The anatomical basis of amphibious hearing in the American alligator (Alligator mississippiensis).

The different velocities of sound (pressure waves) in air and water make auditory source localization a challenge for amphibious animals. The American alligator (Alligator mississippiensis) has an extracolumellar cartilage that abuts the deep surface of the tympanic membrane, and then expands in size beyond the caudal margin of the tympanum. This extracolumellar expansion is the insertion site for two antagonistic skeletal muscles, the tensor tympani, and the depressor tympani. These muscles function to modulate the tension in the tympanic membrane, presumably as part of the well-developed submergence reflex of Alligator. All crocodilians, including Alligator, have internally coupled ears in which paratympanic sinuses connect the contralateral middle ear cavities. The temporal performance of internally coupled ears is determined, in part, by the tension of the tympanic membrane. Switching between a "tensed" and "relaxed" tympanic membrane may allow Alligator to compensate for the increased velocity of sound underwater and, in this way, use a single auditory map for sound localization in two very different physical environments.