G. Social Stress and the Proneness to Enter Traps. 
The demonstration that the avoidance response 
to traps approached a normal distribution was 
not anticipated. Had the population been com- 
posed of individuals with considerable genetic 
variability such a normal distribution would 
have been anticipated on the basis of its probable 
dependency upon multiple factor inheritance. 
However, in the light of the fact that the original 
stock had been selected in such a way as to insure 
the maximum restriction of genetic variability, 
it was believed that variability in this behavior 
was mainly attributable to environmental in- 
fluences. Since the presence of food in the traps 
served as a reward or positive reinforcement in 
overcoming the avoidance response toward the 
trap, it might be presumed that the proneness to 
enter traps was associated with the degree of food 
deprivation. This cannot be ruled out, since data 
was not obtained on the food consumption of indi- 
viduals. The general impression was gained that 
all individuals were able to procure all the food 
that they needed, but that the degree of social 
conflict experienced varied between individuals. 
Variability in social conflict is reflected in the 
number of wounds characterizing rats as of May 
1949 (Tables 44 to 48, 50 to 55): Mean wounds 
for males 14.2 (a, 11.5); for females 5.7 (<r, 6.6). 
Range for males 0 to 61, with a median of 11; 
range for females 0 to 26, with a median of 4. 
Several lines of evidence indicated that the 
degree of stress became accentuated through later 
periods in the history of the colony. These pe- 
riods have been arbitrarily designated: 1. Rats 
born during 1947; 2. Rats born from March 
through May, 1948; 3. Rats born from June 
through October 1948. Comparative data for 
the rats born during these three periods and 
which lived for at least 277 days are shown in 
table 14. For both sexes there are consistent 
trends toward an increasing proneness to enter 
traps as births occurred later in the history of 
the colony. With males there were consistent 
increases in the mean number of wounds per rat 
through these periods as well as consistent de- 
creases in the rate of maturation. As discussed in 
a later section (pp. 220 to 221), slower rates of 
maturation are indicated by a higher Maturation 
Index. The frequency of being wounded is a 
very direct indication of social conflict among 
males and the resulting state of stress. The inhi- 
bition of rates of maturation is also considered to 
result from increased social stress. Among fe- 
males the relationship of these two indices (wounds 
and rate of maturation) of stress do not bear as 
consistent a relationship with the proneness to 
enter traps as they do for males. Apparent trends, 
however, are consistent with the situation for 
males. At the younger age, 150 to 277 days of 
age, the lowest incidence of wounds characterized 
those females born during 1947, whereas the 
highest incidence of wounds for the older age, 
278 to 400 days of age, characterized those females 
born during the later time. The picture for 
females was somewhat obscured by the fact that 
among the eleven females born during 1947 there 
Table 14. — The relationship between time of birth and (7) proneness to enter traps , (2) number of wounds, and ( 3 ) 
rate of maturation 
Time of birth 
Number 
of rats 
Mean num- 
ber of periods 
of exposure 
Index of 
trap 
proneness 
Number of wounds be- 
tween age in days of: 
Maturity 
index 
to traps 
per rat 
(median) 
150-277 
(median) 
278-400 
(median) 
(median) 
MALES 
1. 1947 
10 
11. 1 
. 430 
1 . 0 
5 0 
1.55 
2. March-May 1948 
34 
8.0 
. 817 
2. 4 
8. 1 
1.67 
3. June-October 1948 
29 
5.0 
1. 000 
3. 4 
10. 5 
11.25 
FEMALES 
1. 1947 
11 
13. 4 
. 462 
*0. 3 
**3. 0 
***1.67 
2. March-May 1948 
28 
7.4 
. 714 
1. 3 
3.0 
1.50 
3. June-October 1948 
25 
4. 9 
. 800 
1. 3 
4. 5 
11.25 
Omitting females 17, 20, and 25 born during 1947: *0.7; **2.2: ***1.55. 
95 
