KLEIBEE 



SUMMARY 



Domestication has not essentially changed the basic responses 

 of animals to challenges from cold or hot environments. 



A reaction to cold exposure, common to man and other homeo- 

 therms, is an increase in metabolic rate called chemical tempera- 

 ture regulation. A more economical response, known as insulative 

 cooling, has been lost by civilized man, but operates in domestic 

 animals and ^^ustralian aborigines. 



In order to adapt to continued cold exposure, animals increase 

 their insulation mainly by growing a thicker fur. Man has replaced 

 this adaptation by technical control of the microclimate. 



Overheating is prevented mainly by evaporative cooling at the 

 body surface or in the respiratory system. Contrary to older be- 

 lief, cattle evaporate more water from the skin than by respira- 

 tion, even though respiratory frequency increases consistently 

 with increase in environmental temperature. 



Evaporation from a wet body surface in a hot environment 

 is uneconomical because it allows an influx of heat from the 

 environment to the surface. 



The camel's fur maintains its outer surface dry and hot, 

 minimizing the influx of heat to the skin which is kept cool by 

 evaporation. Clipping of the fur increases the camel's water 

 loss in a hot environment to a rate per unit area similar to 

 that of a donkey. 



Excessive but time- limited daily heat loads producing heat 

 storage in the body can be endured better the larger the animal 

 because heat load is proportional to body surface area and heat 

 capacity is proportional to body weight. Increase in body tempera- 

 ture for given loads, therefore, is proportional to the reciprocal 

 of the cube root of body weight. 



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