African great apes play that ever-popular kids' game as a way to learn how to keep a competitive edge.
Gorillas and other African great apes have been videotaped playing tag.
The game hones communication skills and helps test the limits of others, researchers believe.
Young dogs, coyotes and wolves also play tag, other research suggests.
Young gorillas and other African great apes play a lot of tag, suggesting that this common childhood pastime has deep primate roots, according to a new study.
Tag is based on hit-and-run behavior, and the study, published in the latest Royal Society Biology Letters, presents the first evidence that non-human species try to maintain their competitive advantage when responding to an unfair situation.
What's unfair about tag? One individual gets tagged -- or when played by gorillas, slugged -- while the other individual doesn't.
"The hitting can be very hard and still be part of play," co-author Marina Davila Ross, a research fellow in the Department of Psychology at the University of Portsmouth, told Discovery News. "Apes play overall much rougher than humans."
She and colleagues Edwin van Leeuwen and Elke Zimmermann studied play-fighting videos of 21 gorillas from six colonies in five European zoos. The researchers filmed and collected the videos over three years.
Like human tag, one gorilla runs up to another and taps, hits, or outright punches the second. The hitter then usually runs away, attempting to avoid being hit back. Davila Ross and her colleagues also noticed that, like kids, the gorillas would reverse roles, so sometimes the first hitter would be the tagger, and vice versa.
All African great apes appear to play tag, and younger apes play it much more often than their elders. Tree-dwelling orangutans likely also play a similar game, but not on the ground, according to Davila Ross.
"I believe that this kind of behavior helps young gorillas to improve their social and cognitive skills," she said. "It is likely to help them to learn how far they can go with others and where the boundaries are for the different members of their groups."
Since kids play nearly the exact same game, she theorizes that the "ability of humans to modify their behaviors for their own advantage in unfair situations might therefore have its roots in non-human primate evolution."
She and her team additionally think the childhood game helps apes, including humans, deal with later real conflict. By role-playing, the chaser and the chased appear to develop more refined and sophisticated communication skills.
Among gorillas, for example, the initial hitter usually shows what is called a "play face." Its eyes and mouth are open, but lips are relaxed and sometimes held over the teeth. This expression in apes is often accompanied by a rhythmic vocalization resembling human laughter.
Marc Bekoff is a professor emeritus of ecology and evolutionary biology at the University of Colorado, Boulder, and is co-author of the book "Wild Justice: The Moral Lives of Animals."
Bekoff told Discovery News that the new study "is very important" because it was "performed in the field and provides interesting information on how gorillas, and, I am sure, other animals, assess inequities and then behave either to maintain them or to reduce them."
"I observed this during play behavior in infant domestic dogs, coyotes and gray wolves and wrote about it in our book," he added, "so it is not surprising that non-human primates also engage in this type of behavior."
"We need these data to assess how animals make choices either to compete or to play fairly with one another," Bekoff said.