ACTIVITY UNITS PER 24- HOURS 
200 
I 60 
I 20 
80 
40 
0 
ACTIVITY THROUGH THE SOUTH PASSAGE 
5-DAY MOVING AVER AGE □; TREND 
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10 20 30 10 20 30 
NOVEMBER DECEMBER 
1947 
9 19 29 
JANUARY 
1948 
20 
Figure 95. — Activity through the south passage of the Food Pen. In this case there is no apparent relationship of intensity of 
activity to temperature. Likewise by comparing this figure to figures 93 and 94, it may be seen that there is also no 
apparent relationship of intensity of activity to fluctuations in amount of postmidnight activity or to the amount of activity 
through this passage in comparison with that through the north passage. 
north side of the pen as they approached sexual 
maturity. Thus, as is documented later in the 
paper, the disproportionate density between the 
two sides of the pen was reestablished. Table 22 
further documents the conclusions that, in com- 
parison with the South Passage which was utilized 
by socially higher-ranking rats, there were more 
socially lower-ranking rats utilizing the North 
Passage, and that these latter rats were more 
active after midnight. Furthermore, intensity 
and periodicity of activity through the East 
and West Passages was in general intermediate 
between that of the North and South Passages. 
B. Time-Space Utilization by Rats in Relation to 
Population Growth. In order to obtain a more 
accurate index of such arrhythmicity than is af- 
forded by a mere inspection of the figures 81 to 90, 
the 100 days represented in this sample were 
examined with reference to the variance in the 
percentage of activity among the half-hour inter- 
vals of each night. As a background to this analysis 
there first will be presented certain observations 
and inferences concerning activity rhythms. 
For any individual day each of the two major 
periods of activity consists of shorter intervals of 
increased activity separated by periods of decreased 
activity. While the colony was small, there were 
frequently periods in which no animals were 
active. This fact does not show on the graphs 
(figs. 81 to 90) since arbitrary half-hour periods 
are considered rather than the actual periods of 
activity. From observation of the activity of the 
rats it has been frequently noted that nearly simul- 
taneously many of the rats became active regardless 
of the locations of their places or harborage. Two 
factors were probably involved in the initiation of 
this simultaneity of behavior: 
(1) The physiological rhythms of each individual 
which induces initiation of activity following 
rest is similar. 
(2) There is some means of social communication 
whereby the first few individuals which 
become active transmit this fact to rats living 
in other locations, whereupon these also be- 
come active. It is suspected that vocaliza- 
tion-audition is the mode of communication, 
although there is no conclusive evidence for 
this assumption. 
The reason for making this second inference 
regarding communication is as follows: Slonaker 
and Richter have shown [see review by Calhoun 
(13)] that the relative lengths of periods of inactivity 
and activity vary with age. Despite this fact, rats 
of all ages, when living as members of a colony, 
123 
