On Trails (10 page)

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One year after his initial caterpillar experiments, Charles Bonnet was outside hunting for a new batch of caterpillars when he happened across a prickly flower called a teasel, whose head harbored a colony of tiny red ants. Ever curious, he plucked the flower, carried it back to his study, and planted it upright in an open powder jar.

One day Bonnet returned to discover that a number of the ants had deserted the nest. Searching about, he found them marching up his wall to nibble the wood at the top of his window frame. In his journal, Bonnet described watching one ant as it climbed down the wall, up the side of the powder jar, and back to the nest. At the same time, two ants emerged from the teasel head and climbed to the top of the window frame, following precisely the same route that the other had just descended.

“Instantly, it came to my mind that these ants which I had in front of me, like the caterpillars, left a trace that directed them in their course,” he recalled.

Of course, he knew that ants did not emit a thread. But they did give off a strong smell, which is sometimes described as being reminiscent of urine. (This odor lent ants their archaic name, “pismires,” and later, “piss-ants.”) The substance, Bonnet theorized, could “more or less adhere to objects they touch, and then act on their sense of smell.” He compared those “invisible traces” with the trails of wildcats, which are imperceptible to humans but plain as blood to dogs.

His suspicion was easily tested: as before, he rubbed his finger across the ants' pathway. “Doing so, I broke the path on a width equal to that of my finger, and I saw precisely the same spectacle the caterpillars had given me: the ants were diverted, their walk was interrupted, and their confusion amused me for me some time.”

Bonnet had stumbled on an elegant explanation for how ant trails form, which required neither powerful memories, strong eyesight, nor simple language (as Huber and Fabre later proposed). Bonnet theorized, correctly, that ants ordinarily follow trails that lead to their homes and to food sources. However, some ants wander off track, “attracted by certain smells or other sensations to us unknown,” spawning new side roads. If that rogue ant finds food, it will leave a new trail on its return to the nest, and other ants will follow. So, wrote Bonnet, “a single ant can lead a large number of its companions to
a certain place without any need of a particular language whereby it announces the discovery that it has just made.”

Judging from his journals, Bonnet seems not to have realized how historic this discovery was. Scientists had long suspected that ants deposit chemicals when they walk; in the sixteenth century, two German botanists, Otto Brunfels and Hieronymus Bock, discovered that ants produce formic acid after noticing that a blue chicory flower, when thrown onto an anthill, turns a vivid red. But no one properly connected the dots until Bonnet.

Around the time of Bonnet's death in 1793, the zoologist Pierre André Latreille confirmed Bonnet's suspicion that ants sniff their way through the world. He learned this by amputating the antennae from a number of ants; at once, he wrote, they began wandering aimlessly about, as if in “a state of intoxication or a kind of madness.” Then, in 1891, Sir John Lubbock, the English polymath, performed a groundbreaking series of experiments involving Y-shaped mazes, bridges, and rotating platforms. Through painstaking experimentation, he showed that
Lasius niger
ants navigate primarily by using scent trails.

In the late 1950s, E. O. Wilson solved the riddle by locating the gland in fire ants that secretes trail pheromones. He had a hunch that the trail substance resided somewhere in an ant's abdomen, so he split the abdomen open and, using a pair of sharpened watchmaker's forceps, carefully removed all the organs. Then he smeared each organ across a piece of glass. After each stroke, he checked to see if it had any effect on a nearby colony of ants. Line after line, organ after organ—the poison gland, the hindgut, the little blob of lipids called a “fat body”—prompted no response. Finally he smeared out a tiny, finger-shaped organ called Dufour's gland. “The response of the ants was explosive,” Wilson later recalled. “As they ran along they swept their antennae from side to side, sampling the molecules evaporating and diffusing through the air. At the end of the trail they milled about in confusion, searching for the reward not there.”

By the year 1960, our fuzzy understanding of ant trails had snapped into sharp focus. Two crucial new terms were born concurrently: a pair of German biologists coined the term
pheromone
—chemical triggers, or signals—and Pierre-Paul Grassé introduced the notion of “stigmergy.” Stigmergy is a form of indirect communication and leaderless cooperation, using signals deposited in the environment. Termites, for example, organize their massive construction efforts stigmergically: there is no foreman, and no direct communication between the termites. Rather, the termites respond to a series of simple cues in the environment (
if dirt here, move dirt there
), which in turn impel them to further
alter the environment. This behavioral feedback loop can result in structures of stunning efficiency and resilience, like the towering termite mounds of Australia, which, proportional to their makers, are three times taller than our highest skyscrapers. With a combination of pheromones and stigmergy, even the simplest insects could build labyrinthine trail systems.

In the 1970s a biologist named Terrence D. Fitzgerald, being familiar with Wilson's work, intuited that tent caterpillars might also use trail pheromones. At the time, biologists believed that tent caterpillars followed their nest mates' silk, which is expelled from their mouthparts, but he had a hunch that they were secreting trail pheromones onto the silk from their back ends, as ants do. So he folded a plain piece of paper in half and ran its edge along the underside of a caterpillar's abdomen. Then he unfolded the paper and placed some caterpillars on it. Sure enough, the caterpillars marched back and forth along that crease, following the invisible line of pheromones just as Wilson's fire ants had. (Like Wilson, Fitzgerald was later able to isolate and synthesize these trail pheromones.) This discovery lent a neat symmetry to the path of inquiry Bonnet had started: We learned ants follow pheromone trails by studying tent caterpillars, then we learned tent caterpillars deposit pheromone trails by dissecting ants.

It may seem odd, then, that neither Wilson nor Fitzgerald cites
Bonnet's discovery. In fact, many of Bonnet's writings, including the story of how he discovered the true nature of ant trails, have never been published in English. Though his career showed a promising start, it ultimately veered off on an ill-fated path. In his twenties, Bonnet became a celebrated naturalist: the first person to witness a virgin birth among plant lice, the first to describe regeneration among worms, the first to learn that caterpillars breathe through holes in their skin, and the first to prove that leaves exhale. Then, in a cruel twist, his vision began to cloud with cataracts. Unable to practice observational science, he turned to more cerebral fields, like philosophy, psychology, metaphysics, and theology. Much of the latter half of his life was spent trying to reconcile the confusing new findings of the biological sciences with his deep religious faith, which held that the world was divinely engineered. Bonnet's magnum opus—an all-encompassing theory of the universe called the “Great Chain of Being,” which posited that all species were slowly progressing toward a state of perfection over the course of eons—had some influence on later evolutionary theorists like Jean-Baptiste Lamarck and Georges Cuvier. But in the broader span of scientific progress, it proved little more than a theoretical side road, which was later made obsolete by Darwin's theory of evolution by natural selection. By the end of his life, Bonnet's blindness caused him to suffer from phantasmagoric visual hallucinations, which are now known as Charles Bonnet syndrome.
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Today, that syndrome is primarily what he is remembered for, when he is remembered at all.

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Every trail tells a story, but some trails tell it more eloquently than others. The trails of Despland's forest tent caterpillars, for example, are blunt—they are essentially able to shout just one phrase:
This way!
The trails of certain ant species are more sophisticated: they can whisper as well as shout. The strength of the chemical trail tells the colony how desirable the trail's destination is, which allows for more nuanced communication and nimbler collective decision making. Scientists have long pondered how ants, which are individually quite stupid, can behave so intelligently as a colony. “The reason is,” E. O. Wilson once wrote, “that much of the ‘spirit of the hive' is actually invisible—a complex of chemical signals we have only now begun to reveal.”

Consider the fire ant: Once a scout has found a food source, excited by its discovery, on its return trip it presses its stinger to the ground to release a stream of pheromone, like ink from a fountain pen. The more food it finds, the more pheromone it deposits.
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Other ants follow this trail to the food, and then they lay more trails home. So if there is a large store of food, the trail will emerge quickly and blaze bright (chemically speaking), which will attract more ants. Then as long as food remains, the trail will continue to draw more ants. But once the food runs out, the trail evaporates, and the ants gradually abandon it for another, stronger trail. This process neatly illustrates how stigmergy allows simple beings to arrive at elegant solutions to complex problems all on their own.

The basic mechanism at work here is the feedback loop: cause leads to effect (an ant finds food, and deposits a trail as it returns to the nest), then that effect becomes a new cause (that trail attracts more ants), which then leads to an amplified effect (they lay down their own trails, recruiting more ants), ad infinitum. Feedback loops can be divided into two types: the desirable kind, known as a
virtuous
circle
, such as when ants leave stronger and stronger trails to a food source; or the undesirable kind, called a
vicious
circle
, like when a microphone is placed too close to an electronic amplifier, which allows minor sounds to self-amplify into those terrible, high-pitched shrieks familiar to any concertgoer. (Scientists used to poetically refer to the latter phenomenon as a “singing condition”; today, we simply call it feedback.)

In the circling of tent caterpillars Bonnet and Fabre both witnessed, in a strikingly literal form, how the same mechanism that gives rise to a virtuous circle can also give rise to a vicious one. The animal psychologist T. C. Schneirla witnessed this grim transformation in 1936, while working at a laboratory on an island in the middle of the Panama Canal. One morning, the resident cook, Rosa, approached Schneirla in a state of feverish excitement. She led him outside, where he found, on the cement walkway in front of the library, hundreds of army ants marching in a circle about four inches across.

Army ants, which are blind, rely heavily on pheromone trails to navigate the world. Most of the time, they march in thick raiding columns, consuming everything in their path, a habit that has garnered them the nickname “the Huns and Tartars of the insect world.” Schneirla could tell that something had clearly gone wrong with this colony. Instead of a marching column, the swarming mass resembled a ragged vinyl record, with concentric black rings spinning frantically around a hollow center. The circle widened as the day wore on. In the afternoon, rain began to drum the pavement, which divided the mass of ants into two smaller vortices, each rotating until nightfall. The next morning, Schneirla awoke to find that most of the ants had died; those that remained continued to plod in slow, tragic circles. A few hours later, all were dead, and other species of scavenging ants had arrived to carry them away.

Schneirla was careful to point out that the doomed loop had most likely formed because the ants were walking on perfectly flat cement; otherwise, the undulations of the jungle floor might have disrupted it.
However, looping trails had been recorded under different conditions by other prominent scientists, like the entomologist William Morton Wheeler, who once watched a group of ants circle the base of a glass jar for forty-six hours. (“I have never seen a more astonishing exhibition of the limitations of instinct,” he wrote.)

In 1921 the explorer-naturalist William Beebe described running across a colony of army ants marching in an enormous circle through the Guyanese jungle. Beebe followed the procession for a quarter mile, under buildings and over logs, only to find that their trail ended where it began. Astounded, he traced the crooked circle again and again. The procession continued to circumambulate for at least a full day, “tired, hopeless, bewildered, idiotic and thoughtless to the last.” By the time that a few stragglers at last broke from it and wandered away, most had fallen dead from starvation, dehydration, or exhaustion.

“This peculiar calamity may be described as tragic in the classic meaning of the Greek drama,” wrote Schneirla. “It arises, like Nemesis, out of the very aspects of the ant's nature which most plainly characterize its otherwise successful behavior.”

Beebe was more succinct. “The masters of the jungle,” he wrote, “had become their own mental prey.”

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There is a simple reason why we find the image of circling ants or caterpillars so troubling. The first instinct of humans who are lost in the wilderness is to cling to any trail they find and never leave it. Indeed, authorities on wilderness survival commonly recommend this tactic: “When you find a trail stay on it,” declares a backpacking guide published by the U.S. Forest Service, in a section titled “If You Get Lost.” A trail, the naturalist Ernest Ingersoll once wrote, is a “happy promise to the anxious heart that you are going
somewhere
, and are not aimlessly wandering in a circle.” A circular trail, then, is a cruel trick, a breach of logic, almost a kind of black magic.