blue jay, the caws of crows and the screams of cats. 
During the first summer the rats, which consisted 
mainly of those introduced into the pen, would run 
for cover upon hearing the click which occurred at 
the time of unlocking the observation tow T er door 
or opening the latch to the observation tower 
window. 
Strange noises which occurred within the pen 
produced a more marked response. The click of a 
camera shutter is a good example. During July 
1948 a camera was suspended about 6 feet above 
Path 1 over some food nailed to a board. Four 
hundred and forty-seven day old male No. 16, who 
had already become adjusted to eating at this new 
food source, was here alone when the shutter was 
first released. As the shutter clicked it jumped and 
dashed rapidly 20 feet to the South Alley Burrow 
where it stopped to turn around and look. A 
short while later a picture was taken after several 
rats had settled down to eating. Upon the shutter 
click the group of rats jumped in all directions as if 
there had been an explosion in their midst. This 
was the rats’ first experience with this noise. Dur- 
ing the spring of 1949, when the U.S. Army 
Signal Corps was preparing to take extensive mov- 
ing picture footage of the rats, it was realized that 
this reaction to strange noises would interfere with 
the behavior of the rats. In order to overcome this 
difficulty, a nonsense sound track was prepared 
which included talking, motors, and miscellaneous 
mechanical sounds. This was played for 2 days 
before the filming began and nearly continuously 
during the night for the next 6 weeks while the 
filming was in progress. The rats rapidly adjusted 
to this nonsense sound and insofar as I was able to 
detect all activities occurred in a perfectly normal 
fashion. This even included responses which were 
elicited by the vocalizations of other rats. It was 
concluded that the rats were able to sort out these 
vocalizations from the miscellaneous noises on the 
sound track. 
Such vocalizations by other rats may elicit a 
response similar to that produced by strange 
mechanical noises. As an example take the field 
note of June 28, 1948, 6:40 p.m.: “Some rat gave 
a loud squeal and all other rats ran into burrows 
or other hiding places. However, hardly a minute 
had elapsed before they began coming out.” 
E. The “ Chain-fright Reaction .” By July 1948 
the increasing population size caused the rats to 
exhibit frequently what might be termed a “chain- 
fright reaction.” Fights, chases, rustling of dry 
leaves, in fact anything that produced a sudden 
or unusual noise, made rats scatter even when they 
were some distance away. 
This transfer of the state of being disturbed from 
one individual to another presented a means for 
a more continuous state of disturbance as the 
population continued to increase in size. The 
operation of this phenomenon is exemplified in the 
characteristic behavior of shifting the place of 
harborage upon being disturbed. This occurred 
when the disturbance was by the intrusion of 
another rat or by some outside agent such as the 
observer. In each case where an initial resident 
was joined by another rat, either one must leave 
or they both must make some mutual adjustment. 
Depending upon (a) the degree to which the in- 
coming rat is disturbed, (b) the ability of the 
meeting individuals to make mutual adjustments, 
and (c) the distribution of other individuals through 
the environment, there will inevitably result 
secondary and other sequential disturbances. 
This general inference is based upon the following 
types of observations. Marked rats, who were 
undisturbed by the observer have been noted to 
enter a harborage box or burrow in which another 
rat was known to be at that time. Sometimes 
both rats remained in the harborage with no 
resultant disturbance having been detected by the 
observer. When this happened it was concluded 
that some mutual adjustment had been made 
despite the fact that this may have produced some 
stress. Very frequently though, the sounds of 
fighting within the harborage, or the squeals 
accompanying it, were heard by the observer 
in the tower. Even so both rats might remain in 
the harborage box although more often either the 
invader or the resident rat would depart. A 
similar sequence of events sometimes occurred 
again if the departing rats entered another harbor- 
age containing a rat. Similar events accompanied 
the release of trapped rats into portions of the pen 
outside their normal range. On their return home 
such rats entered harborages along the way. 
Usually they were chased out of these alien harbor- 
ages, but at the same time one or more of the 
resident rats might also depart. On a number of 
occasions the observer opened all the harborage 
boxes and identified pelage marked rats without 
handling them. Although the area in which 
harborage boxes were initially opened varied it 
was customary to travel around the pen in a 
counterclockwise direction. It frequently hap- 
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