How Smart Are Dolphins? What Research Really Shows

How big is a dolphin’s brain?

A bottlenose dolphin’s brain weighs about 1.5–1.7 kg (3.3–3.7 lb), slightly more than a human brain of about 1.3–1.4 kg and roughly four times the brain of a chimpanzee. Relative to body size it is also large. Scientists compare this with the encephalization quotient, or EQ, which measures how much bigger a brain is than expected for an animal of that size. Bottlenose dolphins score around 4 to 5, among the highest of any animal; chimpanzees score about 2.5 and humans about 7.

Size is not everything. The dolphin cortex, the outer layer linked with complex thought, is thinner than ours, and a large share of the brain handles sound. Birds such as crows and parrots show that a small brain packed densely with neurons can also be very capable. A big brain is a clue to intelligence, not proof of it.

Mirror self-recognition

In 2001, Diana Reiss and Lori Marino tested two bottlenose dolphins at the New York Aquarium. When the researchers drew temporary marks on their bodies, the dolphins swam to a mirror and twisted to inspect exactly the marked spots, something they did not do after sham marking that left no visible mark. This suggests they understood the reflection was themselves.

Very few animals pass versions of this test, among them great apes, an Asian elephant and Eurasian magpies. Some researchers question how much it reveals about self-awareness, especially for animals without hands, but dolphins are consistently placed in this small group.

Tools and culture in Shark Bay

In Shark Bay, Western Australia, some Indo-Pacific bottlenose dolphins tear off cone-shaped marine sponges and wear them over their beaks while probing the sandy bottom for fish. The sponge seems to protect the beak from scrapes. This sponging was first described in 1997 and is done mainly by females. A 2005 genetic study found that the habit runs in families, passed from mothers to their young, which makes it a strong case for culture in dolphins.

The same dolphins also use large empty sea snail shells to trap fish, then lift the shell and shake the fish into their mouths. A 2020 study found that this trick spreads mostly between companions rather than from mother to calf. Other bottlenose populations have their own traditions, such as strand feeding on the muddy banks of South Carolina and Georgia, where dolphins briefly beach themselves to grab fish they have driven ashore.

Understanding language and symbols

From the late 1970s, psychologist Louis Herman taught two bottlenose dolphins in Hawaii artificial languages: Akeakamai learned words made of hand and arm gestures, while Phoenix learned words made of computer-generated sounds. Both could carry out instructions they had never seen before and understood that word order changed the meaning, just as “the dog chased the cat” differs from “the cat chased the dog”. They also followed human pointing and gazing.

Dolphins are skilled vocal learners. Each bottlenose dolphin develops its own signature whistle, which others copy to call it, and dolphins remember the whistles of former companions after more than 20 years apart. Read more in How do dolphins communicate?

Cooperation and social intelligence

Many scientists think dolphin intelligence evolved mainly to handle a complicated social life. Bottlenose dolphins live in fission-fusion societies, where companions change often, so they must keep track of many individuals and relationships. In Shark Bay, males form alliances within alliances, a multi-level system of cooperation rarely seen outside humans. In Sarasota Bay, Florida, pairs of males stay allied for many years.

They also cooperate across species. In Laguna, southern Brazil, wild dolphins and local fishers have worked together for more than 150 years: the dolphins drive fish toward people standing in the shallows and signal when to cast their nets, and both sides catch more.

Claims that go too far

Some popular beliefs about dolphins are not supported by research:

• “Dolphins are smarter than humans.” No study shows this. Dolphins have no writing, no technology and no evidence of a language with grammar.
• “Dolphins have a secret language we will soon translate.” From the late 1950s, the neuroscientist John Lilly suggested dolphins might learn to speak with us. Decades later, researchers using computers and machine learning still have not decoded words or grammar in dolphin sounds.
• “Dolphins are always gentle.” Bottlenose dolphins can be aggressive. Males fight with each other, and some populations, including those in Scotland’s Moray Firth, attack and kill harbor porpoises.
• “Dolphins have an IQ of …” There is no IQ test for animals, so any such number is made up.

How do dolphins compare with apes and crows?

Different animals are smart in different ways. Chimpanzees are better tool users than dolphins, making stone hammers and termite-fishing sticks, and they learn symbols too. New Caledonian crows shape hooked tools from twigs, and ravens plan ahead, despite brains the size of a walnut. Dolphins excel in sound-based communication, vocal learning and cooperation, all without hands.

Most scientists now avoid ranking species on a single ladder. In comparisons such as our list of the smartest animals, dolphins sit alongside the great apes, elephants and crows near the top; for the bird side of the story, see How smart are crows? To meet the dolphin behind most of this research, visit our common bottlenose dolphin profile.