Dolphins are
marine 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 m (4 ft) and 40 kg (90 lb) (
Maui's Dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 LT; 11 ST) (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 δελφύς (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 reintroduced 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". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".
[2]
Hybrid dolphins
In 1933, three strange dolphins were beached off the
Irish coast; these appeared to be
hybrids between Risso's Dolphin and the Bottlenose Dolphin.
[3] 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.
[4] A Common-Bottlenose hybrid lives at
SeaWorld California
[5] Various other dolphin hybrids live in captivity around the world or have been reported in the wild, such as a Bottlenose-Atlantic Spotted hybrid.
[6] 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.
[7]Evolution and anatomyThe Anatomy of a Dolphin showing its skeleton, major organs, tail, and body shape
Evolution
See also:
Evolution of cetaceansDolphins, along with whales and porpoises, are 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 2.5 cm (1.0 in) long.
Modern dolphin
skeletons have two small, rod-shaped pelvic bones thought to be
vestigial hind limbs. 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 limbs.
[8]AnatomyDolphins have a streamlined
fusiform body, adapted for fast swimming. The tail fin, called the
fluke, is used for
propulsion, while the
pectoral fins together with the entire tail section provide directional control. The
dorsal fin, in those species that have one, provides stability while swimming.
Though it varies per species, basic colouration patterns are shades of grey usually with a lighter underside. 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. Dolphins breathe through a
blowhole located on top of their head, with the
trachea being
anterior to the brain. The
dolphin brain is large and highly complex and is different in structure from that of most land mammals.
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 shortly after birth, in some cases even before they are born.
[9] The only exception to this is the Boto river dolphin, which does have some small hairs on the rostrum.
[10]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 perception of sound extends ten times or more above the upper limit of adult human hearing.
[11] 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. It is believed that 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.
[12] The dolphin's sense of touch is also well-developed, with free nerve endings being densely packed in the skin, especially around the snout, pectoral fins and genital area. However, dolphins lack an
olfactory nerve and lobes and thus are believed to have no
sense of smell,
[13] 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.
[14] The small hairs on the rostrum of the Boto river dolphin are believed to function as a
tactile sense however, possibly to compensate for the Boto's poor eyesight.
[15]Behaviour
A pod of Indo-Pacific Bottlenose Dolphins in the
Red SeaSee also:
Whale surfacing behaviourDolphins 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. Compared to many other species however, dolphin behaviour has been studied extensively by humans, both in captivity and in the wild. See the
cetacean intelligence article for more details.
Social behaviour
Dolphins surfing at
Snapper Rocks,
Queensland,
AustraliaDolphins 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, even actively helping them to breathe by bringing them to the surface if needed.
[16] This
altruistic behaviour does not appear to be limited to their own species however. A dolphin in
New Zealand that goes by the name of
Moko has been observed to seemingly help guide a female
Pygmy Sperm Whale together with her calf out of shallow water where they had stranded several times.
[17] They have also been known to seemingly protect swimmers from sharks by swimming circles around the swimmers
[18][19] or charging the sharks to make them go away.[
citation needed]
Dolphins also show
cultural behaviour, something long believed to be a quality unique to humans (or to humans and some other
primate species). In May 2005, a discovery was made in Australia which shows this cultural aspect of dolphin behaviour: Some dolphins, such as the
Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) teach their young 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.
[20] Another such behaviour was discovered amongst river dolphins in Brazil, where some male dolphins apparently use objects such as weeds and sticks as part of a sexual display.
[21]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.
[22]Reproduction and sexualityDolphin
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, 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.
[23] Various dolphin species have been known to engage in sexual behaviour with other dolphin species,
[23] 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.
[23][24] Occasionally, dolphins will also show sexual behaviour towards other animals, including humans.
[25]FeedingVarious 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 a 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.
[26] 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 philosopher Pliny the Elder.
[27] A modern human-dolphin fishery still takes place in
Laguna,
Santa Catarina,
Brazil. Here, dolphins drive fish towards fishermen waiting along the shore and give them a signal when they can cast their nets. The dolphins then feed off the fish that manage to escape the nets.
[28][29]VocalizationsDolphins 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.
[30] 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.
[31]Pacific White-Sided Dolphins breaching
Jumping and playing
Dolphins occasionally 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 fairly important part of dolphins' lives, and they can be observed playing with
seaweed or play-fighting with other dolphins. At times they also harass other local creatures, like
seabirds and
turtles. Dolphins also seem to enjoy riding waves and frequently 'surf' coastal swells and the bow waves of boats. Occasionally, they're also willing to playfully interact with human swimmers.
SleepingBecause dolphins need to come up to the surface to breathe and have to be alert for possible predators, they do not
sleep in the same way land mammals do. Generally, dolphins sleep with only one brain
hemisphere in
slow-wave sleep at a time, thus maintaining some amount of consciousness required to breathe and keeping one eye open to keep a watch out for possible threats. The earlier stages of sleep can be observed in both hemispheres of the brain, however.
[32][33][34]However, in captivity, dolphins have been observed to seemingly enter a fully asleep state where both eyes are closed and the animal does not respond to mild external stimuli, respiration being automatic with a tail kick
reflex keeping the blowhole above the water. If not needed to keep the blowhole above the water, the tail kick reflex may subside. Dolphins kept
unconscious using
anesthetics initially show a similar tail kick reflex.
[35] Though a similar state has been observed with wild
Sperm Whales, it is not known if this state is ever reached in the wild amongst any dolphin species.
[36]ThreatsNatural threats
Except for humans (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.
Dead
Atlantic White-Sided Dolphins in
Hvalba on the
Faroe Islands, killed in a drive hunt.
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.
[37]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.
[38] Accidental by-catch in gillnets and incidental captures in antipredator nets used in marine fish farms are common and poses a risk for mainly local dolphin populations.
[39][40] Dolphin safe labels have been introduced to reassure consumers that the fish sold has been caught in a dolphin friendly way. In some parts of the world such as
Taiji in Japan and the Faroe Islands, dolphins are traditionally considered as food, and killed in
harpoon or
drive hunts.
It is believed that loud underwater noises, for example resulting from
naval sonar use, live firing exercises or certain
offshore construction projects such as the construction of offshore
wind farms may be harmful to dolphins, distressing the animals, damaging their hearing and possibly forcing them to the surface quicker resulting in
decompression sickness.
[41][42]Human–dolphin relationships
Mythology
A sketch of the goddess Ganga on her
Vahana (mount)
MakaraSee also:
Dolphins in mythologyDolphins 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. In
Hindu mythology, the Ganges River Dolphin is associated with
Ganga, the deity of the
Ganges river.
Popular culture
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 commands 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.
DolphinariumsSee also:
DolphinariumThe 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 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.
WelfareDolphins 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.
[43] However, this study was criticized on several grounds; for example, it is not known whether dolphins are more effective than common pets.
[44] 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.
[45]MilitarySee also:
Military dolphinA 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 by the
United States Navy to kill Vietnamese divers.
[46] However, no evidence to support these rumors ever surfaced, and the United States Navy denies that at any point Dolphins were trained to do harm. Dolphins are still being trained by the United States Navy as part of the
U.S. Navy Marine Mammal Program. The Russian military is believed to have closed its marine mammal program in the early 1990s. In 2000 the press reported that dolphins trained to kill by the
Soviet Navy had been sold to
Iran.
[47]LiteratureDolphins 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.
Art
