Many animals interact with their surroundings through sound. While the ability to detect sound is widespread, the structures used for hearing are remarkably diverse. Some animals navigate their world using sound without familiar external ear structures, raising questions about how they perceive their auditory environment.
Defining “Ears” in the Animal Kingdom
The term “ear” refers to a biological system designed to detect and interpret sound vibrations. For many mammals, this includes a visible external part, the pinna, which funnels sound waves. Hearing fundamentally relies on internal structures like a tympanic membrane (eardrum) and an inner ear, converting sound vibrations into nerve impulses. All animals capable of hearing possess these internal components, even without a visible outer ear. The external ear primarily collects and directs sound; its absence does not mean an inability to hear.
Animals Without Visible Outer Ears
Many animal groups lack the visible external ear structures common in mammals, yet they still possess hearing. Reptiles, such as snakes, crocodiles, and lizards, along with amphibians like frogs and salamanders, do not have external ears. Fish, living in an aquatic environment, have no external ears. Many invertebrates, including most insects and spiders, also fall into this category. Even some marine mammals, like dolphins and other toothed whales, have evolved without external ear flaps.
How These Animals Perceive Sound
Animals without external ears employ a variety of specialized mechanisms to detect sound, often utilizing vibrations transmitted through their bodies or surroundings.
Snakes
Snakes lack eardrums and primarily sense vibrations through their jawbones when resting on the ground. These vibrations travel from the jaw to a single modified middle ear bone, the columella, which transmits sound to the inner ear. Snakes can also detect airborne sounds through vibrations in their skull bones.
Fish
Fish perceive sound through internal ear structures, including dense calcium carbonate otoliths. Sound waves in water cause these otoliths to vibrate, stimulating hair cells that send signals to the brain. Many fish also use their lateral line system, a row of sensory cells along their body, to detect water movement and low-frequency vibrations. Some fish enhance hearing sensitivity using their swim bladder, a gas-filled sac, to amplify sound vibrations and transmit them to the inner ear.
Insects
Insects have diverse hearing organs, often involving thin, vibrating membranes called tympanal organs. These can be located on various body parts, such as the legs in crickets or the abdomen in some moths. These membranes function like eardrums, converting sound waves into mechanical vibrations. Some insects and spiders also use specialized sensory hairs (hair sensilla) on their bodies or legs to detect vibrations.
Dolphins
Dolphins, despite lacking external ear flaps, possess a specialized auditory system for underwater hearing. They primarily receive sound through their lower jawbone, which contains a fatty substance that efficiently conducts sound vibrations to the middle ear. This bone conduction pathway, along with small ear openings, allows them to process sound for communication and echolocation, a biological sonar system.