Desert snakes hear mouse steps with jaws

Lisa Zyga

Only a few decades ago, some scientists suspected snakes could not hear. Snakes don't have ears and ears outside, so it's hard to find out why this reptile can pick up sound waves.

However, snakes also have inner ears and cochlea, scientists have observed this animal responding to sound stimulation. But accurate information about snakes that can be heard without external ears has not been clarified. In a new study, physicist Paul Friedel and J. Leo van Hemmen of Technische Universitat Munchen University (Germany), along with biologist Bruce Young of Washburn University (Kansas), came up with a model explaining how The horned vipe snake ( Cerastes cerastes ) can be heard with its jaw .

Although the method of hearing thanks to the jaw has been widely known, research has used naval techniques to explain how vibration from the jaw passes through the snake's head and sends sound to the animal's brain.Scientists also explain one of the interesting parts of the jaw-hearing method, which is the right and left side of the jaw can move independently to locate the sound source , such as position. footsteps of the mouse.

Friedel told PhysOrg.com's reporter: 'Until now, no one could think of a thing that snakes could use jaws to listen to stereo sound. However, it is very important to hear the stereo sound to determine the sound source. Therefore, we explained how hearing with jaws brings a lot of information to snakes, and it is not simply a system that says something is there. '

The footsteps of a mouse on desert sand will create surface waves (specifically Raleigh waves) with a wavelength of about 15 centimeters and an amplitude of 1 micrometer. This surface wave is similar to water waves, sand grains forming elliptical motions. The speed of the wave is about 45 meters per second. Its frequency is in the range of 200 to 1000 Hz - falling into the correct sensitivity of the snake (frequencies in the range of 300 Hz).

The horned vipe snake living in the desert is pressing its head to the sand to listen to its prey.A wave on the sand surface vibrates independently on the right and left side of the jaw.This shock reaches the square bone, stapes and inner ear.(Photo: Friedel, et al)

When the horned vipe serpent presses its jaw to the sand, the shock from the mouse's footsteps passes under both sides of the jaw . The tremor passes through the snake's head, through two bones: the square bone and the stapes, and then stimulates the cochlea. Its auditory organ via jaw movement can sense the size of angstrom (one atom). Scientists have determined that the lower jaw amplitude is only about half the amplitude of a surface wave (1 micrometer) - so the amplitude of the wave is quite large for the snake to hear effectively.

From the cochlea, the sound signal is relayed along axon delay lines to a group of neurons arranged according to the terrain on the brain. Scientists have modeled this neural neural network, in which each neuron receives an excitation of up to one millionth of a second for time differences in specific ears, or the difference in time between signals received from the right and left jaws. When the neuron receives information, it is like an input indicator that helps the snake determine the exact location of the prey.

This way of listening has shown that snakes have a different way of listening. And that method is not only real but also an effective skill to survive. As Friedel explains, the method of hearing by jaw brings several advantages over the normal external hearing method.

Friedel said: 'This involves a problem called coordinated impedance. If the sound produced by air comes into contact with the tissue surface, most of the energy will be reflected. This is because a negative impedance (a unit used to determine how easily a sound wave can be emitted) of air is much smaller than the negative impedance of tissue (or inner ear). To solve this problem, the mammalian middle ear possesses three small bones that transmit sound from the eardrum to the inner ear. The snake does not have the middle ear as well as the 3 small bones, but by using the jaw line - square bone - stapes, it avoids the problem of coordination impedance '.