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DOLPHIN,
any aquatic mammal of the family Delphinidae, closely related
to whales and porpoises. Sleek and powerful swimmers found in all
seas, dolphins are distinguished from porpoises by well-defined,
beaklike snouts and conical teeth. The PORPOISE has a blunt snout,
chisel-shaped teeth, and a stouter body.
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The family Delphinidae contains about 32 species. Typical
examples are the bottle-nosed dolphin, Tursiops truncatus, a
popular performer in seaquariums, and the common dolphin, Delphinus
delphis, which inspired many Mediterranean folk legends.
Both often appear in open waters, making their characteristic arched
bounds, often before the bow waves of ships. Several freshwater
species inhabit river estuaries in Asia and South America. The small,
graceful buffeo dolphin, Sotalia fluviatilis, has
been sighted more than 2000 km (more than 1250 mi) up the Amazon
River. The buffeo, the smallest dolphin, is less than 1.2 m (less
than 4 ft) long; the largest, the bottle-nosed dolphin, reaches
a length of 3 m (10 ft). The killer whale, Orcinus orca, is sometimes
considered a dolphin despite its greater length of 9 m (30 ft).
The pilot whale, in the genus Globicephala, is
also considered a dolphin.
Dolphins once were hunted commercially, especially for the
small quantity of valuable oil extracted from parts of the head
and used to lubricate delicate watch mechanisms. Cheaper oils have
now been found from other sources, and dolphins are no longer hunted.
Many dolphins, however, become accidentally trapped and drowned
in tuna nets. Under pressure from animal rights activists and U.S.
consumers, both domestic and international tuna canners have refused
to accept shipments from fishing fleets that do not protect dolphins.
Concern has also been expressed about the treatment of dolphins
on display in public aquariums and in “swim with the dolphins” programs.
The Marine Mammal Protection Act of 1972, amended in 1988 and 1992,
was passed to prevent exploitation of dolphins and related aquatic
animals. The National Marine Fisheries Service is the principal
regulatory agency.
In one day dolphins eat nearly one-third of their weight,
mostly in fish and squid, which these swift mammals can easily outdistance
and seize with jaws that have from 200 to 250 sharp teeth. They
follow schools of fish in groups of varying size. Some species,
such as the white-sided dolphin, Lagenorhyncus obliquidens, make
up aggregations estimated at tens of thousands of members. Less gregarious
species, such as the bottle-nosed dolphin, join in groups that often
contain only a few members.
Dolphins, like whales, breathe through a blowhole at the top
of the head. As they travel they break surface about every two minutes
to make a short, explosive exhalation, followed by a longer inhalation
before submerging again. The tail, like that of other aquatic mammals,
strokes in an up-and-down motion, with the double flukes driving
the animal forward while the flippers are used for stabilization.
Dolphins are superbly streamlined and can sustain speeds of up to
30 km (up to 19 mi) per hour, with bursts of more than 40 km (more
than 25 mi) per hour. Their lungs, which are adapted to resist the
physical problems created for many animals by rapid changes in pressure,
enable them to dive to depths of more than 300 m (more than 1000
ft).
Adults of the bottle-nosed dolphin—the best-studied
species—come to sexual maturity after six or seven years.
They mate in the spring; after a gestation period of 11 or 12 months,
a single calf is born, tail first. Calves swim and breathe minutes
after birth; they nurse up to 18 months. They are able to keep up
with the mother by remaining close and taking advantage of the aerodynamic
effects of the mother’s swimming.
Dolphins almost constantly emit either clicking sounds or
whistles. The clicks are short pulses of about 300 sounds per second,
emitted from a mechanism located just below the blowhole. They are
used for the echolocation of objects and are resonated forward by
the so-called oily melon, which is located above the forehead and
acts as an acoustic lens. The returning echoes are then picked up
by the bony forehead, which acts as a reflector. This echolocation
system, similar to that of a bat, enables the dolphin to navigate
among its companions and larger objects and to detect fish, squid,
and even small shrimp. The whistles are single-toned squeals that
come from deeper in the larynx. They are used to communicate alarm,
sexual excitement, and perhaps other emotional states.
Because of the ability of dolphins to learn and perform complex
tasks in captivity, their continuous communications with one another,
and their ability, through training, to approximate the sounds of
a few human words, some investigators have suggested that the animals
might be capable of learning a true language and communicating with
humans. Most authorities, however, agree that although the dolphin’s problem-solving
abilities put the animal on an intelligence level close to that
of primates, no evidence exists that dolphin communications approach
the complexity of a true language.
The name dolphin is also applied to food
fish of the genus Coryphaena, which have long,
continuous dorsal fins. The fish, found worldwide in tropical marine
waters, can reach a length of 1.5 m (5 ft).