Dolphins are extremely and almost constantly vocal. They are capable of two
kinds of sounds. A specialized mechanism in the nasal passages just below the
blowhole enables them to emit short, pulse-type sounds. These sounds, called
clicks, can be produced in such rapid succession as to sound like a buzz or even
a ducklike quack. The clicks are beamed forward, with the oily melon serving as
an acoustic lens and the bony forehead as a reflector. The clicks are used as a
form of sonar, in which echoes of sounds from surrounding objects enable the
animals to detect obstacles, other dolphins, fish, and even tiny bits of matter
in the water. This ability is termed ECHOLOCATION and is found in a few other
animal groups, most notably the insect-eating bats. Some scientists have
speculated that dolphins also use the sounds to deliver an acoustic shock for
stunning or killing small prey.
Deeper in the respiratory system--presumably in the larynx--dolphins produce
another type of sound: a high-pitched whistle or squeal, which is capable of
rapid pitch changes. The whistles differ from the clicks in being essentially
single tones. Apparently the dolphin uses the whistles to communicate a
particular emotional state and thus influence the behavior of other dolphins.
Typically, the squeals denote alarm or excitement.