Figure 59. — Clearly discernable trails. 
X’s represent tile entries to harborage boxes or entries through the soil into burrows. 
Crosshatched areas are mounds to burrows. 
width of trails was not obtained. When an adult 
meets a juvenile along a trail going in the opposite 
direction, they may both maintain their direction 
of travel with the adult walking over the juvenile. 
By November 1947 (fig. 59B) there had been a 
considerable increase in total amount of trails as 
well as an increase in the number of pairs of points 
between which alternate routes had developed. 
For example, from Passage 1 through the median 
barrier fence to Passage 6 into the Food Pen the 
number of well defined routes of travel had in- 
creased from one to three. Similar increases in 
alternate routes were apparent elsewhere. Also 
by this date most of the harborage boxes were 
connected to each of the immediately adjoining 
ones. This occurred even though many of the 
boxes were not used as daytime retreats. A rat in 
traveling from one harborage box to some more 
distant point would frequently enter and remain 
for a few seconds in each harborage box it passed. 
This momentary stopping at any place offering 
overhead cover was a characteristic behavior of 
rats. 
By a year later in November 1948 (fig. 14) the 
pen had developed into a veritable mesh of trails. 
In addition to new trails leading from new bur- 
rows, and additional alternate trails connecting 
most goals, the one major new location of trails 
was adjacent to the inner surfaces of both median 
and limiting barrier fences. 
Surveys, other than the three illustrated ex- 
amples, were made from time to time. From 
these it was possible to determine the total footage 
of trails for successive dates (See curve A in fig. 60). 
These measurements define the degree to which 
rats biologically condition their environment. 
If this increase in trails is considered in an analogous 
fashion to that of the growth of an individual or the 
growth of a population, it may be described in 
terms of a “growth potential” and “environ- 
mental resistance.” 
During the first 3 months the growth potential 
of trails was slightly greater than 100 feet per 
month, but by the end of 9 months it had attained 
an apparently stable and lower rate of 45 feet of 
additional trails per month. However, just as an 
understanding of changes in growth rates of in- 
dividual animals or of populations demands that 
growth be phrased in terms of the number of the 
biological units contributing to the increment, so 
we must consider trails in terms of the rats which 
produced them. 
In terms of rats over 60 days of age each rat pro- 
duced over 10 feet of trails per month during the 
first 3 months but by March 1949 the rate had 
fallen to approximately 0.35 feet per month per 
60 
