On Trails (9 page)

For many centuries, it was a mystery how ants were able to organize themselves so deftly. Some believed each ant was possessed of a tiny special intelligence, which afforded it rationality, language, and the ability to learn, as the naturalist Jean Pierre Huber argued in 1810. Put simply, this view held that ants found their way to food using their wits, and then “spread the word” of their findings throughout the nest. (This highly anthropomorphized notion remains prevalent among folktales and children's stories, from
Aesop's Fables
to T. H. White's
The Once
and Future King.
In many of these renditions, like White's, the worker ants are given their marching orders by an all-­powerful “queen.”) Opponents of this theory, following the teachings of Descartes, held that ants were possessed of no intelligence whatsoever—or, in the language of the time, no “soul”—but were mere machines directed by an almighty deity who either manipulated them like marionettes or engineered them like windup toys. The naturalist Jean-Henri Fabre, an unfashionably late proponent of this theory, wrote in 1879, “Can the insect have acquired its skill gradually, from generation to generation, by a long series of casual experiments, of blind gropings? Can such order be born of chaos; such foresight of hazard; such wisdom of stupidity?” Fabre concluded that it could not. “The more I see and the more I observe, the more does this [divine] Intelligence shine behind the mystery of things.”

On the one side of this debate lie insects blessed with individual wit, on the other, insects cursed with perfect idiocy, but steered by an omniscient hand. It was not until very recently that scientists began to understand that the answer lay somewhere in between: the complex behavior of ants arises not from smart individuals, but from smart systems—a form of wisdom that exists
between
, as well as inside, living things
.

All animals fall somewhere on the spectrum between internalized and externalized intelligence. At one extreme of this spectrum lies the mountain hermit, thoughts swirling about in his lonely head like moths in a bell jar. At the other end lies the slime mold. As sprawling, single-celled blobs, slime molds are about as stupid as an organism can be: they lack even the most basic rudiments of a nervous system. However, they have nevertheless developed a very effective technique to hunt for food: They extend their tentacle-like pseudopods, grope around, and then retract them whenever they come up empty. As they retract, the pseudopods leave behind a trail—or rather, a kind of
anti-trail
—of slime indicating where food has not been found. Then,
continuing their blind search, they head off in a new, slime-free direction. Using roughly this same trial-and-error method, slime molds can solve surprisingly complex problems. When researchers tasked a slime mold with connecting a series of oat clusters mirroring the location of the major population centers surrounding Tokyo, the slime mold effectively re-created the layout of the city's railway system. Linger a moment over that fact: A single-celled organism can design a railway system just as adroitly as Japan's top engineers. Whatever intelligence slime molds have, though, is wholly external. When their enclosure is wiped down evenly with slime—effectively erasing their trails—slime molds will begin to wander aimlessly, as if struck with dementia. They don't retain any information; the trail does.

As a species, humans straddle a line between external and internal intelligence. With big brains and (typically) small clan size, humans have traditionally harnessed individual cleverness to outcompete rivals for food and mates, to hunt and dominate other species, and, eventually, to seize control of the planet. As later chapters will show, we have also externalized our wisdom in the form of trails, oral storytelling, written texts, art, maps, and much more recently, electronic data. Nevertheless, even in the Internet era, we still romanticize the lone genius. Most of us—especially us Americans—like to consider any brilliance we may possess, and the accomplishments that have sprung from it, as being solely our own. In our egotism, we have long remained blind to the communal infrastructure that undergirds our own eureka moments. This egotism extends to our regard for pathways: when we write about trails, we tend to describe them as the creation of a single “trailblazer,” whether it is Daniel Boone blazing the Wilderness Road or Benton MacKaye dreaming up the Appalachian Trail. The reality of how most trails form—collectively, organically, without the need of a designer or a despot—has been increasingly apparent to scientists for centuries, but has remained invisible to most of us for far too long.

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The story of how we grew wise to the wisdom of insect trails begins, oddly enough, with the lowly caterpillar. One spring day in 1738 a young Genevan philosophy student named Charles Bonnet, while walking through the countryside near his family's home in Thônex, found a small, white, silken nest strung up in the branches of a hawthorn tree. Inside the nest were squirms of newly hatched tent caterpillars, which bristled with fiery red hairs.

At just eighteen years old, frail, asthmatic, myopic, and hard of hearing, Bonnet was a somewhat unlikely naturalist. But he was blessed with patience, attentiveness, and a relentless, burning curiosity. As he approached the cusp of adulthood, his father had begun pressuring him to become a lawyer, but he wanted to spend his life exploring the microcosmos of insects and other tiny creatures, a profession that had scarcely yet been invented.

Bonnet decided to cut down the hawthorn branch and carry it back home. At the time, most naturalists would have sealed the caterpillars in a powder jar, called a
poudrier,
to better inspect their anatomy. But Bonnet wanted to observe the caterpillars' natural behavior wholly unobstructed,
en plein air,
yet from the comfort of his home.
He struck upon the idea of mounting the hawthorn branch outside the window frame of his study. That window soon became a kind of antique television, a glass screen displaying a miniaturized world, before which he spent countless rapt hours.

After two days of patiently waiting for signs of life, Bonnet watched the caterpillars emerge from their nest and begin to march in single file up the windowpane. After four hours, the procession had successfully scaled the window; then it turned around. In descending, strangely, the caterpillars followed the exact path they had climbed. Bonnet later wrote that he even traced their route—presumably, with a wax pencil on the windowpane—to see if they
ever deviated from it. “But they always followed it, faithfully,” he wrote.

Each day Bonnet watched as the caterpillars mounted exploratory expeditions across the windowpane. Paying closer attention, he noticed that as they crawled, each caterpillar laid down an ultra-fine white thread, which the others followed. Curious, Bonnet rubbed his finger across their trail, breaking the thread. When the leader of the returning party arrived at the rupture, it turned back, apparently confused. The one behind it did the same, and the one behind that. Each subsequent caterpillar plodded calmly along until it reached the gap in the trail, at which point it either turned around or stopped to feel about for the thread, like a man groping for a dropped flashlight. Finally, one of the caterpillars, which Bonnet deemed “hardier than the others,” dared to venture forward: a thread was extended across the void, and the others followed.

Emboldened, Bonnet collected more caterpillar nests, which he placed on his mantel. Soon, scores of caterpillars were exploring his bedroom, meandering across the walls, the floor, even the furniture. Feeling, no doubt, like a small new god, Bonnet found he could control where the caterpillars traveled simply by erasing certain trails. He delighted in showing this trick to visitors. “You see these little caterpillars who walk in such good order?” he would ask. “Well, I bet you that they will not pass beyond this mark”—and he would swipe his finger across their route, stopping them cold.

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Along the southern stretches of the Appalachian Trail, I too sometimes encountered mysterious little white tents in the crotches of trees. Occasionally they grew to monstrous proportions; I would turn a corner to find a tree wholly enveloped in a polygonal cloud. “Mummy trees,” my fellow hikers called them.

For a reason I couldn't quite place, they gave me the shivers. Tent
caterpillars, I would later learn, are essentially creepy animals. Their faces resemble black masks, and their bodies are quilled over in fine, toxin-tipped spines, which can detach and float for more than a mile on a windy day, causing rashes, coughing fits, and pink eye. Some species of tent caterpillar undergo rampant population booms on a ten-year cycle, covering the countryside like spilled oil. In June 1913 a stream of forest tent caterpillars climbed up onto the tracks of the Long Island Rail Road; the rails were soon so thickly slathered with their remains that the wheels of approaching trains spun in place.

A biologist named Emma Despland once told me about the time she walked into a stand of sugar maples during a tent caterpillar outbreak. She described it as a “ghost forest.”

“It's June and there are no leaves on the trees, and there are these big strands of gunky silk, like Halloween decorations,” she said. “And then you hear this rain falling. Except it's not rain. It's caterpillar poop.”

Even among biologists, tent caterpillars are little loved. And yet for centuries, researchers like Despland have been studying them for one reason in particular: as consummate followers—perfectly faithful, perfectly foolish—tent caterpillars represent a reductio ad absurdum of what it means to follow a path. Despland told me that if you were to remove a younger caterpillar from its nest mates, it would spend all its time waving its head around in confusion, looking for a trail, and probably starve to death. Alone, they are utterly hopeless, and yet collectively they can denude entire forests.

Curious to see firsthand how such timorous creatures manage to bind together and thrive in the world, I took the bus to Montreal to visit Despland's lab, where she studies the forest tent caterpillar. When I arrived she peeled open a Tupperware container to show me the caterpillars: a smattering of fuzzy black critters, like mouse turds come to life. Then, on an old desktop computer in her office, Despland showed me time-lapse video of an experiment she had been conducting to determine how they find food. In the experiment, the
caterpillars were placed in the middle of a cardboard runway. On the extreme left end of the cardboard strip was a Quaking Aspen leaf, which caterpillars especially love to eat. On the right was the leaf of a hybrid poplar,
Populus trichocarpa × P. deltoides
(clone H11-11),
which they find unappetizing. The experiment was simple: It was as if a group of blindfolded children were placed in the middle of a long hallway, which held a piece of chocolate cake at one end and a pile of raw celery at the other. Asked to find and share the more delicious item, children could quickly solve the problem by splitting up and calling out to one another. But how would the caterpillars?

Displayed on Despland's computer monitor were five strips of cardboard, on which five experiments were being conducted simultaneously, but she directed my attention to the second from the bottom, where, over the course of many minutes, a group of caterpillars had mistakenly ventured over to the hybrid poplar leaf. Others followed their trails, and soon the whole nest was crawling on the broad green leaf, though they ate virtually none of it. For an uncomfortably long period of time they failed to correct this initial mistake. They followed their trail back to their “bivouac” (a silken pad, which they construct as a resting place) in the middle of the strip, and then back to the hybrid leaf on the right, but none ventured to the left, where the tasty aspen leaf lay. It seemed each caterpillar would continue to follow the others to the hybrid leaf, leaving more trails, and more feedback, forever.

I recalled a peculiar incident Bonnet had once described witnessing, in which a group of pine processionary caterpillars mistakenly formed a circular trail leading all the way around the rim of a ceramic vase. The details are scant, but it seems they continued marching around and around for at least a whole day. This same phenomenon was later famously observed by Jean-Henri Fabre: to his amazement, the caterpillars walked in circles for more than a week before they finally broke the ouroborosian loop and escaped. In
Pilgrim at Tinker Creek,
Annie Dillard recounts the horror she felt while reading
Fabre's portrait of these soulless, circling automatons. “It is the fixed that horrifies us,” she wrote. “It is motion without direction, force without power, the aimless procession of caterpillars round the rim of a vase, and I hate it because at any moment I myself might step to that charmed and glistening thread.”

Despland's caterpillars seemed to be caught in a similarly brainless loop. For more than an hour, the pattern continued: the caterpillars returned to their bivouac, returned to the hybrid leaf, and returned to the bivouac again. I began to squirm.

Eventually, a small contingent broke away and ventured off in the opposite direction. They traveled slowly, with excruciating hesitancy, inching, ducking, cowering, stalling, nudging one another forward, and frequently turning back. Despland guessed that their hesitancy springs from a genetic aversion to ending up away from the pack, alone, where they could get picked off by a bird.

By the end of the second hour, the scouting party had finally made it to the aspen leaf, and others subsequently followed the trail they had blazed. Despite their initial misstep, by hour four all the caterpillars had found the correct leaf and gnawed it to a husk.

The foraging technique of these caterpillars is remarkably simple, even idiotic, but it works. The fail-safe, Despland explained, is that hunger induces restlessness, which eventually compels them to abandon the well-worn trails and go looking for something else. “The leaders tend to be the hungry ones,” she explained. “Because they're the ones who are willing to pay the cost.”