Is an Orca a Whale or a Dolphin? Orca Classification

How whales, dolphins and porpoises are classified

All whales, dolphins and porpoises belong to one group of mammals, the cetaceans (infraorder Cetacea), with about 90 living species. Their ancestors were land mammals, and their closest living relatives are hippos, as explained in Are hippos related to whales?

Cetaceans are split into two main groups:

The toothed whales are divided into several families. The largest is the oceanic dolphin family, with more than 30 species, including the orca, the common bottlenose dolphin, common dolphins and pilot whales. Other families include the porpoises, the river dolphins, the beaked whales, the sperm whales, and the beluga and narwhal.

In everyday English, “whale” and “dolphin” are not scientific categories. Large cetaceans tend to be called whales and smaller toothed ones dolphins or porpoises, which is why one family can contain animals with both names.

Why the orca is a dolphin

Scientists place the orca among the oceanic dolphins because it shares their anatomy and, more importantly, their DNA. Like other members of the family it has:

• Cone-shaped teeth in both jaws, used to grip prey. Porpoises, by contrast, have flattened, spade-shaped teeth.
• A single blowhole and a rounded, fatty forehead called the melon, which focuses the clicks used in echolocation.
• The body and skull features that define the family. Carl Linnaeus already placed it in the dolphin genus in 1758, naming it Delphinus orca.
• A similar sound repertoire of clicks, whistles and pulsed calls, and complex social groups.

Genetic studies agree that the orca sits inside the oceanic dolphin family, although they still disagree about exactly which dolphins are its closest relatives. Its genus, Orcinus, contains only one living species.

Then why is it called a killer whale?

The name is older than the science. Sailors gave it to an animal that was big, breathed through a blowhole and hunted whales, long before anyone compared skulls or DNA. Size also matters: a male orca can reach 6–8 m (20–26 ft) and 6 t (13,000 lb) or more, bigger than several animals people call whales, such as the beluga and many beaked whales.

The orca is not the only “whale” in the dolphin family. Long-finned and short-finned pilot whales, the false killer whale, the pygmy killer whale and the melon-headed whale are all dolphins too. For the full history of the name, see Why are orcas called killer whales?

How the orca differs from other dolphins

Although it is a dolphin, the orca stands out from its relatives in several ways.

The orca is the only dolphin that regularly hunts other marine mammals, including other dolphins and large whales. It also has one of the most stable family lives of any mammal: in some populations sons and daughters stay with their mothers for life. And orcas are one of very few animals, along with humans and a handful of other toothed whales, in which females go through menopause and live for decades afterward.

Orcas are mammals, not fish

Whatever name we use, the orca is a mammal. It breathes air through its blowhole, is warm-blooded, keeps warm with a thick layer of blubber and nurses its calf with milk for at least a year. Its tail flukes are horizontal and move up and down, unlike the upright tail of a fish or shark. The reasons whales are classed as mammals are explained in Are whales fish?

People sometimes ask whether an orca is closer to a shark than to a dolphin because of its size and predatory habits. It is not: the orca’s nearest relatives are other dolphins, and sharks are fish that last shared an ancestor with mammals more than 400 million years ago. To learn more about the species, see our orca profile, or compare dolphins with their small, blunt-headed cousins in dolphin vs porpoise.