Some species of bats hibernate, and others are active all year. Bats that live in areas where insects die out during the winter tend to hibernate because of depleting food sources, and bats in tropical areas with reliable year-round prey don't usually hibernate.
Bats roll up into balls while hibernating, and they usually hibernate on the walls or ceilings of caves. Some species of bats hibernate for up to six months, subsisting on just a few grams of fat reserves. Their ability to drop their body temperature and slow their metabolism and heart rate allows them to hibernate for so long. Bats typically lose approximately half of their total body weight during hibernation.