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Wednesday, April 4, 2012

CAN SNAKES HEAR?

Their poison [is] like the poison of a serpent: [they are] like the deaf adder [that] stoppeth her ear;Which will not hearken to the voice of charmers, charming never so wisely.[Psalms 58:4-5]








Can snakes hear, you ask?

A few decades ago the answer was no, for - obviously - snakes don't have external ears. And any way, snakes don't appear to respond to loud noises. Further support for this view is found in some current zoology texts, which still report that snakes lack the sense of hearing. But research begun about 35 years ago, especially the extensive investigations over many years by E.G. Wever and associates at Princeton University, has shown that snakes have a hearing capability(at least in an electrophysiological sense) comparable to that of lizards.

This should not be too surprising, for snakes and lizards share some common features and are thought to have common ancestors.


So how can a snake hear, lacking external ears? By having equivalent structures on each side of its head. The skin and muscle tissue on each side of the head cover a loosely suspended bone, called the quadrate, which undergoes small displacements in response to airborne sound. The quadrate motion is transferred by intermediate structures to the cochlea, which produces electrical signals on its hair cells that correlate with the airborne sounds (within a range of intensity and frequency determined by the ear system) and are transferred to the brain.


Cochlear signals are present in functioning ears of all classes of vertebrates from fish to mammals, while animals that are congenitally deaf produce no such signals, so their presence in response to sound is taken as an indication of the hearing sense. Wever and co-workers [1] developed techniques to measure the hair-cell signals in lizards, snakes, and amphibians, which involved anesthetizing the specimen, inserting a very thin wire probe into contact with a hair cell, and measuring the acoustic signal level needed to produce a specified hair-cell signal (typically 0.1 microvolt). Various experiments were performed to demonstrate that the hair-cell signals were in direct response to airborne sound and not to mechanical vibrations from the medium on which the specimens were placed.


According to Porter [2], the auditory response of snakes in the range of 200 to 300 Hz is superior to that of cats. Hartline and Campbell [3] investigated the transmission of airborne sound through the snake's skin and lung into the inner ear. Wever's results show that this type of transmission, called the somatic mode, is much reduced compared to that through the skin to the quadrate, which is the main mode of hearing.


How are the cochlear responses to be interpreted? Wever points out that it is often difficult to determine the role of hearing in lower forms such as reptiles. It is possible that snakes make less use of the auditory sense than other animals. He notes that the maximum sensitivity occurs in the frequency range of noise made by movements of large animals, so detection of such sounds could function as a warning to snakes to be motionless, a common defensive action with animals. (Although not discussed in the references I was able to check, there is also the question of how the cochlear signals are used in the snake brain. Is it possible that the ability to process this information has been or is being lost?) So the next time you meet a snake on the Reserve trails, be careful what you say to it, for the snake may hear you.

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