Peacemakers likely intervene in squabbles to ensure they do not spiral out of control.
That in turn stops violence affecting other apes in the group and helps the animals live harmoniously together.
Wild orangutans usually prefer to lead relatively solitary lives.
"Orangutans spend most of their time alone in the wild," says Dr Tomoyuki Tajima of Kyoto University in Japan, who recorded the behaviour along with colleague Mr Hidetoshi Kurotori, a keeper at the Tama Zoological Park in Tokyo.
"However, they sometimes come and travel together, so researchers regard them as semi-solitary."
Yet in captivity, orangutans socially interact far more often, sometimes becoming aggressive toward one other.
So Tajima is studying how the apes cope with this novel situation.
He was surprised by what he found.
That conclusion stems from observing five orangutans, comprising two adults, two juveniles and an infant, housed at the Tama Zoological Park.
During this time, one of the juveniles, a six-year-old female named Kiki, was introduced to the group.
Over 13 days of observations, another much older female called Chappy, thought to be 34 years old, became repeatedly aggressive toward Kiki, either chasing or physically attacking her on 28 separate occasions.
During 19 of these interactions, another orangutan intervened, physically stepping between the two squabbling apes to separate them.
Most of the time the peacemaker was an elder female orangutan called Gypsy, who is thought to be 51 years old.
However, a young juvenile male called Poppy also stepped in to quell the trouble.
On every occasion bar one, the peacemaking orangutan was not attacked by Chappy, the aggressor.
Similar peacemaking behaviour has been seen in gorillas and chimpanzees, but these are natural group-living apes, say the researchers.
For orangutans to mediate in this way shows just how flexible they are in adapting to new environments, says Dr Tajima.
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