Evolution / Adaptation
Mammals are surprisingly adaptive; they have managed to make themselves fit into many evolutionary niches. Aquatic, flying, tree-climbing, burrowing, and many other varieties are good examples of biodiversity in mammals. During the 165 million years mammals have been around they have evolved a lot. For example Giraffes have evolved to avoid competition for food by growing long necks. As a class, mammals have been more successful than any other vertebrate when adapting to different environments.
Evolution of the WhaleMammals began as land dwelling creatures, but as time went on some of them where forced into change. As the environment around them changed, so did the mammals. Over a very long period of time, mammals became fully aquatic. They couldn't breath using gills, which is what kept the aquatic mammals from being classed as fish.
As the first few mammal species began to explore the waters, many other species of mammals where created through natural selection. Some mammals even became Semi-Aquatic.