Physiologic studies of the groundhog (Marmota monax).
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An investigation of hibernation was carried out with groundhogs in order to study their unusual tolerance to very low body temperatures. The normal carbon dioxide content of the blood of groundhogs, even while they are in the nonhibernating phase, is higher than that of dogs and humans. In groundhogs, the transition from the nonhibernating to the hibernating phase involves a fall in body temperature to as low as 3° C. and appears to be associated with (1) an increase in the oxygen content and the carbon dioxide content of whole blood, (2) an elevation of the serum magnesium and serum potassium, (3) an increase in hematocrit values, and (4) a decrease in the blood sugar level. Direct microscopic examination of the peripheral circulation in vivo showed a free-flowing stream with no intravascular agglutination of red blood cells or vascular stasis in groundhogs, dogs, and rabbits at a body temperature of 20° C. or in groundhogs during hibernation at temperatures as low as 5° C.