TIME STRATIFICATION OF THOMPSON'S 10 RATS 
INTENSITY 3 1 Z -°J p s 4 N U 
6 19 
TIME 
Figure 155. — Time stratification of Thompson's 10 rats. 
Relative time was taken as the sum of the two mean rank 
orders in table 70. 
10 . The Role of the Physical Structure of the 
Environment 
McCabe and Blanchard (98) and Harris (77) 
have shown that within the genus of mice, Peromys- 
cus, each species and even each subspecies may 
respond to and be attracted by remarkably re- 
stricted physical characteristics or configurations 
of the microhabitat. Since such characteristics 
typically exhibit a mosaic rather than a uniform 
distribution, their absence in portions of the en- 
vironment imposes restrictions on the potential 
population density of each species. Specificity for 
attraction to particular configurations of the en- 
vironment has also been shown for the Norway rat. 
Attraction to isolated or continuous vertical objects, 
to flat surfaces when digging is initiated, or to 
various low overhanging surfaces, etc. are observed 
for the Norway rat. Within the Towson colony 
pen, locations of tree trunks, fences, passages 
through fences, preplaced harborage boxes, etc., 
certainly did control development of trails, and 
burrows, and thus the association patterns among 
rats. These lines of evidence suggest that we should 
diligently seek those configurations or conditions of 
the physical environment, which, by heredity or 
culture, attract or repel the species we are interested 
in. Once these are ascertained, our general course 
of action is dictated. This is to systematically vary 
the distribution of these configurations or condi- 
tions through space and time. Within each set of 
conditions a population may be allowed to develop 
and its members permitted to interact. Observa- 
tions may then be made with the objective of de- 
lineating the dependent variables under two 
categories: fa) the population and its subgroups 
or (b) the characteristics of the individual. Under 
the former fall: fl) population growth and limi- 
tation; (2) the formation size, composition and 
stability of subgroups; and (3) the interrelations 
between subgroups. Under the latter fall: (1) 
growth, maturation and aging; (2) physiology, 
such as adrenal or cardiac function, and many 
others; (3) the kinds, frequencies and durations of 
interactions with other individuals; and (4) repro- 
ductive performance and survival. All of these 
dependent variables may be quantified and related 
in some causal sequence to the manner in which 
we have structured the environment with 
independent variables. 
I view the importance of the present paper on 
the life history of the Norway rat, not from the 
specific findings and interpretations, interesting 
though some of them may be, but rather from the 
insight it provides to the opportunity and feasi- 
bility of developing an experimental approach to 
problems of social biology and population dynam- 
ics on the vertebrate level. The naturalist and 
ecologist can gain insight, and even formulate 
principles about those conditions which govern the 
life of the individual and the population of which 
it is a member. Anyone who has studied animals 
(including man) in their native habitat is cognizant 
of the fact that the characteristics of the environ- 
ment are so irregularly distributed as to make 
evaluation of their effect difficult. Not until we 
begin systematic experimental manipulation of the 
environment can we hope to develop principles 
of precise predictive value. On the vertebrate level 
such systematic manipulations have been extremely 
rare, possibly because of technical and financial 
difficulties. However, the need for such studies 
imposed by a “shrinking” world, as human popu- 
lations expand at the expense of most of the rest 
of the vertebrate fauna, is paramount. One 
question stands out: What are the configurations 
and spatial limitations requisite for an optimum 
state of existence of each species? Space require- 
ments may be restricted with appropriate struc- 
tural configurations, but restrictions beyond certain 
limits is certain to be accompanied by deleterious 
effects. For many years we have been concerned 
with the role of availability of food for static or 
expanding populations. Just as McCabe and 
Blanchard found for mice of the genus Peromyscus 
and I for Norway rats, that needs for space and 
particular configurements impose limitations on 
population growth in the presence of adequate 
food supplies, so it is likely to be found for other 
animals including man. Experimentation on ani- 
mals as to these needs and how they may best lie 
fulfilled in relation to spatial limitations would 
provide valuable insight into their application to 
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