MEAN NUMBER OF ADULT RATS PER EXIT 
o 
Figure 50. — Relationship between number of exits, the size 
of the burrow, and the number of rats inhabiting the bur- 
row. In the estimation of densities of rats from sign sur- 
veys, a rough rule of thumb is that there is slightly less 
than one rat for each active burrow exit. There is no reg- 
ularity of the relationship between chambers per exit and 
number of exits. Therefore the mean of 0.66 chambers 
per exit must be used for any further computations. 
quite disintegrated. In fact, it remained as five 
completely isolated segments and two separate 
arms which connected with the newer portion of 
the burrow system. 
The relationship between elongate burrows de- 
veloped along fences and the ability of rats to 
maintain them through time is reflected by two 
burrows which arose about the same time in 
November 1948 (see fig. 14). One began in the 
South Alley near Passage 2. The second began 
in the North Alley near Passage 3. On the basis 
of the four criteria mentioned above each of 
these colonies was less well integrated socially 
than the neighboring older colony. 
This instability of the social structure was no 
doubt partially responsible for the rapid disinte- 
gration of these two burrows following their initial 
expansion to a size approximating that of the 
hypothetical elongate burrow system discussed 
above. By May 1949 the North Alley Passage 3 
Burrow had so completely disintegrated that no 
rats were then living in it. At this time the South 
Alley Passage 2 Burrow had fragmented into three 
separate units, despite the fact that it was inhabited 
by one adult male, four adult females, and five 
juveniles. It is believed that the elongation of 
these burrow systems was a factor contributing to 
the disintegration of both the social structure of the 
group and the physical structure of the burrow. 
The later discussion of colonies c and k (tables 46 
and 55, pp. 206 and 214) as they existed in the 
spring of 1949 pertain to the rats inhabiting these 
burrows. 
L. Harborage Boxes and Nests. The wooden har- 
borage boxes placed in each triangular area of the 
pen permitted observations that were not possible 
in the burrows. At this point comments will be 
confined to nests. Other activities relating to the 
usage of harborage boxes will be noted under other 
topics. 
Periodically the observer removed nests and 
other material deposited by the rats in the harbor- 
age boxes. This was done to prevent the boxes 
from becoming too fouled with rotting organic 
matter. Rats rarely cleaned out such material from 
the harborage boxes as they did regularly from 
burrows. In fact removal of old nesting material 
from harborage boxes was noted on only three 
occasions. 
Descriptions were prepared of a few of the nests 
observed. The number of times each of the five 
major materials were observed was as follows: pine 
fasicles (32), honeysuckle vine (38), leafy stems of 
HYPOTHETICALLY IDEAL BURROW STRUCTURE 
Figure 51 . — The hypothetically ideal burrow structure for the 
Norway rat. As discussed in the text, this pattern was 
derived from several separate approaches as to what con- 
stituted an ideal burrow system. 
51 
