In order to investigate in more detail the nature 
of the contacts of members of the population with 
members of ingroups, it was decided to examine a 
few groups of rats, with respect to which there was 
particular reason to believe that they each formed 
a real ingroup. These four groups were as follows: 
Group 7: East Alley Ladder Association of males. 
On March 4, 1949, six males, Nos. 54, 56, 657, 662, 
669, and 873 were captured under the paint- 
bucket stand of a small stepladder, which had been 
laid on the ground in the East Alley during the 
previous January. This was certainly a low-rank- 
ing group of males (see p. 214 and tables 55 and 
57). This harborage site was continuously used 
until the study was terminated in May 1949. On 
April 25 three marked rats, males 57, 873, and 662 
were noted using this harborage. On May 17, 
1949, males 56, 83, 662, 868, and 873 were cap- 
tured at this location. Thus nine of the rats, which 
later analysis (table 39C) showed to be members 
of this ingroup, were actually observed using this 
particular harborage. However, the analysis of 
the group stemmed directly from the initial six 
members. 
Group 2: Rats associated with female 44 on Janu- 
ary 6, 1949. On this date females 44, 62, 58, 48, 
922, and males 29, 49, and 670 were caught in 
Harborage Box 12. Each of these rats was at least 
6 months old. Four, 48, 49, 670, and 922 were 
young of female 44, and one, 29, was her sib. 
Following my disturbance of them, four shifted to 
a new and very unusual harborage location. These 
four rats, numbers 44, 48, 29, and 670, built a nest 
in the recorder shelter at one of the four entrances 
into the Food Pen. Until I disturbed them 2 days 
later, they maintained a clean nest here which 
indicated that they had prevented other rats from 
utilizing this passage into the Food Pen. The 
entire eight rats were utilized as the presumed 
ingroup. 
Group 3: The five females living in the burrow 
adjoining harborage box 25 on February 17, 1949. 
The nine rats living here on this date were: males 
742, 909, 918, and 87, and females 37, 915, 71, 75, 
and 352. Four of these five females were still here 
in May 1949 (see table 48). Only the five females 
in this group were utilized as the members of an 
ingroup, with which the paired associations of the 
population were analyzed. This was done to keep 
the members of the ingroup as near six as possible. 
Group 4: The six lactating females taken from the 
central (SW) portion of the South Alley Burrow 
on May 17, 1949 (see fig 21). All of these six 
females were born in this burrow in the spring and 
summer of 1948. During 1949 they formed part 
of the harem of 13 females associated with the most 
dominant male, No. 49. 
These four groups were assumed to be representa- 
tive of the organization of the population. In 
order to assess the reality and function of group 
organization it is necessary to know how this con- 
trasts with a population whose members move 
about at random, and assemble in groups purely by 
chance. An approximation of this situation was 
made by selecting at random six rats and desig- 
nating them as a “random group.” Four such 
groups were selected. 
A tabulation was prepared of the number of 
paired associations which each member of the popu- 
lation had with each member of these groups. The 
capturing of two rats on the same date at the same 
place was defined as a paired association. Contacts 
between rats were assumed to be proportional to 
such paired associations. For example, a rat might 
have three paired associations with one member of 
a group, two with another, one with a third, and 
none with the other three. This rat was then said 
to have six contacts with the group. In these tabu- 
lations, the population was limited to the 155 rats 
who were born in the pen and who lived over 150 
days. Contacts were only counted when the young- 
est rat of the pair was over 60 days of age. Results 
are shown in figure 119. These curves are for the 
average of the four random groups and for the 
average of the four ingroups. The curves for the 
separate groups closely approximated that for the 
respective averages. 
Even with groups of random membership, some 
rats had more contacts with the members than did 
others. There is an approximate exponential 
decline in the number of rats which have fewer and 
fewer contacts with the groups of random member- 
ship. Organized groups show a similar relation- 
ship of frequency of contacts with the members of 
the population, who are not members of their 
ingroup. However, members of the ingroup ex- 
hibit a marked increase in the number of contacts 
they have with each other. Even here there is an 
approximate exponential decrease in the number 
of rats which have fewer and fewer contacts or 
paired associations with the other members of the 
ingroup. Because of the difference in the rate of 
decline of the exponential curves of association by 
the ingroup with itself in contrast to that with other 
173 
