Snake bioacoustics

Young (2003) reviews the ability of snakes to produce and detect sounds, while leaving unresolved the existence of acoustical communication within any snake species. Relative to other vertebrates, snakes have reduced ear morphology: the external ear is absent, the middle ear contains a single ossicle, and the inner ear lacks the round window. Vibrations in the stapes (the sole remaining middle ear bone) create pressure waves in the perilymphatic fluid of the inner ear, and it is suggested that high frequencies create an impedence-matching problem in the middle ear; however, the function of this vibration transmission system is incompletely understood. Confounding understanding of snake auditory perception is the fact that ear morphology shows no clear phylogenetic or ecological pattern. Cochlear amplification (increasing sensitivity and frequency range by altering populations of hair cells) has not been investigated in snakes. Due to differences in the propagation of sounds through different media, fossorial, aquatic, and arboreal snakes are expected to display different ear structures; no study has yet investigated the responses of snakes to aquatic pressure waves.

Studies of snake auditory perception indicate that sound waves are perceived directly; these vibrations do not transmit to the substrate well because of the impedance mismatch between air and ground. Additionally, because medium absorption is very high for substrate-propagated vibrations, detection distances for ground vibrations are low. Auditory sensitivity for most species occurs in a narrow range of low frequencies (200 to 400 Hz), although snakes also display somatic hearing (the perception of vibrations from the body surface rather than the inner ear), which is characterized by less amplitude sensitivity but a greater frequency range. Interestingly, in a study from 1923, a researcher played pure tones from a telephone to a group of rattlesnakes: “A momentary sounding of the telephone would instantly start the rattling.” Due to the large latency and refractory periods of their auditory neurons, snakes are poorly suited to make time-domain analyses of vibrations or spatial localization of sound through temporal encoding.

Sound production by snakes can occur by tail vibration (with or without a rattle), cloacal popping (compressing and releasing air out of the anal opening, which may create a monopole), scale abrasion (with or without specialized rasping scale projections), hissing (which generally shows no evidence of temporal patterning, harmonics, or amplitude or frequency modulation, although one species a laryngeal septum [“vocal cord”]), or growling (in which tracheal divirticula function as resonators). In rattlesnakes, the rattle functions as a dipole and produces a broadband sound from 2 to 20 kHz (up to 50 kHz in one study). In common with the sound-producing muscles of other vertebrates, the tail muscles of rattlesnakes are very energetically efficient.

Young, B. A. 2003. Snake bioacoustics: Toward a richer understanding of the behavioral ecology of snakes. Quarterly Review of Biology. 78:303-325.