8 
Figure 20 . — South Alley Burrow, May 7 , 1949 . (See legend above fig. 15 .) 
response from that of the thigmotropic response 
if the burrow is initiated at the base of the slope. 
Such instances have been observed near the shore 
of Chesapeake Bay. 
Whereas thigmotropic, phototropic, and geo- 
tropic (slope) responses may be considered as the 
primary ones determining the site of initiating a 
new burrow, the particular site selected will be 
influenced by its proximity to other goals, toward 
which the rats have previously oriented their 
movements. Of these, the proximity to food and 
water appear to be the principal ones. Such 
goals may be considered as secondary influencing 
factors. Social and cultural factors modify the 
use of space. A “no-man’s-land” devoid of bur- 
rows tends to develop between the centers of areas 
utilized by adjoining colonies. Such processes 
restrict the areas in which burrows may develop. 
These social factors may be considered as tertiary 
influencing factors. They are discussed in more 
detail under “Home Range” (pp. 160-170) and 
“Local Colonies” (pp. 203-213). 
A final factor which influenced the location of 
the site of initiation of burrows in the pen was an 
artifact of the structure of the barrier fences within 
the experimental pen. The barrier fences forming 
the limit of the central Food Pen, and separating 
the alleys from the areas was made of %-inch 
mesh wire cloth sunk into the ground to a depth 
that the rats did not dig under. However, the 
rats could easily detect the presence of other rats 
or the conditions of the physical environment* 
through this barrier. During the latter part of 
June 1948 a burrow was started against the barrier 
fence in the North Alley just opposite and across 
the fence from Harborage Box 23. While this 
burrow still consisted of a single entry and a 
single mound, an entry and a mound were devel- 
oped in exactly the same position on the opposite 
side of the fence in Area III. Within 2 weeks the 
expansion of the burrow on the North Alley side 
of the fence led to the formation of a second entry 
with its adjoining mound. This development was 
immediately followed by a parallel development 
on the opposite side of the fence. Similar instances 
of parallel burrow construction were observed on 
23 
