This accompanied the drop in population to 127 
rats (fig. 146) due to mortality and an increase in 
average weight to 437 grams. At this latter date 
the number of feet of trails per 1,000 grams of rats 
was near an asymptotic level of 34.3 grams (fig. 
60B). Consideration of this number of feet as a 
fair approximation of the asymptotic level leads 
to another estimation concerning density of rats. 
The number of feet of trails per 1,000 grams of rats 
as a function of total feet of trails is shown in figure 
61. This curve presumably results from an inter- 
action between (a) the ability of available rats to 
construct trails (growth potential), and (b) the 
tolerance of rats to too close approach by other 
rats on neighboring trails (environmental resist- 
ance). Beyond the last trail survey in March 1949 
one might extrapolate the curve. Such extrapola- 
tion indicates a marked diminution in the amount 
of trails per unit weight of rats. However, we have 
already seen from figure 60B that one could not 
expect the number of feet of trails to drop much 
below 34.3 per 1,000 grams of rats. Thus an upper 
limit of approximately 2,200 feet of trails results 
beyond which trailway expansion cannot exceed. 
This amount of trails is that which should have 
existed by the beginning of the breeding season 
in March 1950 with 146 adults. 
By two lines of reasoning the usage and develop- 
ment of trails by the rats led to estimates of an upper 
level of density characteristic of the initiation of 
future breeding seasons. Despite the several crude 
approximations inherent in this reasoning, a later 
self-restriction of the population to 146 to 200 
individuals is quite in harmony with other lines 
of evidence regarding population growth. 
It has long been recognized that competition for 
food is a potent factor in contributing to the 
force of environmental resistance in limiting 
population size. More recently Davis ( 6 ) has 
pointed out that territoriality is also an important 
phenomenon contributing to environmental re- 
sistance. Similarly the manner in which rats 
construct and use trails modifies the intensity 
£ INTERACTION OF GROWTH POTENTIAL AND ENVIRONMENTAL RESISTANCE 
Figure 61 . — Trailway systems as indicating the interaction between growth potential and environmental resistance. Growth 
potential is here considered in the light of the ability of rats to construct trails,- whereas environmental resistance is con- 
sidered as all those factors which tend to inhibit the expression of this behavior. Environmental resistance was prob- 
ably near maximum at 2,200 feet of trails, since according to figure 60 the minimum number of feet per 1 ,000 grams of 
rats was 34.3. Thus one might anticipate little increase in density of either rats or their trails beyond the termination of 
the study in June 1949. 
62 
