A vocal sac is a flexible membrane of skin that most male frogs possess, usually to amplify mating or advertisement calls. The presence or development of the vocal sac evinces the sex of many frog 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 inflates its lungs and shuts its nose and mouth.[clarification needed] Air is then expelled from the lungs, through the larynx, and into the vocal sac. The vibrations of the larynx emits a sound, which resonates within 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.
The development of the vocal sac differs among most species is mostly similar. The development of the unilobular vocal sac begins with two small growths on the floor of the mouth. They grow until they form two small pouches, which expand until they meet in the centre of the mouth and form one large cavity, which grows until it is fully developed.
The vocal sac amplifies the advertisement call of the male to attract females from larger area. Species of frog without vocal sacs may only be heard within a radius of a few metres, whereas some species with vocal sacs can be heard over 1 km (0.62 mi) away. Modern frog species (Neobatrachians and some Mesobatrachians) which lack vocal sacs tend to inhabit areas close to flowing water. The sound of the flowing water overpowers the advertisement call, so they must advertise by other means.
An alternative use of the vocal sac is employed by the frogs of the Rhinodermatidae family. The males of the two species of this family will scoop recently hatched tadpoles into their mouth, where they will move into the vocal sac. The tadpoles of Darwin's Frog (Rhinoderma darwinii) will remain in the vocal sac until metamorphosis, whereas the Chile Darwin's Frog (Rhinoderma rufum) will transport the tadpoles to a water source.
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