Arctic Ground Squirrels Can Lower Their Body Temperature Below Freezing

Arctic ground squirrels are — for part of the year, at least — quite literally the coolest mammals on Earth. During hibernation, which can last up to eight months in the frigid Alaskan winter, these squirrels lower their core body temperature to approximately 26 degrees Fahrenheit — below the freezing point of water. They’re the only known warm-blooded mammals capable of supercooling their bodies this way.

They achieve this feat of suspended animation by slowing their metabolisms to a fraction of the normal rate. Their bodies become so cold, and their breathing and heartbeat so slow, that it’s hard to tell if they’re still alive. Many of their brain cells and synapses die off during this process but rapidly regenerate as the animals emerge from hibernation.