Figure 37 . — South Alley Burrow hole 18 . This shows the excavations made during April 1949 at the time a female had a 
lifter here. Except for once in early October 1948 (fig. 19 ), no other excavations took place here. Shortly after this 
photograph was taken this female began sealing the opening (figs. 38 to 41 ). The position of hole 18 with reference to 
the tunnel system may be seen by examining figures 20 and 21. Photograph by U.S. Army Signal Corps. 
Twenty-four inches from the center of the nest an 
accessory exit or bolt hole had been formed. Dur- 
ing a snowfall in mid-December 1948 this bolt hole 
was sealed. The entry was initially plugged with 
grass. Following this the rats made small balls of 
mud inside the burrow and brought them out of 
another entry (as shown in the right hand illustra- 
tion of fig. 23), crossed over the snow making a 
muddy trail, and deposited them on top of the grass 
plug. This sealing is shown in figure 42. On 
January 6, 1949, four rats born in Area II and one 
born in Area III were in the nest. 
During the heavy rain of the night of May 16-17, 
1948, the roof of a nest cavity at the North Alley 
Burrow partially caved in. It had not collapsed 
sufficiently for a rat to enter the burrow at this 
point. For this reason the resultant response of the 
rats was assumed to be unrelated to the exclusion 
of other rats from direct entry into the nest cavity. 
Rather, this sealing was more likely related to the 
exclusion of light or air from direct access to the 
nest. The rats attempted to close this hole by 
dragging nine goldenrod stalks over it. Each stalk 
contained several leaves. This is shown in figure 
43. A similar response is shown in figure 36 where 
pine fasicles were used to seal the hole. 
With regard to the effect of lactation on this 
sealing behavior, it may be stated that 16 of the 
80 observed sealings occurred at burrows where 
rats were known to have young less than 2 weeks 
old. In most of the cases the evidence indicated 
that the sealing occurred very shortly after par- 
turition. Certainly the association of this behavior 
with the time at which the young lacked fur and 
37 
