Porpoises are a group of fully aquatic marine mammals that belong to the family Phocoenidae, parvorder Odontoceti (toothed whales) . They are similar in appearance to dolphins, but are more closely related to narwhals and belugas than to true dolphins. There are eight extant species of porpoise, all of which are among the smallest of the toothed whales. Porpoises range in size from the vaquita, at 1.4 meters (4 feet 7 inches) in length and 54 kilograms (119 pounds) in weight, to the Dalls porpoise, at 2.3 meters (7 feet 7 inches) and 220 kilograms (490 pounds) .

Here are some key differences between dolphins and porpoises:

• Dolphins tend to have prominent, elongated “beaks” and cone-shaped teeth, while porpoises have smaller mouths and spade-shaped teeth.
• The dolphin’s hooked or curved dorsal fin differs from the porpoise’s triangular dorsal fin.
• Generally speaking, dolphin bodies are leaner, and porpoises’ are portly.
• Dolphins are more talkative than porpoises. Dolphins 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.

Porpoises are abundant and found in a multitude of environments, including rivers (finless porpoise), coastal and shelf waters (harbour porpoise, vaquita) and open ocean (Dalls porpoise and spectacled porpoise), covering all water temperatures from tropical to polar. They feed largely on fish and squid, much like the rest of the odontocetes. Porpoises use echolocation as their primary sensory system, and some species are well adapted for diving to great depths. Most porpoises are social animals and live in groups called shoals that can have just a couple members to thousands.