ABSTRACT 
Five males and five female wild Norway rats, 
Rattus norvegicus were introduced into a 100-foot- 
square enclosure. Food and water were almost 
continuously and abundantly available. Preda- 
tors were excluded. The history of these rats and 
their descendants was followed for a period of 27 
months, the primary objective of the study being 
to determine in what way social interaction might 
inhibit population growth. 
Early in the history of the colony there was a 
space-social class stratification in which the more 
dominant individuals inhabited the southeast half 
of the enclosure. This division was maintained 
throughout the entire study. 
Individuals tended to remain near their place of 
birth. As the population increased, local compe- 
tition forced rats from the socially higher ranking 
colonies in the southeast half of the pen to invade 
the northwest half. This migration was primarily 
by males. There was no reverse migration from 
socially lower ranking colonies into higher ranking 
ones. All changes in social status were downward. 
These migrations led to an increased number of 
social interactions among members of the socially 
lower ranking colonies. 
Females were quite aggressive when lactating. 
At these times they excluded most other rats from 
the vicinity of their burrow, with the exception of 
other reproducing females of the same local colony. 
In a few instances a highly dominant male 
established a territory about a burrow containing 
several reproducing females. He usually excluded 
all other males from the vicinity of the burrow. 
The most important consequence of such male 
territorial behavior was that it reduced the fre- 
quency with which other males were able to invade 
the area and make advances on a female when she 
was in estrous. Females in such situations were 
characterized by marked success in conceiving and 
rearing young. Effective male territorial behavior 
developed only about colonies of females which 
historically were of high rank. 
In the northwest half of the pen there developed 
an excess of males as a result of the migration to it 
of those excluded from the southeast half. Mem- 
bership in local groups was here less stable as 
evinced by the greater number of places of origin 
represented by the members of a local group at a 
particular time. There was never any clear-cut 
expression of territoriality by any individual in 
such groups, although occasionally one male dom- 
inated others. A female member of such a group, 
when in estrous, was continually followed by a 
pack of males. In rapid succession she would be 
mounted for a total of several hundred times 
during a single night. The behavior of such 
females indicated they experienced considerable 
stress. Conceptions were reduced and few young 
were raised. The normal preparturient behavior 
patterns of self-isolation, burrow construction, and 
nest building, which are requisite to successful 
rearing of young, were reduced or entirely in- 
hibited. 
As the population increased a larger proportion 
of the colonies were characterized by this low 
social rank and instability, with the concomitant 
reduction in reproduction. This is basically the 
process by which a population of rats limits its 
numbers in the absence of other controlling factors. 
All evidence indicated that no more than 200 
adult rats would ever have existed in the quarter 
acre pen at any one time. At one end of the social 
spectrum there was a colony consisting of a single 
dominant male and a harem of 13 females, most 
of whom were reproducing successfully. At the 
opposite extreme there were all male aggregates 
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