Snakes are remarkably sensitive to most stimuli, but there is one sense that they seem to have almost done away with: hearing. They have no visible means for detecting airborne sound, having lost the tympanum and the external ear. Equipped with only an inner ear linked to the jaw apparatus by a single middle ear bone, the columella auris, it would seem that snakes have little hope of hearing the world as we know it. They would have to rely on sensing vibrations transmitted through the jaw. Yet the debate about whether snakes hear airborne sounds raged, with evidence stacking up on both sides of the argument. Intrigued by the mystery, Christian Christensen, Jakob Christensen-Dalsgaard, Christian Brandt and Peter Madsen decided to find out whether snakes detect sound via sound pressure or sound-induced mechanical vibrations through the body (p. 331).
Playing sounds ranging in pitch from 80 to 1000 Hz at volumes between 50 and 110 dB re. 20 μPa to 11 royal pythons, Christensen recorded electrical responses in one of the snakes' cranial nerves and their brain stems. Increasing the sound volume until he recorded a measurable electrical signal in the brain stem, Christensen found that the snakes could hear very loud airborne sound (10,000 times louder than the softest sounds heard by people). They were also most sensitive to low frequencies between 80 to 160 Hz and their sensitivity decreased at higher frequencies, falling from 78 dB re. 20 μPa at 160 Hz to 96 dB re. 20 μPa at 800 Hz.
But how were the sounds transmitted to the snake's vibration-sensitive inner ear? As low-frequency sounds are efficiently carried by solid materials, the team wondered whether sound vibrations might be transmitted from the ground into the snake's body.
Christensen measured vibrations generated in the surface upon which the snakes were lying by a loudspeaker suspended above the platform. Meanwhile, he recorded the animals' auditory electrical response to the vibrations. He found that the animals responded well to 80 Hz vibrations, but at higher frequencies, the vibrations produced in the surface by the airborne sound were too weak for the snake to respond.
So, how were the snakes able to sense the higher pitched sounds that they hear? ‘Some suggested that they could use the lung as fish use the swim bladder. Also, we humans still hear by bone conduction in water, that would be another way of sending the sound’, says Christensen. So the team decided to test whether the animals could sense their own skulls' vibrating in response to airborne sounds.
Attaching minute vibrometers to the snakes' heads, Christensen measured the mechanical vibrations induced in the head by loud airborne sounds that were just above the snakes' hearing thresholds. He found that these skull vibrations were the same intensity as the minimum mechanical vibrations that the animals could sense. So instead of responding to sound pressure, snakes respond to vibrations transmitted directly from the air to the skeleton.
Having shown that snakes are sensitive to sound-induced vibrations rather than sound pressure, the team is keen to investigate the hearing of other earless animals.