STEEL TRAP PLACED HERE 
INSULATORS o, 
HOLDING 
ELECTRIC 
"FENCE" 
SCREEN GLASS 
GROUND 
LEVEL 
1/2 INCH MESH 
HARDWARE CLOTH 
EIGHT 
FEET 
CROSS SECTION OF RAT AND PREDATOR PROOF 
LIMITING FENCE 
Figure 6. — Cross section of rat and predator-proof limiting 
fence. Both the overhanging screen glass shelf and the 
underground shelf of hardware cloth were directed into 
the pen. 
In order to facilitate the systematic recording 
of data with reference to their place of occurrence, 
the following system as shown in figure 3 was 
devised. (1) The triangular areas were designated 
as Areas I to IV in a counterclockwise fashion 
starting with the south corner. (2) The passages 
through the outer of the two barrier fences were 
numbered from 1 to 4 in a counterclockwise 
fashion starting with the southwest passage be- 
tween Areas I and IV. (3) The region between 
the Food Pen barrier fence and the second barrier 
fence was called the alley with the four segments 
of it being specifically called the South, East, 
North, and West Alleys. (4) The four passages 
through the Food Pen barrier fence were numbered 
from 5 to 8 in a counterclockwise fashion starting 
with the south passage. (5) In order to facilitate 
observations the Food Pen was always kept clear 
of any plant growth. Likewise a 3-foot wide path 
was kept cleared around the outside of the Food 
Pen and from the corners of the Food Pen toward 
the passages through the second barrier fence. 
The limitations of these pathways are shown by 
dashed lines in figure 3. The paths were num- 
bered from 1 to 4 in a counterclockwise fashion 
beginning with the southwest path. 
3 . The Rats Introduced into the Pen 
The colony in the pen was begun with five pairs 
of rats (table 28; pp. 136—143), which were 
assumed to have been as genetically homozygous 
as it is possible to trap them in the wild state. 
This assumption is based on the following grounds. 
The rats were trapped in February 1947 on Parsons 
Island, a 150-acre tract in the Chesapeake Bay. 
A natural causeway connecting the island with 
the mainland was severed about 1900. Since 
this time the rats have been effectively isolated. 
The number of rats on the island has fluctuated 
widely both on an annual basis and on the basis 
of changes in agricultural practices. An experi- 
mental reduction by poison in 1923 was conducted 
by the Fish and Wildlife Service. A second ex- 
perimental reduction was conducted in the spring 
of 1946 by J. T. Emlen and D. E. Davis, of the 
Rodent Ecology Project. Estimation of numbers: 
(a) prepoisoning, 670; (b) postpoisoning, 220. 
At the time of trapping in February 1947 surveys 
indicated that there were probably no more than 
150 rats on the island. Under these circum- 
stances of fluctuation in population size, it might 
be anticipated that considerable homozygosity 
would have been reached though gene drift (7). 
It is from this inference that variability of results 
in this paper are judged to arise from environ- 
mental conditions rather than from the hereditary 
variability among the rats. 
Figure 7. — Cross section diagram of the construction of the 
wooden harborage boxes. 
4 . Objectives of the Investigation 
The central objective of the investigation was to 
determine the manner and extent to which social 
interactions might influence population growth. 
However, the nature of the experimental situation 
presented the opportunity for simultaneously 
investigating several subsidiary aspects of the 
biology of the rat. These include. 
8 
