Animals of all varieties hibernate, including mammals, reptiles and insects. Hibernating mammals include bears, squirrels, groundhogs, raccoons and skunks; hibernating reptiles include frogs, turtles, lizards, snakes and snails.
Technically, hibernation only occurs when animals become severely inactive or sleep during the short and cold days of winter. It is very common for animals to gain weight in the summer and fall in preparation for hibernation, as the extra fat helps keep them alive as they lie dormant. Black bears sometimes pack on an extra 30 pounds a week in preparation for hibernation. When not fattening themselves up during the fall, many animals prepare nests for themselves for sleeping and food storage.
Some animals sleep throughout the entire course of their hibernation, while other animals wake briefly to eat, drink or move to a different location. Female bears sometimes give birth during hibernation. It is common for hibernating animals' heart rates to slow down beneath 10 beats per minute. As their heartbeats slow, many animals' breathing slows as well; some species of bats can go an hour between breaths while hibernating. Many hibernating animals have internal controls that prevent their body temperatures from falling dangerously low by waking them up if it gets too cold.