Mammals, unlike fish, feed their young with milk that is produced in the mother's body. Many scientists believe that dolphins rank among the most intelligent animals, along with chimpanzees and dogs. Dolphins, whales, and porpoises are members of a group of mammals called cetaceans. Dolphins and porpoises are very similar in appearance. Their chief differences occur in the snout and teeth.
Dolphins have a beaklike snout and cone-shaped teeth.
Porpoises have a rounded snout and flat or spade-shaped teeth. Whales are much larger than most dolphins and porpoises. Scientists apply the term dolphin to two families of cetaceans, marine dolphins and river dolphins. There are 32 species of marine dolphins. They are found in nearly all oceans, and most of them live only in salt water.