BEHAVIORISTIC STUDY OF THE RAT 
17 
by changes in external temperature. These modifications affect 
chiefly the position of the period of maximum activity—the 
“hump” in the curve. In a comparatively low temperature 
10° to 15°C. this period of maximum activity begins almost 
immediately after the daily feeding period. The usual interval 
of relative quiescence following the taking of food is absent. 
This may be seen in figure 9. This curve is based on the records 
of four animals of from 200 to 300 days old, taken in an external 
temperature of 12° to 13°C. In a temperature considerably 
higher than the normal 29° to 30°C. on the other hand, the onset 
of the period of activity is very much delayed. The interval of 
quiescence following the taking of food is greatly lengthened and 
the “hump” of activity is not as conspicuous as before. This 
may be seen in figure 10 based on activity records from animals 
of middle age about 300 days in an external temperature of 
29° to 30°C. 
It will be noticed that the form of the activity curve for middle 
aged animals taken in a cold external temperature is very similar 
to the activity curve for very young animals taken at a normal 
temperature. In both cases the period of maximum activity 
begins immediately after the taking of food. Compare figure 8 
and figure 7. Conversely in higher temperature it will be noticed 
that the form of the activity curve of the young animals becomes 
very much like the normal curve for middle aged animals. In 
both cases the period of maximum activity falls near the middle 
of the twenty-four hours. 
The attempt was made to throw some light on quite another 
phase of the relation of activity to external temperature by the 
determination of the external temperature in which an organism 
is most active. In the previous experiments 23°C. was chosen 
quite arbitrarily as the normal temperature. It was found that 
both below (13°C.) and above (30°C.) this temperature the 
amount of activity per day was diminished. Somewhere between 
these two extremes there must be a point at which the animals 
are most active, a “critical” point of activity. It was not pos¬ 
sible to make the determination of this point for the rat for the 
reason that the means at hand of regulating the temperature of 
