tion in reproductive performance. According to 
him these consequences operate to insure an ade- 
quate food supply for members remaining in their 
original home ranges. For these phenomena to 
develop the number of individuals mutually 
j. tolerated within a given home range, as well as 
j the proximity of groups inhabiting adjoining home 
ranges, must have definite limits. In the present 
study there was no evidence of death of Norway 
rats by shock disease. Although extensive emigra- 
- tion was precluded by the fence surrounding the 
pen, limited movements from the place of birth 
( regularly occurred. These movements were ac- 
companied by formation of new groups, usually 
not exceeding 10-15 rats, such that the minimal 
distance between centers of adjoining colonies was 
approximately 25 feet (see fig. 128). Intolerance 
to further crowding gave rise to stressful interactions 
that indirectly curtailed reproduction. From these 
conditions, two elaborations to Kalela’s general 
concept are suggested. 1. For the Norway rat 
under the experimental circumstances home range 
primarily applies to that region about a burrow 
which is used nearly exclusively by the group 
inhabiting that burrow. The point being made 
here is that there appears to be a minimal distance 
tolerated between home range centers which is 
independent of insurance of an adequate food 
supply. This, however, does not obviate the pos- 
sibility that within the evolutionary history of 
commensal association with man, this tolerance 
distance between colonies was of survival value for 
the species in relation to adequacy of the food 
supply. 2. Kalela at least implies that density 
control by emigration, shock disease, or decreased 
reproduction, equally protects all members in- 
habiting favorable habitats. That is, within each 
home range the members enjoy equal opportunity 
for access to food and performance in reproduction. 
In the present experimental situation groups pos- 
sessing comparable home ranges met quite different 
fates. For example, compare the rats inhabiting 
Area I versus those in Area IV, or those in the South 
Alley versus those in the North Alley. Despite the 
physical comparability of these locations the inhabi- 
tants of Area I and the South Alley were leaving 
many descendants, while the inhabitants of the 
other two localities were leaving practically none. 
The important difference between the two regions 
is that of cultural history which produces differen- 
tial segments of the population. These we may 
logically designate as belonging to different social 
classes. 
These social classes are not to be construed as 
synonymous with social hierarchies, although both 
social hierarchies and the phenomenon of home 
range and its defense contribute to its origin. The 
defense of home ranges, that is the development of 
territory, is an outgrowth of both home range and 
social hierarchies ( 6 , 94 ) and it also contributes 
to population control and survival of the species. 
Formation of social classes is then a third and higher 
order phenomenon. Its contribution to survival of 
the species is that it provides assurance of the 
maintenance of a few favored groups. These not 
only enjoy an adequate food supply but also relative 
freedom during maturation and reproduction from 
the marked stresses of social interaction encountered 
by most of their associates. 
9 . Strange Object Reaction and its Rela- 
tionship to Periodicity of Activity 
In the study of the rats in my Towson colony a 
number of examples were cited of avoidance of 
strange objects, reluctance to reenter traps, caution 
in initiating an excursion, slowness in exploring a 
new portion of the habitat, and retreat into the 
burrows upon detection of any sudden strange 
stimulus. Chitty and Southern (37) cite many 
similar experiences with the wild Norway rat. 
They measured food consumption of a group of 
rats during 17 nights. On six of these nights some 
strange object, such as a tin can, was placed near 
the food hopper between 6 p.m. and midnight. 
During these 6 nights only 45 percent of the food 
consumed was eaten before midnight in contrast 
to the 60 percent before midnight during the 11 
control days. Where access to a food pen was by 
a single entrance, the placing of any new object 
at the entrance, for a few minutes completely in- 
hibited passage during such time. Even replace- 
ment of one food hopper with an apparently 
identical one caused delay in initiating feeding. 
Prebait stations in the form of small wooden sheds 
or clay drain tiles produced a typical avoidance 
reaction in which the amount of food eaten per 
night normally did not reach a maximum until 
2 weeks had elapsed. Once the habit of feeding 
at such a station was established its removal to a 
new location only a few feet away again produced 
an avoidance, but of shorter duration. The fre- 
quent approach and withdrawal from the hopper 
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