Food Pen, two over the burrow system which 
developed in the South Alley, and two which flooded 
Path 1 and portions of the West Alley. Practically 
all observations of free ranging rats were made from 
the tower just outside of the pen above the base 
of Path 1 . The rats never appeared to be disturbed 
in the slightest by these lights. The use of a moving 
spot light had to be discontinued since it did disturb 
the rats sufficiently that they would cease their 
activities and enter their harborages. During 
April and part of May 1949, when the activities 
of the rats were filmed by the U.S. Army Signal 
Corps ( 8 ), it was possible to make rather extensive 
observations of rats at several places in the pen 
from blinds whose platforms were only 6 feet above 
the ground. At times there were as many as five 
2,000-watt lamps flooding a rather limited area. 
After a short period of adjustment the rats would 
enter these brightly lighted areas without any 
hesitation and proceed with their normal activities. 
Whenever any considerable noise accompanies 
the observation, it is necessary to drown it by 
broadcasting a continuous nonsense sound tract. 
This was found to be the effective solution of 
obscuring the distracting noises occurring at the 
time of the filming. Within a day or two after 
first using this nonsense sound-tract broadcast, no 
further disturbance to the normal activities of the 
rat could be discerned. 
About once every 6 weeks (see fig. 10) the attempt 
was made to capture every rat in the pen. The 
main device used in the capturing of the rats was a 
simple box trap (9) (fig. 31). The most effective 
baits used were sunflower seed, a horse-feed mixture 
(oats, cracked corn, and alfalfa impregnated with 
molasses), oranges cut in eights, garbage, and sec- 
tions of sweet potato. The trapping usually was 
more effective when the bait was merely thrown 
into the back of the trap and not placed on the 
trigger. Rats were removed by allowing them to 
run into a cloth sack which was held over the 
opening to the trap as the door was removed. 
With adult rats it is best to elevate the trap by 
setting it on a neighboring trap. This allows the 
sack to hang downward, and the rats run into the 
sack. With juvenile rats it is frequently necessary 
to tilt the door end of the trap upward, while at 
the same time holding the attached sack also 
extending upward. The entering of the sack from 
the trap is an escape behavior which is positively 
geotropically oriented in adults, but negatively 
oriented in juveniles. During these periods of 
TRAPPING PERIODS AND NUMBER OF RATS EXPOSED TO TRAPS 
(NO. OF RATS) 
. INTERVAL 
I s - 
& INITIAL DATE 
INITIATION OF STUDY 
TRAPPING PERIOD 
V 
1 
h 36 -4- 
( 8 ) 
47 
(23) . (22) , (22) 
-j(f- 33 -^-27-f- 37 -sj^- 
A ' 
3 
. / 
1 
* < 
3 
8 
5 
5 
5 
_ J L 
1 
L L_ 
1 1 
ll 
1 ■ 
1 '1 
(NO. OF RATS) 
50 INTERVAL 
ff> INITIAL DATE 
TRAPPING PERIOD 
(22) 
— 55 
1 
-+- 
9 
A 
7 
i_h 
(21) ,(64), (70) , (74) , (82) i (133) , (140) 
75 — — 18 27 -)p — 49 — >j<— 56 — j)t— 36 — a(t 75- 
24 12 8 1722 2 7 21 27 
A / 
2 
V 4 
3 
V A 
6 
A / 
3 
3 
k / 
6 
4 
_ll_ 
L_l_ 
_L 
ill 
] 1 
] L 
(NO. OF RATS) 
_ INTERVAL 
<T> 
£ INITIAL DATE 
TRAPPING PERIOD 
, (136) , (7 1*) , 
-f- 43 50 — J) 
10 22 13 
A A A 
A 
3 
k A 
2 
J 1 
L i 
t — TERMINATION OF STUDY 
J I I I I I L 
MONTH: JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. 
Figure 10. — Trapping periods* * and number of rats exposed to traps. The number of rats exposed to traps was calculated as 
the number of rats 30 days or more of age at the beginning of one trapping period which lived through the following 
trapping period. This figure is shown in parenthesis above each trapping-period histogram. The interval in days between 
adjacent periods of trapping is indicated by the figures between the arrows. The duration of each trapping period in 
days is shown immediately to the right above the respective histograms. 
*The April 1949 trapping period was the only time of trapping when all rats were not exposed to traps. At this time 
traps were set only in the north quadrant of the pen. It was estimated that only 60 percent of the 119 rats were exposed to 
these traps. 
11 
