Sunday, February 17, 2008

How Snakes Hear

It is often believed that snakes cannot hear. This presumption is fed by the fact that snakes lack an outer ear and that scientific evidence of snakes responding to sound is scarce. Snakes do, however, possess an inner ear with a functional cochlea.

Batman and Copperhead © DC Comics
Now scientists have evidence that snakes use this structure to detect minute vibrations of the sand surface that are caused by prey moving. Their ears are sensitive enough to not only “hear” the prey approaching, but also to allow the brain to localize the direction it is coming from.

Any disturbance at a sandy surface leads to vibration waves that radiate away from the source along the surface. A snake can detect these small ripples and if it rests its head on the ground, the two sides of the lower jaw are brought into vibration by the incoming wave. These vibrations are then transmitted directly into the inner ear by means of a chain of bones attached to the lower jaw. This process is comparable to the transmission of auditory signals by the ossicles in the human middle ear. The snake thus literally hears surface vibrations.

The extraordinary flexibility of the lower jaw of snakes has evolved because being able to swallow very large meals is a big advantage if food is in short supply and competition fierce. Moreover, the separation of the sides of the lower jaw also allowed this very interesting form of hearing to develop. press release

Auditory Localization of Ground-Borne Vibrations in Snakes. 2008. P. Friedel et al. Physical Review Letters, 100, 048701.