MAXIMUM AND MINIMUM WEIGHTS 
NORWAY GRAY ? $ 
Figure 149. — Maximum and minimum weights of feral 
(Calhoun's) and captive (King's) female gray Norway rats. 
1 . My rats grew faster, although only the females 
attained greater maximum weights than 
King’s. 
2. My rats were less variable. 
3. The maximum weights of King’s rats of both 
sexes were heavier at 30 days of age for the 
later generations. 
4. Only King’s males of the later generations 
attained greater weight at 30 days of age than 
did my rats. 
5. Minimum weights of all generations of King’s 
rats were less than mine for all ages. 
6. Where changes in rate of growth are evident 
near the age of sexual maturity, the point 
of inflection arises at a later date for the mini- 
mum weights than it does for the correspond- 
ing maximum weights. This applies to both 
my rats and to King’s. 
The slower growth rate, the greater variability 
in weight, and the lower maximum and minimum 
weights of King’s rats are logical consequence of 
the crowded conditions of caging in which subor- 
dinate or younger rats could not avoid the undue 
stress exerted upon them by dominant and larger 
individuals. Under such conditions one would 
anticipate the following sequence of events: In- 
creased emotionality, increased firing of the hypo- 
thalmus, increased secretion of ACTH by the 
anterior pituitary, increased output of adrenal 
cortical hormones, and as a consequence decreased 
secretion of growth hormone by the anterior 
pituitary. Even the most favored rats felt the 
impact of this crowded social setting more than 
did my wild rats, who though dominant, less 
frequently found the necessity of engaging in 
interpersonal conflicts. The lowered minimum 
weights of King’s rats at weaning shows that the 
opportunity for stressful experiences were greater 
than for wild young. More favorable growth for 
the later generations of King’s rats may be attrib- 
utable solely to their more phlegmatic tempera- 
ment, which enabled them to respond less 
emotionally to stimuli within and without the 
cage. 
b. Other relevant data from King’s studies. Birth 
weight: Birth weights were significantly lower for 
rats conceived during July and August when 
temperatures in the animal rooms sometimes 
exceeded 90° F. King found adult weights to be 
positively correlated with birth weights. There- 
fore, higher summer temperature, as well as in- 
creased social interaction, may have contributed 
to the lower weights of rats born in my colony 
during the summer of 1948, as a result of both the 
hotter summer weather and the increase in number 
of rats over that of 1947. 
Survival: It is a general rule that females live 
longer than males. This was true for my rats 
(figs. 153 and 154) as well as for feral rats on a 
farm (28), and for domestic rats (65). King’s data 
is an outstanding exception to this rule. Of 1,353 
males studied 27.1 percent had died by 20 months 
of age and 28.8 percent of the 1,384 females. 
The sexual difference in the number dead or alive 
at the end of 20 months is not significant (X 2 = 
1.137). However, it may be recalled that King 
used a highly select group of rats of which no 
member of the litter died before 60 days of age. 
This suggests that some portion of the span of life 
from conception to 60 days of age (the age of loss 
of juvenile pelage) is critical to the determination 
of survival time for males, and that all later 
experiences produce an equivalent eflect upon the 
survival time for the two sexes. 
E. Maturation, Maintenance, and Decline of Repro- 
ductive Rerjormance in Female Rats. Biologists dealing 
with animals in their native habitat have been 
concerned primarily with reproduction as the 
264 
