That is exactly my belief. There is something primordial about a bone, the predator's "dessert" after a kill, that brings out the predator even in puppies.
Toss a shankbone in there with 6 week old pups, and you will have a free-for-all in no time
this happened to me by accident. Brother and sister, two littermates were romping around and she happened to find a bone just as he was approaching her. It went from wrestling around to dead serious in about half a second.