CHAPTER VI 

 FACTORS INFLUENCING RATE OF METABOLISM 



Metabolism is the chief source of the heat which main- 

 tains the constant temperature of homoiotherms. An 

 animal may promote the retention of heat within its body 

 by insulation or the transfer of body fluids and it may, 

 if febrile, lose heat by surface conduction, radiation, or 

 evaporation. The chief source of the energy which con- 

 tinually supplies animal heat is the oxidation of carbo- 

 hydrates, fats, and proteins. If the surroundings of a 

 homoiotherm become colder, its body usually responds 

 by a greater degree of internal chemical change and 

 energy is thus supplied to keep the body temperature 

 constant. 



The quality of metabolism is quite similar in poikilo- 

 therms and homoiotherms, but the quantity is strikingly 

 different. The daily energy requirement of a fish is only 

 4 per cent of that of a mammal of the same weight. The 

 life intensity, or rate of living, of a rabbit is 25 times 

 that of a pike (Rubner, 1924). Because a homoiotherm 

 lives at a more rapid rate, it must continually have more 

 fuel for its metabolic processes and this is largely sup- 

 plied in its food. A small animal requires porportionally 

 more energy than a large animal of the same type be- 

 cause its surface permits more rapid dissipation of 

 energy. On account of the comparatively slow rate of 

 metabolism in poikilotherms such mass-surface relations 



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