A German Shepherd just rolled belly up and handed a pile of Doberman puppies its own throat, then stood up and did it again.Rolling onto its back puts the throat and the stomach, the two spots a dog guards hardest, out in the open. In a serious fight, no dog offers those up. During play, the bigger, older, stronger animal flops over and takes the losing position anyway. Researchers who study dog play call this role reversal. The animal that could easily win chooses to look like the one that lose