Reptile Hearing

Brief Overview of Ear Structures and Function...Simplified. Really.

Anatomy
In reptiles with external ear structures, the tympanic membrane is visible, either nearly contiguous to the surface of the skin (as with iguanids such as the green iguana), or recessed deeper into the head (as with some scincids, such as the blue-tongue skink, and agamids, such as the bearded dragon). The tympanic membrane covers the middle ear cavity. In fact, it is the outer boundary of this cavity which is linked, on its other side to the pharynx and eustachian tube. In general, the inner boundary of the middle ear cavity has two openings. There is a round one, covered by a thin membrane, and, farther back towards the neck, an oval opening which is uncovered. The stapes crosses the middle ear cavity, from the inside of the tympanic membrane, its inner end fitted inside the oval opening. The outer end of the stapes has a cartilage cap which comes into contact with the tympanic membrane. In some reptiles, this cartilage, called the extrastapes, is attached to the quadrate, the primary support of the lower jaw.

Beyond the round and oval openings of the middle ear cavity is the inner ear cavity. Here are located the organs related to balance (the semicircular canals, utricle, and saccule) and hearing (cochlear duct). The cochlear duct and the saccule are both suspended in perilymphatic fluid; the cochlear is also filled with this fluid. The inside of the duct has two specialized regions, the papilla basilaris and the smaller macula lagenae. Both of these areas are actually clusters of sensory cells. These areas also have cilia which are embedded in a membrane within the cochlear duct. These sensory cells give rise to the auditory nerve (the VIIIth cranial nerve).

Function
Airborne vibrations are picked up by the tympanic membrane. Substrate (ground or other conducting surface on which the reptile is in close contact) vibrations are detected by the quadrate. The tympanic membrane or quadrate vibrates, in turn vibrating the extrastapes and thus the stapes. This causes the vibrations to be conducted through the middle ear cavity, through the windows to the fluid-filled inner ear cavity housing the sensitive cochlear duct, whose sensory clusters then transmit the information along the auditory nerve.

This means, of course, that even without a surface or subsurface-mounted tympanic membrane, many "earless" reptiles can indeed "hear", though to varying degrees. The tympanic membrane is absent in many fossorial (burrowing) and semi-fossorial lizards, such as the legless Anniella, as well as in other reptiles, such as the tuatara, amphisbaenians, and, of course, snakes.

There is a great variation in the tympanic membrane and sensitivity of the inner ear amongst those lizards and chelonians with tympanic membranes ("eared"). Morphological variations include the depth of the structures from the surface of the head, the sizes of the structures, thickness of the various membranes, etc. Some eared lizards, as mentioned above, have surface-mounted tympanic membranes. Others have a recessed membrane, rather like the human tympanic membrane is recessed inside the head. Whereas our ears are marked by a rather visible flap of cartilaginous skin which helps conduct vibrations into our ear, other eared reptiles don't have the significant structure as do ours, though some species have angled recesses, or scales that grow farther out from the head just in front (cranially) of the recess, which may serve to channel vibrations or, more likely, protect the recessed membrane further from getting poked by sharp objects such as twigs and claws.

Crocodilians and geckos have a small muscle that is next to or upon the stapes, the stapedius, which may function in the way the mammalian stapedius muscle does: dampening strong vibrations. However, given the number of humans whose hearing has been permanently impaired by listening to loud music, or loud engine or other machinery noise, one should not assume that the stapedius provides full protection against such damage in humans, nor in those reptiles who have this muscle.

In the tuatara, the stapes is longer, coming into contact with the quadrate as well as the hyoid and squamosal. Their middle ear cavity is filled with loose tissue, mostly adipose. Crocodilians, on the other hand, have a complex of bony air-filled passages and a branching eustachian tube. Amphisbaenians show at least two variations of extrastapes-stapes morphology, both connecting more closely with the lower jaw.

In those reptiles lacking the tympanic membrane, what would be the middle-ear cavity is divided, by a bony partition, into two chambers. The extrastapes passes through the outer chamber, into which opens the eustachian tube. The inner chamber is called by different names, depending upon whose skull it is in:

This inner sinus, in turtles and lizards, is filled with perilymphatic fluid; in snakes, the recess is filled with air.

In many reptiles, including turtles, snakes, and amphisbaenians, the round window leading to the inner ear, is missing. Instead, other ways have evolved to dissipate the vibrations in the perilymphatic fluid. In crocodilians, the cochlear duct is elongated and differs in other ways amongst this group.

The cochlear duct in turtles differs from other reptiles in that that the two sensory areas are not as far apart from one another. In studies of the cochlear duct's papilla basilaris macula lagenae, as well as their cilia and nerve fibers, the patterns found are often so significant that they can help trace taxonomic and phylogenetic relationships. Some of the differences point to other functions, such as the enlarged papilla basilaris in those geckos that vocalize, an area that are larger than the same area in their more fossorial cousins. Contrary to this, however, is that fossorial snakes which have the largest papilla basilaris areas.

Okay, that was all very interesting, but what do they really hear?
As with the morphological differences in the ear structures, there is a diversity in the sensitivity of their hearing, in the decibel ranges reptiles can detect - hear. While we don't have data on all species, there is some, gathered from tests which measured the charge on the perilymphatic fluid, recorded indirectly at the round window or directly from the fluid itself. Use of both techniques enable one to quantify the frequency range as well as the amplitude required to evoke the response.

Amphisbaenians
Amphisbaenia manni, like many amphisbaenians, is responsive to low frequencies, below 2,000 Hz, with sensitivity of 50 dB at 1,000Hz. When the extrastapes was severed in amphisbaenians, the airborne sensitivity dropped to 30 dB, but that made no difference on the amphisbaenians ability to detect and respond to groundborne (somatic) vibrations, transmitted though the tissues of the lower jaw. The front tip of the lower jaw is most sensitive. Amphisbaenians, not surprisingly, share some other features of hearing - detecting groundborne vibrations - with snakes. See the section on Snakes below for more information.

Chelonians
In those species studied, they respond to low frequency sounds in the 50-1,500 Hz range, similar to that of crocodilians. Aquatic species studied show some difference from terrestrial species. For example, Clemmys guttata (spotted turtle) shows a peak sensitivity of 4 dB at 80 Hz, while Geochelone carbonaria (red-footed tortoise) exhibits a much lower sensitivity, with a peak of 50 dB at 300 Hz.

Crocodilians
As with chelonians, they respond to low frequency sounds in the 50-1,500 Hz range. They are not restricted to sound vibrations picked up by their ears or even their jaw bone. In addition to this sensory equipment, crocodilians have apical pits on the scales of their face and bodies which are sensitive to vibrations traveling through water. For more information on this, see Adam Britton's Crocodilian Biology Database > Integumentary Sense Organs.

Lizards
Most of the lizards for whom data has been collected show that most hear in the same range as does the green iguana (Iguana iguana), whose picks up sounds in the 500-4,000Hz range, with a peak sensitivity at 700 Hz, equal to about 24 dB. With fossorial forms (such as Holbrookia maculata) (lesser [Northern] earless lizard) and others lacking a tympanic membrane, hearing is limited to lower frequencies and requires louder sounds (stimulation) to be detected. Other eared species, such as Gerrhonotus (alligator lizards) have both high sensitivity over a wider range, while others, such as the Lepidophyma sylvaticum (Madrean tropical night lizard), has the high sensitivity but over a smaller range in the lower frequencies. Gekkonids who vocalize have both high sensitivity and high frequency, up into the 10,000Hz range.

Snakes
When mechanical vibrations are applied to the body, they result activation of the inner ear just as do airborne vibrations detected by the tympanic membrane and extrastapes do in eared reptiles. Responses to groundborne vibrations are low in sensitivity and frequency, in the 50-1,000Hz range; their peak sensitivity is at 200-300 Hz range, superior to cats. Like the crocodilians, and other reptiles with linkages of their inner ear structures to their jaw and other structures in the head and throat, snakes have another way to conduct sound to their ear. Vibrations picked up by mechanoreceptors in the skin of their bellies (and bodies?), and possibly their venter, are transmitted to the quadrate via the spinal nerves and from there into their inner ear structures. In other words, most snakes can hear a person speaking in a normal tone of voice in a quiet room at a distance of about 10 feet (3 m). So, if you think your snakes recognize their names, you are probably right. Researchers debate whether the snake's receptors cannot tell the difference between airborne or groundborne (somatic) stimuli, but that higher level processing could enable the snake to tell if the stimulus was airborne or groundborne.

Tuatara
These earless reptiles show a frequency response from 100-800 Hz, with peak sensitivity at 40 dB at 200Hz.

And this means...?
In comparison, human hearing is in the range of 20-20,000 Hz, with intensity at roughly 120 dB. The approximate threshold of pain is 130 dB, with a rock concert coming in at 130 dB, and hearing damage occurs at >90 dB Normal conversation is between 60-70 dB The typical background noise in a classroom is 20-30 dB A motorcycle going 5 mph is about 100 dB, busy traffic 70 dB, rustling leaves 20 dB, and a human breathing normally is 10 dB.

Groundborne vibration sensitivity has not been well studied in terrestrial or arboreal lizards and chelonians. It would not be surprising to learn that they, too, have some mechanism by which vibrations detected when they are are recumbent on a branch or, in the case of chelonians, on the ground.

Can Reptiles Communicate Other Than Behaviorally?
There are reptilian species who vocalize (other than a rapid expellation of air resulting in a hiss): crocodilians, many gekkonids, and chelonians. There is some evidence that some (or possibly all) true chameleons produce very low-wave sounds that may be used to communicate. In crocodilians and chelonians, vocalizations are part of the courtship and/or mating. Crocodilians have a wide range of other vocalizations, as well (listen to vocalizations at Adam Britton's Crocodile Talk site). Gekkonid vocalization has not been well studied, but indications are that, besides alarm calls, some species may play a roll in territoriality and social groupings, similar to the use of vocalizations in some "higher" species.

It was not all that long ago that researchers figured out that elephants communicate with each other - often over incredible distances - in frequencies undetected by human ears. To assume that other animals aren't communicating just because we can't hear them would be foolish. So, too, would be assuming that animals can't hear us, or our televisions and stereos when they are cranked up.

Health Concerns
Since the eustachian tube connects the outer ear structure with the inner ear cavity, sinus or recess, and from there to the pharynx, there is risk of pathogens getting in there that shouldn't. Infections of the eustachian tube, inflammation of the cochlear duct, and infection of the oral mucosa can all result from such infections. Since the inner ear also contains the structures helping to maintain balance, ear and eustachian infections can cause loss of balance or the inability to right oneself.

The most common causes of such infections seem to be related to prolonged periods of suboptimal care - inappropriate temperatures and other care, and malnutrition - leading to a compromised immune system unable any longer to fend off infection. Another source of abscessing may be due to the accumulation of shed squamous cells that collect and form plugs or other blockages in the cavities. Tympanic membranes may be punctured, accidentally as the lizard or chelonian moves through its environment. Large lizards, such as iguanas, may be hooked by an untrimmed claw, their own or belonging to a cagemate, or the family cat. Cats and other household pets may get ahold of the reptile, causing injury to the head. Left untreated, the wounds could become infected.

While humans who have ear infections for the most part go on about their daily business, we cannot be so cavalier about such infections in our reptiles. Along with getting them checked and the necessary treatment initiated by a reptile vet, we needed to assess the reptile's captive setup to make sure we identify any problems and rectify them immediately so as to enable the sick reptile to recover at all possible speed.

Cool Stuff
When you have some time on your hands, or even if you don't, put a green iguana's head in between you and a bright light, then look into the tympanic membrane. You will see some movement in there as the iguana breaths and moves its lower jaw.

In lizards with tympanic membranes, there is a layer of skin covering the membranes which shed when the body sheds. In lizards with recessed membranes, when the skin on the membrane and surrounding walls of the recess come off in one piece, it's like a little skin cup.

Crocodilians (alligators, crocodiles, caiman, gharial) are the only reptiles with an outer ear that moves. A mobile flap of skin allows the crocodilians to close their external ears to a thin slit when they are under water.

While this article is really about reptiles, amphibians have some cool adaptations, too. The first known vertebrate to send sound though the air, they needed some good receiving apparatus as well as a strong transmitter. Frogs and toads have well developed ears. In some species, the lower frequencies are transmitted to the inner ear through the forelegs, while the higher frequencies are picked up and transmitted by the tympanic membrane. Larvae and aquatic adults have a lateral sensory line that detects water movement.

Murray, Michael J. 1997. Aural Abscesses. In, In, Reptile Medicine & Surgery, pp. 349-352. Douglas Mader DVM, editor. WB Saunders, NY.

Young, Bruce A. 1997. Hearing, taste, tactile reception, and olfaction. In, The Biology, Husbandry and Health Care of Reptiles, Vol I, pp 185-213. Lowell Ackerman DVM, editor. T.F.H. Publishing, Neptune City NJ.

Wright, Kevin M. 1997. Amphibian husbandry and medicine. In, Reptile Medicine & Surgery, pg. 440. Douglas Mader DVM, editor. WB Saunders, NY.

For those desirous of further research, Young cites, amongst the 214 or so references he at the end of his chapter, three in particular in regards to the structure and function of the reptilian ear:

Baird, I. The anatomy of the Reptilian ear. In, Biology of the Reptilia, Gans, C.; Parsons, T (Eds.) Academic Press, New York, NY. 1970, pp. 193-275

Wever, E. The reptile ear: Its structure and function. Princeton University Press, Princeton. 1978.

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