The immature stage of the pelage lasts to the 
age of 83 days. This is the age when rats are just 
beginning to exhibit behavior indicative of phys- 
iological sexual maturity. The prime pelage 
lasts from 83 to 277 days of age. At about 130 
days of age all rats have developed the prime 
pelage. It is also interesting to note that this is 
the age by which most rats have also exhibited 
behavior indicating physiological sexual maturity. 
At this age the integration of the individual into 
adult society is usually just beginning. I am here 
speaking of the development of a stable adult 
role. Until this role is achieved the individual 
is behaviorally not sexually mature. This adult 
role becomes established between 130 and 277 
days of age, and accompanying it there are pro- 
gressively more signs of a loss of prime pelage. 
By 277 days of age physical growth has for practical 
purposes ceased. After this age there were pro- 
gressively more and more signs of an inability to 
maintain a prime pelage until by 450 days of age 
all rats had a thin pelage all over their bodies. 
In the figure this peroid was designated as that of 
the senescent adult pelage. More properly it 
might be called the senescing adult pelage stage. 
The true senescent stage probably began shordy 
after 450 days of age. One category, very thin all 
over, is not represented in the figures. The in- 
crease in the percentage of the population so 
characterized are as follows with the age in paren- 
thesis: 3(290); 10.5(340); 17(390); 31.5(440). The 
number of observations in each age were repec- 
tively 119, 65, 52, 29. On the basis of the last 
three percentages, a projection of the trend indi- 
cated that all rats surviving to 560 days of age 
would haVe a very thin pelage all over. 
There is one trend in these changes of pelage 
which upon first inspection appeared anomalous. 
Prior to assemblying the data I had anticipated 
that prime pelage would be maintained as long as 
bodily growth was proceeding. However, despite 
the fact that growth was still quite apparent 
between 130 and 277 days of age, there was 
progressive loss of prime pelage. It occurred to 
me that loss of prime pelage was due to alterations 
in the physiology as a result of stresses experienced 
in making social adjustments. In order to explore 
this notion the population surviving to May 1949 
was divided into categories of low- or high-social 
rank as previously discussed. Data on the pelage 
of these two categories were plotted as shown in 
fig. 145. 
The data for males are particularly instructive. 
Initiation of loss of pelage began over 2 months 
earlier for the socially lower ranking group, and 
likewise the 50 percent point of change occurred 73 
days earlier than for the higher ranking group. 
For females there is no such clear-cut difference 
between social rank and pelage changes. There 
is an indication that the socially low-ranking 
females began to lose the prime pelage sooner. 
On the other hand in later life they lose the prime 
pelage less rapidly. This latter is in conformity 
with an inadequately documented suspicion that 
many lower ranking rats develop a behavior of 
avoiding competitive situations. Between sexes 
the difference is mainly one of females maintaining 
their prime pelage longer. In all categories the 
amount of scar tissure present certainly contributes 
to the rapidity of loss of prime pelage. To what 
extent it is the determining factor, the data are 
inadequate to verify. However, since the thinning 
with many individuals also includes portions of 
the body not developing scar tissue, I am inclined 
to believe that differentials in the loss of prime 
pelage do represent some physiological change 
affecting regeneration of hair, which arises as a 
result of the stress experienced. Furthermore, 
more of the socially low-ranking rats exhibit thin- 
ning of the pelage at an early age (under 140 days 
for males and under 200 days for females) than 
can be accounted for by scar tissue (see fig. 122). 
C. Health and Mortality as Observed under the 
Field Conditions oj the Experimental Colony. 
a. Rats found dead or dying. During the course of 
the regular rounds of inspection of the pen several 
rats were found which were dying or recently dead. 
Thirteen rats (males 6, 8, 45, 53, 79, 651, 685, 
768, 868 and females 7, 17, 27, 78) were character- 
ized by the following conditions. They were 
very emaciated; they had difficulty in breathing — 
as they attempted to do so they wheezed quite 
audibly; locomotion was difficult — they wobbled 
as they walked or just made jerky hops on all 4 
feet; the pelage was in poor condition — the fur 
was sparse, matted, and not recently groomed; 
usually the back was so permanently arched that 
I could not force it back into its normal curvature; 
the gut was bloated; and most of these rats had been 
excluded from reentry to the burrows by their 
more healthy companions. Two of these rats 
were found dead in a hunched position with their 
noses thrust in a slight depression, which had been 
scratched out of the grass and soil immediately 
237 
