Zeke, a seven-year-old male, was seen playing "airplane" with a two-year-old bonobo named Bo (short for Bolingo).
Their keeper, Jeremy Phan, recorded a video that shows Bo balancing on Zeke's upright foot. At times, Zeke bobs his leg, bouncing Bo up and down. Zeke also reaches up and pats Bo on the back repeatedly.
"Zeke started tickling him," says Phan. You can't hear it on the video, but Phan says Bo was laughing. Their laughs are lower-pitched and breathier than that of humans. In some frames, Bo can be seen smiling with his lips pulled up and his gums exposed.
It's the first time Phan has seen them engage in this specific behavior, but playing is something he sees all 12 bonobos at the zoo do often. It's an important component to the animals' social structure.
YOUNG BONOBOS "PLAY AIRPLANE" AT CINCINNATI ZOO
"It strengthens their relationship in the society," he says. "You see play at all ages and all sexes."
He's observed them engaging in affectionate behavior to resolve conflict or tensions.
"Their whole thing is make love not war," he adds.
THEY CAME HERE TO MAKE FRIENDS
Bonobos are in fact known to be more peaceful than some of their species cousins.
They have a matriarchal social structure that Phan says is relatively egalitarian. It's not often, he says, that the keepers see displays of aggression.
"Most of the time we see aggression if we see a male picking on the kids too much," he notes. Females will often band together to chase the male off.
While the bonobos play with each other often, it's not uncommon for them to get annoyed or simply not be in the mood. Phan notes they're similar to humans in this way.
And that's not the only way they're similar to humans. Studies have shown that bonobos share 99 percent of our DNA, making them our closest species relative along with chimps.
Chimps, for contrast, behave quite differently. They organise in patriarchal societies and really don't take too kindly to strangers. One 2010 study found they even murder for land.
A study performed in 2016 found bonobos are quite the opposite. They readily welcome new strangers into their fold, and they share food easily. Scientists think this may at least partially attributed to the fact that they're great at reading emotional cues and communicate well with each other.
The Cincinnati Zoo notes on their website that not many zoos in the U.S. have bonobos. Many of the ones in Ohio came from the San Diego Zoo, as part of a breeding program.
The Democratic Republic of Congo is the only country where bonobos still exist in the wild. There, the species is endangered, due to threats from poaching and habitat loss.