Adventures among Ants. Mark W. Moffett

robotics experts, who have discovered that it’s cheaper and easier to achieve a goal such as piling up small objects with a group of simple robots responding to the work done thus far than with one large, more intelligent robot.⁴ Stigmergy is at work in such websites as Wikipedia and Google as well, where many people add their insights to the statements and choices of others.⁵

      Major workers of the marauder ant serve the role that humans reserve for heavy-duty construction equipment. I have called the largest of these individuals “giants” since the day I first saw one lumber from that nest in Sullia to the cheers of Mr. Beeramoidin and other forestry officials. Imagine a man and an elephant working together to build a road; the size difference between the giant and the minor worker is nearly ten times that great.

      Relatively scarce, the giants tackle jobs that, though infrequent, require their prowess. While the smaller ants are so omnipresent that their jobs invariably get done, removing just a couple of giants from the work crew can cause a trail to degrade.⁶ Fallen objects such as twigs and leaves snarl traffic and must be cleared for the roadway to remain open for use. When one of these giants arrives at such an obstacle, she pushes beneath it, then lifts her head high while standing on tiptoe. Ultimately, she shoves the object to one side, if not on the first attempt, then on the second or third, in a manner similar to that used by elephants to clear human paths.

      When the soil roofs of the arcades sag, the large marauder ants respond to the pressure against their heads as they pass underneath with the shoving technique as well. Captain Charles Thomas Bingham, the Irish officer stationed in Burma, called the majors “the trowels and rammers of the Ant’s Public Works Department.” Their actions raise the drooping arcades and conceivably increase their structural integrity by binding the soil particles. The soil covers are finely granulated on the outside and are smoothed internally by the majors’ battening.

      In addition to enclosing their roads, marauder ants build thicker soil edifices over prize fruit or meat bonanzas, structures that facilitate the business of feeding. Workers guard the outer walls while others eat in a narrow gap between this exterior layer and an inner scaffold, which absorbs any moisture in which the diners might otherwise become mired.

      Covered-over passages and encased food bonanzas are kept tidiest in areas of dense litter or vegetation that provide physical support so that less caretaking is required to maintain them. To what use is all this effort? Not, it seems, as protection from the elements. The earthworks fall apart in rain, and disintegrate when the earth is dry. Arcades are thin enough to puncture with a tap of a finger, which means a route is weatherproof only when it travels through an underground tunnel, perhaps dug and then abandoned by other animals. Alternatively, near the nest the ants may make a subterranean route of their own: over time, construction crews can scratch away so much soil from the trail surface that the highway sinks from view, at which point the ants seem to be able to construct a thicker, rainproof cover that becomes flush with the surrounding land.

      A marauder ant trunk trail with soil sides and partial soil cover, extending through the leaf litter in Johor, Malaysia.

      DEFENSE

      The main function of this relentless building is defense. Because trunk trails extend for dozens of meters, they travel through territory controlled by other ant species. Marauder ants must therefore be organized to protect the trunk trails from aggressive neighbors or even from hapless passersby such as the Diacamma.

      Strangely enough, when the soil ramparts are absent or breeched, the job of defending the trails goes to the most expendable ants in the colony—the maimed and the decrepit. At the spot where I saw the Diacamma killed, a row of minor and small medias stood along either side of the trail, ready to fight off any more of her comrades who might wander by. Marauders darken with age, changing from creamy brown to a dark cocoa color, and I could tell that many of these guards were old from their near-black integuments. Amputees and the infirm struggled to stay upright as they jabbed at additional chance intruders from the Diacamma nest nearby.

      Among ants generally, the risks taken by workers tend to increase with age, demonstrating that their long-term value to a colony diminishes as they get older. Months-old fire ants engage in fighting in battles with neighboring colonies, for example, whereas weeks-old workers run away and days-old individuals feign death.⁷ The old and wounded marauders often serve in the worst occupations, such as trail guards. They also throw garbage from the nest onto the community refuse heap, or midden, where they work until they fall over, their bodies joining the rest of the colony’s waste.

      For the marauder colony bothered by Diacamma, all the fuss over the contentious stretch of trail became moot within hours, after an arcade had been completed: the Diacamma workers could now walk over the trunk trail, blissfully ignorant of the industry below them. If a trail should sink underground, it is as protected as a passage in an army bunker, safe even from human footfalls.

      Bulwarks constructed over trails and provisions prevent battles among competing marauder ant colonies as well. Where they are absent, combat can last a day and engage thousands of minor workers, which pour along the line of contact between the armies. Sometimes the tangle of ants stretches a meter wide. Compared to the free-for-alls that erupt during prey capture, the fights unfold with extreme care. At first the minor workers examine each other more like dancers than combatants. Brawls begin when pairs interlock mandibles, then grapple for several minutes before disengaging and maneuvering for better position. Fatalities escalate as additional workers pull on one of the locked ants. Fighters can tuck their hind ends beneath their trunks, making it difficult for others to grasp them at their fragile waists. Meanwhile they wave their abdomens in an action called stridulation, in which a ridged surface like a nail file on the undersurface of their abdomen rubs against a scraper located below their slender waist to produce a rasp like the sound made by strumming a comb; it is barely audible when a large worker is squeezed lightly and held up to the ear. This may be a call for help, though ants, being deaf, detect the rasps only as vibrations through the substrate. After some minutes of struggle, one of the ant’s limbs will pop off like the arm of a medieval torture victim stretched on the rack. Slowly, surely, the workers pull each other apart.

      Among ants in general, most lethal fights are variants on this hand-to-hand combat. Some species avoid prolonged tussles, instead taking a hit-and-run approach, inflicting damage fast and then dashing away. Many of these use a sort of chemical mace, spraying insecticides from their abdomens. Otherwise ants have not developed techniques to safely inflict damage from a distance—a development in human conflict that began with the invention of the spear. In one remarkable exception, workers from cone ant colonies stop their opponents from foraging by surrounding the enemy nest and dropping stones down the entrance and onto their heads as they attempt to leave, a nonlethal, but effective, technique.⁸

      Which marauder ant colony wins? One especially sizzling afternoon in the Singapore Botanic Gardens I conducted an experiment with bottles of spray paint. By spritzing a different neon color lightly on the traffic moving along the trunk trail, I was able to mark a small portion of several colonies’ worker populations. Three days later I came upon a battle between two of the nests. Scanning the thousands of grappling ants, I watched as the pink colony’s larger battalion eventually swamped the greens, which retreated. With only a hundred casualties on both sides, there was no further commotion. In fights between honeypot ant nests in Arizona, special “reconnaissance workers” move through the battlefield to assess the size of the opposing armies, then draw out more troops or organize a hasty retreat depending on the situation.⁹ I have no idea if that’s how the greens “knew” to give up—that the odds were against them. But at some point, the green army clearly decided to leave the field of battle rather than fight on.

      Because marauder ants lack scouts that could monitor intrusions around their nest, conflicts among them have little to do with territoriality—the control of land. Fights occur only by accident, when one colony’s raid contacts the raid or exposed trail of another, and may be avoided, even near a foreign nest, when trails are sealed over. Because the size of a marauder ant army is likely to increase the closer the battle is to its nest or to