K* 263 (MAXIMUM) 
AGE IN DAYS 
Figure 1 33. — Growth in body length of females. See figures 1 30-1 32 for comments on procedures in developing these curves. 
Shortly after 80 days of age the mean data as well as the minimum data indicated an abrupt change toward a slower 
rate of growth. The maximum weights (the upper 20 percent), which presumably represents rats living under more 
favorable conditions did not show this depression in rate of growth. 
both weight and body length are presented for a 
few litters. In most cases where there was a 
change in the rate of growth in weight there was 
a corresponding change for body length. 
b. The Maturity Index. I soon noted that young 
rats were proportionately thinner than older ones. 
In other words, as rats become older the ratio, 
B/W, of body length (B) to weight in grams (W) 
became less. Furthermore, slower growing rats 
at any age were thinner than faster growing ones. 
This suggested that the B/W ratio might serve as 
an index to physical maturity. 
Four curves of the B/W ratio were calculated: 
a. Maximum B/ divided by maximum W. 
b. B at one-third range from mean to maxi- 
mum divided by W at one-third the range 
from mean to maximum. 
c. B at one-third the range from mean to 
minimum divided by W at one third the 
range from mean to minimum. 
d. Minimum B/ divided by minimum W. 
As shown in figures 134 and 135 this resulted in 
three growth channels. These were designated as 
growth channels I, II, and III. I represents the 
most favorable growth (fast-growing rats with a 
large adult weight), whereas III represents the 
least favorable growth. From the three repre- 
sentative rats shown in each figure it may be seen 
that each individual tends to follow a particular 
channel of this growth index. 
The B/W ratio at each handling was calculated 
for each rat. Each B/W ratio was assigned to one 
of the three Maturity Index growth channels. 
Thus, an individual which was handled 10 times 
might fall in growth channel I five times, II four 
times, and III once. An average channel position 
was obtained; in this case 1.6. In other words 
this individual fell on the average along a channel 
0.6 of the way between the centers of channels I 
and II. Such means were calculated for each rat 
and are designated as its Maturity Index, or more 
briefly its M.I. Rats frequently lost considerable 
weight during a period of trapping or excavating 
burrows. Normally this weight loss was recovered 
by the next period of handling. In order not to 
unduly bias the Maturity Indices by these disturb- 
ances introduced by the observer, all noninitial 
captures in a trapping period, which revealed loss 
220 
