The world of mammals is wonderfully rich and varied. It includes thousands of species -- everything from orangutans to muskrats to squirrels. Some animals, such as chimpanzees, build their homes in just a few minutes. Others, such as beavers, will take many days to make sure their lodge or dam is both sturdy and safe. Mammals are highly intelligent, and each time one of these animals builds a new home, it may be built a little differently, a little faster, a little stronger, a little better.