to learn all portions of the pen. 2. The popula- 
tion density was lower. This condition provided a 
minimum restriction due to social action. 3. The 
handling of the introduced rats at each capture 
was frequently a sufficiently disturbing experience 
to induce a shift in place of residence from the 
proximity of the place of capture. 
This latter point is amply documented in figures 
106 to 108. In these figures, it may be seen that 
there is a marked tendency for a capture to be in 
a different area or portion of the alley from the 
preceding one. This continual shifting about was 
quite in contrast to the greater localization of home 
range exhibited by rats born in the pen such as 
shown in figures 109 and 110. 
B. Dispersal With Reference to Age. Until 30 days 
of age the rat remains very close to its home burrow. 
In fact, there is practically no emergence from the 
burrow until about 25 days of age. I have never 
seen a 25-30-day-old rat go more than 10 to 15 
feet away from its home burrow. Although rats 
of this age may play among themselves about the 
surface of the burrow, as well as follow their 
mother about while nudging under her as if to try 
to nurse, their attachment to the home burrow is 
sufficient to prevent farther wanderings. However, 
with the onset of weaning there begins a gradual 
process of extension of the area encompassed by 
wanderings from the home burrow. Coupled with 
this there is frequently a shift in place of harboring. 
A measure of the dispersal was obtained from the 
records of place of capture in traps or from the 
burrow or harborage box in which the rats were 
removed. Since traps were most frequently placed 
near burrows, and since rats were usually observed 
to enter traps nearest to the burrows from which 
they initially emerged following the setting of 
traps, these records primarily reflect proximity to 
the current place of residence. These records 
(table 39B) were tabulated with reference to their 
distance in a straight line across barriers from the 
place of birth of the rats (see pp. 164 to 170). 
During the juvenile period of 30 to 55 days of 
age rats were found more frequently (0.65) within 
20 feet of the place of birth than at any later age. 
There is one peculiarity of dispersal at this age 
that requires further comment. It will be noted 
in table 39B that these young rats occurred more 
frequently at a distance of 50 feet from their place 
of birth than at 30 feet. An examination of the 
individual records indicated that the major reason 
for this increase at the 50-foot distance (i.e., 49-59 
feet) was the habit of the young of this age of 
changing their place of harborage to that which 
was the same or close to that occupied by their 
mother. Thus, when she exhibited a change in 
place of residence, her young at this age tended to 
make a similar change. This was discussed in 
more detail in the section on “mother and young 
interrelationships” (pp. 144 to 148). 
Through the next two periods of subadult life, 
56-85 days and 86-115 days, there was a marked 
alteration of the dispersal pattern such that an 
increasing proportion of the records occurred at 
Table 39B. — Dispersal of rats with reference to their age 
Age in days 
Sex 
Total 
Proportion of total captures with respect to distance across 
barriers from the place of birth to the point of later capture. 
captures 
10 ft. 
30 ft. 
50 ft. 
70 ft. 
80 ft. 
30-55 (juvenile) 
M + F* 
138 
0. 645 
0. 116 
0. 145 
0. 087 
0. 007 
56-85 ~ *. 
M + F* 
208 
. 510 
.250 
. 144 
. 091 
. 005 
86-115 (adolescent) 
M+ F* 
197 
. 411 
.279 
.213 
. 081 
. 015 
Total nubadult 
jM 
IF 
345 
. 496 
. 226 
. 183 
. 084 
. 012 
198 
. 530 
.227 
. 146 
. 091 
. 005 
Over 116 (adult) 
(M 
262 
. 076 
. 405 
. 271 
. 214 
. 034 
Ik 
185 
. 330 
. 314 
. 135 
. 086 
. 000 
Total 
{?.. 
607 
. 315 
. 303 
.221 
. 140 
. 021 
358 
. 464 
. 288 
. 151 
. 095 
. 003 
and irii i.ilr . showed nik h siiiiil.ii result* within each of the subadult age groups that they warranted grouping 
together. 
170 
