368 



Comparative Animal Physiology 



on the desert he may evaporate 1.5 1. per hour.^-^ Output of sweat increases 

 20 gm./hr. for each 1 ° F. rise in air temperature. Man can withstand very high 

 air temperatures if allowed plenty of water for perspiration.^ In secreting 

 sweat man loses much salt, particularly sodium chloride, and in acclimatiza- 

 tion to high temperatures the chloride concentration in sweat decreases while 

 that in urine increases. In warm moist air, as in the jungle, evaporation is 

 much less than in the desert. A burro working on the desert does not pant 

 and it secretes a sweat which contains little chloride. A panting dog, on the 



inbULflTION 



'^(h 



CJ tOTTon 



JO 



OALL SHEEP 



WOLr 

 &RIZ2LY BE.AR 



POLMR BEAR. 



10 IF 



Fig. 97. Maximum insulation of fur of arctic and tropical mammals as a function of 

 fur thickness. Each dot represents one measurement. Values for aquatic animals are 

 given in air (dots) and in ice water (open circles): values for terrestrial animals are given 

 for air only. The insulating value of medium weight cotton is given by the broken line. 

 From Scholander (unpublished). 



other hand, evaporates much water from its respiratory tract, and this may 

 result in increased chloride concentration in the blood and in the development 

 of some alkalosis due to the blowing off of CO^."'^ In birds, vaporization 

 occurring as air passes through the air sacs may have a cooling effect. 



When the body temperature is cooled below "optimum," shivering may 

 begin. 1 his is associated with or follows vasoconstriction and piloerection. 

 Shivering increases oxygen consumption, hence heat production. The abilities 

 to panl and to shiver develop in the young animal during the period when 

 temperature regulation appears."'"- '^- When the "thermostat" becomes set 



