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Water-dwelling
mammals, especially orcas and other cetaceans, depend on sound to
be able to perceive their world. Water visibility is not very reliable,
whereas sound travels through water more efficiently than it does
in air. Orcas use sound to navigate, find food, and to communicate.
They are both great listeners and great sound producers. Sound is
created when a whale forces air through the sinus passages in its
head. The sound is then directed out into the ocean after passing
through the specialised fatty tissue in the melon located on the whale's
forehead. Orcas produce three types of sound. Echolocation clicks
are "sonar" signals which allow the whale to find food and navigate
through the water. The function of the pure tone high frequency whistles
is unknown but are quite noticeable during social encounters between
whales. The third type of signal, the discrete vocalisation or call,
is a pulsed signal which has a distinctive form and duration. These
calls are thought to be primarily used for communication as they enable
the whales to recognise each other and stay in touch, even at a distance
of 10km or more.
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