148 Temperature 



ground squirrel hibernates in burrows made in local areas of unfrozen 

 ground under deep snow. Other small mammals remain active in 

 lined nests made in the snow itself. Measurements at Fairbanks, 

 Alaska, showed that at less than 1 m beneath the snow surface a micro- 

 climate temperature of — 5°C existed at a time when a value of 

 — 50°C was recorded in the air just above the snow. 



Less is known about the physiological adjustments of aestivation 

 in mammals, but this type of dormancy similarly involves a shift of 

 metabolism into low gear and a complete change in the control of 

 internal temperature (Hamilton, 1939). An example of mammalian 

 aestivation is found in the summer dormancy of the ground squirrels 

 of southern California. By remaining torpid in their burrows for 

 several months these animals not only avoid the high summer tem- 

 peratures but also tide over periods of water and food shortage. 



A special device for dealing with extremes of temperature tvJiile in 

 the active condition is homoiothermy, or warm-bloodedness. Birds 

 and mammals are able to maintain a remarkably constant internal 

 temperature despite great variations in the outside world. Limits 

 exist, of course, beyond which these animals cannot maintain their 

 temperature control, but by allowing a regulated amount of evapora- 

 tion to take place from their bodies they can keep their own tempera- 

 tures down to normal values under the highest environmental tem- 

 peratures ordinarily encountered. Under extremes of cold weather 

 birds and mammals are able to maintain their relatively high internal 

 temperature through the insulating action of fur, feathers, and fat, as 

 well as by suitable physiological adjustments ( Scholander et al., 1953 ) . 

 The remarkable ability of mammals to maintain their normal thermal 

 level under severe winter conditions is exemplified by a varying hare 

 whose internal temperature was found to be at its normal value of 

 38°C on a day when the thermometer stood at — 46°C. 



Thermal Migrations. Another method for dealing with excessively 

 high or low temperature conditions is to move out of them. This 

 method is obviously available only for locomotory forms, and it cannot 

 be used by the majority of plants. Journeys taken by animals that 

 enable them to escape from extremely hot or cold situations are referred 

 to as thermal migrations. Some of these migrations are relatively 

 short trips involving movements of only a few meters or even a few 

 centimeters. Thermal migrations are made on a small scale from ex- 

 posed positions to the shade to avoid the scorching heat of the desert 

 and from shade to sun in cold regions. We commonly think of desert 

 animals as being able to withstand extremely high temperatures. As 

 a matter of fact most desert animals have become nocturnal in their 



