Frogs hear through a combination of a visible eardrum, a single middle ear bone, and two specialized inner ear organs that split the work of processing different frequencies. But their hearing system goes well beyond that. Frogs also pick up sound through their lungs, their mouth cavity, and even vibrations traveling through the ground.
The Eardrum Sits Right on the Surface
Most frogs have a round, flat disc visible on each side of the head just behind the eye. This is the tympanic membrane, a thin patch of non-glandular skin stretched over a ring of cartilage. Unlike the human eardrum, which is tucked inside a canal, the frog’s eardrum is flush with the skin’s surface and fully exposed to the environment. It vibrates when sound waves hit it, just like ours does, but there’s no outer ear or ear canal funneling sound inward.
One Bone Does the Work of Three
Humans have three tiny bones in the middle ear. Frogs have a two-part chain that functions as one. A small piece of cartilage connects loosely to the center of the eardrum. That cartilage links to a partially bony rod that widens into a flat plate at its far end, pressing against the fluid-filled inner ear. The whole structure works like a piston: vibrations from the eardrum travel along the rod and push into the inner ear fluid.
This system solves a fundamental physics problem. Sound travels easily through air but poorly into liquid, because air and fluid have very different densities. The frog’s middle ear compensates in two ways. First, the eardrum is much larger than the small plate pressing against the inner ear, so pressure concentrates onto a smaller area. Second, the bone acts as a lever, pivoting along its bottom edge to amplify force. Together, these mechanisms boost the pressure enough to drive vibrations into the inner ear fluid efficiently.
Two Inner Ear Organs Split the Frequency Range
This is where frog hearing diverges sharply from mammalian hearing. Instead of a single coiled structure (the cochlea), frogs have two separate sensory organs inside each inner ear, each tuned to a different slice of the frequency spectrum.
The first organ handles low and mid-range frequencies, roughly 100 to 1,250 Hz. It contains hair cells that respond to sound vibrations by generating electrical signals sent to the brain. Within this organ, different regions handle different pitches. Hair cells at one end are tuned to frequencies as low as 50 Hz, while cells farther along respond to progressively higher frequencies. The tuning comes from the electrical properties of the cells themselves: some oscillate naturally at specific rates, acting like tiny tuning forks.
The second organ picks up the higher frequencies, extending the frog’s hearing up to about 8 kHz in most species. For context, most frog calls contain energy below 5 to 8 kHz, so these two organs together cover the full range of sounds that matter for communication. Having two distinct organs rather than one continuous structure means each can be independently specialized, which is part of why frogs are so efficient at picking out mating calls from a noisy swamp.
Lungs and Mouth Help Filter Sound
Frog eardrums are not isolated from each other. The two tympanic membranes connect internally through the mouth cavity via wide, permanently open tubes (similar to human Eustachian tubes, but always open). This means sound hitting one eardrum also reaches the inner surface of the opposite eardrum through the head, creating a pressure-difference system that helps frogs localize where a sound is coming from.
The lungs play a surprising role too. Sound enters the body wall, passes through the air-filled lungs, travels up through the airway into the mouth, and reaches the inner surface of the eardrums from behind. Laser measurements of eardrum vibration have shown that inflated lungs selectively dampen the eardrum’s sensitivity to certain frequencies, specifically those between the two main peaks in a species’ mating call. In practical terms, this means the lungs act as a filter, helping the frog ignore background noise at irrelevant frequencies while staying sensitive to the frequencies that carry meaningful information from other frogs of its own species.
Ground Vibrations Travel Through the Shoulder
Frogs don’t just hear airborne sound. They also detect vibrations traveling through the ground using a completely separate pathway. A small muscle connects the shoulder blade to a disc-shaped structure that presses against the inner ear fluid, right next to where the middle ear bone attaches. When vibrations travel through the ground, the frog’s shoulder girdle and skull move at different rates. That difference in motion pulls on the muscle, which pushes the disc and creates waves in the inner ear fluid.
Experiments on bullfrogs showed that removing this muscle reduced the frog’s sensitivity to ground vibrations by up to 18 decibels at certain frequencies. The system is particularly effective for vertical vibrations, the type produced by natural surface waves traveling along the ground. In a frog’s normal sitting posture, shoulder motion is much greater than head motion during ground vibration, maximizing the differential that drives this pathway. This gives frogs a way to detect approaching predators, footsteps, or other environmental disturbances even when airborne sound carries no useful information.
Frogs That Hear Without Eardrums
Not all frogs have visible eardrums or even a functional middle ear. Some species have lost these structures entirely over the course of evolution, yet they can still hear and respond to calls from other members of their species. Gardiner’s frog, a tiny species from the Seychelles Islands, is one of the best-studied examples.
Researchers used high-resolution X-ray imaging and computer simulations to figure out how these earless frogs manage. The answer turned out to be bone conduction amplified by the mouth. The frog’s oral cavity resonates at exactly the dominant frequency of its species’ mating call, acting like an echo chamber that boosts those specific sound waves. The tissue between the mouth cavity and the inner ear is extremely thin, sometimes only a few cell layers, and earless species have fewer tissue layers in this region than frogs with normal eardrums. Sound enters through the head, gets amplified by mouth resonance, and passes almost directly into the inner ear. The lungs in Gardiner’s frog are too underdeveloped to contribute meaningfully to this process, so the mouth pathway does nearly all the work.
A Few Species Hear Ultrasound
Most frogs hear in a range topping out around 3 to 8 kHz, but a handful of species have pushed far beyond that limit. At least three species can detect ultrasonic frequencies, sounds above the 20 kHz ceiling of human hearing. Two of these, both from noisy stream habitats in Southeast Asia, actively use ultrasound to communicate. One species, Huia cavitympanum from Borneo, produces exclusively ultrasonic calls and can hear frequencies up to about 33 kHz.
The anatomical basis for ultrasonic hearing lies in miniaturized features of the high-frequency inner ear organ. Compared to a typical frog, ultrasound-detecting species have a smaller chamber housing the organ, lighter overlying membranes, shorter sensory hair bundles on the receptor cells, and smaller cell bodies overall. No single modification accounts for the expanded range. Rather, the combination of all four changes shifts the organ’s sensitivity upward, much like shortening and thinning a guitar string raises its pitch. These modifications appear to have evolved independently in multiple lineages, suggesting that ultrasonic hearing is a convergent adaptation to environments where low-frequency sounds are drowned out by rushing water.