porpoise, small whale of the family Phocaenidae, allied to the dolphin. Porpoises, like other whales, are mammals; they are warm-blooded, breathe air, and give birth to live young, which they suckle with milk. They are distinguished from dolphins by their smaller size and their rounded, beakless heads. Porpoises are 4 to 6 ft (120–180 cm) long and are black above and white below. Traveling in schools, porpoises prey on fish, often pursuing them long distances up rivers.
The finned porpoises, species of the genus Phocaena, have a dorsal fin. They are distributed throughout the world and include the common porpoise, Phocaena phocaena, found throughout the Northern Hemisphere. The finless porpoise, Neophocaena phocaenoides, is found in the Indian and W Pacific oceans and in the Chang (Yangtze) River.
The fat of the porpoise yields a lubricating oil, and the flesh is sometimes eaten. In North America the dolphins (family Delphinidae) are sometimes called porpoises and the bottle-nosed dolphin is sometimes called the common porpoise. True porpoises are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Cetacea, family Phocaenidae.
See W. N. Kellogg, Porpoises and Sonar (1961); K. S. Norris, ed., Whales, Dolphins and Porpoises (1966) and, as author, The Porpoise Watcher (1974); R. Ellis, Dolphins and Porpoises (1989).