Why Sound In The Sea Is Important
Dolphins rely heavily on sound production and reception to navigate, communicate, hunt, and avoid predators in dark or limited vision waters.
A human vocalizes (makes sound) by exhaling — our lungs force air through our larynx. Vocal cords in the larynx vibrate as air flows across them, producing sounds. Our throat, tongue, mouth, and lips shape these sounds into speech.
A dolphin does not have vocal cords in its larynx. Sounds are probably produced by air movements in the nasal passage.
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. This complex, called the dorsal bursa, includes "phonic lips" — structures that project into the nasal passage. As air pushes through the nasal passage and past the phonic lips, the surrounding tissue vibrates, producing sound.
A dolphin has two dorsal bursa/phonic lip complexes, which can operate independently and simultaneously. Bottlenose dolphins can produce both clicks and whistles at the same time.
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 whistles and sounds that resemble moans, trills, grunts, squeaks, and creaking doors. They make these sounds at any time and at considerable depths. Sounds vary in volume, wavelength, frequency, and pattern.
The frequency of the sounds produced by a bottlenose dolphin ranges from 0.2 to 150 kHz. The lower frequency vocalizations (about 0.2 to 50 kHz) are likely used in social communication. Social signals have their most energy at frequencies less than 40 kHz. Higher frequency clicks (40 to 150 kHz) are primarily used for echolocation.
A bottlenose dolphin identifies itself with a signature whistle. The signature whistle is so distinct that scientists can identify individual dolphins by looking at their whistle shapes on a sonogram.
Signature whistle frequencies typically range from 7 to 15 kHz and last less than one second.
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.
A dolphin develops its signature whistle as young as one month old.
Dolphins may mimic each other's signature whistles and scientists have hypothesized that dolphins use the whistles for social interaction.
A mother dolphin may whistle to her calf almost continuously for several days after birth.
Scientists have found no evidence of a dolphin language.
The term echolocation refers to an ability that odontocetes (and some other marine mammals and most bats) possess that enables them to locate and discriminate objects by projecting high-frequency sound waves and listening for echoes as the sound waves reflect off objects. Odontocetes echolocate by producing clicking sounds and then receiving and interpreting the resulting echo.
Bottlenose dolphins produce directional, broadband clicks in sequence. Each click lasts about 50 to 128 microseconds. Peak frequencies of echolocation clicks are about 40 to 130 kHz.
The click train sequences pass through the melon, which consists of special fats (sometimes called acoustic lipids). The melon acts as an acoustical lens to focus these outgoing sound waves into a beam, which is projected forward into water in front of the animal. These sound waves bounce off objects in the water and return to the dolphin in the form of an echo.
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.
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 (16–656 ft.) for targets 5 to 15 cm (2–6 in.) in length.
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.
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.
Dolphins echolocate by producing high frequency clicks that pass through the melon, then receiving and interpreting the resulting echo.
By this complex system of echolocation, dolphins 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.
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.
Many of the details of echolocation are not completely understood. Research on echolocation continues.
Loud Impulse Sounds
Loud impulse sounds recorded from bottlenose dolphins may serve to stun prey or confuse predators; however this suggestion has not yet been confirmed.
Dolphins produce sounds above the water surface. Dolphins also make sounds when they jump, breach, or strike the water surface with flippers and flukes. These sounds may function in communication.