What’s more, he found that canines “role-reverse” or “self-handicap” during play. When a big dog played with a smaller one, for example, the big dog often rolled on her back to give the smaller dog an advantage, and she allowed the other dog to jump on her far more often than she jumped on him.

Bekoff also spotted a number of other blink-and-you’d-miss-them behaviors, such as a sudden shift in the eyes — a squint that can mean “you’re playing too rough” — and a particular wag of the tail that says, “I’m open to be approached.” Humping a playmate during a romp, meanwhile, was often an invitation to nearby dogs to come join the fun.

Such signals are important during play; without them, a giddy tussle can quickly turn into a vicious fight.

In the wild, coyotes ostracize pack members that don’t play by the rules. Something similar happens in dog parks: If three dogs are playing and one bites or tackles too hard, the other two are likely to give him the cold shoulder and stop playing with him, Bekoff says. Such behavior, he says, suggests that dogs are capable of morality, a mind-set once thought to be uniquely human.