Why Can Some Animals Hear Noises That Others Cannot?

For humans and other land animals, sound is vibrations traveling through the air. As the vibrations enter the pinna, the outer portion of the ear on the head's surface, they're funneled to the eardrum. The eardrum is a thin membrane that vibrates when it's hit by sound waves; at high frequencies it vibrates quickly, and at low frequencies, it vibrates slowly. The vibrating eardrum pushes against a group of bones, which transfer the vibrations to fluid in the inner ear. After traveling through the inner ear's fluid, sound vibrations make their way through the spiral-shaped cochlea, which is filled with hair cells. Sound waves make the hairs on the hair cells move, sending signals to the brain, which interprets the vibrations as sound.

An animal's hearing level depends on the make-up of his ear. Size, shape and placement all work together to determine how sound waves are gathered. A large pinna is a more effective funnel to the eardrum, capturing more vibrations. The inside of the ear matters, too. An animal with a very thick, heavy eardrum doesn't hear high frequencies well because the extra weight prevents fast vibrations. For example, bats have very thin eardrums, allowing them to hear up to 110,000 Hz. The size of the bones in the ear impact an animal's ability to hear; heavy bones don't favor high frequency vibrations, so an animal with heavier bones hears low frequencies well. Light bones allow for faster vibrating, and animals with lighter bones hear better at high frequencies.

A snake uses the bones of its skull and jaw to transmit low-frequency vibrations to its inner ear. If you've ever seen someone kneel and put his head to the ground to hear vibrations, you get the idea how this works. Since he's a vertebrate, he has an inner ear. Invertebrates don't have ears; they use other organs to recognize sound waves. Insects may not have ears, but they have tympanal organs that give them superior hearing to humans. Hair helps bugs detect sound; spiders and cockroaches have "hearing hairs" on their legs and the caterpillar has them the length of his body.

You've never seen a fish with ears, but he doesn't need them. Fish have lines running the length of their bodies allowing them to perceive sound. The dolphin, among the animals with the best hearing, has no ears, either. Instead, he relies on external ear drums outside his body. This guy also uses echolocation, a type of sonar, to enhance his hearing. He'll emit tiny chirps, sensing the objects the chirps bounce off when the sound returns. Echolocation helps him learn about things he can't see, such as an object's location and size. Bats use echolocation as well and are able to find an insect 30 feet away in darkness.