Figure 30. — The relationship of flat surfaces adjoining the surface of the soil to the initiation of burrows. The traps shown in 
figure 29 have been moved aside. Note the shallow tunnels which were under the traps. This is typical of the tendency 
of rats to burrow under any object laid flat on the ground. The stick passing through the center of this picture may be 
used for orientation with figure 29, where it protrudes from the right hand side of the center trap. 
times the mounds of excavated dirt become hollow 
shells. 
The second behavior, that of excavating materials 
previously left in the burrow, prevents a more rapid 
catabolism of the burrow system as a result of the 
first behavior. Both food and nesting material 
may be thrown out. Usually the excavation of 
such substances was at those times when dirt was 
also excavated upon enlargement of the burrow 
system. However, nesting material may be ex- 
cavated with little accompaniment of dirt. Al- 
though both the food and nesting material which 
were excavated usually showed some sign of decay, 
very fresh material of both kinds was occasionally 
excavated. 
The third and most complex behavior associated 
with the alteration of the appearance of the burrow 
was that of the sealing of entrances into burrows. 
Directed rather than accidental sealing is referred 
to here. The latter occurred occasionally when 
the earth excavated from one entry spilled over 
into and eventually covered an adjoining one. 
However, attention is here focused on the trans- 
portation of objects by rats and their deposition 
in and over the entries. In these cases the primary 
result was the sealing of entries. 
There were two general types of sealings. These 
two types seem to depend more upon the material 
selected than upon any other response associated 
with the sealing. The first type is a transient seal. 
Vegetation piled over the hole is the simplest ex- 
pression of this type (see fig. 36). The specific ma- 
terial used at any one time depends upon what 
happens to be available within a few feet, certainly 
32 
