Figure 28. — Burrow initiated beside fence. The only burrows, of the many formed in the pen, which were not initiated along 
fences, under piles of traps, or from the inside of harborage boxes, were the North and South Alley Burrows. These 
were both started about April 1 , 1 947, following a period of 6 weeks when I excavated all burrows in the loose dirt beside 
the recently placed fence. 
rocks of over 200 grams while burrowing, they 
merely tunneled around them. Unfortunately, 
measurements were not kept of the larger rocks 
which were moved out of the burrows. Con- 
sidering that the median width of tunnels is 83 
millimeters it is possible that the upper range of 
rocks moved out of the burrows is as much de- 
termined by the dimensions of the rock with refer- 
ence to the dimensions of the tunnels as by the 
weight of the rocks. 
G. Alteration oj Burrows. Three behaviors, re- 
lating to usage rather than expansion of burrows, 
altered the appearance of the burrows. These 
were: (a) elevating the nest cavity, (b) excavating 
materials previously left in the burrow, and (c) 
sealing openings into the burrow. 
There appears to be some optimal distance be- 
tween the floor of the nest and the roof of the 
cavity within which the nest lies. After nesting 
material is deposited in the cavity the rat finds 
itself closer to the roof than it will tolerate. The 
rat then removes some of the dirt from the roof and 
transports it out of the burrow. As the nest be- 
comes soiled more nesting material is brought 
in, which, of course, places the rat in the previous 
situation of being too close to the roof. Through 
a repetition of this process the roof of the cavity 
approaches so close to the surface that its thin wall 
collapses. Where a nest cavity lies under a 
mound the cavity may become so elevated that the 
floor of the nest may be on a level with the surface 
of the ground. In some instances these cavities 
and tunnels in mounds are made directly into the 
mound above those previously constructed. At 
30 
