of their potentials of behavior if all members are 
to maintain an adequate state of health. 
It was quite apparent that the behavior of most 
rats did become inhibited or modified as a result of 
the interaction with other rats. Whatever the 
nature of the alteration in behavior it ultimately 
entailed avoidance of more dominant individuals. 
Whereas the forfeiture of behavioral potentiality 
enabled the individual to live longer, there were 
concommitant negative effects upon the individual 
which reduced both conception and the survival 
of newborn rats. 
This appears to be the process involved whereby 
a population of Norway rats may limit its own 
growth in the absence of predation, and where food 
harborage, and space for construction of burrows 
were always in excess. It suggests certain general 
procedures that will facilitate reduction of popula- 
tion density in this and similar deleterious species 
which serve as reservoirs of diseases transmittable 
to man, or cause property damage. These are: 
(1) Restrict the space available for habitation. 
Irrespective of other conditions, territoriality will 
immediately decelerate population growth. (2) 
Concentrate the locations of sources of food or 
harborage. This serves to increase competition 
and thus to cause stress to the rat’s physiology. (3) 
Increase the probability of one individual contact- 
ing another, particularly in proximity to goals. 
For example, where there is a single place of access 
to a source of food, contacts near the food will 
increase, and it will also be easier for a single 
individual to prevent the accesss of others. (4) 
Disrupt social organization through migration and 
the restructuring of groups (5, 24 ). These four 
procedures need not reduce the total amount of 
food or harborage. Instead, their action is exclu- 
sion from access to goals or production of physi- 
ological stress — both as a result of social interaction. 
Restriction of the amount of goals such as food 
and harborage, when their distribution is unaltered 
may be anticipated to reduce density or prevent 
growth of a population only when the requirements 
of the existing population are no longer fulfilled. 
In urban areas of Baltimore inhabited by rats, re- 
moval of board fences, trash, and food (either 
separately or together) have usually been followed 
by reduction in the population of rats ( 25 , 26 ). 
These reductions were no doubt due to: (1) De- 
creased amounts of environmentally supplied re- 
quirements, and (2) social interaction stressors 
operating through (a) migration, (b) restriction of 
available space, (c) spatial concentration of goals, 
or (d) restricted routes of access to goals. 
246 
