28 
CURT P. RICHTER 
The data from this second method gave the following results. 
In the revolving drums at thirty days the rat is quite active, as 
is demonstrated by the high average at that age of 9000 complete 
revolutions per day. Its activity increases however from this 
age until at 100 days it reaches its maximum. This confirms the 
work of Slonaker (2), who found period of maximum activity lies 
between 81 to 120 days. After 100 days the rat’s activity falls 
off very rapidly so that at 240 days it is already less active than 
it was at thirty days, and at 600 days it is almost completely 
inactive, averaging at this age about 1000 revolutions per day. 
In the third method the relation of activity to age was deter¬ 
mined on the basis of the completeness with which the rat builds 
a nest for itself at different ages under ordinary conditions. For 
this purpose a standard situation was arranged in which the rat 
normally builds itself a nest. A square cardboard frame 3 feet 
wide and 1 foot high was placed on the floor and covered with 
wire cloth. A definite number of small strips of crepe paper 
(200) all of the same size and shape were evenly distributed over 
the floor on the inside of the frame. Four such frames were 
used. A single rat was placed in each frame for a given length 
of time, usually about twelve hours. At the end of this time the 
number of strips of paper gathered into a nest was counted. The 
ratio of this number to the total number available (200) was 
taken as measure of the animars activity. The nests included all 
strips within a radius of four inches of the densest spot. During 
this experiment all external conditions were kept constant. It is 
particularly important to keep the temperature constant for it is 
well known how easily the nest-building activity is changed and 
influenced by changes of temperature. 
In this way the curve marked “Nest-building” in figure 14 
was obtained. The number of strips formed into a nest is given 
in the column headed “Nest-building Units” at the right side of 
the curve. This curve is based on the records for three con¬ 
tinuous days of twelve animals of six different ages. The irregu¬ 
larity of this curve is very largely due to the very small number of 
records taken. 
