Mammals are vertebrates (animals with a backbone) that share characteristics of nursing their young with milk, breathing air, having hair at some point in their lives, and being warm-blooded. Marine mammals are the species of mammals that depend on the oceans for all or most of their lives. There are about 115 different species of marine mammals. Marine mammals vary from the small sea otter to the giant blue whale. Some of them live in groups, like dolphins, while others are solitary, like polar bears. All marine mammals share four characteristics:

• They have a streamlined body shape that makes them excellent swimmers.
• They maintain heat in their bodies with layers of fat called blubber.
• They have respiratory (breathing) systems that allow them to stay underwater for long periods of time.
• They have excretory (waste) systems that allow them to survive without drinking freshwater. Instead they obtain the...