RAT A 
I st ! 2nd 
RAT B 
EFFORT 
PER 
UNIT 
OF 
LENGTH 
dM 
d 
AREA A* 
LENGTH OF TUNNEL LENGTH 
d 2 M d 2 M 
2 4 
IS PROPORTIONAL TO EFFORT 
OF TUNNELS 
Figure 46. — Diagrammatic explanation of the transportation factor. Effort involved in transporting excavated dirt increases 
proportionately more rapidly than does the corresponding distance over which transport takes place. M = rate of using 
effort. Arrows indicate removal of dirt at entrance to funnel. See text. 
decrease until they form one-third of the total 
tunnels. In this structural balance internuncial 
tunnels cannot appear until there are at least 
three segments. As the size of the burrow increases 
internuncials increase in frequency until on the 
average they form approximately one-half of all 
tunnels. The development of this structural bal- 
ance reflects certain behavioral characteristics of 
the rat: (a) The blind tunnels reflect the tendency 
to dig. (b) The frequency of exit tunnels is 
bound to the previously discussed response (see 
fig. 48) of locating the chambers not too far 
removed from the outside, (c) The origin of 
internuncials probably reflects several components 
of behavior including: (1) The cessation of a 
particular pattern of work as a function of the 
increasing magnitude of effort required for its 
continuance. (2) The inhibition of one behavior 
in favor of its replacement by another (i.e. nest 
cavity construction versus tunnel digging). (3) 
Redirection of digging toward a parallel tunnel. 
(4) Maintenance of contact with both ends of a 
tunnel segment. 
In the light of these considerations we can make 
an approximation of the optimum size of a burrow. 
Certainly optimum conditions cannot be said to 
be complete until the proportion of blind tunnels 
has become constant. This beginning has been 
arbitrarily assigned as 16 tunnel segments. The 
upper limit of the optimum range has been assigned 
at 40 segments, the burrow size where internuncial 
and exit tunnels approach an asymptotic level. 
Even under the most favorable conditions, the 
crowding of rats about burrows consisting of more 
than 40 segments begins to produce marked 
strains on the social stability of the groups. 
From the curve of number of rats per exit, 
shown in figure 50, the number of adult rats 
inhabiting burrows within this optimum range 
may be calibrated. 
47 
