Frogs in Helium: The Anuran Vocal Sac Is Not a Cavity Resonator

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Abstract
A widely cited yet unproven function for anuran vocal sacs is that of cavity resonance and call tuning during vocalization. Because resonant frequency is influenced by the speed of sound within the cavity, we replaced a substantial proportion of the air in the anuran vocal tract with heliox (80% He/20% O₂), a medium in which the speed of sound is approximately 75% faster than in a normal atmosphere. Such a manipulation will increase energy in higher call frequencies, provided that cavity resonance is present. In a heliox atmosphere, no consistent or predictable change in the frequency distribution of sound energy was observed in the advertisement calls of two hylids and one leptodactylid or in the release call of a bufonid. Therefore, the call characteristics of the anurans in this study do not arise from cavity resonance in the vocal sac or other supralaryngeal structures. The anuran vocal sac may have a variety of acoustic and nonacoustic functions, but that of cavity resonator can now be excluded in the species examined here and is unlikely to be found in other anurans.

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