Dolphins and other whales descend from land-living mammals, and several lines of evidence point the same way. Among the other mammals, whales, including dolphins, have the hippopotamus as their closest living relative. A deer-like mammal called Indohyus, about the size of a cat, may be even more closely related to whales than hippos are. The ankle bone of Pakicetus has a double-pulley shape seen elsewhere only in even-toed hoofed mammals, one clue that linked whales to that group. 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. In dolphins and other whales the nostrils moved to the top of the head, which makes breathing while swimming easier.

Many dolphin and other toothed-whale species have brains much larger than expected for their body size, second only to modern humans. A 2004 study found that toothed whales' relative brain size jumped in two phases: when toothed whales split from earlier whales and, much later, when the dolphin superfamily Delphinoidea began. Marino said that "size isn't everything" and pointed to an expanded neocortex and human-like behavior as other lines of evidence for dolphin intelligence. In the mark test, researchers put a mark on a dolphin where it could not see it, then watched whether the dolphin used a mirror to look at the mark. 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. Wild bottlenose dolphins often copy each other's signature whistles, and in a 2013 experiment each wild dolphin reacted only when it heard a computer version of its own signature whistle. Janik argued that because voice features change with water pressure, names that do not depend on the voice might be a reliable way to convey identity. 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.

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." A 1994 report quoted an expert saying that dolphins taken from the sea can use parts of speech in tests, but that this does not mean they use actual language to communicate.

Scientists have two hypotheses about how dolphins produce sound underwater, and the most widely accepted is the nasal sac theory. No one has ever been able to see inside a dolphin's head while it makes sounds, which is why scientists are not sure how the sounds are produced. Dolphins find things by making sounds that bounce off objects and return as echoes, and a waxy, lens-shaped part in the forehead, called the melon, focuses the clicks into a beam. Echoes from the clicks 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 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. As of November 2026, researchers calculated that the Laguna dolphin-fisher partnership may fade out in the next 40 to 60 years as mullet numbers fall.

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. The baiji was a species of China's Yangtze River that used sonar. As of November 2026, the record shows that in December 2006 an expedition on China's Yangtze River found no baiji, and its leader said the species was functionally extinct. Overfishing and ship traffic, whose engines interfere with the sonar baiji use, were likely the main reasons for the baiji's decline. 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.

Since October 1970, the Sarasota Dolphin Research Program has studied the dolphins of Sarasota Bay, Florida, and as of November 2026 it is the longest-running study of a wild dolphin population. 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. Scientists tell individual dolphins apart by the nicks and notches on their dorsal fins, and some bottlenose dolphins live for more than 50 years. A female bottlenose dolphin named Nicklo from Sarasota Bay lived to age 67, and as of November 2026 the program calls her the oldest documented dolphin in the world. Because dolphins live into their 50s, even simple facts such as how many calves a female has in her lifetime take decades of study.

Words to know

• hypotheses: proposed explanations
• neocortex: thinking brain layer
• convergent: arrived separately
• embryos: unborn young
• synthetic: made, not natural