The difference between a porpoise and a dolphin can be subtle, as they are two closely-related groups of species that are grouped together in the Cetacea order and Delphinidae family along with whales. As of 2011 however, at least 32 species of dolphins have been identified in the wild compared to only six species of porpoise. This is due in part to the fact that dolphins tend to be much more gregarious and receptive to encounters with humans in or near the water than porpoises, making them easier to classify. Dolphins also appear to be more highly social animals that communicate by underwater sounds with their young, whereas porpoises have not been observed to do this.
Physically, the characteristics of a porpoise and a dolphin vary in a few key common areas. The central dorsal fin on most dolphins has a sloping backward shape and inner curve on its back side, whereas a porpoise dorsal fin is a rounded version of the triangular shape that a shark's dorsal fin has. Dolphins are also known to have cone-shaped teeth while porpoise teeth take on a spade shape. Porpoises are one of the smallest groups of cetaceans as well, rarely exceeding 7 feet (2.1 meters) in length. While dolphin sizes vary considerably, they are often over 10 feet (3.05 meters) in length, with the biggest dolphin, popularly known as the Orca or killer whale, reaching up to 32 feet (9.8 meters) in length.
Another key difference between a porpoise and a dolphin is their range of habitat. While the porpoise has only been observed in saltwater ocean environments, there are two broad ecosystems for dolphins, including the larger group of ocean dolphins and a select group of river dolphins. Five species of dolphins are known to live within freshwater river systems. They are usually named after the rivers themselves, and these include such species as the Amazon river dolphin in South America and the Ganges river dolphin in northern India.
The Chinese river dolphin has traditionally made its home in the Yangtze river in China. Unfortunately, it has not been spotted there since at least the year 2006 and has been declared extinct. The difference in how readily a porpoise and a dolphin will approach human boats has led to many dolphins being inadvertently killed by drowning when ensnared in fishing nets, while shy porpoise behavior has protected them from such deadly human encounters.
While the Phocoenidae family of porpoises is characterized by smaller dorsal fins and a more blunt-shaped face than that of dolphins, they are not without their own distinctive features. A porpoise and a dolphin can often be mistaken for each other because they share sometimes confusing traits. The Dall's porpoise is a good example of this. It has the black-and-white patchwork skin that strongly resembles the Orca, while much smaller in size at a maximum length of 7.5 feet (2.3 meters) for an adult, versus 32 feet (9.8 meters) for that of the Orca. An adult Dall's porpoise can weigh up to 350 pounds (159 kilograms) as compared to a newborn baby Orca that weighs about 395 pounds (179 kilograms).