52 HOMOIOTHERMISM 



water (Anderson, 1928). The amount of heat given off 

 by an animal varies according to the relation between 

 mass and surface area. On account of this, small homoio- 

 therms require more energy to maintain body tempera- 

 ture than larger. This is not the case among poikilo- 

 therms because they expend comparatively little energy 

 to maintain body heat. 



Food. — Foods vary in their caloric value and carbo- 

 hydrates are the best sources of heat. More sugar is 

 needed by animals at lower temperatures (Mansfield and 

 Pap, 1920). It is present in greater quantity in the blood 

 and, of course, furnishes more heat. Associated with 

 this increased consumption there is an increased use of 

 oxidase. Fat and protein may be used for heat produc- 

 tion in an animal's body but both are less productive 

 of calories than carbohydrates, and protein is a rather 

 poor source of heat because of the difficulty with which 

 it is broken up and oxidized. 



A developing hen's egg uses foods in the following 

 order: carbohydrates, proteins, fats. Needham (1926) 

 discovered this fact and believes that it has phylogenitic 

 significance as an indication of the order in which foods 

 have been utilized by vertebrates in the past. However, 

 Gray (1926) finds that a fish egg derives its energy 

 throughout development largely from protein. He also 

 calls attention to the fact that a land animal must have 

 a considerable supply of water provided for the develop- 

 ing egg. This need for water may be related to the early 

 utilization of carbohydrate in the hen's egg. 



Enviromnent. — The temperature of the environment 

 influences the temperature of animals living in it. A 

 poikilotherm tends to have a body temperature which is 

 the same as, or close to that of, the surroundings. At 

 very low or very high temperatures however, the tem- 



