The Arctic ground squirrel (Spermophilus parryii) hibernates for eight months, allowing its body temperature to drop below water freezing point. It reaches -3ºC while it sleeps, which is the lowest body temperature registered in a mammal.
Fish are good at this but they have antifreeze proteins to assist them with that. This is not the case of the Arctic ground squirrel that somehow keeps the blood liquid below 0ºC.
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RMach