Anthill

by Edward Osborne Wilson

  There should be nothing surprising about the looming crisis of Supercolony. Every species walks a tightrope through ecological time. Launched upon it, there is only one way to keep going, and a thousand ways to fall off. That is the way evolution works, and that is how the natural world as a whole runs itself. The instincts driving the anthill are those that succeeded in the past. The genes that programmed them were selected by particular events in the past. Neither the instincts nor the genes, however, had any way to plan for the future. A major change in the immediate environment, or a mutation of the kind that occurred in Supercolony, could quickly lead to disaster. For a species to continue on in a particular environment indefinitely required precision and luck.

  Supercolony had fallen off the tightrope. In this vital way it resembled the great human anthill above and around it. When its end came, it would be at the hands of the moving-tree gods.

  25

  ON A CLOUDLESS AFTERNOON in late August the denizens of Supercolony, blind to any danger facing them, prepared for the greatest event of their yearly cycle. They were about to hold a mating swarm. It was to be the culmination of all their activities, the central purpose of their existence as a colony. It made possible the immortality of their kind. The season and the weather were right. Two days previously a thunderstorm moving northeast off the Gulf of Mexico had dumped a full two inches of rain on the Lake Nokobee tract. This morning the ground was still moist, and plants wilted by the midsummer drought had begun to regain a little of their springtime turgidity and greenness. The sun heated the soil of the sprawling nest, and humid air weighed heavy and still upon its surface.

  At midmorning a biological clock in the brains of the Supercolony ants triggered the nuptial event. Workers by the thousands came pouring out of the entrance holes. They spread over the nest surface, milling about in excited chaos. Within minutes they were joined by a horde of virgin winged queens and males. None flew off. Mating started immediately. Multiple males piled on top of each virgin queen, jostling one another, forcing their way closer to the object of their desire, each struggling to be the one who copulated. Even when one succeeded in joining his genitalia solidly to that of a queen, the tumult around the pair continued. The failed males were frantic. To mate, just once, at this precise time and at this place, was the only purpose for which they existed. Locating a queen a few seconds too late or making less than a do-or-die effort spelled defeat and a childless death.

  Winners and losers alike among the males flew away, but only to die. That night thousands of the corpses piled up beneath the porch light of a nearby farmhouse on the Lake Nokobee road. At dawn small birds came to feast on them. Later in the morning the owner, an elderly lady, murmuring "What on God's earth is this all about?," swept the remainder off the porch into the front yard.

  Queens, in contrast, had no reason to be desperate. Each one was all but certain to be mated. After copulating with one or more males, they broke off their dry, membranous wings and climbed back down into the nest interior. If their luck held, they would soon join the egg-laying reproductive force of Supercolony.

  That would not happen this day, however. Wholly by chance, the gods seen as moving trees had chosen the day and hour of mating to fix the environment. The nuptial frenzy was dying down on the Supercolony nest domes when the divine presence arrived. In one instant there was no sign of the moving-tree gods, but in the next instant they were there, bodies towering into the sky, tree-trunk appendages moving swiftly, their shadows and odors sweeping over the Supercolony nest. There were many of them this time. Large objects floated through the air by their sides. The ground trembled where they stepped. Strange noises descended from above, not like thunder, more like a strong wind blowing through the tree branches.

  The apparitions moved on, beyond the eastern reaches of the Supercolony nest area, until a few minutes later no sign of them remained.

  An hour later the ants still out on the nest sensed that the gods were coming back. But they did not see the giants this time. Instead they detected a strange and unpleasant odor. The ants were alarmed. They responded to the smell as though it were an alarm pheromone of their own, released from some among them in an odorous shout against impending catastrophe. They began to run about in loops and circles, searching for enemies. Nothing was there. Then, in less than a minute, a faint chemical cloud passed silently over them, a fog rolling off a poison sea. Those ants who remained aboveground lifted their heads and waved their antennae in curiosity. Within seconds they collapsed, paralyzed. In minutes they were all dead. As the fog settled closer to the ground, the gods arrived with their accoutrements floating by their sides, and poured streams of deadly liquid down into the nest entrances.

  The next morning the great ant conurbation was an environmental dead zone, a killing field, the site of a myrmicide, a cemetery, all of it lifeless and still. Not an ant, not any other kind of small creature, moved within it. No birds or lizards or squirrels, which still abounded beyond the perimeter, visited the ruined land. Clusters of vegetation not trampled by the divine visitors still stood intact, but not a trace of insect life stirred upon or around them. The only sounds were the whispers of wind over the lake through the canopy of the surrounding pines, and the soft lapping of waves at the water's edge.

  26

  HUDDLED IN A NATURAL HOLE between two roots of a gallberry bush, the tiny Woodland Colony had survived the destruction of its overbearing neighbor. The kill zone in which the Supercolony died, now still and silent, had stopped just short of the Woodlander retreat along the shore of Dead Owl Cove. When first the Trailhead and then the Streamside Colonies ruled this region, their scouts sometimes ventured far enough into the forest to pass close by the Woodlander nest entrance. They never found the little colony. But a few came near, and often enough to frighten the inhabitants. The Woodlanders stayed close to home, forced to survive on a few small scraps of food, mostly dead insects. After Supercolony took over, their situation worsened. Forays by Supercolony scouts were more frequent than those of the earlier colonies, and some approached dangerously near. Woodlander foragers were forced to stay still closer to the entrance of their little nest. Even then, a few were picked off by the Supercolony scouts.

  At the time of the destruction of Supercolony, the Woodland Colony was also dying, although in a different way. The number of workers had dropped from close to a hundred in the Trailhead Colony era to the twenty that huddled inside now. There were no soldiers at all. The Queen was starving. Her ovaries had shriveled and she no longer laid eggs. The death rate of workers was inexorably rising, while the birth rate fell to zero. The little colony, it seemed, could not last the remainder of the warm season as a close neighbor of Supercolony.

  Then came the moving tree trunks, the ant gods, who miraculously wiped Supercolony off the face of antdom. Their lethal pressure was lifted instantly from the imperiled Woodlanders. A scattering of Supercolony scouts had still been exploring outside the kill zone when the gods came, but they offered no further threat. All died within hours, as soon as they tried to return home and unknowingly touched the still-toxic soil. Now, within a week, not a trace of their scent remained around the Woodlander nest.

  Woodlander workers, timid at first, began to venture farther than ever before from their nest. They found more and better food, mostly in the form of dead or easily captured insects of the kind preempted earlier by their dominant neighbors. Also newly available in generous quantity were droplets of sugary excrement that had fallen from aphids living on nearby understory plants.

  By late September, with the weather still warm and the vegetation green, the ovaries of the Woodland Queen revived. She was laying eggs, and healthy young larvae filled the brood chamber of the hidden nest.

  By the following April, with the last of the winter's chill lifted from the deeper recesses of the soil and the spring renewal of plant growth well under way, the Woodlander foragers began traveling farther afield. The colony resumed its growth with greater vigor.
In only a matter of days, the first Woodlander scouts entered the wasteland that was once the Supercolony territory. The pesticide had entirely dissipated from the kill zone, and insects and other small invertebrates were infiltrating the area to explore it on their own. Many were easy prey for the Woodlanders.

  Wasteland it had been, but now it was like a newly planted garden, because ironically, it had flourished from the former occupation by Supercolony. Its soil, having been aerated by the tunnels and chambers of their nests and then enriched by their decomposing bodies, was ideal for plant growth. Grasses and herbs native to the longleaf pine flatland reasserted themselves among the species that had survived the ant Armageddon. By June, a resurgent ground vegetation formed a thick green carpet over the entirety of the old nest surface. The fastest growing of the summer herbs came into full bloom by early June, and a full cast of pollinators visited to serve them--flower beetles, syrphid flies, sweat bees, wood nymph butterflies, sulphurs, whites, blues, skippers, swallowtails swarmed in as though there had never been an episode of violence and death.

  By the height of summer, the grassroots jungle teemed with hundreds of insect species, variously adapted to every major niche. A multiplicity of spiders had settled there to feed on them. Individual species snared their prey in orb webs or tangle webs, or sprinted out from silk-spun tunnels to pounce on unsuspecting passersby. A few lay motionless and camouflaged on flower heads, waiting to ambush bees and other pollinators landing there. The arachnids came in many shapes and sizes, from linyphiid dwarf spiderlings less than a pinhead in size to wolf spiders half the span of a human hand.

  Spiders, although wingless, were strangely among the first animals to colonize the regenerating Supercolony tract. A few walked in, but even more oddly, other pioneers arrived by ballooning. The method is widespread and ancient among their kind. When an immature spider possessing this ability wishes to travel a long distance, it crawls to an unrestricted site on a blade of grass or twig of a bush, lifts the rear part of its body to point the spinnerets at the tip upward, and lets out a line of silk. The delicate little thread is the spiderling's kite. The air current lifts and pulls at it until the young spider, feeling the tension, gradually lengthens the thread. When the strength of the pull exceeds its own body weight, it lets go with all eight feet and sets sail. A flying spiderling can reach thousands of feet of altitude and travel miles downwind. When it wishes to descend, it pulls in the silk thread and eats it, millimeter by millimeter, heading for a soft if precarious landing. The risk it takes offers good odds. Sailing aloft under its silk balloon, the spiderling can reach land still uncrowded by competing spiders. Such openness, for a while at least, was the condition of the newly vacated Supercolony territory.

  27

  THE WOODLANDER ANTS were like human explorers coming ashore on an uninhabited island. The abandoned Supercolony terrain presented the starving colony with a rich and temporarily boundless food supply. The Woodlanders were free for a while of competition from other ant colonies. But their bonanza could not be harvested easily. The moving, nervous prey that ants hunt are not the same as low-hanging fruit on a bush ripe for the picking. They can only be subdued with skill and swiftness. As Woodlander patrols penetrated the ground cover, their target species met them with protective devices of anatomy and behavior perfected by millions of years of evolution. Such defenses are legion in variety, and some are designed specifically to thwart ants. Many are ingenious even by human military standards. The Woodlander huntresses encountered slow-moving oribatid mites, which resembled a cross between a spider and a turtle. They seemed to be convenient morsels, but were protected by hard shells not easily broken even by the powerful jaws of an ant. Millipedes--thousand-leggers--were prize catches, but they too were armored, in a different way. Their elongated bodies were covered, like those of medieval knights, with hinged plates that gave a measure of flexibility. If those proved insufficient, the thousand-leggers unleashed poisons, including cyanide, on their attackers. Pillbugs, which are land-dwelling crustaceans, had similarly jointed armor and could also roll themselves into an almost impenetrable sphere. Springtails had tiny soft bodies ready for the eating, but were intensely alert and nervous in manner. They were equipped with a spring-loaded lever on their undersides that launched them into the air over a distance equivalent in human terms to the length of a football field, thence out of danger. Nematode roundworms, the most abundant animals on earth, were everywhere in the soil but too small for ants to gather efficiently. Ground beetles, the archrivals of ants in the kinds of food and space they require, proved to be all-purpose fighters. They were not only armored, poisonous, and swift on the ground, but also capable of taking flight if stressed--and, finally, as a last resort, they had jaws sharp and powerful enough to chop an ant in two during insect-to-insect combat.

  The Woodlander huntresses did best with the few prey available that were soft, slow, and tasty. When a Woodlander came upon a fallen caterpillar or some other invertebrate that met these exacting standards--in other words, not poisonous, did not strike out with slashing jaws or flee with the equivalent speed of a drag racer, and not least was big enough to be worth the effort--her response was enthusiastic. She reported her find to the colony by laying a chemical odor trail back to the nest and tapping her nestmates with rapid strokes of her forelegs. Out came the alerted workers, and if they found the prey attractive themselves, they attacked. If the prey was so big it could fling a single worker away, it could still be overcome by a swarm of workers rushing it simultaneously.

  In one episode that occurred during the exploration of their new territory, a Woodlander force was summoned to the nymph of a mole cricket. While immature, it was still the same size compared to a worker as a cow is to a human. The first dozen workers who arrived proved sufficient for the kill. Several of the attackers were able to pin the still-active cricket nymph by seizing and spreading its legs, while others stung it in vulnerable seams between its chitinous armor plates.

  Another target discovered by the Woodlander scouts, the prize of the week, was the grown caterpillar of a cecropia moth, the equivalent in size to the ants as a whale is to humans. Hundreds of workers, accompanied by a score of soldiers this time, were needed to subdue the monster and drag it back to the nest.

  The Woodland Colony used the same method to claim and retrieve large, already dead animals before rivals could preempt them. The reward in calories for winning such a bonanza could be enormous. On one occasion a worker force was gathering around the newly discovered corpse of a lizard. It had been led there by one of its most enterprising elite scouts. The prize contained enough food to support the colony for days. Suddenly, however, the bonanza was discovered by a squad of fire ants. The enemy, recruited by their own scouts from a distant colony, did what fire ants do best. They quickly gathered in strength and attacked anything that moved. And they were formidable in combat, especially in groups. A battle was joined, and soon the dead and injured piled up on both sides. The Woodlanders managed to prevail, mainly because their nest was close by and their buildup of fighting forces faster. The presence of soldiers also helped: the members of this larger caste not only stung the fire ants fatally but also used their razor-sharp jaws to clip them into pieces. The fighting Woodlanders were able to clear the lizard's body of fire ants and drag it inch by inch to their own nest.

  By this time the Woodland Colony had an adequate military force. Each of its hoplite soldiers was a formidable fighting machine, not only in defense of the colony, but as an escort for workers retrieving food. Its body was thick and muscular. Its head was huge in proportion to the body and was heart-shaped. The swollen posterior lobes were filled with adductor muscles that closed the jaws with enough force to cut through the chitinous exoskeleton and muscle of most kinds of insects. The inner margin of each jaw was lined with a row of eight sharp teeth. The tooth at the tip of the mandible was the longest, serving as a dagger to stab opponents and a hook to seize and hold them while others moved in.
The pair of spines extending backward from the upper surface of the middle of the body protected the soldier's thin waist, making it difficult for the ant to be cut in half during combat.

  The brain of a soldier was wired for battle. In a gland at the base of each jaw, each carried quantities of alarm pheromones ready to be spritzed into the air when the ant met an enemy. When challenged, the soldier not only produced more of these substances than did an ordinary worker, but was also more sensitive to them. On detecting even a faint trace of the chemicals, the soldier rushed about in search of enemies. Soldiers in combat attracted other soldiers. When they entered a high concentration of the alarm signal at the site of the action, they ran in frenzied loops and threw themselves at any alien object that moved. Deadly threats and overwhelming opposition were of no concern to the soldiers. They were the suicide warriors of the colony.

  The Woodland Colony could not afford to raise soldiers while it was small. The investment would have increased the defense capability of the colony and lessened the risk of total destruction by a dangerous enemy. But additions to the worker force were more important for colony growth, and rapid growth was crucial. The colony could gamble that no lethal enemy would appear during its infancy, but it could not gamble with its growth. The larger the population of workers specialized to gather food and feed the young, the faster the colony grew. And the faster its growth, the better chance it had to live another day, and yet another day, until it could afford to convert some of its time and energy into the production of virgin queens and males, thus creating new colonies of its own kind. Soldiers were added along the way, but the number of these military specialists was kept under strict control. Too few, and the colony was at higher risk of destruction by enemies--especially during wars with colonies of the same species. Too many, and the colony grew more slowly. It harvested less food from its territory, and, again, failure was imminent. A colony out of kilter in its military investment could not compete for long with colonies of the same species that kept their investment closer to the optimum for both survival and growth. To reach the right balance between defense and productive labor was a matter of life and death.