102 THE SOCIAL LIFE OF ANIMALS 



weeks the same relations held, showing that it was 

 the state of aggregation rather than individual dif- 

 ferences between mouse and mouse which was impor- 

 tant in producing the differences in growth rates. 



Mr. Retzlaff, (105) the student who brought this 

 work into our laboratory, tried first to repeat Vetu- 

 lani's experiments in a room held at relatively high 

 temperatures (29-30° C). Under these conditions he 

 found that insofar as significant differences existed 

 they showed that most rapid growth occurred with 

 the isolated mice. When, however, he lowered the 

 room temperature to about 16° C. he obtained the 

 same general effect as reported by Vetulani. It would 

 seem then that in this case the opportunity to keep 

 warm in a chilling temperature is one of the main 

 factors in promoting growth of the crowded, but not 

 too crowded, animals. This conclusion is strength- 

 ened by recent analyses of the temperature relations 

 of mice, made by French physiologists, (30) which 

 show that a mammal as small as a mouse has great 

 difficulty in maintaining a constant temperature and 

 rarely does so for extended periods of time. A change 

 of external temperature from 30° to 18° C. will cause 

 a lowering of 0.4° in the body temperature of a 

 resting mouse. 



With such temperature lability it is easy to see that 

 a few mice huddled together as is their habit could 



