A friend of mine ended up with 5 JRTs in a large fenced in yard. He calls them his "yard piranhas". They got that name when a large neighbor dog, that had been harassing them, got in his yard and was going to beat up on one of them. All five of his Jacks picked different a spot to bite on that 80 lb dog and started trying to eat their way in. That dog couldn't get out of their fast enough, dragging them along with him, and never ever made a peep again.
