Dolphins and other whales descend from land-living mammals. Among the other mammals, whales, including dolphins, have the hippopotamus as their closest living relative. In dolphins and other whales the nostrils moved to the top of the head, which makes breathing while swimming easier. The embryos of whales and dolphins still show traces of land ancestors, with a distinct neck, a narrow tail and hind limbs that protrude from the body. The word dolphin itself comes from an Ancient Greek word that means "fish with a womb."
Many dolphin and other toothed-whale species have brains much larger than expected for their body size, second only to modern humans. In a 2001 study, two bottlenose dolphins used a mirror to examine marks on their bodies, and at that time only humans and great apes had shown convincing mirror self-recognition. One researcher, Marino, said dolphin and primate brains are so different that the two groups have not shared a common ancestor for at least 90 million years, so mirror self-recognition in both is convergent evolution.
Each bottlenose dolphin develops its own signature whistle, and scientists think a signature whistle works like a name. Signature whistles make up about half of a bottlenose dolphin's whistles, and wild bottlenose dolphins often copy each other's signature whistles. In 2006 researchers played synthetic whistles to Sarasota Bay dolphins, and in 9 of 14 cases a dolphin turned toward the speaker more often when the whistle resembled a close relative's. In a 2013 experiment, each wild dolphin reacted only when it heard a computer version of its own signature whistle.
Using new or learned sounds to label things is rare in the animal kingdom, but it is at the heart of human language. Two dolphins in Louis Herman's lab learned a vocabulary of about 50 words, which they used to build more than 1,000 different commands of up to five words. In Herman's tests, dolphins understood word order: they could tell apart commands such as "get the Frisbee and take it to the surfboard" and "get the surfboard and take it to the Frisbee."
Scientists have two hypotheses about how dolphins produce sound underwater, and the most widely accepted is the nasal sac theory. Dolphins find things by making sounds that bounce off objects and return as echoes. A waxy, lens-shaped part in the forehead, called the melon, focuses the clicks into a beam, and the echoes are picked up through the lower jaw. Low-frequency clicks travel farther but give less detail, so a dolphin raises the frequency of its clicks as it moves closer to an object. Dolphins are also good at detecting air spaces, so they can find fish with air-filled swim bladders by echolocation.
In Shark Bay, Australia, some dolphins wear sea sponges on their snouts while they search the sea floor for food, and calves can learn to use sponge tools from their mothers. Sponge-using dolphins in Shark Bay choose to spend time with other sponge-users rather than non-users. At Laguna in southern Brazil, wild bottlenose dolphins chase mullet toward fishers standing in shallow water, and 86 percent of the mullet that fishers caught there came from catches synchronized with dolphins. Dolphins that cooperated with fishers had a 13 percent higher survival rate than non-cooperative dolphins.
Several bottlenose dolphin die-offs have been linked to harmful algal blooms such as red tide, and as of November 2026 getting tangled in fishing gear is one of the main threats to bottlenose dolphins. As of November 2026, in Sarasota Bay a boat passes by a dolphin every 6 minutes, and 1 in 20 dolphins there has a scar from a boat collision. As of November 2026, the IUCN Red List lists the common bottlenose dolphin as Least Concern. In the United States, bottlenose dolphins are protected by the Marine Mammal Protection Act of 1972. Feeding wild dolphins is harmful because it teaches them to come close to boats and people, and as of November 2026 people should stay at least 50 yards (46 meters) away from wild dolphins.
Words to know
- convergent: arrived separately
- embryos: unborn young
- synthetic: made, not natural
- hypotheses: proposed explanations
- echolocation: finding by echoes
