1. The Social Use of Space 123 



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water. Furthermore, the entire population in the quarter-acre pen sur- 

 rounded by a rat-proof fence made a closed system out of the entire popu- 

 lation. At 123 adults (Table X), marked disturbance was in evidence, 

 witli only a minority of the females reproducing successfully. In the closed 

 systems contained within a smaller area, described in Section XII, C, 

 severe reproductive disturbance characterized an A'' of slightly less than 80 

 adults. Therefore, for the Norway rat at least, when the actual A'' ap- 

 proaches the increase above Nb presumed to represent the tolerance limit, 

 a degree of physiological disturbance of sufficient magnitude arise as to 

 indicate a necessity for some evolutionary adaptation to the increase in 

 group size for continued survival. Parenthetically, I might add that this 

 tolerance limit for dd and 6/ was arbitrarily assigned simply with reference 

 to the slope of the curve, and without prior knowledge that this level 

 would lead to tolerance limit A^'s so closely approximating those observed 

 in my experimental studies. 



One type of evolutionary change which will reinstate 6 to its optimum 

 level, that will eliminate dd, involves reducing intensity, i, of interaction. 

 It will be recalled that i- = a, the refractory period following interaction. 

 Now if Nb = 12, and the tolerance limit A^ is 123, a reduction of a from 

 0.091, the a appropriate to Nb = 12, to 0.0082, the a appropriate to Nb = 

 123, will return d to its optimum value of 0.25. The change in i is 0.3 of its 

 level at the former A^'b, that is, from i = 0.3 to i = 0.09. 



With a hereditary change in behavior amounting to a reduction of i to 

 0.3 its former level, A^6 changes from 12 to 123. These would then represent 

 two species, the stem one having an A^^^^ = 12 and residing under those 

 environmental conditions not necessitating an evolutionary change in 

 physiology and behavior, and the derived one having an A^^^^ = 123 and 

 residing in the presence of those environmental conditions forcing the 

 maintenance of group size far above that of the stem species. If this process 

 is repeated each time 9d reaches a tolerance limit of approximately 0.607 of 

 the minimum dd possible, there arises what might be termed a "satiation 

 deficit saltatorial series of basic N's" which are as follows, starting with 

 A^,^^^ = 12. 



