Table 60. — Representative maturity indices according to 
time of birth during the 1948 breeding season, and the 
approximate weights and measurements represented by 
these indices at 175 days of age 
Season Born 
Maturity index 
Early 
1.40 
Middle 
1.92 
Late 
11.20 
Males: 
Weight 
458 
429 
412 
Length 
252 
247 
244 
Females: 
Weight 
378 
356 
338 
Length 
237 
231 
227 
In order to gain insight into why birth during 
a later period of the breeding season causes in- 
hibition of growth, it is necessary to examine the 
growth charts (figs. 136 to 141) in the light of the 
circumstances surrounding the life of the litter. 
Near optimum rates of growth are represented 
by male 690 (chart 1, fig. 136) and female 48 
(chart 12, fig. 136). All other individuals, as 
well as litter means, exhibited less favorable 
growth. Inhibitions of growth, which arose at 
least by the time of weaning, as well as inhibitions 
arising later produced such depression from the 
optimum. 
d. Changes in rate of growth. There is a marked 
sexual difference in the likelihood of a change in 
rate of growth. For the 25 charts of growth in 
weight of the litters born during 1948 (figs. 136 to 
141), 13 of the 16 charts for males show one or 
more changes in rate of growth, whereas only 3 of 
the 9 charts for females exhibit such changes. 
The evidence that males experience more stress 
than do females (see data on wounds, fig. 122, 
migration, fig. 116 and health, table 64) suggests 
that the particular times when changes occur in 
the rates of growth are associated with changes 
in the intensity of stress. Males will be considered 
first. 
Five (Nos. 1, 11, 13, 19, and 27) of the eight 
charts exhibiting a decrease in the rate of growth 
were for the first litters born in their respective 
localities. Litter 10 (chart 3) may for the present 
purposes be considered as a first litter. It was 
born at the South Alley Burrow 8 days after litter 7 
(chart 1). The decrease in rate of growth fell 
between 100 and 125 days of age (mean 113) for 
those six litters. This is the age, based on be- 
havioral criteria, when the rats become sexually 
mature. This age for these litters fell between 
June 17 and August 7. This was a time when 
many females, particularly those born in 1 947, were 
in estrous. As a result there was competition 
among these males for the estrous female. Such 
competition was even more pronounced between 
them and the older males born during 1947. 
Prior to attaining sexual maturity, males of these 
litters were exposed to relatively much less slaressful 
situations. In particular they were not forced to 
compete with older, but not sexually mature, 
juveniles. The inference is that growth rate 
mirrors the degree of stress currently being ex- 
perienced. 
Males of two other litters (charts 9 and 30) 
showed a decrease in growth rate later in life. 
The decrease by Litter 25b was associated with 
the initiation of the spring breeding season. 
However no other litter showed a decrease during 
this time. I can associate no events with the 
decrease in growth rate of Litters 28-29 which 
began about January 3. 
There are eight clear-cut instances (see growth 
charts 3, 5, 10, 13, 16, and 25) where rate of 
growth increased. I can see no importance of the 
age at which changes occurred. However, seven 
of the eight inceptions of increase took place 
during the fall and winter when strife declined. 
One other litter deserves particular note. This is 
L-25b, chart 9. From 40 through 180 days of age 
there was a persisent increase in rate of growth. 
No other litter showed such a growth curve. 
Although one might assume that they were 
continually encountering more favorable conditions 
or making more favorable adjustments to them, 
there is no corroborative evidence for this. 
The five changes in rate of growth for females 
(charts 8, 17, and 24) represent too small a sample 
upon which to base any inferences as to cause. 
e. Further remarks on the chronological order of 
birth. As indicated previously the data on Maturity 
Indices show that litters born later in the breeding 
season tend to exhibit inhibited growth. The 
charts of growth in weight show this more precisely. 
We shall now examine a few of the sequences of 
litters born in specific localities. 
South Alley Burrow: Males of the first two 
litters (charts 1 and 3) grew considerably larger 
than the average, and the first-born litter attained 
the higher adult weight. The third litter shown 
223 
