TUNNEL SYSTEM 
OF 
SOUTH ALLEY BURROW 
MAY 17, 1949 
(WITH ITS TILE I EXTENSION) 
!*i ■ FOOD CACHE 
uVj'NEST 
p> CT 
5 
FOOD PEN 
5 LITTERS 
L-LACTATATING P* PREGNANT 
Figure 21. — The pattern of the tunnel system developed at the South Alley Burrow. The more circular outline of the older 
portion of the burrow with its many interconnections is typical of burrows which develop away from a limiting barrier, 
such as a fence or wall. The crosshatching is merely for making the tunnels more obvious. 
several occasions. In a few cases the burrows on 
both sides of the fence were excavated and it was 
found that the tunnels paralleled with the wire 
mesh fence forming a partition between. On 
December 20, 1948, a survey of the pen showed 
that there were 30 locations where passages into 
burrow systems lay immediately in contact with 
either Food Pen fence or the median barrier fence. 
Of these thirty locations 18 were characterized by 
having openings into parallel burrows on either 
side of the fence. 
In each of these 18 instances the initial burrow 
construction was adjacent to the inner surface of 
the fence nearer the source of food. Burrows on 
the opposite sides of these fences, that is away from 
the food source, were only initiated secondarily. 
Presumably this secondary burrow construction 
reflected the attempt of rats to enter these burrows 
from the outer surface of the fence. This restriction 
of initiation of burrows to inner surfaces of fences 
must be a derivative of the fact that rats frequent 
the inner surfaces of the fences much more often 
because trails (fig. 14) occur more often adjacent 
to the inner surface. The presumed origin of this 
differential placement of trails is discussed on pages 
67-70. 
D. Orientation of Mounds and Tunnels. Dirt ex- 
cavated from a burrow is dispersed about the 
entrance to form an arc subtended by an angle 
of slightly less or more than 180°. The chord 
across this arc normally lies at right angles to the 
tunnel leading into the ground. Where the terrain 
slopes, the initial tunnel tends to be in the general 
direction of the upward slope. I do not know 
the minimum slope necessary for this response. 
In the pen there was an approximately 5-foot 
increase in elevation from the northeast to the 
southwest side of the pen (a slope of circa 3.5°). 
The direction in which the mound was most 
frequently formed from the burrow entries indicates 
that the rats were at least able to perceive this slope. 
For 26 of the mounds shown in figure 14, surround- 
ing barriers did not conflict too much with the 
direction of excavating dirt. Of these, 24 gave 
24 
