February 24 to March 6, and is most comparable 
to Period 4, February 20 to March 7, 1948. 
Again we find an increase in arrhythmicity of 
nocturnal activity during this later period. In 
this instance the increased population density 
accentuated the antagonism which accompanies 
sexual activity. 
During the final Period 10 (fig. 90), March 20 
to April 3, 1949, the action of lactating females in 
accentuating the arrhythmicity of nocturnal ac- 
tivity, when superimposed upon the increase of 
antagonistic relationships accompanying the in- 
crease in numbers of mature and sexually active 
rats, resulted in a nearly complete arrhythmicity 
in the general patterns of nocturnal activity. This 
increase in arrhythmicity of nocturnal activity 
also held true for individual days. 
We may summarize as follows those conditions 
which modify the basic bimodal rhythm toward 
arrhythmicity: 
1. Increase in density of mature rats (i.e. those 
over 3 to 4 months of age). 
2. Increase in sexual activity. 
3. Increase in the number of lactating females. 
4. Increase in the number of juvenile rats (i.e. 
25 to 30 through about 90 days of age). 
The degree of shift from the bimodal nocturnal 
activity with its major premidnight peak toward 
a complete arrhythmicity of nocturnal activity may 
in part be considered as an index of the degree of 
stress experienced by the members of the popula- 
tion. 
Another modification of the 24-hour activity 
rhythm, which accompanies the increase in in- 
tensity and frequency of social interactions, is the 
spreading out of activity into the daylight hours. 
From Periods 1 through 6, November 1947 through 
May 1948, when the population density was low or 
contained a large proportion of juveniles, there was 
essentially no activity before 6 p.m. or after 6 a.m. 
However, by Period 7, June and July 1948, when 
the juveniles born during March 1948 were be- 
ginning to attain sexual maturity, there was an 
augmentation of activity in the afternoon between 
3:30 and 6 p.m. During the three terminal 
Periods 8, 9, and 10 of the winter and spring of 
1949, when the density of adults was high, there 
was an increase in the afternoon activity in contrast 
to comparable Period 3, 4, and 5 to 7 of 1948. 
Thus the increase in intensity of social interaction 
produces a greater utilization of the daylight hours 
as well as increases the arrhythmicity of nocturnal 
activity. 
It is difficult to sort out the specific effect of light 
intensity on limiting activity from the counter 
effect of the spreading out of activity into the day- 
light hours resulting from social interaction. The 
effect of light intensity may best be judged by com- 
paring Periods 2 and 3 with Periods 4 and 5. Dur- 
ing the former two periods the sun set before 5 
p.m. and rose after 7 a.m. With these conditions 
there was considerable activity during the hours: 
5 to 6 p.m. and 6 to 7 a.m. However, in the latter 
two periods when sunset and sunrise were near 
6 p.m. and 6 a.m., the length of the activity cycle 
had made a comparable contraction of 2 hours so 
that very little activity was exhibited during the 
hours of 5 to 6 p.m. and 6 to 7 a.m. During most 
of the later history of the colony, Periods 5 to 10, 
the intensity of social interaction was sufficient to 
override the limiting effect of increased light 
intensity. 
By equally weighing the activity rhythms for each 
of the 10 Periods an average picture (fig. 91) of 
the activity rhythm for free ranging rats may be 
obtained. The bimodality of the nocturnal rhythm 
stands out. 
The four passages through the Food Pen fence 
were fairly consistently utilized by different groups 
of rats. Therefore, the data for particular pas- 
sages provide insight into alterations in activity 
associated with the social structuring of the colony. 
South Passage (No. 5) : Used mainly by the socially 
high-ranking rats of the South Alley Burrow. 
It was also used considerably by the rats in- 
habiting Area I. These rats had considerable in 
common with the South Alley Burrow rats with 
regard to their social and genetic history. It was 
also occasionally used by the middle social 
strata of Area II. 
East Passage (No. 6): The greatest amount of 
usage was by the socially low-ranking rats of 
Area III. It was also used mainly by the Area 
II rats who were fewer in number. 
North Passage (No. 7): It was utilized the majority 
of the time by the socially moderate to low- 
ranking rats of the North Alley Burrow. It was 
also utilized considerably by the Area III rats 
and by the socially moderate to low-ranking 
rats of Area IV. 
West Passage (No. 8): The utilization of this pas- 
sage was quite varied through time. Initially it 
was used most by North Alley Burrow rats. 
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