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 come in many sizes. The Maui's dolphin is about the size of a human, measuring 1.7 metres (5 ft 7 in) long and weighing 50 kilograms (110 pounds). The largest is the apex predator called the orca, which can grow to 9.5 m (31 ft) long and weigh 10 tonnes (11 short tons). In some species, males are bigger than females. Dolphins have streamlined, fish-like bodies. Their two forelimbs have become flippers, and they have lost their back legs (only a small vestigial pelvic skeleton remains). Their tail forms a flat, horizontal fluke. They are not as bendy as semiaquatic seals, but they are faster and more agile in water. Some dolphins can swim up to 29 kilometres per hour (18 mph) for a short time, or leap about 9 metres (30 ft) into the air. They use their cone-shaped teeth to catch fast-moving prey such as forage fish, coleoids and shrimps. Their hearing is very good and works both in air and in water. Some species are good at diving very deep. 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 all over the world, mostly in pelagic marine environments, though some live 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 places. Like most nektonic animals, dolphins are carnivorous. They eat 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 usually mate with several females each year, but females mate only every two to three years. Calves are usually born in the spring and summer, and mothers take care of them. Mothers of some species nurse their young for a fairly long time.

Dolphins are very social animals. They live in complex "fission-fusion" groups. They form loose groups, called pods, that often 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, though this is much less common than 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 in many cultures around the world, such as in art and folklore. Some dolphins are kept 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), which means "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. This 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, which are called oceanic dolphins. It can also refer to river dolphins in several 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.

Dolphin and porpoise are sometimes used to mean the same thing. However, porpoise usually refers to the Phocoenidae family. Porpoises have 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 come from land-living mammals of the artiodactyl order, which includes animals with even-toed hooves. They are related to Indohyus, a small extinct animal that looked somewhat like a chevrotain. Dolphins split from it about 48 million years ago. The first archaeocetes, or early whales, went into the sea about 49 million years ago. Over the next 5 to 10 million years, they came to live completely in the water. Archaeoceti is a group of ancient whales, which were the ancestors of today's whales. Over time, their bodies changed to suit life in the marine environment. They gained better hearing, as seen in Ambulocetus 49 million years ago. They developed a streamlined body and tail flukes, as seen in Protocetus around 43 million years ago. Their nostrils moved toward the top of the cranium, and their front limbs became flippers, as seen in Basilosaurus 35 million years ago. Their back legs shrank and disappeared about 34 million years ago.

The modern dolphin skeleton has two small pelvic bones. These are thought to be vestigial remains of back legs. In October 2006, an unusual bottlenose dolphin was caught in Japan. It had small fins on each side of its genital slit. Scientists believe these fins are an unusually strong growth of those leftover back legs. Today, the closest living relatives of whales and dolphins are hippopotamuses. They share a semi-aquatic ancestor that split off from other artiodactyls some 60 million years ago. About 40 million years ago, a common ancestor split into whales and anthracotheres. Anthracotheres died out about two and a half million years ago. This left only one surviving group: the two species of hippo.

Dolphins have streamlined, torpedo-shaped bodies. Their necks do not bend much. They have flippers made from their front limbs, a tail fin, and rounded heads. Their skulls have small eye sockets and long snouts, and their eyes sit on the sides of their heads. 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 cone-shaped teeth, unlike the spade-shaped teeth of porpoises. They use their teeth 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. Calves are born with only a thin layer. Their stomach has two or three chambers, the fundic and pyloric chambers.

Dolphin skin is smooth and rubber-like. 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. 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 do not have such a big difference between the outside and inside, so they take in sound through the throat instead.

Dolphins make clicks that pass through a fatty melon in the head. This 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. However, 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, work together, plan, and grieve. Many species have long, thin spindle neurons in the neocortex. These are brain cells that, before 2007, were known only in hominids. In humans, these cells help with 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 big sign of an animal's intelligence. Much of the brain controls the body, so a larger brain for the body's size may leave more room for complex thinking. Orcas have the second largest brain of any animal on earth, after the sperm whale. Their brain to body mass ratio is second only to humans.

Some people see self-awareness as a sign of advanced thinking. It is not clearly defined in science, but it is believed to lead to more advanced thinking, such as meta-cognitive reasoning, which means thinking about thinking. Research suggests that cetaceans, along with some other animals, have self-awareness.

One common test is the mirror test. 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. They say the results can be read in different ways and may be affected by the Clever Hans effect. Dolphins cannot touch the mark or the mirror the way primates can, so their results are less certain. In 1995, researchers Marten and Psarakos used television to test dolphins. 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. Pod sizes vary between species and places. When food is plentiful, several pods may join together into a temporary superpod of more than 1,000 dolphins. Dolphins form strong bonds and stay with members who are hurt or sick. They bring 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. Dolphins have also 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. The calf is usually born tail first. 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. The exact age depends on the species and whether the dolphin is male or female.

Hybridization

Dolphins of different species sometimes have 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 can have young of their own. Two live at the Sea Life Park in Hawaii.

Sleeping

Further information: Sleep in animals

Dolphins usually rest one half of their brain at a time while the other half 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. A group of dolphins pushes fish into a tight ball called a bait ball. In Shark Bay, Australia, some dolphins use conch shells to catch fish. A dolphin lifts the shell and shakes it. 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. Dolphin echolocation clicks are among the loudest sounds made by marine animals. Bottlenose dolphins each have a signature whistle that works like a name. It helps other dolphins know who is calling.

Jumping and playing

Dolphins often leap above the water. Jumping can save energy while traveling. It is also used for communication, play, and other reasons. Dolphins play with objects and make bubble rings. They sometimes interact with other animals, including humpback whales and dogs. Young 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. She had been rescued and spent time with captive dolphins, and other dolphins copied her. Scientists think social learning explains how the behavior spread. It gives no clear advantage and uses a lot of energy.

Threats

Dolphins have few enemies in the sea. Some kinds have none, so they are apex predators. Most smaller dolphins face only a few large sharks, such as the bull shark, dusky shark, tiger shark and great white shark. These sharks can be a danger, especially to young calves. Some large dolphins, especially orcas, may hunt smaller dolphins, but this seems rare. Dolphins can also get sick from many diseases and parasites. The Cetacean morbillivirus has caused epizootics in some areas, and many animals have died. Sick dolphins often have pneumonia, encephalitis and damage to their 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. This may help people understand the disease and find new treatments. Dolphins can also recover from serious injuries, such as shark bites. Their healing is fast, 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 by fighting back in clever ways and teaming up against the shark.

Humans

See also: Dolphin drive hunting and Cetacean bycatch

Some dolphins are at risk of dying out, especially river dolphins such as the Amazon river dolphin and the Ganges and Yangtze river dolphin. These are in serious danger. 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 animals such as dolphins. Collisions with boats and their propellers can cause injuries and deaths. Some fishing methods, such as seine fishing for tuna and the use of drift and gill nets, also catch many dolphins by accident, which is called by-catch. In places such as Taiji in Japan and the Faroe Islands, dolphins are traditionally eaten and are hunted with a harpoon or in 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 many dolphins. Loud underwater noises from naval sonar, live firing and offshore construction projects such as wind farms may stress dolphins, harm their hearing and cause decompression sickness. Dolphin drive hunts herd dolphins into a bay or onto a beach. These hunts take place in places such as the Solomon Islands, the Faroe Islands, Peru and Japan, mostly for their meat. Some dolphins end up in dolphinariums. Many 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. More harmful algae blooms have caused many bottlenose dolphins to die. Warming waters also reduce the fish that dolphins eat, which can make them less healthy and lower their numbers. In the Mediterranean, less food led to a steep decline in the short-beaked common dolphin group, which was listed as 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 strongly affected by climate change, as water evaporates faster, becomes warmer and becomes more acidic.

Relationships with humans

Dolphins have long played a role in human culture. In Greek mythology, dolphins were seen as helpers of people. Dolphins also seem to have been important to the Minoans, based on art from the ruined palace at Knossos. In 2009, digging at a major Mycenaean city at Iklaina uncovered a wall painting piece showing a ship, three people and dolphins. Many coins from Ancient Greece show a man, a boy or a god riding on a dolphin's back. Seeing dolphins swim alongside a ship was thought to be a good sign. In both old and later art, Cupid is often shown riding a dolphin. A dolphin saved the poet Arion from drowning and carried him to land at Cape Matapan, the southernmost point of the Peloponnesus. There was a temple to Poseidon there, and a statue of Arion riding the dolphin. The Greeks linked the Phoenician god Melqart with Melicertes, the son of Athamas and Ino. Melicertes became the sea god Palaemon, and his mother became Leucothea. At Corinth, he was closely linked to the worship of Poseidon. The Isthmian Games, first held in Poseidon's honor, came to be seen as his funeral games. Phalanthus was another legendary figure carried safely to shore by a dolphin, according to Pausanias. Dionysus was once captured by Etruscan pirates, who thought he was a rich prince they could hold for money. He changed the oars into serpents, so the sailors jumped overboard. He then transformed them into dolphins so they would help people in need. Dolphins were sacred to Aphrodite and Apollo. The noble "Dolfin" family of the sea-going Republic of Venice had as its most famous member the 13th-century Doge Giovanni Dolfin. In Hindu mythology, the Ganges river dolphin is linked with Ganga, the goddess of the Ganges river. Her animal companion, the Makara, is sometimes shown as a dolphin. The Boto, a river dolphin of the Amazon River, is believed by some people to be able to change shape, or encantados. There are few myths about dolphins in Polynesian cultures. These cultures honor animals such as sharks and seabirds, but dolphins are more often seen as food. 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 in Dorset, England, first recorded in 1563, includes a dolphin.

Dolphins became popular again in the 1960s, which led to many dolphinaria around the world. Criticism and animal welfare laws later forced many of them to close. The best-known in the United States are the SeaWorld marine mammal parks. 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 fairly 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. Only a few 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. As of 2017, about 60 captive orcas were kept in aquariums. They are valued for their intelligence, their ability to be trained, and their size. Between 1976 and 1997, some whales were taken from the wild in Iceland, Japan and Argentina. By 1999, about 40% of the orcas on display had been born in captivity. Groups such as the Mote Marine Laboratory rehabilitate sick or stranded dolphins. 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. The Vikramshila Gangetic Dolphin Sanctuary was created to protect them. Some militaries have used dolphins to find mines and help rescue people. The military use of dolphins drew attention during the Vietnam War, when the United States Navy was accused of training them for harmful tasks. The Navy denies this claim. The Navy still trains dolphins for other tasks through the U.S. Navy Marine Mammal Program. 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. A 2005 study found them helpful for mild to moderate depression, but the study was criticized. Reviews of dolphin-assisted therapy have found no strong evidence that it is a real therapy. Whether captive whales and dolphins are treated well is debated. 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. On average, they live only into their 20s. Wild females that survive infancy live about 46 years on average. Critics say captivity is stressful. They also say that orcas, which can travel up to 160 kilometres in a day, are too big and intelligent for it. Although dolphins generally get along well with people, some orcas have been involved in dangerous attacks on humans in captivity. The most well-known is Tilikum, whose behaviour inspired the documentary Blackfish. Dangerous incidents involving other species are rarer. A bottlenose dolphin named Tião was involved in a death off Brazil in 1994. Visitors had bothered it before this, including by poking things into its blowhole. Some scientists worry about careless human-dolphin interaction. Some have proposed that dolphins should be seen as non-human persons, and that capturing them for entertainment is morally wrong. Chile, Costa Rica, Hungary, and India have declared dolphins "non-human persons." They have also banned capturing and importing live dolphins for entertainment. In some places, such as Taiji, Japan and the Faroe Islands, dolphins have traditionally been hunted for food. 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. In Japan, it is often eaten raw as sashimi. Cooked dolphin meat tastes similar to beef liver. Tests showed high levels of mercury in Japanese dolphin meat. No known cases of mercury poisoning have resulted, but the government advises children and pregnant women to avoid it. In the Faroe Islands, unborn children were exposed to methylmercury and PCBs, mostly from pilot whale meat. According to the World Health Organization, this exposure has been linked to neuropsychological difficulties in children.