People use the term dolphin, porpoise and whale to describe marine mammals belonging to the order Cetacean (from the Greek work ketos, “large sea creature”), and often use them incorrectly. Cetaceans are divided into two groups; baleen whales (Mysticeti) and toothed whales (Odontoceti). The main differences between these two groups are that the baleen whales are filter feeders, have two blow holes and are generally very large in size, whereas toothed whales only have a single blowhole and a relatively small body size (except for the Sperm whale). The toothed whales include all species of dolphin and porpoise.
Dolphins are toothed whales and the largest dolphin is the Orca (generally mistaken for a whale due to its name ‘killer whale’). A related family to dolphins are the porpoises. People often confuse dolphins and porpoises, but there are subtle differences to look out for Dolphins tend to have prominent, elongated rostrums or “beaks” and cone-shaped teeth while porpoises have smaller, rostrums and spade-shaped teeth. The dolphin’s hooked or curved (falcate) dorsal fin (on the back) also differs from the porpoise’s more triangular shaped dorsal fin. A dolphin’s body is lean and a porpoises’ body is rounded.
Dolphins are also more vocal than porpoises. They make whistling sounds through their blowholes to communicate with one another underwater. Scientists are pretty sure that porpoises do not do this, and some think this may be due to structural differences in the porpoise’s blowhole.
As all animals differ both in looks and genetics so do whales, dolphins and porpoises. It’s not easy to distinguish between the many cetacean species found worldwide, but remember: all dolphins are whales, but not all whales are dolphins.
– Whales dolphins and other marine mammals of the world (2006) by Hadoram Shirihai and Brett Jarrett. Prinston field guides 2006
– Jefferson T. Webber M. Pitman R. 2007 Marine mammals of the world: A comprehensive guide to their identification 1st edition