Dolphin

Dolphins are aquatic mammals which 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

Suborder Odontoceti, toothed whales
Family Delphinidae, oceanic dolphins
Genus Delphinus
Long-Beaked Common Dolphin, Delphinus capensis
Short-Beaked Common Dolphin, Delphinus delphis
Genus Tursiops
Bottlenose Dolphin, Tursiops truncatus
Indo-Pacific Bottlenose Dolphin, Tursiops aduncus
Genus Lissodelphis
Northern Rightwhale Dolphin, Lissodelphis borealis
Southern Rightwhale Dolphin, Lissiodelphis peronii
Genus Sotalia
Tucuxi, Sotalia fluviatilis
Genus Sousa
Indo-Pacific Hump-backed Dolphin, Sousa chinensis
Chinese White Dolphin (the Chinese variant), Sousa chinensis chinensis
Atlantic Humpbacked Dolphin, Sousa teuszii
Genus Stenella
Atlantic Spotted Dolphin, Stenella frontalis
Clymene Dolphin, Stenella clymene
Pantropical Spotted Dolphin, Stenella attenuata
Spinner Dolphin, Stenella longirostris
Striped Dolphin, Stenella coeruleoalba
Genus Steno
Rough-Toothed Dolphin, Steno bredanensis
Genus Cephalorynchus
Chilean Dolphin, Cephalorhynchus eutropia
Commerson's Dolphin, Cephalorhynchus commersonii
Heaviside's Dolphin, Cephalorhynchus heavisidii
Hector's Dolphin, Cephalorhynchus hectori
Genus Grampus
Risso's Dolphin, Grampus griseus
Genus Lagenodelphis
Fraser's Dolphin, Lagenodelphis hosei
Genus Lagenorhyncus
Atlantic White-Sided Dolphin, Lagenorhynchus acutus
Dusky Dolphin, Lagenorhynchus obscurus
Hourglass Dolphin, Lagenorhynchus cruciger
Pacific White-Sided Dolphin, Lagenorhynchus obliquidens
Peale's Dolphin, Lagenorhynchus australis
White-Beaked Dolphin, Lagenorhynchus albirostris
Genus Orcaella
Australian Snubfin Dolphin, Orcaella heinsohni
Irrawaddy Dolphin, Orcaella brevirostris
Genus Peponocephala
Melon-headed Whale, Peponocephala electra
Genus Orcinus
Killer Whale, Orcinus orca
Genus Feresa
Pygmy Killer Whale, Feresa attenuata
Genus Pseudorca
False Killer Whale, Pseudorca crassidens
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
Chinese River Dolphin (Baiji), Lipotes vexillifer (considered functionally extinct)
Genus Platanista
Ganges River Dolphin, Platanista gangetica
Indus River Dolphin, Platanista minor
Genus Pontoporia
La Plata Dolphin (Franciscana), Pontoporia blainvillei
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]

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

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 meaningfully be done 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.

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 willing to occasionally 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 one of the few animals other than humans 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]

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.

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]