Why does a frog croak?

A male frog can attract a mate by using a mating call. Not only do frogs have vocal cords (like us), but most also have a vocal sac, which is an inflatable membrane that acts like an amp. The frog breathes in, closes its nostrils and forces air back and forth between the lungs and the vocal sac, which vibrates the vocal cords. The air resonating in side the vocal sac amplifies he call