In addition to being one of the most common bats of North America, the little brown bat (Myotis lucifugus) is also one of the smallest known mammalian hibernators with a typical body mass of just 5-8 g. During winter this insectivorous bat is subjected to long periods of cold temperatures and a lack of food. To survive these inhospitable conditions these bats undergo hibernation in caves and other sites. When dormant, bats drop their metabolic rate as low as 1-1.5% of the euthermic state and decrease their core body temperature to near ambient temperatures. During hibernation, metabolism becomes primarily lipid-based, in preparation for this, bats accumulate large lipid reserves during summer/autumn feeding seasons and deposit them as both white and brown adipose tissue. During the 7-9 month hibernation season the bats can remain in constant torpor for 2-7 weeks at a time interspersed with brief periods of rewarming to euthermia. Currently, the survival of little brown bats is threatened by white nose syndrome. This fungus disrupts the bats hibernation cycle by stimulating more frequent periods of arousal that depletes their lipid stores before they can be replenished by renewed feeding in the spring. The resulting massive die-offs of bat colonies have led Canada to list these bats as an endangered species.