Several different types of animals lack external ears, including birds, snakes and frogs. Seals and walruses do not have external ears or ear flaps either, possessing only small holes in their heads for hearing.
Birds are incredibly sensitive to sound using their entire head as one external ear. Sound hits the eardrums on either side of their head at different frequencies allowing them to discern the direction of the sound.
One of the main differences between seals and sea lions is that seals have no external ears, while sea lions do. True seals have ear holes. Walruses share many characteristics with both seals and sea lions. They share the ability to rotate their flippers with sea lions, but like seals they also have no external ears.
Instead of external ears, frogs have a tympanum, a membrane for hearing, behind each eye. This membrane or eardrum connects directly to both the inner ear and the lungs. Experts believe that this prevents frogs from deafening themselves when they croak.
Snakes lack not only an external ear but a middle ear and eardrum as well. Snakes sense vibrations in the ground with their jaws and the vibrations are conducted to their inner ears.