Scientist, p.14
Scientist, page 14
Similarly, Wilson worked his way through invertebrate biology, filling in missing information as he went along, correcting misjudgments in earlier papers with new information from more recent research, indicating where further research was required, adding his own field observations, cautiously speculating when such speculation seemed warranted. His informed compendium of the life and world of the social insects has never been surpassed.
Ants, along with their cousin wasps and bees, evolved from solitary wasp ancestors during the geologic era known as the Mesozoic, between about 250 million and sixty-five million years ago. (Termites evolved even earlier, from a primitive cockroach.) The Mesozoic was the period when dinosaurs rose to dominance and birds, mammals, and flowering plants first appeared. It ended catastrophically with the massive impact of a ten-kilometer asteroid incoming at forty thousand miles per hour on the coast of what is now the Yucatán Peninsula. The impact splashed molten rock around the world, starting smoky fires that induced a long, dark, cold asteroid winter that starved out the dinosaurs and most other larger life-forms and made room for the evolution of modern plants and animals, including ants and humankind.
In 1967, Wilson and his colleagues Frank Carpenter and Bill Brown reported the discovery of a primitive Mesozoic ant ancestor preserved in a piece of amber found embedded in a beach bluff in Cliffwood, New Jersey. The amber, from a sequoia tree—sequoias grew in New Jersey a hundred million years ago—held two workers of a previously unknown genus and species which the Wilson team named Sphecomyrma (“wasp ant”) freyi, the species designation honoring the amateur mineral collectors who found the amber, a Mr. and Mrs. Edmund Frey.
S. freyi was a curious mosaic of wasp and ant features: short wasplike mandibles; long wasplike antennae; primitive, antlike body; tough, protruding stinger. The three biologists judged it “truly intermediate between the primitive ants and the aculeate [stinging] wasps.” It reminded Wilson strongly of Nothomyrmecia macrops, the “dawn ant” he had sought unsuccessfully in Australia on his expedition there in 1955. (Bob Taylor, Wilson’s former postdoc, finally rediscovered N. macrops in 1977, at a site far to the east of where Wilson had searched.) Though the two S. freyi specimens appeared as fresh as tomorrow morning, their tomb of sequoia sap had engulfed them and hardened into dark-red amber almost unimaginably long ago. “They are the first undisputed social insect remains of Mesozoic age,” the Wilson team concludes, “and extend the existence of social life in insects back to approximately 100 million years.”
All modern ants are eusocial, “eu-” meaning in this case “truly”: truly social. Wilson lists three traits all eusocial insect species have in common: they cooperate in caring for their young; they divide their reproductive labor, with sterile workers attending fertile queens; and their life cycle is long enough to allow offspring to assist parents.
The first question Wilson asks in The Insect Societies is: Why study social insects? He offers scientific answers as well as practical ones. On the scientific side, the social insects in their divisions of labor present for investigation a sort of distributed body. It isn’t possible to take apart a living animal, study its various systems, and then reassemble the parts into a living animal again. But it is possible to investigate the collective organism that is a society of wasps, ants, bees, or termites that way—as Wilson says, to study “the full sweep of ascending levels of organization, from molecule to society.”
Ecologically, the social insects dominate the land. “In most parts of the earth,” Wilson writes, “ants in particular are among the principal predators of other invertebrates. Their colonies, rooted and perennial like woody plants, send out foragers which comb the terrain day and night. Their biomass and energy consumption exceed those of vertebrates in most terrestrial habitats.” In the tropics, they move more earth than earthworms do; they’re competitive with earthworms even in cold temperate forests.
Charles Darwin, in his delightful last book The Formation of Vegetable Mould Through the Action of Worms, estimates the production of new soil by earthworms in one of his fields at an average .083 inch per year, “i.e., nearly one inch in twelve years.” Wilson, citing a 1963 study of ant soil turnover in one locality in Massachusetts, a colder climate than England’s, reports that ants brought “50 grams of soil to the surface per square yard each year and [added] one inch to the topsoil every 250 years.” Earthworms in this comparison moved twenty times as much soil as ants, but earthworms eat their way through the soil and build it up with castings—feces, fluffy and abundant—whereas ants only move soil in the course of building and maintaining their nests. Termites, specializing in dead wood and leaf litter, contribute significantly to soil production as well.
Ants pollinate plants, feed them, and disperse their seeds. Throughout the world, ants protect certain plants, and plants protect ant colonies, in an evolved expression of mutualism. Plants specialized to house ants in various configurations of shelter include bromeliads, coco plums, laurels, legumes, mulberries, orchids, peppers, buckwheats, ferns, madders, coffees, spurges, figs, sapodillas, cacao trees, verbenas, mangoes, sumacs, milkweeds, palms, mahoganies, nutmegs, pitcher plants, figworts, foxgloves, and leatherwoods. Plants feed ants as well, many with specialized food bodies attached to their seeds that spare the seeds themselves for ant distribution. Particularly in poor soils—in grasslands, deserts, and forest margins—ant nests stimulate plant growth. They “turn and aerate the soil,” Wilson writes, “add nutrients in the form of excrement and refuse, and hold the ambient temperature and humidity at moderate levels.”
So large a group of animals could hardly be only benevolent. Ants are significant pests in some tropical environments and when, like the red imported fire ant, they escape their normal evolutionary setting, where predators and parasites control their population. When it came ashore at young Ed Wilson’s doorstep in North America, the fire ant, weedlike, found few competitors to prevent it from exponential increase.
But even in their natural setting, some types of ants are fiercely destructive. Wilson calls the relentless sweep of Eciton burchelli, one of several species of army ants of the humid lowland forests of eastern South America, “exciting.” It may be for a scientist, but it must be terrifying for any creatures in the path of the swarm that are unable to escape. A “big, conspicuous species,” E. burchelli army ants form nighttime bivouacs on the march by linking themselves together around their mother queen in chains and nets “that accumulate layer upon interlocking layer until finally the entire worker force”—as many as seven hundred thousand individuals—“comprises a solid cylindrical or ellipsoidal mass up to a meter across,” cached in the spaces between the prop roots of the great rain-forest trees. At daylight, the bivouac mass dissolves, the chains and nets break up, and a teeming broil of excited workers tumbles out.
Unlike most army ants, E. burchelli is a swarm raider rather than a column raider. At first, Wilson writes, workers fan out in all directions. Then the density of the mass begins to increase “along the path of least resistance and grows away from the bivouac at a rate of up to 20 meters an hour.” The increasingly fan-shaped swarm is leaderless, advanced by workers laying odor trails as they press ahead a few centimeters and then wheel back into the mass. Wilson quotes a specialist in army ants, Theodore C. Schneirla, an animal psychologist who was curator of the department of animal behavior at the American Museum of Natural History in New York, describing a typical raid:
For an Eciton burchelli raid nearing the height of its development in swarming, picture a rectangular body of 15 meters [50 feet] or more in width and 1 to 2 meters [3–6 feet] in depth, made up of many tens of thousands of scurrying reddish-black individuals, which as a mass manages to move broadside ahead in a fairly direct path….
The huge sorties…bring disaster to practically all animal life that lies in their path and fails to escape. Their normal bag includes tarantulas, scorpions, beetles, roaches, grasshoppers, and the adults and broods of other ants and many forest insects; few evade the dragnet. I have seen snakes, lizards, and nestling birds killed on various occasions; undoubtedly a larger vertebrate which, because of injury or for some other reason, could not run off, would be killed by stinging or asphyxiation.
Unusually in a field report, Schneirla describes the characteristic sounds of an E. burchelli raid. There is first of all, he writes, “a kind of foundation noise from the rattling and rustling of leaves and vegetation as the ants seethe along and a screen of agitated small life is flushed out.” There’s “an irregular staccato” caused by jumping insects knocking against leaves and wood, a sound Schneirla chillingly calls “the collective death rattle of the countless victims.” There’s “the loud and variable buzzing” of the clouds of flies that hover, circle, or dart immediately ahead of the advancing swarm, much as sea birds do above feeding whales. Flies individually or in small squadrons emit short, higher-pitched notes as they swoop down to capture their share of the escaping prey. Behind this cacophony, antbirds call—any of a large number of species of the Formicariid family, small birds with strong legs and heavy, hooked bills. “One first catches from a distance the beautiful crescendo of the bicolored antbird,” Schneirla writes, “then closer to the scene of the action the characteristic low twittering notes of the antwren and other common frequenters of the raid.” The antbird seems misnamed, however: it eats not the ants but the prey the ants scare up in their sweeps, not different in that sense from the similarly opportunistic flies.
The question Wilson is asked about ants more than any other, he writes, is whether army ants are “the terror of the jungle.” No, he answers, not really:
Although the driver ant colony is an “animal” weighing in excess of 20 kg [44 pounds] and possessing on the order of 20 million mouths and stings, and is surely the most formidable creation of the insect world, it still does not match up to the lurid stories told about it. After all, the swarm can only cover about a meter of ground every three minutes. Any competent bush mouse, not to mention man or elephant, can step aside and contemplate the whole grass-roots frenzy at leisure, an object less of menace than of strangeness and wonder, the culmination of an evolutionary story as different from that of mammals as it is possible to conceive in the world.
If army ants are relentless raiders, another large ant family has made the transition from hunting to farming. Leafcutter ants of the genera Acromyrmex and Atta, natives of South and Central America and the Southern United States, cut pieces of leafy vegetation and carry them over their heads like parasols back to their colonies, where they drop them onto the floor of large chambers carved out along a central tunnel. Workers have lined the chambers with the ant equivalent of papier-mâché—chewed-up plant matter—inoculated with a fungus. Smaller workers next clip the dropped leaf pieces into smaller fragments and pass them along to still smaller workers that chew them into moist pellets, anoint the pellets with fecal drops, and plant them in the chambers’ pellet garden. Yet smaller workers then clip strands of fungus from the densest areas of growth on the walls and floor of the chamber and pack the strands among the new pellets. “Finally,” Wilson concludes his description of this consecutive process, which he compares to an assembly line, “the very smallest and most abundant workers patrol the beds of fungal strands, delicately probing them with their antennae, licking their surfaces, and plucking out spores and strands of alien species of mold.” The fungus the ant farmers grow then serves to feed the colony.
The great evolutionary advantage of agriculture over hunting, with ants as with humans, is a more reliable food supply, which can support a larger population. Atta colonies are huge, numbering from hundreds of thousands to as many as eight million individual workers. “A full-grown colony,” Wilson reports, “consumes approximately the same quantity of plant material as a cow.” All the colony’s workers are the daughters of a single long-lived queen, whose lifespan may extend from ten to fifteen years. She establishes the colony alone, starting its fungus garden, laying eggs, and tending them until her first workers eclose—emerge as adults from their pupae—and take over all the nascent colony’s duties except egg laying, which remains the queen’s duty alone. “A rough calculation,” Wilson writes, “reveals that the mother of a mature colony lays on average about 20 eggs per minute, thus 28,800 per day and 10,512,000 per year.”
Human agriculture originated with the domestication of wild grains between seven and ten thousand years ago. Ant agriculture, which evolved across a period of thirty million years beginning around fifty million years ago, depends on single species of fungus transmitted clonally from colony to colony by founding queens, which carry a small plug of fungus cultivar in a special pouch in their mouths when they leave a colony on their nuptial flights and use it to start their new gardens. These ancient fungus clones may be several million years old.
Between extremes of ant evolution, such as the mass raiding of army ants and the specialized farming of the leafcutters, lie some twelve to twenty thousand species of ants in a great variety of sizes, numbers, and behaviors. There are ants that feed exclusively on bugs, on seeds, on insect honeydew secretions, on other ants. There are ants that live deep underground, ants that live in the tops of trees, ants that tenant hollow twigs, ants that use silk drawn from their larvae to construct tentlike nests, parasitic ants that occupy the nests of other ants. Degenerate slave-making ants that have lost all skills other than raiding depend on their slave colonies for care and survival. In one such species, the worker caste has disappeared entirely, leaving free-living ectoparasite queens, Wilson writes, “modified for riding on the backs of the host queens.”
Male ants live short and parasitic lives, fed by workers but performing no worker functions themselves, and scarcely surviving a single summer. Their entire purpose is to inseminate a queen on her initial nuptial flight, when she typically mates with multiple males to collect enough sperm to last her a lifetime. “Flying sperm dispensers,” Wilson calls ant males dismissively, recalling the “dancing dons of the cocktail-lounge set” he had sourly imagined were courting Irene when he first glimpsed her descending the stairs at Shirley Hayes.
An unfamiliar aspect of ant organization is what Wilson calls age polyethism (poly-EETH-ism), the changing role of workers as they age. “Young workers tend to remain in the nest and nurse the brood,” he reports, “while older workers spend more time outside the nest.” Workers aren’t specialized or organized into groups to perform specific tasks; they direct their activities to whatever work they find in front of them. So working inside the nest or outside the nest is a tendency rather than a compulsion, and experiments Wilson describes demonstrate that switching brood workers outside leads them to take up foraging, and vice versa.
Even so, young workers usually spend their first weeks caring for the brood, followed by a similar period keeping up the nest—half their time taking care of other workers, handling dead prey, and cleaning the nest, the other half resting. Older workers move outside to perform riskier duties—foraging, patrolling colony territory, and defending the nest against invasion. Unlike human societies, then, ant societies assign their young to the relative safety of home service and send their older members, which are nearer the end of their lives, out onto the land or off to war.
Beyond the natural history of ants lies the more rugged terrain of ant sociology. The central question in that area of knowledge, Wilson proposes, is how complex social behavior emerges from the simple behavior patterns of individual ants. An ant by itself has only a limited repertoire of responses, “neither exceptionally ingenious,” Wilson writes, “nor exceptionally complex. The remarkable qualities of [ant] social life are mass phenomena that emerge from the meshing of these simple individual patterns by means of communication.”
The meshing is anything but efficient. As Wilson describes it, a collectivity of ants lurching toward a mass action sounds like an episode of the Keystone Kops. “It usually results from conflicting actions of many workers,” he explains. If an ant colony “decides” to emigrate from one nest site to another, for example, workers carrying eggs, larvae, and pupae to the new site have to shove past workers returning brood the other way. Other workers “run back and forth carrying nothing.” The “decision” to settle in a new nest is made collectively, as a preponderance of individual actions draws in more and more members of the colony. (Bees similarly decide through their behavior; bee swarms can be divided or dispersed when the collectivity fails to coalesce around a new site.)
At this point, Wilson introduces a theory a French entomologist proposed in 1959 that has since come to be applied far beyond the world of insects. Pierre-Paul Grassé was studying termite nest-building behavior. It appeared to him that termites responded to actions performed by other termites—even seemingly random actions—by moving to continue the work. Placed in a container along with bits of mud and excrement, the termites first explored the container. (Termites are blind.) Then they began picking up and putting down pellets of the mud and excrement. If several pellets ended up on top of one another, the termites began adding to the nascent column. If no other columns emerged nearby, they eventually lost interest in the single column they were building. But if two columns happened to be developing near each other, they built up both, and any others as well. As the columns lengthened, the termites began arching them inward toward each other, presumably following an inherited script. As the columns met and were connected, they formed structural arches. Eventually, several such constructions, emerging from random initial actions, produced an intricate, cathedral-like termite nest.
