terminating lactation. This trailing behavior is 
most strikingly exhibited in the behavior of adult 
males following estrous females. In fact, it is im- 
possible to make any clear break in the behavior 
of the young trailing the mother to that of rats, 
85 days or older, who trail estrous females. Both 
trailing and oral inspection of the posterior ventral 
region of adult females by juveniles of both sexes 
continues from before weaning right through to 
sexual maturity. Although the trailing behavior is 
gradually lost by sexually maturing females, they 
maintain the behavior of oral inspection of the pos- 
terior ventral region of other estrous females. 
Close trailing of one adult by other adults is 
rarely observed, in other than sexual behavior, 
when it cannot as well be attributed to chance 
simultaneous movement in the same direction 
along the same trail. Despite the absence of close 
trailing by adult rats of the type observed in sheep, 
I suspect that juvenile trailing behavior and the 
attendant duplication of the action of the adult is 
influential in establishing the general characteristic 
of simultaneity of action so frequently observed. 
One of the common examples of this simultaneity 
of action is that of the periodicity of activity and 
rest by a local group of rats. 
Mutual grooming and oral inspection is early 
exhibited among juveniles and adults other than 
with their mother. An example of this is as 
follows: 
The participants were 236-day-old male 8 
and a 55-day-old juvenile. The action occurred 
on the mound of the South Alley Burrow. Male 
8 walked over to the juvenile and pushed his 
head under it in an apparent genital inspection. 
Then several times for 2 to 3 minute intervals 
they stood up facing each other while propped up 
by the multiple bases of a small tree. At these 
times they worked over each other with forefeet 
and mouth. There was no sign of antagonism 
between them. There is another behavior which 
is closely allied to these oral behaviors. This is 
the approach of two rats to the point of touching 
noses momentarily (p 179). This recognition 
behavior occurs between juveniles, between adults, 
and between juveniles and adults. Following 
this nose touching the two individuals may ignore 
each other or may engage in various types of 
interactions. Although the origin of this behavior 
is unknown, its frequent occurrence between 
juveniles and adults indicates its importance in 
the early formation of social bonds. 
Interactions between young sibs or between 
juveniles who are of a similar age, although not 
sibs, are a maturational phenomenon which further 
prepares them for integration into an adult 
society. In addition to the behaviors previously 
mentioned, one of the most common is that which 
may be placed under the general category of play. 
With the exception of those terminal phases of 
fighting, which include biting and loud vocalization, 
juveniles engage in pseudofighting play behavior 
with most of the motor actions characteristic of 
the adult sequence of behaviors from approach 
with nose touching, standing on hindlegs and 
sparring with forefeet, and tumbling fighting in 
which the participants roll over and over while 
grasping each other with legs and teeth, and finally 
the termination of the sequence with a short chase. 
The lack of serious antagonistic relations in these 
activities is shown by the fact that the sequence of 
activities may suddenly be terminated at any point 
with the participants then ignoring each other while 
remaining in close proximity. In fact it is rather 
doubtful that rats under 85 days of age are capable 
of injuring each other in a fight. On several 
occasions juveniles of the same or near the same 
age were captured in the same trap. Usually 
upon the approach of the investigator these 
trapped juveniles would engage in what to all 
appearances was a very severe combat. However, 
upon removing the rats from the traps they were 
never found to be injured from this fighting, 
despite the fact that they most certainly did grasp 
each other with their teeth. Juvenile rats also 
exhibit the adult behavior of passage guarding, 
but in a quite strikingly more playful manner. 
While in a play chase, one juvenile may arrive 
at one of the tile passages through the barrier 
fence. It will remain in the tile and block the 
passages through it of the other rat. From time 
to time the rat occuping the tile will kick out with 
its hindlegs at the other rat trying to push through 
the passage. This differs from the similar adult 
behavior in that the adult which blocks a passage 
with its body will actively chase away most 
approaching rats. 
This playful aggression of juvenile rats prepares 
them more effectively to participate in aggressive 
action with older rats. As may be seen from table 
33 there was infrequent aggressive interaction be- 
tween rats of the same age until they were quite 
mature, in contrast to the amount of aggression 
existing between subadult rats (to the age of 150 
150 
