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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 'handshake'
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.
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