Oceanic dolphins or
Delphinidae are a widely distributed family of
dolphins
that live in the sea. Thirty extant species are described. They include
several big species whose common names contain "whale" rather than
"dolphin", such as the
killer whale and the
pilot whales. Delphinidae is a family within the superfamily
Delphinoidea, which also includes the
porpoises (Phocoenidae) and the
Monodontidae (
beluga whale and
narwhal).
River dolphins are relatives of the Delphinoidea.
Oceanic dolphins range in size from the 5.6-foot (1.7 m)-long and 110-pound (50 kg)
Maui's dolphin to the 31-foot (9.4 m) and 11-short-ton (10.0 t) killer whale, the largest known dolphin. Several species exhibit
sexual dimorphism;
the males are larger than females. They have streamlined bodies and two
limbs that are modified into flippers. Though not quite as flexible as
seals,
some dolphins can travel at 34.5 mph (55.5 km/h). Most delphinids
primarily eat fish, along with a smaller number of squid and small
crustaceans, but some species specialise in eating squid, or, in the
case of the killer whale, also eat marine mammals and birds. All,
however, are purely
carnivorous.
They typically have between 100 and 200 teeth, although a few species
have considerably fewer. Delphinids travel in large pods, which may
number a thousand individuals in some species. Each pod forages over a
range of a few dozen to a few hundred square miles. Some pods have a
loose social structure, with individuals frequently joining or leaving,
but others seem to be more permanent, perhaps dominated by a male and a
harem of females.
[1] Individuals communicate by sound, producing low-frequency whistles, and also produce high-frequency broadband clicks of 80–220
kHz, which are primarily used for
echolocation.
Gestation
lasts from 10 to 18 months, and results in the birth of a single calf.
Some species are well adapted for diving to great depths. They have a
layer of fat, or
blubber, under the skin to keep warm in the cold water.
Although oceanic dolphins are widespread, most species prefer the warmer waters of the tropic zones, but some, like the
right whale dolphin,
prefer colder climates. Some have a global distribution, like the
killer whale. Oceanic dolphins feed largely on fish and squid, but a
few, like the killer whale, feed on large mammals, like seals. Male
dolphins typically mate with multiple females every year, but females
only mate every two to three years. Calves are typically born in the
spring and summer, and females bear all the responsibility for raising
them. Mothers of some species fast and nurse their young for relatively
long times. Dolphins produce a variety of vocalizations, usually in the
form of clicks and whistles.
Oceanic dolphins are sometimes hunted in places such as Japan, in an activity known as
dolphin drive hunting. Besides drive hunting, they also face threats from
bycatch, habitat loss, and
marine pollution.
Dolphins have been depicted in various cultures worldwide. They
occasionally feature in literature and film, as in the Warner Bros film
Free Willy.
Dolphins are sometimes kept in captivity and trained to perform in
shows. The most common species of dolphin in captivity is the
bottlenose dolphin, and less than 50 killer whales were found in
oceanariums in 2012.
Taxonomy
Etruridelphis giulii skull in Bologna
Delphinids, especially bottlenose dolphins, are able to
hybridize with a wide variety of other delphinid species;
wolphins are just one of many possible hybrids.
[2]
Six species, sometimes referred to as "blackfish", are dolphins commonly called whales: the
killer whale, the
melon-headed whale, the
pygmy killer whale, the
false killer whale, and the two species of
pilot whales, but they are classified under the family
Delphinidae.
[3]
Recent
molecular analyses indicate that several delphinid
genera (especially
Stenella and
Lagenorhynchus) are not
monophyletic as currently recognized.
[4] Thus, significant taxonomic revisions within the family are likely.
[5]
| A classification of the family Delphinidae from Perrin (1989) reflecting a traditional view of species interrelationships.
|
Revised classification of the family Delphinidae based on molecular systematic analysis; adapted from LeDuc et al. (1999).
[6][7][8]
|
|
|
|
Biology
Anatomy
The Delphinidae are the most diverse of the cetacean families, with
numerous variations between species. They range in size from 1.2 m
(3.9 ft) and 40 kg (88 lb) (
Haviside's dolphin),
to 9 m (30 ft) and 10 tonnes (killer whale). Most species weigh between
50 and 200 kg (110 and 440 lb). They typically have curved dorsal fins,
clear 'beaks' at the front of their heads, and forehead
melons, although exceptions to all of these rules are found. They have a wide range of colors and patterns.
[1]
Oceanic dolphins have a torpedo-shaped body with an inflexible
neck, limbs modified into flippers, nonexistent external ear flaps, a
large tail fin, and a bulbous head. A dolphin skull has small eye
orbits, a long snout, but not as long as its
river dolphin
counterpart, and eyes placed on the sides of its head. Several species
exhibit sexual dimorphism, with the females being larger than the males.
[9][10]
Breathing involves expelling stale air from the
blowhole,
forming an upward, steamy spout, followed by inhaling fresh air into
the lungs; a spout only occurs when the warm air from the lungs meets
the cold external air, so it may only form in colder climates.
[9][11][12]
All oceanic dolphins have a thick layer of
blubber,
the thickness of which depends on how far the species lives from the
equator. This blubber can aid in protection to some extent as predators
would have a hard time getting through a thick layer of fat and
insulation from the harsh climate or cold depths. Calves are born with
only a thin layer of blubber, but some species compensate for this with
lanugos.
[9][13]
Locomotion
Oceanic dolphins have two flippers on the underside toward the head, a
dorsal fin,
and a tail fin. These flippers contain four digits. Although oceanic
dolphins do not possess fully developed hind limbs, some possess
discrete rudimentary appendages, which may contain feet and digits.
Oceanic dolphins are fast swimmers in comparison to seals who typically
cruise at 7–17 miles per hour (11–27 km/h); the killer whale, in
comparison, can travel at speeds of up to 34.5 miles per hour
(55.5 km/h). The fusing of the neck vertebrae, while increasing
stability when swimming at high speeds, decreases flexibility, which
means they are unable to turn their heads. Oceanic dolphins swim by
moving their tail fin and rear body vertically, while their flippers are
mainly used for steering. Some species
log
out of the water, which may allow them to travel faster. Their skeletal
anatomy allows them to be fast swimmers. All species have a
dorsal fin to prevent themselves from involuntarily spinning in the water.
[9][13]
Senses
The oceanic dolphin ear is specifically adapted to the
marine environment. In humans, the
middle ear works as an impedance equalizer between the outside air's low
impedance and the
cochlear
fluid's high impedance. In dolphins, and other marine mammals, there is
no great difference between the outer and inner environments. Instead
of sound passing through the outer ear to the middle ear, dolphins
receive sound through the throat, from which it passes through a
low-impedance fat-filled cavity to the inner ear. The dolphin ear is
acoustically isolated from the skull by air-filled sinus pockets, which
allow for greater directional hearing underwater.
[14] Dolphins send out high frequency clicks from an organ known as a
melon.
This melon consists of fat, and the skull of any such creature
containing a melon will have a large depression. This allows dolphins to
produce
biosonar for orientation.
[9][15][16][17][18] Though most dolphins do not have hair, they do have
hair follicles that may perform some sensory function.
[19]
Beyond locating an object, echolocation also provides the animal with
an idea on an object's shape and size, though how exactly this works is
not yet understood.
[20]
The eye of oceanic dolphins is relatively small for its size, yet
they do retain a good degree of eyesight. As well as this, the eyes of a
dolphin are placed on the sides of its head, so their vision consists
of two fields, rather than a binocular view like humans have. When
dolphins surface, their lens and cornea correct the nearsightedness that
results from the refraction of light; they contain both
rod and
cone
cells, meaning they can see in both dim and bright light. Dolphins do,
however, lack short wavelength sensitive visual pigments in their cone
cells indicating a more limited capacity for color vision than most
mammals.
[21]
Most dolphins have slightly flattened eyeballs, enlarged pupils (which
shrink as they surface to prevent damage), slightly flattened corneas
and a
tapetum lucidum;
these adaptations allow for large amounts of light to pass through the
eye and, therefore, a very clear image of the surrounding area. They
also have glands on the eyelids and
outer corneal layer that act as protection for the cornea.
[15]
The
olfactory lobes are absent in oceanic dolphins, suggesting that they have no sense of smell.
[15]
Oceanic dolphins are not thought to have a good sense of taste,
as their taste buds are atrophied or missing altogether. However, some
have preferences between different kinds of fish, indicating some sort
of attachment to taste.
[15]
Behavior
Feeding
Most
delphinids primarily eat fish, along with a smaller number of squid and
small crustaceans, but some species specialize in eating squid, or, in
the case of the killer whale, also eat marine mammals and birds. All,
however, are purely
carnivorous.
They typically have between 100 and 200 teeth, although a few species
have considerably fewer. Various methods of feeding exist among and
within oceanic species, some apparently exclusive to a single
population. Fish and squid are the main food, but the
false killer whale and the killer whale also feed on other marine mammals. Killer whales on occasion also hunt
whales larger than themselves.
[22]
One common feeding method is herding, where a pod squeezes a school of fish into a
bait ball.
Individual members then take turns plowing through the ball, feeding on
the stunned fish. Coralling is a method where dolphins chase fish into
shallow water to catch them more easily. Killer whales and bottlenose
dolphins have also been known to drive their prey onto a beach to feed
on it, a behavior known as beach or strand feeding. Some species also
whack fish with their flukes, stunning them and sometimes knocking them
out of the water.
[23]
Vocalizations
Oceanic dolphins are capable of making a broad range of sounds using
nasal airsacs located just below the blowhole. Roughly three categories
of sounds can be identified:
frequency modulated
whistles, burst-pulsed sounds, and clicks. Dolphins communicate with
whistle-like sounds produced by vibrating connective tissue, similar to
the way human vocal cords function,
[24]
and through burst-pulsed sounds, though the nature and extent of that
ability is not known. The clicks are directional and are for
echolocation, often occurring in a short series called a click train.
The click rate increases when approaching an object of interest. Dolphin
echolocation clicks are amongst the
loudest sounds made by marine animals.
[25]
Bottlenose dolphins have been found to have signature whistles.
These whistles are used in order for dolphins to communicate with one
another by identifying an individual. It can be seen as the dolphin
equivalent of a
name for humans.
[26]
These signature whistles are developed during a dolphin's first year;
it continues to maintain the same sound throughout its lifetime.
[27]
In order to obtain each individual whistle sound, dolphins undergo
vocal production learning. This consists of an experience with other
dolphins that modifies the signal structure of an existing whistle
sound. An auditory experience influences the whistle development of each
dolphin. Dolphins are able to communicate to one another by addressing
another dolphin through mimicking their whistle. The signature whistle
of a male bottlenose dolphin tends to be similar to that of his mother,
while the signature whistle of a female bottlenose dolphin tends to be
more distinguishable.
[28]
Bottlenose dolphins have a strong memory when it comes to these
signature whistles, as they are able to relate to a signature whistle of
an individual they have not encountered for over twenty years.
[29]
Research done on signature whistle usage by other dolphin species is
relatively limited. The research on other species done so far has
yielded varied outcomes and inconclusive results.
[30][31][32][33]
Surfacing behavior and play
Pacific white-sided dolphins porpoising
Oceanic dolphins frequently leap above the water surface, this being
done for various reasons. When travelling, jumping can save the dolphin
energy as there is less friction while in the air.
[34] This type of travel is known as porpoising.
[34] Other reasons include orientation, social displays, fighting,
non-verbal communication, entertainment and attempting to dislodge
parasites.
[35][36]
Dolphins show various types of
playful behavior, often including objects, self-made
bubble rings, other dolphins, or other animals.
[37][38]
When playing with objects or small animals, common behavior includes
carrying the object or animal along using various parts of the body,
passing it along to other members of the group or taking it from another
member, or throwing it out of the water.
[37]
Dolphins have also been observed harassing animals in other ways, for
example by dragging birds underwater without showing any intent to eat
them.
[37]
Playful behaviour that involves another animal species with active
participation of the other animal can also be observed, however.
Playful human interaction with dolphins is one example, but playful
interactions have been observed in the wild with a number of other
species as well, including
Humpback Whales and
dogs.
[39][40]
Intelligence
Oceanic dolphins are known to teach, learn, cooperate, scheme, and grieve.
[41] The neocortex of many species is home to elongated
spindle neurons that, prior to 2007, were known only in hominids.
[42] In humans, these cells are involved in social conduct, emotions, judgment, and theory of mind.
[43]
Cetacean spindle neurons are found in areas of the brain that are
homologous to where they are found in humans, suggesting that they
perform a similar function.
[44]
Brain size
was previously considered a major indicator of the intelligence of an
animal. Since most of the brain is used for maintaining bodily
functions, greater ratios of brain to body mass may increase the amount
of brain mass available for more complex cognitive tasks.
Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass.
[45] Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an
encephalization quotient that can be used as another indication of animal intelligence.
Killer whales have the second largest brain mass of any animal on earth, next to the
sperm whale.
[46] The
brain to body mass ratio in some is second only to humans.
[47]
Self-awareness
is seen, by some, to be a sign of highly developed, abstract thinking.
Self-awareness, though not well-defined scientifically, is believed to
be the precursor to more advanced processes like
meta-cognitive reasoning
(thinking about thinking) that are typical of humans. Research in this
field has suggested that cetaceans, among others, possess
self-awareness.
[48]
The most widely used test for self-awareness in animals is the
mirror test
in which a temporary dye is placed on an animal's body, and the animal
is then presented with a mirror; they then see if the animal shows signs
of self-recognition.
[49]
In 1995, Marten and Psarakos used television to test dolphin self-awareness.
[50]
They showed dolphins real-time footage of themselves, recorded footage,
and another dolphin. They concluded that their evidence suggested
self-awareness rather than social behavior. While this particular study
has not been repeated since then, dolphins have since passed the mirror
test.
[49]
Interactions with humans
Threats
Consumption
In some parts of the world, such as
Taiji, Japan and the
Faroe Islands, dolphins are traditionally considered as food, and are killed in
harpoon or
drive hunts.
[51]
Dolphin meat is consumed in a small number of countries worldwide, which include
Japan[52] and
Peru (where it is referred to as
chancho marino, or "sea pork").
[53]
Dolphin meat is dense and such a dark shade of red as to appear black. Fat is located in a layer of
blubber between the meat and the skin. When dolphin meat is eaten in Japan, it is often cut into thin strips and eaten raw as
sashimi, garnished with onion and either
horseradish or grated
garlic, much as with
sashimi of
whale or
horse meat (
basashi). When cooked, dolphin meat is cut into bite-size cubes and then batter-fried or simmered in a
miso sauce with vegetables. Cooked dolphin meat has a flavor very similar to
beef liver.
[54] Dolphin meat is high in mercury, and may pose a health danger to humans when consumed.
[55]
The
Faroe Islands population was exposed to
methylmercury
largely from contaminated pilot whale meat, which contained very high
levels of about 2 mg methylmercury/kg. However, the Faroe Islands
populations also eat significant amounts of fish. The study of about 900
Faroese children showed that prenatal exposure to methylmercury
resulted in neuropsychological deficits at 7 years of age
There have been human health concerns associated with the consumption
of dolphin meat in Japan after tests showed that dolphin meat contained
high levels of
mercury.
[56] There are no known cases of
mercury poisoning
as a result of consuming dolphin meat, though the government continues
to monitor people in areas where dolphin meat consumption is high. The
Japanese government recommends that children and pregnant women avoid
eating dolphin meat on a regular basis.
[57]
Similar concerns exist with the consumption of dolphin meat in the
Faroe Islands, where
prenatal exposure to
methylmercury and
PCBs primarily from the consumption of pilot whale meat has resulted in
neuropsychological deficits amongst children.
[56]
Legally
consuming dolphin meat in the United States would be near impossible for most due to the
Marine Mammal Protection Act,
which forbids "...the act of hunting, killing, capture, and/or
harassment of any marine mammal..." (Exceptions are made for certain
groups of people, such as
Alaska Natives.)
Theoretically, one could only eat the meat of a dolphin which died of
natural causes, which would likely be highly undesirable (and
potentially dangerous).
Fishing
Various fishing methods, like
seine fishing for
tuna and the use of
drift and
gill nets, unintentionally kill many oceanic dolphins.
[58] Accidental bycatch in gill nets is common and poses a risk for mainly local dolphin populations.
[59][60]
Dolphin safe labels
attempt to reassure consumers that fish and other marine products have
been caught in a dolphin-friendly way. The earliest campaigns with
"Dolphin safe" labels were initiated in the 1980s as a result of
cooperation between marine activists and the major tuna companies, and
involved decreasing incidental dolphin kills by up to 50% by changing
the type of nets used to catch tuna. The dolphins are netted only while
fishermen are in pursuit of smaller tuna. Albacore are not netted this
way, making albacore the only truly dolphin-safe tuna.
[61]
Sonar
Loud underwater noises, such as those resulting from
naval sonar use, live firing exercises, and certain
offshore construction projects such as
wind farms, may be harmful to dolphins, increasing stress, damaging hearing, and causing
decompression sickness by forcing them to surface too quickly to escape the noise.
[62][63]
In captivity
The renewed popularity of dolphins in the 1960s resulted in the appearance of many
dolphinaria around the world, making dolphins accessible to the public. Criticism and
animal welfare laws forced many to close, although hundreds still exist around the world. In the
United States, the best known are the
SeaWorld marine mammal parks.
In the Middle East the best known are
Dolphin Bay at
Atlantis, The Palm and the
Dubai Dolphinarium.
Various species of dolphins are kept in captivity. These small cetaceans are more often than not kept in theme parks, such as
SeaWorld, commonly known as a
dolphinarium.
Bottlenose Dolphins
are the most common species of dolphin kept in dolphinariums as they
are relatively easy to train, have a long lifespan in captivity and have
a friendly appearance. Hundreds if not thousands of Bottlenose Dolphins
live in captivity across the world, though exact numbers are hard to
determine. Other species kept in captivity are
Spotted Dolphins,
False Killer Whales and
Common Dolphins,
Commerson's Dolphins, as well as
Rough-toothed Dolphins, but all in much lower numbers than the Bottlenose Dolphin. There are also fewer than ten
Pilot Whales,
Amazon River Dolphins,
Risso's Dolphins,
Spinner Dolphins, or
Tucuxi in captivity. Two unusual and very rare
hybrid dolphins, known as
Wolphins, are kept at the Sea Life Park in
Hawaii, which is a cross between a Bottlenose Dolphin and a
False Killer Whale. Also, two
Common/Bottlenose hybrids reside in captivity: one at
Discovery Cove and the other at
SeaWorld San Diego.
[13]
Killer whales
are well known for their performances in shows, but the number of Orcas
kept in captivity is very small, especially when compared to the number
of bottlenose dolphins, with only 44
captive killer whales being held in aquaria as of 2012.
[64] The killer whale's
intelligence,
trainability, striking appearance, playfulness in captivity and sheer
size have made it a popular exhibit at aquaria and aquatic theme parks.
From 1976 to 1997, 55 whales were taken from the wild in Iceland, 19
from Japan, and three from Argentina. These figures exclude animals that
died during capture. Live captures fell dramatically in the 1990s, and
by 1999, about 40% of the 48 animals on display in the world were
captive-born.
[13]
In history and mythology
![[icon]](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Wiki_letter_w_cropped.svg/20px-Wiki_letter_w_cropped.svg.png) |
This section needs expansion with: Vikings and other coastal civilizations. You can help by adding to it. (November 2015) |
In
Greek myths, they were seen invariably as helpers of humankind. Dolphins also seem to have been important to the
Minoans, judging by artistic evidence from the ruined palace at
Knossos. Dolphins are common in
Greek mythology, and many coins from
ancient Greece have been found which feature a man, a boy or a deity riding on the back of a dolphin.
[65] The
Ancient Greeks welcomed dolphins; spotting dolphins riding in a ship's wake was considered a good omen.
[66] In both ancient and later art,
Cupid is often shown
riding a dolphin. A dolphin rescued the poet
Arion from drowning and carried him safe to land, at
Cape Matapan, a promontory forming the southernmost point of the
Peloponnesus. There was a temple to
Poseidon and a statue of Arion riding the dolphin.
[67]
Dionysus was once captured by
Etruscan pirates
who mistook him for a wealthy prince they could ransom. After the ship
set sail Dionysus invoked his divine powers, causing vines to overgrow
the ship where the mast and sails had been. He turned the oars into
serpents, so terrifying the sailors that they jumped overboard, but
Dionysus took pity on them and
transformed them into dolphins so that they would spend their lives providing help for those in need. Dolphins were also the messengers of
Poseidon and sometimes did errands for him as well. Dolphins were sacred to both
Aphrodite and
Apollo.
Dolphins are sometimes used as symbols, for instance in heraldry. When heraldry developed in the
Middle Ages,
not much was known about the biology of the dolphin and it was often
depicted as a sort of fish. Traditionally, the dolphins in heraldry
still may take after this notion, sometimes showing the dolphin skin
covered with
fish scales.
In the
Middle Ages, the dolphin became an important
heraldic element in the coats of arms of several European noble families, the most noticeable being those of the
Dauphin de Viennois (later
Dauphin of France) through which it passed to the
Counts of Forez,
Albon and other French families, as well as several branches of the
Bourbon family (
Count of Montpensier,
Count of Beaujolais, among others) the
Pandolfini of Florence, and the
Delfini of Venice and Rome also used the dolphin as their "canting" armories. In the
19th century,
Joseph Bonaparte adopted a dolphin in his coat of arms as King of Naples and Sicily.
In contemporary days, a dolphin is stilled used in the coat of arms of many cities, as well as in the
coat of arms of Anguilla and the
coat of arms of Romania,
[68] and the
coat of arms of Barbados has a dolphin
supporter.
[69] Cardinal Angelo Amato, prefect of the
Congregation of the Causes of Saints, a dolphin in his coat of arms, as well as
Cardinal Godfried Danneels, former
Metropolitan Archbishop of Mechelen-Brussels.
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