1. The Social Use of Space 73 



tion. This ^'Ocalization preserved much of the characteristics of A, but 

 entailed an addition of components. Following Broadbent's analysis of 

 stimulus characteristics we may suspect that an attribute of some portion 

 of the added vocal components included an increase in intensity. Likewise, 

 B's filter developed alterations which enabled B to filter out selectively 

 those vocalizations not including the new attributes developed by B. Thus, 

 members of the new species B could develop conditioned associations with 

 the vocal signals emitted by its own kind while ignoring those emitted by 

 species A. At maximum stability of such a two-species community there 

 would exist a 2:1 ratio of A:B in a similar fashion exemplified by the 

 Blarina: Peromyscus community previously described for Dr. Barbehenn's 

 Chadwick Woods study (Section VI, B) and Dr. Webb's Rich Lake 

 Island study (Section VI, A) for Sorex: Clethrionomys. 



The interesting aspect of this 2 : 1 ratio of species A:B is that it enabled 

 three animals to live where only one lived before. In other words, the e\'olu- 

 tion of dominant species B not only enabled as many of species B to live 

 in the habitat as was formerly the case with reference to the time when 

 species A only existed there, but it also enabled twice as many of the more 

 primitive species A to live in the habitat as had been the case when B was 

 absent. For such a pattern of evolution to have transpired, it means that 

 intraspecific dispersal of home range centers in a one-species community 

 as a consequence of the repulsive character of vocal stimuli must have been 

 sufficient to ensure an average utilization of resources far below maximum 

 carrying capacity. To clarify further what is intimated above: Members 

 of A, as a result of antagonistic interactions with others of its own kind, 

 develop conditioned avoidance responses to intraspecific vocalizations. 

 Furthermore, the greater the frequency of these vocalizations, as repre- 

 sented by increases in density of the species, the greater is the probability 

 of outward excursions from home being terminated, thus the smaller home 

 range. Lacking a sufficiently effective neural filter, .-1 responds to B's 

 vocalizations as if they were their own. If B emits signals with the same 

 frequency as does A, it follows that in a stable two-species system A will 

 be responding to three times the signal load as B. For this reason, ^'s 

 home range becomes markedly reduced in contrast to its area when B 

 was absent. 



The next step in the evolution of the social hierarchy of space utilization, 

 resulting in species C, entailed similar alterations to vocalizations and to 

 enhancement of the filter in restricting the spectrum of stimuli which 

 would likely be associated with intraspecific interactions. Judging by the 

 fact that the omnivorous mouse Peromyscus is dominant to both Sorex 

 and Blarina, one may conclude that evolution of altered food preference 

 facilitated further evolution of interspecific social domination of space. 



