(chart 5) grew slower than average during the 
juvenile period, but because of later increases in 
growth rate it actually surpassed the second-born 
litter in adult weight. The fourth-born litter 
(chart 9) fluctuated about the average growth, 
while the last-born litter (chart 10) exhibited a 
growth curve considerably suppressed below the 
average. Adequate data for females were available 
for only two litters (charts 7 and 8). Here also, 
despite a later increase in rate of growth, the 
second-born litter exhibited more inhibited growth. 
North Alley Burrow: Because only one male 
survived in most litters (charts 13 and 16), com- 
parisons are not justified. For females (charts 15, 
17, and 18) there were progressively more inhibited 
growth curves. Despite changes in rate of growth 
for the second litter, the mean maximum adult 
weights for the first to the third litter were respec- 
tively: 511, 464, and 400 grams. 
Area I males (charts 27, 29, and 30): Despite 
a decrease in the growth rate of the first-born litter 
at about 110 days of age, which resulted in a 
lowered adult weight, this litter did exhibit a 
more favored growth until 200 days of age than did 
the second-born litter. The third-born litter 
(chart 30) grew much slower than did either of the 
two earlier born ones. 
Area III: Although both the males and females 
(charts 21 and 24) of the second litter exhibited 
poorer growth than did the first litter (charts 19 
and 23), the third litter males (chart 25) equalled 
the growth of the first litter. An understanding 
of why a later-born litter may have more favorable 
growth requires consideration of the interval of time 
between litters. 
The later in time rats were born, the greater were 
the number of their associations with other rats. 
In order to obtain a more quantitative picture of 
the relationship between time of birth and growth, 
the population was divided into two nearly equal 
groups. These consisted of (1) rats bom during 
1947, along with the first litters born during 1948 
(through April), and (2) rats bom from May 
through October 1948. See section D of table 61 
and figure 142. It may be seen that those born 
while the population was smaller had the most 
individuals in the most favorable Maturity Index 
class interval and continually less in those class 
intervals indicating more inhibited growth. Those 
bom during the later period showed just the oppo- 
site trend. These data were grouped as favorable 
growth (Maturity Index 1.0 to 1.79 and poor growth 
Maturity Index 11.20 to III .0) . Theresultant 2X2 
table of the number of early and late born rats in 
each growth category gave a Chi Square of 14.63, 
which has a probability of such a difference arising 
by change of far less than 0.01. 
From this and the previous data presented, 
there can be little doubt that growth is affected 
by the number of rats with which an individual 
must make adjustments. However, there is one 
question raised by the present data as it is revealed 
in figure 142. This is: Why is there an apparent 
exponential rate of change in the numbers of indi- 
Table 61 . — Time and place of birth in relation to growth 
Number of rats in 
each maturity index class interval 
Location of place of birth 
I.0-I.39 
1. 40-1. 79 
1. 80-11. 19 
II. 20-11. 59 
11.60- 
III.O 
N 
Mean 
A. 
a. d" cf alleys 
15 
15 
12 
10 
9 
61 
1.72 
b. cf c? areas 
4 
10 
10 
8 
19 
51 
11.21 
B. 
a. 9 9 alleys 
9 
11 
9 
3 
6 
38 
1.85 
b. 9 9 areas 
8 
3 
10 
9 
7 
37 
11.04 
C. 
a. Total alleys 
24 
26 
21 
13 
15 
99 
1.77 
b. Total areas 
12 
13 
20 
17 
26 
88 
11.14 
D TIME BORN 
a. 1947-April 1948 
27 
21 
18 
14 
8 
88 
1.79 
b. May-September 1948 
9 
18 
23 
16 
33 
99 
11.18 
230 
