way between the two positions that these forces 
acting independently would have produced a trail. 
Another factor contributing to the fusion of these 
two trail positions into a single trail is the tendency 
of rats not to construct parallel trails too close to 
each other (see pp. 61-63). 
E. Orientation in Space. The usage of trails con- 
structed by the rats and the observed movements 
of rats with reference to goals and the physical 
configuration of the environment provide consider- 
able information relating to the orientation of rats 
in space. Much of the insight into the means by 
which rats orientate their movements was provided 
by the reaction of rats to the obliteration of pre- 
existing trails by a cover of snow. Following snow- 
falls rats reproduced the hidden former trails with 
remarkable exactitude except for a few striking 
errors. Prior to observing these trails developing 
over snow, I had considered the use of trails as a 
very simple type of behavior resulting from direct 
perception of the trail itself. Once there dawned 
the realization that orientation might not be such 
a simple phenomenon, I sat down and tried to 
conceive of all the means of perception of the trail 
itself. These are as follows: 
1. Tactile perception. When no snow exists one 
may assume that tactile perception is at least 
partially responsible for the orientation of rats 
along trails since the trails are kept clear of vege- 
tation for a width just slightly larger than the 
bodies of the larger rats. However, the depth of 
the snowfall, particularly when accompanied 
by an icy crust on top, precluded any possible 
functioning of this type of perception. 
2. Visual perception. At times the snowfall was 
as deep as to preclude the use of low-lying vegeta- 
tion as visual cues. However, the barrier fences, 
the food hopper, the activity recorders at the 
Food Pen passages, as well as the trunks of the 
smaller trees, extend above the snow so that these 
might serve as possible cues for spatial orientation. 
3. Kinesthetic perception coupled with a “ cognitive 
map ” of the environment. It is possible that in the 
total absence of reliably detectable cues that the 
rats can nevertheless duplicate previously estab- 
lished routes of travel. This requires a time- 
distance cognition in which the direction of move- 
ment and the angle of change of direction of 
movement is determined by kinesthetic stimuli 
which must have been responded to in a particular 
pattern or sequence before the rat moves from one 
known position toward another. If the rat can 
detect distance through speed-time relationship it 
will be possible for it to arrive at a second position 
of orientation even though the normal cues for 
orientation are impossible to perceive. Once at 
this second point in space, another portion of the 
pattern of kinesthetic stimuli may come into play 
and be sufficient to induce an altered course of 
travel. Through an orderly sequence of such 
orientations a rat might be able to duplicate a 
fairly complex route of travel despite the absence 
of any obvious cues to facilitate orientation. It 
is realized that I am here postulating a rather 
elaborate mechanism for the rat. The reason for 
believing that such a mechanism is within the range 
of possibility of achievement by the rat is that 
many orientations have been repeated thousands 
of times. Each major section of a route of travel 
was covered between 20 and 50 times each night. 
This is a rather conservative estimate, but even so 
it gives a repetition frequency of 1,000 times per 
month and all the rats living in the pen during 
the latter part of January 1948 had at least 5 
months of experience. 
4. Olfactory perception. Although no conclusive 
proof can be presented to show that olfaction was 
not utilized in the detection of the previous trails, 
the following reasons will be offered as supporting 
the belief that olfaction was not operative in the 
observed orientations over snow: 
a. The trails lay under 5 inches of snow, the 
uppermost portions of which formed a hard 
crust on certain occasions. 
b. Even when a high wind was blowing rats were 
observed to duplicate exactly the trails exist- 
ing under the unmarred snow. 
c. Temperatures were below freezing. 
d. The rate of movement of the rats over the 
snow was rapid. Although it was not the 
fastest run the rats were capable of, it might 
be characterized as a rapid lope. 
Considering these four conditions it is difficult 
to conceive of olfaction playing a significant role 
in the orientation of trails over snow. 
The next six sections (pp. 72 to 82) will cite 
specific examples of orientation from which in- 
terpretations of possible mechanisms involved are 
derived. Although focus of attention upon orienta- 
tion began with snow trails a number of other 
observations relating to this topic will be included 
from other seasons of the year. 
a. Orientation over snow. The conditions of snow- 
fall during the day of January 24, 1948, were such 
71 
