Table 5. — Summary of characteristics of 44 burrow 
systems 
-POINT MOVING AVERAGE OF THE PERCENTAGE FREQUENCY 
DISTRIBUTION 
Item 
Total 
number 
Mean 
1. Exits 
300 
6. 8 
2. Chambers: 
a. Terminal 
38 
.9 
b. Tunnel at either end 
160 
3.6 
c. Total 
198 
4. 5 
3. Tunnel segments: 
a. To exits 
316 
7. 2 
b. Blind 
111 
2. 5 
c. Internuncial 
287 
6. 5 
d. Total 
714 
16. 2 
4. Number of weaned rats 
243 
5. 5 
a. Rats per exit 
. 8 
b. Rats per chamber 
1. 2 
c. Rats per tunnel segment . . . 
.34 
OISTANCE IN MM. OF CENTER OF CHAMBER TO 
NEAREST EXIT 
Table 6. — Characteristics of a presumed optimum burrow 
housing 1 1 adult rats and having 17 exits 1 
Tunnel segments 
Chambers 
Kind 
Number 
Kind 
Number 
To exits 
13 
Terminal 
2 
Internuncial 
20 
Tunnel at either 
Blind 
7 
end 
8 
Total 
40 
Total 
10 
1 See figure 50. 
Table 7. — Structural differences between circular and 
elongate burrows 
Structure 
Circular 
burrow 
Elongate 
burrow 
1. Maximum distance across bur- 
7.5 to 8.0 
18 to 20 
row. 
ft. 
ft. 
2. Balance between “internuncial” 
and “to-exit” tunnels: 
a. Internuncial 
27 
20 
(67.5%) 
(44.4%) 
b. To exits 
13 
25 
(32.5%) 
(55.6%) 
Figure 48. — The distance of burrow chambers from the nearest 
exit. 
These are the two fundamental differences be- 
tween the structure of the two types of burrows. 
These differences must ultimately have profound 
effects upon the lives of the rats inhabiting them. 
Within the circular burrow and on the surface 
above it each inhabitant will be brought into 
frequent and nearly equivalent contact with 
every other inhabitant. On the other hand, each 
inhabitant of the elongate burrow will have 
relatively fewer contacts with its associates and 
these contacts will be such that the farther apart 
in the burrow two individuals live the fewer will 
be their contacts. These conclusions are derived 
from the known differences in burrow structure, 
and upon the observation that each rat tends to 
concentrate its activity within and over a restricted 
portion of the burrow. 
The comments in the above paragraph refer 
particularly to the frequency of social contacts 
that each individual has with its associates. How- 
ever, the differences between the two types of 
burrows may also produce profound effects upon 
the ability of the individual to conceptualize and 
integrate the complexities of its physical environ- 
ment. In the circular burrow, even though an 
49 
