ACTIVITY UNITS PER HOUR ACTIVITY UNITS PER HOUR 
SOCIAL MODIFICATION OF INTENSITY AND PERIODICITY OF ACTIVITY 
200 
150 
100 
50 
0 
NOON 6 PM MIDNIGHT 6 AM NOON 
NOON 6 PM MIDNIGHT 6 AM NOON 
NORTH PASSAGE (NO 7) SOUTH PASSAGE(N0.5) 
EAST PASSAGE (NO. 6) WEST PASSAGE (N0.8) 
Figure 92. — Social modification of intensity and periodicity of activity. Two typical days near the time of maximum competi- 
tion. Each activity unit represents a passage in or out of the Food Pen. The south passage (No. 5) was used by the 
fewer number of socially higher ranking rats, while the north passage (No. 7) was used by a greater number of lower 
ranking rats. 
increase in the differential of postmidnight activity 
between the North and South Passages. This is 
further evidence of the importance of the beginning 
of the breeding season on the future history of the 
life of a colony of rats. 
There were three occasions (circa Nov. 15, 1947, 
Jan. 4, and Apr. 28, 1948) during which the post- 
midnight activity through the South Passage 
exceeds that through the North Passage. These 
three exceptions to the general rule appeared to 
result from a time lag of some other underlying 
periodicity. Only a tentative hypothesis may be 
forwarded concerning this periodicity, which is so 
marked that it produces a doubling in the amount 
of postmidnight activity from the minimum to the 
maximum phase of the rhythm: 
During the winter of 1947-48 there were three 
clear cycles of this rhythm. The peaks of each 
approximately coincided with the time of the new 
moon, that is reduced nocturnal luminosity, 
whereas the minima occurred during the light 
phase of full moon. This is interpreted as meaning 
119 
