1. The Social Use of Space 81 



Hinckley, then a sophomore at Yale University, was assigned to me for 

 supervision on a research project. I assigned him the task of placing eight 

 NACSM traplines in an 80-acre central portion of a much more extensive 

 tract. He ran the 480 traps for 15 consecutive days. At that time it was 

 generally accepted that if one plotted a regression line through points 

 representing catch per day as a function of total prior catch, this line would 

 intersect the abscissa at a point denoting the total population (see Calhoun 

 and Casby, 1958, pp. 15-16 for a summary of this procedure). Previously, 

 Hayne (1949) had made such estimations on the basis of 3 days of consecu- 

 tive trapping as employed by the North American Census of Small Mam- 

 mals. Thus, it was my anticipation that Hinckley's longer-term trapping 

 would merely result in the anticipated continuous decline from day to day, 

 and that by the 15th day essentially no more animals would be entering 

 the traps. Furthermore, by the end of the loth day the total should ap- 

 proximate that predicted by the intersection of the abscissa by the regres- 

 sion line as noted above. During the first 3 days the catch per day did 

 decline. However, on the fourth day Hinckley reported a larger catch than 

 on the first day. Well, I thought this was just due to the vagaries of chance 

 or some unrecognized climatic factor. On every successive day his report 

 was the same — more animals than yesterday. On the 15th day, three times 

 as many animals were taken as on the first day. All of this was very 

 perturbing. 



After several months of reflection I came up with this formulation: 

 During the first 3 days the number of residents in the 80-acre tract had 

 been drastically reduced. Animals at the periphery of this tract would then 

 find themselves with the normal number of neighbors centrifugal to the 

 trapped area but with very few remaining centripetally. Now, suppose 

 that in the normal state maintenance of a uniform distribution through the 

 environment is facilitated by vocalizations and audition. Each individual's 

 customary state would be that of detecting an equal intensity of signals in 

 all directions from the center of its home range. In these terms mice and 

 shrews at the periphery of the trapped area would detect few signals 

 toward it but comparatively many away from it. Their normal response 

 being to move so as to e(iualize signals coming from all directions, their 

 response to the neighboring depleted area would be to move in toward it. 

 They then became exposed to traps still set in the central area and many 

 of these invaders were killed. This left the next peripheral group of animals 

 in the same situation so that they also began moving inward toward the 

 trapped depleted area. By this process, a chain reaction was set in motion 

 in which the entire population for a great distance from the trapped area 

 began moving toward it. If we visualize the trapped area as a circle, rather 

 than its actual rectangular shape, it becomes apparent that if animals are 



