Sounds Wild and Broken, page 12
Singing is only one part of communication, though. Listening is the other. Evolution has addressed the challenge of a noisy environment by honing the ears and brains of listeners. 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.
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. Evolution has performed a double feat here: first, giving each individual an acoustic signature and, second, enabling listeners to extract subtle patterns from a storm of masking and distracting noise. 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. 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. Were we to transcribe and notate such sound, we’d need six-dimensional music paper to record variations across frequency, loudness, time, and the three axes of space.
In the cabin at Tiputini, my night vigil tapers into light slumber until my watch alarm stabs me awake an hour before dawn. Time to head out. 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. The humid air is fat with aroma—spicy root and leaf litter, unctuous mud, and the algal scent of lichen-smothered wet leaves. 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. 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. One of the vines coils and twists: a blunthead tree snake, thinner than my forefinger and nearly a meter long, works through the tangle. 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. Threadlike legs, some topped with pincers, protrude from saucer-sized carapaces. 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. Exhilaration and anxiety build in equal measure: delight in the tumultuous sensory diversity, seasoned with little darts of dread as unexpected creatures leap into earshot and view. This is rain forest awe. Admiration and fear, not as detached ideas but as embodied sensory experiences. The forest slaps my body awake. I am immersed not only in the manifestations of life’s diversity but in the experience of life’s ongoing creativity. The overwhelming press of sound and other senses here is one of evolution’s most powerful generative forces.
Sexuality and Beauty
I hear them from a kilometer away, a sound like thousands of small brass bells, mellowed by passage through the wintry deciduous forest. The ringing cuts through the rumble of traffic from the town’s bypass road and the sputtering growl of a small airplane. I’m standing in the suburbs of Ithaca, a small town in upstate New York. It is late March and I’m hearing one of the first sounds of spring: a chorus of spring peeper tree frogs.
When I first started visiting these woods, three decades ago, I was a recent immigrant from northern Europe, and the winter seemed to me dispiritingly long. I was used to quickening birdsong and the first sign of garden blooms in January, then a drawn-out intensification through May. Here chill, gray days keep a firm lock on outdoor life until well into March. The season of migrant songbirds and spring wildflowers does not start in earnest until late April. Were it not for the ever-present haze of engine noise, the sound levels in late winter might be among the quietest on Earth. On windless days, only the gentle palaver of chickadees or the drumming of distant woodpeckers enlivens the air.
Now, after a tepid late-March rain, spring peepers shout their lust to the air in a jubilation of sound. I approach the forest and the merged quality of distant sound clarifies into thousands of individual voices. Each frog gives a sharp peep, a pure tone, rising slightly, lasting about a quarter of a second. Mingled among these are longer, raspy calls, reeep. I pad along a boardwalk through the swampy woods, moving slowly so that I do not startle and silence the singers. Inside the chorus, the sound pressure level is as loud as the blast of a radio turned up high. Visiting amphibian choruses in the springtime has become a ritual that lifts me out of winter’s despondency. The frogs bathe me in sound. I feel as if every cell of my body is shaken into wakefulness by the force of their voices. I suffuse my body with the energy of a reawakening Earth. We made it. Another winter ended. Thank you.
It is perhaps a measure of how unattuned my senses were to the ecological rhythms of North America that the frogs sometimes brought me to relieved, grateful tears. Something inside me could not believe that the long gray cold would end, anxiety enhanced by geographic displacement. Now, after thirty spring seasons on this continent, the smiling relief still comes every year. I’ve also learned to hear more nuance in the amphibian choruses. The rich woodlands of eastern North America are home to more than three dozen species of frogs and toads. These are productive forests, full of frog-ready insect food, fuel for vociferous breeding displays. Every species has its habitat and rhythms. Many seasons are revealed in these sounds, from the chuckle of wood frogs in icy pools to the ear-ringing tumult of gray tree frogs after a summer rain. Frog choruses mark time into finer divisions than the coarse-scaled human chronometer of “spring” and “summer” and offer a portal into how the year is experienced by other species. American toads—endowed with a sweet, whistling trill—start a little later than peepers and sometimes sing all summer. Eastern spadefoot toads give their choruses of explosive waas for a couple of nights only, after summer thunderstorms.
It is not just the experience of time that changes as we listen to the voices of other species. Through the varied sounds of frogs and toads, and those of birds and singing insects, travel becomes an education in the complex geography of life. We humans seem to do our best to impose uniformity on the land, but the tree frogs and song sparrows calling from behind the parking lot or along the edge of the subdivision speak of the complexities that we smother. Every forest or wetland has a distinctive combination of species. Moreover, the voices of individuals within each species often vary from one place to another, revealing some of the fine-grained differences in the character of each place.
Amphibian calls did not, of course, evolve to bring joy or edification to humans. What delights our ears is the expression of the social and sexual dynamics of each species. Sound making mediates breeding, territoriality, and the alliances and tensions of animal social networks. Every species has its own ecology and history, resulting in behaviors and voices particular to each. Much of the sonic diversity of the world, then, is rooted in the divergent social lives of animals.
Standing on the boardwalk, I flip on a small flashlight, holding it inside a translucent red plastic water cup. Frogs have good night vision and can distinguish green and blue in gloomy light that is, to our eyes, a smear of gray. They’re less sensitive to red, though, and they keep calling as I pass my dim beam through the tangle of wet vegetation around me. At least ten frogs call within a couple of meters of me, but I see only one. He is perched on a partly submerged stick, his head angled up by extended skinny forelegs. Under his chin, thin, partly transparent skin balloons, a wobbly bulb almost as big as the frog. As I watch and listen, his flanks pulse inward and, a split second later, the sac expands with a peep. The frog is about as long as my thumbnail but, at close range, the sound smacks my ears. The spring peeper’s call has been measured at ninety-four decibels half a meter away, the loudness of a vigorous bird. Another push from his flanks and the call comes again, repeating once every two seconds.
The peeper calls by jabbing a slug of air from his lungs over vocal folds in the windpipe. The throat sac receives the blast of sound and puff of air. The sac’s extended skin broadcasts the call in all directions. The elasticity of the air sac then pushes air back to the lungs, allowing the frog to call again without opening his nostrils to inhale. Amphibians lack ribs and diaphragms, and so they push the air with bands of trunk muscles whose bulk makes up 15 percent of a male frog’s body weight.
Why such effort? A single peeper’s call is audible over at least fifty meters, an area of about seventy-eight hundred square meters. His body is only two and a half centimeters long, covering an area of just four square centimeters. By calling, the peeper has extended his body’s presence in the forest by nearly twenty million times, not counting the sound’s vertical reach to listeners in trees. By allowing animals to find one another in complex environments, sound helps species to thrive where otherwise they would struggle. The many ecological roles of vocalizing animals—from frogs, insects, and birds on land, to fish, crustaceans, and marine mammals in the seas—are indirectly made possible by the benefits of acoustic communication.
The peeper not only broadcasts his presence and location but also reveals his size, health, and perhaps individual identity. This information mediates social interactions at a distance. Rival male frogs space themselves in the swamp and reduce the dangers of bodily confrontation. Females not only find mates but assess them without coming close and risking either injury or disease transmission. Sound therefore increases the physical range and subtlety of meaning in animal behaviors, substituting for direct combat in territorial situations and allowing more extended and nuanced evaluation of mates than is possible in the tussle of skin-to-skin contact.
When a female spring peeper emerges from her winter hideaway under the leaf litter, thawing a body that was steeped with antifreeze sugars, she listens for the bells that locate the breeding swamp. She likely also remembers the contours and aromas of the land, having lived in the forest, eating spiders and insects, for two or more years before maturing into a breeding adult. In other species, experimenters have shown that frogs have excellent spatial memory and navigation abilities, especially for breeding sites. The same may be true for peepers. Guided perhaps by memory and certainly by sound, the female peeper sets out for the wetland. At this stage in her journey, sound is a guide to the location of potential mates. Finding mates in a vast environment is likely the original function of breeding sounds. For tiny animals in a forest, sound can reduce search time for a mate to minutes, rather than the weeks it would take to wander the forest seeking other individuals by eye. Scent trails also help in this task in some species, leaving hints for keen-nosed suitors to follow, but sound is especially far-reaching and easy to track. Species-specific sounds also increase the precision of mate searching and reduce the risk of predation. To come close enough to mate is to come close enough to be eaten. Sound reveals species identity at a distance, making the search for mates far less dangerous. Exploiters of mating signals underscore the perils of mistaken identity. Predatory katydids in Australia mimic the mating sounds of female cicadas, luring amorous males to their deaths.
The function of sound changes when the spring peeper’s trek across the forest floor brings her to the wetland. She now listens for information embedded in individual voices. Males are spaced every ten to one hundred centimeters and so she lops and swims through an array of pealing sound and bulging throat sacs. Most of the calls are peeps, but if males get too close to one another, they joust with rough reeeps, grappling for territory with sound. The female’s inner ear, like those of all frogs, has three separate clusters of sound-sensitive hair cells, unlike the single membrane in our ears. One cluster is tuned to the frequency of the male’s sound. A second has a wider range, presumably for detecting the diverse sounds of the forest. The third picks up only low-frequency vibrations. Males, curiously, have ears tuned higher than their calls, perhaps to better tolerate many nights sitting in the cacophony or to listen for the higher rustling sounds of approaching danger. It is also possible that the males’ ears are seeking subtle differences in acoustic structure that reveal the identity of neighbors. Bullfrogs recognize familiar calls and respond more vigorously to strangers. Male peepers remember how aggressive their neighbors are, reeeping to those that suddenly ramp up the pushiness. They also call antiphonally with neighbors, synchronizing the timing of their calls so that one frog leads and the other immediately follows—peep-peep peep-peep. These synchronous duets occasionally expand into groups of up to five males with closely aligned tempi. We do not yet know whether spring peepers recognize individual voices.
Female spring peepers prefer calls that are loud and rapidly repeated. The sonic vigor and pacing of the peep have their evolutionary origins in this preference. Loud males are easy to detect and locate. Evolution has thus cranked the blast of sound about as loud as it can from lungs the size of a pea. At temperatures just above freezing, males give about twenty peeps per minute. On balmy nights, their peeping rate increases to eighty peeps every minute. But regardless of whether the night is cool or warm, some males vocalize at up to twice the rate of others. Females sense these differences and swim or hop over to the faster callers. In doing so, they select the healthiest males in the swamp.
Calling is taxing. Some males deliver more than thirteen thousand peeps in a night, each one powered by strong muscular contractions. Fats stored in these muscles supply 90 percent of the energy needed for calling. Males who cannot supply ample fat to their muscles have little stamina. Compared with their languid neighbors, rapid callers are, on the average, heavier and older, with larger hearts, blood cells better supplied with hemoglobin, and muscles more richly stocked with fat-burning enzymes. They also tend to show up night after night, rather than sporadically through the spring.
