FOOD PEN 
Figure 19. — South Alley Burrow, October 6, 1948. (See legend above fig. 1 5.) 
About human habitations and within the experi- 
mental pen the most typical site of origin of a 
burrow was against vertical walls or fences. Such 
locations may be seen in figure 14. A typical 
example of such a location is shown in figure 28 
in which a rat built its burrow directly against 
the outer limiting fence. Whenever any object 
with a flat ventral surface is laid directly on the 
ground, rats in the vicinity begin burrowing under 
it. Between periods of trapping, the wooden 
traps used to capture the rats were placed in small 
piles. Figure 29 shows such a group of traps 
within a few days after they were placed in a com- 
pact group. The shallow tunnel constructed under 
the traps is shown in figure 30 following the 
removal of the traps. When such a group of traps 
is left in place for several weeks, the initially 
shallow tunnels are frequently elaborated into an 
underground burrow. Such a situation is shown 
in figure 31 . Here a large mound of earth may be 
seen in front of the traps. Since these traps were 
piled against the barrier fence the initiation of the 
burrow at this place represents a dual thigmotropic 
response to both vertical and horizontal surfaces. 
Where no opportunity for thigmotropic responses 
is available, the location is primarily influenced 
by the presence of overhead protective cover. 
Presumably this is a negative phototropic response. 
Within such locations there is possibly a minor 
thigmotropic attraction toward narrow vertical 
objects such as tree trunks or posts, but this reaction 
is not particularly pronounced. Away from man- 
made structures dense shrubbery satisfies this re- 
sponse. About human habitations the floors of 
porches and outhouses replace the more natural 
conditions; at least where these floors are elevated 
from the ground by several inches or a few feet. 
An abrupt slope in the terrain will be utilized 
in preference to more flat terrain if other conditions 
of the environment are equivalent. This response 
favors the development of a burrow with greater 
structural support, better drainage, and a more 
constant temperature. Where a nearly vertical 
slope begins abruptly from horizontal terrain, it 
is impossible to separate the influence of the slope 
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