Supercool squirrels go into the deep freeze

In the Arctic winter, it is not even worth getting up in the morning. It's freezing cold and the sun never rises, making it impossible to tell night from day. So each autumn, when the Arctic ground squirrel (Spermophilus parryii) heads underground to hibernate for eight months, it doesn't even bother setting its circadian clock. During hibernation, the squirrel goes into a state akin to suspended animation. It cuts itself off from the world and allows its body temperature to drop to -3 °C while it sleeps – the lowest ever body temperature recorded in a mammal. Once it wakes up for the summer, however, the squirrel can switch its daily clock back on. The squirrels' sub-zero tolerance was first discovered almost 25 years ago. Curious how the animals manage to survive the frigid Arctic winter where temperatures regularly drop to -30 °C, Brian Barnes of the University of Alaska in Fairbanks implanted radio transmitters into the stomachs of captive squirrels, which transmitted information on their body temperature, before letting them build burrows for the winter. Once the squirrels went into their deep sleep, Barnes found that their core body temperature dropped from about 36 °C to -3 °C. To prevent their blood from freezing, the squirrels cleanse it of any particles that water molecules could form ice crystals around. This allows the blood to remain liquid below zero, a phenomenon known as supercooling. It's still unknown how the squirrels do this, says Barnes, because they lack the "antifreeze" proteins that allow fish to survive at low temperatures. But their extremely low metabolism probably allows them to make best use of their fat stores.
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