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Porpoises and Dolphins
Nature Bulletin No. 648-A  September 24, 1977
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Years ago, we were privileged to watch the training of four "porpoises" at Brookfield Zoo where they now perform daily in the Panorama of Seven Seas. Actually those remarkably intelligent, friendly and fun- loving animals that exuberantly play games and do acrobatic stunts are Bottle-nosed Dolphins, called porpoises in Florida.

Porpoises and dolphins, closely related, are members of a group of ocean-dwelling streamlined mammals -- the cetaceans -- that include the whales. Outwardly they resemble fish but, being mammals, they are warm-blooded, bear their young alive -- only one each time -- and suckle them with milk. Cetaceans have lungs and surface periodically to breathe air through a blowhole (nostril) located on top of the head.

In contrast, fish are cold-blooded and most species lay eggs. A few kinds bear their young alive but cannot suckle them. Except for the few kinds that have lungs, fish obtain oxygen from water passing through their gills. Most of them have five types of fins including the tail which is set vertically.

A cetacean's forked tail is set horizontally and it has only a dorsal fin, if any. Its flippers are really hands, with fingers, encased in thick "mittens. " It is propelled mostly by the tail. Our racing swimmers now combine the "butterfly stroke" of arms and hands with a "dolphin stroke" of the legs and feet.

Porpoises and dolphins are remarkably fast and graceful swimmers. The cruising speed of a bottle-nosed dolphin is from 10 to 12 miles per hour but they have been clocked at 28 for short distances. The skin on a dolphin's body is loose and flexible so that, when swimming, it ripples and the friction drag is reduced by as much as 90 percent from that of a tight unyielding surface. Since this was discovered, submarines with their steel hulls encased by loose plastic skins have been enabled to travel at greater speeds.

Scientists cooperating with the U.S. Navy are also studying this dolphin's amazing ability to obtain its favorite food, certain fish, and avoid obstacles such as transparent sheets of glass or plastic, by means of what is known as echo location. It has no "smeller" and no vocal cords but, apparently by means of valves in its blowhole (nasal) passages, makes various sounds. Those audible to us, when they "converse, " are staccato clicks and clacks, barks, and catlike mewing noises. However, it also makes supersonic sounds.

This animal is more intelligent than a chimpanzee. Its brain and ears are highly developed. They provide a marvelously accurate mechanism for receiving and interpreting echoes from those supersonic sounds -- bouncing back from a fish or an obstacle -- which, if successfully imitated, would revolutionize our underwater detection devices such as sonar.

There are several species of dolphins. Some have blunt rounded heads but in most kinds the jaws are elongated to form a beak. On one they resemble a pair of flat paddles. The bottle-nosed dolphin, so common along our Atlantic coast, becomes from 8 to 12 feet long.

Porpoises are smaller and have blunt snouts. The common species, called a "sea pig" in England, inhabits our North Atlantic waters and seldom ventures far from the coasts. From 5 to 8 feet long, it has a conspicuous triangular back fin. The "porpoises" frequently seen gamboling or swimming in long graceful arcs along-side ships at sea, are usually dolphins.

In Florida a "dolphin" is a large swift gamey fish with brilliant hues of blue, olive green and gold. Beware of common names!

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