height, width, and depth of tunnels. A few addi- 
tional measurements were available for a single 
large burrow on a 30° to 40° slope (see table 4). 
Table 4. — Typical internal dimensions of burrows 
Dimensions in millimeters 
Characteristic 
“Mini- 
Median 
“Maxi- 
mum” 1 
mum” 1 * * * 
Tunnel: 
Depth of floor from 
surface 
140 
194 
291 
Height 
52 
75 
99 
Width 
64 
83 
110 
Depth of roof from surface . . 
Chamber: 
88 
119 
192 
Depth of floor from 
surface 
170 
251 
320 
Height 
90 
145 
195 
Width 
146 
184 
248 
Length 
Chamber on 30° to 40° slope: 
155 
221 
298 
Depth of floor from 
surface 
300 
520 
650 
1 The “minimum” and “maximum” measurements repre- 
sent an attempt to designate the more typical extremes. 
Actual extremes were quite atypical. Therefore, the 
“minimum” measurement was that for which 10 percent of 
the items were smaller. Similarly, 10 percent of the items 
were larger than the “maximum” measurement. 
1. An adult rat has a standing height of ap- 
proximately 67 mm. at the shoulders. Here its 
width is 59 mm., or circa 70 mm. if the layer of 
loose fur is included. It is thus quite apparent 
that most tunnels will only accommodate the 
passage of a single rat. 
The greatest depth to which rats were observed 
to dig a burrow was 460 mm. This burrow was 
located in the loose soil in the pen adjoining the 
peripheral barrier fence shortly after the soil had 
been excavated in order to lay the wire mesh apron 
underground. Once this soil had become com- 
pacted only shallow tunnels were constructed here. 
The inference is that the depth to which a rat will 
burrow is in some way dependent upon an inter- 
action between the soil structure and the effort 
required by the rat in making the excavation. In 
other words, the greater the effort required to dig 
a given distance of tunnel, the sooner will the 
positive geotropic response diminish to the point 
that the rat will burrow along on a horizontal 
plane. 
2. Mensuration data representing 35 chambers is 
shown in table 4. From them inferences may be 
made concerning the maximum size of groups 
which might inhabit individual chambers. 
Sixty by one hundred mm. approximates the 
minimum floorspace to which an adult rat may 
accommodate itself in the nest cavity. Thus, the 
number of rats per nest cavity are approximately: 
Minimum sized cavity, 3 rats; median, 7 rats; 
maximum, 11 rats. This range is interesting, 
because it suggests that rats construct nest cavities 
no larger than is sufficient to accommodate a group 
of adults approximating the litter size. The 
greatest number of adult rats taken from a single 
chamber was 1 1 . One may use this observation 
as an inferential basis for supporting the belief that 
the number of individuals (mainly sibs) with which 
a rat makes its first associations determines the 
size of those groupings of adults in which each 
individual has its more intimate associations. 
I. Pattern of Work as Reflected in Burrow Con- 
struction. Forty-four of the excavated burrows 
were drawn to scale on cross-section paper at the 
time of excavation. From these drawings it was 
possible to make close approximations of the length 
of each tunnel segment. The frequency distribu- 
tion for 780 such measurements is shown in figure 
44. With the exception of the first class — interval, 
1 to 100 mm. — these reflect the frequency with 
which rats engage in tunneling operations of 
increasing magnitude of effort. An inspection of 
this frequency distribution indicates an exponential 
1 
curve of the type F=-^e x , where C is a constant. 
L/ 
The data was replotted as a three-point moving 
average on semilog paper (figure 45) in order to 
obtain a more precise understanding of the nature 
of the distribution. A straight line of the equation 
f= „ 1 , where d is the length of the tunnel 
J 1,040 ’ 6 
segment, makes a close approximation to this 
distribution. Having derived this equation to fit 
the data, the question is: Can we derive any 
insight into the behavior of the rat which might 
produce this distribution. 
In this discussion the frequency (/) of tunnel 
segments shall be considered to be inversely 
proportional to the work ( W ) or effort involved 
in constructing the tunnels. That is /=-—• The 
proportional amount of W required increases with 
44 
