1. The Social Use of Space 89 



dominant male (as in colony a, Table X) his actions kept away most other 

 males at times when his harem females were in estrus. This reduced the 

 stress experienced by females in estrus, such as followed the thousands of 

 mountings or attempted mountings experienced by females in colonies 

 lacking such dominant males. In this sex-ratio restructuring of the popula- 

 tion, a few colonies contribute most of the young. 



Although this readjustment ensures the survival of the species, it is not 

 the conseciuence most important with regard to further social evolution. 

 Each time a rat is excluded from one colony it attempts to join another 

 aggregate in order that it will again find itself in a group of a size most 

 compatible with its physiology. As soon as a group exceeds the optimum 

 size, some of its members are excluded from it. This results in a marked 

 reshuffling of the population from the time of puberty of young born in 

 one season up to the beginning of the next breeding season. At this time 

 the population is relatively stable with regard to membership of each 

 colony. 



In the process of attaining colony stability, the social environment is 

 in a constant state of turmoil. Each individual is forced to make many 

 adjustments to such changes. It continually has to learn new social rela- 

 tionships. These are learned so well that a group can shift its place of resi- 

 dence over a sufficient distance that passage by or through other colonies 

 is necessitated. Even so, they can maintain their group integrity. I am 

 convinced that the necessity of making such changes of membership from 

 one colony to another required for reproductive survival of the species 

 has resulted in the evolution of the Norway rat into a species which is not 

 only highly perceptive of changes within the environment but has the 

 capacity to learn required adjustments of behavior. 



Although such capacities for perception and learned adjustment must 

 have arisen in the context of a changing social environment, these same 

 capacities then become available for perceiving and adjusting to nonsocial 

 changes in the environment. Among ecologists this extreme awareness to 

 changes in their environment by Norway rats has been termed the "strange 

 object reaction" (e.g., Chitty and Southern, 1954). Among psychologists 

 it is reflected in the studies falling under the broad rubric of ''open-field 

 emotional behavior" (e.g.. Hall, 1934, Schneirla and Tobach, 19G2). 



If I were to make my evolving thesis concerning the social use of space 

 complete, I should substantiate the role of vocal communication at the 

 compact colony level of social evolution. Unfortunately, I cannot cite any 

 adequate proof regarding the nature of its function. Norway rats do have 

 a wide scope of vocalization ranging from the loud signal accompanying 

 the termination of a fight to the low chirping and whining one can detect 

 if one lies on the surface of a burrow with his ear at an entrance hole. Many 



