Figure 42. — Sealing of a burrow entrance with balls of mud. Previously this entry had been plugged with grass. After the 
snowfall the rats formed small balls of mud inside the burrow, came out of another entry, crossed the snow, and deposited 
them on top of the grass plug. 
form down in the burrow and bring out and deposit 
in the opening. The Purina food checkers were 
about the size of these wads of mud and were 
observed to be similarly used: February 11, 1949. 
A nest cavity at the burrow by Passage 3 in the 
North Alley caved in. The rats had plugged it 
with grass, pine needles, and Purina checkers. 
The other behavior was that of sealing the 
warped lids of the harborage boxes by lactating 
rats. Usually the lids of the harborage boxes 
fitted quite tightly, but occasionally they became 
warped so that a crack developed along a portion 
of the lid. In addition to the crack at the lid two 
other conditions characterized each of the ob- 
servations: (A) There were young in the box; 
in one case they were 17 days old, and in the other 
instance 10 days old. (B) Moist dirt was present 
on the floor of the box. It had been excavated 
from a tunnel leading from a hole gnawed through 
the box. In one instance the crack was sealed 
for a distance of 10 inches and in the other instance 
of 28 inches. The mud seal extended for an inch 
away from the crack both over the inner surface 
of the lid and against the side wall. In such cases 
the female rat had to stand on her hind legs in 
order to place the mud in position. Three hundred 
and seven-day-old female 43 at Box 19, and 257- 
day-old female 734 at Box 9 were the two in- 
dividuals involved. This behavior is probably an 
elaboration of that of sealing burrow entries with 
mud. 
H. Some Quantitative Aspects of Burrow Systems. 
During the excavation of 44 burrow systems on 
level ground 82 measurements were made of the 
42 
