RESPONSES OF HOMOIOTHERMS TO VARIATIONS 35 



hibernate. Man enjoys more advantages from his homes 

 than any other animal and of course provides special 

 means to prevent extreme changes in temperature. 



Clothing. — Man alone among animals has become in- 

 dependent of temperature fluctuations by inventing 

 clothes. In cool countries he endeavors to protect him- 

 self by keeping his skin covered. Only about 20 per cent 

 of his skin is normally exposed to air. He varies the 

 amount of surface exposed and the thickness of this arti- 

 ficial covering with the climatic changes. The air in and 

 within clothes is a much poorer conductor of heat than 

 the fibers from which fabrics are made and furnishes 

 excellent insulation. This is especially true of that in the 

 furs which man finds ready-made on the bodies of other 

 mammals. The greater the thickness of clothing, the 

 thicker is the layer of contained air and the greater is 

 the prevention of heat loss from the body. Moisture ab- 

 sorbed by the clothes and filling up the air spaces de- 

 creases the effectiveness of the clothes. Savage races 

 living in Tierra del Fuego in a cold moist climate usually 

 do not wear clothes but a layer of oil over their bodies.. 



Temperature Regulation. — The mechanism of tem- 

 perature regulation is not fully developed in many 

 species of mammals until after birth. Some mammals, 

 however, have the ability to maintain a very constant 

 temperature from the first. For example this is true of 

 the guinea pig, which has a well developed nervous sys- 

 tem at birth. Other species, like the rat and the pigeon 

 which are born helpless and require parental care for 

 some time, do not come into possession of the power to 

 regulate their loss and gain of heat for many days after 

 birth. A newborn child is not able to control its bodily 

 temperature well for several weeks. It is probable that 

 this post-embryonic development of the regulatory mech- 



