Anthill

by Edward Osborne Wilson

  Still, the instinct machine could not be turned off. The pheromone messages continued. They flowed ant to ant, spreading the latest news, the gossip, the health and wealth of the commonweal. The worker castes performed as before, and foragers still left in the early morning to search for food in the surrounding terrain. But the Trailheader workers had begun to change in subtle ways. The throttle of the colony was easing, a little at a time.

  The Queen's health had been declining weeks before her death. The clues were all around her. Her egg production had plummeted, then halted. There were fewer and fewer larvae to feed. More nurse workers were idled, and colony growth slowed. The number of foragers taking the field dropped.

  Yet there was hope for the colony. Even when the Queen still lived, the thinning of her pheromones had caused subtle changes in the bodies of young soldiers headquartered in the nest. These largest members of the worker caste had massive heads filled with powerful muscles, which slammed their sharp-toothed jaws together like serrated wire cutters. They were the iron, the physical power, the instinctual viciousness of the colony. They usually served only to defend the nest from intruders. Sometimes they went out along the odor trails with the ordinary workers to guard large food sources against rival colonies. But they also had the ability to reproduce. Their capacious abdomens contained a half dozen ovaries that, when enlarged by further growth, could produce viable eggs. Amazons all, they could change from warriors to mothers.

  As the Queen pheromone declined, the soldiers were alerted. The sensory cells in the outer segments of their antennae noted the change. The information was relayed along nerve cells to the soldiers' brains. Circuits within the brains transmitted instructions to endocrine glands located elsewhere in the head. The hormones released from these glands stimulated growth in the ovaries of the young soldiers. Lines of eggs then appeared inside the ovaries. They began as microscopically small masses near the outer tips of the ovaries. The eggs grew in size as they migrated downward toward the openings of the ovaries, reaching maximum size just before they were laid.

  The soldiers with the potential to become the new queens of the Trailhead Colony and no longer inhibited by the mother Queen pheromone abandoned their regular duties. With their ovaries swelling with eggs, they moved deeper into the nest interior and closer to the dwindling piles of larvae and pupae. As the last shards of the old Queen's body were carried into the cemetery, several of her rival successors began to lay eggs. They were now soldier-queens, and the only hope the colony had to restart its own growth.

  The ordinary workers around them accepted the new status of the soldier-queens. Their tolerance represented a profound shift in the behavior of the colony as a whole. If the mother Queen had remained alive and well, and she had continued to broadcast her special scent, the response to any usurper would have been swift and violent. The Trailhead Colony had previously obeyed a basic rule of antdom: to reproduce in the presence of a healthy queen is strictly forbidden. The odds against success of such an affront to authority are long. The gamble is dangerous, and only a very few make the attempt. When a usurper starts to lay her own eggs and place them among those of a healthy queen, or even when she just becomes capable of doing so, she is harassed by her nestmates. Her sisters refuse to regurgitate food to her. They climb and stand over her, pulling at her legs and antennae. They may use their stings to cripple or kill her, or else spray her with a poisonous secretion. And they eat any eggs she manages to lay. Only when the Queen dies is the taboo lifted--and then only for a few individuals.

  When the taboo ended in the Trailheader nest, a second crisis arose. The candidate royals began to quarrel among themselves for control. They converged on the brood chambers and jostled for position there. They struggled to climb on top of their rivals. Winners in these encounters seized their opponents' legs and antennae and dragged them away from the brood chambers. Unlike the thousands of their ordinary nestmates, they recognized one another as individuals. In time a dominance hierarchy formed, similar to pecking orders among chickens and rank orders among wolves. The Trailheader female who emerged as the alpha contender, in other words was able to chase away all her rivals, won the reproductive role. Egg-laying and larval growth resumed in a reduced but orderly manner. The crisis had ended by combat.

  If the Trailhead Colony could not understand the history of its own species, how much did it know of its current condition? How could it make the right decision for survival? In fact, the Trailhead Colony knew a great deal. Worker ants are far more than just automated specks running around on the ground. Even with a brain only a millionth as big as that of a human, an ant can learn a simple maze half as fast as a laboratory rat, and remember the directions to as many as five different destinations when she forages away from the nest. After exploring a new terrain, a worker can integrate all the seemingly haphazard twists and loops she made and, amazingly, return to the nest in a straight line. She can learn and recall the special odor of the colony to which she belongs. In some species, she can recognize her own personal smell in odor trails over the ground to which she has contributed her own pheromones.

  The Trailhead Colony, when all the learning and thought of its workers came together, was very smart by insect standards. With the unifying power of its Queen taken away and its population growth plummeting, it needed to act with all its group intelligence to regain its balance.

  When one of the soldier-queens dominated its rivals and became the new Queen, the recovery of the colony seemed to get under way. A stream of eggs were laid. Larvae began to fill the empty brood chambers. Their odor and hunger signals joined with the pheromones of the new Soldier-Queen and spread through the nest. The power was returning. The workers found new energies. More foragers took the field.

  One of the ants that led the way in restoring order was an elite worker that had served in the Queen's entourage during the final days. About ten percent of the worker force deserved this status, which they kept all their lives. All achieved it by labor; none belonged to the soldier caste, which was specialized for combat and called into action only when the colony was threatened. The elites were nervous and vigorous in movement. They initiated more tasks. They worked harder and more persistently, and they usually stayed on the job until it was finished. Other colony members were stirred to join them at the tasks they began. They were not just statistically at the upper end of the activity curve. They were a distinct group all on their own, forming a bump on the high end of the curve, and important to even a temporary prolonging of the life of the colony.

  This particular elite worker was typical of her class in initiative and energy. After leaving the dead Queen, she proceeded directly to the nest entrance in search of new duties. Food was low, and fewer ordinary workers, grown lethargic, were leaving in search of new supplies. The defense of the colony had been weakened by the thinning of the sentinel force spread around the nest perimeter. Sensing the negligence of its nestmates, the elite left on solitary patrols, circling first close to the nest, then farther and farther away.

  The renewed activity, led by the elites, was short-lived, however. The colony was destined to die, doomed by a hereditary trait even more basic than the altruism of the workers and the pheromonal ties that bound them together. The trait is the following. The Trailheaders, along with all ants of all kinds that ever existed back to the birth of ants in the late Jurassic period, used a strange but elegant genetic method to fix the sex of an individual at birth. Fertilized eggs develop into females, which can become queens or workers, and unfertilized eggs develop into males, which can do nothing but inseminate females.

  The Soldier-Queen had never mated. Her children all arose from unfertilized eggs and were therefore male drones, contributing nothing to the welfare of the colony. They had weak mandibles and small brains but huge eyes and genitalia. They were wondrously adapted for mating after flying, up in the air with virgin queens, but even if that occurred it would do nothing for the Trailhead Colony. Those created by the Soldi
er-Queen would not mate with her or other potential Soldier-Queens. They were programmed to mate during nuptial flights away from the nest.

  No way out existed for the Trailheaders; the colony was in a terrible fix. The linchpin of its social existence was gone and could not be replaced. Like a player in a Greek tragedy, it had been undone by the unfolding of events prescribed by its own unalterable nature. The source of its early success had become its fatal flaw. The colony could for a while contribute, through its production of males, to the gene pool of the population of colonies all around Dead Owl Cove, and in that way tweak out one more bit of Darwinian profit. But it could do nothing more for its own physical existence. With each passing day it became more vulnerable as a superorganism.

  As the Trailhead Colony struggled in this pitiless world, its territory and even its very flesh were coveted by others. It would not merely fade away, holding on until a final worker sat alone in the nest. On the contrary, the neighboring colonies were likely to learn of its decline, and when that happened there would be war. And when war came, there was only the slimmest chance that the queenless Trailhead Colony could win.

  21

  FOR A WHILE the Trailhead Colony, while stricken, still retained most of its military strength. Fifteen percent of its adult members were soldiers. They were contracted to be hoplites, or heavily armored infantry. Twice the size of an ordinary worker, a soldier's exoskeleton was literally heavy armor: thick, tough, and pitted in places like a shield for resilience and strength. A pair of spines projected backward from the midsection of the body to protect the waist. Spikes extended forward from the midsection to protect the neck, and the rear margin of the head was curved forward, turning that part of the surface into a helmet. When attacked, the hoplite soldier could pull in her legs and antennae and tighten up the segments of her body in order to turn her entire body surface into a shield.

  The ordinary Trailheader workers, while built for labor, were also available for combat. Then they served as the equivalent of light infantry. Because their exoskeletons were much thinner than those of the hoplites, they were not inclined to stand fast in battle. Instead, they used the swiftness and agility of their supple bodies, running around their enemies, darting in and out, seizing any leg or antenna available, holding on to it, slowing the opponent enough for nestmates to close in and seize another body part. When the adversary was finally pinned and spread-eagled, others piled on to bite, sting, or spray it with poison. This swarm attack, in which a crowd of fighters rush a formidable opponent simultaneously, was the same as used by wolves circling a moose, or infantrymen attacking an enemy firebase.

  Such was the force, originally ten thousand strong, that had protected the Trailheader nest against all enemies. Now the number of able-bodied adults had begun to decline, and the survivors were growing old.

  The decline of Trailhead Colony was being closely watched by its closest neighbor, the Streamside Colony. This younger and now more powerful superorganism was prepared to take advantage of its neighbor's misfortune.

  Early one morning, an elite Streamsider worker, followed by a squad of her nestmates, left her nest to assess the strength of the Trailhead Colony. Precise monitoring of the enemy strength was not easy. The two nests were separated by about two thousand ant lengths, or a distance of twenty yards. The scout, if allowed to travel in a straight line on a smooth surface, might have covered that distance in under six minutes. But a straight run was not possible, because the terrain was filled with obstacles that were scarcely noticeable to a human being but were daunting to a ten-millimeter-long ant. In the miniature world of antdom, clumps of grass were like groves of trees and bushes, and dead leaves and twigs like fallen timber. A surface of sand smooth to humans was to the ants a jumble of rocks, and pebbles were large boulders. Rain was a deadly threat. One drop striking an ant had the human-equivalent force of a firehose jet. A rivulet of rainwater trickling through a crease in the soil was the equivalent of a flash flood raging down a desert ravine.

  As the elite ant left on her journey, she remembered the route more or less precisely. She had been to the Trailhead territory before, and remembered the way. She carried a compass in her head, using the sun as a lodestar. This reliance could have been the source of a huge error for an ant, because the sun travels across the sky and so the correct angle constantly changes. However, each ant also carried in her head a biological clock set on a full day's twenty-four-hour cycle, run with a precision far beyond the capacity of an unaided human brain. Using her clock, the scout continuously changed the angle to the sun needed to keep her on track.

  The trajectory of the sun by itself is completely reliable in space and time. At Nokobee it traced a geometrically perfect arc across the sky, rising through the pines on the eastern lakeshore, passing directly above the anthills of Dead Owl Cove, and finally disappearing westward into the forest lying beyond. The azimuth read by the ant, however, unlike the transit of the sun, was less than perfect. So the scout occasionally stopped and gazed at prominent features she had memorized during earlier trips. A pair of pine seedlings were one such signpost, a circular opening in the canopy a second, a dark shadow beneath a holly shrub a third.

  Then there was the odor terrain, parts of which the scout memorized from chemical cues she had encountered on earlier trips. In exercising this ability, she was as different from a human being as it is possible to imagine. The scout smelled the ground continuously and precisely as its surface rushed by two millimeters' distance beneath her body. Her nose was the outer segments of her paired antennae--the two feelers on her head. She turned these hypersensitive instruments downward, enough to almost touch the ground, and swung them from side to side. The odors she detected as she ran, specific in their mix, intensity, and gradient, provided detailed information of her location and direction of travel. They were her combined field guide and topographic map.

  The nearby pine-leaf litter conveyed its acrid scent to mingle with that from the humus beneath the colony foraging grounds. A surge of one particular blend greeted her here, a countersurge of another kind there. The prevailing background was overpowered now and then by a flashing scent of something radically different--quickly gone but remembered for a while.

  The olfactory world of the running Streamsider contained much more than an invisible road map. Bombarding the ant from below and from all sides above were the odors of organisms that inhabited the soil--so densely as to make up a large part of the physical bulk of the soil. There were endless local profusions of fungal hyphae and bacteria. Each gave up its signature smell. There were the rising odors of the animals the size of the ant or smaller, a quarter million packed into every square meter. They were the insects, spiders, pillbugs, nematode roundworms, and other invertebrates that dominate in numbers. One trace within the mix picked up by the sweeping antennae could disclose a potential prey, another a waiting spider or some other ambush predator.

  The human mind cannot imagine the tumult of chemical stimuli by which such a traveling ant guides every moment in her life, and thus survives. It cannot conceive of the constant enormity of the deadly risks she must skillfully evade, instantaneously at every moment.

  The Streamsider scout hurried undistracted through this olfactory cosmos. Her destination was in the direction of the enemy nest but not the nest itself. She was consciously headed for a flat, open area half the distance there. On arriving, the scout mingled with a group of nestmates who had preceded her, and--an extraordinary event for ants--they also mingled freely with scouts from the Trailhead Colony. One of these enemies was the newly arrived elite and former Trailhead Queen attendant.

  In a short time the representatives from the two colonies appeared to dance with one another. The ants were not performing in any human sense, however. They had come here instead to conduct a tournament between colonies. The scouts were gathering information that allowed them to assess the strength of the opposing Trailhead Colony. They could use this information and simultaneously adver
tise their own strength to the enemy without risk of death or injury. The dance, in short, was not that, but a highly formalized probe and communication that reinforced the security of the two colonies.

  At the time of this day's tournament, the Streamside Colony was at its peak as a superorganism. It was strong enough to challenge any neighboring colony, and especially the declining Trailhead Colony. The Streamside Queen was only six years old, the equivalent of thirty years in a human life-span. She was in her prime, bursting with eggs, and she reeked of sweet-smelling royal pheromone. Her colony's nest was on firm, productive ground at the edge of an undisturbed patch of deciduous scrub woodland. Close by in the woods a small stream gave the nest protection on one side. On the other side a miniature ravine dropped away, too steep to harbor nests of potential rivals. The Streamsiders had not chosen this site for their own protection. They were just lucky that their mother Queen had landed there.

  As the Streamsider scouts gathered in the arena, they found their Trailheader counterparts also assembling in almost equal numbers. A few had climbed up on the tops of pebbles to serve as sentinels. The first scouts on both sides to encounter the enemy ran home to recruit reinforcements. They laid odor trails to excite and guide their nestmates, and they carried faint smears of the enemy odor on their own body surface to identify the opposition. Within an hour hundreds of ants from both colonies were milling around one another. The original scouts, all of whom were relatively small and thin, were soon joined by contingents of the more massively built soldiers.

  The opposing forces were careful not to start a battle. Their strategy was the opposite: the displays were the equivalent of competing military parades by human armies. They wanted their performance to be viewed by the enemy.