Ears can be floppy and leathery like an elephant’s, pointed and fluffy like a cat’s, or flat, round disks like a frog’s. But no matter their shape or size, vertebrates use their ears to magnify incoming waves of sound and transform them into signals the brain can interpret. The result allows us to hear the elephant’s trumpet, the cat’s purr and the frog’s croak. Also, of course, our favorite songs.
Sound travels through the air in waves that compress, stretch and then repeat. The compression exerts a push on objects, such as ear tissue. As a wave stretches back out, it pulls on the tissue. These aspects of the wave cause whatever a sound hits to vibrate.
Sound waves first hit the outer ear. That’s a part often visible on the head. It’s also known as the pinna or auricle. The outer ear’s shape helps to collect sound and direct it inside the head toward the middle and inner ears. Along the way, the shape of the ear helps to amplify the sound — or increase its volume — and determine where it’s coming from.
From the outer ear, sound waves travel through a tube called the ear canal. In people, this tiny tube is about 2.5 centimeters (1 inch) long. Not every animal has an outer ear and ear canal. Many frogs, for example, just have a flat spot behind their eyes. This is their ear drum.
In animals with an outer ear and ear canal, the ear drum — or tympanum — is inside the head. This tight membrane stretches across the end of the ear canal. As sound waves slam into this ear drum, they vibrate its membrane. This triggers pressure waves that swell into the middle ear.
Inside the middle ear is a small cavity with three tiny bones. Those bones are the malleus (which means “hammer” in Latin), the incus (which means “anvil” in Latin) and the stapes (which means “stirrup” in Latin). In people, these three bones are known as ossicles. They are the smallest bones in the body. The stapes (STAY-pees), for instance, is only 3 millimeters (0.1 inch) long! These three bones work together to receive sound waves and transmit them on to the inner ear.
Not all animals, however, have those ossicles. Snakes, for instance, lack both the outer ear and the middle ear. In them, the jaw transmits sound vibrations directly to the inner ear.
Inside this inner ear is a fluid-filled, snail-shaped structure. It’s called the cochlea (KOAK-lee-uh). Inside it stand ranks of microscopic “hair” cells. They contain bundles of tiny, hair-like strands embedded in a gel-like membrane. When sound vibrations enter the cochlea, they make the membrane — and its hair cells — sway to and fro. Their movements send messages to the brain that register the sound as any of many distinct pitches.
Hair cells are fragile. When one dies, it’s gone forever. So over time, as these disappear, people begin to lose the ability to detect certain sounds. Hair cells that respond to high-pitched sounds tend to die off first. For example, a teen may be able to hear a sound with a very high frequency of 17,400 hertz, while someone with older ears may not. Want proof? You can test it yourself below.