slope and a heavy turf of grass distinguished this 
situation from any others of the many I examined. 
At this time the burrow was inhabited by 20 rats, 
and I strongly suspect that it had been in active 
use for several years without any additional en- 
largement. The configuration of the tunnels of 
this burrow is shown in figure 27. It satisfies the 
basic requirement of circular movement character- 
istic of complex well-integrated burrow systems. 
Most of the initial tunnel segments led directly 
into the embankment so that their terminal ends, 
where nest cavities or other tunnel segments 
branched off, were covered by several times the 
amount of dirt (see table 4) that characterizes 
burrows constructed on essentially flat terrain. 
This greater overhead cover of earth must provide 
greater structural support in preventing cave-ins 
as well as providing a more constant temperature 
with respect to that of the external air. Further- 
more, the fact that the entrances into the burrow 
were located on a sloping surface prevented as 
much rain water from entering as would normally 
funnel in on a flat terrain. In addition better 
drainage was facilitated by the sloping tunnels 
which connected one level of the burrow with 
another. These factors of greater structural sup- 
port, more constant internal temperature, and 
better drainage form what I believe are the more 
favorable conditions provided by a sloping terrain. 
C. The Initiation and Expansion of Burrows. Ac- 
cording to my experience with the rats in the pen 
and from studying burrows in various locations 
through Baltimore County, there are four cate- 
gories of conditions or responses which influence 
the location of the site of the initial entrance 
passage into a burrow: 
1. Thigmotropic response. 
a. Against a vertical surface. 
b. Under a horizontal surface. 
2. Overhead cover. 
a. Shrubs, low trees, etc. 
b. Floors of buildings, or other such surfaces 
which lie above the actual contact distance 
by rats. 
3. Slope of the terrain. 
a. An inclined terrain is preferred over a 
flat terrain. 
4. Proximity to major goals such as sources of 
food and water. 
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