Hibernation

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In early autumn, the 13-lined ground squirrels regularly start disappearing from the Midwestern farm landscape for half a year.

The active little animals of summer begin hibernating, curled up in their underground burrows below the frost line. There they can remain, without eating or drinking, until spring.

On sunny summer days, farm children frequently saw the ground squirrels digging burrows in the pasture, or running along narrow trails in the grass. Sometimes they heard the ground squirrels' high-pitched whistles or watched as the animals nibbled seeds or stood on their hind legs for a better view of the area.


    Thirteen-lined Ground Squirrel

    (Spermophilus tridecemlineatus)

    The distinctive 13-lined pattern of the ground squirrel's coat helps conceal it from predators and gives the animal the last part of its scientific name, tridecemlineatus. The alternating lines of dark brown and tan stripes, with light dots, provide camouflage from predators, as the small animal moves through the grass.

 Seeds are a large part of the ground squirrel's diet. The first part of its name, Spermophilus, means seed lover. However, these ground squirrels are considered omnivorous, eating both plant and animal material. They commonly eat seeds, grasses, roots and insects.

The animal is about 6 inches long, with a thin tail adding another three inches or so to its total length. The ground squirrel's weight changes throughout the year, as it stores, then uses body fat reserves.

Its small feet have long nails that are very good for digging. The animal is active in spring and summer daylight hours, but also spends a great deal of time in its burrows. They are not chipmunks or tree squirrels.

The 13-lined ground squirrels dig three kinds of burrows in the soil: shorter hiding burrows, temporary nesting burrows and longer, deeper hibernating burrows. These solitary animals may live in an area where there are informal colonies of ground squirrels.

There can be from two to 12 ground squirrels born in a litter within a burrow. By 6 weeks of age, the young ground squirrels are independent and finding food to eat outside, to build up their own body fat for winter.

Ground squirrels often close their burrow openings with a plug of earth. Loose soil at the burrow entrance, left over from digging, is spread out and patted down by the little animals. The burrow opening is generally 2 to 2 1/2 inches across.

These small mammals bring nesting materials into their burrows and store seeds there to eat after hibernation. The animal's body fat, built up through its active months, is gradually used during the months of inactivity.

The ground squirrel's hibernation in its underground burrow is not sleep in the usual sense, but an inactive state called torpor. During this time, the animal's metabolism slows dramatically and its body temperature lowers to just above freezing.

From time to time during hibernation, the ground squirrel arouses and its body temperature increases, re-warming itself before the animal once again returns to its inactive state. Much of the ground squirrel's stored fat reserves are used by these periodic changes. The processes that take place within the animal during this time and the reasons for them have been sources of scientific speculation.

The mysteries of 13-lined ground squirrels' activities and hibernation continue to be subjects of research by scientists, as they study how these little animals so successfully spend their lives in such a unique way.