more distant points from the place of birth. 
Through these subadult periods both sexes ex- 
hibited similar patterns of dispersal. However, 
there developed a marked sexual difference in 
dispersal as adults. Adult females exhibited no 
increase in dispersal from the time of maturing 
sexuality (85-115 days). Adult males, on the 
other hand, exhibited a marked shift away from 
their place of birth. The character and causation 
of this latter shift by males will become more 
apparent in the discussion of “male dominance” 
(pp. 198 to 202). 
Four choices of dispersal were open to the matur- 
ing rat: (1) It might remain about its place of 
birth; (2) It might invade an area inhabited by 
adults whose social rank was higher than that of 
the adults near its place of birth. Such dispersal, 
which required development of subordinate status, 
was rarely observed. (3) It might invade an area 
inhabited by adults who had subordinate status 
with reference to the adults about its own place of 
birth. Such a situation favored the development 
of favorable social status by the invading rat in its 
new home. This was a regular phenomenon 
involved in the invasion of the northwest half of the 
pen by rats born in the southeast half. (4) It 
might invade an uninhabited area. 
Thus, dispersal from region of birth is a function 
of inability of certain individuals to compete 
successfully with their associates. Furthermore, 
dispersal occurs in those directions which result in 
either a reduced frequency of social contacts or in 
which the invading rat is more likely to dominate 
resident ones. 
C. Intercolony Affiliations. As discussed in the 
previous section on home range, each individual 
tends to remain near the place of its birth. Here 
it becomes a member of a local colony formed 
by its mother, its sibs, and a few other adults along 
with their progeny. Later some individuals may 
be forced to emigrate, and a few others emigrate by 
choice in seeking more favorable localities for 
living (see discussion of female 44 and her sibs 
pp. 173 and 199 to 200). Although such shift 
from the place of birth do occur, as do shifts during 
later life, they are still of infrequent occurrence. 
For the most part each individual belongs to a 
local colony whose membership is stable over 
periods of weeks or months. This stability of aggre- 
gations particularly applies to the period of rest 
within the place of harborage. 
At some times and places intermingling between 
members of different colonies occurs. This inter- 
mingling occurs particularly where there is some 
common goal to which they are attracted. In 
the pen, the single central Food Pen formed 
such a place of frequent associations. Yet, despite 
their membership in different colonies, there was 
much less antagonism expressed among them, so 
long as the supply of food and water was available, 
than there was when rats from one colony ap- 
proached the place of harborage of another colony. 
Each local colony habitually developed a single 
route of travel to the Food Pen which was used 
more than alternate routes. Finally these routes 
were such that there would be the least opportunity 
of encountering members of other colonies. An 
example of this is shown in figure 118. On 
April 3, 1948, a 3-foot wide strip was hoed bare 
of vegetation around the outside of the Food Pen, 
from the corners of the Food Pen to the passages 
through the median barridr fence opposite to each 
corner, and for a 10-foot strip from each of these 
passages to the outside limiting fence (fig. 3). Over 
this hoed earth trails were reestablished. The rel- 
ative intensity of their usage was determined by 
such criteria as the amount of compaction of the 
earth, and the amount of feces and urine deposited 
along them. 
From time to time the pattern of the most used 
routes changed as a consequence of changes in the 
location and membership of local colonies. There 
was, however, an overall symmetry to these routes 
with respect to the rats inhabiting the corner 
areas. Area I rats most frequently used Passage 
1; Area II rats, Passage 2; Area III rats, Passage 
3; and Area IV rats, Passage 4 (fig. 118). In each 
area the harborage boxes nearest this most used 
passage were more frequently used than were those 
nearer the lesser used passage at the opposite 
comer of the area (see table 18 and fig. 74). This 
pattern of movement reduced to a minimum the 
number of contacts between rats inhabiting 
different areas, while on their way to and from the 
Food Pen (see discussion on pp. 83 to 85). 
On the positive side, affiliations between colonies 
were enhanced by the tendency of a few adult 
females to visit their place of birth. The most 
striking instance of this was for female 28. She 
was born at the South Alley Burrow during August 
1947, but during the following winter settled in 
the adjoining Area I, where she was the dominant 
female. During the spring and summer of 1948 
171 
