74 John B. Calhoun 



In fact, it was this difference in food preference which led me to conchide 

 that the interspecific aspects of dominance in the utihzation of space most 

 hkely fails to involve direct physical interaction between members of 

 different species. Objects desired by species holding opposite ranks in the 

 hierarchy gradually became more and more different. At this level of evolu- 

 tion species A responds not only to vocalizations of its own kind but also 

 responds to vocalizations of both B and C as if they were by members of 

 species A. Species B can ignore signals from A but treats both its own 

 signals and those from C as B signals, while C "filters out" A'& and 5's 

 signals and responds only to those of its own species. In other words, C 

 functions as if it alone were in the environment. 



The fourth step in\'olving evolution of species of type D, which is similarly 

 dominant to species types A, B, and (', again is accompanied by further 

 specialization toward a nearly total use of plant material as food. Judging 

 by the results of field studies presented earlier in this paper, the red- 

 backed mouse, Clethrionunujs, represents a species at the D level. In most 

 situations where both it and Peromyscus are present, Clethrionamys domi- 

 nates. Following the previous line of reasoning we may anticipate that 

 such a D-type species emits vocalizations having not only the basic char- 

 acteristics of species A, B, and C, but, in addition, possesses vocalization 

 characteristics peculiar to itself. 



At each level a species responds not only to its own vocalizations, but 

 also to those of all species ranked above it, as if they were emitted by its 

 own kind. The lower the rank of a species, the greater are the number of 

 vocal stimuli to which it responds and thus the smaller its home range. 

 Reasoning back from present day simall-mammal communities to the 

 probable course of evolution, it appears that there is a correlation between 

 (a) social rank in the domination of the use of space, and (b) the shift 

 from carnivorous to herbivorous diet. I do not believe that an herbivorous 

 diet per se contributes in any way to social dominance. Rather, it has 

 relevance only because of the later development of flowering plants and 

 grasses. Evolution of more advanced types of plant permitted evolution of 

 small mammals specialized to utilize these new resoiu'ces. To a certain 

 degree such feeding specialization would enable an incipient species to 

 avoid direct competition with its progenitor. Once removed from overt 

 competition with its progenitor, psychological dominance by the incipient 

 species could then proceed through the process of increased complexity of 

 vocalization and development of a more effective filter. 



A major aspect of my thesis is that psychological dominance, resulting 

 from a greater complexity of vocalization and an increased effectiveness 

 of the neural filter, far outweighs all niche specializations in determining 

 the relative abundance of species comprising the small mammal commimity. 



