Hearing in birds
- Key People:
- Georg von Békésy
The avian auditory structure
Ears of birds show considerable uniformity in general structure and are similar in many respects to those of reptiles. The outer ear consists of a short external passage, or meatus, ordinarily hidden under the feathers at the side of the head. Most birds have a muscle in the skin around the meatus that can partially or completely close the opening.
The tympanic membrane bulges outward as in most lizards. In the songbirds, however, it consists of two separate membranes, with the outer one apparently serving to protect the inner one from injury. From the inner surface of the tympanic membrane an ossicular chain transmits vibrations of the cochlea. As in lizards, the chain consists of an osseous inner element, the columella, and a cartilaginous extracolumella that extends the columella peripherally and connects with the tympanic membrane.
The cochlea of birds is similar to that of crocodiles, consisting of a short, slightly curved bony tube within which lies the basilar membrane with its sensory structures. The length of the basilar membrane varies between 2.5 and 4.5 millimetres (0.1 and 0.2 inch) in most birds, but in the owls it may reach 10 millimetres (0.4 inch) or more. At the end of the cochlea is another ending with a different function, the lagena and its macula.
Auditory sensitivity in birds
Using the conditioned-response method to study auditory sensitivity in a small songbird, the bullfinch, responses over a frequency range from 100 to 12,800 hertz have been observed. The electrophysiological method was first applied to the study of hearing in birds in 1936. In this study impulses from the cochlea of pigeons were recorded for tones usually up to 10,000 hertz and occasionally as high as 11,500 hertz. Although this method has been used since 1936, few detailed and quantitative results have been obtained; nevertheless, one striking characteristic revealed by these studies has been the high degree of sensitivity in the low and middle range and the very rapid decrease in the high tones.
Uses of hearing in birds
Like other animals, birds use hearing to warn them of enemies and other kinds of danger. To a degree hardly equalled in lower species, they also use hearing in social relations and communication. Many male birds sing to hold their territories and to attract mates. Some birds also use vocalizations to identify their mates or group members. During the breeding period of the emperor penguin, for example, the male leaves his mate for a journey taking many days in order to obtain food. Upon returning to the general area where his mate has remained with a pack of hundreds of birds, the male is able to locate and to recognize his partner by an interchange of calls.
There is good reason to believe that certain birds, including the swiftlets (Collocalia) of Asia and Australia, the oilbirds (Steatornis) of tropical America, and possibly a few others, are able to use echolocation when flying in the dark caves that they inhabit. Moreover, it is well established that many owls locate and catch their prey by auditory cues. On a dark night, an owl perched in a tree can hear the rustling sounds made by a mouse in the grass and leaves on the ground below; by accurately localizing this signal, he can make his strike and capture the prey without any visual aid.
Hearing in mammals
Auditory structure of mammals
In the mammals the ear reaches its highest level of development, with well-differentiated divisions of outer ear, middle ear, and inner ear. Except in some of the sea mammals, in which certain modifications and degenerations have taken place, these structures carry out their functions in a remarkably regular manner.
The outer ear consists of pinna (or auricle) located behind the ear opening and partially enclosing it and an auditory meatus that leads inward. The pinna varies greatly in size relative to the size of the animal, being large enough in many species to serve a useful purpose in the collection and reflection of sounds. Many mammals can move the pinna back and forth to regulate in some degree the entrance of sounds to the auditory meatus, which transmits the sounds inward to the tympanic membranes. In some mammals, such as many of the marine types, the external opening can be closed to keep out water when the animal dives, and in certain species of bats the tube itself contains a valve that can be closed to protect the ear against undesirable sounds.
The middle ear of mammals consists of a tympanic membrane, an ossicular chain of three elements, and two tympanic muscles. The tympanic membrane bulges inward, unlike the usually outward-bulging membrane of reptiles and birds. The elements in the ossicular chain are the malleus (hammer), incus (anvil), and stapes (stirrup), so named because of the resemblance of the bones to these objects. The malleus is attached to and partly embedded in the fibrous layer of the inner surface of the tympanic membrane. It connects to the incus, which connects in turn to the stapes, the footplate of which lies in the oval window of the cochlea.
One tympanic muscle extends from an attachment to the skull to an insertion on the malleus. Another muscle has its insertion on the neck of the stapes. By their contractions, both muscles add friction and stiffness to the ossicular chain, thereby reducing its mobility and protecting the inner ear from excessive sounds. The contraction of the muscles is a reflex action and occurs in both ears at the same time in response to loud sounds.
The inner ear is called the cochlea because in humans this structure is a complex tube coiled into about 2.5 turns, thus bearing some resemblance to a snail’s shell, from which the term is derived. The name cochlea has now been extended to include the auditory portion of the labyrinth in all animals, even when the structure is not coiled, as in reptiles, birds, and egg-laying mammals. In the mammals in which it is coiled, the number of turns in the cochlea varies with species from a little less than two to as many as four. The guinea pig and its relatives have the largest number of cochlear turns. Extending along the inside of this coiled passage is the basilar membrane, bearing on its surface the sensory structure known as the organ of Corti, which contains the hair cells.
In mammals a uniform system is employed in the stimulation of the hair cells by sounds. A relatively thick tectorial membrane, anchored securely on one edge to the supporting structure (the limbus), lies with its free portion over the hair cells and with the cilia of these cells firmly attached to the lower surface of this portion. When vibratory movements of the basilar membrane cause the bodies of the hair cells to move, the tips of the cilia are restrained by their attachments to the tectorial membrane. Hence the relative motion between the bodies and cilia of the hair cells stimulates them.
The sizes, shapes, and spatial relations of many otic structures vary in the different mammalian species, but it is thought that the same basic principles of operation are involved. This uniformity contrasts with their situation in reptiles, in which different systems are present both in different species and sometimes within one ear.
A number of features are of particular significance in determining the sensitivity and frequency range, which vary with species. Because large masses involve great resistances when moved at high frequencies, the size and mass of the moving parts determine to some degree the variations of sensitivity with frequency and the frequency limits within which the ear operates. The ossicular chain is a mechanical lever, and its lever ratio and the difference in area between the tympanic membrane and the stapedial footplate determine the efficiency of sound transmission from air to the cochlear fluid. The mechanical characteristics of the cochlea and the degree of variation of these characteristics along its extent determine the frequency range of hearing and the degree to which different tones can produce different response patterns. Finally, the numbers and distribution of hair cells along the basilar membrane and the density and specificity of innervation of these cells determine the delicacy and precision with which their periodic activity and spatial patterns are registered by the central areas of the auditory nervous system.
These anatomical features have been studied in detail in a few animals—among the mammals, mainly in cats, guinea pigs, and to a lesser degree in humans. The functional aspects, as shown in responses to sounds and to discriminations among different sounds, have been considered principally in humans and to a much more limited extent in other mammals. Some of the auditory characteristics of mammals below humans are described in the sections that follow.
Hearing in subhuman mammals
Primates
The hearing of other species in the division of mammals to which humankind belongs has always been of special interest. A number of species have been studied, including monkeys, marmosets, and chimpanzees among the primates considered as the most advanced, the anthropoids; and tree shrews, lemurs, and lorises among the more primitive.
By using a variety of training methods with chimpanzees, monkeys, and marmosets, behavioral thresholds have been recorded in response to sounds of different intensities and frequencies. When compared with each other and with humans, it has been found that the hearing sensitivity of these animals and humans is remarkably similar over a range of frequencies from 100 to 5,000 hertz, after which the sensitivity begins to differ. The differences observed at the higher frequencies, however, may be partly attributed to variations in experimental procedures. Thus, the results for the chimpanzee stop at 8,192 hertz because this was the highest tone used in the tests. Other observations have shown that chimpanzees can hear tones up to about 33,000 hertz and that young human subjects often hear tones as high as 24,000 hertz. It is also evident that monkeys and marmosets of the species studied can hear still higher tones.
Common laboratory animals
Certain mammals have long been favourite subjects for various kinds of biological studies in the laboratory, largely because of their convenient size, hardiness under caged conditions, and gentle temperament. Familiar among these are cats, dogs, guinea pigs, rats, mice, rabbits, and, more recently, hamsters, chinchillas, and gerbils. Auditory sensitivity functions have been obtained in these animals by a variety of behavioral and electrophysiological methods.
When measured behaviorally by conditioned responses and then plotted on a curve, the auditory threshold sensitivity of cats, guinea pigs, and chinchillas is much the same—a progressive improvement in sensitivity as the frequency is raised until the middle tones (about 500 to 5,000 hertz) are reached, at which point sensitivity tends to remain the same, and then shows a rapid loss in the upper frequencies. There are differences, however, in the maximum sensitivity attained in the middle region, with the guinea pig the least sensitive and the cat the most sensitive of the three species.
Sensory responses in the cochlea of mammals have been measured electrophysiologically by placing an electrode on the round window membrane. Unlike behavioral curves, however, the curves obtained by plotting the sound required to produce an arbitrary amount of electrical potential of the cochlea do not represent auditory thresholds. Instead, their usefulness is largely in their shapes, which indicate in a relative way the regions of good and poor sensitivity. In addition, these curves represent the performance of the peripheral portion of the auditory mechanism up to the point at which the sound stimulus activates the sensory hair cells in which the potentials are generated. Hence, unlike the curves obtained by behavioral responses, those obtained by cochlear potential methods do not indicate the performance of the central auditory nervous system (the nerve connections between the ear and brain and those parts of the brain in which neural impulses from the ear are processed to produce behavioral responses).
In the simpler animals, the two types of curves are much alike, judging from the very limited evidence available. In mammals, however, the behavioral curves differ from the cochlear potential curves in three ways. In the behavioral curves there is (1) an exaggerated gain in sensitivity to tones of low frequency, (2) a greater sensitivity to the medium-high tones, and (3) a more rapid loss of sensitivity to the extreme-high tones and a lower frequency of the upper limit. These differences are believed to arise mainly through the elaborate neural processing that takes place in the more highly developed mammalian nervous system, a processing that improves the sensitivitity to high-frequency tones but reaches a limit of effectiveness and finally fails above some frequency limit. With these conditions in mind, the electrophysiological curves can be used to predict reasonably well an animal’s behavioral responses to sound waves.
Large mammals
Because most of the mammals in which hearing has been studied by laboratory methods are small, much less is known about the auditory capabilities of large ones, even of such domesticated animals as horses and cows. Nevertheless, it is usually assumed that the auditory capabilities of these animals are much like those of humans. At least they hear sounds in the human vocal range, because they seem to respond to verbal signals. Elephants, for example, trained as working animals, are said to obey as many as 30 different commands. A number of wild animals of medium and large size—raccoons, opossums, and several members of the cat and dog families—have been studied electrophysiologically by the cochlear-response method. Their sensitivity curves are fairly similar in form and in the upper limits attained.
Marine mammals
Of special interest are the sea mammals, which have been derived from early land species and which have undergone certain changes in order to adapt themselves to at least a partially aquatic existence. In the course of adapting to marine conditions, however, some sea mammals, such as seals and sea lions, seem to have made only limited alterations in their ear structures. In addition to being able to close the meatus when diving, their pinnas have been greatly reduced or essentially lost, a feature of streamlining for rapid progress through the water.
There are three possible ways that the hearing of marine mammals might be adapted to an aquatic environment: (1) unchanged aerial hearing, with no aquatic adaptation, (2) conversion to an aquatic type of hearing with loss of good hearing for aerial sounds, and (3) development of some kind of double system, with at least serviceable reception of both aerial and aquatic vibrations. In a study of hearing in the common seal, in which responses to aerial and aquatic stimuli were compared, it was found that this animal has a greater sensitivity to aquatic sounds, especially in the upper frequencies, which extended to the remarkably high frequency of 160,000 hertz. Yet, although the seal has made an adjustment for hearing in water, it has not sacrificed the quality of its aerial hearing, which remains at an excellent level, especially for one frequency around 2,000 hertz and another around 12,000 hertz. These differences in auditory senstivity suggest that the mechanisms in this animal for aerial and aquatic hearing are somehow different, but no complete explanation of the adaptations has yet been found.
Whales, on the other hand, have converted their ears to a truly aquatic form, apparently with some sacrifice of aerial reception. The study of their ears and hearing has been carried out in only a few species of the toothed whales, which produce sounds and use their ears in the process of echolocation (see next section).
The ear of whales has undergone extensive changes. The pinna is absent and the external ear opening has been reduced to such a minute size, almost a pinhole in some species, that it no longer serves as a path for the entrance of sound. The eardrum, although present in a modified form, seems to serve no useful purpose; it is connected to the malleus only by a ligament, and this connection can be cut without an ensuing loss of sound reception. The usual three ossicles of the middle ear are present, with the footplate of the stapes resting in the oval window. These ossicles are much more massive than the ordinary mammalian ossicles.
It appears that the whale ear has been converted to a true aquatic type, functioning according to principles similar to those found in the ears of fishes, as described earlier. Sound vibrations in the water readily pass through the tissues of the head and reach the deep-lying middle- and inner-ear structures. Probably the ossicles represent an inertial mass in somewhat the same way that the otolithic body does in fishes. Because of their inertia, the ossicles tend to move with smaller amplitudes and in different phase relations when the tissues of the head, including parts of the cochlea, are set in vibration. This difference in relative motion produces an alternating displacement of the cochlear fluid, which is in contact with the footplate of the stapes and which can be set in motion because of the presence of a pocket of gas in the region of the round window. The performance of the whale ear has been measured in an exact manner throughout the frequency range in one species, the bottle-nosed dolphin (Tursiops truncatus). By a conditioned-response method, it has been found that this animal possesses excellent auditory sensitivity that extends well into the high frequencies.
