Figure 1 31 . — Growth in weight of females. The three curves are as described for males in figure 1 30. K’s represent the mean 
adult weights at 387 days of age (i.e. all records between 340 and 450 days of age). As for the males, the smaller 
females were also slower growing. Note also that females not only were smaller than males but that they attained 
0.9 K at a later age. 
line could be drawn through them such that there 
was little scattering of the observed point about it. 
K — y t0 and K—y m were determined from this line. 
y s for 15 representative ages between 40 and 520 
days were calculated as above oescribed. In 
actual procedure this was a time consuming process. 
A close approximation of y may be made by 
reading K — y from the fitted straight line for a 
particular age and subtracting this from K. This 
more rapid procedure was followed in the later 
development of growth curves for individuals or 
litters (fig. 136 to 141). 
Rate of growth may be judged by the age at 
which 0.9/f or 0.9 postweaning increment is 
attained. 
Age, jt, at 
0?jr - >-log (*—.9 K)) 
f40. 
These x's are shown in figures 130 to 133. Both 
weight and body length exhibit a consistent trend 
of greater age at 0.9 K (or 0.9 postweaning incre- 
ment) associated with lower K’s. In other words 
the larger an animal the faster it grows. It must 
be realized that this conclusion rests upon the 
pooled data for many individuals. Confirmation 
of this conclusion from growth curves of individ- 
uals or of small groups, such as litters, could not 
be made because of the reduced reliability of K 
and because of the obscuring of any such trend by 
the changes in rate of growth apparent in these 
smaller samples (see growth charts in figs. 136 
to 141). 
Males not only grow larger than females but for 
the same K’s, males grow faster than do females. 
That is for the same K, x at 0.9/T is lower for males. 
Growth of rats appears to be reduced from the 
optimum rate only at the time and to the extent 
that their physiology is upset due to stress. This 
stress interferes with maintenance of weight 
appropriate for a given age. Data on body length 
(table 58) are particularly pertinent 
218 
