Figure 25. — The “metabolism” of burrows. Each horizontal row of dots represents the excavations from a single hole or entry 
into the burrow. No excavations were observed at same entries. Rate of metabolism is equated with frequency of repair 
(i.e. fresh excavations at an old entry), and with expansion of the burrow (i.e. opening of new entries). 
the neighboring barrier fence or the nearest har- 
borage box. In addition, there was a lesser tend- 
ency for these tunnels to approximate the route of 
the trails existing on the surface. There were 14 
such clear-cut examples. One has already been 
mentioned (pp. 18-19) and is illustrated in the 
right-hand drawing of figure 23. Another very 
good example is illustrated in figure 33. This 
shows two adjoining harborage boxes with tunnels 
surrounding them and with a single very straight 
tunnel connecting them across a distance of 10 feet. 
All dirt from these burrows was excavated into the 
harborage boxes. The burrows about these two 
boxes were initiated independently during the latter 
part of November 1948 and it was not until in 
December that the connecting link between them 
developed. It is not known whether this tunnel 
developed from only one of these boxes, or whether 
tunnels were simultaneously constructed from each, 
and met half way between at a point where an 
entry from the surface was then built. 
E. The History of a Typical Burrow During its First 
Few Weeks. The history of a particular burrow 
and of the female who constructed it may well be 
considered as typifying the conditions surrounding 
the origin of a new burrow system. A full under- 
standing of some of the conditions or behaviors 
discussed is dependent upon more detailed accounts 
presented in various of the later sections. 
A map of this burrow as it appeared on May 24, 
1949, is shown in figure 34. It was constructed 
by female No. 378 who was born at the South 
Alley Burrow on September 4, 1948. This latter 
burrow was the most favorable location for birth 
of any in the pen. Even so, the large numbers of 
individuals born at this location forced many to 
emigrate. During the winter of 1948-49, when 
sexual activity was nearly nonexistent, female 378 
remained at her home burrow. With the advent 
of spring she shifted to Area II where relatively few 
rats were residing at that time. Her first concep- 
tion occurred about February 26, 1949. As is 
27 
