Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Video: Squirrel begins to wake up

As cold as ice (Image: Nick Norman/NGS/Getty)

Species: Spermophilus parryii

Habitat: The frozen North American tundra, in labyrinthine burrows

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.

How low can you go?

To determine the limit of the squirrels’ supercooled sleep, Melanie Richter from Barnes’s lab slowly decreased the temperature inside a freezing wind tunnel with a hibernating squirrel in it. As the mercury dropped, the squirrel stayed asleep, keeping its body temperature around -4 °C until the room temperature hit -26 °C.

At this temperature, the squirrel started to shiver and wake up (see video above), and its body temperature rose, suggesting that this is the lowest temperature at which it could survive while asleep.

Richter presented her work at the Society for Integrative and Comparative Biology meeting in San Francisco earlier this month.

These extreme temperatures aren’t the only thing that makes hibernators want to stay in bed. The constant light of the Arctic summer and constant darkness of the winter make it impossible to tune circadian clocks to any environmental cues. As a result, many Arctic species such as ptarmigans and reindeer lack circadian rhythms at all.

Hit the snooze

To test whether squirrels have such clocks, the group gave their tagged animals light-sensitive collars, allowing the researchers to know when the squirrels were in and out of their burrows.

Male squirrels woke up about three weeks before females. During this time, they stayed in their burrows eating food they had previously stored. In this constant darkness, their body temperatures remained constant, indicating that their circadian clocks were still off.

But as soon as a squirrel emerged in the spring, its body temperature began rising during the day and falling at night.

Throughout the constantly light summer, the squirrels seemed to create their own sunrise and sunset by popping out of their burrows every day and retreating at night. This tricked their body clocks into responding.

It’s not clear how they tell night from day, Barnes says, unless they’re responding to some cue such as the height of the sun in the sky or how red the light is. The team is now trying to figure out what genes in the squirrels’ brains are turning their rhythms on and off.