I briefly had an oversized Sheltie that was a great watchdog and a pesonal protection dog, at that. When pressed, he leapt into the air and went after the face. Sort of like a demon furball. The one time this occurred it literally scared the crap out of the would be mugger, who fell into the gutter in the rain, covering his head and face. Monte was leashed, so he never actually made contact...but his teeth clicked in the air literally an inch from the guys face as i dragged him back. It was quite impressive.