days) and rats which were considerably older than 
they. 
Table 33. — Number of paired aggressions in relation 
to age 
Age of the younger rat 
Difference in age 
Under 
1 50 days 
150 to 
300 days 
Over 
300 days 
Less than 50 days of age be- 
tween combatants (in- 
cluding sibs) 
3 
15 
19 
Over 50 days between com- 
batants 
30 
36 
24 
The young-to-mother bond serves another func- 
tion in the socialization of the young rat. When a 
female is in the terminal stages of lactation — this 
might be designated as the 25th to 38th day fol- 
lowing parturition — she is less antagonistic toward 
other rats including the young of other females. 
Where there are such females in the vicinity of 
other litters whose mothers have recently ceased 
lactation or who are nursing younger sibs, they 
attract such recently weaned rats. This leads to 
the formation of multilitter aggregations of juve- 
niles. One such case has already been mentioned 
in the history of the litters born to female 43. This 
case referred to the joining of female 44’s Litter 9 
by female 43’s Litter 8. Another case was observed 
on May 23, 1949. Female 734 was in Box 9 with 
her six 33-day-old young. She was still lactating. 
With them were eight 56-day-old rats and one 
70-day-old rat. The mothers of both of these in- 
vading litters had ceased to lactate. These close 
associations between members of litters which were 
born in the same general locality must facilitate the 
formation of more harmonious aggregations as 
adults. 
D. Period of Initial Sexual Behavior of Sub-Adult 
Rats. The age of rats at which definite sexual 
behavior first makes its appearance is about 80 
days. The terminal age of this initial period has 
arbitrarily been set at 115 days. By that time 
much of the full pattern of sexual behavior has been 
established. Although females of this age have not 
been seen in copulation, this must at times be 
accomplished since both females 15 and 75 were 
known to have conceived at the age of 94 days. 
Males of this age show nearly the complete sexual 
behavior, including smelling and pawing of the 
genital region of the estrous female, following the 
female, rolling at burrow entrances and passages 
through fences where the estrous females have left 
their scent, and even occasional attempts to mount. 
It is my impression that the sexual rolling is not 
nearly so well developed as among adult males; at 
least it is only fleetingly exhibited among subadults 
of this age, whereas fully adult males engage in 
rolling for periods of 10 to 30 minutes with hardly 
a pause. The 21 cases of sexual behavior observed 
for this age group of subadults fall into four 
categories (table 34). 
Table 34. — Early sexual experience 
Participants Number of cases 
Two subadults 2 
Subadult male and adult female 1 15 
Subadult female and adult female 1 1 
Subadult female and adult male 3 
1 All these adult females on the basis of their interactions 
with adult males were known to be sexually attractive at the 
times of these observations. 
One of the interesting things about this tabula- 
tion is that only 6 of the cases involve subadult 
females whereas 17 involve subadult males. Actu- 
ally only five cases involve subadult females since 
one observation of a subadult female being chased 
simultaneously by adults as well as a subadult was 
placed in both the first and last category. The age 
structure of the population during June and July 
1 948, when the 1 948 observations on sexual behavior 
were made, was interesting with reference to the 
frequency of sexual behavior. During this time 
there were 11 males and 10 females over 300 days 
of age as well as 23 males and 11 females in the 80 
to 115 day old range. Only two of the subadult 
females were noted in sexual activity and one of 
these advances was upon an estrous adult female in 
a similar fashion to a male. Considering the fact 
that there was a nearly equal number of adult and 
subadult females, and that 7 of the 10 adults were 
judged to have come into estrous during this time, 
as judged by their behavior, it is concluded that up 
until the age of 115 days most females are insuffi- 
ciently matured to be sexually attractive to males. 
The initial sexual experiences observed for fe- 
male 15 form an interesting set. At the age of 81 
days she was observed to be followed to the South 
Alley Burrow by 320-day-old male 12. After she 
entered the burrow, he engaged in a series of sexual 
rolls at the burrow entrance. As male 12 began to 
151 
