(0 
Figure 1 34. — Maturity Index Growth channels for males. The lower curve is the B/W ratio for the upper 20 percent of 
the records, while the upper curve is that for the lower 20 percent of the records. The two central curves represent the 
B/W ratio one-third the way toward the respective extremes from the mean. The standard age was assigned as that 
at which the fastest growing individuals attained 0.9 of their mature weight. 
in weight, were excluded in the calculation of the 
lifespan Maturity Index. 
This index has the advantage that it enables 
comparison in growth between sexes, the averaging 
of both sexes of the same social characteristics, or 
the comparison of rats who died young with those 
who lived to maturity. No further discussion of 
the Maturity Index is given at this point. In 
many other places in this monograph it is utilized 
to show the relationship between growth and other 
characteristics of the rats or the circumstances 
surrounding their life. 
A standard age has been assigned as 196 days for 
males and 283 days for females. This is the age at 
which the maximum growth curve for weight 
attains 0.9 K. (i.e., 0.9 of the mature weight). At 
this age males characterized by the minumum 
growth curves have a B/W ratio which was attained 
112 days earlier by males characterized by the 
maximum growth curves. A similar situation is 
true for females. Thus many individuals maintain 
a juvenile body type far into the period which may 
be termed adulthood for other rats. Such evidence 
as reduced aggression or sexual behavior in rats 
who maintain a high B/W ratio suggests that 
slenderness of body type in some way determines 
or is correlated with the persistence of juvenile 
behavior in adulthood. 
c. Chronological order of birth. An inspection of 
table 59 reveals that for 10 of the 16 comparisons 
of a litter with the preceding litter born in the 
same locality, the litter born later grew more 
slowly. Only two litters grew faster than the 
preceding one, while for four comparisons there 
were no appreciable differences in average growths 
(i.e., less than 0.1 variation in the Maturity Index). 
The general magnitude of the differences in 
growth associated with order of birth is revealed 
by assembling the Maturity Indices for litters in 
terms of birth as early litters (Litters 7, 11, 14, 9, 
and 8), middle season litters, (Litters 10, 15, 17-19,- 
22a, 16, 25a, 13, 18, and 30), and late litters (Lit- 
221 
