AGGREGATIONS OF HIGHER ANIMALS IO3 



help each other maintain their internal temperatures, 

 conserving energy for growth, while if isolated they 

 must use much of their energy in keeping warm. 



Vetulani observed another factor at work. Some 

 of his mice had lesions of the skin which they treated 

 by licking. When these were in the head region they 

 could only be treated by another individual. Some 

 of his isolated mice had such lesions when at the end 

 of the first experimental period they were re-grouped 

 for further observation; these wounds were soon 

 cured by their new nest mates. 



When one turns from studying the rate of growth 

 of individuals to that of populations of these higher 

 sexual animals, many of the same principles can be 

 observed working as were outlined in the last chap- 

 ter for the growth of asexual populations of proto- 

 zoans in which overcrowding retards population 

 growth, while optimal crowding, at least in many 

 instances, favors it. 



With experimental populations of mice, for exam- 

 ple, three long, laborious experiments made in Scot- 

 land (36) and in Chicago (106) have indicated that, 

 under the conditions tried, the least crowded mice 

 reproduce most rapidly. The same holds true for the 

 well-studied fruit-fly, Drosophila. (96) 



Neither with these flies nor with the mice is there 

 any indication to date of a more rapid rate of repro- 



