reared litters, while the latter two did not. I shall 
only consider the extremes, Nos. 75 and 76, since 
the most conclusive data is for them. Female 75 
was an extremely aggressive rat, and with the 
exception of avoiding a few older females during 
1 947, she moved about without being disturbed by 
other rats. Information concerning female 76 
consisted entirely of recapture data. However, 
the location of her terminal capture and the rats 
with whom she was associated are particularly 
instructive (see table 53). She was a member of 
the socially lowest ranking colony which contained 
female members. There, two of her three female 
associates were the surviving sisters of Litter 13. 
Like them she was of stunted growth and never 
conceived. Because of the marked similarity of 
these three rats, and because of the general tendency 
of rats of similar history to aggregate, I suspect 
that female 76 also exhibited marked avoidance of 
her higher ranking associates. 
The information in the above five paragraphs 
forms the strongest supporting evidence that 
growth is modified by the stress associated with 
the development of the behavior of avoiding more 
dominant associates. The conditions which appar- 
ently favor the development of avoidance behavior 
to the point where it exerts a deleterious effect upon 
growth are as follows. They are listed in the order 
of the chronological age at which growth may be 
inhibited. 
1. Nursing competition with older sibs. 
2. The “copying” or “mimicing” of the 
avoidance behavior exhibited by their mother. 
3. Competition with older nonsib juveniles. 
4. Adjustment to the priority rights of adults at 
the source of food. 
5. Aggressive action of lactating females. This is 
particularly effective if a rat is forced to pass 
near the burrow of females who are not 
members of their own local colony. 
6. Competition of recently sexually matured 
males with adult males over estrous females. 
7. Territorial competition. These are the 
behaviors associated with exclusion and 
migration. 
Once avoidance behavior is developed, the 
hesitant actions expressed as a result of it are more 
likely to elicit attack by dominant superiors, and 
thus the behavior is maintained, and therefore the 
stress associated with such interaction maintains the 
suppression of growth below the optimum. 
234 
In conclusion it may be remarked that fighting 
to the point of receiving wounds is not the prime 
cause of inhibition of growth, despite the fact that 
such encounters are probably stressful to both 
participants. Most of the decrease in growth rate 
for males began about 113 days of age. Yet from 
115 to 180 days of age there was a low (mean 1.81) 
number of wounds received (see fig. 122). 
h. Trapping and handling rats as a source of stress 
inhibiting the maintenance of weight. Any stress 
exerted upon the rats, particularly when mediated 
through social interactions, appears to inhibit 
growth. One such situation followed trapping 
rats, or handling them after excavating their 
burrows. In either case following release the rats 
tended to shift their residence to some other local- 
ity from that in which they were captured. This 
necessitated the formation of new social relation- 
ships. The reason for believing that the formation 
of such new social relationships were stressful was 
that whenever a rat, following release, was noted 
to enter the harborage of a colony other than its 
own, fighting ensued. 
A number of records were available of weights 
at first and second handling during a period of 
capturing the rats. With few exceptions there was 
a decrease in weight between the first and second 
capture (see fig. 143). The point of particular 
note was that twice as much weight had been lost 
when 3 to 10 days had elapsed between capture 
than when only 1 to 2 days intervened. During 
the longer interval there was more opportunity for 
the effects of social disturbance to be realized. 
A criticism of the validity of these results and 
conclusions was that a small portion of the second 
captures followed a period of withholding food at 
the Food Pen. Thus, one might expect that some 
of the loss in weight might have been as a result of 
the exhaustion of food caches in the burrows and 
harborage boxes. Fortunately one portion of the 
sample was unbiased by any chance of being in- 
fluenced by food deprivation. These data pertain 
to the capture of rats between December 6, 1948, 
and January 6, 1949, when rats were only captured 
by removing them from their burrows. Food was 
continuously available in the Food Pen. However, 
the rats were exposed to extreme social disturb- 
ance since their burrows were destroyed and they 
were forced to join other rats in harborage boxes, 
or in burrows not yet excavated. Twenty-two rats 
were captured twice. There was an average loss 
