Mammals
Arctic Ground Squirrel
Urocitellus parryii · also called parka squirrel, siksik
The Arctic ground squirrel spends most of the year frozen into the tundra, or nearly. For weeks at a stretch its body core sits below the freezing point of water, colder than any other mammal is known to go, and its blood somehow stays liquid.
Overview
Urocitellus parryii lives in the tundra and subarctic meadows of northeastern Siberia, Alaska, and northwestern Canada, in burrows dug into well-drained ground above the permafrost. It is a herbivore that feeds heavily through the short summer to lay down fat.
Below freezing
In hibernation it lets its core temperature fall to about minus 3 degrees Celsius — the lowest recorded for any mammal — entering a supercooled state where the body fluids stay liquid below their normal freezing point. Its winter lasts seven to eight months, cycling between weeks of deep cold torpor and brief, costly rewarming bouts driven by shivering.
Feeding the tundra
It is a staple prey of grizzly bears, which dig out whole burrow systems, as well as foxes, eagles, gyrfalcons, and snowy owls, so its numbers move through the whole food web.
Status
Least Concern and widespread, though warming and changing snow cover are altering its hibernation conditions in parts of the range.
Arctic Ground Squirrel: common questions
How can an animal survive its body temperature dropping below freezing?
By supercooling rather than freezing. Water can stay liquid below zero degrees Celsius if it is very pure and undisturbed, with nothing present to start ice crystals forming. During hibernation the Arctic ground squirrel appears to clear its blood and tissues of the particles and proteins that would normally seed ice, so its body fluids cool to around minus 3 degrees and remain liquid. If ice did form inside its cells it would be fatal, as it is for most animals. The squirrel holds this state for weeks at a time, its heart beating only a few times a minute, then warms back up. Exactly how it avoids ice formation is still being studied, and it is of interest to medicine for preserving organs.
How long does an Arctic ground squirrel hibernate?
In the coldest parts of its range, roughly seven to eight months, from about September or October until April or May. That is among the longest hibernation of any mammal. The hibernation is not one continuous sleep. The squirrel cycles between long bouts of deep cold torpor, lasting two to three weeks, and short arousals of a day or less during which it shivers its temperature back up to normal before cooling down again. Those arousals use a large share of the animal's total winter energy, which is why the deep cold in between, with its very low metabolism, is thought to be an energy-saving adaptation.
What eats Arctic ground squirrels?
Almost every tundra predator. Grizzly bears in particular spend a lot of late summer and autumn digging out ground squirrel burrows, and in some areas the squirrels are a major part of a bear's diet before its own hibernation. Arctic and red foxes, wolves, wolverines, golden eagles, gyrfalcons, rough-legged hawks, and snowy owls all hunt them, and larger ground squirrels will kill and eat smaller ones. Because so many predators depend on them, a good or bad ground squirrel year is felt across the tundra, affecting fox litter sizes and raptor breeding success.
Is it the same as a prairie dog or a marmot?
They are all ground-dwelling members of the squirrel family and all dig burrow systems, but they are different animals. Marmots are much larger and heavier, the size of a house cat, and include the groundhog and the alpine marmot. Prairie dogs are stocky, highly social burrowers of the North American grasslands with complex colonies. The Arctic ground squirrel is a true ground squirrel, slimmer and smaller than a marmot, living in loose colonies in the Arctic and alpine north, and it is the largest and most northerly of the North American ground squirrels.