conclusion is based upon the fact that there were 
only half as many young per female in colonies e 
and f as in colonies a to d, despite the fact that the 
number of conceptions, as indicated by placental 
scars, were approximately the same. Second, as 
the stress increased, there was a reduction in the 
ability to conceive; and such individuals reared 
none of their young. 
These two phenomena are believed to be the 
most important ones through which population 
growth became inhibited in the presence of an 
abundance of food and ample space for the elabo- 
ration of new burrows. 
A concomitant effect of stress is the inhibition 
of growth (see table 56). By May 1949 there was 
a decreasing mean weight associated with decreas- 
ing social rank of the adult rats bom between 
March and October of 1948. This lower weight 
by members of lower ranking colonies was not 
merely a result of their having lost proportionately 
more weight during the 1949 spring breeding sea- 
son. Rather, the most important interrelationship 
between growth and social rank was that the lower 
the social rank of a group of rats the slower was 
their mean rate of growth during their entire life 
spans. This is shown by the higher Maturity 
Index ratings for the lower ranking rats. 
As a final evaluation of rank the mean of the five 
separate indices for each colony was calculated 
and designated as the “Composite Index” of stress 
(table 57). Now we might ask “What does this 
index mean?” Only an intuitive interpretation 
may be given. An index of nearly 1.0 means that 
the members of the colony encounter few diffi- 
culties in fulfilling their needs. As the index ap- 
proaches zero the members of the colony have on 
the average found it more difficult to satisfy their 
needs. In fact, there is a nearly ninefold differ- 
ential between the highest and lowest ranked colo- 
nies, with the intervening colonies forming a 
gradual transition between them. 
This social ordering of degree of stress quite 
obviously favors survival of the species. Colonies 
having a Composite Index of stress less than 0.5 
are destined to leave no progeny. Furthermore, 
the average index -calculated across all members 
of population — is well below 0.5. Thus, if rats 
failed to develop a class structured society they 
would not long survive as a species. One cannot 
help but believe that the evolution of social be- 
havior in the Norway rat has been such as to assure 
the development of class-structed society. 
6 . Growth , Health and Mortality 
A. Growth of the Individual. Measurements of 
weight and body length were regularly taken. 
Body length refers to the nose-to-anus span. 
Little variation in these for a particular age should 
have resulted from genetic variability, if the 
assumption of near homozygosity were actually 
correct. Therefore, differences in growth should 
mainly be attributable to environmental effects. 
The major environmental variables anticipated by 
the original experimental design were those which 
would arise from the interaction among rats. 
Social stratification, or differential social experi- 
ence, such as did develop, should include the major 
interactions. These might well affect growth 
through altering the amount of food attained and 
consumed, or more indirectly as a result of an 
upset in the endocrine growth physiology of the 
adrenal cortex-pituitary growth hormone balance. 
Such imbalance might arise as a result of stresses 
associated with social interaction. Such was the 
rationale with which I looked forward to relating 
growth to conditions affecting the members of the 
developing population. 
No data was obtained on how much food individ- 
ual rats consume. As discussed previoulsy (pp. 
99 to 105) considerable circumstantial evidence 
developed that all rats obtained access to more 
food than required to fulfill demands for optimum 
growth. There were signs of emaciation on only a 
few sick or old rats just prior to their death. 
Even stunted rats had good supplies of fat desposited 
through their mesenteries at autopsy. Such fat 
deposits in rats exhibiting stunted growth is just 
what might be anticipated as a result of stress. 
Evidences of stresses did develop in the form of 
altered activity cycles (pp. 117 to 126); avoidance 
of dominant associates (pp. 179 to 180); approach 
and withdrawal from places where encounters 
with dominant associates occurred, (pp. 194 to 
196); increases in the total interindividual con- 
tacts with population growth as indicated by 
amount of available trails (pp. 54 to 63) in relation 
to the number of rats present (pp. 244 to 246); 
subordinate status upon direct aggression from 
dominant associates (pp. 180 to 183); and sibling 
competition (pp. 147 to 148). All following doc- 
umentation of evidence supporting this contention 
that stress arising from social interaction was the 
major condition altering growth, must be viewed 
merely as supporting this hypothesis. The reason 
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