producing this exception to a general rule. 
Vegetation was kept cleared for 12 inches 
away from the fence to prevent a shorting 
of the adjacent electric barrier wire. Rats 
most usually ran along this cleared pathway 
in the direction of the corners of the pen. 
While so traveling their heads were usually 
bent slightly toward the fence as if they were 
attending to stimuli outside the pen. Then 
the rat would stop. Presumably defecation 
took place at these times. All I can say here is 
that it always looked as if the rats were halted 
by some event random in onset with reference 
to the outward motion of the rat along the 
fence. Once the behavior of motion was in- 
terrupted, the intervening stopping was fol- 
lowed either by a retracing of the prior 
route along the fence or a turning out into the 
adjoining area as soon as an intersecting trail 
was encountered. Feces in piles adjacent 
to the outer fence were often in greater num- 
ber, up to 100, than would be deposited by a 
single rat at a single period of defecation. 
This further suggests that though initial 
defecation here is on a chance basis, each 
defecation biases the probability of recurrence 
at the same spot. It is this sequence of be- 
havior which leads to the decreased frequency 
of defecation with increases in distances from 
a center of orientation as was discussed under 
the topic of orientation and shown in figure 75. 
5. Feces tend to be concentrated on either side 
of the passages through the fences. Stopping 
here is the consequence of two related behaviors. 
One is that a dominant rat spends a con- 
siderable amount of time at passages where 
it prevents others from passing through. As 
a result of having received punitive action 
near these passages the subordinate rats 
hesitate before going through. 
6. Feces becomes concentrated in any sheltered 
area, particularly if there are vertical objects 
toward which the rat may orient. Thus, the 
activity recorder shelters at the passages into 
the Food Pen and the harborage boxes them- 
selves became foci for defecation. 
7. Defecation which occurred in the harborage 
boxes was normally away from the nest. Oc- 
casionally feces were found on the edge of a 
nest. In only one harborage box, No. 36, did 
defecation on the edge of the nest regularly 
occur (see fig. 54). However, in this case the 
rats regularly relined the nest and kept it 
clean. When the number of rats inhabiting a 
single nest exceeds five or six adults, there is 
a beginning of cessation of maintenance of 
repair of the nest, and an accompanying depo- 
sition of both feces and urine in the nest. 
With aggregates of 15 or 20 adults the harbor- 
age site becomes extremely foul. 
The presence of feces recorded on the surface of 
the ground has been utilized by the group at Johns 
Hopkins to assist in their sign-survey technique of 
estimating the abundance of rats. Although dif- 
ferent workers can make very close estimates of the 
number of rats present, the actual number of feces 
were rarely recorded. In this pen study, however, 
both accurate counts of feces and close estimates of 
the number of rats alive are available (table 21). 
Table 21. — Variability in the number of above-ground 
feces per rat 
Date 
Number of 
weaned rats 
Feces per 
rat 
June 1947 
15 
12. 6 
September 1947 
13 
40. 4 
November 1 947 
26 
72. 7 
January 1948 
25 
3.8 
May 1948 
66 
52. 5 
November 1948 
147 
72.2 
February 1949 
135 
8. 8 
It is quite obvious that the actual number of 
feces is quite misleading with reference to any rela- 
tionship to population density. One factor that is 
involved is the severity of cold weather. The 
January 1948 survey includes only those feces left 
on the snow-covered ground during two nights 
following the snowfall. At this time when the mini- 
mum temperatures dropped below 15° F. there 
were only approximately two feces above ground 
per rat per night. The February 1949 survey also 
occurred when there was a snow cover. However, 
the count was made after only one night had 
elapsed following the termination of the snowfall, 
and at a time when the minimum temperature was 
only slightly less than 31° F. Thus, with the 
warmer air temperature approximately four times 
as much defecation occurred per night above 
ground. At other seasons of the year the attempt 
was made to count only those feces which were 
probably no older than 1 to 4 days. However, 
111 
