"BURROW METABOLISM" AS SHOWN BY 
EXCAVATIONS AT THE NORTH ALLEY BURROW 
Figure 26. — See legend for fig. 25. 
frequently the case with primiparous females this 
first litter did not survive. On the night of April 9- 
10 she began the burrow. At the time of the birth 
of the second litter on April 12, 1949, the burrow 
consisted of a single short tunnel terminating in a 
large nest cavity. The nest was made entirely of 
fresh green grass cut from a location only 10 feet 
from the burrow entry. A path was trodden into 
an extensive plot of grass but the material for the 
nest was acquired by completely denuding a patch 
of grass only a foot in diameter. This was an 
accentuation of the tendency of rats to secure 
nesting material close to the harborage site. 
Within a few days after the birth of the young, a 
second tunnel segment was constructed from the 
terminal end of the nest cavity and extended to the 
surface to form a second entry. While the young 
were between 10 and 17 days old, the mother made 
an extension of the burrow. The dirt from this 
burrowing was excavated from the second entry. 
Both entries to the burrow system were kept covered 
with piles of grass cut by the mother and deposited 
over them. Most of the new burrow extension 
consisted of a cavity which served as a place for 
storing food. Shortly after this food-cache cavity 
was completed the secondary entry was perma- 
nently sealed with dirt by the mother rat. 
There is considerable anticipatory implication 
to the sequence of burrow construction in such 
instances. Initiation of the burrow and the build- 
28 
