Many male frogs amplify mating calls with a vocal sac.
A vocal sac is a flexible membrane connected to the mouth and throat. When a frog calls, air moves back and forth between the lungs, mouth and sac. The sac expands and resonates, making the call louder without requiring the frog to push out extra air each time.
Vocal sacs vary greatly. Some frogs have one central sac, while others have paired sacs on the sides of the throat. Their shape and color can also help females identify calling males. The sound itself usually advertises species identity, sex and location.
A common mix-up is treating the vocal sac as the organ that creates the sound. The larynx produces the basic vibration; the vocal sac mainly acts as a resonating chamber and amplifier. Calling is energetically expensive, so frogs often call from water, leaves or other places that improve sound transmission.