The Forest Unseen: A Year's Watch in Nature (27 page)

Now reds, purples, blues, oranges can mix in thousands of tones and hues: ash skies, sand and saffron leaves, blue-green lichens, silver and sepia slugs, and tree limbs in dun, russet, and slate. The forest’s National Gallery has unlocked its collections. After living a season swimming in the yellow and green light of Van Gogh’s sunflowers and Monet’s lily pond—masterly works, but only a small portion of the full collection—we’re allowed to roam the galleries reveling in the full depth and range of visual experience.

My strong unconscious relief at the changed forest light suggests something about our visual sense. We crave rich variegations of light. Too much time in one ambience, and we long for something new. Perhaps this explains the sensory ennui of those who live under unchanging skies. The monotony of blank sunny skies or of an endless cloud ceiling deprives us of the visual diversity we desire.

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The mandala’s light environment affects far more than my human sense of aesthetics. Plants’ growth is mediated by light, as are the feeding and breeding of most animals. Sensitivity to variations in illumination is therefore a central part of the lives of forest creatures. Herbs living on the forest floor in autumn grow and embrace the wavelengths of light that were formerly intercepted by tree leaves. Limbs on trees use the strength and color of light to direct their growth into sunny openings and away from other limbs. Inside plant cells, light-harvesting molecules respond to changing light minute by minute, assembling then disassembling as needed.

Animals also modulate their behavior as light changes. Some spiders adjust the color of their web silk to the particularities of brightness and color in different parts of the forest. Tree frogs melt into their background by moving pigments up and down within their skin, tuning the brightness and color of the skin to the surface on which the frogs find themselves. Displaying birds position themselves in the light environments that best show their feather colors.

Birds with red plumage have a particularly rich set of opportunities in and under the forest canopy. Birds such as cardinals and scarlet tanagers seem gaudy and conspicuous when we see them isolated on the page of a field guide. Yet the red part of the light spectrum is weak in the green gloom of forests. A “bright” red bird appears dusky and dull in the forest shade. But when the bird steps into a patch of direct sunlight, colors blaze and feathers dazzle. By hopping into and out of sunflecks, red forest birds can transform themselves from sulkers to show-offs, then back again, all in an instant. In my experience, woodpeckers are particularly adept at this trick. All seven woodpecker species here have red crests or crowns, and all are experts at manipulating the light. When woodpeckers are feeding quietly they are devilish to locate, but when they are advertising their ownership of a patch of forest, or displaying to a mate, they are like burning torches at dusk, unmissable.

Conspicuous displays are impressive, but they are not the most
masterly of the animals’ adaptations to light. Obscurity is much harder to achieve. Not only must a camouflaged animal match the hue and tone of its environment, but the rhythm and scale of the textures on its surface must echo those of the background. Any deviation from the visual properties of the surroundings produces visual dissonance and a potential failure of disguise. There are thousands of ways to stand out in the forest but only a few ways to blend in.

The evolution of camouflage is a finicky process in which the particularities of place matter a great deal. Therefore, animal species whose lives are played against just one visual backdrop, moths that rest only on hickory bark for example, are more likely to evolve camouflage than species that move among backdrops, such as moths that flit from hickory bark to maple twigs to spicebush leaves. Mobile animal species rely on other forms of defense—fast escape, noxious chemicals, and protective spines.

For camouflaged species, concealment within a particular microhabitat is a great short-term adaptation. In the longer term, such specialization can be a trap; the fate of the camouflaged species is tied to the background on which it rests. Moths that are beautifully disguised on hickory bark thrive as long as hickories are plentiful, but if hickories should decline, these otherwise poorly defended moths will be picked off by sharp-eyed birds in new visual environments. Even if hickories continue to be abundant, moths that specialize on hickory bark are ecologically constrained and are less likely to evolve new ways of living in the world. Their cousins that rely on other methods of defense can explore new habitats without incurring the severe penalties that failed camouflage entails. In some ways, therefore, the textbook example of camouflage evolution—English peppered moths evolving dark wings as trees in their environment changed from unpolluted gray to sooty black—is unrepresentative of the evolutionary pressures experienced by moths. Seldom does a lucky mutation allow a camouflaged animal to switch backgrounds so easily. The complexity of the visual environment and the sophistication of predator eyes
make the evolution of camouflage more fraught and constrained than the textbooks suggest.

The colors on the slug meandering across the mandala match the colors of the lichens and wet leaves through which the animal moves. This straightforward form of camouflage is augmented by an additional layer of visual trickery. The irregular flames of dark pigment that lick up from the mantle’s edge serve to break up the outline of the slug. These disruptive patterns deceive the eye by creating the perception of edge where there is no edge, thus distracting the neural processors in predators’ eyes and brains, concealing the real edge with seemingly meaningless patterns. This decoying of pattern-recognition systems is surprisingly effective. Experiments with feeding birds show that disruptive patterns, even when the patterns are formed by conspicuous colors, can match or beat the performance of simple color-matching camouflage.

Disruptive patterns do not depend on an exact match between the animal and the color and texture of its background. Animals with disruptive patterns therefore stay hidden against many different backgrounds, avoiding the constraints imposed on animals with camouflage that perfectly matches just one habitat. The slug remains protected on green moss, even though the slug has no green on its skin. Its fake edges give no hint of the true, edible shape of the animal. Only a prolonged gaze unmasks the deception. Scanning predators cannot afford to sit watching one small patch of moss for an hour or more, as I have done.

Predators are not without countermeasures. Some peculiarities in our human visual ecology may be partly explained by the visual sparring between predator and prey. Military planners in the Second World War noticed that color-blind soldiers were better at seeing through camouflage than were soldiers with normal vision. More recent experiments have confirmed that dichromats (people with two types of color receptors in their eyes, so-called red-green color-blind) are better camouflage breakers than are trichromats (people with three receptor
types, the more common condition in humans). Dichromats detect boundaries in texture that are missed by trichromats, who are fixated on and misled by variations in color.

The superior pattern-finding abilities of dichromats may seem a peculiar but unimportant quirk of an unlucky mutation. Two facts suggest otherwise. First, the frequency of dichromatism in humans, two to eight percent of all males (the genetic change is on the male sex chromosome), is much higher than would be expected if the condition were a maladaptation. Such commonness suggests that evolution may, in some circumstances, smile on the condition. Second, our cousins the monkeys, specifically the New World monkeys, also have both dichromats and trichromats living together within the same species. Dichromats in these species make up half or more of the population, again suggesting that dichromatism is not just an accidental defect. Feeding experiments with marmosets found that dichromats have an advantage over trichromats when light is dim, perhaps by seeing patterns and texture that were missed by trichromats. In bright light, the advantage is reversed; trichromats can find ripe red fruit faster than can dichromats. The diversity of these monkeys’ ways of seeing may therefore be a reflection of the diversity of light conditions in the forest.

New World monkeys generally live in cooperative groups, so it is to everyone’s advantage to have both types of vision in the same group—food can be found in all kinds of lighting conditions. Whether the same explanation applies to humans is not known. We also evolved in a social context of extended groups, so it is possible that dichromatism now exists in humans because of past natural selection. Perhaps groups with some dichromats fared better than exclusively trichromatic groups, thus passing the genetic propensity for dichromatism to future generations. These are interesting speculations, but no one has tested them by examining human visual performance under conditions that approximate those of our ancestors.

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My response to the changing light in the forest was subconscious and manifested itself in my aesthetic sense. It is tempting to brush away such aesthetic responses as merely human inventions, unconnected to the forest. What could be less natural than the overcultivated tastes of a human? But it turns out that our human aesthetic faculties do reflect the forest’s ecology. Our sensitivity to the tone, hue, and intensity of light is bound up with our evolutionary heritage. Even the diversity of our visual abilities may echo our ancestors’ ecology.

We live in a civilized world where light is usually as unsubtle as a flashing computer screen or billboard. The mandala’s changing autumn light stirred in me an awareness of the forest’s more subtle illumination. I am coming late to this awareness. Sweet cicely knew about the autumnal light weeks before I did and has unfolded new leaves. Many generations of natural selection taught the slug about light and scribed markings onto its mantle. Spiders, cardinals, woodpeckers, and frogs all know how the forest is lit and tune their behavior, their silks, feathers, and skins to the forest’s rich light. As rain brings down the last golden leaves, I also start to see.

W
e have crossed a seasonal threshold. Ice has returned to the mandala, coating the leaves of low-growing herbs with a fuzz of crystals. Frosts have brushed the canopy intermittently for a week or so, but this is the first autumn freeze to have reached the ground. Unlike deciduous trees that drop their leaves to avoid freeze damage, many herbs persist through the cold, loading their cells with sugars to act as antifreeze. They also suffuse their leaves with purple pigment that protects the cells, shielding them from sun damage when the cells’ usual light-absorbing machinery is iced up. Herbs that were formerly entirely green,
Hepatica
and leafcup, are now edged with deep purple, the mark of approaching winter. These leaves will hold on through the entire winter, eking out small amounts of photosynthesis on warm days and dying back only when fresh springtime growth supersedes them.

Although mornings are frosty, there is still plenty of animal life in the mandala. As the day’s temperature climbs, small insects swarm into the air, and the leaf litter is populated by ants, millipedes, and spiders. These invertebrates are a rich source of nourishment for birds, some of which have recently arrived here, fugitives from the snowstorms that have choked off their food supply in more northerly forests. One of these birds, a winter wren, visits as I sit at the mandala. It lands next to me, poking its needle beak into the folds of my bag and the hem of my jacket before darting to a viburnum shrub. There, it hangs from
a twig, regarding me with one black eye from a tilted head, then takes to the air and flies into a tangle of downed branches a few meters away. Its tiny dusky body disappears into the thicket, moving more like a mouse than a bird. The chattering calls of these wrens have been common for at least a week, but I feel lucky to have been investigated so closely by this bird; they are usually much more wary.

Unlike the migrant warblers that have left the mandala and are now in Central and South America, these wrens make a relatively short journey, lingering in North American forests all winter. In most years, this is a successful strategy, saving a costly transcontinental flight and allowing the birds to get back to their breeding ground swiftly. But the winter wrens’ preference for feeding on the ground and on fallen wood makes them vulnerable to hard winters. The combination of cold and deep snow in southerly forests can cause drastic die-offs in some years.

This visit from an inquisitive wren was my second unusual bird encounter of the day. On my walk into the forest, a flash of cobalt blue sprung vertically from the center of the mandala. The sharp-shinned hawk’s wings and tail splayed out as she curtailed her swoop and, in a blink, she rose twenty feet, bouncing off air. The wings twisted, the body leveled, and the bird swung in an ascending arc, alighting on a maple branch. She sat briefly, holding her back and long tail vertically, then slipped down the slope with her wings and tail held in a motionless T.

Like a pebble skimmed over ice, the bird’s motion seems effortless and smooth. As she slid out of sight into the haze of trees I felt gravity like a restraining strap, leashing me to earth. I am rocklike, a clumsy boulder.

The hawk’s mastery relies on a careful proportionality between weight and power. The sharp-shinned hawk likely weighs just two hundred grams, several hundred times lighter than me. Her pectoral muscles are several centimeters thick, meatier than many human chests and
forming one-sixth of the bird’s weight. A contraction of the hawk’s muscles therefore sends her soaring, like a beach ball launched skyward by the kick of a powerful leg.

Humans have tried to follow the hawks, but medieval tower-jumpers and Haight-Ashbury trippers were all given the same hard, unyielding answer to their requests for airy freedom. Only by transcending our bodies’ limitations with a slug of power from fossil fuel have we been able to break the strap that holds us to earth. To do so under our own power would require grotesque modifications of our bodies: either chest muscles six feet deep, or an impossible reduction of the bulk of the rest of our body. We are just too puny for our leaden frame. The story of Icarus’s flight from Crete might therefore be instructive about the dangers of hubris but is a poor teacher of aerodynamics. Gravity would have taught him humility long before the sun could deliver its melting judgment on his wings of wax and feathers.