Figure 70. — The inability of rats to reproduce a curved trail 
in the absence of immediately adjoining visual or tactile 
cues. The solid line represents the position of the trail 
prior to the 5-inch snowfall of January 2 4, 1948. The 
dashed lines indicate in a diagramatic fashion the position 
of trails initially developed over the snow. During the 
next two nights activity was concentrated on two of these, 
neither of which coincided with the underlying original 
trail. 
sumed that there had been a complete suppression 
of above ground activity. However, upon entering 
the pen and proceeding to the food hopper it was 
noted that three tunnels emerged from the snow 
into the food hopper which was protected overhead 
by a roof wide enough to prevent the drifting of the 
snow on top of the food. 
This indicated that the rats were active under 
the snow. Beginning at the food hopper these 
three trails were traced outward by sectioning 
through the snow with a hoe at approximately 
2-foot intervals. By this procedure the subnivian 
trail system shown by the solid lines in figure 71 
was revealed. The thickness of the snow above 
the floor of these tunnels averaged 9.7 inches for 
14 measurements. Eleven measurements were 
made of the height and width of the tunnels 
(compare with table 4). For each, these ranged 
from 60 to 75 mm., with each having a mean of 
67 mm. There was no obvious extension of these 
diameters of the tunnels at the many other unmeas- 
ured points where the tunnel was sectioned. It 
is obvious from this that only one rat at a time had 
traveled any specific section of this system of 
tunnels. 
The floor of the tunnel was formed by a thin 
compacted layer of the new snow. This was 
sufficiently thick to completely mask the muddy 
trail preexisting on the former snow at points 
where the subnivian trails and the former trails on 
top of the snow coincided. This indicated that 
direct visual perception of the trail was precluded. 
It is not known whether the rats could detect the 
previous trail by olfactory means through the thin 
layer of compacted snow. However, we shall 
examine the data for enlightenment on this point. 
As had already been noted, there is considerable 
evidence that rats utilize vertical objects at a dis- 
tance toward which to direct their locomotion. A 
typical distance between vertical objects is 5 feet. 
In order to detect an object 5 feet away and 2 feet 
above the surface of the tunnel would require 
seeing it through approximately 23 inches of snow. 
It seems rather unlikely that this could be accom- 
plished at night. Therefore, it will be concluded 
that neither visual perception of the former trail 
nor visual perception of neighboring vertical cues 
were in operation. 
By a cursory examination of the system of sub- 
nivian trails, the maximum number of sets of 
responses that is revealed by these data can be esti- 
mated. There were four boxes and five burrow 
systems (the latter counting the North Alley Burrow 
twice because of the two widely separated exits) 
in which rats were harboring, and there was one 
food source. Counting each segment of tunnel 
between harborages, between a harborage and the 
food source, or the blind tunnels as a response by a 
rat there is a maximum of 20 responses. In all 
probability a much fewer number of rats was 
involved in these initial responses. Once the tun- 
nel was developed by one rat the response of other 
rats which followed it might be a simple tactile 
one to the walls of the tunnel. This was the only 
instance during the history of the colony for which 
conditions were suitable for the development of 
subnivian trails. Since the orientations shown dur- 
ing the development of these trails appear to be 
quite revealing as to the modes of orientation, 
separate sections of the trail will be commented 
upon separately. Figures 14 and 71 must be com- 
pared in order to follow the interpretations 
presented below. 
A. The trail from Box 9 to Box 3. The move- 
ment without a doubt began at Box 9. There are 
three general directions toward which movement 
from Box 9 might be oriented: 1. Box 3, which 
served as a way station during travel, and the food 
hopper; both of these were in the same direction. 
2. Box 1 and/or Passage 1; these are secondary 
points of orientation which lie along the route 
which, has to be traveled in order to get to the 
main goal, the food source. 3. The South Alley 
Burrow. Animals from Area I occasionally stopped 
in this burrow system along the way to the Food 
75 
