dolphin

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dolphin aquatic mammal, any of the small toothed whales of the family Delphinidae, numbering more than 50 species. These include the true, or beaked, dolphins, the killer whale , the pilot whale, and 12 freshwater species found in rivers of South America and S Asia. Most species are highly gregarious. The name dolphin, meaning "beaked," is also applied to a species of fish (see dolphin , fish). In the United States dolphins are often mistakenly called porpoises, a name correctly applied to small, blunt-nosed whales of another family. Until recently dolphins formed the basis of a widespread fishing industry; only the Japanese continue to hunt them for food on a large scale. They are accidentally caught and killed in large numbers in tuna seining operations.

Characteristics and Species

Dolphins are fishlike in form, with streamlined, hairless bodies. Their powerful, horizontal flukes, or tail fins, drive them through or out of the water, while their forefins and dorsal fin are used for steering. Constantly shedding their skins, dolphins accumulate no barnacles or other external parasites. A layer of blubber protects them from cold and seals small wounds. Dolphins breathe air through a single, dorsal blowhole.

The dolphin's intelligence, playfulness, and friendliness, its built-in smile and merry-looking eyes have been a source of interest and enchantment to human beings from earliest times; it is a common figure in mythology and literature and has been much depicted in art, especially in the posture of its graceful, arched, 30-ft (9-m) leap. Dolphins have long been famous for riding the bows of ships, and it is now known that they also ride the bows of large whales. Today they are valued and exploited as entertainers in more than 40 water shows around the world and have thus become available for extensive study.

The best known species are the common dolphin ( Delphinus delphis ), of worldwide distribution, and the bottle-nosed dolphin ( Tursiops truncatus ), found in coastal waters of the North Atlantic Ocean and the Mediterranean Sea. The bottlenose has been particularly intensively studied; it is presumed that much of what is known about this species applies to other dolphins and even to the large whales.

Common Dolphin

The common dolphin averages 8 ft (2.4 m) in length and 165 lb (75 kg) in weight. It has a dark blue or black upper body, a white underbody, golden stripes on the sides, and a sickle-shaped dorsal fin. Its pronounced, slim beak, holding 100 teeth, is separated from its snout by a deep groove. A fast swimmer, it travels in large schools in warm waters and is noted for leaping alongside boats for long distances. Its life span is about 50 years.

Bottle-nosed Dolphin

The bottle-nosed dolphin is blue-gray with a dorsal fin and white belly. Its average length is 9 ft (2.7 m) and its average weight 350 lb (160 kg). Its domed forehead, called the melon, contains an oily substance thought to protect the brain case and to act as an acoustic lens. With age the 200 or more teeth of the bottlenose wear down, hence the name truncatus. Members of this species live about 25 years. Bottle-nosed dolphins swim in large schools with a social organization and hierarchy, hunting the small fish, crustaceans, squid, and cuttlefish that make up their diet. They have been clocked swimming at 30 mi (48 km) per hour, although 20 to 24 mi (32-39 km) per hour is their usual speed. They can dive 70 ft (20 m) and remain underwater for 15 minutes. They sleep by night, just below the surface of the water, rising for air every three or four minutes.

Their aquatic natural enemies are sharks and killer whales; these they attempt to outswim, using complex evasive strategy, or batter to death, acting in a group. If one of their number is injured or sick they make every effort to rescue it, holding it above the water for air. Play behavior is highly developed in the bottlenose from infancy through old age, and in this connection it displays considerable tool-making, tool-using, and manipulative ability; for example, a dolphin has been observed to kill a fish, strip its skeleton, and use the bones, held in the mouth, to pry another fish out of a crevice. Sex play is frequent and is initiated by any individual toward any other, without regard to size, age, sex, relationship, or even species; approaches to human beings and to turtles are common.

Courtship and impregnation occur mainly in spring, when males vie for the attention of the females. A single calf, 3 1/2 ft (97 cm) long and weighing 30 lb (14 kg), is born tail first after a gestation of 12 months. The mother or a female assistant bites the umbilical cord in two and pushes the calf to the surface to breathe; it is nursed for one to two years. One female may watch over several calves while the mothers hunt, or during battle.

The senses of the bottlenose have been subjected to intensive investigation, as have their intelligence and their remarkable systems of echolocation and communication. In relation to body size, the brain of the adult bottlenose is comparable in size to that of humans; it is twice as convoluted and possesses 1 1/2 times as many cells. The bottlenose has partially stereoscopic vision that is keen both in water and in air; when the animal leaps from one medium to the other, its brain corrects for the difference in refractive index. The eye has a glowing layer for night vision and a brownish filter that is lowered over the iris in bright sunlight. The brain has no olfactory lobe and the sense of smell is presumably missing, but the taste buds are well developed and are used to detect underwater chemical traces, as when the dolphin tracks fish.

Echolocation and Communication

Dolphins produce an enormous variety of sounds, up to frequencies ten times those heard by human beings. The sounds are apparently produced by a complex of anatomical structures including the blowhole with its air sacs and valves. Each dolphin has a signature whistle with which it identifies itself; a calf soon learns to recognize its mother's whistle. Clicking and rapid creaking sounds are the basis of the echolocation mechanism (sonar) with which the dolphin gathers extremely precise information about the size, location, and nature of surrounding objects. Dolphins communicate by means of a demonstrably descriptive language understood by more than one species, using all the sounds in their repertory. They are observed to converse, and it has been repeatedly shown that one animal can convey instructions to another. Computer-aided efforts are being made, so far without success, to learn the dolphin language and to teach dolphins human speech, either in its normal form or translated into whistle combinations.

Interaction with Humans

Dolphins are capable of imitation and memorization; they demonstrate foresight, learn from observation, communicate experience, solve complex problems, perform elaborate tasks, and learn multiple procedures simultaneously. Their so-called training is in fact a discipline structured around play, using their natural behavior as the basis for involved maneuvers; they appear to perform primarily for their own enjoyment. In situations of great stress in captivity they have been known to commit suicide by starvation, battering against walls, or drowning. There are many reports of dolphins rescuing people from drowning.

The United States and Russian/Soviet navies have spent vast sums to reach a greater understanding of dolphin echolocation, which could have countless military applications. The U.S. navy has trained dolphins to act as messengers to underwater stations, to rescue wounded scuba divers and protect them from sharks, to locate and mark underwater mines, and to seek and destroy submarines, using kamikaze methods; this last project met with considerable public criticism.

Classification

Dolphins are classified in the phylum Chordata , subphylum Vertebrata, class Mammalia, order Cetacea, family Delphinidae.

Bibliography

See W. N. Kellogg, Porpoises and Sonar (1961); K. S. Norris, ed., Whales, Dolphins and Porpoises (1966); E. Devine and M. Clark, The Dolphin Smile (1967); R. Stenuit, The Dolphin, Cousin to Man (1968); D. K. and M. C. Caldwell, The World of the Bottlenosed Dolphin (1972); M. M. Bryden and R. Harrison, ed., Research on Dolphins (1986).

dolphin.
1. A large wooden pile, or collection of piles, serving as a mooring post for ships, or occasionally as a beacon.

2. Small brass guns carried in a ship and fitted with two lifting handles over the trunnions. They were used mainly as anti-personnel guns during the 15th, 16th, and early 17th centuries, much like a murderer or robinet.

3. The plaited rope strap (also known as dolphin of the mast) round the mast of a square-rigged ship to prevent nip between the lower yard and the mast and at the same time to secure the puddening round the mast which prevented the lower yard falling to the deck if the jeers and slings were both shot away.

4. A small light rowing boat of ancient times. It was from this name that arose the story recounted by Pliny of a boy going daily to school across the Lake of Lucerne on a dolphin.

5. Small toothed whales. There are 34 known species, which include killer whales or orcas (Orcinus orca). Many of the smaller species of dolphin have the habit of riding the bow waves of ships under way. They often associate with large schools of tuna or whale sharks, and this makes them vulnerable to being accidentally caught in purse seine nets set around fish shoals. It was estimated that in 2003 as many as 8,000 dolphins were accidentally killed in this way in the North Sea, and the British government is currently (2004) experimenting with nets fitted with escape grids.

Dolphins usually associate in extended family units, some of which can be quite large, and occasionally large numbers of them become stranded. Sometimes the cause of their deaths is evident from clear signs of injuries from fishing nets but often there are no obvious reasons for their stranding. Being top predators dolphins tend to accumulate high concentrations of pollutants such as heavy metals and PCBs, one of today's environmental issues, and these may disorient them, as may the powerful sonars used by naval vessels. However, recently dolphins stranded off Southern California have tested positive for domoic acid, a natural toxin produced by a red-tide diatom Pseudonitschia, so one cause may be the results of eutrophication, another environmental issue.

In orcas the pods are led, and the hunts are organized, by the senior females, whereas the males, recognizable by their tall dorsal fins, tend to move between the family units. Like all dolphins they are highly vocal, using sounds both to communicate within the pod and as sonar to find and track their prey. Each killer whale group tends to have an identifiably different dialect for communication; even more interesting is that the dialects of groups of orcas that specialize in hunting other marine mammals are distinct from those that specialize in hunting fish. Off southern Argentina a group of orcas has developed a unique hunting technique of beaching themselves to snatch cubs from a breeding colony of fur seals.

Dolphins, particularly the bottlenose or bluenose (Tursopis truncatus), are highly intelligent. For example, those kept by the Institute of Marine Mammals in Mississippi are trained to keep their pools clean by holding on to any litter until they see a trainer, and the litter is then traded for fish. They have a very sophisticated sonar system, which enables them to acquire detailed information about an object in the water, so some have been trained by the US Navy to detect mines, either floating in the water or lying on the bottom. They are taught not to touch the mines, but mark them with electronic floats, while cameras on their bodies transmit images to their handlers.

See also pelorus jack.

M. V. Angel

dolphin Family of small-toothed aquatic whales, there are salt and freshwater species. The best-known are the dark blue-backed common dolphin, the blue-grey bottle-nosed and the killer whale. Larger than a porpoise, a dolphin has a distinct beak and slender body, a tail fin for propulsion, and a dorsal fin for steering. A dolphin breathes through a single blowhole, and can remain underwater for 15 minutes. It is the fastest and most agile of the whales, achieving speeds up to 39km/h (24mph) and leaps of 9m (30ft). Dolphins swim in large, hierarchically organized schools, feeding on fish and crustacea. Their intelligence and playful behaviour have contributed to a wealth of maritime literature and mythology. They communicate through a complex language and map their environment by echolocation. Dolphins have a gestation period of 12 months, and the mother nurtures her calf for the first two years of life. Length: to 4m (13ft). Family Delphinidae; species Tursiops truncatus.

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dol·phin / ˈdälfin; ˈdôl-/ • n. 1. a small gregarious and highly intelligent toothed whale that typically has a beaklike snout and a curved fin on the back. Dolphins inhabit seas (family Delphinidae) and rivers (family Platanistidae). ∎  a dolphinlike creature depicted in heraldry or art, typically with an arched body and fins like a fish. 2. (also dolphinfish) another term for mahimahi. 3. a bollard, pile, or buoy for mooring. 4. a structure for protecting the pier of a bridge or other structure from collision with ships.

dolphin [L from Gk. delphis]. The porpoise-like marine, warm-water mammal appears often in early Celtic iconography but not later. It may be found on the Gundestrup cauldron and on Gaulish coins, as at various pre-Christian British sites, notably Lydney Park. The ModIr. word for dolphin is deilf; ScG an leumadair; Manx perkin; W dolffin, morwch; Bret. delfin.

dolphin XIII. Three types of form have been current:
i. delfyn, delphin — L. delphīnus — Gr. delphís, -īn-;

ii. dalphyn — OF. daulphin (see DAUPHIN);

iii. dolfyn, dolphin, app. Eng. alts. of (ii).

dolphin n.
1. a bollard, pile, or buoy for mooring.

2. a structure for protecting the pier of a bridge or other structure from collision with ships.

dolphin (Delphinus) See DELPHINIDAE.

dolphin •Aladdin • stand-in •Dunedin, lead-in •Blondin, Girondin •Odin •paladin, Saladin •Borodin • Baffin • elfin •biffin, griffin, tiffin •boffin, coffin •dolphin • endorphin • bowfin •yellowfin •muffin, puffin •ragamuffin • paraffin • perfin •bargain • Begin • Kosygin •hoggin, noggin •imagine • margin • engine •pidgin, pigeon, smidgen, wigeon •stool pigeon • wood pigeon • origin •Pugin • virgin