Dolphins probably rely on sound production and reception to navigate,
communicate, and hunt in dark or murky waters. Under these conditions,
sight is of little use.
B. Sound production.
Toothed whales can produce sounds for two overlapping functions:
communi- cating and navigating. A bottlenose dolphin can communicate
and navigate at the same time.
The odontocete larynx does not possess vocal cords, but researchers
have theorized that at least some sound production originates
in the larynx. Early studies suggested that "whistles" were
generated in the larynx while "clicks" were produced
in the nasal sac region (Au, 1993).
Technological advances in bioacoustic research enable scientists
to better explore the nasal region. Studies suggest that a tissue
complex in the nasal region is probably the most likely site
of all sound production (Au, 1993).
Sounds are probably produced by movements of air in the trachea
and nasal sacs. During some vocalizations, bottlenose dolphins
actually release air from the blowhole, but scientists believe
that these bubble trails and clouds are a visual display and
not necessary for producing sound.
Bottlenose dolphins produce clicks and sounds that resemble
moans, trills, grunts, squeaks, and creaking doors. They also
produce whistles. They make these sounds at any time and at considerable
depths. The sounds vary in volume, wavelength, frequency, and
pattern.
The frequency of the sounds produced by a bottlenose dolphin
ranges from 0.25 to 1 50 kHz. The lower frequency vocalizations
(about 0.25 to 50 kHz) probably function mainly in social communication.
Social signals have their most energy at frequencies less than
40 kHz. Higher frequency clicks (40 to 150 kHz) probably are
used primarily in echolocation. Peak frequency of typical echolocation
clicks is about 1 00 kHz, but frequency varies considerably with
specific echolocation tasks..
C. Communication.
Bottlenose dolphins identify themselves with a signature whistle.
However, scientists have found no evidence of a dolphin language
(Caldwell, Caldwell, and Tyack, 1990).
A mother dolphin may whistle to her calf almost continuously
for several days after giving birth. This acoustic imprinting
helps the calf learn to identify its mother (Caldwell, Caldwell,
and
Tyack, 1990).
D. Echolocation.
The term echolocation refers to an ability that odontocetes
(and some other marine mammals and most bats) possess that enables
them essentially to "see" with their ears by listening
for echoes. Odontocetes echolocate by producing clicking sounds
and then receiving and interpreting the resulting echo.
a. Dolphins produce directional clicks in trains. Each click lasts
about 50 to 128 microseconds (Au, 1993).
b. The click trains pass through the melon (the rounded region
of a dolphin's forehead), which consists of lipids (fats). The
melon acts as an acoustical lens to focus these sound waves into
a beam, which is projected forward into water in front of the animal
(Barnes, 1990).
c. Sound waves travel through water at a speed of about 1.5 km/sec
(0.9 mi/sec), which is 4.5 times faster than sound traveling through
air. These sound waves bounce off objects in the water and return
to the dolphin in the form of an echo.
d. High frequency sounds don't travel far in water. Because of
their longer wavelength and greater energy, low frequency sounds
travel farther. Echolocation is most effective at close to intermediate
range, about 5 to 200 m (1 6-656 ft.) for targets 5 to 15 cm (2-6
in.) in length (Au, 1993).
e. The major areas of sound reception are the fat-filled cavities
of the lower jaw bones. Sounds are received and conducted through
the lower jaw to the middle ear, inner ear, and then to hearing
centers in the brain via the auditory nerve.
f. The brain receives the sound waves in the form of nerve impulses,
which relay the messages of sound and enable the dolphin to interpret
the sound's meaning.
By this complex system of echolocation, odontocetes
can determine size, shape, speed, distance, direction, and even
some of the internal
structure of objects in the water.
Bottlenose dolphins are able to learn and later recognize the
echo signatures returned by preferred prey species (Herman, 1980).
Despite the effectiveness of echolocation, studies show that
a visually-deprived dolphin takes more time to echolocate on
an object than a dolphin using vision in tandem with echolocation
(Akamatsu, et al., 1995).
Many of the details of echolocation are not completely understood.
Research on echolocation continues.
Dolphins produce high frequency clicks that pass through the
melon. These sound waves bounce off objects in the water and
return to the dolphin in the form of an echo.
