Bats are the only mammals that can fly, but vampire bats have an even more interesting distinction—they are the only mammals that feed entirely on blood. These notorious bats sleep during the day in total darkness, suspended upside down from the roofs of caves. They typically gather in colonies of about 100 animals, but sometimes live in groups of 1,000 or more. In one year, a 100-bat colony can drink the blood of 25 cows.