WARM-BLOODED ANIMALS 79 



energy out in the form of work as is consumed in the 

 way of foodstuffs, we should expect to find the balance 

 putting in an appearance in the form of heat, and that 

 is exactly what happens; wherever metabolism goes on 

 which is at all wasteful, heat is produced. Sometimes 

 this heat serves a useful purpose, as it does in the warm- 

 blooded animals. 



Warm-Blooded Animals make up the two highest 

 animal groups, birds and mammals. These differ from 

 all other living things in that their bodies are maintained 

 at substantially the same temperature day in and day out, 

 through summer and through winter. Since this means 

 that their bodies during most of this time are warmer 

 than their surroundings, they must produce a great deal 

 of heat. The animal that is warmer than the air around 

 it will tend to cool off just as would a stove or any other 

 inanimate object in the same situation. This cooling off 

 which is going on nearly all the time is counterbalanced 

 by the production of large amounts of heat, which from 

 the standpoint of efficient metabolism would be consid- 

 ered wasteful since it represents a large consumption of 

 food for which no useful work — in the engineering sense 

 — is accomplished. Warm-blooded animals profit by this 

 large heat-production, because it enables their bodies to 

 stay at a favorable temperature for activity at all times. 

 On the coldest days of winter as well as on the pleasantest 

 days of summer, the temperature within their bodies is 

 favorable to metabolic activity. 



Cold-Blooded Animals, on the other hand, produce 

 much less heat, and in consequence their bodies are at 

 all times very nearly at the temperature of the surround- 

 ings. It follows that whenever the surrounding tem- 

 perature falls to a point unfavorable for metabolism, the 

 cold-blooded animal must suffer a great diminution or 

 an almost complete suspension of activity. During a 

 large part of the year, in cold climates, the cold-blooded 

 animals are hibernating, living over to the next season 



