But the female's voice is not nearly so loud as the male's. A frog produces sound by means of its vocal cords. When a frog forces air from its lungs, the vocal cords vibrate and give off sound. Among many species, the males have a vocal sac, which swells to great size while a call is being made. Species that have a vocal sac produce a much louder call than do similar species that have no sac.
Some species have a vocal sac on each side of the head.
Others have a single sac in the throat region. Frogs that live in regions with cold winters hibernate. Some species hibernate in burrows. Others spend the winter buried in mud at the bottom of a pond or stream, breathing through their skin. During hibernation, a frog lives off materials stored in its body tissues.