The Forest Unseen: A Year's Watch in Nature (3 page)

The basic hexagonal shape of snowflakes is elaborated in varied ways as the ice crystal grows, with 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.

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 taken 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.

The energy used by chickadees has been measured both in the laboratory and in free-living birds. On a winter day, the birds need up to sixty-five thousand joules of energy to keep themselves alive. Half this energy is used to shiver. These abstract measures become more understandable when they are converted into the currency of bird food. A spider the size of a comma on this page contains just one joule. A spider that fits within a capitalized letter holds one hundred joules. A word-sized beetle has two hundred and fifty joules. An oily sunflower seed has more than one thousand joules, but the mandala’s birds have no seed-filled bird feeder. Chickadees must daily find hundreds of food morsels to meet their energy budget. Yet the mandala’s larder looks utterly empty. I see no beetles, spiders, or food of any kind in the ice-blasted forest.

Chickadees can coax sustenance out of the seemingly barren forest in part because of their outstanding eyesight. The retinas at the back of the chickadees’ eyes are lined with receptors that are two times more densely packed than are mine. The birds therefore have high visual acuity and can see details that my eyes cannot. Where I see a smooth twig, birds see a fractured, flaking contortion, pregnant with the possibility of hidden food. Many insects pass the winter ensconced inside tiny cracks on tree bark, and the chickadees’ discerning eyes uncover these insect hideaways. We can never fully experience the richness of this visual world, but peering through a magnifying lens gives us an approximation. Details that are normally invisible snap into view. Chickadees spend most of their winter days passing their superior eyes over the forest’s twigs, trunks, and leaf litter, sleuthing concealed food.

Chickadee eyes also perceive more colors than mine can. I view the mandala with eyes that are equipped with three types of color receptor, giving me three primary colors and four main combinations of primary colors. Chickadees have an extra color receptor that detects ultraviolet light. This gives chickadees four primary colors and eleven main combinations, expanding the range of color vision beyond what humans can experience or even imagine. Bird color receptors are also equipped with tinted oil droplets that act as light filters, allowing only a narrow range of colors to stimulate each receptor. This increases the precision of color vision. We lack these filters, so even within the range of light visible to humans, birds are better able to discriminate subtle differences in color. Chickadees live in a hyperreality of color that is inaccessible to our dull eyes. Here in the mandala, they use these abilities to find food. Ultraviolet light reflects from the dried wild grapes that are sparsely scattered across the forest floor. Wings of beetles and moths are sometimes tinged with ultraviolet, as are some caterpillars. Even without the advantage of ultraviolet vision, insect camouflage is unmasked by slight imperfections detected by the birds’ precise perception of color.

The visual abilities of birds and mammals differ because of events
in the Jurassic, one hundred and fifty million years ago. At that time, the lineage that gave rise to modern birds split from the rest of the reptiles. These ancient birds inherited the four color receptors of their reptilian ancestors. Mammals also evolved from reptiles, splitting away earlier than the birds. But, unlike birds, our protomammal ancestors spent the Jurassic as nocturnal shrewlike creatures. Natural selection’s shortsighted utilitarianism had no use for sumptuous color in these night-dwelling animals. Two of the four color receptors that the mammals’ ancestors bequeathed to them were lost. To this day, most mammals have just two color receptors. Some primates, including those that gave rise to humans, later evolved a third.

The chickadees’ acrobatic bodies let them put their vision to good use. A wing-flick takes a bird from one branch to another. Feet grasp a twig, then the bird falls, swinging from a branch tip. The beaks probe as the bird’s body pivots, still hanging, then wings flash open and the bird flits up to another small twig. No surface is left unexamined. The birds spend as much time upside down, peering under twigs, as they do upright.

Despite the vigor of their search, the chickadees catch no prey while I watch. Chickadees, like most birds, give a distinctive backward flick of their heads as they swallow or, if they find a bigger morsel, will hold the food in their feet as they pound it with their beaks. The flock stays in my sight for just fifteen minutes, finding no food. The chickadees may need to call on their fat reserves to survive the cold. These reserves are essential to winter survival, and they allow chickadees to make good use of winter’s variability. When the weather warms, or when birds find a cluster of spiders or berries, the flush of food is turned into fat that carries the birds through times when the feeding is poor and the weather is cold.