360 



Comparative Animal Physiology 



the period of warming-up but are constricted once an optimal temperature is 

 reached."'-' Desert animals restrict their foraging to those periods of the day 

 when the temperature is neither too cold nor too hot. When the body tempera- 

 ture of the desert iguana, for example, approaches 43° it goes into the shade 

 of bushes or underground. The horned toad (Phrynosoma) shows discomfort 

 at 40° •'' and burrows at temperatures above 40° or below 20°.^^*^ An important 

 factor in limiting survival of reptiles at temperatures above 45° is the decreased 

 affinity of their hemoglobin for oxygen (Ch. 9, p. 312). At 50° the blood of 

 a chuckwalla (Sauromalus) could not become more than 50 per cent saturated 

 with oxygen at atmospheric tension,^'* whereas the tension for 50 per cent 

 saturation of the blood of a gila monster (Heloderma) rises from an O^ tension 

 of 32 mm. Hg at 20° to 60 mm. Hg at 37.5 °.-*-* 



Little is known about thermal sense in reptiles, but there appear to be 

 epidermal receptors, stimulation of which causes snakes to approach warm 

 objects. The pit vipers (Crotalidae) have facial sensory pits and some boas 

 have labial pits, both of which are temperature receptors. These snakes strike 



.. 10- 



- 6- 



4- 



Z Z 



ex. 







^ Species 



Fig. 93. Comparison of change in body temperature of several species of amphibians 

 and reptiles when the relative humidity was lowered from 100 per cent to 7 per cent 

 saturation at 20°. From Hall and Root.™ 



at a warm object and distinguish slightly warm from air-temperature objects 

 only if the pits are intact and even when other sensory structures in the head 

 are inactivated.^-'*^" 



Poikilotherms lack efficient mechanisms of heat retention; hence in low 

 environmental temperatures their metabolic activity is limited, and as the 

 temperature rises heat distribution from the body surface depends largely on 

 an active circulatory system. It has been suggested that the giant reptiles were 

 adapted nietabolically to high temperatures and that their circulatory systems 

 were sluggish at the low temperatures of the Pleistocene, so that they could not 

 make suflicient use of radiant heat. Smaller reptiles would be less dependent 

 on their circulation, and their temperature would have less of a lag behind 

 that of the environment. 



