Adult female Arctic ground squirrels can begin winter hibernation as early as early August.
The Arctic ground squirrel (Urocitellus parryii) lives across Arctic and Subarctic regions of North America and Eurasia. Its short summer is devoted to feeding on plants, seeds, fruit, and other foods while building the fat reserves needed for a long period underground. Females enter hibernation earlier than males, reflecting differences in their annual reproductive and energetic schedules.
Adult females may hibernate from early August until late April. Adult males generally enter later, around late September, and emerge earlier, around early April. Juveniles follow their own timetable as they grow and accumulate enough reserves for winter. The exact dates can vary with latitude, weather, sex, age, and local conditions.
This species is famous for tolerating extraordinary cold. During hibernation, its core body temperature can fall from about 37°C to as low as −3°C. It avoids freezing partly by removing ice-nucleating substances from its body fluids, allowing them to remain supercooled. Researchers study this adaptation for possible lessons in preserving human organs.