The general form of dolphins is a classic example of streamlining. The body is sleek and smooth; the hairless skin is rubbery to the touch. Most species have jaws that protrude into a beaklike snout. Above the upper jaw is a large mass of fat and oil-containing tissue forming the so-called "melon" that looks much like a bulging forehead.

The anterior appendages contain the skeletal remnants of five digits that form the flippers, which the animal uses primarily as stabilizers, although occasionally in an oarlike fashion. The hind appendages are virtually absent and consist of a pair of small pelvic bones, deeply embedded in the connective tissue at the base of the tail. The dorsal fin is formed from subcutaneous dermal tissue and is not movable by muscle action. The caudal, or tail, fin is also primarily dermal in origin, rather than skeletal, and consists of a pair of horizontally extending flukes. The locomotion of dolphins is typical of the whale. The main thrust comes from vertical oscillations of the tail and flukes, and most species tested are capable of sustained swimming speeds of up to 30 km/h (18.6 mph). In shorter bursts dolphins can attain a speed of 37 to 40 km/h (23 to 25 mph), and in a few instances a speed of 48 km/h (30 mph) has been reported. Shipboard observers commonly see dolphins swimming in the bow wave of a moving vessel. The animals seem to rest motionless but actually ride the bow wave by using the thrust of the ship.

Because dolphins are mammals, they must breathe air and maintain a high body temperature. The maintenance of internal temperature, which has been measured at 36.5 deg to 37.2 deg C (97.9 deg to 99 deg F), is aided by a thick layer of dense fat (blubber) under the skin. Air is breathed through a specialized single nostril, or blowhole, situated almost directly on top of the head. The dolphin normally comes to the surface to breathe about every two minutes, and each breath consists of a short, almost explosive exhalation, followed by a slightly longer inhalation. Dolphins can hold their breath for up to several minutes and are capable of rapid and deep dives of more than 300 m (1,000 ft).

Lost? Click here for the main index page.