An order of wholly aquatic mammals occurring in all the oceans and adjoining seas of the world, as well as in certain river systems. Some cetaceans are the only animals other than elephants that have a brain larger than man's. most have eyes well adapted to underwater vision and can also see well above water. They produce numerous underwater sounds used in echolocation. They feed generally on fish, cephalopods, and crustaceans. Most are gregarious and most have a relatively long period of parental care and maturation.