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Ancient Greek mariners listened to the sounds of dolphins through the hulls of their ships.

People have been fascinated by dolphin sounds for millennia. Yet, we still don't know what 'they' are talking about!

Dolphin sounds fall into several main categories:

Whistles: which are unique to each individual animal - much like our own voices. It appears that dolphins use these signature whistles like we use names. You often hear a loud whistle from a nearby dolphin, followed by a similar sounding whistle from another dolphin. Sort of like a sound 'hand shake' or greeting.

Clicks: which are generally used for some form of echolocation. Echolocation works like radar and is used by dolphins to find food - like schooling fish. The dolphin makes a 'click' which travels through the water, bounces off an object like a fish, and then hears the echo.

Chirps: which are tones of varying frequency - their purpose is not known.

These dolphin sounds are well within the hearing range of people. While echo location clicks can range up to about 150,000 Hz (about 8 times higher than the normal human hearing range), a lot of these clicks occur at frequencies as low as about 2,000 Hz. So people can easily hear them with the proper hydrophone (underwater microphone).

It is reported that cetaceans have a large portion of their brains devoted to auditory senses. Therefore they may be able to convert sound into an acoustic image in a section of their brains which allows them to 'see' in the darkness of the ocean, or in the murky waters of river deltas. There are many 'noise' sources in the ocean that could act to 'illuminate' objects with sound that cetaceans detect. For example, in shallow tropical and semi-tropical waters, snapping shrimp product continuous 'clicking' noises. These may allow cetaceans to 'see' fish without the need to use their own echo location - which might alert fish of their presence. Further out in the ocean, ambient sounds from wave action may serve the same purpose.