The Forest Slaps My Body Awake

Animals that live in noisy places are very good at focusing on the sounds of their kind and ignoring others. Their ears cut through sonic confusion to find what they need. A study of poison dart frogs in the Peruvian Amazon forest found that the auditory discrimination of each species was correlated with how many other frog species made similar sounds. These tiny frogs give repeated peep notes from breeding nooks in the leaf litter. After the eggs hatch, the male carries the tadpoles on his back to nearby water. Each species’ song has a different rhythm and frequency, although there is extensive overlap among calls. Species whose songs are very similar to one another have much more discriminating ears than those whose peep is unique. The same is true for some rain forest crickets. Their auditory nerves are tuned to the precise frequency of the song of their own species. These nerves respond to their own species’ song in a rain forest filled with dozens of similar insect sounds. In contrast, the nerves from cricket species from uncrowded meadows in Western Europe have broad sensitivity, firing off in reply to a wide range of frequencies. Acoustic competition, then, seems to have shaped not the calls, but the nerves and behavior of listeners.

Isn't That Peculiar

Likewise, birds that live in noisy, dense aggregations can extract acoustic details from a hubbub. European starlings can identify the voices of individual flockmates. In the lab, they can pick out their companions’ sounds from a confusion of four or more simultaneously singing birds. Penguin chicks have similar abilities, recognizing their parents’ calls even when the calls of other adults are much louder, a skill that no doubt ensures the chicks’ survival in colonies of thousands. Vocal individuality and auditory discrimination are common in sociable birds and mammals, including, of course, in humans. Infants pick out their parents’ voices from a crowd, and adults focus on single conversations in the racket of a cocktail party. Scans of human brains show that listening to voices in noisy environments is demanding. Multiple control and attention centers, brain networks that have only minor roles when we hear speech in quiet surroundings, activate when we listen in noisy places. Animals use the complex structures of forests to their advantage. Sound delivered from elevated twigs and branches travels farther than it would on the ground. The canopy’s crown offers a fine place to broadcast from, especially in the calm of dawn. The forest’s structure also allows animals to negotiate social competition among singers with similar songs.

Think Solution, Not Problem

By spacing themselves across the forest’s complex structure, animals can reduce acoustic masking and competition. Such a process seems at work in the dusk chorus of crickets and katydids in the tropical forests of the Western Ghats in southern India. There, fourteen species call at once, overlapping the annual cycle of their breeding seasons, daily coming into voice as the sun sets. Yet a detailed study of their spacing and hearing abilities showed low sonic overlap between individuals, even for species whose songs had similar frequencies and timing. By singing from perches sufficiently far enough away from others, each individual finds itself an acoustic space. What seems at first a smothering throng of sound contains within it a spatial structure, a microgeography of sound. While most human music blends sound into a single experience that varies through time in pitch and amplitude, but usually not over space, sound in forests and other habitats lives in rich spatial patterns. In the cabin at Tiputini, my night vigil tapers into light slumber until my watch alarm stabs me awake an hour before dawn. The trail is a mire of sludgy clay and puddles, winding over ground made uneven by tree roots. The beam of my headlamp lurches as I walk. The wet surfaces of waxy leaves flash then disappear, dozens of forms reeling toward me and slipping away. I pass through a knot of calling frogs, ack ack, then on into a cloud of insect sound, a dozen pure cricket tones layered onto one another.

Get It Right The First Time

The crickets encase me in sound, as if I were within a ringing metal bell. A few minutes later, the timbre of insect sounds changes, adding more rough scrapers and whirs to the purer notes. As my light beam weaves and jogs, furry spiders the size of my fist jump into view on the trail. A bush cricket, its orange abdomen looking oily in the wet air, pounces onto my rubber boot with a click and thud, then leaps away into the dark vegetation. All around, thick ropes of vine and a fine tracery of dangling aerial roots are vivid in the artificial light against the dark surrounds. Its head is swollen with two large eyes that gleam then slide into shadow. Further on the trail, the dark pools of two more large eyes face me from a low branch. The gecko swallows as it gazes at me, then bobs its head. I pass the buttresses of a giant tree, arching walls that disappear upward into the dark. In the cleft between two buttresses, five whip spiders sit immobile in the glare. I know they’re harmless to me, but my body sends a pulse of adrenaline as the beam of my lamp suddenly brings them in view. Above, a lone screech startles me. Does a macaw see the first easing of darkness? Over the next half hour, a net of sound weaves across the upper layers of the forest as the predawn gray seeps onto the highest branches. I stand in the gloom below, listening as the light ignites the growl of howler monkeys, the ringing clamor of parrots, the first sawing cicadas, and the incessant piping of flycatchers. As I walk in the dark, I feel as if I’ve shrunk to the scale of a mouse struggling through tangled leaf litter. The night forest encloses me in a throng of sound and aroma. This is rain forest awe. Admiration and fear, not as detached ideas but as embodied sensory experiences.