fence. The lack of trails on the area side of the 
median barrier fence was not a result of the rats 
in the areas failing to build trails adjacent to fences. 
This is revealed by the fact that trails were located 
nearly entirely around the pen adjacent to the 
inner surface of the limiting barrier fence. 
A general hypothesis will now be presented to 
account for these differences in the location of 
trails adjacent to barrier fences. This hypothesis 
is actually a formulation of the factors influencing 
the movement of the rat through space. The 
placement of trails along the barrier fence is 
merely a special case where the movement through 
space is modified by the intervention of a barrier. 
This latter response will be designated the “ barrier 
wall response” in contrast to the “vertical surface 
wall response”. 
D. The Movement-Through-Space Hypothesis. Move- 
ments through space fall into two major categories: 
1. Goal directed movements. Such movements 
are restricted to those directed toward those places 
of harborage and sources of food which are suf- 
ficiently permanent for learned responses to have 
developed. Movements toward such goals tend 
to be along the shortest route or the route requiring 
the least expenditure of energy. 
2. Investigatory movements. A rat which is at 
some point of orientation and which has tem- 
porarily satisfied those basic needs, that may be 
fulfilled by responding to localized goals, will tend 
to make movements in a random direction away 
from the point of orientation. In the absence of 
any objects in the environment along the outward 
route of travel, locomotion will nevertheless be 
interrupted from time to time. This interruption 
presumably arises from the need of the animal to 
defecate, urinate, or rest. Once having stopped 
the animal tends to redirect its movement toward 
(a) the prior point of orientation, (b) some localized 
goal, or the point of stopping will itself become a 
point of orientation from which outward travels 
will radiate in a random direction. Such outward 
movements from a point of orientation will be of 
decreasing frequency of increasing length. This 
is discussed in more detail on page 82 under the 
section titled, Utilization of Space by Rats. 
Where cues, goals, or minor obstructions arc 
abundantly dispersed through the environment 
the rate of decrease in the occurrence of ever longer 
movements will become accentuated. In other 
words, there will be even still fewer of the longer 
movements. This arises from the fact that these 
cues, goals, and minor obstructions will more 
rapidly bring about an inhibition of movement 
away from the point of orientation. Once having 
been inhibited the movement will more likelv be re- 
directed toward a prior goal or point of orientation. 
These formulations on orientation will permit 
a more exact insight into what is meant by “ home 
range” in mammals, than a mere statement that 
it is the area inhabited by the individual. Those 
few localized places of harborage and sources of 
food toward which the individual has developed 
learned orientation responses will form an approxi- 
mate “center” of the home range. Outward 
movements from this center will be in accordance 
with the interaction of (a) the need for resatiation 
of a basic need at one of the localized goals, and 
(b) the density of more widely dispersed goals, 
cues, and minor obstructions. The boundary of 
the home range may be considered merely as some 
arbitrary statistical probability of encountering 
the individual at a given distance from the center. 
When a physical barrier occurs not too far from 
the center of an animal’s home range ihere will be 
a piling-up or concentration of activity at the 
barrier which is presumably equivalent to the 
amount of activity that would have occurred 
beyond the barrier had it not been present. This 
is the explanation forwarded for the presence of 
trails adjacent to the Food Pen side of the median 
barrier fence. It is also the explanation for the 
close proximity of the trail along the inner surface 
of the limiting barrier fence. 
However, this leaves unexplained the fact that 
the trails along the outer surface of both the Food 
Pen fence and the median barrier fence do not 
nearly so closely approach the fence. In general, 
these trails approached no closer than eighteen 
inches of the fence, whereas, the trails on the inner 
surface of both the limiting and median barrier 
fences were usually less than 6 inches from the 
fence. 
The orientation dependent upon the investiga- 
tory travel stipulates that there should be a moder- 
ately used trail immediately adjacent to the outer 
surface of the median fence. On the other hand, 
the more goal directed locomotion between the 
sources of food and harborage stipulates that there 
should be trails leading directly between the 
harborage boxes and the passages through the 
median barrier fence. Apparently what happens 
is that these two forces affecting the position of a 
trail are combined to produce a trail about half- 
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