Dolphin is a widely distributed and diverse aquatic mammal. They are an informal group of cetaceans that do not include whales and porpoises , so to zoologists, this group is collateral. Dolphins include the extant Delphinidae (marine dolphins), Delphinidae (Indus river dolphins), Delphinidae (New World river dolphins) and Delphinidae (brackish water dolphins), as well as the extinct Delphinidae ( Baiji or Chinese river dolphins ). There are 40 species of dolphins in existence. Dolphins, like other cetaceans, are artiodactyls. Their closest relative is hippopotamus , which began to diverge about 40 million years ago.
Dolphins range in size from the 1.7 meters (5.6 feet) long, 50 kilograms (110 pounds) Maui dolphin to the 9.5 meters (31 feet), 10 tons (11 short tons) killer whale. Some species exhibit sexual dimorphism in that males are larger than females. They have streamlined bodies and two limbs modified into flippers. Although not as agile as seals, some dolphins can travel at speeds of 55.5 km/h (34.5 mph). Dolphins use their conical teeth to catch fast-moving prey. They have well-developed hearing, are adapted to air and water, and are so developed that some can survive even if they are blind. Some species are great for diving to great depths. They have a layer of fat, or blubber, under their skin to keep them warm in cold water.
Although dolphins are widely distributed, most species prefer the warm waters of the tropics, but some species, such as right whales dolphins, prefer colder climates. Dolphins mainly feed on fish and squid, but some, like killer whales, feed on large mammals such as seals. Male dolphins typically mate with multiple females each year, but females mate only once every two to three years. Calves are usually born in the spring and summer, and the female takes on all the responsibilities of raising them. Mothers of some species fast and nurse their young for relatively long periods of time. Dolphins make a variety of sounds, usually clicks and whistles.
In places such as Japan, dolphins are sometimes hunted, an activity known as dolphin hunting. In addition to hunting, they face threats from bycatch, habitat loss and ocean pollution. Dolphins are described in different cultures around the world. Dolphins occasionally appear in literature and film, such as in the film series Mermaid Fairy Tales. Dolphins are sometimes held in captivity to train for performance skills, but breeding success is low and the animals often die within months of capture. The most common dolphins are killer whales and bottlenose dolphins.
