemies, prey, and good sites for leaf nests.
For humans, it’s thought that when communities and their institutions, from the government to the marketplace, evolve beyond a threshold size, the potential arises for more complex social mechanisms.22 Weaver ants, with their intricate transport and communication systems, conform to this expectation of emergent complexity with greater social size.23
HUNTING, GATHERING, AND ANIMAL HUSBANDRY
Weaver ants forage everywhere, while incessantly protecting every leaf, twig, and branch. With ready access to other weaver ant foragers in the canopy and to a surfeit of ants in nearby nests, the workers are able to handle unpredictably scattered prey and enemy incursions. If a dense army ant raid is like a powerful net trawling a limited area in a narrow swath, the workers spread across a weaver ant territory act as an immense, and only slightly weaker, fixed net. The space that weaver ants occupy is so huge that the influx of prey into their territories is enormous. Indeed, a mature colony processes millions of victims each year.24 Each worker typically stands at one spot, using something like the sit-and-wait strategy of the bumpy Proatta ants I watched in Singapore, but with greater effectiveness. They keep their jaws open and body erect, pivoting occasionally. Unlike the sightless driver ants, weavers are so visually acute that they can follow the flight paths of tiny fruit flies and snatch them out of the air before they alight.
Even when a weaver ant is capable of subduing an insect on her own, capture of all but the smallest prey is almost always a group enterprise.25 Attracted in part by the struggle, in part by the sternal gland pheromone, weavers use the spread-eagle technique I saw them apply to driver ants. This is how many belligerent ants make their kills, and this technique may have been in the repertoire of the ancestors of humans as well; two or more chimpanzees, for instance, will pin a male from a competing group while others bite, tear, and beat him with their fists.26
A scorpion being carried into the canopy by weaver ants at Kirirom National Park, Cambodia.
One day during my stay in Cambodia, I was overturning rocks in the forest when out from under one scrambled a 5-centimeter-long scorpion. Though fierce enough that I couldn’t grasp it with my forceps, the scorpion was immediately seized and held in place by a single weaver ant—doubtless one-thousandth its weight—until backups arrived to pin its limbs. The expanded soft pads on weaver ant feet allow them to maintain their grip on their substrate, even when their bodies seem stretched to the breaking point.27 According to one report, a lone weaver ant was able to support the full weight of a 7-gram baby bird that hung below her.28
Two dozen workers ganged together to haul the scorpion 6 meters up a tree in about the same number of minutes. When I extracted the leviathan from their grip, he was groggy but alive. Weaver ants don’t gnaw off limbs to immobilize their quarry, as marauders do. Death is thought to come from being pulled with enough force to dislocate appendages, though few limbs break free. But I suspect the ant bites, with their acidic secretions that leave me feeling dizzy after a day in the field with these ants, deliver toxins as well.
Against powerful invertebrates like scorpions, short-range recruitment and drawing-andquartering techniques are very effective, and such prey could represent a larger part of the food intake of weaver ants than it does of army ants. Though small arthropods are the foundation of their diet, weaver ants may even target vertebrates: one colony in Cameroon contained the remains of two lizards, a snake, a bat, and three birds. Though we don’t know if the ants slew the animals or found their corpses, the workers can kill a bird by pulling it taut, their dozens of bodies linked in chains like those formed when they build nests. Perhaps to avoid theft from even larger animals, weaver ants conceal hefty prey under leaf litter while they subdue it and organize transport on the ground.29 When the carcass has reached its final canopy destination—as Conservation International biologist David Emmett told me happened in Phnom Penh with a 15-centimeter-long dogfaced fruit bat—workers often construct a leaf encasement around food, similar to the soil barricades made by marauder ants. This permits both species to eat in privacy—in the case of the dogfaced bat, down to and including many of the bones.
No adult ant can swallow the prey she kills because solids can’t pass through her impossibly narrow waist to reach her stomach. Instead ants drink the fluids oozing from its body, perhaps after some chewing. Many species use child labor—just as weavers do to build their nests—to transform prey into a form the workers and queen can ingest. Workers of Arizona’s Pheidole spadonia “big-headed” ants place chunks of prey in a bowl-like depression on the bellies of their larvae; the larvae spit out digestive enzymes that dissolve the meat into something like a protein shake for the adults, which slurp up what they want and feed the rest back to the larvae.30 The Adetomyrma “Dracula ant” of Madagascar takes a more gruesome approach. The workers immobilize prey such as centipedes with their virulent stings, then move their larvae to the food, which chew it up. After a larva has eaten, the workers pierce its thin skin to drink the hemolymph that leaks out, leaving their young literally scarred for life.31 Larvae of Asian Leptanilla avoid such blood-letting: they are fitted with spigots from which the adult ants can obtain a drink.32
Weaver ant workers have other sources of liquid sustenance. Not nearly as meat reliant as army ants, they are partial vegetarians, which has advantages, as they are always walking around on an excellent source of nutrition. Why look farther than the plant underfoot? While workers can’t digest the cellulose and have no taste for seeds and fruit, they eagerly lap up sap.
Although plant sap is low in nutrients, it offers energy and sufficient nourishment, provided the ant drinks enough of it. The most successful canopy ants are therefore built to tank up.33 The workers of some species transport droplets between their jaws to other ants, which drink the sap as if from a bucket. Other species carry the liquid in a thin-walled internal sac called a crop, from which they take food into their stomachs as needed or regurgitate it for their sisters. This makes a convenient storage place, and it leaves the mandibles free for other work.34 However it’s done, fluid meals are transferred from ant to ant so that each receives a sampling of the nutrients passing through the colony at the time and is aware of the colony’s general food needs.35 The ant superorganism has, in effect, a social stomach—an approach that even Napoleon, whose army traveled on its stomach, never imagined.
Nutrients aren’t everything, however; that stomach needs to hydrate, too, and drinking water can be in short supply. I discovered that the nest entrances of a certain south Indian ant resemble dead birds because the workers decorate them with feathers, which collect dew each morning—a sort of proto-tool for harvesting moisture.36 Even in a wet rainforest, rain quickly drips out of trees, leaving the canopy parched much of the time. For weaver ants and other species, sap is a prime water source. Workers often drink from wounds on vegetation. Typically, they prefer the watery sap from leaves and twigs over the unpleasantly viscous fluid from the bark that prevents infection of the trunk and can harden and fossilize as amber. (Ants can get caught in this sap, which is bad for them but sometimes useful for us. In one deposit in Kenya, a population of thirty-million-year-old weavers, preserved in crystalline form, revealed that the division of labor, based on similar minor and major workers, was probably organized then much as it is today.)37
When it comes to nutrition, though, much more desirable than everyday plant sap are the enriched, sweet fluids secreted by glands on the plant surface. Even more than the ground-strewn bonanzas of fruit and meat frequented by the marauder ant, these nectaries tend to be both persistent and distributed in patches.
Traveling around tropical Asia, I noticed blemishes on the leaves of many dipterocarps, the local tree giants. Later I learned that these “green spots” exude nectar.38 Unlike the nectar-producing organs inside flowers, which attract bees, butterflies, and other pollinators, so-called extrafloral nectaries like the green spots occur on leaves and stems of diverse plants and are tailored to ant cravings. Oecophylla are among the many ant species that guard such nectaries, and in so doing they protect the plants by making lunch out of any foliage-eating insects nearby. Some nectaries even develop close to flowers or fruit, thereby ensuring protection for the plant’s reproductive parts during its breeding season.39
Weaver ants don’t just seek out food sources such as fruit flies, bats, and green spots. They also, fascinatingly, farm them—in the form of such insects as mealybugs, scale insects, and plant hoppers, which many tropical ants care for the way ants tend aphids in the temperate zones.40 Classified by biologists as Homoptera, these sap-sucking species excrete excess plant fluids in a condensed form known as honeydew—often directly into the mouths of ants.41 Their excrement, which is more nutritious than nectar, is considered delectable even by some humans: in the Bible, it’s called manna.
Weaver ants tend many kinds of Homoptera, as well as certain caterpillars that produce similar sweet secretions. These “cattle” range from species that do fine without the ants to a few that are found only where weaver ants thrive. The ants are as protective of their livestock as any cowboy is of his herd, keeping them from harm and even moving them to fresh pastures when the plant sap runs dry—preferably to a site with young foliage, which is more easily penetrated by their mosquito-like mouthparts. Often, the ants construct a “pavilion” nest, a kind of holding pen, over the insects, as they do also with nectaries, before settling in to exploit them over the long haul.42 Such leaf enclosures protect the Homoptera from predators, parasites, and weather and may even be essential to the ants’ claim on their herd, since workers are less active outside their nests at night, when other species could steal from resources not under heavy guard.
In short, their continuous social dialogue enables weaver ant workers to exploit resources no matter how they are distributed. Throughout their absolute territories, the workers scatter widely to hunt prey on foliage or cluster densely to feed at nectaries or the sites of their homopteran herds. Colonies grow big and strong with a balanced meat and vegetable diet, so they are most vigorous where all of these sources of food are bountiful, such as in the young and succulent vegetation along forest edges.43
Among ants such as the weaver, the flow of food and other goods is likely to be regulated by what’s available and what’s needed, a supply-and-demand market strategy.44 This is best observed in the workers of the red imported fire ant, which monitor the nutritional needs of the other adult ants and of the larvae and change their actions as necessary.45 When scouts and their recruits converge on the nest laden with a variety of foods, they hawk their merchandise by regurgitating samples into the mouths of “buyers” in the nest chambers, who in turn roam through the nest to distribute the meals to the larvae and queen. If the buyers find their “customers” have become sated on meat, they peruse the marketplace for other commodities, until they find, perhaps, a seller offering nectar. When the market becomes glutted and the sellers can no longer peddle their wares, both buyers and sellers wander off to engage in other jobs, or take a nap. This is an excellent way to run a superorganism: if only our digestive systems served us this well, rejecting any excess fats arriving in our meals!46
The diet choices of weaver ants affect their anatomy. Compared to the meat-sustained driver ants, the largely vegetarian adult Oecophylla are thin-skinned, with no special armaments. This may be because nectar and honeydew derived from plants are poor in the nitrogen needed to build proteins. In the economics of weaver ant existence, the carbohydrates in these readily available liquids are the fuel that the adults burn on their labor-intensive hunts for prey, while the prey themselves provide the bulk of the protein the larvae need to complete development and keep the superorganism growing. Even with their feeble armor, few adult weaver ants are killed in encounters with these prey; they are so fleet-footed that even army ants succumb to them.
Both the weaver ants and the army ants are predatory titans, but the two approach their lives differently. The army ants’ narrowly concentrated raids comb wide areas to gain enough of the protein they require, especially from large and aggressive prey.47 But weaver ants remain settled in one area and minimize travel within it by harvesting a steady and more varied local supply of plant foods and honeydew in addition to small and large prey. These differing tactics have allowed both ants to flourish with colony populations reaching into the hundreds of thousands.