Figure 76. — Avoidance of a strange situation. A hole from the Norlh Alley Burrow is in the left foreground. Prior to placing 
the two tunnel traps in the positions shown, the rats had developed two trails to the Food Pen from this hole. One went 
toward passage 7 in the left background. The other crossed toward the right center of the photograph in the direction of 
the northwest corner of the Food Pen. There were no tracks made by rats in the snow on either side of these two trails 
during the several nights preceding the placing of the two traps. There was a door at either end of these traps. These 
were propped open with sticks and placed over the trails so that it was possible for the rats to pass through without diverg- 
ing from their previously established routes. Each trap contained a layer of loose snow. The next morning no tracks 
were visible over this snow within the traps. Instead the rats had diverged to either side of these traps as the tracks through 
the snow indicate. 
away as soon as the rat’s nose nearly reached the 
open door. Finally the rat would desist and 
engage in some other activity. I am also con- 
vinced that rats soon learned to detect whether 
or not a trap was set. Frequently traps were 
prebaited for 1 to 3 days before setting. At such 
times most or all of the bait was removed each 
night. When the traps were set three changes in 
appearance took place: (a) a wire trigger pro- 
truded down into the center-rear of the trap near 
the bait, (b) A wooden trigger extended parallel 
above the roof of the trap connecting the wire 
trigger and the door. (c) The door stood up 
in its sliding groove and also obscured the upper 
one-third of the doorway. Then many traps 
remained set with the bait undisturbed, even 
though during the preceding days, when traps 
were unset, the rats regularly removed the bait. 
The reason that I believe that trap-avoidance is 
a learned response is that most young rats were 
captured during their first period of exposure to 
traps, in contrast to older rats who frequently 
avoided capture through an entire 3 to 8 day 
period of continuous exposure to set traps. 
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