acea order. Because they are mammals, all cetaceans must breathe air from time to time. For this purpose, they are equipped with blowholes instead of the gills which are generally found in most finned aquatic creatures. In addition, like mammals, dolphins and other cetaceans are warmblooded, bear live young, and nurse their young with milk (Reeves and Leatherwood, 1987, p. 10). Scientists believe that the cetaceans used to live on land during prehistoric times. Like the other mammals, they could have continued adapting themselves for life on land during the course of their evolution. However, for some reason, they returned to the sea and became adapted to life in that type of environment. This explains why the dolphins and other cetaceans combine the characteristics of the mammal family with certain qualities of fish, such as the existence of fins.
There are various different types of dolphins found throughout the world, including common dolphins, bottle-noses, spinners, and spotters. Some dolphins live exclusively in rivers. River dolphins are only found in the large rivers of South America and Asia, such as the Amazon, the Yangtze, and the Ganges (Patent, 1989, p. 9). In addition to river dolphins, there are certain types of dolphins which live in coastal waters or along the continental shelves. There are also oceanic or deep-sea dolphins which exist beyond the continental shelves. However, these types of dolphins are comparatively rare, and they are generally clustered near "seamounts," or submerged islands, where they are able to find sources for food (Reeves and Leatherwood, 1987, p. 68). All dolphins, regardless of their specific environment, have a capability which is known as "echolocation." Echolocation is similar to sonar, in that it involves the use of sound vibrations in order for the dolphin to find its way through the water and to seek out fish to eat. The dolphin emits a clicking sound which echoes back to ...