Dolphin

A dolphin is any one of the 40 extant species of aquatic mammal from the cetacean families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the brackish dolphins), and the probably extinct Lipotidae (baiji or Chinese river dolphin). All these families belong to the parvorder Odontoceti, which are toothed whales. This group also includes the closely related families Monodontidae (beluga and narwhal) and Phocoenidae (porpoises), as well as the more distant families Physeteroidea (sperm whales) and Ziphiidae (beaked whales).

Dolphins vary greatly in size. They range from the human-sized Maui's dolphin, which is 1.7 metres (5 ft 7 in) long and weighs 50 kilograms (110 pounds), to the apex predator called the orca, which reaches 9.5 m (31 ft) and 10 tonnes (11 short tons). Various species show dimorphism, where the males are larger than the females. Dolphins have streamlined, fish-like bodies. Their two forelimbs have evolved into flippers, they have lost their hind limbs (with only a vestigial pelvic skeleton remaining), and their tail forms a horizontal fluke. They are not as flexible as the semiaquatic seals, but they are faster and more agile in water. Some dolphins can briefly travel at speeds of 29 kilometres per hour (18 mph) or leap about 9 metres (30 ft). They use their conical teeth to catch fast-moving prey such as forage fish, coleoids and shrimps. Their hearing is well developed and works in both air and water. Some species are well adapted for diving to great depths. Like other aquatic mammals, they have a thick layer of subcutaneous adipose tissue, called blubber, which keeps their bodies warm in cold water.

Dolphins live across the world, mostly in pelagic marine environments, though some thrive in brackish and freshwater bodies. Most species prefer the warm waters of the tropic zones, but some, such as the right whale dolphin, prefer colder climates. Like most nektonic animals, dolphins are carnivorous. They feed mostly on fish and squid. A few large dolphins, such as the orca, hunt bigger sea animals such as seals, other dolphins and porpoises, large deep-sea fish such as sharks, billfish and bluefin tuna, and sometimes even seabirds. Male dolphins typically mate with several females each year, but females mate only every two to three years. Calves are usually born in the spring and summer months, and females take on all the responsibility for raising them. Mothers of some species fast and nurse their young for a fairly long time.

Dolphins are highly social animals that live in complex "fission-fusion" societies. They form fluid groups, called pods, that constantly change in size and makeup. Like other whales, dolphins make a variety of vocalizations, usually high-pitched clicks and whistles. They are also very skilled at echolocation.

Dolphins are sometimes hunted, although this is relatively uncommon compared to whaling. Hunting happens in places such as Japan and parts of Latin America, where dolphin meat is called chancho marino, meaning "sea pork", in Peru. This activity is known as dolphin drive hunting. Dolphins also face threats from bycatch, ghost net entanglement, habitat loss and marine pollution. Because of their grace and high intelligence, dolphins appear prominently in many cultures around the world, such as in art and folklore. Some dolphins are kept in captivity in dolphinariums for research, conservation, or training to perform tricks in marine mammal parks. The most common dolphin species in captivity is the bottlenose dolphin, and there are also around 60 orcas in captivity.

Etymology

The name comes from the Greek word δελφίς (delphís), meaning "dolphin." This word was related to the Greek word δελφύς (delphus), meaning "womb," so the animal's name can be understood as "a 'fish' with a womb." The name passed through the Latin delphinus, the romanization of the later Greek δελφῖνος (delphinos). In Medieval Latin it became dolfinus, and in Old French it became daulphin, which brought back the ph spelling used in the word dolphin. The term mereswine, meaning "sea pig," is also used.

The word dolphin can refer to most species in the family Delphinidae (oceanic dolphins) and to the river dolphin families Iniidae (South American river dolphins), Pontoporiidae (La Plata dolphin), Lipotidae (Yangtze river dolphin), and Platanistidae (Ganges river dolphin and Indus river dolphin). The mahi-mahi fish is called the dolphinfish. In everyday use, whale refers to the larger cetaceans, while smaller ones with a beak or long nose are called dolphins. Dolphin is also used casually for the bottlenose dolphin, the most common and familiar dolphin. Six dolphins that are often thought of as whales are known as blackfish: the orca, the melon-headed whale, the pygmy killer whale, the false killer whale, and the two pilot whales. All of them belong to the family Delphinidae and are dolphins. Although dolphin and porpoise are sometimes used interchangeably, porpoise usually refers to the Phocoenidae family, which has a shorter beak, spade-shaped teeth, and different behavior. A group of dolphins is called a school or a pod. Male dolphins are called bulls, females are called cows, and young dolphins are called calves.

Evolution

Main article: Evolution of cetaceans

Dolphins are descendants of land-living mammals of the artiodactyl order (even-toed ungulates). They are related to Indohyus, an extinct chevrotain-like ungulate, from which they split about 48 million years ago. The first archaeocetes, or primitive cetaceans, went to sea about 49 million years ago and became fully aquatic 5 to 10 million years later. Archaeoceti is a group of ancient whales, the ancestors of modern whales. Their bodies changed over time for life in the marine environment. These changes included better hearing through Ambulocetus 49 million years ago, a streamlined body and tail flukes with Protocetus around 43 million years ago, and nostrils that moved toward the top of the cranium and forelimbs that became flippers with Basilosaurus 35 million years ago. The hind limbs shrank and disappeared 34 million years ago.

The modern dolphin skeleton has two small pelvic bones, which are 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 is an unusually strong development of these vestigial hind limbs. Today, the closest living relatives of cetaceans are hippopotamuses. They share a semi-aquatic ancestor that branched off from other artiodactyls some 60 million years ago. About 40 million years ago, a common ancestor branched into cetaceans and anthracotheres. Anthracotheres became extinct at the end of the Pleistocene, two and a half million years ago, leaving only one surviving lineage: the two species of hippo.

Anatomy

Dolphins have torpedo-shaped bodies, necks that do not bend much, flippers made from their limbs, a tail fin, and rounded heads. Their skulls have small eye sockets and long snouts, and their eyes sit on the sides of the head. They have no outer ear flaps. Dolphins range from Maui's dolphin, about 1.7 m (5 ft 7 in) long and 50 kg (110 lb), to the orca, which can reach 9.5 m (31 ft 2 in) and 10 t (11 short tons). Compared with other Cetartiodactyls, they are usually smaller. In several species, females are larger than males. Dolphins have conical teeth, unlike the spade-shaped teeth of porpoises, and they use them to catch fast prey such as fish, squid, or large mammals like seals. To breathe, a dolphin pushes stale air out through its blowhole in an upward blast, then takes in fresh air. All dolphins have a thick layer of blubber, which mainly keeps them warm, and calves are born with only a thin layer. Their stomach has two or three chambers, the fundic and pyloric chambers. Their reproductive organs are inside the body, with genital slits on the belly. Males have two slits, one holding the penis and one further back for the anus. Females have one slit for the vagina and the anus, with a mammary slit on each side. Dolphin skin is smooth and rubber-like, with no hair or glands except mammary glands, and it has two main parts, the epidermis and the blubber. The epidermis has rete pegs that help hold it to deeper layers. Nerve endings in the skin are highly proprioceptive, which helps a dolphin sense where its body is and how it moves. Dolphins have two flippers, a boneless dorsal fin that helps keep them steady, and a tail fluke that pushes them through the water. Orcas are faster than seals, and can reach 55.5 km/h (34.5 mph). Most dolphins cannot turn their heads because their neck bones are fused, but river dolphins can turn theirs up to 90°. For deep dives, some dolphins can slow their heart rate to save oxygen, and they store oxygen in hemoglobin and myoglobin.

Dolphin ears are adapted to life in the marine world. In humans, the middle ear helps match the low impedance of air with the high impedance of the cochlear fluid. Dolphins and other marine mammals have no such great difference between outside and inside, so they take in sound through the throat instead. Dolphins make clicks that pass through a fatty melon in the head, which helps them use echolocation to find objects and judge their shape and size. Dolphins have few hairs, but hair follicles may help them sense things. The small hairs on the snout of the boto, a South American river dolphin, are believed to act as a tactile sense, possibly because its eyesight is poor. A dolphin's eyes are small for its size but still see well. Their eyes are on the sides of the head, so they have two separate fields of view. They have both rod and cone cells, so they can see in dim and bright light, but they have far more rods, so their colour vision is more limited than in most mammals. A tapetum lucidum helps large amounts of light reach the eye, and glands protect the outer corneal layer. The olfactory lobes and nerve are absent, suggesting dolphins have no sense of smell. Their sense of taste is thought to be weak, though some prefer certain kinds of fish.

Intelligence

Further information: Cetacean intelligence and Cetacean surfacing behaviour

Dolphins are known to teach, learn, cooperate, scheme, and grieve. Many species have long, thin spindle neurons in the neocortex, a type of brain cell that, before 2007, was known only in hominids. In humans, these cells are involved in social behaviour, emotions, judgment, and understanding others' minds. Dolphins have these cells in brain areas similar to where humans have them, which suggests they may do something similar. Brain size was once seen as a major sign of an animal's intelligence. Most of the brain controls the body, so a larger brain compared to body size may leave more room for complex thinking. Orcas have the second largest brain mass of any animal on earth, after the sperm whale, and their brain to body mass ratio is second only to humans.

Self-awareness is seen by some as a sign of advanced, abstract thinking. It is not clearly defined in science, but it is believed to lead to more advanced thinking, such as meta-cognitive reasoning (thinking about thinking). Research suggests that cetaceans, among other animals, have self-awareness. One common test is the mirror test, in which an animal is shown a mirror and then marked with a temporary dye. If the animal goes to the mirror to look at the mark, this is strong evidence of self-awareness. Some scientists disagree, arguing that test results can be read in different ways and may be affected by the Clever Hans effect. Because dolphins cannot touch the mark or the mirror the way primates can, their results are less certain. In 1995, researchers Marten and Psarakos used television to test dolphins, and they concluded that the results pointed to self-awareness rather than social behaviour. Dolphins have since passed the mirror test, although some researchers still argue that self-awareness has not been clearly shown.

Behavior

Socialization

Dolphins are very social animals. They often live in groups called pods, which can have up to a dozen members, though pod sizes vary between species and places. When food is plentiful, pods may join together into a temporary superpod of more than 1,000 dolphins. Dolphins form strong bonds and will stay with injured or sick members, bringing them to the surface to breathe if needed. This altruism is not only shown toward their own species. The dolphin Moko in New Zealand guided a pygmy sperm whale and her calf out of shallow water, and dolphins have been seen protecting swimmers from sharks.

Reproduction and sexuality

See also: Bottlenose dolphin § Reproduction, Dusky dolphin § Social behavior and reproduction, and Short-beaked common dolphin § Reproduction

Dolphins usually give birth to a single calf, which is born tail first in most cases. The gestation period differs by species. It is about 11 to 12 months for the small tucuxi dolphin and about 17 months for the orca. Dolphins often reach maturity at a young age, and the age varies by species and sex.

Hybridization

Dolphins of different species sometimes produce hybrid young, both in captivity and in the wild. The best known hybrid is the wholphin, a cross between a false killer whale and a bottlenose dolphin. Wholphins are fertile, and two live at the Sea Life Park in Hawaii.

Sleeping

Further information: Sleep in animals

Dolphins usually rest one brain hemisphere at a time while the other stays alert. This helps them keep breathing and watch for danger. The Indus river dolphin sleeps very differently. It lives in water with strong currents and floating debris, so it must keep swimming. It sleeps in short bursts lasting between 4 and 60 seconds.

Feeding

Fish and squid are the main foods of dolphins. Some dolphins, such as the false killer whale and the orca, also eat other marine mammals. One common method is herding, in which a group of dolphins pushes fish into a tight ball called a bait ball. In Shark Bay, Australia, some dolphins also use conch shells to catch fish. A dolphin lifts the shell, shakes it, and the fish inside fall into its mouth.

Vocalization

Dolphins make many sounds, including frequency modulated whistles, burst-pulsed sounds, and clicks. The clicks are used for echolocation, and dolphin echolocation clicks are among the loudest sounds made by marine animals. Bottlenose dolphins each have a signature whistle that works like a name, helping other dolphins identify them.

Jumping and playing

Dolphins often leap above the water. Jumping can save energy while traveling, and it is also used for communication, play, and other reasons. Dolphins play with objects, make bubble rings, and sometimes interact with other animals, including humpback whales and dogs. Juvenile dolphins off Western Australia have been seen chasing and chewing on blowfish.

Tail-walking

Tail-walking is very unusual in wild dolphins. Some Indo-Pacific bottlenose dolphins in the Port River, north of Adelaide, South Australia, have been seen doing it. A dolphin named Billie started the behavior in 1988 after she was rescued and spent time with captive dolphins, and other dolphins copied her. Scientists think social learning explains how the behavior spread, even though it gives no clear advantage and uses a lot of energy.

Threats

Dolphins have few enemies in the sea. Some kinds have none, which makes them apex predators. Most smaller dolphins face only a few large sharks, such as the bull shark, dusky shark, tiger shark and great white shark, which are a possible danger, especially for calves. Some large dolphins, especially orcas, may hunt smaller dolphins, but this seems rare. Dolphins also suffer from many diseases and parasites. The Cetacean morbillivirus has caused regional epizootics that have left hundreds of animals dead. Infected dolphins often have pneumonia, encephalitis and damage to the immune system. A study at the U.S. National Marine Mammal Foundation found that dolphins, like humans, can develop a natural form of type 2 diabetes, which may help people understand the disease and find new treatments. Dolphins can also recover from extreme injuries, such as shark bites. Their healing is rapid, and even very deep wounds do not cause them to hemorrhage to death. A study in the journal Marine Mammal Science suggests that some dolphins survive shark attacks using everything from sophisticated combat moves to teaming up against the shark.

Humans

See also: Dolphin drive hunting and Cetacean bycatch

Some dolphins are at risk of extinction, especially river dolphins such as the Amazon river dolphin and the Ganges and Yangtze river dolphin, which are critically or seriously endangered. A 2006 survey found no Yangtze river dolphins, and the species now appears to be functionally extinct. Pesticides, heavy metals, plastics and other pollutants build up in predators such as dolphins, and collisions with boats and their propellers cause injuries and deaths. Fishing methods such as seine fishing for tuna and the use of drift and gill nets unintentionally kill many dolphins through by-catch. In places such as Taiji in Japan and the Faroe Islands, dolphins are traditionally eaten and are killed by harpoon or drive hunts. Shark control programs in Queensland (a shark culling program), New South Wales and KwaZulu-Natal use shark nets and drum lines, which have also trapped and killed many dolphins. Loud underwater noises from naval sonar, live firing and offshore construction projects such as wind farms may stress dolphins, damage their hearing and cause decompression sickness. Dolphin drive hunts, which herd dolphins into a bay or onto a beach, take place in places such as the Solomon Islands, the Faroe Islands, Peru and Japan, mostly for their meat, though some dolphins end up in dolphinariums. Thousands of dolphins are still caught in these hunts each year.

Impacts of climate change

See also: Effects of climate change on oceans

Climate change affects dolphins in many ways. Rising ocean temperatures have caused many species to move toward cooler waters, and more harmful algae blooms have caused a mass die-off of bottlenose dolphins. Warming waters also reduce the fish that dolphins eat, which can lower their health and numbers. In the Mediterranean, falling food supplies led to a steep decline in the short-beaked common dolphin subpopulation, which was deemed endangered in 2003. In California, the El Niño warming event of 1982–83 drove market squid away, and their predator, the pilot whale, left as well. The Pacific white-sided dolphin has also declined in the southwest Gulf of California. River dolphins are highly affected by climate change, as high evaporation, warmer water and increased acidification occur.

Relationships with humans

Dolphins have long played a role in human culture. In Greek mythology, dolphins were seen as helpers of humankind. Dolphins also seem to have been important to the Minoans, judging by art from the ruined palace at Knossos. In 2009, excavations of a major Mycenaean city at Iklaina uncovered a wall painting fragment showing a ship, three human figures and dolphins. Many coins from Ancient Greece show a man, a boy or a god riding on a dolphin's back, and seeing dolphins ride in a ship's wake was considered a good omen. In both ancient and later art, Cupid is often shown riding a dolphin. A dolphin rescued the poet Arion from drowning and carried him to land at Cape Matapan, the southernmost point of the Peloponnesus, where there was a temple to Poseidon and a statue of Arion riding the dolphin. The Greeks reimagined the Phoenician god Melqart as Melicertes, son of Athamas and Ino. He drowned but became the sea god Palaemon, and his mother became Leucothea. At Corinth, he was closely linked to the cult of Poseidon, and the Isthmian Games, first held in Poseidon's honor, came to be seen as his funeral games. Phalanthus was another legendary figure brought safely to shore on a dolphin, according to Pausanias. Dionysus was once captured by Etruscan pirates, who mistook him for a rich prince they could ransom. He turned the oars into serpents, so the sailors jumped overboard, but he transformed them into dolphins so they would help people in need. Dolphins were sacred to Aphrodite and Apollo. The aristocratic "Dolfin" family of the maritime Republic of Venice had as its most prominent member the 13th-century Doge Giovanni Dolfin. In Hindu mythology, the Ganges river dolphin is linked with Ganga, the deity of the Ganges river, and her mount, the Makara, is sometimes shown as a dolphin. The Boto, a river dolphin of the Amazon River, is believed to be a shapeshifter, or encantados. There are few myths about dolphins in Polynesian cultures, which otherwise honor animals such as sharks and seabirds, since dolphins are more often seen as food than as symbols. Dolphins appear in Rapa Nui Rongorongo and in traditions of the Caroline Islands. When heraldry developed in the Middle Ages, the stylised heraldic dolphin was drawn like a fish, sometimes with fish scales. The coat of arms of the Dauphiné in southern France gave rise to the title Dauphin of France, which means "The Dolphin of France." Dolphins also appear in the coat of arms of Anguilla, the coat of arms of Romania, and the coat of arms of Barbados, which has a dolphin supporter. The coat of arms of the town of Poole, Dorset, England, first recorded in 1563, includes a dolphin.

The renewed popularity of dolphins in the 1960s led to many dolphinaria around the world, though criticism and animal welfare laws forced many to close. The best-known in the United States are the SeaWorld marine mammal parks, and in the Middle East they include Dolphin Bay at Atlantis, The Palm and the Dubai Dolphinarium. Bottlenose dolphins are the most common captive dolphins because they are relatively easy to train, live long in captivity and look friendly. Other captive species include spotted dolphins, false killer whales, common dolphins, Commerson's dolphins, and rough-toothed dolphins, while fewer than ten pilot whales, Amazon river dolphins, Risso's dolphins, spinner dolphins, or tucuxi are kept. A very rare hybrid called a wolphin, a cross between a bottlenose dolphin and a false killer whale, lives at the Sea Life Park in Hawaii. About 60 captive orcas were held in aquaria as of 2017, valued for their intelligence, trainability and size. Between 1976 and 1997, whales were taken from the wild in Iceland, Japan and Argentina, and by 1999 about 40% of the orcas on display were captive-born. Groups such as the Mote Marine Laboratory rehabilitate sick or stranded dolphins, while the Whale and Dolphin Conservation and the Hong Kong Dolphin Conservation Society work to protect them. India named the dolphin its national aquatic animal to protect the endangered Ganges river dolphin, and the Vikramshila Gangetic Dolphin Sanctuary was created for their protection. Some militaries have used dolphins to find mines and help rescue people, and the military use of dolphins drew scrutiny during the Vietnam War, when the United States Navy was accused of training them to attack divers, a claim the Navy denies. The Navy still trains dolphins for other tasks through the U.S. Navy Marine Mammal Program, and the Russian military is believed to have closed its program in the early 1990s. In 2000, the press reported that dolphins trained by the Soviet Navy had been sold to Iran. Dolphins are also a popular choice for animal-assisted therapy, but a 2005 study that found them helpful for mild to moderate depression was criticized, and reviews of dolphin-assisted therapy studies have found no compelling evidence that it is a legitimate therapy. Welfare of captive cetaceans is debated, and organizations such as World Animal Protection and the Whale and Dolphin Conservation oppose keeping them in captivity. See also: Captive orca

Captive orcas often develop health problems, such as dorsal fin collapse, and on average live only into their 20s, while wild females who survive infancy live 46 years on average. Critics say captivity is stressful, and that orcas, which can travel up to 160 kilometres in a day, are too big and intelligent for it. Although dolphins generally interact well with people, some orcas have been involved in fatal attacks on humans in captivity, most notably Tilikum, whose behaviour inspired the documentary Blackfish. Fatal attacks by other species are rarer, though a bottlenose dolphin named Tião was involved in a death off Brazil in 1994, after visitors had harassed it, including by trying to put ice cream sticks into its blowhole. Some scientists worry about careless human-dolphin interaction, and some have proposed that dolphins should be seen as non-human persons whose capture for entertainment or killing is morally unacceptable. Chile, Costa Rica, Hungary, and India have declared dolphins "non-human persons" and banned the capture and import of live dolphins for entertainment. In some places, such as Taiji, Japan and the Faroe Islands, dolphins have traditionally been hunted for food, and dolphin meat is eaten in a small number of countries, including Japan and Peru, where it is called "sea pork." Dolphin meat is dark red, with a layer of blubber, and in Japan it is often eaten raw as sashimi, while cooked dolphin meat tastes similar to beef liver. Tests showed high levels of mercury in Japanese dolphin meat, though no known cases of mercury poisoning have resulted, and the government advises children and pregnant women to avoid it. In the Faroe Islands, prenatal exposure to methylmercury and PCBs, mostly from pilot whale meat, has been linked to neuropsychological deficits in children, according to the World Health Organization.