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.During the darkest part of the night, common vampire bats emerge to hunt. Sleeping cattle and horses are their usual victims, but they have been known to feed on people as well. The bats drink their victim's blood for about 30 minutes. They don't remove enough blood to harm their host, but their bites can cause nasty infections and disease. Vampire bats strike their victims from the ground. They land near their prey and approach it on all fours. The bats have few teeth because of their liquid diet, but those they have are razor sharp. Each bat has a heat sensor on its nose that points it toward a spot where warm blood is flowing just beneath its victim's skin. After putting the bite on an animal, the vampire bat laps up the flowing blood with its tongue. Its saliva prevents the blood from clotting. Young vampire bats feed not on blood but on milk. They cling tightly to their mothers, even in flight, and consume nothing but her milk for about three months. The common vampire bat is found in the tropics of Mexico, Central America, and South America.