Sounds Wild and Broken, page 4
The distorted scaling of subjective experience adapted us to subtle differences in the quiet sounds of the preindustrial world. The meanings in human speech, especially the textures of emotion, are conveyed through tiny changes in sound intensity. The same is true for information gleaned from the sounds of wind, rain, plants, and nonhuman animals. Our ears evolved to pay attention to quiet voices and are out of place in persistently loud environments. In an industrial society—surrounded by engines, power tools, and amplified music—it would be helpful to have a more nuanced experience of the upper end of the loudness scale. We’d better appreciate the sonic variegations of this new world and be empowered to protect our inner ears from permanent damage.
We also have biased perceptions of sound frequencies. Our sensitivity is like a one-humped dromedary, with highest sensitivities in the midranges and duller perception in the low and high extremes, tuning our ears to environmental sounds most relevant to human survival: the sounds of our prey and predators, and the movements of flowing water and wind in vegetation. As we age, the hump sags at the high-frequency end or splits into a two-humped beast. Our ears’ specialization on the intermediate frequencies works well for hearing the speech of other humans and some of the voices of nonhuman animals. But although we can hear many low and high sounds, we have an erroneous sense of their vigor. What we hear as faint, high trills from insects or dull, low roars from waves on a shore are in fact as intense as a strong-voiced human talking next to us. It is the biases of our ears and nerves that have cranked down the perceived loudness of these high and low frequencies. We live embedded within sensory distortion.
There are also many sounds beyond the ken of our cochleas. We hear, at best, from about 20 to 20,000 hertz (sound waves per second). Some whales and elephants hear down to 14 hertz. Pigeons can hear as low as half a hertz. Porpoises hear well up to 140,000 hertz and some bats all the way to 200,000. Domestic dogs hear up to 40,000 hertz and cats up to 80,000. Mice and rats chatter and sing to one another up to 90,000 hertz. If my feet represent the lowest sounds heard by animals and the top of my head the highest, we humans hear from just above the skin of my feet to the top of my hiking boots. Compared with most mammals, humans and our primate cousins live within a restricted aural world.
Thunder clouds, ocean storms, earth tremors, and volcanoes all sing and moan, calling out with sound waves as low as one-tenth of a hertz, far too low for our ears to detect. These low sounds carry for hundreds of kilometers, revealing the dynamics of seas, skies, and Earth. But we cannot hear them and thus live in a sonic world unaware of what stirs over the horizon. A similar limitation exists at the other end of the frequency scale. High frequencies attenuate very quickly in air and travel only short distances. We miss close-range dynamics of high-pitched songs of insects, cries of bats, much of the creaking of tree wood, and the quiet sounds of water fizzing through plant veins. There’s a poignancy in these limitations. The world is speaking, but our bodies are unable to hear much of what surrounds us.
Our culture falsely divides us into those who can “hear” and those who are “deaf.” But there is no sharp biological divide between hearing and deafness. We are all insensitive to most of the world’s vibrations and energies. And every human body, regardless of our ears, can feel some sound in our body tissues and skin. Yet from the small portion of the sound waves that the majority of humans can hear, we have erected a sharp cultural divide. The “hearing” population relies on spoken language to such an extent that those who communicate by sight and gesture are too often excluded. A thriving Deaf culture rightly rejects the prejudice and aspersion that often accompany this exclusion, and has built communities united by rich nonvocal visual and gestural languages.
The limitations of human hearing reveal a paradox. As biological evolution endowed living creatures with a sense of hearing, connecting them to others, it simultaneously built perceptual walls. The bodily mechanisms of hearing work only because they focus on specific tasks. In becoming sensitive to the vibrations of the world, cells must narrow their abilities. Middle ear bones amplify sound and translate from air to water, but only within a particular range of frequencies. Proteins in hair cells pump up and down, but only at a speed set by their looping structure in the cell membrane. Hair cells boost quiet sounds, but that limits their finesse for louder ones. The cochlear membrane is too short to pick out very low and high tones, and too stiff to allow any finer discrimination of frequencies.
Like any other superlative achievement of evolution, mastery comes through specialization, and specialization circumscribes power. Hearing, like other senses, both reveals and distorts. It opens us to the multifarious sonic waves of the world. It also, necessarily, conveys warped and edited perceptions of sound energy.
And so evolution has built hearing organs tuned to the ranges of frequency and loudness most relevant to the success of each species. The human range of hearing therefore reveals the sounds our ancestors found most useful. If our ancestors had been eaters of mice and moths, both of which communicate with ultrasound, we likely would have evolved to hear much higher frequencies, as do many smaller predatory mammals like cats. If these forebears had sung underwater across ocean basins, they would have evolved water-adapted ears tuned to low frequencies, as did the whales.
The richer the sensory experience, the more convincing the perceptual illusion. Before I faced my own attenuation of hearing, I lived within that illusion, giving little thought to the limits of my senses. I had no embodied experience to teach me that my ears convey active interpretations of sound energy. Seeing at the audiology clinic the liveliness of my hair cells taught me otherwise. I understood that the price of sensory experience is to live—always, from birth—in a perceptual box, a space much smaller than the diverse flows of energy of the world. The walls of the box bend and filter incoming sound, manufacturing the shape and texture of my sonic perception.
The stab of sadness that I experienced in seeing the marks of my dead and dying hair cells on the audiologist’s graph jolted me into a better appreciation of both the limits and the precious value of my senses. Distortions and narrowing boundaries are the price of nuanced and rich sensation. My hearing connects me to sound, of course, but also to the bargains that evolution struck on its long path from the cilia-covered cells of the primal oceans to the aural wonders of animal inner ears.
PART II
The Flourishing of Animal Sounds
Predators, Silence, Wings
Grasshoppers clatter away from me as I walk the verge of a country road. Crickets chirp from hiding places in the unkempt thatch of grass. A fritillary butterfly wings past. Every minute or two, I pass through a thin cloud of midges and I wave my hands to sweep away their mote-like bodies. The cicadas, loud and persistent yesterday afternoon, give only sporadic croaks and stuttering whines in the cool morning.
On one side of the road, exposed rock the color of raw liver angles up the valley slope. Entombed within this stone are the ancestors of the insects that fly and sing around me. One of this fossilized swarm bears the earliest known sound-making structure of any animal, a ridge on the wing of an ancient cricket. This fossil is the oldest direct physical evidence of sonic communication.
There should be a shrine here. A monument to honor the first known earthly voice. But pilgrimages instead lead away from these mountains in southern France to the chapels and cathedrals of the lowlands. The Camino de Santiago passes by; pilgrims tread the road unaware that the deepest known root of all song and speech lies in the stones under their feet.
I am on the southern edge of the Massif Central, a complex of mountains and steep riverine valleys that curves inland along the Mediterranean coast, then stretches north, covering nearly one-sixth of France’s land area. Unlike the coastal plain, the geography here is rugged and the human population sparse. Volcanism, collisions with the Alps and Pyrenees, and the push of continental plates have wrought a complex mix of rocks across the Massif. Where I walk, the carmine color of the stone alongside the road was born hundreds of millions of years ago in the hot, dry interior of a continent. Iron, leached and oxidized in wind-blown soils, left its mark. These rocks, the Salagou Formation, named for a local river, are made from sediments laid down in a semiarid basin into which heavy rains sometimes carved lakes and rivulets. Scrubby ferns and conifers grew beside these wet areas, adding patches and corridors of green to an otherwise bare landscape. The formation dates from the Permian, 270 million years ago, a time when all Earth’s landmasses were united in one giant continent, Pangaea.
Jean Lapeyrie, a local medical doctor, discovered in the 1990s that the colorful outcrops near his home were, in some places, richly peppered with fossilized insects. He made collections and, through collaborations with researchers across the world, opened a rare window into a time when the earliest members of modern families of insects mingled with now-extinct groups. Mayflies, lacewings, thrips, and dragonflies flew alongside ancient forms, including several relatives of modern crickets and grasshoppers.
Most of the insect fossils are of wings. Insect bodies decompose quickly, but their wings are made of dry, tough protein. Blown or washed into water channels or mud cracks, the wings became entombed in silty ooze. Later, unearthed from their funerary vaults by geologists’ hammers, wing veins and contours are visible as impressions in stone. Every type of insect has its own wing shape and vein arrangement, so a fossilized wing can identify the taxonomic family of its long-dead owner.
In the Permian rocks of the Salagou Formation, one wing bears an unusual feature. Normally wing veins are arranged in a web, supporting a thin membrane. On one fossil specimen, though, a cluster of veins near the attachment point of the wing are thickened and raised. A slightly curved prominent central vein is buttressed by side veins. This convergence of embossed veins is just a couple of millimeters long, the length of a letter on this page, on a wing half the length of my thumb. Such a structure, a raised ridge, had no function in supporting the wing membrane. Instead, it was likely the insect’s singing device. When the wings rubbed together, the raised central vein scraped over the base of the other wing, making a scratchy sound. The large flat surface of the wing may have acted like a loudspeaker, broadcasting the sound.
Modern crickets use a similar wing structure to make sound, although theirs is of a more refined design. Corrugated ridges on the right wing rub against a nub on the left wing. This action of a plectrum against a file is amplified and projected by an adjacent thin, membranous window in the wing. The shape of the file and window is unique to each species, as is the strumming rhythm, producing a great diversity of sound among modern crickets, from mellow chirping, to sustained trills, to whines so high that human ears cannot perceive them. The raised ridge of the fossilized insect lacks the precise series of bumps on the file, and there is no evidence in the wing of an amplifying window. It is likely, then, that the sound the animal made was a simple rasp, without the purity of tone achieved by the precisely tuned structures of today’s crickets.
The scientists who described the fossil in 2003, French paleontologists led by Olivier Béthoux, working with the discoverer, Jean Lapeyrie, named the species Permostridulus—from Permian, the geologic age of the fossil, and stridulate, the zoological term for rubbing body parts together to make sound. The ridge in Permostridulus is made from the union of a different set of veins than those of modern crickets. The species belongs in its own taxonomic family, a clan now extinct, an early, distant relative of modern crickets.
When it was alive, Permostridulus’s arthropod companions were other insects, spiders, scorpions, and, in the temporary pools, swarms of small crustaceans. Our distant ancestors and their kin were there, too, their lizard-like bodies leaving footprints in the mud, preserved as tracks of fossilized prints. These reptiles, known as therapsids, ranged from iguana- to crocodile-sized and stalked the land on legs held vertically, unlike the sprawling gait of most reptiles and amphibians today. Some of their kind would, over the next fifty million years, shrink, gain a furry pelt, and evolve into what we now call mammals. But in the Permian, therapsids were reptilian-skinned browsers and predators, the dominant large animals in many land environments.
These forebears of the mammals most likely could not hear the insects’ sounds. The eardrum and triplet middle ear bones that deliver high-frequency sounds to our mammalian ears had yet to evolve. The sonic world of therapsids comprised only low frequencies, delivered to the inner ear through external ear holes and the bones of the animals’ bodies. The thud of footsteps and the boom of thunder were probably all they could hear. Perhaps, too, they heard the murmurs of other reptiles, although there is no fossil evidence that these animals were vocal. An ear adapted to higher sounds would come later in the evolution of these animals, when forest and plains were filled with edible singing insects and when the therapsid body transformed into the compact insect-hunting frame of the early mammals.
The arthropods of the time, though, could hear Permostridulus’s song. In their miniature world, sensitivity to higher-frequency sounds was a boon. For a spider or scorpion waiting in ambush for prey, feeling the patter of tiny footsteps in the soil, the scrape of an insect limb, the flutter of wings, or even a brush of a tiny body against vegetation can deliver information leading to the next meal. For prey, too, vibrations in air or through the ground are useful, serving as warnings of close-at-hand danger. Sonic awareness of the presence of others also helps in the intimate social negotiations of mating. These sounds of insect bodies and movement—whishes, sighs, and crinkles—are quiet and travel only a few centimeters or, for the heavy rustlings of the largest, a meter.
Ancestral crickets possessed well-developed hearing organs in their legs, arrays of cilia-bearing cells that detect minute vibrations in the ground and pressure waves in air. After Permostridulus’s time, these capabilities were further expanded when evolution added a thin-membraned eardrum to cricket forelegs. This innovation, dating to about two hundred million years ago, was surely precipitated by the evolution of sound-making wings. Once sonic communication started, natural selection favored refinements in hearing.
We don’t know why Permostridulus made its sound. Living crickets sing to attract mates and defend territories. It is possible that the wing’s sound gave the ancient insect an advantage in the breeding season, perhaps by garnering attention, bluffing away rivals, or revealing location to searching mates, just as the chirping of crickets does today. As long as the breeding advantage was larger than the increased risk of predation, the song would have been favored by natural selection.
Perhaps, though, the wing’s sound-making ridge served a defensive purpose. A burst of sound can startle attacking predators, buying time for an escape. This sonic defense would have been especially effective in a world where such calls were rare. Imagine the shock that a pouncing spider experienced on feeling a buzz in its jaws or hearing an unexpected rasp at close quarters. To this day, vibratory startle responses are common. Pluck an arthropod from its home and you’ll often get a short blast of sound. Animals as diverse as lobsters, spiders, millipedes, crickets, beetles, and pillbugs all give defensive vibrations. Experiments with predatory wasps, spiders, and mice show that these vibratory alarms do indeed offer protection, startling attackers enough to allow potential prey to escape.
This uncertainty about the function of sounds highlights a difficulty in human language. In describing the sounds of other species, we project human nouns onto nonhuman beings. Song is anything we judge to have an aesthetic root, a sound made to please or persuade. Most often, we reserve the term for sounds whose repetitions have timbres or melodies that are pleasing to our ears. We name shorter sounds calls: the chirps of begging nestling birds; the sharp high notes of flocking birds; the bell-like exclamations of frogs in breeding season; or the grunts, cries, and sighs of monkeys discovering and sharing food. Calls can unite a flock, communicate from offspring to parents, signal alarm, or mark territories. But the functions of animal sounds are more diverse than our simple classification allows. Often the division between song and call is arbitrary and usually reveals more about the effect of the sound on human aesthetics than its roles among nonhuman animals. I follow common usage, but where social functions are unknown, as in Permostridulus, or only partly known, as in most nonhuman animals, this terminology is a mere sketch.
Whatever its function, the wing ridge in Permostridulus presaged further developments in an insect group whose relatives would become some of the world’s singing champions. Permostridulus is close kin to the taxonomic order named Orthoptera, from “straight wing,” which today comprises more than twenty thousand species, most of which sing. Some, the crickets and katydids, make sound by rubbing files and plectra on their wings. Others, grasshoppers and giant flightless crickets called wetas, rasp hind legs onto ridges on their abdomens. Sound-making wings and legs are supplemented in some species by rasping mouthparts, wheezing air tubes, drumming abdomens, and wings shaped to crackle and snap as they fly.
Permostridulus is, for now, the earliest known singer in the fossil record. But it was surely not the first animal to make a communicative sound. The fossil record is incomplete and gives us only very conservative estimates of the antiquity of evolutionary innovations, especially innovations like tiny ridges on insect wings that do not preserve well in stone. To cast our ears further back in time than the testimony of fossils, we can infer the past indirectly, using evolutionary family trees reconstructed from genetic comparisons among modern species. These trees, when calibrated to the ages of known fossils, give estimates of when groups of species diverged from one another. It seems that the cricket clan appeared around 300 million years ago. Almost all the living descendants of these first crickets sing. It is likely, then, that their common ancestor did too. Other contenders for early singers are the ancestors of treehoppers, cicadas, and other hemipteran bugs. Their common ancestors may have communicated with sound waves transmitted from vibratory organs in their bodies through wood or leaves. Like the crickets, these ancestors date to about 300 million years ago. Stoneflies, common insects in many waterways whose adults breed on stream-side vegetation, communicate by tapping out duets on vegetation, yielding drumming rhythms unique to each species. Their origin dates to nearly 270 million years ago, and so their soft percussion likely was another early animal communicative sound.
