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- Herausgeber: John Wiley & Sons
- Kategorie: Lebensstil
- Sprache: Englisch
In our busy, noisy world, we may find ourselves longing for silence. But what is silence exactly? Is it the total absence of sound? Or is it the absence of the sound created by humans – the kind of deep stillness you might experience in a remote mountain landscape covered in snow, far away from the bustle of human life?
When we listen closely, silence reveals a neglected reality. Neither empty nor singular, silence is instead plentiful and multiple. In this book, eco-acoustic historian Jérôme Sueur allows us to discover a vast landscape of silences which trigger the full gamut of our emotions: anxiety, awe and peace. He takes us from vistas resplendent with full and rich natural silences to the everyday silence of predators as they stalk their prey. To explore silences in animal behaviour and ecology is to discover a counterpoint to the acoustic diversity of the natural world, throwing into sharp relief the grating reverberations of the human activity which threatens it. It is to attune ourselves to a world that our human insensitivities have closed off to us, to take a moment simply to breathe and listen to the place of silence in nature.
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Veröffentlichungsjahr: 2024
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CONTENTS
Cover
Table of Contents
Title Page
Copyright
Acknowledgements
Foreword
1 In the Alps
2 The essence of sound
Notes
3 In the tropics
Notes
4 The nature of sounds
Notes
5 In the heart of the Jura
6 The enemy
Notes
7 In the laboratory
Notes
8 Absolute
Notes
9 Natural
Notes
10 Pleyel
11 Music!
Notes
12 Ourselves and others
Notes
13 To hear or not to hear
Notes
14 At the museum
Notes
15 Past
Notes
16 Hiding
Notes
17 Solar days
18 Romantic
Notes
19 Together
Notes
20 Sharing
Notes
21 Battles
Notes
22 Where?
Notes
23 Great silences
Notes
24 Silence, lockdown!
Notes
25 Preserving silence
Notes
Conclusion
End User License Agreement
Guide
Cover
Table of Contents
Title Page
Copyright
Acknowledgements
Foreword
Begin Reading
Conclusion
End User License Agreement
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Natural History of Silence
JÉRÔME SUEUR
Translated by Helen Morrison
polity
First published in French as Histoire naturelle du silence © Actes Sud, 2023
This English edition © Polity Press, 2025
This book is supported by the Institut français (Royaume-Uni) as part of the Burgess programme.
Polity Press65 Bridge StreetCambridge CB2 1UR, UK
Polity Press111 River StreetHoboken, NJ 07030, USA
All rights reserved. Except for the quotation of short passages for the purpose of criticism and review, no part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher.
ISBN-13: 978-1-5095-6403-3
A catalogue record for this book is available from the British Library.
Library of Congress Control Number: 2024937922
The publisher has used its best endeavours to ensure that the URLs for external websites referred to in this book are correct and active at the time of going to press. However, the publisher has no responsibility for the websites and can make no guarantee that a site will remain live or that the content is or will remain appropriate.
Every effort has been made to trace all copyright holders, but if any have been overlooked the publisher will be pleased to include any necessary credits in any subsequent reprint or edition.
For further information on Polity, visit our website:politybooks.com
Acknowledgements
Thanks to …
inspiring and enlightening conversations with Dominique Dupuy;
time spent exploring natural history and acoustics in the company of Julien Barlet, Adèle de Baudouin, Pablo Bolaños, Camille Desjonquères, Manon Ducrettet, Pierre-Michel Forget, Amandine Gasc, Philippe Gaucher, Elie Grinfeder, Sylvain Haupert, Laurent Lellouch, Félix Michaud, Sarah Obaid, Stéphane Puissant, Marie-Pierre Reynet, Frédéric Sèbe, Simon Targowla, Juan Sebastian Ulloa;
careful and thoughtful re-readings by Thierry Aubin, Laure Belmont, Bernie Krause, Sabrina Krief, André Nel, Emmanuelle Vin;
messages of support, advice and friendship from Stéphane Durand;
little notebooks made of paper and silence given to me by Caroline, Chloé and Julia;
… I have successfully completed this silent little exploration.
Many, many thanks to you all!
Foreword
What a book!!
Natural history of silence … What a wonderful title … Is nature sometimes silent? Yes, but not very often! In 2011, at the Muséum national d’histoire naturelle in Paris, Jérôme Sueur introduced me to someone who was subsequently to become a friend – the extraordinary American bioacoustician Bernie Krause, who had just published The Great Animal Orchestra. Bernie was in Paris to introduce the exhibition of the same name at the Fondation Cartier. This exhibition would make history and would be a way of saving Bernie’s sound recordings lost during the great fire in California.
Since the end of the 1960s, Bernie Krause, frequently cited by the author in his book, has been a major pioneer in the study of the sounds of nature, contributing to the development of a new discipline in the United States – bioacoustics. Together, he and Jérôme initiated me into ‘the sounds of nature’ … Even if, ever since the dawn of time, humans – hunters and gatherers and then farmers and stockbreeders – have listened to nature, scientific and technological approaches have been slow to take off and, until very recently, these sounds have been scientifically little exploited. During the first half of the twentieth century, sounds were recorded using the phonograph, and then, gradually, new technological developments began to appear, largely thanks to the boom in electronics. Oscilloscopes, acoustic cameras, microphones and hydrophones ever more finely tuned (and often used to record whale ‘songs’) allowed increasingly sophisticated approaches and prepared the ground for the expansion of this new discipline.
Most recent progress concerns the impact of anthropogenic noise (the sound of human activities or the ‘anthropophony’) on the living and examines the relationships between humans, ‘non-humans’ and their acoustic environment, often referred to as a ‘soundscape’. New research also focuses on a non-invasive technique for measuring biodiversity (for certain species of frog, for example, which are indistinguishable in appearance but which produce totally different sounds), leading to a new discipline, ecoacoustics, born just a few years ago in France at the Muséum national d’histoire naturelle.
In his book, Jérôme Sueur writes: ‘Omnipresent outside, sound also rumbles continuously inside us. It enters through our ears and does not re-emerge, it travels effortlessly through our bodies, it reaches our organs, touches our unborn children. Our bodies are themselves a source of sound, pulsing constantly with the beating of our hearts, grinding in our sleep, creaking as our joints stretch and gurgling with gastric rumblings when we are seized by hunger.
At each moment, day and night, we are therefore both the target and the source of these sound arrows. All we need to do in order to tune into this omnipresent sound is to close our eyes, concentrate for a few seconds, and analyse the acoustic landscape that is playing out all around us. Voices, music, bodies, plants, winds, rains, storms and objects send or re-send the sounds which continuously bombard all of us, all the time, and which rarely come in isolation but are often mixed with others.’
This book is therefore a wonderful incitement to listen to all the ‘soundscapes’ which surround us. The author sets himself the task of describing to us, sometimes with a great deal of poetry and always with a solid technical and scientific base, a profusion of species and environments, whether mainland or oceanic, terrestrial or aquatic. Are we capable of detecting these sounds and understanding them? We have a very clear memory of the sights and smells of our childhood but does the same apply to sounds? It is true that we have lost a good deal in terms of our abilities to perceive our immediate environments, our senses dulled by lives which are so very different and so cut off from the nature in which we were immersed not so very long ago. We have only to look, for example, at Australian Aborigines who are capable of hearing a lizard burrowing in the desert sand, an essential capacity for survival in such hostile environments.
‘Noise, the enemy of silence, makes no attempt to disguise itself. It is an insidious and large-scale enemy which makes itself heard at all times and in all circumstances …’ And this situation triggers a great many harmful effects and debilitating conditions in humans, in spite of the fact that the vast majority of noises are created by strictly human activities (anthropophony). ‘Man does not like noise but likes to make noise.’ Just listen to the sarabands of jet-skis along our shores or on our lakes during the summer! If I make noise, then I exist! Is silence therefore ‘a sound which does not make a noise’? What a lovely definition, formulated by a nursery school child!
What then must be done to put a stop to these endless cacophonies of anthropophony? A major lockdown such as we have recently experienced with a more than 99 per cent drop in noise pollution in areas close to airports? Evidence clearly indicates that overflights represent the most significant source of noise pollution in certain uninhabited ‘wilderness’ areas. Or should we go in search of that zone of silence set up by the ONF around the Chartreuse monastery in the Alps, where it is forbidden to … make any noise? Vladimir Nabokov tells us that silence is multiple and full of secrets, but it is by no means a void, an emptiness … So, as in Saint-Martin-du-Canigou, that splendid abbey in the Catalan region of the Pyrenees with those sculpted monks in the cloisters, should we too make our mouths smaller and our ears bigger?
Indeed, alongside the omnipresent pollution of our civilizations, of chemical, physical (including radioactive) and biological (with species distributed all over the world) contaminations, should we also include light and sound pollution? The sounds produced by living organisms (biophony) have been of such vital importance since the origins of life, and these, along with those of geophony (the sounds of the physical world such as volcanos, thunder, waves …), have shaped life as we know it. And they have often been a source of inspiration for our music … So, as in that wonderful song by Paul Simon and Art Garfunkel, let us learn to listen to The Sound of Silence …
Bergerac, 14 September 2022
GILLES BOEUF
Professor at the Sorbonne University, former president of the Muséum national d’histoire naturelle, visiting professor at the Collège de France
1In the Alps
It is an easy drive from Saint-Pierre-de-Chartreuse down to the flat plains of the Dauphiné region. This former track leading to the Chartreuse monastery is now a tranquil little mountain road, winding gently downhill, with only a few sharp bends to negotiate. There is a single narrow section under overhanging rocks where those heading cheerfully uphill find themselves almost rubbing shoulders with the unhappy travellers on their way back down.
On the occasion in question, we were ourselves part of the less fortunate group, heading back down to the plain, and, worse still, to the straight lines and heavy traffic of the main roads leading towards the flat and dreary Parisian basin.
Feeling the need for fresh air, snowy expanses and high, empty landscapes, we had decided to take a family break in the Chartreuse region, the nearest mountain range accessible from the western lowlands and one with a name that conjured up all the mystery of a place cut off from the rest of the world.
In that year, and those that followed, we would accumulate many memories on the slopes of these snowy mountains. That first week turned out to be a rather hectic one. Wonderful moments of noisy games in the virgin snow, far from the smoothly ploughed slopes, were interspersed with a number of minor family catastrophes: the car with its alarmingly slippery tyres getting stuck in the ice and snow, our three-year-old daughter falling head (and teeth) first down the steep staircase of a former barn now cosily converted and rented out on a weekly basis by an unerringly kind couple, and a nasty stomach bug passed from children to parents and vice versa, through some strange form of family altruism.
It was during this short stay – a time in which everything felt strangely compressed, punctuated with the to-ing and fro-ing of medical visits and often confined within the cosy though rather small chalet, our feet snug in thick new socks – that I found myself with an unexpected opportunity to make a brief escape into the surrounding pine forest.
Mountains have almost always scared me with their sheer mass and the threat of landslips or avalanches weighing heavily on the frail shoulders of my pallid city-dwelling self. Until then, I had never spent much time on rock faces or on snowy slopes. Only a handful of winter trips where I had skied – as people did thirty years ago, and still do today – a bit mindlessly, or at least in a tight group and always at high speed, without paying much attention to either the distant peaks or, nearer to hand, the animal and plant lives which might survive among the pylons and the ski-lifts. A few summer trips too, in the Pyrenees, trying to track down a mysterious frog on the slopes of Mount Canigou, or in the Swiss Alps, on that occasion in quest of an almost equally mysterious red cicada. My perception of the mountains was a very blinkered one, a vision which did not extend much beyond what could be seen through a ski mask or a cheap pair of goggles purchased in a large Paris store in the section tucked between horse-riding and aqua-gym. A stranger in the Chartreuse mountains, this escape from the chalet thanks to an enforced siesta for the rest of the family, still suffering the effects of the stomach bug, turned out to be a real moment of discovery for me.
I got dressed – even perhaps, like a true Parisian, overdressed – and quickly thrust my feet into a pair of snowshoes, grabbing the poles thoughtfully provided by the chalet owners. Under a grey but luminous sky, I took the first path to the right of the chalet, heading up the slope and into trees, dark against the snow.
In the course of this very short walk, I quickly realized that something was happening, or rather, not happening. True, I had escaped from a house full to the brim with activity and loud voices, but suddenly and without any prior expectation on my part, I found myself stepping into a completely different sound environment.
Advancing slowly along the snowy track, I ventured a short way from the path, carefully memorizing a few visual markers so as to avoid getting lost. As I walked, I could feel the crisp crunch of my snowshoes and poles on the hard snow, the rustle of my waterproofs and the rasp of my breath inside my scarf.
It was only when I stopped, after a few hundred metres, that I finally realized exactly what was happening.
Deep in the Chartreuse mountain range, far from the laboratory, far from the bustle of Paris life, nature was more alive, colder and more vibrant than ever before. Stripped of human noises, it presented itself to the passing hiker in its raw, unadorned state. In the cold air, there was only the flutter of a coal tit in search of something to eat and the muffled slither of a slab of snow sliding from a branch and falling slowly to the ground, leaving no trace of its presence.
It was there, at the end of that very short walk, standing alone and motionless in the snow, cut off from the relentless pace of time, that finally, for the very first time, I experienced silence in a natural world teeming with life.
And so, during the drive home, in the course of which my eye would be caught by a curious sign at the side of the road, I resolved that, from then on, I would set myself the task of trying to listen to what no one else was listening to, to listen to what was supposedly emptiness, while others concentrated their attention on what was ostensibly full.
2The essence of sound
In the space of just a short walk in my snowshoes, I had experienced the deep silence of a snowbound mountain landscape. Was this therefore what silence meant? A moment of solitude and of peace in a natural environment encased in ice and cold? Was silence an acoustic stillness where only a few flakes of sound still drifted past – a rustling of feathers, the slither of snow falling from branches? Was silence an absence of sounds and, as a result, a lack of information emanating from the surrounding landscape?
Yet, if this is indeed the case, how can we set about identifying an absence, a sort of emptiness? How can we define this antimatter, silence, without turning our attention to matter, in the form of sound? In order to understand silence, we must necessarily be familiar with sound. Who or what exactly is sound? What is this quivering yet formless creature which constantly makes its presence felt, even as we sleep, in the circumvolutions of our outer ears, tapping softly against our eardrums, gently stirring the stapes, the malleus and the incus in our inner ears, circling through the spirals of our cochlea, travelling along our nerves like a train advancing on its tracks, and finally awakening the neurones in our brains?
According to the laws of physics, sound is a modification of the pressure or density of a gas, fluid or solid caused by the endogenous or exogenous vibration of an object. Regardless of what that object is – a piano, a toaster, a copper beech or a whale – sound travels through air, water, vegetable or mineral matter. Without its medium, sound is nothing. In fact, sound is an integral part of the medium through which it is propagated: sound is air, water, plant or pebble and has been there since gases, liquids or solids first existed on earth. If sound fades with distance, bounces back and is diffracted by obstacles, it can also pass from one medium to another, from air to water, from water to rock, from rock to plants and from plants to air. Sound knows no frontiers; it disperses, permeates and then spreads out in all directions. All routes are possible as long as sound retains sufficient energy and the matter it travels through is capable of distortion.
Today, as I write, everything around me is vibrating in an atmosphere of hypersensitivity; the blue tit on the cherry tree in the garden, the radio in the kitchen, the neighbour’s cat. Far away from me, sound is also vibrating. At exactly the same moment, sound is travelling to and fro in the humidity of tropical forests, in the frozen wastes of taigas, in the arid expanses of deserts. It plunges into the water of streams, lakes, rivers, ponds, seas and oceans, it passes through flowers, shoots, buds, fruit, leaves, trunks and branches and it travels through rocky ground, across sandy beaches and deep into the earth’s crust.
Omnipresent outside, sound also rumbles continuously inside us. It enters through our ears and does not re-emerge, it travels effortlessly through our bodies, it reaches our organs, touches our unborn children. Our bodies are themselves a source of sound, pulsing constantly with the beating of our hearts, grinding in our sleep, creaking as our joints stretch and gurgling with gastric rumblings when we are seized by hunger.
At each moment, day and night, we are therefore both the target and the source of these sound arrows. All we need to do in order to tune into this omnipresent sound is to close our eyes, concentrate for a few seconds, and analyse the acoustic landscape that is playing out all around us. Voices, music, bodies, plants, winds, rains, storms and objects send or re-send the sounds which continuously bombard all of us, all the time, and which rarely come in isolation but are often mixed with others.
Sound is an essential element of our lives both inside and outside the womb, and a key to our survival. Sound in the form of the spoken word is the fundamental basis of our family and social relationships and keeps us constantly informed about the state of our environment and any changes to it. The framework of an immense network which connects and informs living creatures, sound is also a source of distraction, of relaxation. It alerts us, stimulates us and sometimes controls or irritates us. Sound penetrates our bodies for better as well as for worse.
In the air, sound is the relatively regular and relatively complex alternation of the longitudinal compressions and dilations of atmospheric particles. These particles oscillate around their position, never moving much further away than a few dozen nanometres. This small and very precise movement gives rise to an important phenomenon: the nano-oscillations of particles are transmitted from one to another in such a way that the alternating compression-dilation is capable of being transmitted over long distances, sometimes as much as several kilometres. The movements of the particles are small and weak, but the sound wave is big and powerful and the differences in pressure which occur can have a dramatic effect on us. How strange that we are incapable of seeing sound even though it is sometimes so intense as to be painful. In order to do that, we are obliged to resort to a little help from sensitive receptors in the form of microphones, accelerometers, laser vibrometers and mathematical manipulations capable of converting sound into image.
The physical description of a sound wave is made up of four main dimensions: amplitude, duration, frequency and phase. Amplitude is the strength, pressure, energy, power and intensity of the sound. All these terms from mechanical physics are linked to each other by variables of acceleration, mass, surface, density, celerity, impedance and time. Beyond the underlying mathematics, which are in fact relatively straightforward, it is simply a matter of remembering that the greater the amplitude of a sound, the more that sound distorts its medium of transport. The trumpeting of an African elephant displaces air particles to a greater extent than do the wingbeats of the mosquito buzzing around its ears.
The second dimension, a purely temporal one, is easier to grasp. A sound is not an indefinite event. It has a beginning and an end, a duration. While each sound contains its own history, made up of complex events of transduction, diffusion, dispersal, propagation, refraction, reflection and diffraction, sound is essentially transient, an event of the moment. The birth of a sound is relatively easy to determine given that it corresponds to the beginning of the vibration which occurs when the particles of the medium pass from a state of rest to a state of activity. This process is usually a rapid one, marked by a clear increase in amplitude. The death of a sound, on the other hand, is more difficult to pin down. Sound can often take a certain time to fade completely. The length of time required for a sound to die depends first of all on the physical absorption properties of its source and, in particular, its mass and elasticity. A resonating object such as a church bell takes longer to stop vibrating than a sound which is more spontaneously produced, like the snapping of fingers. The duration of a sound also, and even more importantly, depends on the inherent physical properties of the medium, such as its volumetric mass and impedance, and the extent to which the physical space is encumbered by the presence of other objects which can reflect the sound, creating echoes, or, on the contrary, absorbing and stopping it.
Subtle, discreet, hidden behind the broad shield of temporal variations and amplitude, phase nevertheless plays an essential role in the production, propagation and reception of sound. Phase is a temporal property which is a little more difficult to grasp since it is expressed in an unfamiliar unit, the radian, and with the golden number of π. Phase is used to define periodic sounds, or in other words, sounds which have repeated or cyclical time patterns. It is a way of describing position and indicates the location of the wave within the wave cycle which is circular in form.
Finally, frequency, perhaps the best-known aspect of sound, corresponds to the number of cycles travelled by a sound wave in a second, a number expressed in hertz (Hz). The greater the number of cycles, the higher the sound, and conversely. The sounds which surround us are very rarely made up of a single frequency but are more often a whole range of sounds covering a wide spectrum of frequencies, ranging from low frequencies of just a few hertz to very high frequencies measuring several thousand hertz. Sounds inaudible to humans are referred to as infrasounds when frequencies are lower than 20 hertz and as ultrasounds when they are in excess of 20,000 hertz.
Since a musical metaphor is unavoidable when it comes to describing sound, it is possible to see amplitude in terms of nuances such as pianissimo, forte, fortissimo, duration in the form of notes, quavers, quarter notes, whole notes, and frequency in terms of pitch – C, D, E. Only phase, eternally forgotten, does not seem to have any equivalent in the western system of musical notation, even if it is an important factor in the craftsmanship of instruments and in the engineering behind sound recording and reproduction.
Amplitude, duration, frequency and phase are not independent properties – amplitude is measured in a predefined temporal window, frequency is the opposite of a temporal period, duration is a specific time, and phase is a characteristic of both amplitude and time. Time is therefore the fundamental dimension of sound.
Hearing means listening to the passing of time.
Soft or violent, short or prolonged, low or high-pitched, sound does not come from nowhere but emerges from a vibrating object.
Green tree frogs (Hyla arborea) are endearing-looking frogs which conjure up an image of a plump rounded body, a broad grin at the water’s edge, a fly trapped like a sandwich in its mouth and skin the colour of a Granny Smith apple. The tree frog is also known for its capacity to leap over water lilies the size of flying saucers and for its croaking sounds, which are as insistent as they are incomprehensible.
Straight out of the pages of a fairy tale, the little green frog with sticky toe pads occupies a privileged place in our imaginations. But if the tree frog croaks loudly in our bedtime stories, few of us venture out at night to listen to it in the ditches of a waterlogged lane or around the edges of a pond. True, it requires a certain amount of courage to switch off our screens and head out into the darkness, dirtying our boots in muddy water. And yet it is an experience which is both moving and rewarding. We should make the effort, spend some time searching in the long grass, on stones or in damp earth in quest of this little animal, crouched like a cat ready for its nap, until finally we find it, fully concentrated on its nightly task of calling and calling, again and again, its throat swollen almost to bursting point as it proclaims its romantic longings to the stars turning far above its tiny, fragile head.
The male tree frog is a tireless singer. Like all vertebrates, its song relies on the vibration of delicate membranes attached to its larynx, the equivalent of our vocal cords. At the start of the process, the frog takes a breath in, then, contracting its abdominal muscles, it expels the air from its lungs towards its mouth. The air passes over the larynx and forces the vocal cords to vibrate. While most terrestrial singing animals take a breath between two bouts of song, the tree frog functions in a closed circuit. The air in the lungs travels across the larynx and the vocal cords, passes over the glottis and enters the mouth cavity, kept firmly closed by the male frog, his jaws clenched and nostrils tightly sealed. Air then travels back in the opposite direction towards the lungs and this process is constantly repeated. This closed ventilation system allows the tree frog to sing for a long period without needing to take in air, therefore avoiding the tiring movements of external respiration.1
The tree frog therefore never deflates during the mating call, with air alternately dilating the lungs and the mouth. The roof of the mouth is pierced ventrally by two small openings leading to a delicate, flexible membrane forming the throat. Under the pressure of air coming from the lungs, this membrane, the vocal sac, swells up like a balloon. Sporting this curious double chin, the frog takes on a strangely aristocratic look. Indeed, the vocal sac plays a crucial role in sound production since it enhances amplification by facilitating the transfer of vibrations from the frog’s body to the outside air, improves tone by concentrating the sound energy on just a few specific frequencies and helps acoustic diffusion by projecting the sound in all directions. Without the vocal sac, the sound produced by the tree frog would be distorted, fainter, more difficult to locate and probably less agreeable to listen to along the edges of a lane.
The sac swells up and vibrates. Our eyes are sadly not quick enough but it is almost possible to see the vibrations on the surface of the taut membrane, on the interface between the amphibian body and the air. This is the origin of the airborne sound, the sound wave which reaches our ears. If, by chance, the tree frog is immersed in water, the sound can also travel from the vocal sac to the water and at that point a regular series of little ripples form around it in concentric circles. Sound becomes visible and radiates out from the frog.
Seeing sound in the white throat of a tree frog sitting in a puddle gives us an opportunity to try to understand the mechanics and the rationale of an animal sound. The frog makes throaty sounds, since it is the throat that is doing the work here. It distorts its body in order to distort the air, water or ground around it and, for a certain period, takes control of all of this. Singing is a competitive sport.
For the croaking to make sense, for it to be the outcome of an evolutionary process, it must of course be noticed, detected, recognized, identified and localized and it must have a function other than that of simply making the frog visible to passing predators. There must be a very good reason behind this behaviour. And it is a simple one: the drive to reproduce, to pass on its genes to another generation. Like almost all animals, the tree frog becomes vocal when love is in the air, when it is time to find a partner. The male sends out a communication signal which contains various different elements of information, essentially for the benefit of any nearby females. That guttural croak divulges information about identity: ‘I am a green tree frog’; gender: ‘I am a male’; current state: ‘I am ready to reproduce’; location: ‘I am here’; and quality: ‘I swear that with me you will have the most beautiful tadpoles.’ This communication is an essential element in the tree frog’s life: the croaking is the keystone of its life cycle. Removing the sound would result in the loss of the species.
Sound exists in its own right but it takes on a value, a behavioural, social or ecological purpose, when it is detected and interpreted by a living creature possessing a sense of hearing and an integral nervous system capable of reading and interpreting information coded in the properties of amplitude, duration, frequency and phase. The first stage in the process of hearing is the transfer of sound, which must travel from the outside to the inside of the body, whether human, pink dolphin or tree frog. In this particular case, there is a physical problem in that air offers little resistance to sound, or in other words, the impedance of air is relatively weak, whereas the biological tissue of the frog, essentially made up of water and fatty matter, offers a certain amount of resistance to sound and therefore a high level of impedance. The transition from one to the other is no easy matter. One solution to facilitate transmission from outside to inside is to diminish the impedance of the receiving body, notably by reducing the thickness of the external tissues as much as possible. The tree frog has a very fine membrane called a tympanum, behind which is concealed an air-filled cavity. Delicate, light and flexible, the tympanum receives the sound vibrations from the air, thereby allowing sound to enter.
In frogs, as in most animal species, the hearing process involves a series of mechanical and physiological stages. Air is distorted and transformed into movements of the tympanum which are then transmitted to the columella, an ossicle equivalent to our stapes bone, and this in turn connects with the oval window of the cochlea in the inner ear. Mechanical vibrations are then transformed into electrical impulses, the basis of nerve signals, in a cluster of sensory neurones which together form the auditory nerve. The auditory nerve projects into the brain, notably in a nucleus called the torus semicircularis, which processes information coded within the parameters of time, amplitude, frequency and phase. The frog on the receiving end of the song is therefore capable of detecting, identifying, localizing and decoding the message emitted by the croaking frog, a process which is fundamental to any system of communication and therefore of socialization.
But sound does not always pass through the main tympanic entrance. It can also be picked up by other parts of the body and transmitted to the brain. The American green tree frog (Dryophytes cinereus), cousin of the European green tree frog, uses its lungs as well as its ears to listen to other frogs of the same species. The sound reaches the tympanum through their external surface via a direct route and travels to their internal surface by a roundabout, visceral one. Sound can in fact pass through the lungs, the glottis, the mouth or the Eustachian tubes in order to make the internal surface of the tympanums vibrate. The Gardiner’s Seychelles frog (Sechellophryne gardineri), one of the smallest amphibians in the world, has neither external nor internal ears and therefore no tympanum or columella. At first sight, therefore, it should not be able to hear. And yet, it sings. Experiments conducted on the idyllic island of Silhouette and morpho-anatomical analyses have shown that it reacted strongly to the songs of its own species and that sound was clearly finding another pathway. In the case of this frog, sound is transmitted through the mouth cavity, which acts as a resonator. The waves then travel through the jaw bones and reach the inner ear.2
In this way, certain frogs hear with their bodies, their lungs or their mouths. Animals have multiple means of receiving or emitting sound. Flies and mosquitoes hear with their antennae, which are also their nose, cicadas broadcast their song in the blazing heat of the Provençal garrigue with the aid of their tymbals, fish hear thanks to their swim bladder which acts as a ballast for vertical navigation, kangaroo rats chat by tapping their feet and crustaceans listen with the help of their sensilla, sensory hairlike structures covering their bodies. If we are to enter the realm of the acoustic lives of animals, we need essentially to set aside everything associated with the processes we ourselves use for emitting or receiving sound and avoid any comparison between our anatomy, our vocal capacities and our auditory properties and those of the animals around us. Sound is not necessarily a voice, and ears are not necessarily located on the head.
But what do frogs hear? Without going into the neurobiological details, frogs, like other animals, do not hear in the way we do and it is hard for us to imagine exactly what their perception of sound is like. The auditory capacities of frogs may appear more restricted given that their frequential sensitivity, that is to say the frequencies they are most likely to pick up, is very often limited to a window corresponding to the frequencies of their song. This means that there is some form of connection between the frequencies emitted and the frequencies picked up which effectively excludes any sounds which are of no interest from a survival point of view. The ear, from the tympanum to the cerebral nuclei, functions partly as a series of filters only permitting the transmission of those frequencies which carry information and enable a female frog to react to the declarations made by the emitter of the sounds, for example those revealing his identity, such as ‘I am a green tree frog’. What applies for frequencies also applies to the time structure of the signals in question. The neurones adjacent to the ear and the central neurones of the brain only become active if the rhythm of any vocalizations, or of the impulses which constitute these, corresponds to a recognized rate of repetition, for example thirty beats per second. In this way frequential and temporal filters play a role in decoding the information concealed within the frog’s croaking.
Each frog, each species, but also each individual therefore perceives their surrounding environment in their own way, and in a way which is very different way from our own, effectively ruling out any generalization based on our own acoustic perception and, in consequence, any anthropomorphism or anthropocentrism.
Sound is a universal mechanical wave which travels through space without leaving any trace but which carries information and sensations unique to each living creature. Weak in terms of the movements of its particles, scarcely visible, the sound wave has a fundamental effect on the tree frog and, of course, on all species relying on sound. The tree frog uses sound in the reproductive cycle in an attempt to encounter a frog of the opposite sex, but, as we shall see later, sound also plays a role in numerous other situations: in the relationship between parents and their young, in the cohesion within family groups, in locating a prey or in escaping from a predator.
How can we listen to animal sound when we are also animals ourselves, with our own auditory capacities, our bodies, our life experience, our feelings? There are two distinct but compatible ways of listening to the sounds of nature: an aesthetic listening where sensations are of prime importance, and an analytical listening where questioning and knowledge take precedence. We can listen to a concerto or a symphony without understanding them and we can also study them, analyse them and discuss them according to the rules of musicology. The same is true when it comes to the sounds of nature: we can allow ourselves to be transported by the sound of a forest without understanding what we are hearing, and then we can analyse the sounds in detail in order to enhance our understanding.
Henry David Thoreau, the North American naturalist and philosopher, envies the unsophisticated way children hear sounds and admires their ability to love a sound purely for its own sake and for its intrinsic value.3 We need to be sensitive to sound in its own right, unadorned, and without any artifice such as music or orchestration, like the creak of packed snow, the rustling of a white poplar or the sudden crackle of lightning. Most importantly we should love sound for sound’s sake, whatever form it takes. We should love sound for its shape, its silhouette, its multiple dimensions, its depth, its delicacy, its power, its discretion, and come to accept its sometimes harsh and even abrasive quality. We should love the way sound takes unexpected directions, how it bounces back and beats out its rhythms. We should embrace its sudden changes of mood and its surprises. Enjoy its creaks, whistles, crackles and rattles, its ability to be melodic, rhythmic and reedy. We should embrace sound full-on, standing tall, our lungs full, on the summit of a mountain or in the bed of a river. We should be joyously attentive to the complete and all-encompassing sound emanating from an entire landscape and then seek out the details and the different elements. We should relish its sudden surges, its trills, vibrations, glissandi, its sharp spikes and muffled softness. We should listen to sound as a whole and then identify the individual sounds, stoop down to gather up little sounds from the forest floor and reach upwards to touch the sounds coming from the foliage overhead. Listening to sound in this manner means applying Pierre Schaeffer’s principle of reduced listening, it means forgetting about the reasons behind the sound and concentrating instead on what is unique about it.
