The Chase
Based on research, the canine turned out to be a hunting dog, and the story was possible that he was chasing something up the tree. Discoveries noted that the dog was about four years old in 1960, so he could have possibly been chasing a squirrel or raccoon who slipped up the tree. It must have gone through a small opening in the tree and followed it up.
It is also believed that he was trying to dig his way through to the top, but not realizing the space was getting smaller, the higher it went. This piece of information was a shocker to everyone, especially after noticing how high in the tree it went.

The Chase
