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.
Listen to a dolphin
1 . Toothed whales can produce sounds for two overlapping functions: communi- cating and navigating. A bottlenose dolphin can communicate and navigate at the same time.
2. 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).
3. 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).
4. 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.
5. 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.
6. 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.
1. Bottlenose dolphins identify themselves with a signature whistle. However, scientists have found no evidence of a dolphin language (Caldwell, Caldwell, and Tyack, 1990).
2. 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).
1. 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.
2. 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.
3. Bottlenose dolphins are able to learn and later recognize the echo signatures returned by preferred prey species (Herman, 1980).
4. 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).
5. 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.
Longevity and Causes of Death
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