Purpose
The primary purpose of the vocal sac is to amplify the advertisement call of the male, and attract females from as large an area as possible. 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 that 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 metamorphisis, whereas the Chile Darwin's Frog (Rhinoderma rufum) will transport the tadpoles to a water source.
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