1. Snakes possess two sensory systems which respond to both air-borne sound and infstrate vibration as shown by extracellular slow evoked potentials recorded in the midbrain. One involves the VIII cranial nerve and inner ear (designated ‘auditory system’); the other requires an intact spinal cord and probably originates in skin mechanoreceptors (designated ‘somatic system’).
2. In species of the families Colubridae, Crotalidae, Boidae, the auditory system has a typical U-shaped frequency-threshold curve, similar to those found in other animals, but restricted to a narrow frequency range, c.150 Hz to 600 Hz. The U-shaped curve has a distinct minimum threshold or best frequency. Frequency-threshold curves for sound and head vibration have only minor differences.
3. The auditory system is not remarkably sensitive to sound. It is about 20 dB less sensitive than the human auditory system for air-borne sound between 200 and 400 Hz. It is remarkably sensitive to head vibration: at the best frequency, 1 Å peak-to-peak amplitude is suprathreshold.
4. The auditory system responds not only to stimuli at the head but to sound and vibration delivered to the body alone. Responses to body stimulation by sound are caused by the same sensory end-organ that responds to head vibration. The role of the body in picking up sound for the auditory system can be infstantial; thus the lung plays an important part in snake hearing, a novel situation among land vertebrates.
5. The somatic system is not as sensitive to sound or to vibration as is the auditory system over most of the latter's frequency range. The somatic system has a relatively flat frequency-threshold curve which lacks a distinct best frequency. The frequency range extends both above and below the range of the auditory system, 50-1000 Hz. The somatic system is insensitive to vibration of the head.
6. Evoked slow potentials fail to show that either spinal or auditory system distinguishes between vibratory energy from the air and from the infstrate. It is proposed that intensity information from the auditory system could be compared by the snake's brain with intensity information from the snake's somatic system in order to determine the relative amounts of air-borne sound and infstrate vibration in an unknown natural stimulus.