In 1551, it was Pierre Belon du Mans who classified dolphins as "fish with lungs." He was the first to notice that these animals weren’t like fish but more like land mammals. It wasn’t until the 1700’s, however, that these animals were classified taxonomically and were given the scientific name "Tursiops truncatus".

Bottlenose dolphins range in length from 2-3 m and can weigh 150-650 kg. Dolphins have a layer of skin and blubber, which provide both warmth and favorable hydrodynamics. At low speed the skin and blubber stay relatively smooth. But at high speeds skin and blubber begin to ripple, diminishing water’s friction by reducing turbulence, resulting in laminar flow around the animal. The importance of laminar flow was discovered when a plastic replica of a dolphin was placed in a flow tank with water moving at the speed of a live dolphin. It was found that drag due to turbulence slowed the motion of the replica 10 times more compared to a live dolphin.

There are two different types of sound produced by dolphins. One type of sound is a whistle, which is produced in the larynx. The other type of sound is produced in the nasal sacs and focused and directed through the melon, the concave head of the dolphin that is filled with lipids (fats). These sounds are used for echolocation. Echolocation is like "seeing" with ears. The sound waves are reflected from objects and come back as an echo. The echo is detected by the lower maxillary (lower jaw), is translated through the inner ear and processed into a mental picture in the brain. Echolocation is a useful sense to have in water because sound waves travel 4.5 times faster in water than in air. This sense is also helpful when locating fish. It has been found that dolphins can find a fish without any visual cues. A dolphin’s sense of hearing covers a much wider range of frequencies than that of a human. Dolphins can detect sound waves from 1-150 kHz and a human can hear .02-17 kHz. The dolphin’s ears can detect sounds of about 1- 20 kHz, and the lower maxillary detects echolocation clicks above 20 kHz.

Bottlenose dolphins are very social animals and live in groups called ‘pods’. These pods range in size from 2-10 inshore and 2-25 offshore, but more than 500 dolphins have been sighted in a pod. A dolphin will spend most of its life with a pod. While in these pods they communicate constantly. There are three types of communication between dolphins, direct body contact, vocalization and bubbles. An example of direct contact is called a pectoral fin rub. Two dolphins touch each other’s pectoral fins, similar to a handshake among humans. This has been observed when dolphins first meet and has been classified as a form of greeting. The second type of communication is vocalization. An example of this is called a signature whistle, which characterizes each dolphin. These signature whistles were studied by Vincent Janik and Peter Slater who discovered that when dolphins were separated from each other, each would use their own whistle signature. They also found instances of whistle copying, a phenomenon that needs more research to be fully understood.

The third type of communication is bubbles. These can be either bubble trains or bubble clouds. The bubble trains are often displayed when young dolphins are playing. The bubble clouds can be used to express anger or warning, and can be used to shield one dolphin from another’s vocalizations.

Communication between dolphins is important for group cohesion, and is used to locate lost or disoriented dolphins as well as alerting others of nearby danger. The communication between mother and calf strengthens the bonds between the two and becomes crucial when mother and calf are separated.

There are major threats to dolphin’s communication posed by underwater noise by large fishing vessels, whale watching boats, and underwater drilling stations. Social bonds may become weakened as a result, as well as the dolphin’s ability to find food. The social bonds between dolphins are important not only in a social aspect but as a survival strategy. The animals protect one another and if the social group isn’t working right this protective aspect is compromised.

Reference:

Cahill, Tim. Dolphins. Washington, D.C.: National Geographic Society. 2000.

Sylvestre, Jean-Pierre. Dolphins & porpoises: a world wide guide. New York: sterling Publishing Company. 1993.

Carwardine, Mark. Whales, dolphins and porpoises. New York: Dorling Kindersley. 1992.

Janik, V. M and P. J. B. Slater. 1998. Context-specific use suggests that bottlenose dolphins signature whistles are cohesion calls. Journal of Animal Behaviour 56, 829-838.

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http://www.geocities.com/CollegePark/Den/4433/structure.html
Accessed: March 23,2003