Range 
Number of tunnel segments 16 to 40 
Number of exits 8 to 1 5 
Number of adult rats 7 to 12 
Number of chambers 4 to 1 1 
These estimates are interesting in their indication 
that the presumed optimum range of size of bur- 
rows accommodates a number of adult rats over 
the range of normal litter size. This is only one 
line of evidence which suggests that the group 
size which adults tolerate is determined by the 
number of associations made prior to weaning 
among sibs. 
From figures 49 and 50, and from table 5, one 
can derive something of the characteristics of an 
optimum burrow (table 6). We shall take 40 
tunnel segments which is at the upper limit of the 
optimum range. 
When these considerations are coupled with the 
tendency of rats to develop circular burrow systems, 
which have a higher frequency of internuncial tun- 
nels toward the central, older section of the burrow 
(see fig. 21 for the South Alley Burrow), it is possible 
to arrive at a hypothetically ideal burrow struc- 
ture. This is shown in figure 51. From this figure 
it is possible to visualize the altered structure of a 
burrow system when it develops for a similar period, 
but with an orientation along a fence or similar 
elongate vertical structure. If the outer circle is 
severed at “A” and the inner circle of the burrow 
system severed at the three points designated “B” 
and stretched out into an essentially straight line 
the two burrow systems then have the differences 
in structures shown in table 7. 
CONTRIBUTION 
TO 
TOTAL 
EFFORT (W) 
Figure 47. — Relative contribution by successive behaviors to the total effort involved in constructing a tunnel. According to 
this notion the effort in constructing a tunnel of a given length is equivalent to the sum of the values for each curve where 
a vertical line drawn from the abscissa intersects them. 
48 
