RESPONSES TO THE PHYSICAL 
ENVIRONMENT 
The social behavior of rats and the social organi- 
zation of the group take place in a physical environ- 
ment. Therefore, before going into the social 
aspects of the life of the rat it will be best to provide 
a background of the reactions of rats to their physi- 
cal environment. In doing this it will be impossible 
to avoid reference to the interaction of the physical 
environment and the social aspects of the life of 
the rats. In fact a dual generalization arises from 
this longitudinal study of the ecology of the Norway 
rat. This is that the physical structure of the 
environment modifies the expression of social 
behavior, while on the other hand social experiences 
modify the responses to the physical environment. 
1 . Burrow Systems and Harborages 
A. The Growth of the Burrow System. The rats fre- 
quently constructed burrow systems as places of 
harborage, even though there were nine sunken 
wooden harborages provided in each of the four 
triangular areas. It was possible to follow the 
growth of burrow systems by examining the mounds 
of excavated dirt and the origin of passages into the 
burrow. Periodically the surfaces of the two main 
burrow systems were mapped by the technique 
shown in figure 13. These two burrow systems 
were named the North Alley Burrow and the South 
Alley Burrow. In the text I shall refer frequently 
to these two burrows simpley as the NAB and the 
SAB, respectively. They were the two main bur- 
row systems developed in the alleys. Both were 
begun during the first few days of April 1947. All 
other burrow systems in the alleys were smaller and 
both historically and socially were subsidiary to 
them. The position of these two burrows are 
shown in figure 14. 
The changes in surface appearance of the South 
Alley Burrow, from April 22, 1947, to May 17, 
1949, are shown in figures 15 to 20. Initially, the 
burrow consisted of a single entrance hole with 
an adjoining mound. Very shortly two changes 
became apparent. First, additional entrance holes 
appeared beyond the edge of the mound. No 
excavated dirt occurred about these new entrances, 
which resulted from the rats digging toward the 
surface from the terminal ends of tunnels radiating 
out from the initial underground tunnel. Second, 
trails on the surface radiated outward from the 
mound toward various goals. These initial altera- 
tions may be seen in figure 15B. At this time, 
June 14, 1947, this burrow was inhabited by a 
single female and the young of her litter born 
April 16. Major directions of travel were toward 
the Food Pen and up Path 1 toward Areas I and 
IV. This latter route of travel is the result of a 
centrifugal type of movement toward less intensely 
used portions of the environment. 
Up until July 11, 1947, this South Alley Burrow 
was still mainly utilized by the original female 
and her five young, which were then nearly 3 
months old. All of these six rats participated in 
the enlargement and use of the burrow. As will 
be seen from figure 16, the mound was increasing 
in size. Some of the newer entrance holes into the 
burrow began serving as foci for the excavation 
of dirt as tunnels continued to be extended in a 
radial fashion from the original site. The number 
of trails on the surface leading from the burrow 
also increased in number. By October 7, 1947, the 
surface area of the mound had continued to in- 
crease and in addition there were three new en- 
trances into the burrow system beyond the mound. 
At this time the burrow was inhabited by some of 
15 
