Some ocean-dwelling dolphin species are: spotted, common, striped, bottlenose (the species most people think of when they refer to dolphins) and hourglass. Freshwater species include the Ganges River (or "susu"), which is nearly blind; the largest freshwater species, the Amazon River ("boutu"); and the Irrawaddy dolphin, one of the slower swimmers. True porpoise species include the harbor and Dall's porpoise.

Dolphins are small-toothed whales that migrate through the tropical and temperate oceans of the world; freshwater dolphins live in the rivers of South America and tropical Asia. Unable to do without food for long, they migrate year round to ensure adequate food supplies.

The largest adult dolphins measure 12 feet in length and can weigh up to 1500 pounds. The fastest swimmer, the common dolphin, has been clocked at 70 miles per hour, but the average dolphin travels at anywhere between 17 and 22 miles per hour. These speeds are achieved by "running," wherein they continuously leap forward out of the water. They can achieve greater speeds when assisted by the bow wave of a passing ship and they actively seek this mode of transportation. They can dive from 1000 to 1600 feet beneath the ocean surface (as opposed to the sperm whale which can reach depths of 3300 feet).

Most dolphins are dark-backed, light-bellied and built along the sleek lines of a racing yacht or submarine. They have crescent-shaped dorsal fins, dark-ringed eyes and mouths curved into the classic dolphin "smile." The tail, too, is crescent-shaped, moving up and down to propel it through the water, unlike a fish's upright rudder which moves from side to side. The single nostril, or blowhole, is located on top of the head and must be kept clear at all times, lest the animal drown.

Dolphins live in herds, the size varying from species to species -- from as few as five (the harbor porpoise) to herds numbering 100,000 (deep sea dolphin species). Groups are matriarchal, with an "auntie" system similar to that of elephants and lions. Babies remain close to their mothers for three to six years. Mothers, aunts and other related females often establish safe "playpens" of circling adults around a group of calves.

Dolphins are born weighing 30 to 50 pounds and are 35 to 50 inches in length. They are born without teeth, not growing them for several weeks thereafter. When males mature, they leave their birth communities to form bachelor groups of their own, hunting and feeding communally. They return to female groups to mate. Dolphins of both sexes aid each other in times of trouble or infirmity, i.e., two dolphins will support a third by each extending a flipper beneath a sick or injured comrade, and bring the ailing animal to the surface frequently to allow it to breathe.

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