Squirrel.

[Bigbad]

I’ve been watching a squirrel run up and down the chestnut tree, (he doesn’t seem interested in what’s already fallen) then either open the case and scoff the nut on the ground, or run off with the case in his mouth.
As the cases are so prickly, how come he doesn’t hurt himself?

[Old_Geezer]

Maybe it's a masochistic squirrel.