September 15 (108 days old): Females 17, 20, 
and 25 together in Box 23. They had built a small 
nest of green herbs. 
The above events show that at least up until the 
20th day rats do not of their own action depart 
from the home harborage, although they may be 
transferred by the mother from one place of resi- 
dence to another. During the next 9 days they 
develop the ability to make short excusions from 
the home harborage. Between the 35th day and 
56th day after birth extensive independent excur- 
sions take place. Even so, close association with 
the mother, insofar as place of harborage was con- 
cerned, was maintained until 2 months of age. 
Beyond this time there was continued close asso- 
ciation among the three surviving female sibs. On 
February 4, 1948, all three (250 days of age) were 
in Box 9 in Area I. During this period of mid- 
winter they were the only rats inhabiting Area I. 
c. Three litters of female 43. The first three litters, 
L-8, L-13, and L-18, which this female had during 
1948 reveal additional information about mother- 
and-young relationships as well as the competitive 
relationships which may develop between members 
of consecutive litters. Figure 102 may be examined 
to determine the membership of these litters. The 
later sections on growth and on social behavior 
should be examined for further data concerning the 
fate of these litters. 
The six members of her first litter, L-8 were born 
in Box 20 on March 16, 1948. By the time they 
were 1 7 days old she had transported several hun- 
dred Purina food pellets from the Food Pen into 
this box with her young. They were not further 
disturbed until April 16 (31 days old) when they 
were observed in Box 28. On this date the mother 
was in a nest cavity in the ground beside Box 28. 
A hole gnawed through the side of the box con- 
nected the two nests. With her were the seven 
8-day-old members of her second litter, L-l 3 . One 
hundred forty food pellets carried from the Food 
Pen the previous night were in Box 28. Litter 13 
was born 23 days after Litter 8. This indicates that 
the mother conceived during the day following the 
birth of her first litter. This is the only case where 
such conception was known to occur, although on 
several occasions females were noted to be followed 
by males engaging in sexual advances on the day 
following parturition. 
On April 23 (L-8, 38 days old ; L-l 3,15 days old) 
all 13 young of the 2 litters were still with the 
mother in Box 28. The three-plus condition of all 
14 of her teats suggests that all members of both 
litters were still nursing. Certainly it was highly 
probable that nursing competition existed during 
the first 10 days of life of L-13 and perhaps a week 
or more longer. Following this disturbance by the 
observer examining the rats, the mother moved her 
15-day-old young to Box 21, where on April 24, 
they were found nursing. The six 38-day old L-8 
rats had followed her here, and though they were 
not nursing at the time of the momentary observa- 
tion they were huddled up against their mother. 
April 28 (L-8, 43 days old; L-13, 20 days old): 
The L-l 3 young were nursing their mother in Box 
21, where she had brought at least 100 food pellets 
during the past 4 days. The L-8 young were not 
in any of the boxes in Areas II and III. Pre- 
sumably they had just moved to the harborage 
where they were noted 15 days later on May 13. 
By this date, when the L-8 young were 58 days old, 
and for the previous few days of trapping, they were 
mainly concentrated about the burrow along the 
northeast outside fence in Area II opposite Box 
17. Here they joined Litter 9, which was a few 
days younger. At this time the mother of L-9, 
female 44 was in the terminal stages of lactation, 
and had not conceived a second litter. This female 
transported food into this burrow and its vicinity 
until these litters were between 60 and 70 days of 
age. Following this she shifted her range slightly 
over into the burrow by the fence opposite Box 10 
in Area II, although most of the members of both 
L-8 and L-9 remained about the burrow opposite 
Box 17 until about 100 days of age. Other in- 
stances of juveniles initiating their adoption into 
other litters will be cited later. 
May 13 (L-13, 35 days old): Female 43 was in 
Box 29 with three of this litter. The condition of 
teats indicates that she was still lactating despite 
being in the fourth day of pregnancy. 
May 14 (L-13, 36 days old): Female 43 was in 
Box 26 with one of her L-l 3 young. Three others 
were in Box 22 10 feet away. These observations 
of May 13 and 14 indicate that there was still con- 
siderable attachment between the L-l 3 young and 
their mother, and that perhaps she was still nursing 
them. 
On the night of June 1-2, 1948, 12 young of L-l 8 
were born to female 43 in Box 19. During the 
following night she must have been away from the 
young most of the night, since she was observed 
over long periods to be followed by a pack of males 
attempting to copulate with her. 
147 
