Adventures among Ants. Mark W. Moffett
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signals. In response she runs ahead, drumming the unmarked ground with her antennae and depositing a smear of pheromone that guides those behind her. She then returns hastily to her “comfort zone” within the pheromone-saturated land behind. Such timidity is crucial to keeping the troops functioning as a unit, the equivalent of human boot-camp training. It vividly contrasts with the pluck the same worker shows when she joins the wanton melee around prey.
In the marauder ant, as in army ants, every worker is in effect shackled to a nexus of social signals generated largely by individuals who happen to be nearby. Thus it is not so much the proximity of individuals but their lack of autonomy that makes the army and marauder ant superorganisms nonpareil. No matter how much individuality may be prized, there may be times when, for a society—ant or human—to function productively, it pays to march in lockstep.
OTHER ANIMALS THAT HUNT IN GROUPS
There are other members of the animal kingdom that mass forage. Some spiders are sit-andwait socialists who weave a communal web. The more spiders, the larger the catch, with dozens bearing down to secure, say, a large moth.15 Harris’s hawks of New Mexico hunt in families of up to five, leapfrogging between perches until they see a rabbit. Then they converge for a simultaneous kill or attack it in relay. If the quarry finds cover, one or two hawks flush it out while others wait in ambush.16 Among mammals, lions, wild dogs, wolves, and killer whales also hunt in groups, staying in range of one another while seeking prey too large or agile for them to catch unassisted. Some bacteria move in similarly voracious swarms called wolf packs, with pioneers advancing and retreating in army ant style.17 By secreting enzymes together, they can digest prey far larger than a lone bacterium would have any chance of killing.18
Species that bring down large prey are not the only ones that forage as a group. Many bird species can mix together in a flock that, according to Ed Wilson, “behaves like a giant mower, leaving a pattern of well-trimmed areas juxtaposed to relatively untouched areas.”19 While birds act separately to glean insects, in a flock they can take advantage of their companions’ guidance to avoid enemies such as hawks and to track the best bug-hunting locations.
Mass foraging can also be a tool for mass transit, as with cellular slime molds. After they eat an area clean of bacteria, hundreds of thousands of amoeba-like cells join together to produce a sluglike creature that resembles a blob of petroleum jelly. This slug can journey far greater distances than a single amoeba and can pass over pockets of air between grains of soil that would stop the lone amoeba cold. As it goes, the slug sheds individual amoebas, which feed on the local bacteria.20 The slug is searching not for food, however, but for areas of low moisture and high illumination, where it casts off spores.
Another group, the “true” slime molds, grow by the expansion of one amoeba into a fan-shaped body called a plasmodium, which hunts for decaying matter. In high school, I kept an orange species that resembled a swarm raid shrunk to a few centimeters across. If there was little food, my pet crept over its Petri dish slowly but steadily. A sizable bonanza could bring it to a halt as it set about gorging itself; if a patch of food was more modest, part of the slug gathered to eat while the rest continued searching, its fanlike front reduced. A slime mold isn’t as dumb as its brainlessness suggests: one variety can find the shortest route through a maze.21 I admit, though, that a person must be very patient to find it interesting as a pet.
Some of the most army ant–like strategies are deployed by vegetarians. Workers of a few termite species spread out in a loose network while foraging, each walking ahead a centimeter or two and laying an exploratory trail before she retreats and another takes her place. The advance resembles the progression of a marauder ant raid, though it’s less methodical and more dispersive than cohesive.22 A forager who detects wood at a distance, likely by scent, will abandon its search and move straight to the food. Usually she explores the wood alone, then lays a recruitment trail back to the nest. Being defenseless and easily dehydrated, termites expire fast when lost. Staying in the columnar networks helps them find their way back home and hastens the construction of the galleries the termites require to survive on exposed ground.
Another vegetarian engages in mass hunts that have a protective as well as a nutritive function. Whereas an unaccompanied eastern tent caterpillar can easily lose its grip on a tree, several together will lay a silk mat that engages their feet and keeps them from falling. These leaf eaters then find meals in a procession, with the pioneers pushing ahead short distances before retreating, to be replaced by the ones behind.23 A group can follow an old silk trail or strike out over new terrain. A lone caterpillar finding satisfactory greenery will lay an especially attractive—perhaps chemically stronger—recruitment trail back to the silk tent housing the colony, in some cases drawing out the entire population.
This is where all other animals that search for food in groups differ from ants like the marauder: whether caterpillar or bird, bacterium or wolf, individuals are fully capable of moving away from the pack or flock and foraging without companions. And with rare exceptions, “alone” in these species really means alone, because few animals have the capacity to recruit assistants from a distance. A few birds and primates call one another to food: for example, in Africa chimpanzees draw others to bonanzas of fruit in trees by uttering loud hoots, and pied babblers lead their novice fledgling offspring to feeding spots with a “purr” sound.24 But such social actions are virtually unknown in most species, where signals such as the yelp of the coyote or the singing of whales more often function in maintaining appropriate spacing between individuals, in combat, courtship, or group bonding, or to keep pack members together when they are on the hunt, than in calling in the troops.
One rare exception is the naked mole rat, an African rodent with antlike colonies that include queen, small worker, and soldier castes. The worker rodents lay odor trails to the root tubers their colonies feed upon.25 Another remarkable exception, involving a symbiosis between animals who have little in common, is the raven, who will call out to guide wolves to prey; the wolves share the prey with the ravens after the kill.26
COMPARATIVE MARTIAL ARTS
Even though marauder and army ant campaigns are directed at predation rather than military conquest, the byzantine structure of their pillaging and the frequency with which they do battle with other ants make it tempting to conceive of their “armies” in martial terms. Predation and combat have been linked in human history as well, the tools for one often serving handily for the other, with battles occasionally ending in cannibalism.27
Swarm raids compare neatly to the deployment of Roman heavy infantry and other early battalions that swept forward in a broad front. One Roman innovation was to spread troops a bit more widely than did previous armies, which gave each man a few square meters in which to defend himself. Though their workers are never far apart, marauder and army ants similarly tend to remain a few body lengths away from each other, right up to the front lines, a spacing most likely maintained by the ants in order to avoid treading on one another.28
Naturally, there are differences between the Roman armies and ant armies. Roman troops fell into formation only in times of active conflict, when soldiers on the front lines served as a defensive shield against another army open to view, protecting the soldiers behind them and slowing the advance of the opposing army before them. Among marauder and army ants, in contrast, the foremost workers serve as a contiguous search party to flush out prey. Rarely are the ants’ opponents arranged in a similar configuration; rather, they are discovered and overtaken in sporadic fights.
Despite their tactical responsiveness to prey, marauder raids can seem regimented when compared to the flexibility of Roman legions. Deployed in formations arrayed three deep, the Roman troops could be reconfigured in response to changes in an enemy’s assault. The phalanx might be preceded by cavalry that harried the enemy in advance, for example, or by scouts sent ahead to report on the lay of the land so that the day’s plan could be adjusted accordingly.
My painstaking observations of the marauder ant raids left me with several unanswered questions. Animals as diverse as wolves, birds, and bacteria are able to mass forage in organized groups and then to move off in isolation. Why aren’t marauder ants and army ants similarly able to employ