Dolphins and Porpoises

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By Linda | March 24, 2008

Yesterday’s post about rabbits and hares brought to mind another pair of species that are often confused.

Dolphins and porpoises belong to the same scientific order, Cetacea. This order includes all whales, even the great whales, to which both dolphins and porpoises are related. All cetaceans are completely aquatic mammals, have a streamlined body, a tail fluke, and a blowhole - which is what these air-breathing animals use to breathe. The dolphins and porpoises are also classified in the scientific suborder, Odontoceti, which are the toothed whales. All odontocetes also have the ability to echolocate, detecting objects in their underwater environment using the echoes of a sound, much like sonar.

Bottlenose Dolphin

Porpoises and dolphins are classified into two different families. The porpoises are in the family Phocoenidae and the dolphins are in the family Delphinidae. When separated at the family level, dolphins and porpoises are physically as different as cats and dogs.

In comparison to dolphins, porpoises are small. Porpoises seldom exceed seven feet in length, whereas many dolphins can exceed ten feet. Porpoises are also stouter than dolphins. Dolphins have a lean sleek body, whereas porpoises appear more chubby. The dorsal fin (the fin on the back of the animal) in porpoises is triangular, looking more like a shark’s. The dorsal fin of the dolphin is less symmetrical. Porpoises lack a rostrum or “beak”. The rostrum is very prominent in dolphins. The teeth of a porpoise are spade-shaped, whereas the teeth of the dolphin are conical.

Harbour Porpoise

Many porpoises do not live past their mid-teens. Porpoises maintain an aggressive reproductive schedule which may play a role in their lack of longevity. A porpoise can become pregnant each year, give birth, and then become pregnant again just five to six weeks later, meaning that it is often nursing and pregnant at the same time. Although this can also happen in dolphins, it is far less common. A dolphin’s lifespan can be upwards of fifty years.

There are behavioral as well as physical differences between the species too. For the most part, porpoises are shy animals. They do not often approach people or boats. The dolphin, on the other hand, is often seen riding the bow wave of fishing boats. We rarely see porpoises at the water’s surface unless they are coming up for a breath.

It’s dolphins rather than porpoises that we see in marine animal shows. This comes back to the dolphin’s bolder nature and fearlessness of man. This is also why it’s dolphins, rather than porpoises that get trapped in tuna nets. Because we encounter the dolphin more frequently, it is more widely studied and understood than the porpoise.

The American Cetacean Society website features an interactive picture chart that provides specific and fascinating information about each species and subspecies as well as illustrations of each. I had no idea that the Orca (Killer Whale) is actually a dolpin until I read about it on the ACS Website!