The Arctic Ground Squirrel, the largest and most northern of the New World ground squirrels, is a hibernator. Just what does that mean? During hibernation it's brain and core body temperature drops to below freezing! And it stays this way from late September through April! Peripheral, colonic, and blood temperatures become subzero by means of supercooling. Body temperatures drop as low as 27 degrees Fahrenheit, the lowest known naturally occurring core body temperature of any mammal. The Arctic Ground Squirrel is truly "built" for it's environment.