FACTORS INFLUENCING RATE OF METABOLISM 43 



present in the respiratory center itself. In primitive 

 protoplasm respiration is the direct result of and condi- 

 tional for metabolism, but in more specialized animals 

 there is much circumlocution in control. Individual 

 idiosyncrasy, racial pecularity and physiological, en- 

 vironmental, psychological, and other factors may mod- 

 ify its rate or method. 



Food. — Food supplies the fuel for metabolism. At low 

 temperatures poikilotherms require little or no food, but 

 homoiotherms under the same circumstances need an 

 increased supply. However, an isolated heart of a poi- 

 kilotherm requires two or three times as much sugar at 

 low as at high temperatures. In a normal animal the 

 sugar in the blood varies in accordance with such needs 

 (Mansfield and Pap, 1920). This indicates that the 

 tissue cells of poikilotherms, as well as those of homoio- 

 therms, tend to oxidize more food at lower temperatures, 

 but in the former the rate of metabolism is so much 

 slower at lower temperatures that there is an actual 

 decrease. Another characteristic of metabolic processes 

 which intensifies this effect is the general tendency of 

 animals to store fat during warm seasons and utilize this 

 for metabolism during cooler parts of the year (Fage 

 and Legendre, 1914; Pearse, 1924). 



The amount of food required to carry on a particular 

 amount of metabolism at a given temperature does not 

 appear to differ markedly in poikilotherms and homoio- 

 therms. The protoplasms in the two types of animals 

 have similar needs. Page (1895) determined that a 

 trout could subsist on food equal to about one per cent 

 of its own body weight daily. Centrarchid and silurid 

 fishes apparently need about six per cent (Pearse, 1924). 

 The former eat about three times as much at 20 deg. C. 

 as at 10 deg. C, and if kept at 10 deg. C. for several 



