122 John B. Calhoun 



3. Saltatorial Chaxges in the Basic Group Size 



Within most orders, and many lesser taxonomic categories, related 

 species may be found between which there exist marked differences in the 

 typical group size. Caribou and elk characteristically maintain large 

 herds in contrast to the small groups or even isolated pattern of living by 

 mule deer or moose. During the active breeding season, bats of the species 

 Myotis lucifugus and M. yumanensis roost singly or in small clusters, 

 whereas M. grisescens and il/. velifer maintain large assemblies even during 

 the breeding season. Woodchucks, Marmoia monax, tend to live in isola- 

 tion, whereas black-tailed prairie dogs, Cynomys luchviciamis, live in large 

 colonies. 



Obviously these represent a select group of comparisons. Although I 

 shall not attempt to substantiate here the typical group sizes found within 

 any fairly closely related series of species, examination of many series 

 suggests that there are within each series several discrete basic group sizes 

 with an extensive range between any two nearest sizes not represented by 

 any species. For the present purpose, this conclusion will be accepted as 

 approximating reality. Then the question follows: "What characteristics 

 of physiology and group interactions might lead to saltatorial steps in 

 group size which become fixed by natural selection or cultural evolution?" 



In the first place, there must be some condition which induces animals 

 to assemble in far greater group sizes than their Nb, and this condition must 

 remain sufficiently strong to prevent splitting of the group as it approaches 

 2Nb — 1 [see Eq. (85)]. Any spatially restricted but locally abundant 

 resource might well so act, particularly if response at the source favored 

 the establishment of a behavioral sink as described in Section XII, C. In- 

 crease in group size beyond 2A^6 — 1 would accentuate social discomfort 

 and stress in the sense of increasing 6d and 6/ (Fig. 3o) . At some point these 

 factors must become so intense as to produce sufficient decrements in re- 

 production and survival to threaten the survival of the species. There is 

 no a priori basis for judging what this threshold might be. Beyond 2A^6 — 1 

 each increment in N produces a smaller increment of 9d and df. Examination 

 of the curves in Fig. 35 reveals that when dd is about 0.70 of its maximum 

 and df is slightly over twice its optimum level, any further increments to 

 N produce little further change in dd and 6/. The horizontal line through all 

 curves defines this point on each curve and shall be considered empirically 

 as a limit beyond which further increases in A'^ cannot be tolerated. 



If Ni'^ = 12, the limit is Ni^^ = 82 for Of and A^f ^ = 123 for dd. It will 

 be recalled from Section XII, A that Nb for the Norway rat appeared to 

 be about 12. Although local colonies approximated this number, all mem- 

 bers of all colonies were forced to interact at the single source of food and 



