the ears of river dolphins have specific adaptations to their aquatic environment. in humans, the middle ear works as an impedance equalizer between the outside air's low impedance and the cochlear fluid's high impedance. in river dolphins, and other cetaceans, there is no great difference between the outer and inner environments. instead of sound passing through the outer ear to the middle ear, river dolphins receive sound through the throat, from which it passes through a low-impedance fat-filled cavity to the inner ear. the ear is acoustically isolated from the skull by air-filled sinus pockets, which allows for greater directional hearing underwater. dolphins send out high frequency clicks from an organ known as a melon. this melon consists of fat, and the skull of any such creature containing a melon will have a large depression. this allows river dolphins to produce biosonar for orientation. they are so dependent on echolocation that they can survive even if they are blind. beyond locating an object, echolocation also provides the animal with an idea on the object's shape and size, though how exactly this works is not yet understood. the small hairs on the rostrum of the amazon river dolphin are believed to function as a tactile sense, possibly to compensate for their poor eyesight.