Two topics from the excellent book THE SEVEN MYSTERIES OF LIFE by GUY MURCHIE
If the eye is the most notable organ for detecting radiation, many of whose sense variations we have been describing, the ear is hardly less important or complex as the outstanding organ of feeling - in this case feeling sound waves that mechanically vibrate the eardrum - and it may have an even longer evolution. At least its earliest discovered form seems to have been that of a simple balance indicator in primordial planktonic sea creatures who could not hear a sound. Known as a statocyst, it was a microscopic hollow cell containing an even smaller pebble of limestone, called a statolith, balanced on sensitive bristles so that, whenever the swimming creature got tilted, the pebble would roll toward the cell's down side, instantly triggering the down bristles' nerves and making them signal the animal how to shift back to an even keel. Such an organ hardly seems to have anything to do with hearing, yet, consistent with the interrelatedness of all senses, it evolved during hundreds of millions of years into the central gas bladder of modern fish which acts as a buoyancy balance or float to keep them at their accustomed depth in the sea. But it also vibrates when reached by sound waves, serving as an eardrum to convey hearable patterns through amplifying bones to the liquid channels of the inner ear, where they are transformed by hairs (more sensitive than the statocyst's bristles) into nerve impulses that go to the hearing center of the brain.
This successful fish ear in some species can even be shifted into reverse, giving the fish a voice through muscular control of little drumstick bones that beat rhythmically on the bladder to "talk back" to whoever has addressed it. All fish and some amphibians in addition have an earlike organ along their sides known as the lateral line, through which they hear low-pitched sounds and feel water pressure fluctuations, including faint waves made by other fish - a sense indispensable to formation maneuvering in fish schools.
By the time birds evolved some 140 million years ago, hearing had taken another step forward with invention of the cochlea, an improved inner ear shaped like a spiral seashell and containing ducts of fluid separated by delicate membranes that vibrate and move around a kind of microscopic empyrean harp called the organ of Corti in which (as adapted to the human ear) some 23,500 hair cells somehow transform sound's mechanical motion into electric current that conveys it through about an equal number of fibers of the auditory nerve to the brain, where it is consciously heard. The organ of Corti presumably has thousands of resonators, each of which, like a harp string, responds only to one precisely pitched note, and there is evidence that it works on the piezoelectric principle (page 45!). Certainly it is comparable to the retina, which transforms light waves from the eye into electrical optic nerve impulses and vision, but such organs are far too complex (not to say controversial) for detailed discussion in a book of this scope.
Before the cochlea appeared, insects, living on land and in the air, evolved their own type of ear. Unlike the ears of fish, birds and man (all of which originated underwater), this bug ear had no fluid transmission of sound and therefore developed a simpler and more direct transition by running an auditory nerve from the eardrum directly to the brain. And it was this simple ear, I like to speculate, that heard the first messages ever uttered in the atmosphere of Earth. Three hundred and fifty million years ago it undoubtedly was much simpler than it is now, for it has evolved unceasingly, no doubt still increasing its sensitivity range in species after species, still trying out new body locations. The ears of moths and butterflies, for instance, are often in the base of their wings, mosquitoes hear with their antennae, and many kinds of insects have ears in their midsections, usually at the lowest point so they can detect the ground reverberating with the tread of terrestrial predators. Katydids, tree crickets and some grasshoppers, however, have slit-shaped ears just below their knees which are efficient for directional hearing because they can be widely separated and aimed in different directions, this being particularly important at mating time when males and females are out calling and sometimes work themselves up to the point of desperation to provoke positive and explicit responses.
Some ears, as you've probably noticed, serve more than one purpose like the gigantic ones of the African elephant and the dainty fennec, a fox of the Sahara (see chapter heading, page 177), which not only hear acutely but act as radiators to dissipate excessive body heat. This must be true also of the spotted bat of Mexico whose pink ears are as long as himself, not to mention the earlike petals and leaves of plants which may "hear" music (page 640), while they transpire moisture, refract and reflect light, attract pollinators and function in ways still unknown. Most aspects of vision have counterparts in hearing and other senses, you see, including aural camouflage and smell illusion.
The range of pitches heard by various kinds of ears varies widely, each animal family tending to evolve the range that will enable it to hear its predators or prey soon enough to escape or attack, and between times to converse and mate with its own species. Humans, incidentally, have a hearing range from about 20 to 20,000 vibrations a second, with maximum sensitivity at around 3000, which, significantly, is the pitch of a woman's scream! But hearing capacity is not constant, varying from man to man to woman to child and diminishing with age and masculinity for, although a young girl may hear bats twittering at an ultrasonic 25,000 cycles a second (as an acoustic engineer would put it), her mother can hear only a warbler singing at 15,000 and her aging father barely catch the top note of the piano at 4100. In fact, tests on men in their forties have shown that the upper limit of human hearing descends inexorably at an average 160 cps (cycles per second) for every year lived.
THE SONAR OF BATS
The thought of inaudible, ultrasonic frequencies naturally brings to mind the echo-location technique called sonar that is used by so many high-pitched chirpers like bats, night birds and sea mammals. Bats, the most fantastic of these, have lived on Earth for 60 million years evolving some 1300 species from chickadee-sized ones to the "flying foxes" of Java with wingspreads exceeding five feet. Although they are mammals without feathers and cannot fly as fast as birds, they are champions of maneuverability - superior even to hovering chimney swifts and backward-flitting hummingbirds - for they can turn at right angles at full speed in little more than their own length.
Most of the time a hunting bat emits from ten to twelve chirps or beeps each second at a pitch averaging 50,000 cps, but when he detects a moth, say, five feet away, his output suddenly accelerates as he closes in on it, reaching what's called a buzz of some 200 beeps a second, which greatly increases his accuracy until he snags it - the whole pursuit and capture taking a scant third of a second with the bat's beeps (recorded on film) looking as you will see in the next illustration. Many bats thus catch gnats or moths when they're plentiful at a rate of one or two per second, which includes time for a reasonable percentage of misses.
Although the bat cannot possibly be conscious of it himself, his brain determines the moth's direction by comparing the echo's arrival time at one ear with that at the other - while the range is sensed in the lapse between each pulse out and its echo back, neither interval exceeding a thousandth of a second when the prey is close. It is probably not possible for a human to visualize accurately the kind of awareness a bat must have of the shape, size and motion of a flitting moth or of twigs, wires and other obstacles in his path, all sensed in darkness through his ears, but his sonar is certainly attuned to his specific needs, the distance between the sound waves about a tenth of an inch (just right for a small twig or a bug) and the aural images of these details all conveyed to his brain in electrical impulses of the auditory nerve (just as visual images are conveyed electrically by the optic nerve), combining into a sensation so complete it must be mentally equivalent to a visual impression.
Indeed the reason these remarkable creatures are not really "blind as a bat" (as tradition would have it) is not because they have eyes (which they all do) but because they have ears. For their ears give them what amounts to vision. In fact it has been proven experimentally that bats can fly with their eyes taped shut, but they cannot fly when their ears are plugged. So, in effect, they "see" with their ears. It is almost as if you had hundreds of ears, each one unbelievably sensitive to the exact direction of any sound you heard and that, while blindfolded, you listened to an orchestra until you could visualize the position and instrument each member was playing, all instantaneously, continuously and automatically in three dimensions, so that you could see the whole orchestra in action. Even if you can't imagine your hearing getting so intense that it could tune itself into an image as graphic as seeing, the evidence clearly shows that ultrasonic sonar accomplishes exactly that. And the bats, porpoises and other animals that use it discriminate between their own echoes and those of their companions, even where the frequency of the pulses is the same. This is hard to explain, but ultrasonic researchers reason that as the time dimension replaces some of the spatial dimensions when hearing replaces seeing, the creatures involved must begin to perceive both the amplitude and phase of the sound waves, discriminating among them in reference to a coherent background of ultrasound - which would mean that they have evolved a natural but unique bioholographic technique. This hypothesis in fact was tested in 1968 by Paul Greguss of the RSRI Ultrasonic Laboratory in Budapest when he made a model of a bat's brain which, operating at a frequency of one megacycle, produced actual ultrasonic holograms that could be recorded on soundsensitive photographic plates, scanned with a microdensitometer and used to reconstruct three-dimensional images of objects "heard" by the "bat".
Although the ultrasonic voices of bats vary from jet engine intensity (in the swift ones) to a dainty whisper (in the hovering ones), some of the moths and smaller insects they prey on can hear them from more than a hundred feet away and take effective evasive action, sometimes diving to the ground where they may be completely out of sonar
range. But, more amazing, is the fact that certain advanced moths not only dodge the bats but emit countersonar ultrasonic signals to confuse them when they get close. In one test, made with a high-speed movie camera synchronized with an ultrasonic tape recorder for playing back as audible slow motion, 85 percent of the bats hunting such moths actually abandoned the chase in the critical last second. And, as if jamming bat sonar weren't enough, parasitic mites have also been discovered living on the same moths, where they have quietly evolved a curious appetite for the soft flesh of one ear - only one ear, mind you, not both, because all the greedier mites that partook of the second ear have thereby long since doomed their breed into extinction by becoming suicidal passengers on a deaf moth who, through this very act, turned into easy bat fodder.
MELODY OF LIFE
When we perceive the world thus as melody, I feel, we mortals are about as close as we can get to understanding its abstract essence, harking what Thoreau called that "most glorious musical instrument," whose generally unheard notes, grandnotes, great-grandnotes and succeeding harmonic posterity continuously join the chorus of evolution that may never end. I try not to assume the music of my native sphere is better than the music of other worlds unknown (even though it seems far too complex to have much likelihood of being identical to theirs just by chance) - so I listen to the saw-bows of insects and crustaceans as if they were unique to the universe. I marvel at the horns of earth fashioned by the mole cricket who stridulates underground (as loud as 90 decibels at one meter's range) to serenade his mate from the lonely sky. I hear the weedy warbling of the toad and the wistful whistling of the lovestruck turtle - even the almost inaudible whisper of the courting fruit fly, whose thousands of species are sorted almost entirely by the nuance of song. Diving off a reef, I discover the teeth-clicking conversations of fish, their oboeing of blown air and their drumming with special muscles upon their tuned air bladders (as were once the ancestors of our lungs).
Drumming is widely broadcast on land too, I note, by thumping rabbits and mice, by the wings of grouse and prairie chickens, by the fists of gorillas on their chests. ... Even leeches tap on leaves, as do termites on tunnel floors, while amorous earthworms beep faintly in their coded cadences. The ultrasonic twitter and aural "vision" of bats are well known (page 206), though not the haunting bell-like tones bats ring out while hanging at rest upside down in dark glades in the deep woods. And an old-time ship's doctor describes the shrill skirling of the humpback whale as "filled with tensions and resolutions" - all these but a meager sampling of the endless sequence of living, breathing parts of the song of Earth.
If such voices do not individually attain the full character of music, in concert they belong to something vastly greater than themselves. And could we tune in on all of them at once, fully orchestrated as integral parts of the whole planet's symphony, we would surely hear sophisticated counterpoint, blended dissonant harmonies and all sorts of subtle sonorities just above the background pastiche of insignificant gabbing. A lot of small talk naturally has to be included in Earth's total chorus, because it comprises everything voiced by everybody from bacteria to whales.
Birds probably produce the most sophisticated music of any class of animals and put both meaning and emotion into many of their songs, which are much more individual than we usually think, a fact suggested by one ornithologist's count of 884 different songs sung by the American song sparrow and another's recording of chord variations including a four-note chord intoned repeatedly by a woodthrush. An Englishwoman named Len Howard, the professional musician and bird lover who wrote Birds as Individuals, says she can easily recognize dozens of individual birds by their voices, even though a song may change from day to day, partly through imitation. And she can tell whether the singer is happy, dejected or perhaps struggling with something unusual on his mind. At migration time the birds in her native Sussex, in effect, tell her not only when they are ready to take off for the winter but where they are going. They can't help it, for Spain, Italy, Morocco, East Africa or whatever place else is woven right into their music.
The English blackbird, called Amsel in German-speaking countries, sings songs that more closely resemble human compositions than does any other bird with the possible exception of the wattle-eyed flycatcher of East Africa, credited with a theme of Salome that presumably antedated Strauss. Miss Howard heard an Amsel sing a phrase from Bach one day that he may or may not have heard her playing a few weeks earlier on her violin. In any case, he had to work at it, making many attempts before he got a certain trill right. Then he began experimenting with doubling the length of the trill and adding new ones with a result Miss Howard called "flutelike" and "very lovely." Another young cock Amsel "actually composed the opening phrase of the rondo in Beethoven's violin concerto," something she cou!d categorically "vouch for his not having heard." On the other hand, who knows what Beethoven may have picked up from Amsels in his day? Music is in the air and most certainly in the world's genes, and it was curiously reminiscent of Beethoven the way this bird composed the rondo by trial and error, pertinaciously trying numerous variations before reaching final satisfaction.
Many birds copy others or try to, even others of very different families, while it has been estimated that a catchy tune may spread across the landscape in the mating season at a fairly constant mile and a half per day. Group singing is a factor in this process of course, being a common practice among certain species. Two dozen linnets, for example, will fly purposefully into a tree, all keyed up with anticipation. When one bursts into song, the others quickly join in, some twittering, some trilling, many slurring their notes up and down until all the voices unite in a great crescendo that can be heard across the fields, attracting other flocks of feeding birds, field by field, the effect sometimes traveling miles in a few minutes.
Other birds do antiphonal singing to maintain close communication in dense foiliage, different birds singing different notes of the same tune in responsive sequence. This has been reported of pairs of tawnybreasted wrens high in the jungled Andes of Colombia who sang their parts back and forth "like' the twanging of liquid wires," diminuen-doing then to gurgling babbles, which blended into the murmurs of a nearby brook. And there is the well-documented case of an "elegant trio" sung repeatedly by three boubou shrikes in overlapping territories on the shore of Lake Bunyoni in Uganda. The third bird was a male perched on a different branch from the first two, who may have been mates, but he inserted his note every time with professional precision.
A pair of these small shrikes has been heard by Dr. W. H. Thorpe of Cambridge University to sing as many as seventeen different duets on the same day. They use the orthodox western diatonic scale in at least five keys and know each other's parts. When the male shrike is away, the female will sing the complete duet by herself. But hearing her sing "his" part, even from a distance, often is enough to bring him flying back to reclaim his place in what seems to be a musical as well as a sexual marriage and which, should an outsider participate, can arouse musical jealousy, although trusted friends usually are more than musically welcome.