NEEDS OF THE ANIMAL 347 



needs? We can live without air for only a few minutes, 

 without water pr about a week, without food for over 

 a month. We can get along without light for a good 

 while, but we cannot get along without warmth even 

 for a moment. To keep up this warmth, food must 

 be burned in the body all the time, without stopping 

 even for an instant. And so the first and greatest 

 problem is to get the best sort of food and plenty of it. 

 The cold-blooded animals such as fishes, frogs, snakes, 

 and worms do not need so much warmth. A frog may 

 even be frozen for a time in a cake of ice without injury. 



For what is food needed? We have seen (page 84) 

 in what way food is used in the plant. It is (1) burned 

 up, (2) built up into the growing body, (3) stored up 

 for future use. Just so with the animal. The animal 

 burns up a much greater part of its food to keep warm 

 (except in the case of cold-blooded animals) than the 

 plant does. 



What are the kinds of food? First, protein; second, 

 fats, including oils; third, sugar and starch (in the 

 process of digestion starch is changed into a kind of 

 sugar, which is then taken up by the body). 



What kind of food is best for burning in the body, in 

 order to keep warm? In cold weather we always eat 

 more fat ; this is good, for fat gives out a great deal of 

 heat when it burns. So do sugar and starch. Protein 

 gives out only about half as much heat as fat or sugar. 

 Fats and sugars are known as "heat-making foods." 



