Arctic ground squirrels have developed a unique way to cope with the extreme conditions they encounter every winter.
They avoid the harsh weather and lack of food by escaping to underground burrows and slipping into a deep sleep for around eight months.
Once inside their nests, Arctic ground squirrels' body temperatures drop as low as -2.9C – the lowest measured in a mammal – and their major organs slow while others shut down entirely, making them appear dead to the world.
During this potentially lethal hibernation, or torpor, Arctic ground squirrels stop their bodily fluids from freezing by a clever process known as supercooling.
The BBC Two programme Alaska: Earth’s Frozen Kingdom used thermal imaging to demonstrate how the mammals occasionally wake-up and warm themselves for short periods of time, characterised by bouts of shivering (see clip above).
Scientists think that these re-warming phases, which happen every three weeks, are necessary to maintain certain processes like brain function and to allow the Arctic ground squirrel to properly sleep.
They are only awake for around 12 days during the winter months but even in torpor they need to generate heat to make sure they don’t get any colder. They do this by using reserves of "brown fat", or brown adipose tissue, shown by the heat spot between the shoulder blades in the clip above.
Before entering hibernation Arctic ground squirrels almost double their body weight. They are larger than other squirrels, meaning they can put on more fat and therefore have bigger energy reserves to cope with the demands of their extreme hibernation.
It takes Arctic ground squirrels three hours to fully warm up from hibernation and bodily functions return to normal within 24 hours.
Males, who wake up first, are thought to know when it’s time to begin the warming process thanks to a circannual clock in their brains and also by detecting soil temperatures.
They lose almost a third of their body weight and have to rely on stores of food built up before hibernation. Females emerge a few weeks later and mating begins immediately as the season is only a fortnight long.
Arctic ground squirrels remain active for around three months of the year, part of which is spent preparing for their next hibernation.
The highly specialised survival process isn't without risk: lowering metabolic rates to dangerous levels means many Arctic ground squirrels don’t ever wake up.
This clip was taken from Alaska: Earth’s Frozen Kingdom, first broadcast in the UK on BBC Two in 2015.
You can follow BBC Earth on Facebook, Twitter and Instagram.