DOLPHIN(aquatic mammal), fast-swimming mammal belonging to the order Cetacea, which also includes whales and porpoises. Sleek and powerful swimmers, dolphins are found in seas throughout the world; some inhabit freshwater rivers and lakes. Characteristic features of most dolphins are long snouts with rows of sharp teeth, and rounded foreheads with a nostril on top, known as the blowhole.

There are at least 40 species of dolphins. Dolphins resemble fish in many ways, but they exhibit a number of true mammalian characteristics:

They are warm-blooded, breathe air, and nurse their young on milk. Dolphins and porpoises have a similar appearance, but dolphins can be distinguished from porpoises by their more prominent snouts and conical teeth. Porpoises have blunt snouts, chisel-shaped teeth, and a stouter body than dolphins.

Sailors have long considered the presence of dolphins cruising alongside the bows of ships as a good omen and a promise of fair weather. Dolphins also figure prominently in folklore, often appearing in works of art, on coins and currency, and on stamps. Ancient Greek coins depicted the son of Poseidon seated on a dolphin, and the ancient Roman author Pliny the Elder wrote a story of a dolphin that carried a poor man's son to school each day. Many classical writers described how dolphins harnessed to chariots helped maidens in distress. A dolphin was on the coat of arms of the dauphin, the title given the eldest son and heir of the king of France.

Dolphins throughout the world are threatened by habitat destruction and pollution. Many cultures have hunted dolphins for food and for the oil found in small quantities in the animal’s head. (The oil was once widely used as a lubricant.) Although the hunting of dolphins greatly declined in the 20th century, many dolphins are still killed inadvertently when they become entrapped in huge nets used to catch tuna.

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