GROWTH OF MOUNDS 
SOUTH ALLEY BURROW 
MOUND AREA = O 
BURROW OPENINGS ' □ 
Figure 24. — Growth of the surface of burrows. It will be 
noted that there were periods of constant growth rale, but 
that the later periods had slower rates. Very little addi- 
tional growth should have taken place had the burrows 
been followed beyond the third year. 
evidence of the dirt having been thrown downhill; 
and from most of these, observations showed that 
the tunnels were directed uphill. Observation of 
many other burrows started in the pen substantiate 
these generalizations. 
The initial tunnel segment usually terminates in 
a nest cavity. A second tunnel segment is usually 
constructed within a few days. It begins from the 
opposite end of the nest cavity or from the point 
where the nest cavity joins the initial tunnel seg- 
ment. The terminal end of the second tunnel 
segment, or perhaps a later one in the series, is 
directed upward to form a second entry into the 
burrow. All dirt from the burrow up to this 
point is carried back and excavated at the original 
entry. This second burrow exit has been termed 
a “bolt hole” (JO). The interpretation given by 
these writers to this entry is that it serves as an 
escape point for the rats in the burrow when they 
are faced with invasion by other rats or predators. 
Although the availability of such additional entries 
may serve for escape, I suspect that their origin is 
related to the distance which rats are otherwise 
forced to transport excavated dirt back to and out 
of the original opening. Such a secondary entry, 
or “bolt hole,” presents the appearance shown in 
figure 32. Initially it may be surrounded by much 
more dense vegetation. Most of these later open- 
ings serve as foci from which dirt is excavated as 
further expansion of the burrow occurs. 
These later elaborations of the burrow system 
follow the pattern of the initial expansion. How- 
ever, there are three responses which influence the 
pattern of the developing system of tunnels: 
a. The thigmotropic response to walls and other vertical 
barriers which extend underground. Whenever a bur- 
row is initiated against such a barrier, or later 
comes into contact with it, the subsequent exten- 
sion is in very close association with it. There is a 
great reduction in the frequency of tunnels extend- 
ing at right angles to the barrier. Since this is so, 
there is little opportunity for cross connections 
between tunnel segments. Thus, travel within the 
system is limited to going up and down the burrow 
with the occasional opportunity for going up to 
the surface along the way (See fig. 22). 
b. The deflection or bifurcation response in tunnel con- 
struction. As shall be discussed in more detail later 
there is a decreasing frequency of longer tunnel 
segments (See figs. 44 and 45). The median length 
of 780 measured segments was 298 millimeters. 
After such a distance the tunnel segment either 
(a) terminates in a nest cavity, (b) abruptly bends, 
or (c) bifurcates. By such a process tunnel seg- 
ments which are initially blind will come in contact 
with previously existing tunnel segments. When 
this happens circular movements by the rats are 
possible within their burrow (See fig. 21). Although 
interconnection may develop purely by random 
directions taken by the bent or bifurcated tunnel 
segments, there remains the possibility that the rats 
actually direct their tunneling toward previously 
constructed tunnels. 
c. Directional orientation response toward prior goals. 
As the wooden harborage boxes aged and became 
soft, the rats gnawed through them. They then 
dug a tunnel around the sunken box by keeping in 
direct contact with its outer walls. Excavated dirt 
was pushed into the box. In most cases radiating 
branches developed from these initial tunnels. 
During December 1948 and May 1949 all the bur- 
rows in the areas were excavated and mapped. It 
was noted that longer than normal tunnel segments 
tended to extend directly toward or actually reach 
