©2002 Melissa Kaplan
Brief Overview of Ear Structures and Function...Simplified. Really.
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).
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.
that was all very interesting, but what do they really hear?
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.
Reptiles Communicate Other Than Behaviorally?
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.
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.
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.
Kaplan, Melissa. 1994. Did you hear the one about...?
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
Bellairs, A. The life of reptiles. 2 vol. Universe Books, New York. 1970.
Wever, E. The reptile ear: Its structure and function. Princeton University Press, Princeton. 1978.
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