MOST USED ROUTES OF TRAVEL TO THE FOOD PEN 
APRIL 8 1948 
Figure 118. — Most used routes of travel to the Food Pen on 
April 8, 1948. Thickness of lines is roughly proportional 
to the intensity of usage of each route. At this time there 
were six aggregates of rats. There was one in each of the 
areas, and one each in the large burrows in the North and 
South Alleys. Note that the routes of travel from the areas 
were such that there was a reduced opportunity of rats 
from the aggregate in one area contacting those from 
another in passing through the passages of the median 
barrier fence. Those rats living in the alleys selected the 
shortest route to and from the Food Pen. These alley 
inhabiting rats were at that time of higher social rank than 
their nearest neighbors in the areas. Near the Food Pen 
these area inhabiting rats diverged away from the higher 
ranking alley rats, and entered the Food Pen by passages 
which reduced the probability of their contacting these 
alley inhabiting rats. 
she frequently utilized the South Alley Burrow as 
a way station, while on her way between Area I 
and the Food Pen. Occasionally her sisters, who 
had young at the South Alley Burrow, would 
chase her away. However, for the most part their 
relations were amicable. Between 35 and 65 
days of age female 28’s young of litter 14 spent as 
much time residing at the South Alley Burrow as 
they did closer home in Area I. Visitations by 
adults to neighboring colonies was less frequently 
observed during the breeding season for rats living 
elsewhere in the pen. 
However, during the winter, when sexual activity 
was inhibited, there was a more marked inter- 
mingling of members of local colonies. This took 
two forms: (a) There was a reduced constancy of 
membership in local colonies. That is any par- 
ticular rat would shift its place of residence between 
several of the places of harborage available to it 
within the region of the pen it was then inhabiting. 
Thus, there was a moderate amount of flux of the 
composition of a group harboring at a particular 
place, (b) There was a marked attraction between 
members of different local colonies. The following 
record typifies this situation: 
1. January 23, 1948; snow on ground; observa- 
tions were begun before any rats were active 
above ground. 
First exchange: 5:48 p.m. A rat, which re- 
sided in Area III or in the burrow opposite 
Box 14, came through Passage 3, and ran 
over the runway through the North Alley to 
the North Alley Burrow, which it entered. 
Second exchange: 6:07 p.m. A rat left the 
South Alley Burrow and proceeded through 
the Food Pen without stopping, and went to 
and entered the North Alley Burrow. 
Third , 4th and 5th exchanges: 6:08, 6:13, and 
6:20 p.m. Three rats as for first exchange. 
Sixth and 7th exchanges: Just before 6:25 p.m. 
Two rats as for second exchange. 
Similar behavior was observed on a number of 
occasions during the winter of 1948-49. Thus, the 
North Alley Burrow served as a way station or 
assembly point for rats during the period just pre- 
ceding the first intensive period of feeding in the 
Food Pen. This temporary association included 
members of colonies of both lower and higher 
social rank than that of the North Alley Burrow 
itself. Whatever the cause of this intermingling at 
the North Alley Burrow, such a situation certainly 
contributed to the disorganization of the social 
structure of the North Alley Burrow, so apparent 
by the spring of 1949. 
D. The Function of Group Structure in Altering the 
Frequency of Contact Between Rats. A number of 
observations lead to the belief that adult rats 
assemble in groups, and that these maintained 
their identity through as much as several months 
time despite shifts in place of harborage. Such 
assemblies have been designated as “ingroups”. 
If such ingroups have any reality other than that 
which may be produced occasionally, even in a 
population whose members are moving at random, 
it might Ik: anticipated that the members of the 
ingroup have more contacts with other members 
of the same ingroup than would have been the 
case had there been no ingroup organization. 
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