Prairie Dawg

It must be great being a Prairie Dog. When you are just a few months old you can copulate and begin the next generation. The ultimate mammal would be one where the time between generations is reduced from months to days to hours to seconds. Imagine throwing 2 Prairie Dogs off a tall building and by the time they reach the ground their numerous generations of offspring would have evolved to grow wings and could fly safely away. Of course all the earlier generations would die a horrible death, but that's natural selection for you.