Dolphins and other whales descend from land-living mammals. Among the other mammals, whales, including dolphins, have the hippopotamus as their closest living relative. One change in their bodies helps them breathe while swimming: in dolphins and other whales, the nostrils moved to the top of the head. A dolphin breathes air through this blowhole when it comes to the surface. Dolphins are warm-blooded mammals, not fish.

The word dolphin comes from an Ancient Greek word that means "fish with a womb." Dolphins are closely related to whales, and the orca, or killer whale, is the largest member of the dolphin family. Dolphins and porpoises are both toothed whales. Dolphins tend to have cone-shaped teeth, and porpoises have spade-shaped teeth. As of November 2026, the Society for Marine Mammalogy's April 2026 list counts 38 species of ocean dolphins and 7 species of porpoises.

Dolphins find things by making sounds that bounce off objects and return as echoes, which is called echolocation. A waxy, lens-shaped part in the forehead, called the melon, focuses the clicks into a beam. The echoes are picked up through the lower jaw. Some dolphins also find fish by listening for the sounds the fish make. Dolphins also make whistles.

When a dolphin sleeps, half of its brain stays awake. A bottlenose dolphin near shore comes up to breathe about two times a minute. Dolphins can see well both underwater and above the water. A bottlenose dolphin can swim about 18 miles an hour for a short burst, and many dolphins can leap out of the water. A bottlenose dolphin can be 12 feet long and weigh more than 1,000 pounds.

Each bottlenose dolphin develops its own signature whistle, and a calf starts to develop its whistle in its first months of life. Scientists think a signature whistle works like a name. Bottlenose dolphins form nursery groups of mothers and calves, groups of young dolphins, and bonded pairs of adult males. Calves learn how to hunt mostly by watching their mothers. Some dolphins in Shark Bay, Australia, wear sea sponges on their snouts while they search the sea floor for food.

As of November 2026, the IUCN Red List lists the common bottlenose dolphin as Least Concern, but getting tangled in fishing gear is one of the main threats to bottlenose dolphins. In the United States, they are protected by the Marine Mammal Protection Act of 1972. Large sharks are the main natural predators of bottlenose dolphins. Feeding wild dolphins is harmful, because it teaches them to come close to boats and people. As of November 2026, people should stay at least 50 yards (46 meters) away from wild dolphins.

Words to know

• descend: come from
• echolocation: finding by echoes
• nostrils: nose holes
• melon: forehead lump
• predators: hunting animals