Vocal sac

The vocal sac is the flexible membrane of skin possessed by most male frogs and toads. The purpose of the vocal sac is usually as an amplification of their mating or advertisement call. The presence or development of the vocal sac is one way of externally determining the sex of a frog or toad in many species.

The vocal sac is open to the mouth cavity of the frog, with two slits on either side of the tongue. To call, the frog first inflates its lungs as usual. Then, before expelling the air from its lungs, it shuts its nose and mouth. When it does expel the air through the larynx and into the vocal sac, the vibrations of the larynx emit a sound which resonates on the elastic membrane of the vocal sac. The resonance causes the sound to be amplified and allows the call to carry further. Muscles within the body wall force the air back and forth between the lungs and vocal sac.