Dolphin and Porpoise, common names for small, toothed MAMMALS of the order CETACEA which also includes WHALES. They are known from all oceans and many major river systems. In N America dolphins, with long snouts, are distinguished from porpoises, with blunt heads, but the terms are sometimes applied interchangeably to many small toothed whales, including members of the families Delphinidae, Phocoenidae, Stenidae and Platanistidae. 

 Harbour Porpoise   Five species are common in Canada. Harbour porpoise (Phocoena phocoena) is especially abundant in the lower Bay of Fundy, parts of the Gulf of St Lawrence and along the west coast. This, one of the smallest Northern Hemisphere cetaceans, has a relatively high metabolic rate and a short life span (about 13 years). Unlike some dolphins and porpoises, it does not live in large social aggregations, nor does it normally engage in aerial displays. 

 Dall's Porpoise   Dall's porpoise (Phocoenoides dalli), native to the N Pacific, occurs along the BC coast. Its colour pattern is reminiscent of the killer whale (Orcinus orca) which, at 9.5 m and 8000 kg, is the largest delphinid. 

 Other Species   Three species of temperate-region dolphins occur in Canadian waters; all are gregarious and sometimes acrobatic. White-beaked dolphins (Lagenorhynchus albirostris) and Atlantic white-sided dolphins (L. acutus), both endemic to the N Atlantic, have overlapping ranges between Cape Cod and Davis Strait. Their N Pacific relative is the Pacific white-sided dolphin (L. obliquidens). 

 The bottlenose dolphin (Tursiops truncatus), popular in animal exhibits, generally does not range into Canadian waters. Most captive specimens come from Florida or the Gulf of Mexico region. 
The dolphin's intelligence, playfulness, and friendliness, its built-in smile and merry-looking eyes have been a source of interest and enchantment to human beings from earliest times; it is a common figure in mythology and literature and has been much depicted in art, especially in the posture of its graceful, arched, 30-ft (9-m) leap. Dolphins have long been famous for riding the bows of ships, and it is now known that they also ride the bows of large whales. Today they are valued and exploited as entertainers in more than 40 water shows around the world and have thus become available for extensive study. 
The United States and Russian-Soviet navies have spent vast sums to reach a greater understanding of dolphin echolocation, which could have countless military applications. The U.S. navy has trained dolphins to act as messengers to underwater stations, to rescue wounded scuba divers and protect them from sharks, and to seek and destroy submarines, using kamikaze methods; this last project has met with considerable public criticism.