Dolphins are aquatic mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen
genera. They vary in size from 1.2 metres (4 ft) and 40 kilograms (88 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and ten
tonnes (the Orca or Killer Whale). They are found worldwide, mostly
in the shallower seas of the continental shelves, and are carnivores, mostly eating
fish and squid. The family
Delphinidae is the largest in the Cetacea, and relatively recent: dolphins evolved about
ten million years ago, during the Miocene. Dolphins are considered to be amongst the most
intelligent of animals and their often friendly appearance and seemingly playful attitude have made them popular in
human culture.
Origin of the name
The name is originally from Ancient Greek δελφίς
(delphís; "dolphin"), which was related to the Greek δελφυς (delphys; "womb"). The animal's
name can therefore be interpreted as meaning "a 'fish' with a womb".[1] The name was transmitted via the Latin delphinus,
Middle Latin dolfinus and the Old French
daulphin, which reintrodued the ph into the word.
The word is used in a few different ways. It can mean:
- Any member of the family Delphinidae (oceanic dolphins),
- Any member of the families Delphinidae and Platanistoidea (oceanic and river
dolphins),
- Any member of the suborder Odontoceti (toothed whales; these include the above families and some others),
- Used casually as a synonym for Bottlenose Dolphin, the most common and familiar species of dolphin.
In this article, the second definition is used. Porpoises (suborder Odontoceti, family
Phocoenidae) are thus not dolphins in this sense. Orcas and some closely related species belong
to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language. A group of
dolphins can be called a "school" or a "pod".
Taxonomy
Killer Whales, also known as Orcas
The Boto, or Amazon River Dolphin
- See also: List of
dolphins
- Suborder Odontoceti, toothed whales
- Family Delphinidae, oceanic dolphins
- Genus Delphinus
- Genus Tursiops
- Genus Lissodelphis
- Genus Sotalia
- Genus Sousa
- Genus Stenella
- Genus Steno
- Genus Cephalorynchus
- Genus Grampus
- Genus Lagenodelphis
- Genus Lagenorhyncus
- Genus Orcaella
- Genus Peponocephala
- Genus Orcinus
- Genus Feresa
- Genus Pseudorca
- Genus Globicephala
- Long-finned Pilot Whale, Globicephala melas
- Short-finned Pilot Whale, Globicephala macrorhynchus
- Family Platanistoidea, river dolphins
- Genus Inia
- Boto (Amazon River Dolphin), Inia geoffrensis
- Genus Lipotes
- Genus Platanista
- Genus Pontoporia
Six species in the family Delphinidae are commonly called "whales" but are strictly speaking dolphins. They are sometimes
called "blackfish".
- Melon-headed Whale, Peponocephala electra
- Killer Whale, Orcinus orca
- Pygmy Killer Whale, Feresa attenuata
- False Killer Whale, Psudorca crassidens
- Long-finned Pilot Whale, Globicephala melas
- Short-finned Pilot Whale, Globicephala macrorhynchus
Hybrid dolphins
In 1933, three abnormal dolphins were beached off the Irish
coast; these appeared to be hybrids between Risso's Dolphin and the Bottlenose Dolphin.[2] This mating has since been repeated in captivity and a hybrid
calf was born. In captivity, a Bottlenose Dolphin and a Rough-toothed Dolphin produced hybrid offspring.[3] A Common-Bottlenose hybrid lives at SeaWorld California.[4] Various other
dolphin hybrids have also been reported in the wild, such as a Bottlenose-Atlantic Spotted hybrid.[5] The best known hybrid however is the Wolphin, a
False Killer Whale-Bottlenose Dolphin hybrid. The Wolphin is a fertile hybrid, and two such Wolphins currently live at the Sea
Life Park in Hawaii, the first having been born in 1985 from a male False Killer Whale and a
female Bottlenose. Wolphins have also been observed in the wild.[6]
Evolution and anatomy
The Anatomy of a Dolphin showing its skeleton, major organs and body shape.
Evolution
- See also: Evolution of
cetaceans
Dolphins, along with whales and porpoises, are thought to be descendants of terrestrial mammals, most likely of the
Artiodactyl order. The ancestors of the
modern day dolphins entered the water roughly fifty million years ago, in the Eocene epoch.
Hind Limb Buds on Dolphins An embryo of a Spotted Dolphin in the fifth week of development. The hind limbs are present as
small bumps (hind limb buds) near the base of the tail. The pin is approximately 1 inch (~2,5 cm) long.
Bottlenose Dolphin with vestigial hind flippers, captured 2006 in Japan.
Modern dolphin skeletons have two small, rod-shaped pelvic bones thought to be
vestigial hind legs. In October 2006 an unusual
Bottlenose Dolphin was captured in Japan; it had small fins on each
side of its genital slit which scientists believe to be a more pronounced development of these vestigial hind legs.[7]
Anatomy
Dolphins have a streamlined fusiform body, adapted for fast swimming. The basic colouration
patterns are shades of grey with a light underside and a distinct dark cape on the back. It is often combined with lines and
patches of different hue and contrast.
The head contains the melon, a round organ used for echolocation. In many species, the jaws are elongated, forming a distinct beak; for some species
like the Bottlenose, there is a curved mouth which looks like a fixed smile. Teeth can be very numerous (up to two hundred and
fifty) in several species. The dolphin brain is large and has a highly structured
cortex, which often is referred to in discussions about their advanced intelligence.
Unlike most mammals, dolphins do not have hair, but they are born with a few hairs around the tip of their rostrum which they lose after some time, in some cases even before they are born. The only exception
to this is the Boto river dolphin, which does have some small hairs on the rostrum.
Their reproductive organs are located on the underside of the body. Males have two slits, one concealing the penis and one further behind for the anus. The female has one genital slit, housing
the vagina and the anus. A mammary slit is positioned on either side of the female's genital
slit.
Senses
Most dolphins have acute eyesight, both in and out of the water, and their sense of
hearing is superior to that of humans. Though they have a small ear opening on each side
of their head, it is believed that hearing underwater is also if not exclusively done with the lower jaw which conducts the sound
vibrations to the middle ear via a fat-filled cavity in the lower jaw bone. Hearing is also
used for echolocation, which seems to be an ability all dolphins have. Their teeth
are arranged in a way that works as an array or antenna to receive the incoming sound and make it easier for them to pinpoint the
exact location of an object.[8] The dolphin's sense of
touch is also well-developed. However, dolphins lack an olfactory nerve and lobes and thus are believed to have no
sense of smell,[9] but
they can taste and do show preferences for certain kinds of fish. Since dolphins spend most of
their time below the surface normally, just tasting the water could act in a manner analogous to a sense of smell.
Though most dolphins do not have any hair, they do still have hair follicles and it is
believed these might still perform some sensory function, though it is unclear what exactly this may be.[10] The small hairs on the rostrum of the Boto river dolphin are believed to
function as a tacticle sense however, possibly to compensate for the Boto's poor
eyesight.[11]
Behaviour
- See also: Whale
behaviour
Pacific White-Sided Dolphins breaching
Dolphins are often regarded as one of Earth's most intelligent animals, though it is hard to say just how intelligent dolphins
are, as comparisons of species' relative intelligence are complicated by differences in sensory apparatus, response modes, and
nature of cognition. Furthermore, the difficulty and expense of doing experimental work with large aquatics means that some tests
which could yield meaningful results still have not been carried out, or have been carried out with inadequate sample size and
methodology. Dolphin behaviour has been studied extensively by humans however, both in captivity and in the wild. See the
cetacean intelligence article for more details.
Social behaviour
Dolphins are social, living in pods (also called "schools") of up to a dozen individuals. In places with a high abundance of
food, pods can join temporarily, forming an aggregation called a superpod; such groupings may exceed a thousand dolphins.
The individuals communicate using a variety of clicks, whistles and other vocalizations. They also use ultrasonic sounds for
echolocation. Membership in pods is not rigid; interchange is common. However, the cetaceans can establish strong bonds between
each other. This leads to them staying with injured or ill individuals.
In May 2005, researchers in Australia discovered a cultural aspect of dolphin behaviour: Some dolphins (Tursiops
aduncus) teach their children to use tools. The dolphins break sponges off and cover their
snouts with them thus protecting their snouts while foraging. This knowledge of how to use a tool is mostly transferred from
mothers to daughters, unlike simian primates, where the
knowledge is generally passed on to both sexes. The technology to use sponges as mouth protection is not genetically inherited
but a taught behaviour.[12]
They are also occasionally willing to approach humans and playfully interact with them in the water. Dolphins have also been
known to seemingly protect swimmers from sharks by swimming circles around them.[13]
Dolphins are known to engage in acts of aggression towards each other. The older a male dolphin is, the more likely his body
is covered with scars ranging in depth from teeth marks made by other dolphins. It is suggested that male dolphins engage in such
acts of aggression for the same reasons as humans: disputes between companions or even competition for other females. Acts of
aggression can become so intense that targeted dolphins are known to go into exile, leaving their communities as a result of
losing a fight with other dolphins.
Male Bottlenose Dolphins have been known to engage in infanticide. Dolphins have also
been known to kill porpoises for reasons which are not fully understood, as porpoises generally
do not share the same fish diet as dolphins and are therefore not competitors for food supplies.[14]
Reproduction and sexuality
Dolphin copulation happens belly to belly and though many species engage in lengthy
foreplay, the actual act is usually only brief, but may be repeated several times within a
short timespan. The gestation period varies per species; for the small Tucuxi dolphin,
this period is around 11 to 12 months, while for the Orca the gestation period is around 17 months. They usually become sexually
active at a young age already, even before reaching sexual maturity. The age at which sexual maturity is reached varies per
species and gender.
Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in acts of a homosexual nature.[15]
Various dolphin species have been known to engage in sexual behaviour with other dolphin species,[15] this also having resulted in various hybrid dolphin species as
mentioned earlier. Sexual encounters may be violent, with male dolphins sometimes showing aggressive behaviour towards both
females and other male dolphins.[15][16] Occasionally, dolphins will also show sexual behaviour
towards other animals, including humans.[17]
Feeding
Various methods of feeding exist, not just between species but also within a species various methods may be employed, some
techniques being used by only a single dolphin population. Fish and squid are the main source of food for most dolphin species,
but the False Killer Whale and the Killer Whale also feed on other marine mammals.
One feeding method employed by many species is herding, where a pod will control a school of fish while individual members
take turns plowing through the school, feeding. The tightly packed school of fish is commonly known as bait ball. Coralling is a
method where fish are chased to shallow water where they are more easily captured. In South
Carolina, the Atlantic Bottlenose Dolphin takes this one step further with what has become known as strand feeding, where
the fish are driven onto mud banks and retrieved from there.[18] In some places, Orcas will also come up to the beach to capture sea lions. Some species also whack fish with their fluke, stunning them and sometimes sending
fish clear out of the water.
Reports of cooperative human-dolphin fisheries date back to the ancient Roman author and
natural philospher Pliny.[19] A modern human-dolphin fishery still takes place in
Laguna, Santa Catarina,
Brazil.[20]
Vocalizations
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 however; frequency modulated sounds which
are usually just called whistles; burst-pulsed sounds and clicks. Whistles are used by dolphins to communicate, though the nature
and extent of their ability to communicate in this way is not known. Research has shown however that at least some dolphin
species are capable of sending identity information to each other using a signature whistle; a whistle that refers specifically
to the identity of a certain dolphin. The burst-pulsed sounds are also used for communication, but again the nature and extent of
communication possible this way is not known.[21] The
clicks are directional and used by dolphins for echolocation and are often in a short series called a click train, the rate
increasing when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by animals in the sea.[22]
Jumping and playing
Dolphins often leap above the water surface, sometimes performing acrobatic figures (e.g. the spinner dolphin). Scientists are not always quite certain about the purpose of this behaviour and the
reason for it may vary, it could be to locate schools of fish by looking at above-water signs like feeding birds, they could be
communicating to other dolphins to join a hunt, attempting to dislodge parasites, or simply doing it for fun. Play is a very
important part of dolphins' lives, and they can often be observed playing with seaweed or play-fighting with other dolphins. They
even harass other locals, like seabirds and turtles. Dolphins also seem to enjoy riding waves and frequently 'surf' coastal
swells and the bow waves of boats.
Threats to dolphins
Natural threats to dolphins
Except for mankind (discussed below), dolphins have few natural enemies, some species or specific populations having none at
all making them apex predators. For most smaller species of dolphins, only a few larger
species of shark such as the bull shark, dusky shark,
tiger shark and great white shark are a potential
risk, especially for calves. Some of the larger dolphin species such as Orcas may also prey on some of the smaller dolphin
species, but this seems rare. Dolphins may also suffer from a wide variety of diseases and
parasites.
Human threats to dolphins
- See also: Dolphin drive
hunting
Some dolphin species face an uncertain future, especially some of the river dolphin species such as the Amazon River dolphin,
and the Ganges and Yangtze River dolphin, all of which are critically or seriously endangered. A
2006 survey found no individuals of the Yangtze River dolphin, leading to the conclusion that the species is now functionally extinct.[23]
Contamination of environment - the oceans, seas, and rivers - is an issue of concern, especially pesticides, heavy metals, plastics, and other industrial and agricultural pollutants which do not disintegrate
rapidly in the environment are reducing dolphin populations, and resulting in dolphins building up unusually high levels of
contaminants. Injuries or deaths due to collisions with boats, especially their propellers,
are also common.
Various fishing methods, most notably purse seine fishing for tuna and the use of drift and gill nets, results in a large amounts of dolphins being killed inadvertently.[24] Accidental by-catch in trout
nets is common and poses a risk for mainly local dolphin populations. In some parts of the world, such as some areas in Japan and
the Faroe Islands, dolphins are traditionally considered as food, and killed in harpoon or
drive hunts.
Human-dolphin relationships
Mythology
- See also: Dolphins in
mythology
Dolphins have long played a role in human culture. Dolphins are common in Greek
mythology and there are many coins from the time which feature a man or boy riding on the back of a dolphin. The
Ancient Greeks treated them with welcome; a ship spotting dolphins riding in their wake
was considered a good omen for a smooth voyage. Dolphins also seem to have been important to the Minoans, judging by artistic evidence from the ruined palace at Knossos. In Hindu mythology, the Ganges River Dolphin is associated
with Ganga, the deity of the Ganges river.
Therapy
Dolphins are an increasingly popular choice of animal-assisted therapy for
psychological problems and developmental disabilities. For example, a 2005 study with 30 participants found it was an effective
treatment for mild to moderate depression.[25] However, this study was criticized on several grounds; for example, it is not
known whether dolphins are more effective than common pets.[26] Reviews of this and other published dolphin-assisted therapy (DAT)
studies have found important methodological flaws and have concluded that there is no compelling scientific evidence that DAT is
a legitimate therapy or that it affords any more than fleeting improvements in mood.[27]
Entertainment
- See also: Dolphinarium
The famous Orca Keiko from the Free Willy movies being prepared for transport.
In more recent times, the 1963 Flipper movie and the subsequent popular
Flipper television series, contributed to the popularity of dolphins in
Western society. The series, created by Ivan Tors,
portrayed a dolphin in a friendly relationship with two boys, Sandy and Bud; a kind of seagoing Lassie. Flipper, a Bottlenose Dolphin, understood English unusually well and was a marked
hero. A second Flipper movie was made in 1996,
which was based on the story of the original movie. A Bottlenose Dolphin also played a prominent role in the 1990s
science fiction television series seaQuest DSV in which the animal, named Darwin, could communicate with English speakers using a
vocoder, a fictional invention which translated the clicks and whistles to English and back.
More well known from this time period is probably the movie Free Willy however,
which made famous the Orca playing Willy, Keiko. The 1977
horror movie Orca paints a less friendly picture
of the animal. Here, a male Orca takes revenge on fishermen after the killing of his mate. In the 1973 movie The Day of the Dolphin trained dolphins are
kidnapped and made to perform a naval military assassination using explosives.
The renewed popularity of dolphins in the 1960s resulted in the appearance of many dolphinariums around the world, which have made dolphins accessible to the public. Though criticism and
more strict animal welfare laws have forced many dolphinariums to close their doors,
hundreds still exist around the world attracting a large amount of visitors. In the United
States, best known are the SeaWorld marine mammal parks, and their common Orca
stage name Shamu, which they have trademarked, has become well known. Southwest Airlines, an
American airline, has even painted three of their Boeing 737 aircraft in Shamu colours
as an advertisement for the parks and have been flying with such a livery on various aircraft since 1988.
Occasionally, dolphins make an appearance in computer games. Best known is the
Ecco the Dolphin game series. The games are named after their main character,
Ecco, a young Bottlenose Dolphin. The Ecco the Dolphin games hinge on the idea that cetaceans are sapient beings and have their
own underwater society.
A well known American National Football League (NFL) team is named the
Miami Dolphins. Their logo depicts an aqua-coloured Bottlenose Dolphin wearing an
American football helmet and jumping in front of a coral-coloured sunburst.
Military
A number of militaries have employed dolphins for various purposes from finding mines to rescuing lost or trapped humans. Such
military dolphins, however, drew scrutiny during the Vietnam War when rumors circulated
that dolphins were being trained to kill Vietnamese skin divers. Best known today is the
U.S. Navy Marine Mammal Program.
Literature
Dolphins are also common in contemporary literature, especially science fiction
novels. A military role for dolphins is found in William Gibson's short story
Johnny Mnemonic, in which cyborg
dolphins are used in war-time by the military to find submarines and, after the war, by a group of revolutionaries to decode
encrypted information. Dolphins play a role as sentient patrollers of the sea enhanced with a deeper empathy toward humans in
Anne McCaffrey's The Dragonriders of
Pern series. In the Known Space universe of author Larry Niven, dolphins also play a significant role as fully-recognised "legal entities". More humorous is
The Hitchhiker's Guide to the Galaxy, in which dolphins are
the second most intelligent creatures on Earth (after mice, and followed by humans) and tried in vain to warn humans of the
impending destruction of the planet. However, their behaviour was misinterpreted as playful acrobatics. Their story is told in So Long, and
Thanks for All the Fish. Much more serious is their major role (along with chimpanzees) in David Brin's Uplift series. A talking Dolphin called "Howard" helps Hagbard
Celine and his submarine crew fight the evil Illuminati in Robert Shea and
Robert Anton Wilson's Illuminatus
Trilogy.
Dolphins also appear frequently in non-science fiction literature however. In the book The Music of Dolphins by author Karen Hesse, a girl is
raised by dolphins from the age of four until she is discovered by the coast guard. Fantasy
author Ken Grimwood wrote dolphins into his 1995 novel
Into the Deep about a marine biologist struggling to crack the code of
dolphin intelligence, including entire chapters written from the viewpoint of his
dolphin characters. In this book, humans and dolphins are capable of communicating via telepathy.
2007: Year of the Dolphin
The year 2007 has been declared as (International) Year of
the Dolphin by the United Nations and United Nations Environment Programme (UNEP) in an effort to promote dolphin conservation.[28] The idea was launched by the UN's Convention
on Migratory Species and the Whale and Dolphin Conservation
Society (WDCS).
References
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online entry at Dictionary.com,
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- ^ a b c Herzing D.L.,
Rogers C.A., for the Wild Dolphin Project, Directionality of sexual aggression in mixed-species encounters between Atlantic Spotted dolphins
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- ^ Scott et. al,Aggression in bottlenose
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- ^ M.B. Santos, R. Fernández, A. López, J.A. Martínez and G.J. Pierce
(2007), Variability in the diet of bottlenose dolphin, Tursiops truncatus, in Galician waters, north-western Spain, 1990 –
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- ^ Clover, Charles. 2004. The End of the Line: How overfishing is
changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7
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- ^ Biju Basil, Maju Mathews (2005).
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