I remember reading once that the term “squirrelly” was coined during the pioneer days, and it was used to describe homesteaders who had been forced to live on a diet of squirrel. I believe the explanation was that squirrel meat is almost all protein so the complete lack of fat caused the rodent-munching pioneers’ brains to misfire, causing them to say things like: “rasfram, gfrrnarlgm chagnm, ghum!” as though everyone could understand it. Though it may just have been the taste of tree rat drove them mad.
Another explanation would be to watch their behaviour during the month of November, right before the snow flies. Not that they aren’t devilishly clever too: man’s greatest enemy. [clip here if the embeddy thing doesn’t work]