Snakes can’t hear as they don’t have an ear, it is often believed. But, a new study has found that the reptiles do possess an “inner” ear with a functional cochlea which they use to detect vibrations caused by prey.

A team of international researchers has carried out the study and found that the ears of the snakes are sensitive enough to not only hear the prey approaching, but also to allow the brain to localise the direction it is coming from.

According to the researchers, any disturbance at a sandy surface leads to vibration waves that radiate away from the source along the surface. These waves behave like ripples on the surface of a pond after a stone is dropped into water.

However, these sand waves propagate much quicker (the speed is about 50 metres per second) than at water surface. But, on the other hand much more slowly than for instance in stone and the amplitude of the waves may be as small as a couple of thousands of a millimetre.

“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,” the study’s lead author J Leo Van Hemmen of the Technical University Munich was quoted by the Science Daily as saying.

In their study, combining approaches from biomechanics and naval engineering with the modelling of neuronal circuits, the team has 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 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 and found 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 results of the study have been published in the Physical Review Letters journal.