Frogs rely heavily on sound for communication, especially during breeding season. Their hearing anatomy is unique and differs significantly from the external ears seen on mammals. The specialized structures that allow amphibians to process sound and detect predators take the place of a traditional outer ear.
The Tympanum: Name and Appearance
The structure that serves as a frog’s eardrum is called the tympanum, or tympanic membrane. Unlike a human ear, this feature is not recessed within a canal but lies flush with the surface of the frog’s head, making it highly visible. It appears as a taut, circular, or slightly oval patch of thin skin located just behind the eye. The external position allows the tympanum to receive sound waves effectively both in air and when the animal is partially submerged.
The size and prominence of the tympanum vary between different species and sexes. In many species, the male possesses a tympanum noticeably larger than his eye, while the female’s may be smaller. This difference relates to the male’s need to hear the specific frequencies of rival males and female responses during breeding. The tympanum acts as a drumhead, capturing airborne sound vibrations from the environment.
The Mechanics of Amphibian Hearing
Once a sound wave strikes the tympanum, the vibration is mechanically transferred to the inner ear, which is protected within the skull. This transfer is accomplished by a single bone in the middle ear called the columella, which is homologous to the stapes in mammals. The columella extends across the air-filled middle ear cavity, connecting the tympanic membrane to the oval window of the inner ear. This single-bone system acts as a lever, amplifying vibrations to overcome the impedance mismatch between the air and the inner ear fluid.
The frog’s inner ear does not contain the coiled cochlea found in mammals. Instead, it features two distinct sensory organs for airborne sound reception: the amphibian papilla and the basilar papilla. These organs are sensitive to different frequency ranges. The amphibian papilla processes lower frequencies, while the basilar papilla handles higher frequencies, including those of their species-specific calls. Specialized hair cells convert the fluid movement into electrical neural signals transmitted to the brain.
Specialized Adaptations for Sound Reception
Beyond the primary tympanum-columella pathway, frogs employ unique methods for sensing sound and vibration. A significant adaptation involves the use of the lungs, which are connected to the middle ear cavity via the Eustachian tubes. This lung-to-ear transmission pathway allows the inflated lungs to function as an internal pressure sensor. In species like the green treefrog, the inflated lungs selectively reduce the eardrum’s sensitivity to frequencies used by other species in a mixed chorus.
This mechanism creates spectral contrast enhancement, allowing the female to filter out distracting background noise and focus on the specific call of her own species. Furthermore, some frogs that lack a visible tympanum, or hear very low-frequency sounds, rely on bone conduction. These vibrations travel through the ground, up the animal’s limbs, and into the inner ear via a small bone called the operculum. This bone is connected to the shoulder girdle by a muscle, demonstrating a versatile and complex auditory system.