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Morphological basis of "growling" in the king cobra, Ophiophagus hannah.

A bioacoustic analysis is conducted on the defensive sounds produced by 21 species of snakes. The "typical" snake hiss is described as having a broad-frequency span (from roughly 3,000 to 13,000 Hertz) and a dominant frequency near 7,500 Hertz. The "growl" of the king cobra (Ophiophagus hannah) differs from the "typical" snake hiss in consisting solely of frequencies below 2,500 Hertz, with a dominant frequency near 600 Hertz. Structural analysis of the upper respiratory tract of O. hannah suggests that the "growl" is produced by tracheal diverticula functioning as low-frequency resonating chambers. This hypothesis is supported in several ways. An acoustic analysis of a mechanical model of the trachea demonstrates the potential for these diverticula to produce resonance effects. A "growl" also occurs in the mangrove ratsnake (Gonyosoma oxycephalum), a species that also has tracheal diverticula. Flushing the respiratory tract of G. oxycephalum with helium produces a shift of over 1,000 Hertz in the "growl," a shift that is indicative of a resonance effect.