Feb. 21, 2008 -- Using techniques borrowed from nautical engineers, scientists in the United States and Germany have for the first time demonstrated how snakes could hear, despite their lack of external ears and internal eardrums.
The research, which shows that snakes actually have two hearing systems, provides valuable insight into snake evolution.
"It's amazing how little we know about the biology of snakes," said Bruce Young, a biology professor at Washburn University and one of the study authors. "It's nice to work in an area that is so under-explored."
For years it was assumed that snakes couldn't hear, that they sensed prey by smell, taste, and, in some species, special heat-sensing pits near the nose. Basic experiments during the 1970's showed snakes could hear, but didn't explain how.
Now we know.
With each tiny footstep, a mouse or other prey radiates waves through the ground and air the same way drops of water ripple through a pool and produce a single drip sound. Just as a ship bobs up and down in response to a wave in the ocean, a snake jaw resting on the ground responds to sound waves carried by the ground.
"The lower jaw of a snake is essentially a ridged cylinder," said Young. "So in that respect it's not terribly different from a ship."
The researchers used the exact equations that measure a ship's movement to model how a snake's jaw would move in response to waves moving through sand or earth. Just as a ship can move in six different directions (heave, pitch, roll, etc.) so can a snake's jaw (up, down, side to side, etc.).
And just as a ship is more stable the deeper it rides in the water, snakes often bury themselves in sand to make their hearing more precise. Buried, a snake can more easily detect the differences in the way its jaw moves.
After a sound is picked up by a snake's jaw bones, it travels into the cochlea, where nerves pick up the signal and transmit it to the brain. By hearing through their jaw bone and through a traditional ear, snakes essentially evolved a second way to hear, say the researchers.
Humans also have a very crude version of this ability. If you hit a tuning fork lightly and place it in the air next to your ear, the sound will be faint. If you lightly hit the tuning fork again and then place the base against the bone behind your ear, the sound becomes much stronger.
While a human jaw is one complete bone, snakes actually have two jaws, an adaptation that allows them to swallow prey larger than themselves, but also apparently lets them hear in stereo.
Catherine Carr, a biologist at the University of Maryland who was not involved in the research, said that the work "was truly interesting. Transmission through the skull may have been how the first land vertebrates heard."
The work was reported recently in the journal Physical Review Letters.
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