with which a particular area can be utilized. 
Therefore, such limitations to how space is used 
must contribute to the action of environmental 
resistance in retarding population growth, despite 
an excess availability of food and harborage in 
satisfying more basic needs. It is apparent from 
figures 14 and 59 that the distribution and density 
of goals affects such orientation responses. Fur- 
thermore, the character of the physical structure 
of the environment may affect the frequency with 
which an individual moves about particular 
portions of its environment when all other aspects 
of the environment are identical (77). These 
considerations point to the fact that the distri- 
bution, type, and density of goals are variables 
which influence population ecology and social 
behavior. Extremely little experimental research 
has been done in this area, although the leads 
which exist indicate that if they are followed up 
systematically they will produce many valuable 
concepts. 
B. The Position of Trails with Reference to Physical 
Objects. Although there is a considerable tendency 
for trails to be developed along the shortest or 
least impeded route between a point of orientation 
and a goal, it is, nevertheless, apparent that 
objects along the route will produce deviations. 
Where no impediment existed along a route and 
no physical objects occurred within 18 inches to 
either side of the shortest route between two points, 
an essentially straight trail usually developed. 
This was the typical situation between the corners 
of the Food Pen and the passages through the 
median barrier fence (see fig. 62). If, however, 
an object such as a box trap was placed within 
18 inches of the straight trail, the rats deviated 
from the preexisting trail toward and along the 
object. Such an example is shown in figure 63, 
in which the trail is first deflected to the right 
toward one group of traps, while a few feet farther 
along it is deflected to the left toward another 
group of traps. Where minor impediments to 
free travel exist, such as grass, herbs, vines, and 
similar vegetation, the rats gnaw and remove 
them. This frequently results in a partially or 
completely arched-over trail (see fig. 64). Where 
an unsurmoun table barrier exists the rats may 
initially start out in the direction of the shortest 
distance between the point of orientation , and 
the goal. When the barrier is encountered, the 
rat follows along it until a passage through or 
around the barrier is encountered. Continued 
experience with the barrier rapidly modified this 
initial behavior to one in which the route of 
travel was directly from the point of orientation 
to the passage through the barrier, whereupon 
the movement was again more directly goal 
oriented provided no other barriers intervened. 
It was such a sequence of events which prevented 
the development of trails adjacent to the outer 
surface of the median barrier fence as rats traveled 
from their places of harborage in the areas toward 
the Food Pen. 
The most characteristic pattern of trails within 
the pen was their orientation toward vertical 
signs, such as tree trunks. Some of the movement 
of rats toward trees is undoubtedly associated with 
the attraction toward them as a source of over-head 
cover. The orientation of trails with respect to 
poles or trees without leaves was sufficiently fre- 
quent to indicate that the vertical object itself is 
the prime factor. An examination of the position 
of the trails with reference to trees in November 
1948 (fig. 14) provides many examples of this 
relationship. A specific example may be more 
instructive. The trail between the North Alley 
Burrow and Passage 3 through the median barrier 
fence will serve as the example. 
A number of surveys are available for this trail. 
The more pertinent ones are shown in figure 65. 
On May 15, 1947, (A) there was no trail through 
this portion of the North Alley. The position of 
the 17 trees are indicated and numbered. By 
November 22, 1947, (B) the basic route of travel 
by these trees had been established, although the 
goals being connected were the North Alley 
Burrow and the Food Pen via the East Alley. ‘At 
this time there was insufficient travel between 
Areas II and III and the North Alley Burrow to 
produce a trail from Tree 13 toward Passage 3. 
Judging by the trail produced on January 14, 1948, 
communication between these two regions had 
begun. During the morning of January 13, 1 inch 
of snow fell. During the afternoon a little more 
snow and considerable sleet fell forming an icy coat 
over the ground which completely obscured prior 
trails. Despite this event the rats reproduced a 
trail (fig. 65C) with essentially the same orientations 
to trees as had existed previously. Since prior cues 
of the trail itself, whether they are visual, tactile, 
or olfactory, must have been essentially absent, 
it is concluded that the present orientation with 
respect to the trees is an indication that the prior 
63 
