A hibernating Arctic ground squirrel can lose about 40% of its body weight. This loss reflects the gradual use of stored fat and, to a lesser extent, other body reserves during a winter when the squirrel spends long periods underground without feeding.
Before hibernation, Arctic ground squirrels undergo an intense period of fattening. Body mass can rise by roughly 40–60% during the preparation season, giving the animal enough fuel to survive months of torpor. Females and males do not always follow identical schedules or lose the same proportion of mass; some studies report females losing more than one-third of their body weight.
The species is one of the most extreme mammalian hibernators known. Its core temperature can fall below the freezing point of water, reaching about −3°C, while its heart rate and metabolism slow dramatically. The squirrel periodically arouses from torpor to restore normal body temperature before returning to hibernation.
The 40% figure is best read as an approximate upper value rather than a fixed loss for every squirrel. Age, sex, season length, starting fat reserves, and local food conditions all influence the final percentage. The squirrel’s famous cold tolerance is only possible because its pre-winter fat stores are so substantial.