1. The Social Use of Space 77 



by Perovnjscus, even those living much farther out in the habitat, altering 

 their movement and passing with increasing frequency and in an altered 

 pattern through the home ranges of Microtus. This trapping was followed 

 by additional removal trapping along several concentric circular traplines 

 between the original one and the bait station. Many more Microtus were 

 trapped, but practically all Peromyscus had been removed in the initial 

 trapping. Furthermore, the drop-off in catch of Microtus in this later 

 trapping, as the concentric lines came nearer the initial one, indicated that 

 only Microtus living within 37 feet of the initial circular line had been taken 

 by it. It furthermore indicated that Microtus were not wandering about, 

 as we had supposed they might be, but regardless of distance from the 

 feeding stations had maintained fixed home ranges despite many strange 

 Peromyscus focusing their movements toward this one central spot. Paren- 

 thetically, it might be added that this altered behavior by Peromyscus did 

 disrupt Sorex home ranges. They were taken in relatively large number 

 beginning with the very first day of trapping along the initial circular 

 trapline. 



Now the question arises, "What do these data indicate?" Since Peromys- 

 cus altered its behavior by readjusting its movements toward the feeding 

 station as if Microtus was not there, I conclude that one of Microtus'^ 

 adaptations has been the loss of vocalization. On the other hand, since 

 Microtus appears to be unaffected by the altered movements of Peromyscus 

 and the periodic concentration of many Peromyscus in one spot, which must 

 increase the vocalizations at that spot, I conclude that Microtus has 

 evolved a filter system so effective that they can ignore signals from other 

 species. This leads me to suspect that Microtus represents what might be 

 called a secondary herd-type species. See later discussion on the evolution 

 of colonialism and herding. 



X. Psychological Dominance as the Primary Component of the Niche 



Hutchinson (1957) includes the relationships one species has with others 

 as comprising aspects of its niche requirements equally to be considered 

 along with food, shelter, and climatic factors. Implicit in his inclusion of 

 interspecific social factors in niche characterization is the relative capacity 

 for any two given species to compete for some given environmental com- 

 modity or condition. The more nearly the identity of their nonsocial niche 

 requirements, the more important become social relationships both as a 

 component of the niche and in leading to the elimination of one species by 

 the other via the principle of "competitive exclusion" (Hardin, 1960). 



In terms of this viewpoint and the marked difference in diet, morphology. 



