Rinning like a mule eating briars

Does anyone know the origin of the phrase, "Grinning like a mule eating briars"? Is it of European, American, African, etc. origin?

It sounds country USA to me.

In south Alabama/north Florida around the turn of the century it was in common use among my father's family. It could have an earlier origin, but if you've ever seen a mule doing this you have no doubt about WHY the phrase originated,