The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  We are Russian dolls, our lives made possible by other lives within us. But whereas dolls can be taken apart, our cellular and genetic helpers cannot be separated from us, nor we from them. We are lichens on a grand scale.

  Union. Fusion. The mandala’s inhabitants are plaited into winning partnerships. But cooperation is not the only relationship in the forest. Piracy and exploitation are here also. A reminder of these more painful associations lies coiled on the leaf litter at the center of the mandala, enclosed by the lichen-coated rocks.

  The reminder unwrapped itself slowly, held back by the torpidity of my powers of observation. My attention was first drawn by two amber ants bustling across the wet leaf litter. I watched their scurrying for half an hour before I noticed the ants’ particular interest in a coiled strand nestled in the litter. The strand was about as long as my hand and was the same rain-soaked brown as the hickory leaf on which it lay. At first I dismissed the curl as an old vine tendril or leaf stem. But as my eyes were about to move on to more stimulating things, an ant paddled the tendril with her antennae, and the coil straightened and lurched. My mind started into recognition: a horsehair worm. A strange creature, with a taste for exploitation.

  The worm’s twisting gave away its identity. Horsehair worms are pressurized from within, and the tug of muscles against this inflated body makes the worm jerk and writhe like no other animal. The worm has no need of complicated or graceful movement, for at this stage of its life it has only two tasks left, to squirm toward a mate and then to lay eggs. Nor did the worm have need of sophisticated motion in its previous life stage, when it lay balled inside the body of a cricket. The cricket did the worm’s walking and feeding. The horsehair worm lived as an internal brigand, robbing then killing the cricket.

  The worm’s life cycle began when it hatched from an egg laid in a puddle or stream. The microscopic larva crawled over the streambed until it was eaten by a snail or small insect. Once inside its new home, the larva wrapped itself in a protective coat, formed a cyst, then waited. The lives of most larvae are cut short at this point, as cysts, never completing the rest of the life cycle. The worm in the mandala was one of the few that make it to the next stage. Its host crawled onto land, died, and was chewed on by an omnivorous cricket. This is such an improbable sequence of events that the horsehair worm’s life cycle requires parent worms to lay tens of millions of eggs; on average, only one or two of this multitude of young will survive to adulthood. Once inside the cricket, the spiny-headed larval pirate bored through the gut wall and took up residence in the hold, where it grew from a comma-sized larva to a worm the length of my hand, coiling upon itself to fit within the cricket. When the worm could grow no more, it released chemicals that took over the cricket’s brain. The chemicals turned the water-fearing cricket into a suicidal diver seeking puddles or streams. As soon as the cricket hit water, the horsehair worm tensed its strong muscles, ripping through the cricket’s body wall, and twisted free, leaving the plundered vessel to sink and die.

  Once free, horsehair worms have a keen appetite for company, and they mate in untidy skeins of tens or hundreds of worms. This habit has given them a second name, Gordian worms, after the eighth-century legend of King Gordius’s monstrously complex knot. Whoever could unbind the knot would succeed the king, but all would-be rulers failed. It took another pirate, Alexander the Great, to loose the knot. Like the worms, he cheated his hosts, slashing the knot with his sword and claiming the country’s crown.

  After the Gordian mating tangle is sated, the worms unwrap and crawl away. They lodge their eggs in soggy pond margins and damp forest floors. Once hatched, the worm larvae will pick up the Alexandrian plunderer’s spirit, first infecting a snail, then emerging to rob a cricket.

  The horsehair worm’s relationship with its hosts is entirely exploitative. Its victims receive no hidden benefit or compensation for their suffering. But even this parasitic worm is sustained by an interior crowd of mitochondria. Piracy is powered by collaboration.

  · · ·

  Taoist union. Farmer’s dependence. Alexandrian pillage. Relationships in the mandala come in multifarious, blended hues. The line between bandit and honest citizen is not as easily drawn as it first seems. Indeed, evolution has drawn no line. All life melds plunder and solidarity. Parasitic brigands are nourished by cooperative mitochondria within. Algae suffuse emerald from ancient bacteria and surrender inside gray fungal walls. Even the chemical ground of life, DNA, is a maypole of color, a Gordian knot of relationships.

  January 17th—Kepler’s Gift

  Ankle-deep snow has smoothed the forest’s fractured, uneven surface into a gentle swell and trough. This covering disguises deep cracks between rocks and makes walking treacherous. I move slowly, bracing myself against tree trunks as I slide and clamber to the mandala. I brush the snow from my rock, then sit, huddling in my coat. Loud cracks, like gunfire, echo down the valley every ten minutes or so, the sound coming from snapping fibers in ice-stiffened branches of the bare, gray trees. The temperature has dropped to ten below, not a hard freeze but the first real cold of the year and enough to stress the trees’ wood.

  The sun emerges, and snow transforms from a soft layer of white into thousands of sharp, bright points of light. I hook a fingertip of this glittering jumble from the mandala’s surface. Seen closely, the snow is a tangle of mirrored stars, each one flashing as its surface aligns with the sun and my eye. The sunlight catches the minute ornamentation of each flake, revealing perfectly symmetrical arms, needles, and hexagons. Hundreds of these exquisite ice flakes crowd onto one fingertip.

  How is such beauty born?

  In 1611, Johannes Kepler took time away from elucidating the motions of the planets to meditate on the snowflake. He was particularly intrigued by the regularity of snowflakes’ six sides: “There must be some definite cause why, whenever snow begins to fall, its initial formations invariably display the shape of a 6-sided starlet.” Kepler searched for an answer in the rules of mathematics and the patterns of natural history. He noted that the honeybee and pomegranate array their combs and seeds in hexagons, perhaps reflecting geometrical efficiency. But water vapor is not squeezed into a rind like pomegranate seeds, nor is it built up by the work of insects, so Kepler believed that these living examples could not reveal the cause of the snowflakes’ architecture. Flowers and many minerals don’t conform to the six-sided rule, further frustrating Kepler’s search. Triangles, squares, and pentagons can also be stacked into neat geometrical patterns, eliminating pure geometry from the list of possibilities.

  Kepler wrote that snowflakes are showing us the spirit of the earth and God, the “formative soul” that inhabits all being. But this medieval solution didn’t satisfy him. He sought a material explanation, not a finger pointing toward mystery. Kepler ended his essay in frustration, unable to peer beyond the door of the icy palace of knowledge.

  His frustration could have been eased had he taken seriously the concept of the atom, an idea that originated with the classical Greek philosophers but had fallen out of favor with Kepler and most early seventeenth-century scientists. Yet the atoms’ two-thousand-year exile was coming to a close, and by the end of the seventeenth century atoms became fashionable again, balls and sticks dancing triumphant across textbooks and chalkboards. Now, we seek out atoms by blasting ice with X-rays, using the pattern of the rays that emerge to discover a world one million billion times smaller than the normal scale of human life. We find jagged lines of oxygen atoms, each atom tethered to two restless hydrogen atoms, electrons flashing. We float around the molecules, examining their regularity from all angles and, incredibly, see atoms arranged like Kepler’s pomegranate. This is where the snowflakes’ symmetry begins. Hexagonal rings of water molecules build on one another, repeating the six-sided rhythm over and over, magnifying the arrangement of oxygen atoms to a scale visible to human eyes.

  The basic hexagonal shape of snowflakes is elaborated in varied ways as the ice crystal grows, wit
h the temperature and humidity of the air determining the final shape. Hexagonal prisms form in very cold, dry air. The South Pole is covered with these simple forms. As temperatures rise, the straightforward hexagonal growth of ice crystals starts to destabilize. The cause of this instability is still not fully understood, but it seems that water vapor freezes faster on some ice crystal edges than others, and the speed of this accretion is strongly affected by slight variations in air conditions. In very wet air, arms sprout from the snowflakes’ six corners. These arms then turn into new hexagonal plates or, if the air is warm enough, they grow yet more appendages, multiplying the arms of the growing star. Other combinations of temperature and humidity cause the growth of hollow prisms, needles, or furrowed plates. As snowflakes fall, the wind tosses them through the air’s innumerable slight variations of temperature and humidity. No two flakes experience exactly the same sequence, and the particularities of these divergent histories are reflected in the uniqueness of the ice crystals that make up each snowflake. Thus, the chance events of history are layered over the rules of crystal growth, producing the tension between order and diversity that so pleases our aesthetic sense.

  If Kepler could visit us today he would perhaps be pleased by our solution to the puzzle of the snowflake’s beauty. His insights into the arrangement of pomegranate seeds and honeybee cells were on the right track. The geometry of stacked spheres is the ultimate cause of the snowflake’s shape. But because Kepler knew nothing of the atomic basis of the material world, he could not imagine the minute oxygen atoms from which ice’s geometry grows. However, in a roundabout way, Kepler contributed to the solution to the problem. His musings on the snowflake prompted other mathematicians to investigate the geometry of packed spheres, and these studies contributed to the development of our modern understanding of atoms. Kepler’s essay is now regarded as one of the foundations of modern atomism, a worldview that Kepler himself explicitly rejected when he told a colleague that he could not go ad atomos et vacua, “to the atoms and the void.” Kepler’s insights helped others to see what he could not.

  I examine again the glassy stars on my fingertip. Thanks to Kepler and those who followed him, I see not just snowflakes but sculptures of atoms. Nowhere else in the mandala is the relationship between the infinitesimally small atomic world and the larger realm of my senses so simple. Other surfaces here—rocks, bark, my skin and clothes—are made from complicated tangles of many molecules, so my view of them tells me nothing straightforward about their minute structure. But the form of the six-sided ice crystals gives a direct view of what should be invisible, the geometry of atoms. I let them fall from my hand, and they return to the oblivion of massed white.

  January 21st—The Experiment

  A polar wind rips across the mandala, streaming through my scarf, pushing an ache into my jaw. Not counting the windchill, it is twenty degrees below freezing. In these southern forests such cold is unusual. Typical southern winters cycle between thaws and mild freezes, with deep chills arriving for a few days each year. Today’s cold will take the mandala’s life to its physiological limits.

  I want to experience the cold as the forest’s animals do, without the protection of clothes. On a whim, I throw my gloves and hat onto the frozen ground. The scarf follows. Quickly, I strip off my insulated overalls, shirt, T-shirt, and trousers.

  The first two seconds of the experiment are surprisingly refreshing, a pleasant coolness after the stuffy clothes. Then the wind blasts away the illusion and my head is fogged with pain. The heat streaming out of my body scorches my skin.

  A chorus of Carolina chickadees provides the accompaniment to this absurd striptease. The birds dance through the trees like sparks from a fire, careening through twigs. They rest no more than a second on any surface, then shoot away. The contrast on this cold day between the chickadees’ liveliness and my physiological incompetence seems to defy nature’s rules. Small animals should be less able to cope with the cold than their larger cousins. The volume of all objects, including animal bodies, increases by the cube of the object’s length. The amount of heat that an animal can generate is proportional to the volume of its body, so heat generation also increases with the cube of body length. But the surface area, where heat is lost, increases by only the square of length. Small animals cool rapidly because they have proportionally much more body surface than body volume.

  The relationship between the size of animals and the rate of heat loss has produced geographic trends in body sizes. When an animal species exists over a large area, the individuals in the north are usually larger than those in the south. This is known as Bergmann’s rule, after the nineteenth-century anatomist who first described the relationship. Carolina chickadees in Tennessee live toward the northern end of the species’ range, and they are ten to twenty percent larger than individuals from the southern limit of the range in Florida. Tennessee birds have tipped the balance between surface area and body volume to match the colder winters here. Farther north, Carolina chickadees are replaced by a closely related species, the black-capped chickadee, which is ten percent larger again.

  Bergmann’s rule seems remote as I stand naked in the forest. The wind gusts hard and the burning sensation in my skin surges. Then, a deeper pain starts. Something behind my conscious mind is trapped and alarmed. My body is failing after just a minute in this winter chill. Yet, I weigh ten thousand times more than a chickadee; surely these birds should be extinguished in seconds.

  The chickadees’ survival depends, in part, on their insulating feathers, which give them an advantage over my naked skin. The smooth upper layer of plumage is plumped by hidden downy feathers. Each down feather is made from thousands of thin protein strands. These tiny hairs combine to make a lightweight fuzz that holds heat ten times better than the same thickness of coffee-cup Styrofoam. In the winter, birds increase by fifty percent the number of feathers on their bodies, adding insulative power to their plumage. On cold days, muscles at the base of the feathers tense, puffing the bird and doubling the thickness of the insulation. Yet all this impressive protection merely slows the inevitable. Chickadee skin does not burn in the cold like mine, but heat still courses out. A centimeter or two of downy fluff buys just a few hours of life in the extreme cold.

  I lean into the wind. The sense of alarm builds. My body shakes in uncontrollable spasms.

  My usual heat-generating chemical reactions are now totally inadequate, and my muscles’ shivering paroxysms are the last defense against a falling core temperature. Muscles fire seemingly randomly, pulling against one another so that my body shudders. Inside, food molecules and oxygen are burned, just as they are when muscles cause me to run or lift, but now this burn produces a rush of heat. The violent shuddering of my legs, chest, and arms warms the blood, which then carries heat to the brain and the heart.

  Shivering is also the chickadees’ main defense against the cold. Throughout the winter, the birds use their muscles as heat pumps, shivering whenever the temperature is cold and the birds are not active. Slabs of flight muscle in the chickadees’ chests are the primary sources of heat. Flight muscles account for about a quarter of a bird’s body weight, so shivering produces great surges of hot blood. Humans have no comparably huge muscles in our bodies, so our experiences of shivering are weak in comparison.

  As I stand shaking, fear surfaces. I panic and dress as fast as I can. My hands are numb, and I grasp my clothes with difficulty, fumbling with zippers and buttons. My head aches as if my blood pressure has suddenly soared. My only desire is to move quickly. I walk, jump, and wave my arms. My brain signals: make heat, fast.

  The experiment has lasted only a minute, just one ten-thousandth of the duration of this week of arctic air. Yet my physiology reels. My head pounds, my lungs can’t grasp enough air, and my limbs seem paralyzed. Had the experiment continued minutes longer my core body temperature would have dropped into hypothermia. Muscle coordination would have fled, then sleepiness and hallucinations would have take
n over my mind. Human bodies normally keep themselves at about thirty-seven degrees Celsius. If the body temperature drops just a few degrees, to thirty-four, mental confusion sets in. At thirty degrees, organs start to shut down. In cold winds like today’s, these few degrees of temperature loss can take place in just an hour of naked exposure. Stripped of my clever cultural adaptations to the cold, I’m revealed as a tropical ape, profoundly out of place in the winter forest. The chickadees’ insouciant mastery of this place is humbling.

  After I’ve waved and stamped my limbs for five minutes, I huddle down into my clothes, still shaking but no longer panicked. My muscles feel tired and I’m winded, as if I’ve just sprinted. I’m feeling the aftereffects of the exertion required for heat generation. When shivering continues for more than a few minutes, it can rapidly deplete an animal’s energy reserves. For both human explorers and wild animals, starvation is often the prelude to death. As long as food supplies last, we can shiver and cling to life, but we cannot survive with empty stomachs and drained fat reserves.

  I will replenish my reserves when I retreat to my warm kitchen, drawing on the winter-defying technologies of food preservation and transportation. But chickadees have no dried grains, farmed meat, or imported vegetables. Survival in the winter forest demands that chickadees uncover enough food to fuel their four-pennyweight furnaces.