Heterogeneity of vocal sac inflation patterns in Odorrana tormota plays a role in call diversity.

Male concave-eared torrent frogs (Odorrana tormota) can emit at least eight distinct call-types. However, the mechanisms by which they are produced are not fully understood. Anatomical analysis revealed that the vocal sacs of male O. tormota comprise two physically distinct compartments (pars lateralis and pars ventralis), residing on two sides of the vocal slits. The goal of the present study was to test the hypothesis that the two compartments play a role in the production of the diverse call-types. For this, audio and video recordings of male vocalizations were made, and sounds were analyzed afterwards. Results showed that the vocal sac inflation pattern was heterogeneous, and the call duration was a major factor determining the differential inflation patterns. Short call-types (duration <200 ms) involved inflation of one of the compartments only; those having a fundamental frequency (F0) of >5000 Hz involved inflation of pars lateralis only, whereas those with an F0 of <4000 Hz (tone-pips and "infant" calls) involved inflation of pars ventralis only. Long call-types (duration >200 ms), e.g., shallow frequency modulation calls, staccato calls, and long calls, involved inflation of both compartments of the vocal sacs. These results give support to the working hypothesis.