Arctic ground squirrels can lower their body temperature below freezing during hibernation without their blood turning to ice.

Arctic ground squirrels (Urocitellus parryii) are the only known mammals capable of surviving body temperatures below 32°F (0°C). During their eight-month hibernation in the Alaskan and Siberian tundra, their metabolic rate drops to just 1% of their normal active state. Researchers from the University of Alaska Fairbanks have documented these squirrels reaching core temperatures as low as 26.8°F (-2.9°C).To achieve this, the squirrels utilize a biological mechanism known as supercooling. They effectively purge their blood of ice-nucleating agents, which are proteins or particles that typically provide a starting point for ice crystals to grow. Without these agents, water can remain in a liquid state even when it is well below its standard freezing point.While their core remains supercooled, their head and neck regions typically stay slightly above freezing, likely to protect brain function. Every two to three weeks, the squirrels undergo a period of shivering to temporarily raise their body temperature back to 98°F (37°C) for several hours. This prevents long-term cellular damage and allows for essential biological maintenance before they return to their frozen state.