Dolphins are highly clever marine mammals and are part of the family of notched whales that includes orcas and pilot whales. They are found worldwide, mostly in shallow seas of the continental shelves, and are carnivores, mostly eating fish and squid. Dolphin coloration varies, but they are generally gray in color with darker backs than the rest of their bodies. Dolphin consumes a variety of prey including fish, squid and crustaceans. It is difficult to estimate population numbers because there are many different species spanning a large geographic area. Most species live in shallow areas of tropical and temperate oceans throughout the world. Five species live in the world's rivers.