by Marlene Zuk
Here my teacher intervened. Surely, she said gently, you are exaggerating. Ants couldn't possibly be that destructive. Perhaps they attacked the animals near the area, or got into a hut or two, but this scale of devastation and carnage seemed a bit much for such tiny creatures.
I dug in my heels. No, I insisted, the book had said (and hence I unswervingly believed) that the ants could tear apart a person in minutes. It wasn't just the odd hen or two, it was An Entire Village. I honestly don't remember exactly how or if this disagreement was resolved, or if my grade on the book report was reduced due to my teacher's suspicion of hyperbole, but I remain convinced that people don't fully appreciate the wonders of ants, perhaps because they refuse to believe the extraordinary things ants can do.
Army ants in particular inspire superlatives. They were described in detail in the late nineteenth and early twentieth centuries by legendary scientists such as William Morton Wheeler and Theodore Schneirla; the latter published a paper in 1934 titled "Raiding and Other Outstanding Phenomena in the Behavior of Army Ants," in the prestigious Proceedings of the National Academy of Sciences, which was, interestingly, placed into the "Psychology" section of the journal, as if it had more relevance to the workings of the mind than to zoology. Wheeler referred to army ants as "filled with an insatiable carnivorous appetite and a longing for perennial migrations, accompanied by a motley host of weird myrmecophilous camp-followers," and Hölldobler and Wilson call them "the unstoppable, superorganismic grim reapers of the tropical forest."
Army ants occur in several parts of the world, including the southern and western United States, but have been best studied in the New World tropics. They lack a fixed nest site, instead creating bivouacs, football-shaped masses that can be nearly a yard wide with anywhere from ten thousand to seventy thousand workers surrounding the queen and larvae, depending on the species. Hölldobler and Wilson estimate that the bivouac contains "a kilogram of ant flesh." The ants link their limbs and jaws to form their shelter, making a kind of savage yet delicate lacework of individuals that supports layers upon layers of brownish bodies. Just after dawn breaks, the bivouac seethes and breaks apart, sending out lines of ants in many directions.
The lines include army ant workers of several shapes and sizes, all female, of course, despite my students' disbelief. The small and medium-sized individuals lay down an odor trail as they walk down the middle of the track, while the larger soldier caste ants, with their scimitar-shaped jaws, lumber alongside. Workers are about the size of many North American ants, perhaps as long as a grain of rice, but the soldiers are three times their size, about as long as a kidney bean. The streams of advancing ants have no leader; individuals hustle back and forth at the edges of the swarm, altering direction as they encounter prey.
As my childhood reading experience suggested, army ants and their relatives the African driver ants are merciless when they encounter an animal in their path. People and other vertebrates such as birds or squirrels, however, are usually able to evade the advancing columns unless they are injured or otherwise prevented from moving out of the way, which vindicates at least some of my teacher's skepticism, though the ants certainly could overpower an immobile human being. Insects, spiders, and other invertebrates usually cannot escape so easily and are surrounded by the eager jaws of the horde. Hundreds of ants sink their mandibles into the prey, their grip so strong that if the ant is torn away, its jaws remain imbedded in the flesh of its victim. Anecdotally, at least, this powerful grasp has led to their use as sutures by the Masai of Africa, who induce the ants to latch onto either side of a wound with their jaws, holding it shut even after the body of the ant is discarded. (I was once asked how the ant jaws are removed once they have served their purpose, and I don't know the answer, save that the process might put that "ouch" at the tug of a Band-Aid to shame.) Small animals are borne away intact, while larger victims such as tarantulas or grasshoppers, or the occasional unlucky mouse or even deer, are efficiently butchered and carried off in chunks that can be managed by one of the medium-sized workers.
The intimidating-looking jaws of the soldiers, like many weapons, are not actually useful for practical tasks, and so all of the work of hauling food back and forth is done by the more modestly equipped smaller workers. Sometimes a group of such ants collaborates to transport a larger prey item, balancing it expertly among themselves so that the load can be carried by the minimum number of individuals needed. Schneirla reported that the entire operation is accompanied by the sound of thousands of tiny exoskeletons tapping against the dry leaves of the forest floor, a sound that according to Hölldobler and Wilson "beats on the ears of an observer until it acquires a distinctive meaning almost as the collective death rattle of the countless victims."
The naturalist and author William Beebe once observed a bivouac of army ants that had taken temporary residence in the outhouse near his laboratory in Guyana. Transfixed by the sight, he determined to observe the insects as they set up their encampment. He first noted the odor of the group, which was "sometimes subtle, again wafted in strong successive waves. It was musty, like something sweet which had begun to mold; not unpleasant, but very difficult to describe." He was deterred from further rumination by "a dozen ants [that] had lost no time in ascending my shoes, and, as if at a preconcerted signal, all simultaneously sank their jaws into my person." Beebe proceeded to take a chair into the outhouse and use the traditional technique of placing each of its legs into a can of disinfectant; he then rushed over to the chair, hung a bag of equipment over the back, and pulled his legs onto the seat. "Close to my face were the lines ascending and descending, while just above me were hundreds of thousands, a bushel-basket of army ants, with only the strength of their thread-like legs as suspension cables. It took some time to get used to my environment, and from first to last I was never wholly relaxed, or quite unconscious of what would happen if a chair-leg broke, or a bamboo fell across the outhouse."
This rhythm of activity, with the ants alternating between going out on raids and forming bivouacs, continues for months. In the Central American army ants that Schneirla studied, the ants sometimes will form a new bivouac every evening and sometimes settle in their self-manufactured housing for a few weeks at a time. Because army ants have no permanent nest site, they do not reproduce as many other ants do, with the release of winged males and females that mate in flying swarms before the newly inseminated queens found new colonies. Instead, at least in the species of army ants that have been the best studied, although both fertile males and females are produced at a certain time of year, only the males can fly. They attempt to join the bivouacs of another colony. At the next raiding period, a group of workers stays with the old queen and moves to a new bivouac, while one of the virgin queens is surrounded by another set of workers and travels to a different site, where she mates with one of the males that had flown into the colony. In some species of army ants the young queen mates with several males in succession, in others with only one. The male ants, like the drones of honeybees and many other social insects, die soon after mating, assuming they get a chance to mate at all. The rest of the handful of reproductive females are abandoned to the company of a small group of workers, but they do not hunt for food, and eventually all die, leaving their mother and sister to carry on in their place.
Army ant queens themselves have dramatically episodic reproductive lives; instead of monotonously laying egg after egg, day after day, for their entire adult life span, the queen's ovaries will develop only while the colony is in their more long-term bivouac. At that point, rapidly making up for lost time, her abdomen distends and she lays up to three hundred thousand eggs in one fell swoop. When the workers that are the product of her labor appear, they seem to perk up the energy levels in the group, and after a while the entire colony starts the migratory phase again, as the queen's labor subsides and she is shielded from harm as the columns of scissor-jawed daughters resume their activity.
The term army ant is not a tr
uly scientific designation; it is used to describe those species of ants that exhibit both incessant migration of the entire colony and coordinated group hunting, including the raids by large numbers of individuals and the carrying of prey back to the nest. Sometimes terms such as legionary or driver ants are used, but virtually everyone who has written about them describes the ants' behavior in the most aggressive terms. While Hölldobler and Wilson, in their monumental tome The Ants, concede, "Yet driver ants are not really the terror of the jungle as popularly conceived," they devote much of their chapter on army ants to the same bloodthirsty details relished by Beebe and other naturalists. Even Maeterlinck, with his enthusiasm for the social virtues exhibited by ants in general and for their food-sharing proclivities in particular, sorrowfully notes, "Even in the ants this universal charity, this perpetual communion, does not prevent wars: though the wars of the ants are less frequent and less cruel than is generally believed." He also saw—or thought he did—a perfect mirror of human foibles: "Every kind of warfare known to ourselves will be found in the world of the ants; open warfare, overwhelming assaults, levies en masse, wars of ambush and surprise and surreptitious infiltration, implacable wars of extermination, incoherent and nerveless campaigns, sieges and investments as wisely ordered as our own, magnificent defenses, furious assaults, desperate sorties, bewildered retreats, strategic withdrawals, and sometimes, though very rarely, brawls between allies, and so forth."
In the midst of this veritable battalion of military metaphors, it might be worth stepping back and considering the actual goal of the army ants themselves. Those forms of warfare Maeterlinck exhaustively details might be better replaced by a far humbler list: going to the grocery store, harvesting vegetables in the garden, or hooking fish in a river. The ants are an army without an enemy. They are predators, and predation is not waging war, it is acquiring food. We seem to like linking hunting live prey with being aggressive, and we seem to especially like linking it to manliness. Predatory animals such as hawks or lions are often depicted as being exceptionally fierce, and so we transfer that to the ants, struggling with the grasshoppers that are elephant sized to them. But the truth is that hunting is a more widespread and less glamorous profession than it is sometimes made out to be. We tend to think of predators as animals that subdue large, warm-blooded prey, usually after a heroic struggle, but there is no a priori reason for us to dismiss animals that catch and kill more modest fare, for example, the ladybug gleaning aphids from a rosebush, or a chickadee nipping an inchworm off of a leaf. Some biologists refer to any food item that comes in a discrete chunk, as opposed to the unending sea of grass in a field, as "prey," and talk about animals such as the seed-eating kangaroo rats as "seed predators." Even if that is going a bit too far for some, is it any less savage to bite a worm than a weasel? Why does a hawk swooping down on a mouse seem more aggressive than a songbird snapping its bill against the hard shell of a beetle?
It's true that hunting, for both humans and other animals, can be risky, and facing up to prey that is bigger than you are and that has sharp teeth or claws can take courage. And in some cultures hunting, because it requires that bravery, is used as a test of manhood. But none of this applies to the ants, not least because, of course, all of the workers—even the ones with the big, bladed jaws—are female and won't get any more kudos from the colony no matter how many tarantulas or pythons they bring down. That they all eat meat doesn't make them any more vicious than the more peaceably named harvester ants that lug heavy seeds back to their nests.
Maybe the emphasis on warfare and aggression in army ants is an effort to counter that idealization of the ants' social harmony that used to be so prevalent. After all, Solomon wanted us as sluggards to look to the ant for inspiration to hard work; he didn't ask us as wimps to look to her for inspiration to violence. Sleigh discusses a book on "natural history and animal morals" published in 1851 by the Society for the Promotion of Christian Knowledge, which is still active. In it, ants are held up as models of prudence and industry from which humans are exhorted to learn. Ant and bee societies were embraced by the natural theologians of the nineteenth century, and their cooperative altruism was used as a model for social organizations that supported the poor. Later, socialists claimed that ants were exemplars of comradely sharing, though they must not have looked too closely at that turgid-bellied queen.
As my childhood flirtation with a career in myrmecology suggests, I am hardly immune to the drama of the ants' activities. But all of this symbolism, and the focus on aggression, runs the risk of seeing the ants as miniature soldiers instead of the skillful predators they are. What's really interesting about army ants isn't whether they show "incoherent and nerveless campaigns" as opposed to "sieges and investments as wisely ordered as our own." It's how and why that one virgin queen is singled out and sequestered from the rest in the colony. (Is she older? Younger? Or does one live and the others die at random? We still don't know.) It's how the colony's internal clock tells it when to migrate and when to make camp, given that research has shown the ants are not simply driven by hunger. Day length may give a proximate cue, while colony size also plays a role. It's how those massive colonies in Africa and South America differ from their less conspicuous counterparts in Texas or Alabama.
Army ants could even serve a more prosaic function, and one potentially useful to humans. Adrian Smith and Kevin Haight from Arizona State University pointed out that because other ant colonies tend to flee with their brood, evacuating the entire colony, queen and all, when army ants approach, researchers attempting to collect the prey species could exploit this behavior and save themselves time and trouble. They cheerfully—and in my opinion, just a tad cold-bloodedly—suggest that fellow scientists use a small batch of army ants to rout a desired species from their subterranean chambers, rather like tiny terriers sent after rabbits in a burrow, saving hours of often back-breaking labor excavating the nest. Their paper even includes links to videos that demonstrate, in eerily infomercial fashion, exactly how nifty and efficient this technique can be. It sounds like a great idea. But somehow "terrier ants" doesn't have much zing.
Adopting the Enemy
IF THE army ants are more hunters stocking their family larder than noble soldiers proving their mettle, what about the slave-making ants with their "wonderful" instinct, as noted by Darwin? The wars, or raids, that these ants undertake are not about getting food, at least not directly. A slave-maker colony is started when an inseminated queen of one species enters the nest of another, kills or expels the resident queen or queens, and begins to lay eggs of her own kind. Her children are then reared by the host species, which accepts them as if they were nest mates. To replenish the host workers, the slave-maker species sets out on periodic expeditions to snatch the larvae and pupae from another host colony, bearing them back to the slave-maker nest to mature and act as normal workers for their hosts. About fifty of the eleven thousand known ant species behave in this manner, with some capable of living on their own and others so specialized that they cannot even feed themselves without the help of their captives.
Despite their rarity, the slave-making ants have attracted a great deal of human attention. Charlotte Sleigh documents the fascination of nineteenth-century observers with the slave-making ants, many of whom were quite overt in their analogy with the human slave trade of the time. Perhaps surprisingly, many naturalists and authors decried, not the practice of exploiting the labor of others, but the "degeneracy" of the slave makers themselves. In characteristically opinionated prose, Maeterlinck disapprovingly notes that the slave makers "cannot eat without assistance, for they cannot take any nourishment save from the mouths of their servants. They are as little capable of rearing their young as of building or repairing their nest. In the depths of their lair they pass their time in besotted idleness, rousing themselves only in order to polish their armor, or to pester their slaves for a mouthful of honey. Without their servants these magnificent warriors, with their bronze armor, the
se superb shock-battalions, these irresistible veterans of great campaigns, are as impotent, as utterly helpless as so many suckling infants." In his 1954 book Ways of the Ant, John Crompton was similarly censorious: "Even if their slaves do not desert them, mental and physical decay will in itself and in its own time exterminate them. There must be many species of slave-making ants that have died out for this reason."
Although his language is rather histrionic, Maeterlinck was scientifically accurate, at least with regard to the obligate slave-making ants; in the early 1800s the great entomologist Pierre Huber had placed a group of ants of one of the slave-maker species in a kind of ant farm, along with honey to eat and some of their own pupae and larvae. Within a few days half had already died and the remainder were on the brink of starvation.
The raids themselves can be quite dramatic to witness. The species whose brood is being taken generally attempt to drag the pale, helpless larvae and pupae away from the nest, only to be pursued by the workers of the host ant species. Raids seem to be confined to certain times of the year, and at least some of the ants studied in this regard use cues from within the nest to decide when to begin raiding behavior. Slave making in ants is confined to temperate regions of the world, and scientists have suggested that the absence of seasons in the tropics explains the lack of raiding and, hence, slave-making behavior, since there is no internal signal that indicates when pupae can reliably be abducted from their nests. Some species also raid at certain times of day. Joan Herbers, a scientist at Ohio State University and an authority on such ants, says that when she was at Colorado State University her students knew exactly when to go looking for raids: "Jeremy [her student] could head to the hills of Colorado around 10 in the morning, knowing he would be done with fieldwork by 3 or 4." Others are not so reliable: "We have set up many experiments in the lab; some days they raid and other days they don't. Some days they raid fiercely and other days the raids fade away. Sometimes it takes an hour and other times 6–8. It's a pain, and has frustrated several journalists who have visited my lab."