Pen, look into one of the recorders without entering, 
and then to make similar responses at one or more 
of the other entrances before going elsewhere in 
the pen. These responses were exhibited by both 
100-day-old rats born in the pen and the adult 
rats introduced the previous winter. Although the 
recorder tape jammed during the night, and thus 
there was no record of possible entrances, the fact 
that the garbage in the Food Pen had been undis- 
turbed during the night forms a good indication 
that no rats had passed through the recorders into 
the Food Pen. During the hour of observation 
there was considerable fighting. This is in con- 
trast to the presumed suppression of conflict which 
accompanied the presence of strange objects on 
May 8, 1947. During the second night two of the 
four recorders operated satisfactorily. These left a 
total of only 11 marks on the tape. Judging by 
the fact that the garbage was still undisturbed on 
the morning of July 30, all that had happened was 
that a few rats had entered the tunnel just far 
enough to break the beam. With the clang of the 
closing solenoid the rats must have turned around 
and run out of the tunnel. During a 20-minute 
observation period on the evening of July 30 be- 
ginning at 7 :05 p.m. only five rats were seen active 
and only two of these even approached the Food Pen. 
They appeared quite nervous. Such a state was 
indicated by an interspersion of standing and snif- 
fing, hesitant approaches, then short dashes, rather 
than the more usual continuous ambling or running 
between goals. On the morning of July 31 there 
was no evidence that any rats had entered the Food 
Pen. This is based upon the fact that food placed 
the previous night directly in front of the tunnels, 
on the Food Pen side of the recorders had not been 
disturbed. This consisted of bread in front of two 
tunnels, table scraps in front of another, and water- 
melon in front of the last. These are all quite 
preferred foods over the normal diet of Purina 
checkers. Also, there was no dew during the night 
July 30-31. The rats may have had a little stored 
food but they certainly had no other source of 
water than that in the Food Pen. The two record- 
ers which were operating on this night showed 
only three marks, again indicating that rats which 
had come in far enough to break the beam were 
frightened away after the clang of the closing sole- 
noid. Although this new object and situation was 
sufficient to prevent any rats from passing through 
for the first three nights, the following five nights 
saw the completion of the adjustment of the rats 
to the point that they evidenced no difference in 
reaction to the tunnel and the clanging solenoid 
than they did to other aspects of their environment 
Rats born in the pen following the conditioning of 
these first few rats never showed any sign of avoid- 
ance of or disturbance by this tunnel activity re- 
corder. Rats just being weaned were observed pass- 
ing through the tunnels with no more concern 
than the adults. This unconcern may possibly be 
attributed to two causes: (1) Any object toward 
which the rat has no inherent aversion, and which 
occurs in the environment into which the rat is 
born, is accepted by it. (2) A rat is less likely to 
become alarmed over new situations towards which 
it associates remain undisturbed. 
B. Food. Food may initially elicit an avoidance 
response providing it is of a new type or at a new 
location. On October 3, 1947, a pile of garbage 
and a pile of cornmeal were placed in the Food Pen 
at 5:31 a.m. At 5:48 a.m. male No. 8 was the 
first rat to approach this source of food. It 
approached each of the piles slowly with neck out- 
stretched and its tail protruding straight behind. 
After the initial approach without untoward con- 
sequences, later approaches were direct with no 
hesitation. This is the characteristic behavior of 
the initial rats encountering a new source of food. 
Normally rats, which encounter the food after prior 
rats have already made a satisfactory adjustment 
and are eating peacefully at this source, exhibit 
nothing of this hesitant approach. This same set 
of responses has been noted to accompany the 
encountering of small strange objects such as rulers, 
jars, etc. which the observer left temporarily in the 
pen. 
C. Birds. Birds alighting in the pen normally 
caused no alteration in the behavior of the rats. 
Presumably this was of such frequent occurrence 
that birds were actually not strange objects. On 
only two occasions was a positive interaction noted. 
On June 22, 1947, as adult female No. 4 stood up on 
top of Box 6, a male yellow-breasted chat flew at 
it, whereupon she ran into the nearby weeds. On 
July 6, 1947, adult female No. 10 was entering the 
South Alley Burrow when a young house wren 
began taking a dust bath on the nearby Path 1. 
The rat turned around and ran at the wren as if 
in an attempt to catch it, but the bird escaped. 
D. Sound. Disturbances outside the pen, which 
may be presumed to have occurred irregularly in 
the past, seem not to disturb the rats. These 
include both the scolding and hawk cries of the 
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