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.
In a recent article in Physical Review Letters* scientists from the Technical University Munich (TUM), Germany, and the Bernstein Center for Computational Neuroscience (BCCN) present 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, i.e., the auditory system, to localize the direction it is coming from. The work was carried out by J. Leo van Hemmen and Paul Friedel, scientists at the Biophysics Department of the TUM and BCCN, together with their colleague Bruce Young from the Biology Department of Washburn University at Topeka (KS, USA).
Any disturbance at a sandy surface leads to vibration waves that radiate away from the source along the surface. These waves behave just like ripples on the surface of a pond after a stone is dropped into the water. The sand waves, however, propagate much quicker (the speed is about 50 meters per second) than at the water surface but on the other hand much more slowly than for instance in stone (or concrete) and the amplitude of the waves may be as small as a couple of thousands of a millimeter.
Yet a snake can detect these small ripples. 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.
Mammals and birds can localize a sound source by comparing the arrival times of sounds that arrive at the right and left ear through air. For sound coming from the right, the right ear will respond a fraction of a second earlier than the left ear. For sound coming from the left, the situation is exactly the other way around. From this time-of-arrival difference, the brain computes the direction that the sound comes from.
Combining approaches from biomechanics and naval engineering with the modeling of neuronal circuits, Friedel and his colleagues have shown that the snake can use its ears to perform the same trick for sound arriving through sand. The left and right side of the lower jaw of a snake are not rigidly coupled. Rather, they are connected by flexible ligaments that enable the snake to stretch its mouth enormously to swallow large prey. Both sides of the jaw can thus move independently, just like two boats floating – so to speak – on a sea of sand, and in this way allow for stereo hearing.
A sand wave originating from the right will stimulate the right side of the lower jaw slightly earlier than the left side, and vice versa. Using a mathematical model, the scientists calculated the vibration response of the jaw to an incoming surface wave. They could show that the small difference in the arrival time of the wave at the right and the left ear is sufficient for the snake’s brain to calculate the direction of the sound source.
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.