June 4 (L-18, 3 days old): After a little over 48 
hours there were only six young left in Box 19. 
The fate of the six which disappeared is unknown. 
Two possibilities exist. They may have been 
eaten by males invading Box 19 while trailing the 
estrous scent of female 43. If this were the case, 
the question arises as to why all were not destroyed. 
The second possibility is that they were destroyed 
by their mother. Whatever the cause of the 
infanticide it must result in the surviving young 
securing a more abundant food supply while 
nursing. The six young were in one corner of the 
harborage box in a nest. There was another 
accessory nest in another corner and a small food 
cache in another. 
June 8 (L-18, 7 days old): Eyes were closed. 
Five hundred food pellets in Box 19 with the young. 
An extensive burrow system joined Box 1 9 through 
a hole in its wall. 
June 12 (L-18, 11 days old): Eyes still closed, 
and their dorsal sides at least are covered with a 
fine brown hair. Still six young. 
June 24 (L-18, 23 days old): Female 43 in Box 
19 with all six of her young. Later two of these 
young rats followed her to Box 24. This is proba- 
bly the earliest age that rats engage in independent 
locomotion outside the home harborage. Since the 
L-13 rats were 53 days old when L-18 was born, 
they produced no nursing competition for the 
younger rats. An occasional older rat other than 
female 43 was seen extending its head out into Box 
19 from the burrow which adjoined it. Although 
the identity of these rats was not known, it indicates 
that the L-18 rats must have had some noncompeti- 
tive associations with older rats during the time of 
their dependence upon their mother. 
June 10-25 (L-13, 63 to 78 days old): There was 
a total of 25 captures of the seven members of this 
litter. Twelve were about the North Alley Bur- 
row, six in Area III and seven in Area II. Since 
all members of L-13 appeared to be ranging over 
this section of the pen, it means that they were 
making associations with members of L-9 and 
L— 1 1 as well as their sibs of L-8. This develop- 
ment of interrelationships with many other rats is 
probably the most important phase of this period 
preceding sexual maturation. 
July 13 (L-18, 42 days old): They remained 
mostly about Box 19 although their mother has 
been noted storing food in Area IV. 
October 7: Female 43 in Box 21 with 180 day 
old L-13 male 54. 
October 25: Female 43 in Box 21 with 198 day 
old L-13 females 58 and 60. These are the only 
two surviving females of this litter. These records 
of 3 of the surviving 6-month-old young of L-13 
being associated with their mother during this time, 
while none of the 10 surviving members of L-8 
and L-18 were, may be purely coincidental. How- 
ever, it suggests a relationship between the nursing 
competition encountered by the L-13 rats and the 
degree of their dependency attachment to their 
mother. The reason for suspecting this is that the 
deprivation with respect to total quantity of food 
or frequency of feeding exerted upon the L-13 
rats by their older L-8 sibs should have enhanced 
the reward value symbolized by the mother in her 
nursing capacity. 
The records for February 1949, when all members 
of these three litters of female 43 were 7 or more 
months old, provide further data on their inter- 
relationships. Female 43 was in Box 20 with 10 
other rats. Two of these were again females 58 
and 60 of L-13 along with male 70 of L-8. The 
other seven members of this aggregation had their 
origin from scattered places and litters. Of the 
12 remaining surviving members of Female 43’s 
first 3 litters, records of their places of habitation 
are available for 8. Seven of these eight were 
centered in the small region between Box 20 and 
the burrow at Passage 3. This concentration of 
mother and adult progeny in a small area is without 
a doubt in part due to the competitive action of 
more dominant rats elsewhere in the pen which 
prevented their expansion of range. However, 
there were at least two other important factors 
which were probably in operation. These were 
the attachment to the physical structure of the 
environment about their place of birth and the 
attachment to other individuals with whom they 
had early associations. 
C. The Social Conditioning of Juveniles by Adults. 
Beginning at about 30 days of age young rats 
begin to wander alone away from the site of their 
birth. These excursions bring them into contact 
with adults with whom they have had little or no 
prior association. These contacts may occur along 
trails, at passages through the barrier fences, in or 
about other places of harborage, or at the source 
of food and water. Along trails adults usually 
ignore the juveniles. They cither walk right over 
them or crowd them off the trail. About har- 
borages, other than the juveniles’ own home, overt 
antagonism was rarely observed until the period 
148 
