92 John B. Calhoun 



in groups of about the size anticipated by theory as most nearly optimum 

 than they do in any smaller or larger sized groups. In fact, the mean number 

 of adults with which the 838 young in these 136 groups had their early 

 social experience was 12.22!, even though Curve C, Fig. 32, is flatter than 

 predicted by the theory of Section VIII, D and Table VIII. 



My selection of the Norway rat and the howler monkey as examples of 

 compact colony types is open to the criticism that I selected those species 

 which would support my theory that group size in higher evolved types is 

 dependent upon their evolution from species which had been characterized 

 by the loose constellation form of social use of space. At present, this 

 criticism cannot be avoided. I wish merely to say that my intensive study 

 of a few groups of Norway rats, and the extensive study by Carpenter and 

 his colleagues of a large number of howler monkey groups, are the only 

 ones known to me that appear adeciuate for the present purpose. 



All we can really say at present is that available data show that optimum 

 group size in some compact colony living species appears to approximate 

 12 adults and that this number is in harmony with that number antici- 

 pated by the physics of communication characterizing more primitive 

 and more dispersed types. 



C. Behavioral Sink Development by the Norway Rat 



IVIuch of the prior sections have been devoted to documenting theory 

 and e\ddence supporting the hypothesis that groups of 12 individuals 

 represent a major category of optimum density. Yet many species cus- 

 tomarily live as aggregates much larger than this. Why such large groupings 

 should have evolved remained a puzzle until insight developed from an 

 unexpected phenomenon arising in the situational content of some experi- 

 mental populations of rats I was studying (Calhoun, 1962a). 



The upper portion of Fig. 33 presents in diagrammatic fashion the salient 

 aspects of the environment affecting the rats. Four 35 square-foot pens, 

 separated by 2-foot high partitions, formed a linear communication net- 

 work through the opportunity of access between adjoining pens via the 

 V-shaped ramps, R, surmounting barriers between pens. F and W represent 

 a superabundant supply of food and water in each pen. "Apartment" 

 houses, H, connected to the floor by ramps provided ample place of retreat 

 and rearing young by most residents. The //'s of the left-hand pens I and 

 11 were 3 feet from the floor, while in pens III and IV a 6-foot distance 

 separated the i/'s from the floor. Height formed an intentional environ- 

 mental factor designed to produce a 2:2:1:1 ratio of density across pens 

 1:II:III:IV as a consequence of the inverse-proportionality-to-effort 

 usage principle. 



