Figure 4. — View of the pen in late April 1948 looking northeast from the observation tower. The South Alley Burrow is 
located just to the right of where the observer is taking notes. At the four entrances to the Food Pen in the center are 
placed the activity recorders in the black tunnel boxes. Photograph by F. Di Gennaro. 
(2), screech owls ( 9 ), sharp-shinned hawks ( 3 ), 
broad-winged hawk (/), skunks ( 13 ), opossums 
( 15 ), weasels ( 3 ). Since the avian predators 
normally alight on the posts rather than fly 
directly in, no loss was attributed to them other 
than by the great-horned owls which would fly 
directly into the Food Pen. Probably five rats 
were lost to them before this problem was solved 
by placing traps on top of 12-foot poles surrounding 
the Food Pen. An opossum managed to get over 
in the pen during the summer of 1948 and was in 
for several days before being detected. It was a 
young animal and the evidence was that it may 
have destroyed one or two litters of unweaned rats. 
The three-strand electrified barrier fence, 10 feet 
peripheral to the main rat-proof fence, effectively 
kept dogs away. Initially cats were quite a prob- 
lem although there was no evidence that any rats 
were killed by them. At first there was only a 
single strand of electrified wire on top of the fence. 
One day a cat was observed to make a leap from 
the ground over the top of the 4-foot fence, and 
over the electrified wire immediately on top of it, 
and land on the overhang from which it jumped 
into the pen. Following that incident three more 
strands of electrified wire were placed on top of 
this fence. This effectively eliminated disturbance 
by cats (see fig. 6). 
A hopper which provided a continuously avail- 
able source of Purina laboratory pellets was placed 
directly in the center of the pen. Here 10 to 15 
rats could obtain food simultaneously without 
crowding. The Food Pen was surrounded by a 
barrier fence through which there was a 3-inch 
diameter opening in the center of each of the four 
sides. This pen was 20 feet in diameter. A single 
water fountain was placed in each corner of the 
Food Pen. These were 2 % gallon chicken-water- 
ing-troughs around which it was possible for several 
rats to drink simultaneously. Food and water 
were continuously and abundantly available at 
this central location. Additional food in the form 
of garbage from the author’s home or neighboring 
grocery stores was occasionally provided. 
Nine artificial harborage boxes (figs. 7, 54, and 
55) were placed in each of the four triangular 
corner areas of the pen. The center of each box 
was 10 feet from the center of adjacent ones. An 
18-inch long and 3-inch diameter clay drain led 
from the surface of the ground down into each of 
of the boxes which were sunk into the ground. 
These 36 harborages were designated as boxes 1 
6 
