dolphin

Any of various highly intelligent aquatic mammals of the family Delphinidae, which also includes porpoises. There are about 60 species. Most inhabit tropical and temperate oceans, but there are some freshwater forms in rivers in Asia, Africa, and South America. The name ‘dolphin’ is generally applied to species having a beaklike snout and slender body, whereas the name ‘porpoise’ is reserved for the smaller species with a blunt snout and stocky body. Dolphins use sound (echolocation) to navigate, to find prey, and for communication. The common dolphin Delphinus delphis is found in all temperate and tropical seas. It is up to 2.5 m/8 ft long, and is dark above and white below, with bands of grey, white, and yellow on the sides. It has up to 100 teeth in its jaws, which make the 15 cm/6 in ‘beak’ protrude forward from the rounded head. The corners of its mouth are permanently upturned, giving the appearance of a smile, though dolphins cannot actually smile. Dolphins feed on fish and squid.

River dolphins
There are five species of river dolphin, two South American and three Asian, all of which are endangered. The two South American species are the Amazon river dolphin or boto Inia geoffrensis, the largest river dolphin (length 2.7 m/8.9 ft, weight 180 kg/396 lb) and the La Plata river dolphin Pontoporia blainvillei (length 1.8 m/5.9 ft, weight 50 kg/110 lb). The tucuxi Sotalia fluviatilis is not a true river dolphin, but lives in the Amazon and Orinoco rivers, as well as in coastal waters.

The Asian species are the Ganges river dolphin Platanista gangetica, the Indus river dolphin Platanista minor (length 2 m/6.6 ft, weight 70 kg/154 lb) (fewer than 500 remaining), and the Yangtze river dolphin or baiji Lipotes vexillifer (length 2 m/6.6 ft, weight 70 kg/154 lb). In January 2000, there were believed to be only five Yangtze river dolphins remaining in the wild.

As a result of living in muddy water, river dolphins' eyes have become very small. They rely on echolocation to navigate and find food. Some species of dolphin can swim at up to 56 kph/35 mph, helped by special streamlining modifications of the skin.

All dolphins power themselves by beating the tail up and down, and use the flippers to steer and stabilize. The flippers betray dolphins' land-mammal ancestry with their typical five-toed limb-bone structure. Dolphins have great learning ability and are popular performers in aquariums. The species most frequently seen is the bottle-nosed dolphin Tursiops truncatus, found in all warm seas, mainly grey in colour and growing to a maximum 4.2 m/14 ft. The US Navy began training dolphins for military purposes in 1962, and in 1987 six dolphins were sent to detect mines in the Gulf. Marine dolphins are endangered by fishing nets, speedboats, and pollution. In 1990 the North Sea states agreed to introduce legislation to protect them.

Also known as dolphin is the totally unrelated true fish Coryphaena hippurus, up to 1.5 m/5 ft long.