During Hibernation, which is most of their life, their body temperature lowers to below freezing.
Arctic Ground Squirrel (Spermophilus parryii) http://www.adfg.alaska.gov/index.cfm?adfg=arcticgroundsquirrel.printerfriendly
Hibernation
As an adaptation for coping with the harsh, inclement weather of the winter months, arctic ground squirrels hibernate for about eight months out of the year. Their chosen hibernacula have coverage provided by vegetation, rather than open, windswept burrows. This vegetation coverage allows for a higher accumulation of snow and warmer soil temperatures.
Females enter hibernation first, beginning in August, and are followed by males throughout the following month. During winter hibernation, arctic ground squirrels enter into a state of torpor in which their metabolic rate and body temperatures are drastically lowered for up to three weeks at a time. Their body temperature drops from about 99 degrees F (37 degrees C) to as cold as 27 degrees F (-3 degrees C), below freezing. Between these states of torpor they arouse and will either shiver or use their stored fat to bring their body temperatures back to a euthermic, or comfortable state of about 34–36 degrees Celsius. This rewarming period usually lasts one to two days and then they return to their state of torpor in order to conserve energy.
Males will emerge from hibernation earlier than females in order to reach sexual maturation before breeding season, since this kind of development is not possible in the extremely cold temperatures of winter months. They will have generally lost almost a third of their body mass by this point, and will begin to consume their food cache. Females emerge around two to three weeks later, experiencing a greater loss in body fat than males, having lost over one third of their body weight.