where the tunnel contacted the underground por- 
tion of the barrier fence the rats dug upward to 
form SAB entrance 24. From this point the burrow 
system rapidly expanded along the edge of the bar- 
rier fence to extend past the tile of Passage 1 and 1 0 
feet over into the West Alley. Sometime between 
October 6, 1948, and May 17, 1949, this long ex- 
tension of the South Alley Burrow became sepa- 
rated from the older section of the burrow. Fre- 
quently in my notes this extension of the South 
Alley Burrow is referred to as the Passage 1 Burrow 
or the Tile 1 Burrow, although historically and 
socially the two sections were one. 
By the terminal survey on May 17, 1949, figure 
20, many of the isolated mounds had fused. In 
this figure are shown the position of 26 former en- 
trances, which were at this date permanently 
sealed. 
On May 17, 1949, the South Alley Burrow was 
excavated. The pattern of the underground bur- 
row is shown in figure 21. It will be seen that the 
tunnels, passages, and cavities form a complex in- 
terconnecting system. In the northeast, older por- 
tion of the burrow the tunnels were shallow, that 
is, they were narrow in their vertical dimensions. 
In addition, the nests here were in disrepair. This 
portion of the burrow was inhabited by a senescent 
female and a younger but nonreproducing adult 
female. The central southwest portion of the bur- 
row was not only the most complex in its structure, 
but it also housed seven females and five litters of 
varying ages. Here the tunnels and cavities were 
larger in their dimensions and the nests were in 
good repair. This portion of the burrow also con- 
tained one adult male, No. 49, the dominant male 
of the entire pen. No other adult males were pres- 
ent in this burrow system. The Passage 1 portion 
of the South Alley Burrow, while rather extensive 
in the total amount of tunnels present, housed only 
two females and their litters. 
This latter is one piece of evidence, among several 
similar ones to be described later, that indicates a 
definite relationship between the physical structure 
of a burrow and the social structure of the group of 
rats which it houses. Each appears to be both 
cause and effect. Where the inception of a burrow 
near a wall causes the burrow to elongate along the 
wall, this separation from each other of the mem- 
bers of the colony appears to reduce the integrity 
of the group and there follows a lowering of social 
rank of the members of the colony. On the other 
hand, where other factors have caused a breakdown 
in the social structure of the group, the rats seem to 
be unable to maintain a complex interconnecting 
burrow system such as is seen in the South Alley 
Burrow system. 
Such a relationship between a poorly integrated 
social structure of a colony and the inability of its 
inhabitants to maintain a well organized burrow 
system is exemplified by the history of the North 
Alley Burrow. The pattern of the underground 
tunnel system is shown in figure 22. This is as it 
was on May 29, 1949. The surface development 
of this North Alley Burrow indicated that at one 
time the underground tunnel system was as com- 
plex as the central portion of the South Alley 
Burrow was on May 17, 1949. The complexity 
which had existed was lost through a fragmenta- 
tion. This left segments of the burrow system 
completely isolated or only as arms with no cross 
connections with other arms. In other words, 
fragmentation of the tunnel system made it im- 
possible for a rat to make a circular movement by 
following the tunnels. 
Figure 14. — Heavily worn trails November 1948. X’s 
represent positions of drain tile entries to harborage boxes 
or entries through the soil to burrow systems. Cross 
hatched areas are mounds of burrow systems. The numer- 
ous entries to the South Alley Burrow (SAB) and the 
North Alley Burrow (NAB) have been omitted from the 
figure. The smaller dots represent the positions of trunks 
of trees. The larger dot between boxes 20 and 21 is the 
large pine tree in area III. The three large dots in the 
Food Pen are the locations of water fountains. The three 
solid rectangles in the West Alley by the median barrier 
fence and the one next to the Food Pen in the East Alley 
are rolls of wire. The row of open rectangles in the West 
and East Alleys are piles of traps. 
17 
