Figure 27. — A nonsenescing burrow system located in an embankment with an approximate 30° slope. Cavities are indicated 
by letters and entries by numerals. Entries 1 to 3 were toward the base of the embankment, while entries 8 and 9 were 
near the top. 
ing of the nest just preceded parturition. Enlarge- 
ment of the burrow and formation of the food 
cache just preceded the weaning of the young. 
This latter enables the young to avoid an excessive 
amount of social conditioning through the punitive 
action of older rats (pp. 148-151), that they 
might have otherwise received away from their 
home burrow. Although these behaviors do 
precede the time at which their consequences have 
survival value, there is no evidence that the rat is 
aware of their future usefulness. 
The initial histories of several burrow systems 
constructed by females paralleled closely the 
course described above. Once formed, such bur- 
rows may serve as foci for the elaboration of 
extensive burrow systems which gradually spread 
from these simple beginnings. For example, see 
the previously described expansion of the South 
Alley Burrow over a period of 2 years (p. 15). 
F. The Weights of Rocks Moved by the Rats. Rats 
remove rocks from their burrows either by carrying 
them out with their teeth or by pushing them 
along with their forefeet. One hundred and 
forty-three rocks were collected from the top of the 
mounds (fig. 35). Only one rock weighed less 
than 20 grams. This weight was the author’s 
estimate of the maximum sized rock carried out 
of the burrow in the rat’s mouth. Above this point 
the mean was 66 grams (a — 37 g). The heaviest 
25 rocks ranged between 100 and 210 grams, with 
a mean of 140 grams. The physical ability of the 
rats, and the frequency of different sized stones 
mixed through the soil are factors influencing the 
choice of rocks moved. In assessing the ability of 
rats in moving such objects it must be remembered 
that these irregular shaped rocks were not just 
rolled out but rather that their flatter surfaces 
were subject to considerable friction against the 
floor and walls of the burrows. Furthermore, 
these objects, which frequently amounted to 15 
to 30 percent of body weight, were pushed up 
inclines of at least 30°. When rats encountered 
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