result of the competition with the older sibs, I 
believe that the sustained poorer growth was a 
byproduct of the abnormal behavior developed by 
these rats as a result of this early experience. As 
already pointed out these L-13 rats developed a 
syndrome of abnormal behavior which reflected 
the stress they experienced in their relationships 
with their associates. 
There are two other cases where nursing rats 
probably competed with older young ,who, how- 
ever, were not their sibs. Prior to weaning, young 
rats do move into the nest cavities housing other 
young rats, if another female nearby has a younger 
litter. These two cases are for L-10 at the South 
Alley Burrow and L-28, 29 in Area I (see table 59). 
In each case the Maturity Index indicates a marked 
inhibition of growth over the prior litter. 
The reason for believing that L-10 at the South 
Alley Burrow would have exhibited more favorable 
growth, had they not been in association with 
other young, who were 8 days older, is that the 
other three litters (L-8, L-9, and L-ll) who were 
born elsewhere in the pen at about this time had 
much more favorable Maturity Indices. 
g. The nature of the stress associated with inhibition 
of growth. I cannot offer any conclusive proof of 
the nature of the situations which are presumed 
to be stressful to the organism. However the 
circumstantial evidence suggests that whenever a 
rat develops an avoidance of its associates, as it 
moves about the pen, it is then under sufficient 
stress to cause inhibition of growth. At least on 
the basis of what is known of the ACTH and 
pituitary growth hormone relationship (17, 18) 
one would anticipate inhibition of growth pro- 
vided this avoidance really does reflect a stressful 
situation (see discussions on pp. 260 to 264). 
A few examples will assist in showing why I 
have come to the conclusion that rate of growth 
depends upon the degree of stress associated with 
social interactions. 
The introduced female No. 11 (see table 28) 
was smaller and subordinate to the other intro- 
duced female, No. 10, who raised litters. She 
had her litter during May 1947 in Area III. 
This peripheral location reflected her lower social 
status. All three males of her litter (L-3) died 
while juveniles. The three female young showed 
an inhibited growth (chart 31, fig. 141) as juveniles. 
Why this should have been so, I can only guess. 
This guess is that they developed an avoidance of 
older rats based upon copying the avoidance be- 
havior of their mother. When they were 77 days 
of age they found themselves in a situation where 
they had to pass by the North Alley Burrow where 
lactating female No. 15 was residing, This female 
chased them whenever they encountered her. 
Furthermore, she persisted in chasing them for 
nearly 2 months after her litter (L-6) was weaned. 
During this time the L-3 females showed a marked 
depression in rate of growth. These three females 
were not bitten during these encounters, which 
fell into the category I have designated as “psy- 
chological drubbing”. After this experience these 
females developed the most marked avoidance of 
other rats that was noted, and they never achieved 
favorable growth. 
There were only two of the introduced males 
who lived very long. These were males 8 and 12. 
At the time of their capture on February 2, 1947, 
male 12 weighed 316 grams and male 8, 296 grams. 
Following the death of the other three introduced 
males, male 12 became an aggressive dominant 
rat, while male 8 became a subordinate rat who 
exhibited many of the characteristics of the syn- 
drome of the social outcast, including marked 
avoidance of other rats. Associated with the be- 
havioral differences, male 12 attained a maximum 
weight of 517 grams, while male 8 reached only 
434 grams. 
The markedly suppressed growth of Litter 13, 
(fig. 139, charts 21, 22, and 24), which was thrown 
into competition with their 23-day-older sibs has 
already been noted. Both the males and females 
of this litter were characterized by the syndrome 
of the social outcast, which included marked 
avoidance of other rats. 
Rats which developed avoidance of their more 
dominant associates were gradually forced to 
become members of less favored aggregates or 
local colonies to the extent that this avoidance 
behavior had developed. The more unfavorable 
the local colony, or as I have called it the lower 
the social class of the colony, the poorer had been 
both their growth and their reproductive success. 
See tables 56 and 57 and the accompanying 
discussion (pp. 214 to 216). 
The females of Litter 10 offer relevant material. 
See growth chart 32 (fig. 141). There was a 
wider range in the growth curves among the 
members of this litter than for any other litter. 
Two (Nos. 75 and 80) became very large rats, 
while the other two (Nos. 72 and 76) exhibited 
markedly depressed growths. The former two 
676-768 O — 63 16 
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