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How Ground Squirrels Lose Weight

June 04, 2002

Spring. First flowers sprout from the ground, and animals wake up from hibernation (dormancy) and come out from their burrows. If not for the hibernation, small rodents would starve or freeze to death in winter. However, the winter slumber is not a kind of rest in a sanatorium. Animals become emaciated during winter, and all they think about in spring is food. How much weight they lose, what controls these losses, which conditions are optimal for dormant ground rodents - these problems are considered by A.I. Anufriev and his colleagues from the Institute of Biological Problems of the Cryolithozone SD RAS (Yakutsk). It is established that ground squirrels are best "preserved" at zero degrees Celsius.

        The winter slumber is typical for almost all small animals. They need much food, which is hard to store in a sufficient quantity to maintain a high temperature of the body within a whole winter. Hence, the animals fatten in summer and hibernate in winter. The Yakutian scientists studied the process of hibernation of long-tailed Siberian ground squirrels (sousliks). The Yakutian climate is harsh: the absolute value of the annual amplitude of air temperature exceeds 100 degrees, a mean daily temperature in January in the environs of Yakutsk varies from -410C to -430C, the minimal temperature is -670C, and the duration of the period with positive temperatures does not exceed 100 days. Within such a short period, a ground squirrel should fatten to increase its weight by 30-40%. In early October, snow covers the ground, and the animals retreat into their winter beds. However, their hibernation is not a steady slumber, but a sequence of alternating inactive and active periods. The intervals of "sleep" are short at the beginning and reach 12-14 days in December and January. Then the frequency of waking increases up to a complete restoration of activity in spring. Ground squirrels, as distinct from hamsters and chipmunks, do not store food and do not eat between "naps"; that is why they become so thin by the spring time.

        The observations of ground squirrels were performed, of course, not in their natural burrows. A few squirrels were trapped in late September and placed into a cellar at the beginning of the hibernation season. In conditions of the cellar lightened for only a couple of hours a day, ground squirrels fell asleep soon. For the whole winter, the scientists weighted them in groups monthly and individually for 20 times per day and took more than 1500 measurements in total. These procedures made no harm to the animals and did not disturb their slumber. As a result, the researchers discovered the following.

        The weight loss rate is highest at the beginning of the hibernation season, when the periods of activity between sleeping are rather long. The heavier is the animal, the more weight it loses. Adults 800-gram-heavy lost in average 32% of the initial weight within 150 days, whereas thinner young squirrels lost only 18% within that period, despite all animals stayed in same conditions.

        The microclimate in the winter chamber of a ground squirrel is relatively stable, in comparison with conditions on the surface. The chamber is located at a depth from 80 to 120 cm, and temperature fluctuations inside it are small: from plus 6-80C in autumn to minus 6-80C in late March and early April, i.e., at the time of activity restoration. At positive temperatures, the weight loss is most rapid, 75 mg per hour in average. As temperature decreases to 0-20C, the rate of fat burning becomes minimal - about 5 mg per hour. The further drop of temperature makes it necessary for the organism to spent more calories and lose weight at a rate of 35 mg per hour. A ground squirrel burns fat for energy very economically, as it loses a quite insignificant amount of water with evaporation. To gain weight again, this animal has to make a strong effort. Ground squirrels do not bother about their figures, as different from many people. So, may they have a good dinner.

Informnauka (Informscience) Agency