Dolphin

Every bottlenose dolphin invents a whistle for itself in its first year, and keeps it for life. Researchers call it a signature whistle, and it works the way a name does.

The part that makes it a name rather than a label is this: dolphins copy each other’s whistles. A dolphin separated from its group will produce another individual’s whistle, and that individual answers. They are not describing each other. They are calling each other by name.

Outside of humans, almost no animal is known to do this.

There are about 40 species of dolphin, so there is no single scientific name. Most belong to the family Delphinidae, the ocean dolphins, with the river dolphins placed in several smaller families of their own.

• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Cetacea
• Family: Delphinidae, and others

The common bottlenose dolphin is Tursiops truncatus. The common dolphin is Delphinus delphis.

And here is the one that catches people out: the orca is a dolphin. Orcinus orca sits inside Delphinidae, which makes the killer whale the largest dolphin in the world, not a whale at all. Pilot whales and false killer whales are dolphins too.

Dolphins are mammals, not fish. They breathe air, are warm-blooded, give birth to live young and nurse them with milk, and their ancestors walked on land about 50 million years ago.

A dolphin makes clicks in its nasal passages, focuses them through a fatty lens in its forehead called the melon, and listens to what comes back through its jawbone, which carries the sound to its ears.

What this gives it is not a vague sense of obstacles. It is detailed.

• It finds buried prey. A dolphin can detect a fish hidden under sand, which no eye could do.
• It sees inside things. Sound passes through soft tissue and bounces off bone and air, so a dolphin gets something closer to an ultrasound scan than a photograph. Dolphins have been observed paying particular attention to pregnant humans in the water.
• It is precise. Trained dolphins can tell apart objects differing only slightly in thickness, at distances of many yards.
• It works in the dark. River dolphins in the Amazon and the Ganges live in water so muddy that eyesight is nearly useless, and some have very poor vision indeed.

This is also why noise matters so much to them. A dolphin’s main sense is hearing, and shipping, sonar and construction fill the ocean with sound it cannot escape.

Dolphins range from about 4 feet and 90 pounds in the smallest species to 30 feet and over 20,000 pounds in a large male orca. A common bottlenose is roughly 8 to 13 feet and 300 to 1,400 pounds.

• Blowhole. A nostril on top of the head, closed by a muscular flap. Dolphins breathe voluntarily, which matters more than it sounds.
• Skin. Smooth, rubbery and replaced constantly. A dolphin sheds its outer skin layer roughly every two hours, which keeps it clean and reduces drag.
• Teeth. Simple cones, all alike, for gripping. Dolphins do not chew.
• Flippers and fluke. The flippers contain the bones of a hand. The tail fluke is horizontal and beats up and down, unlike a fish’s vertical tail.

About the famous smile: it is not one. The upturned line of a dolphin’s mouth is just the shape of its jaw, fixed in place whatever the animal is feeling. It is worth knowing, because that permanent grin has sold a lot of people on the idea that dolphins are always happy, including dolphins in tanks far too small for them.

Dolphins live in every ocean, from the tropics to the edge of the ice, and several species live in fresh water.

Most are coastal or offshore ocean animals. The river dolphins are a separate story: the boto in the Amazon, the South Asian river dolphin in the Ganges and Indus, and the finless porpoise and Yangtze species in China, all of them living hundreds of miles from the sea.

The Yangtze river dolphin, the baiji, is the hard lesson here. A survey in 2006 failed to find a single one, and it is now considered functionally extinct, the first dolphin species lost in modern times and the first large mammal driven out by human activity in decades.

All dolphins are carnivores, taking fish, squid and crustaceans, with orcas also hunting seals, sharks and other whales.

Their hunting techniques are the most interesting part, because many of them are learned rather than instinctive, and different groups do different things.

• Mud-ring feeding. In Florida, dolphins beat the bottom with their tails to raise a ring of mud, and fish panic out of it straight into their mouths.
• Strand feeding. In South Carolina and Georgia, dolphins drive fish onto a mudbank and throw themselves half out of the water after them, always on the same side.
• Sponge tools. In Shark Bay, Australia, some dolphins carry a sea sponge on the beak to protect it while probing the rough seafloor. The habit passes from mother to daughter, which makes it culture, not instinct.
• Working with people. At Laguna in Brazil, dolphins have herded fish toward net-fishermen for well over a century, signaling when to cast. A study published in 2023 showed both sides catch more this way.

Dolphins live in groups called pods, usually a few dozen strong, sometimes gathering into superpods of hundreds or thousands.

They sleep with half the brain at a time. One hemisphere rests while the other stays awake with one eye open, because a dolphin must decide to take every breath it takes, and an unconscious dolphin would drown. They never fully sleep in the way we do.

They are among the few animals that pass the mirror test, recognising the reflection as themselves rather than another dolphin. They play, surf boat wakes, pass objects around, and blow rings of air underwater and then swim through them.

They should not be romanticised, though. Male bottlenoses form alliances that coerce females. They have been recorded killing harbor porpoises for no reason anyone can identify, and killing calves of their own kind. Dolphins are highly intelligent, and intelligence is not the same as kindness.

Dolphin mothers carry a single calf for 10 to 17 months depending on species, and give birth tail first, underwater.

The calf must reach the surface for its first breath within seconds, and the mother, often helped by another female, pushes it up.

Calves nurse for one to three years and stay with the mother for three to six, which is a long apprenticeship. It has to be, because so much of what a dolphin needs to know is taught rather than inherited, including which hunting method its group uses.

Dolphins live 20 to 60 years in most species. Female orcas can reach 80 or 90, and a few are believed to have passed 100.

Large sharks take dolphins, and orcas prey on smaller dolphin species.

The serious threats are all human, and the biggest by a wide margin is fishing gear. Hundreds of thousands of dolphins, porpoises and small whales drown in nets every year, caught by accident in fisheries aimed at something else.

Beyond that: chemical pollution that concentrates in their blubber and passes to calves in milk, underwater noise that interferes with the sense they depend on most, plastic, overfishing of their prey, dams and pollution in the rivers the freshwater species cannot leave, and direct hunting in a few places.

No single status covers 40 species. Widespread ocean dolphins such as the bottlenose and common dolphin are Least Concern. The river species are in serious trouble.

The most urgent case is not a dolphin but its close relative the vaquita, a small porpoise in the Gulf of California reduced to around ten individuals by illegal gillnet fishing. It is the clearest demonstration available of how fast bycatch alone can finish a species.

There is real good news too. The dolphin-safe tuna campaigns of the late 1980s cut dolphin deaths in the eastern Pacific tuna fishery by more than 95 percent, which shows that bycatch is a solvable problem when anyone decides to solve it.

Dolphins are top or near-top predators, and they keep fish and squid populations in check across every ocean.

They are also useful indicators. Because they are long-lived, fatty and high in the food chain, pollutants build up in their bodies, and testing dolphin tissue tells scientists what is accumulating in a whole marine system.

Where orcas hunt, they shape entire communities, and changes in orca behavior have knock-on effects reaching down to kelp and sea urchins.

Their largest contribution may be to how we think. Signature whistles, tool use passed down the female line, self-recognition in a mirror and hunting methods that differ between neighboring groups have all forced a rethink of what counts as culture and what minds other than ours are capable of. The dolphin is the animal that made that question unavoidable.