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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
PORPOISES AND DOLPHINS
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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Update: June 2012
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