Whales are the only group of mammals to have adopted an exclusively aquatic lifestyle. Their entire life cycle, from birth until death, is carried out in an aquatic habitat. The terms "whale" and "cetacean" are usually used interchangeably, cetaceans being the scientific term for whales, dolphins, and porpoises. However, "whale" is also sometimes used to distinguish some of the larger species of the order Cetacea from the other two major groups, the dolphins and porpoises.

Cetaceans are found in almost all oceans and their connecting seas, as well as in estuaries (wide tidal river mouths) and rivers. Whales range in size from the 1-meter-long (3 feet) dolphins and porpoises to the 30-meter-long (100 feet) blue whale. The seventy-seven extant species of whales each belong to one of two suborders, the Odontoceti (toothed whales) and the Mysticeti (baleen whales). The toothed whales—porpoises, dolphins, and...