Sounds Wild and Broken, page 10
To human eyes, the open ocean seems uniform. We might imagine this sameness penetrating all the way to the ocean bottom. Yet for sound, the ocean contains an invisible conduit, a passageway through which sounds can travel for thousands of kilometers. This “deep sound channel” is about eight hundred meters below the surface. Gradients of water temperature and density—cooler and denser in the depths—trap sound within the channel. When sound waves veer up or down, they are bent back into the channel by either warmer water above or denser water below. This watery lens transmits sounds across entire ocean basins, especially low sounds whose passage in water is unhindered by water’s viscosity. Whales take advantage of this channel, and their rumbling, moaning, throbbing calls were, until humans invented the telegraph, the only animal signals capable of crossing oceans.
Sound also travels through solid matter, zipping through wood or rock ten or more times faster than in air. We use these waves in all our musical instruments, but these vibrating sheets and strings of wood, skin, and metal are designed to send their sounds to the air. For many other species, though, solid matter is the primary or only acoustic medium.
All land invertebrates like insects and spiders sense vibrations through nerves in their external skeletons and, especially, in the soft tissues of their leg joints. Imagine if every human toe, foot sole, and finger were an ear. This is the insects’ world. They hear the vibratory energies around them through receptors on their body surfaces and inside their appendages. Most also use this ability to communicate. Spiders tap the ground with the feet, signaling to mates and competitors. Many hemipteran bugs—treehoppers and their kin—use buzzing organs in their abdomens to send complex trains of sound waves down their legs into leaves or twigs. These signals are usually inaudible in air but transmit with speed and clarity to the listening feet and limb joints of companions. Legs are, for these species, the organs of speech and hearing.
Insects live in a parallel world of sound, running alongside the aerial sounds that we humans hear. Only recently has the magnitude and diversity of this soundscape of solids become known. By attaching electronic sensors to vegetation, scientists have discovered that up to 90 percent of insects communicate using some form of vibration through vegetation or the ground. My own initiation into this strange world of insect buzzes, squeaks, and clicks came when I was gathering recordings for an exhibition of tree sounds. I hooked a tiny sensor to a cottonwood twig, capturing the many tremors and bangs that flow inside the windblown tree. Interspersed among the clatters of the tree itself were second-long high buzzes, regularly spaced like the ring of a cell phone set to vibration mode. I sent the sound file to Rex Cocroft at the University of Missouri, a pioneer in the exploration and study of insect communication, and he confirmed that the sounds were of an insect, likely a leafhopper. More precise identification is not possible because, unlike the well-known songs of birds, our knowledge of the diversity of these sounds is so rudimentary that we lack a comprehensive catalog matching sounds to species. For naturalists with an exploratory bent, the insect “vibroscape” offers fertile ground for discovery.
Every plant species and part of a plant has a different physical character. Young leaves are soft and spongy. Mature twigs are brittle and stiff. Bark is a wide sheet, but a leaf petiole, the fine stem that holds the leaf, is a tube of dense material around a more open core. Each of these materials transmits vibrations in a different way, favoring some frequencies over others. We get a crude sense of this when we hear our neighbors in an apartment building. The hardwood floor of the people living upstairs filters out nearly all high frequencies, but acts as an excellent transmitter of midrange footsteps. If our neighbors install cork—a form of tree bark—on the kitchen floor, only the lowest thuds come through. These varied properties of plant matter are the sonic world in which insects live. Such differences have created sonic diversity in insect sounds, just as differences in vegetation have done for airborne bird and mammal sounds.
Treehoppers in eastern North America offer a clear example of how physical differences in vegetation shape vibratory sounds. These diminutive relatives of cicadas suck fluids from tree leaves and stems using piercing mouthparts. A crest on their heads makes them look like little thorns. In the breeding season, male treehoppers whine and click, and females reply with lower grunts. This duet plays out entirely through tremors sent through leaves and stems.
Two-marked treehoppers, named for the yellow dots on their backs, are a group of closely related species that each specializes on a different plant species. This diversity arose when ancestral species expanded their ranges to new host plants. Colonist treehopper species not only encountered new food when they switched to novel hosts, but their sonic environment changed.
Two-marked treehoppers on eastern redbud, a common tree species of forest edges, give a low whine, about 150 hertz, the pitch of a throaty human hum. Treehoppers on wafer ash, another small woodland tree species, call much higher, about 350 hertz. The two varieties of the treehopper are the same size, and they stick with their song type even when plucked from one tree species and put on another. Each tree species has its own sonic qualities, transmitting some sounds better than others. Each treehopper species sings at the frequency that works best for its preferred plant species. Like human luthiers who know and use the subtle differences of woods, these insects have diversified their songs to match the material properties of their homes.
Insects that use many host plants have more widely transmittable songs than do the treehoppers. Harlequin stinkbugs, for example, feed on more than fifty plant species. They call with multifrequency buzzes whose sounds will travel through leaves and stems regardless of plant species. They are wandering troubadors whose songs work in any space, unlike the specialized two-marked treehoppers.
Wolf and jumping spiders attract mates with vibrations whose frequencies match the sound-transmission properties of the leaf litter on which they hunt. Elephants call to one another across great distances by making rumbles that then flow through the ground. They hear these sounds using dense patches of sensory cells in their feet, supplemented with transmission of the sound through their leg bones to the neck and then the inner ear. These rumbles are very low, too deep to be heard by humans, a frequency that transmits especially well over long distances in the soil.
The great diversity of sonic expression across the animal kingdom has its origin, in part, in the varied physical properties of Earth. When we hear a song or cry, we hear the material context in which it evolved. We are also surrounded by sounds inaccessible to our unaided ears, each one tuned to its environment. Our senses live confined in a small part of the whole. Yet we can imagine that under the river’s surface are fish drumming to one another. Off the seacoast, whales sing into the deep sound channel and listen to answers from half a world away. In the trees and on the stems of grasses and flowers, insects duet. In human language, whether actively voiced or transmitted through the page, we hear the legacy of habitat, diet, and the physical nature of air and vegetation on our ancestors’ speech.
In the Clamor
It is two in the morning and I lie awake, listening to the rain forest. The cabin is in a small clearing, the top half of its walls open to the forest save for a shield of mosquito netting. My companions, scientists working at the Tiputini Biodiversity Station in the Ecuadorian Amazon, are asleep, worn out by treks on muddy trails. I woke from deep sleep into a glory of sound, an exultation born in the voices of hundreds of species.
A crested owl growls a sonorous oor, repeating every five seconds. This is the deepest sound in the forest tonight, delivered with the slowest tempo, a languorous bass. In the daytime, a pair of these crow-sized owls roost with their fledgling in low branches near our cabin. Twin white plumes crown the head of each adult, contrasting with their chocolate plumage. The youngster is all white. In the rain forest, we rarely see the animals whose sounds surround us, and so this family group is much photographed by visitors.
It rained earlier in the night, and drips from the soaked vegetation that overarches the cabin enliven our tin roof with snaps and spatters. In the forest, tree frogs yelp from low vegetation. Their call is tight and nasal, yup! yup!, and each singer has a slightly different pitch, perhaps reflecting differences in body size. I hear them all around the cabin, answering one another. I feel caught in the middle of a ball game among half a dozen frogs. On my left, a call smacks the rubbery projectile into the forest, then another frog on my right whacks it in a different direction, to a singer near my head, back and forth, the sound vaulting over me.
The songs of insects are not as easily localized by my ears as owl and frog sounds. I can pinpoint the direction of only a few crickets and katydids, but mostly I’m wrapped in their sonic mists. The clouds of sound are not homogenous, though. Dozens, perhaps more, of pitches, timbres, and rhythms coexist. My ears are used to the relative uniformity of the temperate world: quiet, singleton cicadas in the summer forests of the Rocky Mountains or Maine; the liveliness of field crickets in grassy meadows, a chorus of a handful of species at best. Even the relentless, ear-ringing pounding of katydids in late-summer forests of Tennessee and Georgia is dominated by one species and spiced with occasional bursts from half a dozen others. Here in the Amazon, species diversity is ten or more times higher, a magnificent convergence of sounds.
In the lower registers, a katydid gives short fibrillating bursts. This is overlain with higher, shimmering songs, like dry rice cascading into a steel bowl. Alongside, a hacksaw delivers regular strokes, the harsh bite of teeth on metal. A sweet trill floats over, pulsing once every second. At a faster tempo, another trill comes, higher-pitched and drier. Alongside, three species give continual buzzes, quite close in pitch, one ringing clear and bright, another slightly fuzzy, and the third very arid, like a stick dragging through sand. An irregular sound like the tinkle of metal shavings skips over the buzzes and whirs, so clear and bright that I see silver flashing. Pitched even higher are more pulses, some pumping every second or so, others coming in streams.
There is yet more sound here at higher frequencies, but the human ear cuts it out, a space we call ultrasonic but is, in fact, not “beyond sound” but merely beyond our perceptual abilities. Also evading my ears are many hemiptera—planthoppers, treehoppers, shield bugs, and others—that send songs made of chirps, trills, and pure tones through the solid material in leaves and stems. At least thirty genera of treehoppers live here, comprising an unknown number of species, as do more than four hundred species of planthoppers.
In the audible range, the insect sounds seem to occupy two bands. One is about the frequency of high birdsong. This is where most of the insects sing, a range familiar to anyone who has heard chirping crickets and katydids in parks or forests outside of the tropics. The other is much higher, a fine, crystalline gleam of sound. The lowest frequencies and the midrange seem sparser, save for the lowest insect trills, the owl, and tree frogs.
As I lie in the humid cabin air, sweat easing down my face and neck, pooling in my clavicles, I am befuddled by the experience of listening. I can attend to the insects in one of only two ways. Either let the sound wash over me as a whole or pick out one single species and focus on its shape and qualities. There is too much richness here to hold multiple species in close attention, as I do in temperate forests. In forests in northern Europe or the North American mountains, I can revel in the combination of several singing species, like enjoying the convergence of several spices in a meal. In the tropical forest, hundreds of flavors and aromas coexist at once, an extreme blast of sensory diversity that stuns my auditory palate.
This wonderful but unsettling experience is also radically unlike listening to human music. Whether in a folk song, a jazz improvisation, or a symphony, the human mind crafts sonic layers, each in close relation to the others, all emerging from instruments designed to complement one another. One or, sometimes, a small number of people compose the music. Human music contains complex, divergent, and sometimes discordant narratives but emerges from a narrow generative source, the minds of its composers and the proclivities of the human ear. In the rain forest, there is no single composer and no agreed-upon collection of tonal or melodic rules. Many aesthetics and narratives coexist here. Listening in the rain forest is challenging and delightful because we hear many stories at once, each expressed with a voice suited to the aesthetic of its own species. These stories are connected through bonds of ecology and evolutionary kinship, but each is propelled and shaped by its own history, needs, and context. The anarchic equality of evolution—a process with no controlling central hierarchy—delivers sound that, to my ears, is joyful in its profusion, humbling me when I try to find its inner patterns. Listening here is a liberation from the tight control that we humans like to impose on the flow of sound.
From my cabin, I hear only the sounds of one spot in the forest, a single moment in the rhythm of the seasons and the night or day cycle. Last night, I walked with a small group of researchers to the riverside and then on a trail through the wet forest. The cloud of sound changed every ten meters or so, revealing new insects and, near the water, the varied crepitations, twangs, and tremolos of frogs. As dawn approached, night-calling species dropped out, one by one, replaced by voices of the predawn, then the day. Blue-gray spread into the sky’s black, and howler monkeys suffused the forest with a low rumble and growl. A few birds joined at the first gleam of light, a chorus that peaked just after dawn. As light spread over the rain forest canopy and seeped to the understory, the soundscape filled with krak cries of pairs of macaws flying overhead and sneezy exclamations from flycatchers. As they did at night, insects dominated the new morning with dozens of tempi and pitches.
The cycle of day and night is marked here by shifting combinations of sound as each species calls at its preferred time. Rain and sun modify the shapes of this acoustic cycle. A downpour silences many of the birds, canopy-dwelling insects, and primates, but frogs and ground-dwelling insects persist or have their voices quickened by the rain. The sun-filled hour after a deluge evokes a burst of song, even from species that usually confine their vocal liveliness to the dawn. Midafternoon on a sunny day is the quietest moment for vertebrate animals and even for many crickets, but it is a rousing time for cicadas.
The soundscape varies greatly over the rain forest’s terrain. As we walk the trails or climb on ladders from the ground into the canopy, we move through patches and layers. No two places sound alike. This is radically different from temperate or boreal forests. I can walk for hours in the spruce and fir forests of the Rocky Mountains in summer and hear combinations of the same half dozen bird, two squirrel, and two cicada species. No one knows exactly how many insect species live in the forests around Tiputini, but the count may be near 100 thousand, many of which are sound makers. Frogs and birds are better known. Nearly 600 bird species and 140 frog species live here. The same number of species as inhabit the varied terrain of North America are crammed here into the space of a few square kilometers. The sonic community is thus crowded and richly variegated.
The power and diversity of the rain forest’s animal voices reveal sound’s communicative power. Every species here is advertising presence, revealing identity, and conveying meaning to distant others without the danger of being seen. At night, darkness conceals. In the day, the dense profusion of rain forest foliage is almost as effective as a cloak. This is one of the most visually occluded habitats on Earth, perhaps rivaled only by the impenetrably dense thickets of young boreal forest or turbid seawater near a river mouth. No wonder sound blossoms here. Individuals can communicate through the crowds of leaves, all while remaining hidden from predators that hunt by sight. Hundreds of plants in every hectare, smothered in mosses and algae, create habitats of great visual complexity. This, combined with the cryptic color patterns of many insects and other species, makes seeing animals in the rain forest very challenging, even for dedicated and experienced naturalists. But we hear them.
What started on the arid plains of the late Paleozoic, 270 million years ago or more, with the thin rasp of Permostridulus and its kin, has now diversified into a thick weave of thousands of sounds in a single place. The sonic grandeur of these forests, though, presents challenges. The costs of vigorous sound making are borne by individual singers and also threaten the viability of sonic communication for the whole community. These dangers drive the diversification of sound in the rain forest, spurring evolution’s creativity.
The first cost of singing is the same one that likely silenced ancient animals: making sound risks advertising your presence and location to predators. The risk increases with sustained sound, like the hours-long trilling of crickets or the repeated melodies of songbirds. The solution to this problem in Permostridulus’s time was a swift escape. The same is true now. Immobile or slow animals rarely vocalize. The rain forest’s sounds come mostly from animals with wings, powerful jumping legs, or both: birds, frogs, monkeys, crickets, katydids, leafhoppers, cicadas, and their flighty, springy kin. But predators and parasites have honed their skills since the Paleozoic. Bounding escape is sometimes insufficient.
Singing insects in the tropics, for example, are plagued by tachinid flies. These hunters have paired eardrums on their undersides, just behind the head, that allow mother tachinid flies to home in on victims. Guided by ears tuned to the particular frequency and tempo of her preferred singing insect host, she alights and spills tiny larvae from her abdomen. These wrigglers swarm the victim and burrow through its exoskeleton. Lodged inside, the larvae grow for a couple of weeks, then burst out, killing their host.
