Figure 1 3. — Technique of mapping the surface of burrows. A grid of string forming squares 1 2 inches across was placed over 
the burrow. 
the members of the first litter as well as the 7- 
week-old members of a second litter. 
By May 26, 1948, there had been both a doubling 
of the surface area of the mound as well as changes 
in the number and position of entrances into the 
burrow. See figure 18. By this date only 13 of the 
19 entrances which had been constructed were 
still in use. As a burrow ages some of the passages 
are accidently covered over as dirt is excavated 
from adjoining passages. In other instances, the 
rats intentionally plug passages (see pages 30 to 
42). Two such recently plugged passages are 
indicated by the solid black circles in the figure. 
One of the interesting aspects about the growth 
of the burrow system was that, although it radiated 
from the original site, the main expansion was in 
a southwesterly direction. The terrain of the pen 
sloped gently upward from northeast to south- 
west, covering a vertical rise of approximately 
5 feet over a horizontal distance of 100 feet. The 
direction of new tunnels tended to follow in the 
direction of the upward slope of the terrain, while 
the dirt was excavated and deposited in a down- 
hill direction. 
In the last two surveys of the surface characteris- 
tics of the South Alley Burrow the position of the 
surface trails was omitted. However, figure 14 
gives greater detail of the relationship of surface 
trails to burrow systems and other goals. In addi- 
tion to the continued growth of the burrow system 
in evidence by October 6, 1948, as seen in figure 19, 
the main point of note is the expansion of the bur- 
row system along the edge of the barrier fence. 
During the summer of 1948 there was an under- 
ground tunnel extension from SAB entrance 16 
directly toward the barrier fence. At the point 
16 
