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The terms "dolphin" and "porpoise" have come to be interchangeable when referring to the sea mammal belonging to the Cetacean order and the Delphinidae subfamily.  (There is a species of fish called a dorado or mahi mahi which is also called a "dolphin.")  There are 30 species of dolphin:  25 saltwater, and five freshwater, plus six species of porpoise.  Porpoises differ from dolphins in tooth shape (flat, triangular teeth in porpoises; round, cone-shaped teeth in dolphins); shape of the dorsal fin (triangular in porpoises, crescent-shaped in dolphins) and shape of the head (the porpoise has a much shorter snout).
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.
Dolphin communication is accomplished through clicks, whistles and body language.  They also whine, groan and clap their jaws sharply together.  They orient themselves and find food by emitting a series of clicks, at up to 2000 per second, from an internal nasal sac called a "melon."  The sounds stream through the ocean depths and bounce back upon encountering an obstacle.  Since sound travels more quickly through water than through air, this "sonar" enables the dolphin, in a second's time, to ascertain size, shape, speed and direction of a moving object.  It is theorized that these sound projections can be strong enough to stun or kill the fish and squid upon which dolphins feed; indeed, the sonar is sensitive enough to spot a minnow at ten feet.
Although there is some controversy regarding the relative nature and degree of dolphin intelligence, it has been noted that they respond swiftly to training, delighting aquarium visitors with antics that usually draw fish rewards.  They have complicated brains which may be comparable to those of humans, and learn tricks faster than the brightest monkeys.  They seem to communicate with each other by high-pitched whistles and grunts when feeding or in moments of distress or pleasure.  Some scientists hope eventually to train dolphins to talk.
Aside from disease, parasites and predation, many man-made hazards threaten dolphin populations; one of the most serious is drift-net fishing.  Commercial fishnets, designed for tuna, often entrap dolphins (which, for some unknown reason, often swim directly beneath tuna schools).  Unable to rise to the surface to breathe, the dolphins drown.  Ocean pollution, particularly along coastlines, contributes to ecological breakdown of an environment for all its inhabitants; this is a major threat, since dolphins occupy the top tier of the aquatic food chain.  Two freshwater dolphins -- the Ganges of India and the baiji of China -- are close to extinction because of the heavily contaminated river environments.
Controversies rage over dolphins, including captivity and their training and use by the military.

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