The
majority of small-toothed WHALEs are called dolphins. They are mammals of the order
Cetacea and the families Platanistidae, Delphinidae, and Grampidae and include about 50
species. All have a beaklike snout and sharp, conical teeth. The term porpoise is
sometimes applied to many of the same species, but porpoises, strictly speaking, are
members of the family Phocaenidae and have a blunt snout and spade- or chisel-shaped
teeth. The dolphin fish (mahi-mahi, or dorado), on the other hand, is neither a dolphin
nor a porpoise. It is a sport fish related to the mackerels.
Most dolphin species are about 2 m (6 ft) in length, the males averaging 10 to 20 cm (4 to
8 in) longer than females. The largest is the bottle-nose dolphin, Tursiops truncatus,
well known in many public aquariums. This species may reach over 3 m (9 ft) in length and
200 kg (440 lb) in weight. The smallest species is the buffeo, Sotalia fluviatilis, found
in the Amazon River; the buffeo rarely grows over 1.2 m (3.9 ft) in length and 30 kg (66
lb) in weight.
Dolphins are predators and feed on live food, except when trained otherwise in captivity.
The primary food is fish, mostly open water types such as herring, mackerel, and sardines.
Some species seem to prefer squid; occasionally, shrimp and other crustacea are consumed,
and even mollusk shells have been found in their stomach contents. Food consumption is
estimated at about 30 kg (66 lb) a day for an individual about 2.5 m (8.2 ft) in length
and 100 kg (220 lb) in weight.
Range
Dolphins can be found in virtually all the seas and oceans of the world. Some species are
sharply restricted, but many, like the common dolphin, Delphinus delphis, or the
bottle-nose dolphin, are found worldwide. Several species are found in fresh water,
notably the Ganges River dolphin, Platanista gangetica; the rivers of South America are
the home of the long-snouted dolphin, Inia geoffrensis, and the small, graceful Sotalia
fluviatilis, occasionally seen as far as 2,500 km (1,553 mi) up the Amazon River.
Dolphins are quite abundant in some areas of the world. Off the coast of Japan, for
example, populations of the white-sided dolphin, Lagenorhynchus obliquidens, are estimated
at 30,000 to 50,000 individuals. In many species, schools of up to 1,000 travel together,
while some species, such as the bottle-nose dolphin, tend to be found in smaller groups of
less than 100, or even just a small family group. Solitary individuals are, however, rare.
Physiology
The general form of dolphins is a classic example of streamlining. The body is sleek and
smooth; the hairless skin is rubbery to the touch. Most species have jaws that protrude
into a beaklike snout. Above the upper jaw is a large mass of fat and oil-containing
tissue forming the so-called "melon" that looks much like a bulging forehead.
The anterior appendages contain the skeletal remnants of five digits that form the
flippers, which the animal uses primarily as stabilizers, although occasionally in an
oarlike fashion. The hind appendages are virtually absent and consist of a pair of small
pelvic bones, deeply embedded in the connective tissue at the base of the tail. The dorsal
fin is formed from subcutaneous dermal tissue and is not movable by muscle action. The
caudal, or tail, fin is also primarily dermal in origin, rather than skeletal, and
consists of a pair of horizontally extending flukes. The locomotion of dolphins is typical
of the whale. The main thrust comes from vertical oscillations of the tail and flukes, and
most species tested are capable of sustained swimming speeds of up to 30 km/h (18.6 mph).
In shorter bursts dolphins can attain a speed of 37 to 40 km/h (23 to 25 mph), and in a
few instances a speed of 48 km/h (30 mph) has been reported. Shipboard observers commonly
see dolphins swimming in the bow wave of a moving vessel. The animals seem to rest
motionless but actually ride the bow wave by using the trust of the ship.
Because dolphins are mammals, they must breathe air and maintain a high body temperature.
The maintenance of internal temperature, which has been measured at 36.5 deg to 37.2 deg C
(97.9 deg to 99 deg F), is aided by a thick layer of dense fat (blubber) under the skin.
Air is breathed through a specialized single nostril, or blowhole, situated almost
directly on top of the head. The dolphin normally comes to the surface to breathe about
every two minutes, and each breath consists of a short, almost explosive exhalation,
followed by a slightly longer inhalation. Dolphins can hold their breath for up to several
minutes and are capable of rapid and deep dives of more than 300 m (1,000 ft). The
respiratory system is specially modified to avoid the BENDS.
Reproduction
Reproductive behavior is known mostly from studies on captive individuals, primarily the
bottle-nose dolphin. Copulation normally occurs during the spring months, with the
male-female pair exhibiting courtship for some time prior to copulation. Gestation in the
species studied is between 11 and 12 months, after which a single calf is produced.
Delivery is normally tail first, and the newborn is capable of swimming and breathing
within the first minutes. Some mothers have been observed raising the calf to the surface,
as if to help it, but dolphins apparently play in this fashion with a variety of objects,
living or not. Such play may have provoked the stories of drowning persons being helped to
shore by dolphins.
After birth, the calf follows its mother closely, and suckling takes place frequently,
with the mother rolling slightly and the calf nuzzling the mammary area. The dolphin's two
mammary glands open into a pair of sacs on either side of the anal opening, and the calf's
beak fits into the openings of the sacs. The nipple is grasped between the upper jaw and
the tongue, and muscular contractions by the mother literally squirt milk into the calf's
mouth. Nursing may continue for as long as 12 to 18 months after birth, although weaning
is probably slowed or inhibited in captive animals.
Communication
Dolphins are extremely and almost constantly vocal. They are capable of two kinds of
sounds. A specialized mechanism in the nasal passages just below the blowhole enables them
to emit short, pulse-type sounds. These sounds, called clicks, can be produced in such
rapid succession as to sound like a buzz or even a ducklike quack. The clicks are beamed
forward, with the oily melon serving as an acoustic lens and the bony forehead as a
reflector. The clicks are used as a form of sonar, in which echoes of sounds from
surrounding objects enable the animals to detect obstacles, other dolphins, fish, and even
tiny bits of matter in the water. This ability is termed ECHOLOCATION and is found in a
few other animal groups, most notably the insect-eating bats. Some scientists have
speculated that dolphins also use the sounds to deliver an acoustic shock for stunning or
killing small prey.
Deeper in the respiratory system--presumably in the larynx--dolphins produce another type
of sound: a high-pitched whistle or squeal, which is capable of rapid pitch changes. The
whistles differ from the clicks in being essentially single tones. Apparently the dolphin
uses the whistles to communicate a particular emotional state and thus influence the
behavior of other dolphins. Typically, the squeals denote alarm or sexual excitement.
Intelligence
Because dolphins are highly social and vocalize among themselves with a wide range of
sounds, it has been conjectured that they might possess an almost humanlike intelligence.
In the 1950s and '60s the American neurologist John Lilly conducted well-publicized
experiments based on this concept, in which he attempted to communicate with dolphins in
their own "language," but other scientists have rejected his work as poorly
documented and lacking scientific validity. Most researchers agree that dolphins exhibit a
level of intelligence greater than that of dogs and even comparable to that of some
primates--but not human beings. Research into dolphin intelligence continues at centers
such as Hawaii's Kewalo Basin Marine Mammal Laboratory.
Dolphins and Humans
Dolphins adapt well to human companionship and are readily trained. Bottle-nose dolphins
have become well known performers in many aquariums; they are capable of spectacular
tricks and may mimic the sounds of a few human words. Dolphins are also being studied by
scientists and the military for possible use in undersea operations, but the U.S. Navy has
dropped plans to use them to guard nuclear submarines.
Dolphins at one time were hunted commercially. Some flesh and blubber were used, but the
main product obtained was a fine-quality oil that was extracted from the lower jaw of the
dolphin. Although the oil remains highly prized as a lubricant for delicate watch
mechanisms, similar and cheaper products are available from other sources, and commercial
dolphin fishing is virtually nonexistent. Many dolphins have been destroyed inadvertently
by commercial tuna fishers using purse nets. A tuna boycott pushed U. S. tuna canners to
agree in 1990 to buy tuna only from boats that fish in area where the dolphins and tuna
swim separately.