Science / Medicine : Telling Dolphin From Porpoise

Just what is the difference between a porpoise and a dolphin, anyway?

Not much. At least not much to the casual whale watcher.

According to the American Cetacean Society, “It would be convenient to differentiate dolphins and porpoises, if only there was some rule we could use. . . . Unfortunately, there isn’t.”

“Some animals are always known as dolphins, and some are always called porpoises,” the society explains simply in a guide written by Richard Ellis, author of “The Book of Whales” and “Dolphins and Porpoises.”

Generally, however, small cetaceans with a “beak” are called dolphins; those with a rounded head profile are referred to as porpoises.

Indeed, most small cetaceans conveniently fit this profile. The spinners, spotters and bottlenose are all dolphins, while the harbor and speedy Dall’s porpoises have more rounded heads.

But there are notable exceptions, often quite confusing. The pilot whale, for example, is not really a whale, but a member of the dolphin family. But its head is distinctively rounded, like that of a porpoise.

The killer whale, too, is not a whale but a dolphin. Yet it also has a head more rounded than beaked.

At the same time, some true whales have beaks, such as the giant bottlenose whale or the Cuvier’s beaked whale.

A more specific--though, for most casual observers, less useful--way to distinguish porpoises from dolphins is to look at their teeth.

Scientists say that true porpoises have spade-shaped teeth, while those of most other small cetaceans are tapered and sharply pointed.