Figure 31 . — Mound of a burrow initiated under traps piled against the median barrier fence in the South Alley near passage 2. 
The rats excavated this burrow during September and October 1948. This is a typical reaction of rats to piled traps. 
The burrow systems in the North Alley near passage 3 and in the West Alley near passage 4 were initiated as the result 
of a similar response to piled traps. 
not more than 10 feet away. The vegetation may 
also be stuffed down into the entry and perhaps 
have mixed through it some loose earth. In either 
case the seal is sufficiently light that a rat can lift it 
up or squeeze through it. The second type is a 
more permanent seal, although even these are some- 
times reopened. This type takes on three general 
expressions: (a) loose dirt, perhaps mixed with 
rocks; (b) lumps of dirt; and (c) aggregates of lumps 
of dirt. In the latter case several small lumps that 
have been formed previously into moist wads are 
packed together to form a larger ball before being 
transported to the place of sealing. The loose dirt 
and rock seal is formed by the rat pushing the ma- 
terial from within the tunnel up to the opening and 
and gradually filling it from the inside. Dirt and 
rocks are pushed into place with the forefeet and 
nose or actually put in place with the teeth (see 
figs 37 to 41). Where either small or composite 
wads of dirt are used, the sealing may be done from 
the inside, otherwise the rats carry or push the wads 
or balls of dirt out of one entry and over to the 
neighboring one which is being sealed. 
There are three conditions which appear to in- 
fluence the expression of this behavior. They are: 
(a) a draft of air into a nest cavity; (b) the state of 
lactation; and (c) low social rank. 
With regard to the influence of a draft of air the 
description of specific instances will adequately pre- 
sent the relationship. During the latter part of 
1948 a burrow made contact with Harborage Box 
10 in Area II (see pp. 18-19). The tunnel followed 
around the outside surface of the box and a nest 
cavity was formed at about midway down one side. 
33 
