“Is a porpoise a whale?” sounds like a trick question, and in a way it is. The answer depends on whether you are speaking as a biologist or as someone on a whale-watching boat. Scientifically, every porpoise and every dolphin is a whale, because all of them belong to the same order of mammals, Cetacea. In everyday language, people use “whale” for the big ones and “dolphin” and “porpoise” for the small ones, even though some “whales” are actually dolphins. This page untangles the family tree, explains why the common names are so confusing, and shows how to tell a whale, a dolphin and a porpoise apart. For all seven species, see our porpoise species guide.

The cetacean family tree

All whales, dolphins and porpoises are cetaceans, descendants of land mammals that returned to the sea around 50 million years ago. Their closest living land relatives are hippos.

Living cetaceans split into two main groups. Baleen whales (Mysticeti) have no teeth as adults and filter food through plates of baleen; this group includes blue, fin, humpback, gray, right and minke whales. Toothed whales (Odontoceti) have teeth and hunt individual prey, using echolocation to find it. The toothed whales are by far the larger group by number of species, and they include sperm whales, beaked whales, belugas and narwhals, river dolphins, ocean dolphins and porpoises.

So a porpoise is a toothed whale, in exactly the same way that a dolphin or a sperm whale is. Within the toothed whales, porpoises form their own family, Phocoenidae. Genetic studies show that their closest relatives are not dolphins but the family Monodontidae, the beluga and the narwhal. For how hippos fit into the story, see are hippos marine mammals.

Why the common names are so confusing

Common names for cetaceans grew up among whalers, fishermen and sailors long before anyone sorted out their evolutionary relationships, and they mostly describe size. Big animals were whales; small, fast animals were dolphins or porpoises. That is why some of the most famous “whales” are actually dolphins. The killer whale, or orca, is the largest member of the dolphin family; see orca. Pilot whales, false killer whales and melon-headed whales are dolphins too.

It goes the other way as well. In parts of the United States, especially among older fishermen, bottlenose dolphins were routinely called porpoises, and the name stuck in some places for a long time. Meanwhile, the beluga is sometimes asked about as “a dolphin or a whale”; it is a toothed whale in its own family, more closely related to porpoises than to dolphins. See are beluga whales dolphins or whales. In other words, the names are a rough guide to size, not a guide to family.

How to tell a porpoise from a dolphin

Porpoises and dolphins are the two groups people mix up most. The clearest difference is inside the mouth: porpoise teeth are flattened and spade-shaped, while dolphin teeth are cone-shaped. You will rarely see that at sea, so look at the head, the fin and the behaviour. Porpoises have a short, rounded head with no distinct beak; most dolphins have an obvious beak. A porpoise’s dorsal fin is usually small and triangular, like a little shark fin, and the finless porpoises have no fin at all, just a low ridge; most dolphins have a taller, backward-curving fin.

Porpoises are also smaller, stockier and generally shyer. The harbour porpoise, the most widespread species, is about 1.5 metres long and usually surfaces with a quick, quiet roll, showing its back and fin briefly before disappearing. It rarely leaps or bow-rides. Dolphins are often larger, more social, more acrobatic and more likely to approach boats. Dall’s porpoise is the big exception: it is fast and sometimes rides bow waves, throwing up a distinctive “rooster tail” of spray. Our dolphin vs porpoise page goes into more detail.

How to tell a porpoise from a large whale

This one is easier. Even the largest porpoise, Dall’s porpoise, is only a little over two metres long, while the smallest baleen whale, the pygmy right whale, is around six metres and most great whales are much bigger. Baleen whales have two blowholes and usually a tall, visible blow; porpoises have a single blowhole and a blow that is hard to see at all. A porpoise surfaces with quick rolls, often several in a row, while a large whale tends to surface slowly, blow, show a long stretch of back and sometimes lift its tail before a deep dive. If you are not sure whether you are looking at a porpoise or a whale, the size and the visibility of the blow will usually settle it.

The seven porpoises at a glance

All seven porpoises are small, and most live in coastal or shelf waters. The harbour porpoise ranges across the cool coastal waters of the North Atlantic and North Pacific. The vaquita lives only in the northern Gulf of California and is the world’s most endangered marine mammal, with only a handful left; see vaquita facts. Burmeister’s porpoise lives along the coasts of South America, and the spectacled porpoise lives in the cold waters of the Southern Ocean and is rarely seen alive. Dall’s porpoise lives in the open North Pacific. The two finless porpoises, the Indo-Pacific and the narrow-ridged, live in warm coastal waters and some rivers of Asia, including the Yangtze. All of them use high-frequency echolocation clicks to find fish and squid, and most are vulnerable to accidental capture in fishing nets.

Where the word “porpoise” comes from

The word itself hints at how people saw these animals. “Porpoise” comes through Old French from the Latin porcus piscis, roughly “pig fish”, probably a reference to the porpoise’s stout, rounded body and blunt snout. “Dolphin” traces back to the Greek delphis, while “whale” comes from Old English hwæl, a word once used for almost any large sea creature. None of these names was coined with biology in mind. Medieval writers often classed all of them as fish, and it was only in the eighteenth century, when Carl Linnaeus grouped whales with mammals, that the modern picture began to take shape. Even then, the common names stayed as they were, which is why we still say “killer whale” for a dolphin and still ask whether a porpoise is a whale. Linnaeus himself named the harbour porpoise Delphinus phocoena, lumping it in with the dolphins; it was later moved into its own genus, Phocoena.

Does the label matter?

For conservation and law, the words used matter less than the science. In the United States, all cetaceans are protected under the Marine Mammal Protection Act, whether they are called whales, dolphins or porpoises; see why the Marine Mammal Protection Act was created. The International Whaling Commission, by contrast, has long focused on the large whales, and whether it should also manage small cetaceans such as porpoises has been a long-running debate among its members. In practice, small cetaceans often fall through the cracks: the biggest threat to most porpoises is not hunting but bycatch in gillnets, which is managed, when it is managed at all, by fisheries agencies rather than whaling bodies.

For understanding the animals themselves, the scientific grouping is far more useful than the common names. Knowing that a porpoise is a small toothed whale tells you it echolocates, hunts individual prey, has a single blowhole and shares a common ancestor with sperm whales and belugas, none of which you would guess from the word “porpoise” alone.

My take: watch the fin and the roll

When people ask me how to tell these animals apart, I always come back to two things you can actually see from a boat or a shoreline: the shape of the dorsal fin and the way the animal surfaces. A small, triangular fin that appears briefly in a quick, quiet roll is very likely a porpoise. A tall, curved fin on an animal that leaps, rides the bow or comes over to investigate is very likely a dolphin. A tall blow and a long back, perhaps followed by a tail fluke, is a large whale. That simple checklist will sort out most sightings.

And when someone asks whether a porpoise is a whale, my honest answer is: yes, a small one. It is a reminder that the names we use for sea life often tell us more about the history of the people who named them than about the animals themselves. For the bigger picture, see our marine mammals hub and our guides to whales and dolphins.

For the range and habitat of each of the seven species, see where does a porpoise live?

Like other toothed whales, porpoises hunt individual prey by echolocation; see what do porpoises eat?