Explainer: What is a whale?

Most people think they know what a whale is. It’s one of those enormous animals that cruise the ocean. But ask what distinguishes whales from dolphins (or from porpoises), and things get fuzzy. The answer isn’t just size. A major problem is that “whale” isn’t even a scientific term.

The word probably comes from some ancient European language and originally meant big ocean fish. But in recent centuries, biologists have pointed out that whales aren’t fish. They’re mammals.

The formal term for all of these related mammals is cetaceans (See-TAY-shuns). Things tend to get confusing when people attempt to divide cetaceans into subgroups.

All cetaceans belong to one of two suborders, based on how they eat. The biggest of these animals filter food from the water — often tiny krill and plankton — using big baleen plates. The 15 species of baleen whales belong to the suborder of cetaceans known as Mysticetes (Miss-tuh-SEE-tees). They include such behemoths as the blue, gray and right whales.

The other suborder, Odontoceti (Oh-DON-tuh-SEH-tee), have teeth. These animals include sperm whales, beaked whales, porpoises and dolphins. And about those dolphins: Some are, well, “whales.” Indeed, six kinds of oceanic dolphins have whale in their common name. These include killer whales and pilot whales.

So it’s best to think of whale as the marine mammal equivalent to “bug” (that equally unscientific term that people use when referring to some insect or other small arthropod, such as a spider or tick).