44 HOMOIOTHERMISM 



weeks gradually eat less (Hathaway, 1927). A young 

 smelt in May needs daily 1 mg. of food ; in July, 2 mg. ; 

 and in September, 4 mg. A young herring in July re- 

 quires 26.6 mg. ; in September, 49.9 mg., and in October, 

 38.8 mg. (Putter, 1909). Even with lower body tempera- 

 tures the food requirements of poikilotherms are similar, 

 in some eases at least, to those of homoiotherms. An 

 average man requires organic food equal to only one to 

 two per cent of the weight of his body each day. 



Food is prepared for assimilation by digestion and 

 its energy is released by the activity of catalytic agents. 

 It has long been known that the digestive enzymes of 

 homoiothermic animals have their optimum for activity 

 at about 37 deg. C. and the recent work of Kenyon (1925) 

 is of particular interest because it shows that the same 

 is true for poikilothermic vertebrates. When reptiles 

 gave rise to mammals and birds there were apparently 

 no radical changes in the methods of digestion, assimila- 

 tion, or energy release through metabolism. 



The character of the food which any animal eats may 

 have a profound influence on metabolism. It is well 

 known that various foods have varying values as pro- 

 ducers of energy. For example, Kubner's standard cal- 

 oric, or heat producing, values are, in calories per gram 

 of food; protein, 4.1; fat, 9.3; and carbohydrate, 4.1 

 (Lusk, 1917). Proteins, of course, have greater value 

 as tissue builders than carbohydrates or fats but are 

 more expensive as heat producers because more work 

 is required to release their energy. Certain foods like 

 vitamins, and certain protein compounds such as trypto- 

 phane, do not appear to have their chief value in the 

 amount of energy or material they furnish, but in pecu- 

 liar qualities which are as yet not wholly understood. 

 There are also certain secretions like thyroidin and 



