Myoglobin content in skeletal muscles of hibernating ground squirrels rises in autumn and winter. 
The content of myoglobin (Mb) in skeletal muscles of Arctic Yakutian ground squirrel (Citellus undulatus Pallas) was measured in the active euthermic summer and prehibernating autumn animals as well as in hibernating and awake animals in winter.  The myoglobin content in winter, irrespective of the state of the animal, was found to be about three times higher than in summer.  The content of myoglobin in autumn was also two-fold increased compared to summer, suggesting that high myoglobin level is necessary for hibernation.  Analysis of biochemical data available suggests that the increase in myoglobin content in winter is probably related to a high oxygen demand of muscles at the first stage of arousal (non-shivering thermogenesis) when rectal temperature rises from 0 to 10-12 degrees C. At this stage, the oxygen-dependent processes in muscles proceed under the conditions when peripheral blood flow is blocked and anaerobic glycolysis is switched off.