DOMESTIC MAMMAL ADAPTATIONS 



cooling but also to hide blushing and even to hide non-blushing. 

 Steers do not worry about blushing but they get relief against 

 overheating from fans, as shown on Figure 12, which was con- 

 structed from a table giving observations by Ittner, Bond, and 

 Kelly (1955), in the Imperial Valley of California. The fan could 

 keep the body temperature one degree lower than it would have 

 been without the fan. This type of relief, of course, is given the 

 animal by his keeper. The animal itself, however, also has methods 

 for preventing overheating. The first reaction of an animal ex- 

 posed to a high environmental temperature is to increase the 

 blood flow to the skin, which increases the heat flux from skin 

 to environment. This type of physical temperature regulation 

 is effective only when the skin temperature is higher than the 

 environmental temperature. When the air temperature and the 

 temperature of the objects toward which an animal radiates 

 are equal to, or higher than body temperature, more drastic 

 means of cooling have to be taken. The last resort is water 

 evaporation. Men and horses perspire. Dogs and cattle have 

 discovered a flaw in this method. When the surface is wet and 

 evaporative cooling takes place, there is not only the welcome 

 temperature difference for the flux of heat from the interior to 

 the surface , but also an increase in the flux from the hot environ- 

 ment to the animal surface. The animal therefore spends water 

 to cool its environment. To overcome that disadvantage, cattle 

 and dogs operate an internal evaporative cooler which leaves 

 the surface temperature higher and keeps the heat influx lower. 

 Dogs and cattle increase the evaporative cooling by panting. 

 Increased respiratory ventilation, however, involves the danger 

 of depleting the blood of CO , a condition known as acapnia which 

 causes unpleasant disturbances in the operation of the breathing 

 reflexes. The answer to this danger is shallow breathing, in- 

 creasing the ventilation rate in the upper parts of the respira- 

 tory system only, this provides the desirable increase in evapora- 

 tion, with little change of the CO washout from the alveoli. 



Figure 13 shows the type of breathing in heat exposed cows 

 compared with that at a low (for the cow's taste comfortable) 

 temperature. A threefold increase in respiratory frequency pro- 

 duces a twofold increase in ventilation rate because the volume 

 for each breath is reduced. Cows do not start panting at a certain 



261 



