Arctic Ground Squirrel
NPS Photo / Alex Vanderstuyf
Arctic ground squirrels are the largest and most northern of the North American ground squirrels. This species is common in the ice-free mountainous regions of Denali. Permafrost and soil type are two of the most important factors limiting ground squirrel distribution in Denali.
Arctic ground squirrels are burrowing animals and they establish colonies in areas with well-drained soils and views of the surrounding landscape. Colonies often consist of multiple burrows and a maze of tunnels beneath the surface. Well-drained soils are important, as flooding of these burrows causes considerable problems for squirrels. Accordingly, squirrels usually avoid establishing colonies or excavating burrows where permafrost is close to the surface.
Adapting to the Arctic
Like many other arctic animals, arctic ground squirrels have unique physiological adaptations that allow them to survive during winter. Arctic ground squirrels are obligate hibernators and spend 7 to 8 months in hibernation. Researchers at the University of Alaska at Fairbanks have shown that during hibernation, arctic ground squirrels adopt the lowest body temperature ever measured in a mammal. The body temperature of hibernating squirrels drops below freezing, a condition referred to as supercooling. At intervals of two to three weeks, still in a state of sleep, hibernating squirrels shiver and shake for 12 to 15 hours to create heat that warms them back to a normal body temperature of about 98 degrees Fahrenheit. When the shivering and shaking stops, body temperature drops back to the minimal temperature. This type of hibernation is rare among mammals and scientists are still studying this unique physiological behavior.
In Denali, ground squirrels are active from late April to early October, but the sexes and age-classes show some differences in their annual activity patterns. Adult males are usually the first to emerge from hibernation. They dig their way through the snow and stay relatively close to their burrows until the snow cover melts. Breeding occurs in May and a single litter of 5 to 10 pups is born in June. The young develop rapidly and usually emerge from their burrows in mid-July. By late summer, young abandon their natal burrow and occupy a neighboring, empty burrow or excavate a new one.
Adults start hibernating as soon as they have enough body fat to survive the winter, often in late August when plenty of foods are still available. It is probably safer to enter hibernation early, even when foods are accessible, than to remain on the surface vulnerable to predators. Youngsters, however, take much longer to find foods and put on body fat and they are often active until late September. This means that youngsters are more vulnerable to predation than adults.
Diet and Social Behavior
The diet of arctic ground squirrels is diverse and opportunistic. They eat many types of vegetation including the leaves, seeds, fruits, stems, flowers, and roots of many species of grasses, forbs, and woody plants. They also eat mushrooms and meat from freshly killed animals (including ground squirrels). Because they are active only during the short subarctic summer, arctic ground squirrels must be efficient foragers. As summer progresses, they put on a tremendous amount of fat stores for the winter and often double their body weight by the time they enter hibernation in fall.
The social behavior of arctic ground squirrels is complex. This species is highly territorial and squirrels may kill other squirrels over territorial disputes. However, other related females in the colony often care for orphaned youngsters. Further, territorial behavior lessens during late summer, and male squirrels may move between colonies or establish colonies of their own.
So many different predators eat arctic ground squirrels that Adolph Murie called them the "staff of life" in Denali. They are one of the most important summer food sources for golden eagles, gyrfalcons, foxes, and grizzly bears.