A vocal sac is a flexible pouch of skin found in some species of amphibians, particularly in male frogs. This structure is used to amplify their calls during mating season, helping them attract females. When a male frog croaks, air is pushed into the vocal sac, causing it to expand and produce a louder sound. In addition to frogs, some other animals, like certain species of toads, also possess vocal sacs. The size and shape of the vocal sac can vary among species, influencing the pitch and volume of their calls. This adaptation plays a crucial role in their reproductive behavior.