They were not observed again until they were 40 
days of age on May 26. Just before sunset, while 
I was inside the pen, four of these young rats were 
observed. In contrast to the older rats they really 
scampered about. Their pelage was in the soft 
gray stage. One was seen to drag an orange rind 
from within the Food Pen into the drain tile then 
serving as Passage 7. These young rats seemed to 
be mostly utilizing the North Alley Burrow. 
Two days later (age 42 days) more detailed ob- 
servations were made from the tower. The young 
travelled back and forth between the North and 
South Alley Burrows by crossing through the 
Food Pen and utilizing Passages 5 and 7. More 
time was spent about the North Alley Burrow. 
One was seen on the mound of the South Alley 
Burrow sitting on its haunches cleaning itself. 
Several times these young transported food into 
the drain tile of Passage 7, where they sat and con- 
sumed it. The failure of rats of this age to transport 
food all the way to their burrows — the distance 
to the North Alley Burrow from the center of the 
Food Pen was about 25 feet — is a characteristic of 
rats of this age. Old males 8 and 12 came into the 
Food Pen and ate garbage. These old males 
(circa 223 and 281 days of age respectively) loped 
along with their backs arched, whereas the young 
rats darted back and forth. It took these males 
quite a while to adjust to these youngsters. Every 
time the old males would settle down to eat, a young 
rat would make a rapid movement into or out of 
the Food Pen, at which time the adults would run 
aside a few feet. This was repeated several times 
until the adults finally quit exhibiting avoidances 
every time the young moved rapidly. There 
seemed to be no antagonism between these two 
adult males and the juveniles. In fact they ad- 
justed to eating side by side. Finally all six young 
of Litter 2 came in and were simultaneously 
eating at a pile of corn meal and the two adult 
males departed. 
Other pertinent notes listed chronologically: 
May 30 (44 days old): Males 21 and 22 caught 
in same trap. 
May 31 (45 days old): Nursing had apparently 
ceased since the teats of the mother had regressed 
to the one-plus stage characteristic of non- 
lactating adults. Therefore, weaning must have 
occurred probably a week or 10 days earlier, at 
least. No further dependency relationships were 
observed after this although the four surviving 
members of this litter, males 16, 21, and 22, and 
female 15 spent part of their time, until sexual 
maturity began to set in at about 85 days of age, 
about the South Alley Burrow where their mother, 
female 10, remained. 
b. Litter 3. The seven young of this litter were 
born to female 11 the night of May 30-31, 1947, 
in Box 22, in Area III. When I opened this box 
on the morning of May 31, I could hear faint 
squeaks of the newborn young, but they could not 
be observed because the nest was roofed with 
foot-long, green, leafy stalks of goldenrod. The 
box was closed immediately because I did not 
wish to disturb the mother further. The places 
where eight of the goldenrod stalks had been 
clipped were found within 2 feet of Box 22. 
June 5, 1947 (6 days old): The young are no 
longer in Box 22, although during the previous 
3 days the entrance tile was noted to be plugged 
with soil and grass. It is presumed that this be- 
havior of plugging entrances to burrows or boxes 
by lactating females is one associated with the 
exclusion of other adults from the young. The 
neighboring Box 25 contained the largest nest I 
have yet observed in a harborage box. This nest 
was a hooded mass of green vegetation filling the 
entire front half of the box. The young were not 
heard nor was the nest disturbed sufficiently to 
determine the presence of the litter. 
June 8 (9 days old): The nest in Box 25 had been 
sufficiently compacted to reveal the seven young. 
Black pigment of erupting hair covered their 
dorsal side although the ventral side remained 
pink. They did not emit any sound. Two pieces 
of bread and a piece of meat carried from the Food 
Pen, presumably by the mother, were in the box. 
June 10 (11 days old): When I opened the lid to 
Box 25, the mother was in the opposite end from 
the young. She immediately went to the young 
and crouched over them, but as I continued to 
look she became somewhat agitated and ran out 
and into Box 21. The young were just about 
covered with fine hair. 
June 12, 1947 (13 days old): The young were in 
Box 20. They were well covered with hair but 
their eyes were still closed. 
June 14 (15 days old): Female 11 was crouched 
over her young. Although Box 20 was filled with 
freshly cut honeysuckle shoots, the nest proper was 
at the very rear. Practically the only vegetation 
immediately neighboring this box was honey- 
suckle vine. Similarly the last two nests were 
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