general trend of the tunnels. The second phase 
of development of this burrow was the construction 
of a tunnel directed toward the neighboring Har- 
borage Box No. 10. As soon as the rats contacted 
the edge of this box, they built an entrance to the 
surface and excavated dirt through it as they dug 
a tunnel around the edge of this sunken wooden 
box. Many instances were observed where bur- 
rows initiated away from a fence or a harborage 
box had secondary tunnel segments constructed 
which were apparently directed toward the object 
in question. From these observations it is believed 
that rats in many instances direct tunnel construc- 
tions toward prior points of orientation. Such a 
behavior will account for the development of 
circular routes of travel in burrow systems. Thus, 
where two blind tunnel segments exist, that have 
developed from a common point, there is a tend- 
ency for future extensions from them to bend back 
toward each other. 
A study of the surface area covered by excavated 
dirt and of the number of passages or burrow 
entrances for the North and South Alley Burrows 
provides an index of their rates of growth. For 
the South Alley Burrow it will be seen (fig. 24) 
that there was an initial rapid rate of expansion 
during the first 3 months when the first litter was 
being raised. From the time of birth of the 
second litter during August 1947 through the 
entire breeding season of 1948 a second and de- 
creased trend in the rate of expansion is apparent. 
Although there is only one measurement of surface 
area available for 1949, it is nevertheless apparent 
that the rate of increase was continuing to decline. 
If burrows are considered as a cultural artifact, 
it is evident that the density of a cultural artifact 
follows a similar pattern of growth as does the 
population of rats which produced the artifact. 
The trend of growth of the surface area of the 
North Alley Burrow roughly paralleled that of the 
South Alley Burrow. However, the initial expan- 
sion of the North Alley Burrow appeared more 
rapid until the fall of 1947. From that time on the 
rate of increase in surface area of the North Alley 
Burrow was less than that of the South Alley 
Burrow so that by the termination of the study 
19 
