Temperature: Metabolic Aspects and Perception 



367 



the organism. Under basal conditions man loses about 20 per cent of his heat 

 by vaporization, under exercise the loss is 75-80 per cent.^" Sweating begins 

 in man at an average skin temperature of 34-35° C.^^ The loss by radiation 

 remains relatively constant, its per cent contribution to the total loss is less in 

 exercise. Loss of heat by radiation and conduction is prevented by insulation. 

 Aquatic mammals, many of which are hairless, have an insulating layer of 

 fat. Heat loss by terrestrial mammals— man, dog, and others— in water is twice 

 as great as it is in air.^-^ In air-dwellers the fur coat and feather cover con- 

 serve heat and can be erected to encase an insulating layer of air. Permanent 

 bird residents of cold areas have a thicker feather cover in winter than do 

 migrants to warm climates, and mammals have thinner fur coats in summer 



TEMP, c: 



22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 

 CAL. TEMP. C: 



Fig. 96. Relation of rectal temperatures (2 subjects), skin temperature, and per cent 

 of heat loss by radiation, vaporization, and convection in man, to environmental (cal- 

 orimeter, cal.) temperature. From Hardy and DuBois." 



than in winter. Not only is the fur thicker, but it is better insulation in arctic 

 mammals (Fig. 97). A guinea pig loses 33 per cent more heat if it is shorn,^^® 

 and at 20-24° C. heat loss from a shaved cat was 16 per cent above heat pro- 

 duction, indicating that fur-bearing mammals rely on the fur, in contrast to 

 man, who alters heat conductance of the skin.^^^ When sleeping, many mam- 

 mals coil up, and birds bury the head under a wing and fluff out their feathers. 

 When the body temperature rises above the "optimum" at a given humidity 

 and air temperature, heat loss by vaporization is increased. ^^ Sweat glands 

 are stimulated, and in animals lacking sweat glands and in birds panting 

 appears. Panting begins in dogs at rectal temperatures a few tenths of a degree 

 above the basal rectal temperature.'^^ Breathing and heart rate are accelerated. 

 Man in cool air may lose by evaporation 1 liter of water per day; in hard work 



