Adventures among Ants. Mark W. Moffett
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Singapore Botanic Gardens, founded by Raffles in 1822 to display some of his own exquisite plants, offer plenty of marauders in a manicured setting where they are easily watched. I was introduced to the gardens as an ant haven by D.H. “Paddy” Murphy, a senior lecturer at the University of Singapore. A native of Ireland, Paddy is an autodidact, an entomological genius of a kind that normally falls through the academic cracks. Because he lacked a Ph.D., his prestige-minded colleagues didn’t know what to make of the fact that when any entomologist visited Singapore, he or she called on Paddy. No matter what the researcher’s area of expertise—whether some obscure group of crickets, plant lice, or marauding ants—Paddy would pull out the specimens in his collection and begin gently instructing about the local species. The visitor would leave enlightened, while Paddy seemed to soak up everything his interlocutor knew.
In addition to showing me the Botanic Gardens, Paddy took me in his battered white Nissan on expeditions to Singapore’s watershed, the Bukit Timah Nature Reserve. After several hours rooting in the mud and stuffing specimens into vials, we would finish our day with a stop for a drink. Oblivious to our jungle-rat appearance, he’d drive to one of Orchard Road’s fancy hotels and, shuffling into its gleaming five-story foyer, demand two Tiger beers from the bar, all the while holding his insect net like a national flag. Libation consumed, we would then retreat to his flat, where his wife, a chemistry professor of Indian descent, kept a motley herd of little dogs. Sitting at the kitchen table, Paddy would scrutinize the day’s catch, never raising his eyes from the magnifying glass. Meanwhile the dogs, announced by a rumble of paws on the tile floor, ran in formation like a migration of African wildebeests, circuiting the house every minute or two.
After another round of beers from his fridge, Paddy would drop me off at what came to be my favorite part of the Botanic Gardens, a seldom-visited back section where I began to understand how deep were the convergences between marauder and army ants. The foraging behavior of both displays a specific set of characteristics that, in scientific fashion, form a sequence in my head. In brief, (1) the workers are tightly constrained by one another’s activities, such that while individuals constantly enter and leave the raid on a trail to the nest, (2) those in the raid nevertheless avoid spreading apart, so that the raid retains its existence as a cohesive whole; in fact, (3) adjacent ants stay close enough together that communication between them can be virtually instantaneous. (4) This unit moves along a path that (5) is not controlled by any steadfast leader or leaders within it, (6) nor by scouts arriving from outside. Indeed, (7) their movement does not target a specific source of food, (8) nor is progress dependent on finding food en route, because the ants are drawn forward not just to meals but also to the land ahead; further, (9) the advance can continue across “virgin ground,” because advance doesn’t require cues left by prior raids. Finally, (10) all foraging is collective. No ant sneaks out to grab lunch on her own.
These features basically define what we mean by the words group and forage in these mass-foraging ants. The first three describe a particular sort of group in which proximity turns out to be essential: no marauder ant searches alone for any significant distance.16 That means food never has to be abandoned while help is enlisted. Overpowered quickly, prey is unlikely to be stolen, or escape, or have occasion to defend itself. The other attributes describe a certain kind of foraging in which the searching group has no predetermined destination and need not take any particular course.
All these details took me months to work out in that back section of the gardens. I was largely hidden there from the heavy tourist traffic, though I do recall one passing wedding party that was shocked and then fascinated to see me on my hands and knees, counting ants performing the superman task of carrying a lizard egg. The bridesmaids lifted the bride’s veil as she too stooped to take a look.
2 the perfect swarm
At the end of my first week in Singapore I had my first clear view of a swarm. It was late afternoon in a remote corner of the Botanic Gardens. Paddy Murphy sat nearby, smoking a “fag” and examining a silverfish on a tree. I had spent the previous hour on my hands and knees following a trail of marauder ants that were obviously on a foraging expedition, because they were bringing back all kinds of prey. And there, suddenly, near the base of a Brazil nut tree, was a throng of ants—shimmering with the movements of thousands in the cropped grass. I’d caught glimpses of such mobs in my Indian plantation’s understory brush, but here was one open to scrutiny, a band of ants 2 meters wide and over 7 centimeters from front to back. At the back of this band was a V-shaped network of columns 3 meters long that resembled the web of veins in a human hand and, judging by the slaughtered prey being carried along, served the same purpose of conveying nourishment. The web converged into a single column that was the colony’s aorta to the nest. Workers laden with plunder marched along this route all the way home.
Paddy came over and gave a whistle of astonishment. My hand sped across a waterproof notebook as I penciled a sketch of the action. Afterward, examining my drawing, I realized how closely it resembled illustrations of army ant raids. In fact, the swarm compared point for point with descriptions of the most extreme form of army ant attack, the swarm raid.1
In the terminology of army ant researchers, the advancing margin, where the workers meandered ahead of their sisters, is the swarm front. The swarm is the band of ants behind the front, and the fan is the network of columns farther back still, which converge to form the single base column that extends to the nest. Among army ants, swarm raids are peculiar to some New World Eciton and Labidus ants and to a few African Dorylus species known as driver ants. Skirmishes within a raid appear chaotic when viewed in isolation; but when a raid is seen as a whole, a sense of order and even aesthetic beauty emerges.
The phalanx of ants stayed in tight formation. This made the raid’s anatomy easy to pick out—a boon to humans, whose noses are too poor to register the pheromone scents that the ants prefer to use for communication and that bind the raid together. A century ago, Herbert Spencer saw a “closeness of parts” of this kind as strengthening a society’s similarity to an organism. After all, we recognize a dove or rice grain by its boundaries: each has an inside and an outside. The workers that form a marauder ant or army ant raid may be separate creatures, but they do not drift apart, and therefore they form an entity that is not only cohesive but also distinct and well bounded.
The same was true outside the raids, throughout the colony. Over the next weeks I would learn that while trunk trails and their temporary offshoots could extend for a hundred yards, individual marauder ants stay on these roads and seldom travel more than a few centimeters from their sisters.2 All foraging, I determined, is done in a group: my observations revealed no rogue hunters. (I did come upon strays, though. Some were stragglers, sick or lame, on paths all but abandoned. Then there was the occasional isolated worker that was just plain lost. I spent hours watching these individuals stumble around. But even after I gave one lost marauder a bit of my lunch, she had no idea where to go with it. Presumably these forlorn souls wander until they die.)
Certain things became clear to me as I sketched the raid that afternoon in the Botanic Gardens. Within the raiding horde, there’s little appreciable movement of any ant at the swarm front beyond the ground covered by her nestmates—no exploration of fresh terrain except for a stint at the front of the raid, which is the one time in a marauder ant’s life that can be unambiguously described as foraging. The trailblazers at the front (too temporary and plentiful to be considered scouts, they are appropriately called pioneers, as they are in army ants) cross onto new soil. Pioneers don’t appear to be specialists at this task; whoever reaches the front does the job. Nor do they press ahead and fall back with the precision seen in movies depicting Roman soldiers massed against the Gauls. Sometimes they wander a bit. In any case, their actions are restricted to the vicinity of their neighbors, and the raids as a whole have no ultimate destination.
Marauder and army ant raids differ only in degree. One obvious difference is their speed: marauder raids move at a measly 2 meters an hour, maximum, while army ants can travel ten times that fast, the record being 25 meters in an hour. Scale the ants up to human sizes, and that would be over 800 meters an hour for the marauders, versus up to 8 kilometers an hour for an army ant raid. As a result of their slow speeds, marauder ant raids, which