SUNSET SUNRISE 
(PERIODS I- 10 EQUALLY WEIGHTED BY AVERAGING THE HALF-HOUR 
PERCENTAGES OF ACTIVITY FOR ALL OF THE TEN-DAY PERIODS.) 
Figure 91 . — Typical activity periodism exhibited by a colony of wild Norway rats. It is typical in the sense that the range of 
conditions, which contribute to variability from the more basic activity cycle as shown in figure 82, are represented by the 
samples from which this figure was developed. 
Later on it was mainly used by Area I rats as 
well as those expelled into Area IV. During the 
last few months of the study it was mainly domi- 
nated by South Alley Burrow rats and Area I 
rats. However, a few Area IV rats still utilized 
it. 
Although every rat probably utilized each pas- 
sage at least occasionally, the above gives a suffi- 
ciently accurate picture of the differential usage of 
the passages with reference to the social stratifica- 
tion of the population. 
The activity through each of these passages near 
the height of social interaction, March 1949, is 
shown in figure 92 for 2 consecutive days. With 
regard to intensity of activity, the total number of 
times rats went through the North (No. 7) and 
East fNo. 6) Passages was greater than through 
the other two passages, which were dominated by 
the socially high-ranking rats. This is a direct 
reflection of the ability of higher ranking rats to 
prevent more subordinate ones from utilizing cer- 
tain place in the environment. Differential utili- 
zation of time is also revealed in these figures. The 
South Passage (No. 5) shows the greatest amount 
of premidnight activity. Thus, the more dominant 
rats were able to express more nearly the basic 
bimodal nocturnal rhythm with its major pre- 
midnight peak. On the other hand, the North 
Passage (No. 7) exhibits an arrhythmicity of the 
activity through it by the lower ranking rats. 
I his arrhythmicity of activity by the lower ranking 
rats only partially results from the immediate 
actions of the more dominant rats during the same 
night. What had happened, was that a large 
portion of the population had previously been 
conditioned to utilize the North Passage and to 
avoid the South Passage. This avoidance of the 
South Passage, as a result of conditioning, occurred 
even when the more dominant rats were inactive. 
Among the many socially lower ranking rats there 
was further competition for the use of the North 
Passage, such that some individuals were forced 
to be active mainly after midnight. Activity 
through the East (No. 6) and the West (No. 8) 
Passages tended to fall intermediate between the 
North and South Passages with regard to both 
intensity and rhythmicity. This is as might be 
expected, since these passages were utilized more 
equally by lower and higher ranking rats. 
Because of the marked contrast between the 
activity through the North and South Passages, 
detailed comparisons of them were prepared. 
These are shown in figures 93 and 94. The first 
analysis relates to the time distribution of activity. 
It utilizes the percentages of activity occurring after 
midnight as an index of the degree to which there 
is a modification of the periodicity of activity. 
For most of the course of this comparison 5 to 10 
percent more of the activity through the North 
Passage (No. 7) occurred after midnight than was 
the case for the South Passage (No. 5). This is 
further evidence of the permanency of the greater 
social stress experienced by the rats utilizing the 
North Passage into the Food Pen. During the 
early spring of 1948 and 1949 there was a marked 
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