Figures 81 to 90. 
Periodism of activity as revealed by the number of passages in and out of the Food Pen. Ten sets of figures are shown 
for successively later periods in the history of the colony. An inspection of the series will reveal the following: 
1 . There is a tendency toward a bimodal periodism with the major period before midnight and a minor period after midnight. 
2. As the population increased there was a tendency for the activity of the population as a whole to become more 
arrhythmic. For example, compare periods 2 and 3 with periods 9 and 1 0. 
ACTIVITY 
MIDNIGHT 
PERIOD I 
Figure 81 . — Activity period 1; November 1947; 26 rats 
present in the pen. 
MIDNIGHT 
PERIOD 3 
Figure 83. — Activity period 3, January 1948, 25 adult rats 
present in the pen. This was a time of inhibited sexual 
activity. 
PERCENT 50 
OF 
TOTAL 
ACTIVITY 
ONE DAY 
rl 
DEC. 26-27 
1947 
r 
J 1J 
[M 
PERCENT 50 
OF 
TOTAL 25 
ONE DAY 
1 
DEC. 25-26 r 1 
1947 

Lr 
OF 
TOTAL 
Figure 82. — Activity period 2,- latter third of December 1 947; 
25 rats present in the pen. 
ACTIVITY 
Figure 84. — Activity period 4,- February-March 1948; 24 
rats present in the pen. The 1948 breeding season 
began about February 1 3. 
114 
