SCHMIDT 



certain components of the normal bacterial and viral flora and on the 

 retention of specific viral agents in the intestinal tract of ground 

 squirrels. 



Since hibernation is such an important feature of these investi- 

 gations, a few words aboutsomeof its salient characteristics are in 

 order. First of all, it is important to recognize that hibernation is a 

 natural phenomenon for certain animals and that it is quite different 

 from experimental hypothermia. Hibernation is a physiologically 

 controlled and regulated process, while experimental hypothermia is 

 a result of weakened or overcome normal mechanisms of tempera- 

 ture regulation. Chilling occurs in enforced hypothermia in spite of 

 a concerted effort on the part of the animal to maintain its normal 

 temperature. On the contrary, hibernation is a passive, yet deliber- 

 ate process in which declines in metabolic rate, respiratory rate, 

 and heart rate precede the drop in body temperature. 



Within a limited range, the body temperature of the hibernating 

 ground squirrel parallels that of the environment, usually remaining 

 0,5° C to 3° C above the ambient temperature (Johnson, 1931), Mayer 

 (1960) reported an average rectal temperature of 4.2*^ C for ground 

 squirrels hibernating in an environmental temperature of 1.6° C, 

 The heart rate and respiratory rate are slowed to about 3 per min, 

 and the metabolic rate is between 1/30 and l/lOO of that found in the 

 active "resting" animal. Vascular changes include a decrease in 

 blood pressure as the animal enters hibernation followed by vaso- 

 constriction once the torpid state has been achieved (Lyman and 

 O'Brien, 1960), There appears to be a pronounced leucopenia during 

 hibernation (Svihla, 1953) which has notbeen explained. Although the 

 rate is very slow. Brock (1960) reports that there is actually some 

 manufacture of red blood cells during deep hibernation. 



There are indications that some of the endocrines play a role in 

 hibernation, although the specific data are conflicting. Most workers 

 agree that an involution of the endocrines takes place before the 

 animal enters hibernation. Indeed, Kayser (1955) insists that hiber- 

 nation will not take place without this involution. 



Although hibernation is partof a yearly cycle of events for ground 

 squirrels, it is not clearly understood what specific factors are im- 



400 



