Bottlenose dolphins have very good eyesight and can see quite well both above and below the water. Vision, however, does little good in murky or deep water where light from the surface can not penetrate. To compensate for this dolphins rely heavily on sound to sense their environment.
In addition to very good passive hearing, that is picking up and interpreting the sounds created around them, they have developed a system of "active" hearing or echolocation. It works much in the same way that bats emit sounds and then listen for the echoes to bounce off objects around them.
Dolphins can produce specialized echolocation sounds by actively squeezing air through their nasal passages beneath the blowhole and then focusing the sound it produces out through their melon. The melon is a fat filled area in the dolphin’s forehead that acts like an acoustical lens to focus the sound waves ahead of the animal. If you are in the water with a dolphin while it is actively echolocating you can actually hear and feel some of these sounds as "clicks" and "squeaks".
Once the sounds are emitted by the dolphin, some of them are then reflected off an object, such as a fish, another dolphin, a potential predator, etc., and received by the dolphin through its lower jaw.
Much of the brain is devoted to interpreting these reflected sounds and giving the dolphin a mental picture of what lays ahead. Studies done with trained dolphins have shown that they can determine an object’s distance, direction, speed, density and configuration. Their ability to distinguish between different objects is so good that they can tell one species of fish from another, even though they may be exactly the same size.
While most echolocation studies have been done with bottlenose dolphins, most scientists believe that all Odontocetes (toothed cetaceans) have this superb sensory adaptation. Some Odontocetes, such as sperm whales, may even use this sound production to stun or confuse their prey, thus making catching their food much easier.
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