like mammals, anurans possess a larynx and vocal folds, which are used to create vibrations in sound production. however, frogs also use structures called vocal sacs, elastic membranes in the base of the mouth which inflate during sound production. these sacs provide both amplification and fine-tuning of sounds, and also allow air to be pushed back into the lungs during vocalizations. this allows air used in sound production to be recycled, and is thought to have evolved to increase song efficiency. increased efficiency of sound production is important, as some frogs may produce calls lasting for several hours during mating seasons. the new river tree frog ("trachycephalus hadroceps"), for example, spends hours producing up to 38,000 calls in a single night, which is made possible through the efficient recycling of air by the vocal sac.