Mammals
Dolphin
Delphinidae · also called dolphins, oceanic dolphin
A dolphin is a small whale with a big brain. It hunts by firing clicks and reading the echoes, well enough to pick a fish out of sand it cannot see. It calls its companions by name, remembers those names for decades, and in some bays passes down hunting tricks from mother to daughter that exist nowhere else on Earth.
Overview
Dolphins are small toothed whales — every dolphin is a cetacean, and a whale in the broad sense. The oceanic dolphin family Delphinidae has around 40 species, from the bottlenose, common, and spinner dolphins to the pilot whales and the killer whale, the largest dolphin. Separate families hold the river dolphins and the porpoises. All are carnivores, hunting fish and squid.
Seeing with sound
A dolphin makes rapid clicks in its nasal passages, focuses them through a fatty forehead lens, the melon, and receives the echoes through its lower jaw. From this it builds a detailed acoustic image — enough to tell a fish from a rock, find prey buried in sand, and sense the air spaces inside another animal.
A brain built for society
Bottlenose dolphins have among the highest brain-to-body ratios of any animal. They pass mirror self-recognition tests, use tools, understand pointing and symbolic language, and give each other individual signature whistles that work like names and are remembered for 20 years or more. Some groups keep local hunting traditions — the Shark Bay dolphins that carry a sponge on the beak while foraging teach the trick to their daughters.
Living in water, breathing air
A dolphin must decide to breathe, so it sleeps one brain hemisphere at a time, swapping every couple of hours. Dolphins hunt in coordinated teams, and in a few places, such as Laguna in Brazil, wild dolphins have herded fish toward human fishers for over a century.
Status
Dolphins as a family are Not Evaluated. The common and bottlenose dolphins are abundant, but the Maui dolphin is down to about 50, the humpback dolphins are Endangered, and the river dolphins are among the most threatened cetaceans — mostly from net entanglement, prey loss, dams, pollution, and boat traffic.
Dolphin: common questions
What is the difference between a dolphin, a porpoise, and a whale?
All three are cetaceans, and dolphins and porpoises are both small toothed whales, so in the broad sense every dolphin and porpoise is a whale. In common use, whale is kept for the larger species, dolphin for the sleek, beaked, highly social small toothed whales in the family Delphinidae and the river dolphin families, and porpoise for a separate family of smaller, stockier animals with blunt rounded heads and spade-shaped rather than cone-shaped teeth. The killer whale, despite the name, is the largest dolphin. Porpoises are generally shyer, less acrobatic, and less vocal than dolphins, and there are only about seven porpoise species against roughly 40 oceanic dolphins.
How intelligent are dolphins?
By every measure used for animal cognition, exceptionally. Bottlenose dolphins recognise themselves in mirrors, a test passed by only a handful of species, and they use tools, plan ahead, cooperate in flexible teams, imitate sounds and actions, understand human pointing and gaze, and can be taught to respond to a symbolic gesture language with correct answers to novel sentences. Each dolphin develops a unique signature whistle that works like a name, which others copy to address that individual, and they remember the signature whistles of companions for at least 20 years. Some populations maintain hunting traditions passed from mother to calf. Their brains are large, deeply folded, and rich in the neuron types linked to social processing.
How does dolphin echolocation work?
A dolphin makes a stream of short, sharp clicks by forcing air past structures in its nasal passages just below the blowhole. These clicks pass forward through the melon, a lens of fatty tissue in the forehead that focuses them into a narrow beam aimed ahead of the animal. When the beam hits an object, some sound bounces back, and the dolphin receives the echoes through fat-filled channels in its lower jaw that carry the vibrations to the middle ear. By sweeping the beam and timing the echoes, the dolphin builds a three-dimensional acoustic picture, judging an object's distance, size, shape, density, and internal structure, and it can do this in murky water or total darkness where vision is useless.
Are dolphins dangerous to humans?
Wild dolphins are not generally a threat, and encounters are usually curious and gentle, but they are large, powerful wild predators and should be treated as such. There are documented cases of bottlenose dolphins injuring swimmers, of lone sociable dolphins that habituate to people becoming pushy or aggressive, and of male dolphins showing forceful sexual behaviour toward swimmers. Bottlenose dolphins have also been recorded killing porpoises and their own calves. Feeding or swimming with wild dolphins is discouraged or illegal in many places because it changes their behaviour and puts both animals and people at risk. Killer whales in the wild have no confirmed record of killing a human.