Dolphins are a widely distributed and diverse aquatic mammal. They are an informal group of cetaceans that do not include whales and porpoises, so for zoologists, this group is lateral. Dolphins include the extant dolphin family (marine dolphins), the dolphin family (Indus dolphins), the dolphin family (New World River dolphins) and the dolphin family (brackish water dolphins), as well as the extinct dolphin family (white-capped dolphins or Chinese river dolphins). There are 40 species of dolphins in existence. Dolphins, like other cetaceans, are ungulates. Their close relatives are hippos, which began to diverge about 40 million years ago.
Dolphins range in size from a 1.7 m (5.6 ft) long, 50 kg (110 lb) maui dolphin to a 9.5 m (31 ft), 10 t (11 short tonnes) killer whale. Some species exhibit hermaphroditic heteromorphisms because males are larger than females. They have a streamlined body and two limbs that have been transformed into flippers. While 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 a well-developed sense of hearing, adapt to air and water, and are so developed that some survive even if they are blind. Some species are great for diving deep. They have a layer of fat, or blubber, under their skin that is used to keep warm in cold water.
While dolphins are widely distributed, most species prefer warm tropical waters, but some species, such as right whale dolphins, prefer colder climates. Dolphins feed mainly on fish and squid, but there are also some, such as killer whales, that feed on large mammals such as seals. Male dolphins usually mate with multiple female dolphins each year, but female dolphins only mate once every two to three years. Calves are usually born in the spring and summer, and the females take all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long time. Dolphins make a wide variety of sounds, usually clicks and whistles.
In places like Japan, dolphins are sometimes hunted, and this activity is known as dolphin hunting. In addition to hunting, they face the threat of by-catch, habitat loss and marine pollution. Dolphins are described in different cultures around the world. Dolphins occasionally appear in literature and films, such as the film series Mermaid Fairy Tales. Dolphins are sometimes locked up to train their performance skills, but reproductive success rates are low, and animals often die within a few months of being caught. The most common dolphins are killer whales and bottlenose dolphins.