The word is used in a few different ways. It can mean:
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".
Six species in the family Delphinidae are commonly called "whales" but are strictly speaking dolphins. They are sometimes called "blackfish".
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.
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 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. 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.
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. 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.
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.
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, even actively helping them to breathe by bringing them to the surface if needed. 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. They have also been known to seemingly protect swimmers from sharks by swimming circles around the swimmers or charging the sharks to make them go away.
Dolphins also show cultural behaviour, something long believed to be a quality unique to humans. 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. 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.
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.
Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in acts of a homosexual nature. Various dolphin species have been known to engage in sexual behaviour with other dolphin species, 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. Occasionally, dolphins will also show sexual behaviour towards other animals, including humans.
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. 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. 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.
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.
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. 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.
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.
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. 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. 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.
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. In Hindu mythology, the Ganges River Dolphin is associated with Ganga, the deity of the Ganges river.
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.
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 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.
Ecco the Dolphin is a well known video 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. The science fiction roleplaying game Blue Planet includes genetically "uplifted" dolphins as playable characters.
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.