Thermal Migrations 149 



habits and thus avoid the heat of the day. This is true even of char- 

 acteristic desert reptiles such as the rattlesnake. Ecologists w^orking 

 in the desert areas of southern California have found that rattlesnakes 

 will succumb if forced to remain for more than 15 minutes on the hot 

 desert soil exposed to the midday sun. The red racer is one of the 

 few desert reptiles that venture forth regularly in the daytime. Since 

 this species is one of the fleetest of the snakes, it is able to cross hot 

 areas and get back into the shade before becoming harmfully heated. 



In less severe environments we are familiar with the short journeys 

 of terrestrial amphibians into shaded places in hot weather. How- 

 ever, the chief benefit from a movement out of the sun for salamanders 

 and other forms with wet, permeable skins may be conserving water 

 rather than keeping cool. The excessive evaporation experienced by 

 an amphibian in an exposed, windy location would cause a chilling of 

 the body, and, in addition to avoiding water loss, the animal might go 

 into a sheltered place to keep warm. 



The short trips into or out of the water made by frogs, turtles, and 

 other amphibious forms are familiar to everyone and serve to provide 

 cooling or warming for the animals concerned. An unusual tempera- 

 ture relation is displayed by the white pelicans that nest on islands of 

 the Salton Sea, California. In this extremely hot location the eggs 

 and young in the nest would be killed if exposed for more than about 

 20 minutes to the intense radiation from the sun. The brooding of 

 the mother birds acts to keep the eggs and young cool rather than to 

 keep them warm! At intervals the adult pelicans wet their plumage 

 by a trip into the water and take advantage of the cooling produced 

 by evaporation. 



Burrowing animals escape excessive heat or cold by short journeys 

 deeper into their substrata. Soil organisms avoid summer heat by 

 moving deeper into the earth. Ground squirrels are included in the 

 desert fauna of southwestern United States but they are rarely exposed 

 to the extremes of heat in that area. In the hot season these rodents 

 retire into their burrows where a microclimate of much more mod- 

 erate temperatures prevails (Table 9). At a later season the same 



TABLE 9 



Reduction in Temperature at Increasing Depths beneath the Surface 

 OF THE Desert at Tucson, Arizona 



Air (maximum) 42.5°C 



Surface (maximum) 71.5 



10 cm below surface 41 . 1 



30 cm below surface 29 . 8 



45 cm below surface 27 . 9 



