The Forest Unseen: A Year's Watch in Nature (23 page)

Caterpillars are famously gluttonous. During their lives they increase their weight two or three thousandfold. If a human baby did the same, she would weigh nine tons, the combined weight of several marching bands, by adulthood. And, if the baby kept pace with the caterpillar, adulthood would arrive just a few weeks after birth.

Caterpillars grow fast because their being is focused on just one task: eating leaves. Unlike adult insects they do not grow tough external skeletons, wings, complex legs, sex organs, or elaborate nervous systems. Such accoutrements would blunt the caterpillars’ focus and slow their growth. Defensive bristles are the only nongourmand elaboration that natural selection has allowed. By specializing on the task of
feasting, caterpillars have opened a business in which they have few rivals. In most forests they consume more leaves than all other herbivores combined.

A fat tussock moth caterpillar shuffles into the mandala. The caterpillar is a carnival of colored plumes and hairs, its brightness advertising nastiness from stinging hairs and internal poisons. Four yellow tufts project from its back, like shaving brushes pointed at the sky. These sit in a haze of long silver hairs that sprout from each body segment. Two black sprays of hair jut from either side of the head, and the tail is tipped with a cluster of brown needles. Where it is visible through the haze of hairs, the caterpillar’s skin is striped yellow, black, and gray: a gorgeous and fearsome outfit.

Adult tussock moths do not expose themselves to danger by munching leaves in the open. Thus, their colors can be bland. The female emerges from her hidden cocoon, then clings to it and waits for a male. She is flightless and looks like a furry sleeping bag. Because she has no need to wander, she need not advertise her distastefulness and can rely on camouflage to protect her. The adult male is a strong flyer. He sniffs out female pheromones with his feathery antennae, mating with her, then flying away. Both sexes are inconspicuously colored brown and gray, protected by the female’s utter immobility and the male’s vigorous wings. As is true for many other moths, natural selection’s paintbrush has produced an audacious youngster and a dour adult.

As I watch the gaudy caterpillar, a black ant climbs onto its back, squeezing through the bristles like a man pushing through a thicket of bamboo. The ant pushes down its mandibles, straining unsuccessfully toward the caterpillar’s neck. The caterpillar marches on, seemingly unperturbed by its attacker. The ant moves back from the neck and bites down between the yellow tufts, but again it cannot reach the skin. Then another ant, smaller and honey-colored, climbs aboard and joins the attack. The ants meet and fight each other, grappling on a mat of yellow hairs. The honey-colored ant is thrown off, climbs back on, falls
off, and is followed by the black ant. The caterpillar accelerates, perhaps trying to escape, but the ants circle. The black ant lunges into the caterpillar and attacks again, biting down repeatedly but never making contact with the soft caterpillar skin. The ant falls off, and the caterpillar immediately climbs a dead leaf that arches above the forest floor. The caterpillar stops. Is it outmaneuvering the ants? The ants circle on the floor but cannot find the victim. Their circles finally carry them away from the leaf. The caterpillar climbs down and lumbers toward the trunk of a large maple just outside the mandala. Free!

A smaller tussock moth caterpillar was not so lucky. Ants drag its corpse away to feed their nest mates. Perhaps this caterpillar’s hairs were too short or its evasive maneuvers too slow? Whatever the cause of its demise, it now joins a quiet funeral procession of dead caterpillars flowing into the maws of ant colonies in and around the mandala. One study counted more than twenty thousand caterpillars entering an ant nest each day. Until witnessing the caterpillar’s struggle in the mandala I assumed that birds were the cause of caterpillars’ hairiness. But hairs evidently also keep ant mandibles away from caterpillar skin. The scientific literature confirms what I observed today: ants are major enemies of most caterpillars.

One group of butterflies has turned this ant-agonistic relationship around. The blues, or lycaenids, have evolved a mutualistic relationship with ants. The caterpillars of blues are hairless and supremely vulnerable to ant attacks. But ants generally don’t bite them, preferring instead to feed on the sweet “honeydew” that the caterpillars secrete for them. The caterpillars’ gift to the ants is perhaps akin to mafia-style protection money. For the price of some sugar, the caterpillars are unharmed by the ants. But the ants don’t simply withhold attack in return for food: they actively protect the caterpillars, fending off other predators, especially wasps. So a closer analogy might be that the ants are hired as bodyguards. Ants give lycaenid caterpillars a tenfold survival advantage relative to caterpillars without ant attendants. The caterpillars seem to prefer life with ants, and some have special scrapers that
they use to produce vibrations on leaves. These vibrations attract ants, so the caterpillars literally sing for their guards.

Having escaped the ants, the mandala’s tussock moth caterpillar ascends a maple trunk. There are no ants on the tree, but spiders have painted most of the trunk with sticky strands, and the caterpillar has difficulty pushing through. Patches of moss, still wet from last night’s rain, present another challenge. The hooklets on the caterpillar’s legs lose their grip, and the animal falls back several inches before struggling on.

The caterpillar’s ascent carries it into a world dominated not by ants but by birds. Ants find their prey through touch and smell. Birds use vision. Pigments and patterns are therefore vitally important if one is to escape the attentions of birds. Because humans are so visual, we are fascinated by the extraordinary diversity of caterpillar art. Caterpillars feature prominently in children’s stories, and many naturalists owe their love of nature in part to the caterpillars’ charm. In contrast, the larvae of flies, wasps, and beetles live hidden from the keen eyes of our avian cousins, so these pasty white larvae have little allure for us.

The tussock moth caterpillar in the mandala uses arresting contrast between bright yellow and black to advertise its distastefulness. The brushes of yellow hair present a striking textured contrast to the spiny silver hairs that fuzz over the rest of the body. The display leaves the observer in no doubt that spines, hairs, and toxins are abundantly provided on this animal. Most birds will not even peck at such a display. Similar costumes are found in other poisonous or bristly caterpillars, each species creating its own variations on the themes of hue and contrast.

Caterpillars that lack spines or noxious chemical defenses are deceivers, not advertisers. They mimic bird droppings, dead leaves, twigs, small snakes, or poisonous salamanders. Natural selection used a refined hand as it turned out these animals, giving leaf buds to twig mimics, endowing snake mimics with eyes that have fake reflections in their pupils, and adding small droppings to the surface of leaf mimics.

The gaze of birds has not wavered from caterpillars for millions of years, turning caterpillar bodies into masterworks of visual design. Remarkably, this gaze has molded yet more. Even the pattern of light through the mandala’s nibbled leaves is shaped by the birds’ discriminating eyes. Feeding birds learn to associate ragged holes in leaves with the presence of caterpillars. Because leaves remain damaged long after caterpillars have moved on, birds continually update their feeding patterns based on their recent experience of feeding in particular tree species. Caterpillars that excise obvious holes in leaves, then linger next to these holes, will quickly attract the attention of these smart birds. Therefore, only well-defended caterpillars can afford to be messy eaters. Caterpillars that are more vulnerable to birds, such as those with few hairs, fastidiously pare leaves down from the edges, leaving no telltale holes, maintaining the silhouette of an entire leaf. Some caterpillars curl their leaf-mimicking bodies onto the missing leaf edges, filling out the leaf’s profile and fooling predatory eyes. The leaves above the mandala bear the jagged marks of insouciance, and I suspect the tussock moth caterpillar and its kin were responsible for most of the damage.

Bird eyes have carved and painted the mandala. The form of both nibbling caterpillar and nibbled leaf reflects the evolutionary struggle between caterpillars and birds. The migrating warblers seem ephemeral, but their presence will outlast their physical departure.

M
y study of the chewed leaves in the canopy has drawn my eyes skyward. The summer canopy usually narrows my world, directing my gaze down, but now I peer through chinks in the tree cover. The sky has been cleared of dust by a vigorous rainstorm that gusted through yesterday, leaving glassy blue. Summer’s humidity has dropped, giving the day’s heat a comfortable feel. This is September’s typical weather, long stretches of open skies interrupted by warm, boisterous fronts, often the remnants of tropical storms from the Gulf.

Today, a turkey vulture circles directly above the mandala, its broad wings held like unflapping sails against the sky. After its pivot, the vulture soars eastward, puffed away by a sudden draft.

The mandala sits far enough south that turkey vultures are found here in every month. At this time of year, our local resident birds mix with northerners who migrate over Tennessee to winter on the Gulf Coast and Florida. Some birds winter even farther south, trekking to Mexico and beyond. These long-distance migrants will have company—turkey vultures are permanent residents of Central and South America, making them one of the most widely distributed species of birds in the New World.

Unlike most flying birds, turkey vultures are easy to identify, even from a great distance. They hold their wings in a shallow V with wingtips splayed up, like an airborne curly bracket  }. They fly with a drunkard’s gait, swaying and tipping as they go. This apparent lack of sobriety
has an aerodynamic reason; turkey vultures are masters of soaring flight, seldom flapping their wings and almost never flapping more than ten times in a row. To ride the wind with such energy-saving ease, the great paddlelike wings catch updrafts and eddies, turning to the birds’ advantage every rising puff of air. This results in a slow, teetering flight style whose superficial inelegance hides its extraordinary efficiency. The drunkard is a thrifty genius with no need for maneuverability, grace, or speed. Turkey vultures spend their days leisurely scanning their domain, riding the skies for up to a third of their waking hours.

The turkey vultures eat nothing but carrion, and their frugal flying style allows them to patrol tens of thousands of acres each day in search of corpses. In forested areas, where they do much of their feeding, the tree canopy obscures the view, but even with a clear line of sight, immobile bodies covered by camouflaged fur are hard to spot. Yet turkey vultures sleuth them out with thoroughness and precision. Dead chickens and rats deliberately placed in forests by scientists are usually found within a day or two, even when the bait is covered with leaves and brush. Turkey vultures are evidently sniffing their meals, using their wide nostrils to peer through the forest’s confusion of color.

Finding a rank corpse by smell is hardly an impressive feat, but vultures do much better than this. Indeed, overly putrid meat is not much to their taste. Turkey vultures instead cruise the skies in search of the subtle whiff of recent demise. Unlike the barrage of odor from a festering carcass, the smell of fresh carrion is slight, made up of a few choice molecules exhaled by microbes and the cooling corpse. Soaring vultures catch these wafts and follow them to the ground, pinpointing their destination from the thousands of acres in their field of view.

In the modern world, the turkey vultures’ sense of smell can sometimes lead them up dead ends (figuratively, for a change). They circle over abattoirs that, although they look like ordinary warehouses, send skyward the aroma of the recently departed. Pipelines offer similar
frustrations. Gas companies add a small amount of an odorous chemical called ethyl mercaptan to the otherwise odorless natural gas in their transmission pipes. If a valve or pipe seam fails, the smelly chemical leaks out with the natural gas, alerting human noses to an explosion hazard. But vultures also smell the leak and will congregate around cracked pipes, becoming unwitting assistants in the search for pipeline flaws. This tangle of vulture and human noses is caused by the bouquet of death—ethyl mercaptan is given off naturally by corpses. Because humans have a deep aversion to rotting meat, our noses are extremely sensitive to ethyl mercaptan. We can pick out its smell at concentrations two hundred times lower than our threshold for ammonia, which is itself strongly odorous. Gas companies need therefore add only minuscule amounts of the smelly chemical to their pipes. Unfortunately for turkey vultures, they also can smell these low concentrations and gather in confusion at leaks.

Turkey vultures are the purifiers of the forest, administering ecological last rites, speeding the material transformation of large animals from carcass to freed nutrients. Their scientific name recognizes this:
Cathartes
, the purger.

The apparently humble role of corpse eater seems to us mightily unpleasant. But the forest is rife with competition for what we disdain. Foxes and raccoons sometimes swipe carrion before vultures can get a mouthful. Black vultures gang up on turkey vultures, their larger cousins, and drive them away from meals. Burying beetles drag away and entomb smaller carcasses.

Mammals, birds, and beetles are rivals, but their competitive importance pales when compared to the microscopic death eaters, bacteria and fungi. From the moment of death these microbes start their work, digesting animals from inside and out. At first, this decomposition helps the vultures by releasing plumes of odor that guide birds down from the sky. Once at the carcass, however, vultures compete
with the microbes for the dead animals’ nutriment. In hot weather, that competition is won by the microbes within a few days; vultures have to be quick if they are to feed well.