Table 36 . — Age and seasonal aspects of lactation 
Age In 
days 
March to September 
October to February 
Number of 
observa- 
tions 
Proportion 
lactating or 
recently 
lactating > 
Number of 
observa- 
tions 
Proportion 
lactating or 
recently 
lactating 
83-115. . 
24 
0 
23 
0 
116-196. . 
28 
.22 
86 
0 
197-277. . 
21 
.41 
71 
0 
278-363 . . 
27 
.45 
60 
0 
364-450 . . 
39 
. 54 
1 Rats for whom the teats were marked : -| — f- or -f- -( — |- • 
Table 36 also reveals that October to February 
was a period when young were not raised. Further 
seasonal aspects of the reproductive state of females 
may be seen in figure 104. Since all of these 
females were between 6% and 15 months old, they 
probably were capable of reproduction, except for 
seasonal effects upon their physiology. By Janu- 
ary', the teats of most females had regressed to the 
most inactive stage. From this time until June 
and July, when the teats of practically all females 
w'ere no longer completely inactive, there was a 
gradual increase in the number of individuals 
evincing some hormonal stimulation of the teats. 
Conceptions of weaned rats (table 37) also reveal 
the effect of season. One litter was known to have 
been conceived during November, but it did not 
survive longer than 2 days. If any other litters 
Table 37. — Temperature and season of conception 
Month 
Known 
conceptions of 
weaned rats 
(1947 and 
1948) 
Mean 
minimum 
nightly 
temperature 
(° F., 1948) 
January 
0 
22 
February 
36 
27 
March 
26 
37 
April 
30 
45 
May 
15 
56 
June. 
33 
64 
July 
29 
70 
August 
22 
67 
September 
0 
60 
October 
0 
49 
November 
0 
45 
December 
0 
33 
were conceived between September and January, 
no evidence of this was observed. Three of the 
four litters conceived during February were during 
the last week. Temperature was not the primary 
inhibiting agent. The temperatures in March, 
April, and May, when 71 rats conceived, was 
lower than the corresponding months of November, 
October, and September, when no conceptions 
occurred. It is possible that initiation and ter- 
mination of conceptions was under photoperoidic 
control. Conceptions did not begin until there 
were 11 hours between sunrise and sunset, and 
conceptions ceased as soon as this period was less 
than 13 hours. 
There was also a seasonal cycle in the number of 
males whose testes were scrotal or abdominal (fig. 
105). During December and January the testes of 
many males were retracted into the abdomen. 
When brought into a warm room, descent into the 
scrotum occurred within 30 minutes. Despite this 
immediate effect by temperature, the annual cycle 
showed no close correlation with temperature. For 
example all males had scrotal testes in April but 
less than 50 percent did in October and November, 
when the temperatures were comparable to April 
(table 37). Here again photoperiodic control is 
indicated. During April there was 12.5 or more 
hours between sunrise and sunset, but less than 12 
during October and November. 
In addition to age and season, factors affecting 
growth and physical maturation influence repro- 
ductive success, if not also sexual maturation. This 
is discussed in part under the topic of “Stress and 
reproduction” (pp. 214 to 216). Further perti- 
nent data is presented in table 38. It is quite 
apparent from this table that those conditions 
which inhibit growth also reduce the likelihood of 
conceiving or the success of rearing young. My 
observations are of little help in delineating the 
extent to which altered physiology, behavioral 
immaturity, or the development of abnormal be- 
havior contribute to the lowered reproductive 
success of these slower growing females. 
4 . Utilization of Space 
A. Home Range. Home range refers to the total 
space occupied by the individual as it engages in its 
various activities from day to day. Certain aspects 
of this problem have already been covered in the 
earlier sections on orientation (pp. 71 to 82). 
In these sections emphasis was placed upon the fact 
