The tympanum, or ear drum, on a frog transmits sound waves to the inner ear and also prevents water and debris from entering the inner ear. The tympanum on a frog is a large external structure located on either side of the head behind the eye. It is made up of nonglandular skin.
The tympanum of a frog works very much like the ear drum of a human. When sound reaches the tympanum, it causes it to vibrate. These vibrations are then transferred away from the tympanum by a small bone that attaches the tympanum to the oval window of the inner ear. When the vibrations reach the fluid of the inner ear, they stimulate receptors in the ear. These receptors send the information to the brain to process the sound.
The tympanum may also be used to determine the sex of certain frog species. For example, in male bullfrogs, the tympanum is larger than the eye. In contrast, a female bullfrog's tympanum is about the same size as its eyes. Six species of North American frogs display sexual dimorphism with respect to the tympanum. This term simply refers to the difference in the tympanum's appearance between males and females of the same species.