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. Most frogs that live in tropical and semitropical regions breed during the rainy season. In other regions, most species of frogs breed in spring or in early summer. The majority of frogs, including most species that live on land, mate in water.