Can snakes hear???

Hi folks,

Over the years, working with many different snakes in the field, and in captivity, I have noticed anecdotal evidence of snakes reacting to sound. Or so it would appear. The most recent of which, was about 2 years ago. I was in the snake room, working, when I noticed one of my indigos out and about from her hide, in her enclosure. I stood still for just a few moments, to observe her. She seemed unaware of my presence until I spoke to her. Apparently startled by the sound of my voice, and the sudden realization that she was not alone, she quickly popped her head high into the air, and then began rapid tongue flicking. I then spoke again, and she ducked her head back down, and shot back into her hide.

The recent post about fish, and amphibians got me thinking about this again. I know beyond any doubt that fish can hear. Having been an avid fisherman, snorkeler, and speer fisher all my life. I have seen countless evidence that fish can definitely hear. And they have no externally visible audio perception organs,...just like snakes. So I'm wondering what others have to say about the possibility of sound or airborne vibration perception in snakes. I know they can feel vibration. And I also know that a great many animals can hear, or perceive sounds which are inaudible to humans. Have any studies been done? Has work been published? What internal anatomical structures, if any, are there, which would suggest an ability to percieve sound?

And lastly, I had an idea. If certain sound frequencies can be determined to be disturbing to snakes, such as indigos, it may be possible to keep them away from dangerous areas, and roadways. If this can be done, it might be possible to coax them to safe tunnels, or culverts under roadways, and link vital habitat in such a way as to link otherwise fragmented habitat, without the dangers of crossing over roads. Well, just a thought. Thanks,
Tony Carlisle.

Reprinted from the web site of the Torrey Pines State Reserve.
As reprinted in The Michigan Herpetologist, the newsletter of the Michigan Society of Herpetologists, September, 2001.
--------------------------------------------------------------------------------
Can snakes hear, you ask? A few decades ago the answer was no, for -- obviously -- snakes don't have external ears. And anyway, 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 electro-physiological 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.

Acknowledgements:
My thanks to R. Haase, research associate with the UCSD Biology Dept., for informative discussions and reviewing this article.

------------------------------------------------------------------
http://www.richland.lib.sc.us/archive/quest1102.htm

Although they do not have external ears and do not hear in the same manner as humans or other reptiles with eardrums, snakes do hear sounds transmitted through the air. According to the book, Snakes in Question, sounds are transmitted to a snake through the skin located over the temporal region of its skull to its jaw muscle and then to its quadrate bone. A snake’s ear bone is adjacent to its quadrate bone and the ear bone picks up the sound and transmits the vibrations to the snake’s inner ear. The inner ear contains cells that are sound-sensitive. Snakes hear low frequency sounds best. For further information on hearing or other senses in snakes, see Snakes in Question.

-----------------------------------------------------------------------

Years ago I found a reference to snakes' ability to hear, but, failing to write it down, could never locate it again. Seigel and Collins, in their Snakes: Ecology and Behavior, were not enormously enlightening: "Although there is meager behavioral evidence for hearing of airborne sounds, there is physiological evidence from several species in six families. In addition, attachment of the quadrate to the inner ear suggests that vibrational stimuli could be transmitted by the jaw. However, vibrational cues could just as likely be detected by tangoreceptors on the venter of the snake" (New York: McGraw-Hill, Inc. p. 121, 1993), which most of us already know ('hearing' by feeling vibrations transmitted up from the ground into the belly muscles).

I was pleased to see a short note about snake hearing ability in this month's The Vivarium (6[3]:24-25). In the "Ask the Experts" column, Winston Card responds to a snake hearing question by explaining that, while snakes lack external and middle ear structures (including the tympanum, or ear drum), they do have inner ear structures which have been shown experimentally to receive airborne sound waves.

Thus, like many other animals, snakes have two ways of detecting sounds: earthborne and airborne. The earthborne vibrations are passed through the belly muscles to special receptors along the spine and thus transmitted to the brain. Airborne sounds are transmitted to the lung from the skin receptors to the eighth cranial nerve and inner ear.

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). Two of my snakes have always responded to my calling their names; it's nice to know I wasn't imagining it!

-----------------------------------------------------------------------

Hearing: Snakes can hear but this sense is not as well-developed as its other senses. Unlike lizards, snakes don't have external ears or even a middle ear. They only have a small bone (columella) which connects the jaw bone (quadrate bone--coloured orange in diagram) to the inner ear canals. These inner ear canals work superbly as the snake moves in three dimensions! A snake picks up sound through the skin which passes on to this jaw bone. Indications are that it is not true that a snake can only hear sounds when its head is on the ground. Snakes can hear airborne sounds, though probably not as acutely as some other animals. There are also indications that the lung may act as a sound receptor.

Hey, That was awsome! Thanks very much for all the info!!!

Now,... how can we go about filling in the blanks left by Wever?
T.

I was thinking about this thread the other day. I was wondering about experiences of others? Anyone else have anything to add?

While the jury is out, as to whether snakes can "hear", I think one highly relevant factor has been missed: Acoustic noise produces vibration in solid objects. The closer the harmonic frequency (of the solid object) is to the acoustic frequency, the more intense the vibration.

An unsung genius named Nikola Tesla proved that it was possible to reduce a several story building to rubble, by finding its acoustic resonance point, and then, intensely generating that sound wavelength inside the building.

I don't know if snakes can really hear, or not. But in order to find out, one would have to rule out the possibility that they are simply reacting to movement associated with the sound (ie lips moving), or solid object vibration.