Ears (comparative anatomy)

Sound is vibration in a gas, liquid, or solid medium capable of being perceived by an ear when activating an auditory mechanism. For land-dwelling animals, the medium is air, although some creatures also hear surface vibration transmitted through their skeletons. Aquatic life hears vibrations propagated through water, but sea mammals also perceive airborne sounds.

Due to the variety of structures and physical operating principles that exist among animals to detect sound, the ear will be defined, by its functional specialization, as an organ developed primarily for the purpose of hearing, with the reception of sound as its principal function. Although the primitive ears found in many lower life forms only faintly resemble the complicated hearing organs of higher vertebrates, they may be justifiably termed “ears” if the mechanism evolved to allow sound perception.

Invertebrate Ears

Only a few of the millions of insect species are known to possess organs capable of detecting sound. These include grasshoppers and crickets, cicadas, the waterboatman, moths, and mosquitoes. The ears of invertebrates are located on various parts of their bodies. Crickets and katydids have ears on the first walking legs; grasshoppers’ ears are on the first segment of the abdomen, while the waterboatman hears through its first thorax segment. Moths have uncomplicated ears located either on the first segment of the abdomen or the rear part of the thorax; mosquitoes hear through sensors associated with their antennae. Insects can produce and perceive a variety of different highly species-specific sounds for communication and mating. Although not sensitive to pitch, information is conveyed by changes in intensity, duration, and sound patterns.

One of several different structures may serve as the hearing organ of insects: tympanal membranes, cercal organs, or antennae. The auditory system of grasshoppers and crickets is anatomically connected to the respiratory system, which conducts air from openings in the thorax to the muscles of the legs. Tympana, very thin membranes located on the forelegs or on the abdomen, are found on the body surface of a respiratory tube. Impinging sound waves cause the tympana to flex, which in turn induces tension changes in the attached scolophores, highly specialized sensory structures that transmit nerve impulses to the central nervous system. Moths have simple tympanal organs containing only two or four scolophores, while the highly developed auditory mechanism of cicadas may contain over a thousand scolophores. The grasshopper ear, with about one hundred scolophores, has a tympanic membrane hidden beneath the base of the wing cover.

Roaches and certain crickets respond to a wide range of sound frequencies by means of cercal organs located at the tip of the abdomen. The mosquito ear, located at the base of the antenna where an expanded sac contains many scolophores, is stimulated when the antenna shaft vibrates. The stimulation is greatest when the antenna is pointed toward the source of sound, which enables mosquitoes to determine the direction of sounds. For male mosquitoes, the frequency region which responds with the greatest intensity is the same as the hum of the female’s vibrating wings, which enables him to find her when mating.

The bodies of spiders contain many slitlike openings, called lyriform organs, one of which (located on the next to last segment of each of the eight legs) is close to the joint between this segment and the last leg segment (tarsus). The tarsus is the sensing element which transmits vibrations to the lyriform organ. The small leg segments respond to the changing velocity of oscillating air particles, enabling some species to hear over a frequency range from 20 hertz to 45,000 hertz.

Fish and Amphibians

Although fish can detect very low frequency compression waves as bodily vibration, this is not considered hearing, since no ear is employed. For ears, diverse species utilize different mechanisms, either singly or in combination. These include the lateral line organ, otolith detectors, and the swim bladder. The lateral line organ is a line of sensory hair detectors, spaced along each side of the fish, that responds to water flow. These sensors are inefficient acoustic detectors because sound waves produce little relative motion between the fish and its environment, but the organ’s extension along the body suggests that it locates the direction of a sound. Primitive fish, such as sharks, react to sounds in water with two mechanisms—macular organs, which give information about the orientation of the head, and the lateral-line apparatus.

It is in the bony fishes that an organ of hearing first appears among the vertebrates. Because fish flesh has a density close to that of water, however, sound detection becomes problematic; the waves tend to pass right through the fish’s body. Consequently, accessories to hearing, such as the otolith detector, developed to make the fish hear more effectively. The dense minerals of this detector respond sluggishly to incoming waves, resulting in an out-of-phase vibration with the surrounding tissue; the relative motion stimulates the hair cells, enabling fish to hear, albeit inefficiently. In some species, the efficiency is increased by means of an air sac, a discontinuity whose surface oscillation, responding to incoming vibration, is detected by special receptors. The frequency of greatest sensitivity for most fish species is about 350 hertz, with an upper limit between 1000 and 3000 hertz.

Frogs have no visible external ears, but they possess an otic notch on either side of the skull, which houses the eardrum, and a well-developed middle ear structure. The eardrum consists of a disk of skin-covered cartilage connected to the oval window by a bony rod. When the eardrum vibrates, this rod transmits the vibration to the fluids of the inner ear, which in turn stimulates sensory hair cells similar to those found in other vertebrates. Experiments indicate that bullfrogs are capable of hearing frequencies between 50 and 3,500 hertz.

Salamanders, lacking the otic notch, eardrum, and middle-ear cavity of frogs, transmit sound through the forelimb and shoulder blade to a muscle that passes from the shoulder blade to a bony structure connected to the inner ear.

Reptiles

Lizards are the lowest form of vertebrate possessing cochlea in which different regions respond to different frequencies of sound, thus enabling pitch differentiation. Studies have shown that most lizards have good auditory sensitivity over a range from 100 to 4,000 hertz. As with frogs, a lizard has no visible outer ear, but possesses a tympanic membrane connected to the oval window of the inner ear through a two-part ossicular chain. This chain consists of a bony part ending in a stirrup at the oval window and a cartilage structure embedded into the tympanic membrane. Although the inner ear contains a basilar membrane and hair cells which are stimulated when the fluid of the inner ear vibrates, the form of the basilar membrane is different from that of frogs. Among lizards, hair cell stimulation by two or more different arrangements within the same cochlea is not uncommon. One method provides greater sensitivity, while the parallel system is more resistant to possible damage from very loud sounds.

Although snakes evolved from primitive lizards, the fact that they show no external ear and their seeming indifference to aerial sound has led to the supposition that they are deaf and only perceive vibrations transmitted through the ground. Actually, snakes can perceive aerial sounds through a device consisting of a thin bony plate, detached from the skull but held in place by ligaments, which vibrates in response to sound waves. A bony structure transmits this vibration through a stirruplike plate to the oval window. Because this mechanism is somewhat inefficient, snakes can perceive only low-frequency tones lying in the range from about 100 to 700 hertz. By the relative intensity of sound in each ear, snakes can localize the direction of a sound.

The turtle’s ear is not a degenerate organ as is sometimes assumed; its hearing acuity is quite good in the low-frequency range. A plate of cartilage on each side of the head serves as a tympanic membrane. The ossicular chain consists of a two-element bony structure, ending in a stirrup which covers the oval window. As the stirrup moves inward and outward in response to a sound wave, it causes the fluid in the otic capsule (the inner ear) to move back and forth, thus activating the hair cells. This rather bulky mechanism is quite effective at low frequencies, but the sensitivity to sound decreases rapidly as frequency increases. Experiments indicate that the turtle’s ear can respond to aerial sounds having frequencies between 100 and 1,200 hertz, with the greatest sensitivity for tones below 500 hertz.

Crocodile and alligator ears, while obviously reptilian, have several unusual features. A short, external passageway with a closeable earlid leads to a tympanic membrane on the surface of the middle ear. The left and right middle ear cavities are connected by an internal air channel, enabling sound entering one ear to reach the other. A typical reptilian ossicular chain connects the tympanic membrane to the oval window at the entrance to the otic capsule. The inner ear is highly developed, containing approximately eleven thousand sensory hair cells (seven times as many as found in the most sophisticated lizard ear). Studies indicate that crocodilian ears respond to frequencies between fifteen and twenty thousand hertz.

Birds and Mammals

Avian ears are similar to reptilian ears, but the longer and more sensitive cochlea gives birds an enhanced ability to discriminate pitch. The outer ear consists of a short tube, with a muscle to partially close the opening. The eardrum of songbirds consists of two separate membranes, an outer (which protects the inner), and an inner membrane attached to the ossicular chain. As in lizards, the ossicular chain consists of a cartilaginous structure at the eardrum connected to a bony column ending in a stirrup. The inner ear is similar to that of crocodiles, with a basilar membrane enclosed within the cochlea. Studies of the hearing of small birds indicate a hearing range from about 100 hertz to above 12,000 hertz, with the greatest sensitivity in the low and middle frequencies. It is believed that certain species use echolocation when flying in the dark. Owls are known to locate prey solely by means of auditory cues.

The ear of mammals is typically composed of three parts: an outer ear, a middle ear, and an inner ear. The outer ear is the visible portion (pinna) and ear canal, which terminates in the tympanic membrane, or eardrum. The middle ear is a small chamber containing three small bones (auditory ossicles), the hammer, anvil, and stirrup. The hammer is attached to the eardrum and transmits vibration through the anvil to the stirrup. The stirrup’s footplate covers the oval window—the entrance to the inner ear. The inner ear (cochlea), resembling a snail’s shell, is a fluid-filled chamber which transforms mechanical vibrations into nerve impulses, which are sent to the brain. The cochlea is divided lengthwise into two compartments, separated by a slightly flexible fluid-filled duct. On the duct’s lower surface is the basilar membrane, containing thousands of hair cells, which create nerve impulses when stimulated. Aerial sounds cause the tympanic membrane to vibrate; this vibration is transmitted to the fluid of the inner ear by the ossicular chain. The vibrating cochlear fluid induces traveling waves on the basilar membrane, flexing the hair cells, causing them to send nerve impulses to the brain. The brain decodes this information as sound; the frequency information is encoded by the place along the basilar membrane responding to the vibration, while the loudness is proportional to the number of nerve firings per second.

The sensitivity and hearing range vary among species, as they are dependent upon the size and mass of the moving parts. Dogs and cats can perceive frequencies as high as 40,000 hertz, and bats can detect sounds above 150,000 hertz. Adult human ears are capable of perceiving sound waves having frequencies between 16 hertz and 16,000 hertz; chimpanzees and monkeys can hear frequencies above 30,000 hertz. The delicacy and precision of pitch discrimination, dependent upon the number and distribution of hair cells along the basilar membrane, are most highly developed in primates.

In the course of adapting to the sea, marine mammals have eliminated or greatly reduced the size of their pinna and acquired a movable flap to close the outer ear when diving. Seals have remarkably acute underwater hearing, responding to frequencies up to 160,000 hertz, without sacrificing the ability to hear airborne sounds. The whale, however, has sacrificed aerial hearing for increased underwater discrimination. The external ear opening is a pinhole, the eardrum serves no useful purpose, and the bones of the middle ear are too massive to respond well to high frequencies. Underwater sounds pass through the tissues of the head directly to the middle and inner ear, the fat-filled cavity of the lower jaw helping funnel low-frequency sounds. The ossicle mass functions like the otolithic organ of fish; inertia causes the ossicles to vibrate out of phase with the surrounding tissues when sound energy is present. This relative motion is conducted to the cochlear fluid through the stirrup and excites the hair cells of the inner ear. The middle and inner ear are housed within a heavy solid complex, which is not connected directly to the skull but supported by air sacs connecting to the respiratory tract. These sacs acoustically isolate each ear from the other, and isolate both ears from sounds produced by the animal itself. Because they use echolocation, cetaceans have acute hearing and can perceive sounds up to at least 100,000 hertz; the bottle-nosed dolphin can hear to about 150,000 hertz.

Principal Terms

Basilar Membrane: The flexible partition connected to the cochlea along which are attached neural hair cells

Cercal Organs: Tufts of hair supplied with nerves, located on insects’ abdomens, which respond to aerial sounds

Cochlea: The vertebrate neural organ which transduces sound waves into nerve impulses

Ossicles: The small bones of the middle ear

Otolith: A dense mineral frame supported by sensory hair cells immersed in aqueous fluid; used in the auditory system of lower animals to detect acceleration

Tympanic Membrane: Eardrum or other surface which serves the purpose of converting sound waves into mechanical vibration

Bibliography

Fletcher, Neville. Acoustic Systems in Biology. Oxford University Press, 2014.

Michelsen, A. “Environmental Aspects of Sound Communication in Insects.” In Acoustic and Vibrational Communication in Insects, edited by K. Kalmring and N. Elsner. Paul Parey, 1985.

Stebbins, W. C. The Acoustic Sense of Animals. Reprint. Harvard University Press, 2014.

Strong, William, and George Plitnik. Music, Speech, Audio. 4th ed. Soundprint, 2013.

Ward, Hannah. "10 Animals With The Best Hearing." A-Z Animals, 1 May 2023, a-z-animals.com/blog/10-animals-with-the-best-hearing. Accessed 4 July 2023.

Wever, E. G. The Amphibian Ear. Princeton University Press, 2014.

Wiley, R. H., and D. G. Richards. “Adaptations for Acoustic Communication in Birds: Sound Transmission and Signal Detection.” In Acoustic Communication in Birds, edited by D. E. Kroodsma and E. H. Miller. Academic Press, 1982.