this introduction considerable dislocation of the 
population occurred. This affected both the 
residents and the introduced members. Rats of 
both groups invaded yards where previously they 
had not been observed. Under these conditions 
of disruption of normal population equilibrium 
there was an increase in mortality in both groups, 
but the mortality rate for the aliens was approxi- 
mately three times that of the residents. Emigra- 
tion from the block was negligible. These observa- 
tions suggested a negative correlation between the 
degree of stability of social relationships and the 
mortality rate. Under these conditions of unusual 
disruption of social relationships, mortality 
appeared to be due primarily to increased exposure 
to predation by man and dogs. However, the pre- 
vious experience of the various workers in the 
Rodent Ecology Project had led to the impression 
that predation by dogs and cats and the sporadic 
attacks by the resident human population did 
not appreciably affect the density of the rat popu- 
lation. Although we had no quantitative data on 
this point, this conclusion was arrived at through 
the general observation that city blocks supporting 
high densities of dogs and semiferal cats frequently 
also supported a high population of rats. One 
other general observation was pertinent to the 
formulation of the hypothesis. This was that 
many blocks maintained a stable population 
without further increase even in the presence of a 
superabundance of food in open garbage con- 
tainers and an excess of space, which was poten- 
tially suitable for the construction of burrows but 
which actually was never utilized. 
These observations may be stated as premises: 
1. Mortality rate increased with increases in 
the disruption of established social relation- 
ships. 
2. Gross predation is an insignificant factor in 
the usual determination of the stable level of 
a rat population. 
3. The growth of a population of rats will cease 
(reach the upper asymptote of the growth 
curve) even in the presence of a superabun- 
dance of food and an excess of potential har- 
borage sites. 
The hypothesis derived from these premises: 
The growth rate of a population will be inhibited 
in the absence of predation and in the presence of 
an excess of food and harborage. This inhibition 
arises through the social interaction among the mem- 
bers of the population with respect to the distribu- 
tion of goals and barriers through the environment. 
The genesis of this hypothesis is influenced by 
two sets of concepts in the field of ecology. First, 
it has long been recognized that social interaction 
through both cooperation and competition in- 
fluences population density where either food or 
harborage or some other requisite aspect of the 
environment is limited. Second, the defense of 
territory will limit populations irrespective of the 
density dependent variables which may happen 
to be distributed through the territories. This 
latter situation has been somewhat overlooked, 
but its implication has recently been pointed out 
by Davis (6). 
The exact methodology of executing an experi- 
mental study of a mammalian population presented 
quite a problem. It is obvious that one could not 
utilize the normal man-influenced environment in 
which the rat most characteristically occurs. In 
the first place, the physical configuration and dis- 
tribution of objects through space is so highly ir- 
regular and changing as to preclude any hope of 
sorting out the influence of the variables. In addi- 
tion, human interference in field studies of rats in 
the city situation often proves disconcerting. Even 
such a simple procedure as trapping, marking, and 
releasing rats meets with such obstacles as having 
someone kill them in the traps, release them, or 
move the traps about so as to defeat the original 
aims of the study. Investigation of the hypothesis 
necessitated construction of an artificial environ- 
ment where at least some of the variables could be 
controlled. The space requirements posed prob- 
lems of a magnitude not encountered by investiga- 
tors in experimental population dynamics working 
with such insects as Drosophila or Tribolium. Also, I 
knew of no other experimental studies on the mam- 
malian level where even the space requirements 
necessary to satisfy home range or territorial be- 
havior were satisfied. Therefore, the structure of 
the experimental environment finally decided upon 
had to be made mostly on an empirical basis. 
Although Norway rats accommodate to several 
types of human environments such as ships, ware- 
houses, farms, and residential areas, it was decided 
to model the experimental environment after that 
encountered in the “row house” type of residential 
area (figs. 1 and 2) found through large sections of 
the city of Baltimore, Md. It has been within such 
a habitat that the majority of the Rodent Ecology 
Project research effort has been concentrated. Typ- 
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