Dolphins are a widely distributed and diverse group of aquatic mammals. They are an informal grouping within the order Cetacea, excluding whales and porpoises, so to zoologists the grouping is paraphyletic. The dolphins comprise the extant families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the new world river dolphins), and Pontoporiidae (the brackish dolphins), and the extinct Lipotidae (baiji or Chinese river dolphin). There are 40 extant species of dolphins. Dolphins, alongside other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates. Cetaceans’ closest living relatives are the hippopotamuses, having diverged about 40 million years ago.
Dolphins range in size from the 1.7 m (5.6 ft) long and 50 kg (110 lb) Maui’s dolphin to the 9.5 m (31 ft) and 10 t (11 short tons) killer whale. Several species exhibit sexual dimorphism, in that the males are larger than females. They have streamlined bodies and two limbs that are modified into flippers. Though not quite as flexible as seals, some dolphins can travel at 55.5 km/h (34.5 mph). Dolphins use their conical shaped teeth to capture fast moving prey. They have well-developed hearing which is adapted for both air and water and is so well developed that some can survive even if they are blind. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.
Although dolphins are widespread, most species prefer the warmer waters of the tropic zones, but some, like the right whale dolphin, prefer colder climates. Dolphins feed largely on fish and squid, but a few, like the killer whale, feed on large mammals, like seals. Male dolphins typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period of time. Dolphins produce a variety of vocalizations, usually in the form of clicks and whistles.
Dolphins are sometimes hunted in places like Japan, in an activity known as dolphin drive hunting. Besides drive hunting, they also face threats from bycatch, habitat loss, and marine pollution. Dolphins have been depicted in various cultures worldwide. Dolphins occasionally feature in literature and film, as in the film series Free Willy. Dolphins are sometimes kept in captivity and trained to perform tricks. The most common dolphin species kept is the bottlenose dolphin, while there are around 60 captive killer whales.
The name is originally from Greek (delphís), “dolphin”, which was related to the Greek δελφύς (delphus), “womb”. The animal’s name can therefore be interpreted as meaning “a ‘fish’ with a womb”. The name was transmitted via the Latin delphinus. (the romanization of the later Greek δελφῖνος – delphinos), which in Medieval Latin became dolfinus and in Old French daulphin, which reintroduced the phinto the word. The term mereswine (that is, “sea pig”) has also historically been used.
The term ‘dolphin’ can be used to refer to, under the parvorder Odontoceti, all the species in the family Delphinidae (oceanic dolphins) and 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). This term has often been misused in the US, mainly in the fishing industry, where all small cetaceans (dolphins and porpoises) are considered porpoises, while the fish dorado is called dolphin fish. In common usage the term ‘whale’ is used only for the larger cetacean species,while the smaller ones with a beaked or longer nose are considered ‘dolphins’. The name ‘dolphin’ is used casually as a synonym for bottlenose dolphin, the most common and familiar species of dolphin.There are six species of dolphins commonly thought of as whales, collectively known as blackfish: the killer whale, the melon-headed whale, the pygmy killer whale, the false killer whale, and the two species of pilot whales, all of which are classified under the family Delphinidae and qualify as dolphins.Though the terms ‘dolphin’ and ‘porpoise’ are sometimes used interchangeably, porpoises are not considered dolphins and have different physical features such as a shorter beak and spade-shaped teeth; they also differ in their behavior. Porpoises belong to the family Phocoenidae and share a common ancestry with the Delphinidae.
A group of dolphins is called a “school” or a “pod”. Male dolphins are called “bulls”, females “cows” and young dolphins are called “calves
by simon schofield