94 John B. Calhoun 



If the two movement biasing principles described above operated inde- 

 pendently and equally, the expected ratio of density would be 3:4:3:2 

 across pens I: II: III: IV, thus giving an expected distribution for the 

 September 1958 observation of 347:462:347:232. The observed and ex- 

 pected values are so nearly the same as to support strongly the belief that 

 these two movement biasing principles were in fact the only effective ones 

 operating at this time. These same factors also affected the third generation 

 then maturing. 



Taking mortality into consideration, each population consisted of about 

 80 rats with on the average 20 living in pen I, 27 in pen II, 20 in pen III, 

 and 13 in pen IV. Only in pen IV did the density approximate the ideal of 

 12. Elsewhere, particularly in pen II, density far exceeded this. From this 

 time on a remarkable change in the differential use of space, particularly 

 as reflected by food consumption, set in. In one pen (in three instances it 

 was in pen II and in the fourth in pen III) food consumption increased at 

 the expense of that in the other three pens. See the lower half of Fig. 33. 

 By the time seven months had elapsed, most rats were eating all their 

 food in this "favored" location and all rats were doing most of their eating 

 there. 



The explanation for this change appears rather simple. Gnawing food 

 through the wire mesh of the food hoppers required considerable time. In 

 the one pen w^here more rats fed than in the other pens, the probability of 

 two rats eating side by side increased. Gradually rats redefined the eating 

 situation as requiring presence of other rats. Thus, all rats shifted most of 

 their eating to that pen where this condition was most likely to be met. It 

 must be kept in mind that such a system is stochastic and not deterministic, 

 so it was not unexpected that pen III became the favored place of eating 

 by one of the four groups. However, the likelihood of pen IV ever becoming 

 the favored pen is extremely remote. 



The learned need for social proximity while engaging in an act which 

 might have been expressed alone assumed priority over the simple original 

 hunger drive. Food was not food without the presence of a comrade. This 

 whole process of developing excessive aggregations in order to satisfy a 

 secondarily acquired social drive is what I mean by a "behavioral sink." 

 Gradually more rats also shifted residence to this favored place of eating. 

 Such behavioral sinks result in every member encountering more associates 

 than the ideal, and even more than necessitated by the operation of those 

 principles of spatially structuring the environment which biases movement. 



In this situation marked alterations in mortality and behavior resulted. 

 Males became pansexual in the sense that they mounted other rats ir- 

 respective of sex or age. Nest building and maternal behavior became so 

 disrupted in most females as to preclude the possibility of most young 



