BEHAVIORISTIC STUDY OF THE RAT 
23 
become progressively more active in the dark than in the light 
as they grow older. At 400 days when the animals are fed at 
the beginning of the dark period they are 2.90 times more active 
in the dark than in the light. When fed at the beginning of 
the light period they are only 1.42 times as active in the dark. 
This is shown in a diagram in figure 12. In the first series the 
ratio of the activity in the dark to activity in the light, 2610/900 
is 2.90. In the second series this ratio, 2090/1460, is 1.42. The 
effect of the food was eliminated then by taking the total amount 
of activity in the dark periods in the two series to the total 
amount in the light periods (2610+2090) 4700/(900 + 1460) 
2360 or 1.99. Ratios obtained in this way are shown in curve C 
in figure 11. 
FOOD SERIES a food at beaming of dm period 
LIGHT - 900 
NOON 
Food 
SERIES B 
LIG-H^T ~ i4 6 o 
MIDNIGHT 
FOOD AT BEGIHNW& OF LIGHT PERIOD 
noon 
DARKNESS 2 09 0 
NOON 
MIDN 
WT 
NOON 
Fig. 12 . Diagram Showing How Curves in Figure 11 were Obtained 
This curve shows conclusively that the rat is more active in 
the dark than in the light, in other words that the rat is a noctur¬ 
nal animal. This curve shows further that the rat becomes 
progressively more nocturnal as it grows older. At 60 days it 
is 1.34 times more active in the dark than in the light. From this 
age on this ratio increases rapildy until at 600 days the animal is 
more than twice as active in the dark as in the light. 
Whether or not nocturnal and diurnal tendencies change with 
age in other animals and in man is not definitely known. It is 
generally said that man also becomes more nocturnal in his 
habits with age. There seems to be some truth in this statement 
for it is well known that at birth and for a considerable time 
afterwards the human infant is almost totally inactive in the dark. 
