ADAPTATION 353 



minute under conditions of normal activity. The heart beat dur- 

 ing hibernation averages 17.4 beats per minute as compared with 

 200 to 300 per minute under normal conditions. The body 

 temperature, which under normal conditions varies from 32 to 

 41°C, during hibernation may drop as low as 2°C. The loss of 

 body weight may total 40 per cent; it occurs principally in 

 stored fat. 



When the hibernating animal awakes, the resumption of nor- 

 mal activities is gradual, but the rise in body temperature is 

 abrupt. Observations on the European dormouse record a rise 

 from 13.5 to 35.7°C. in 1 hour and in the case of a bat, a rise from 

 17 to 34°C. in 15 minutes. 



Among warm-blooded animals hibernation occurs only in some 

 mammals and never in birds. It is relatively common in amphib- 

 ians and reptiles and also occurs in some fishes (lungfishes). 

 The lack of a delicately adjusted temperature-regulating mecha- 

 nism in the lower vertebrates fits them for the hibernating habit ; 

 in fact, if a frog is chilled in cold water, it takes on the character- 

 istics of a hibernating animal. In birds and mammals, the 

 normal range of fluctuation of body temperature is rather 

 limited, particularly in forms that do not hibernate. In man a 

 few degrees' rise in body temperature produces a fever condition 

 with deleterious results, while a subnormal temperature beyond 

 a very limited range interferes with metabolism. Hibernating 

 mammals, on the other hand, have a temperature-regulating 

 mechanism that maintains a fairly constant value under condi- 

 tions of normal activity but which ceases to function when the 

 animal hibernates. In such animals the fluctuation of body 

 temperature under nonhibernating conditions, 32 to 41°C, is 

 much greater than in the case of man. The hibernating habit of 

 certain mammals is possible because of the absence of a rigid 

 temperature-regulating mechanism which in turn may be a sur- 

 vival from cold-blooded ancestors. 



Light. — Animals are sensitive to light and react to it by mov- 

 ing toward or away from it, or by developing anatomical features 

 of an adaptive significance. A light-sensitive organ, such as the 

 red eyespot of the protozoan, Euglena viridissima, by increasing 

 the light absorption makes the region of the body in which it is 

 located more sensitive to light than other regions. The various 

 types of eyes found in different groups of animals, climaxed by 



