were more stunted and did less well at maintaining 
their weight (i.e., Maturity Index, II. 2 to III.O). 
These two categories according to the number from 
each place of birth were: 
Maturity Index 
Place bom 
1.0 to 1.79 
II. 2 to III.O 
Alley-bom 
50 
28 
Area-bom 
25 
43 
This gave a Chi Square of 11.49 with a probability 
of such a difference arising by chance far less than 
0 . 01 . 
B. The Condition of the Pelage. The type and 
thickness of the fur changes with age. Before 
weaning the rat acquires a covering of short pliable 
hair which forms a soft gray coat. This is desig- 
nated as the juvenile pelage. As the rat matures, 
this juvenile pelage is replaced by longer darker 
hair. Among these are especially long, guard 
hairs. Color appearance during this state, which 
was designated as prime adult, varied from buff 
to brown or reddish brown. Later on in life the 
pelage began to thin. Thinning normally first 
appeared over the lumbar-sacral region. This 
thinning sometimes was accentuated by the devel- 
opment of scar tissue, where no hair regenerated, 
as a result of combat. In any case the fur gradual- 
ly became thinner all over. A few individuals 
developed such sparse hair after 400 days of age 
that the pelage must have been insufficient to 
provide adequate protection during inclement 
weather. Between 200 and 300 days of age the 
coat became suffused with white or very light hairs. 
This gave a grizzled gray cast to the pelage. These 
general characteristics denote the senescing adult. 
Detailed pelage studies were not made. How- 
ever, whenever a rat was handled a general remark 
concerning its pelage was usually made. They in- 
cluded the following: (1) juvenile, (2) mostly juve- 
nile, (3) one-half juvenile, (4) mostly adult, (5) 
adult (good, excellent, or thick), (6) brown, buff, 
reddish-buff, (7) thinning over lumbar region, (8) 
gray-grizzled, (9) thinning all over. There were 
806 such observations for which the age of the rat 
was known. One and 2 were grouped as juvenile, 
3 and 4 as transition to adult, 5 and 6 as prime 
adult, 7 to 9 as senescing adult. Changes in the 
percentage of these categories with ages are shown 
in figure 144. By taking the 50 percent point 
of the intersection of the curves, three stages may 
be designated. 
236 
AGE I N DAYS 
Figure 144. — Changes in the character of the pelage with age. Both males and females are included. 
