A new paper published in Current Biology suggests a novel twist on an old saying: Instead of “monkey see, monkey do,” the paper proposes “chimp see, chimp pee.” Well, not in so many words. But the study, which involved 20 captive chimpanzees at the Kumamoto Sanctuary at Kyoto University, does suggest that when one chimp urinates, others urinate too.
“In humans, urinating together can be seen as a social phenomenon,” said Ena Onishi, a study author and a doctoral student at Kyoto University, in a press release. “This behavior, which is called tsureshon in Japanese, is represented in art across centuries and cultures and continues to appear in modern social contexts. Our research suggests that this phenomenon may have deep evolutionary roots.”
Read More: Understanding Our Closest Relatives in the Animal Kingdom
Contagious Peeing
When researchers at the Kumamoto Sanctuary saw something strange in the behavior of their captive chimpanzees, they set out to study it. Instead of urinating alone, the chimps appeared to urinate together, at around the same time, with one chimp initiating the behavior and other chimps continuing it.
Calling the behavior contagious urination, the researchers wanted to see whether it was at all similar to contagious yawning, whereby a chimpanzee or a human yawns, causing onlooking chimps or humans to do the same.
“Since there were no prior studies on contagious urination in any species, we drew parallels to contagious yawning, another semi-voluntary physiological behavior,” said Onishi in the release.
Observing the Kumamoto chimpanzees for over 600 hours, then documenting the almost 1,330 urinations that occurred during that time, the researchers determined whether the chimpanzees’ urinations were synchronized. They also determined whether the chimps’ urinations were influenced by their physical proximity and their social closeness.
“We found that chimpanzees, our closest relatives, tend to urinate in response to the urination of nearby individuals,” Onishi said in the release.
According to the researchers, the results reveal important insights into the social groupings of chimps, helping us understand how they get and stay together, through cohesion, connection, and, apparently, urination.
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Synchronized Peeing Patterns
After analyzing their observations, the researchers concluded that the chimps’ urinations were synchronized, occurring at around the same time much more frequently than if they were randomized. But their analysis also identified other trends in the chimpanzees’ urination, too.
For instance, the chimps’ urinations were more synchronized when the chimpanzees were closer together in terms of their physical proximity, though not in terms of their social closeness.
“We initially expected that any social influences might resemble those seen in yawning — such as stronger contagion between socially close pairs,” Onishi said in the release. “However, our results showed no evidence of effects related to social closeness.”
Instead, the researchers revealed a strong influence of social hierarchy, with higher-status chimps urinating alone and lower-status chimps urinating in response to the higher-status individuals.
“We observed a clear influence of social rank, with lower-ranking individuals being more likely to follow the urination of others,” said Onishi in the release. “We were surprised to discover that the contagion pattern was influenced by social rank.”
Though the purpose and mechanisms of contagious urination remain a mystery, the researchers say that their results are unprecedented, representing the first formal study of contagious urination in any species.
“This was an unexpected and fascinating result, as it opens up multiple possibilities for interpretation,” said Shinya Yamamoto, another study author and an associate professor at Kyoto University, in the release. “These findings raise intriguing questions about the social functions of this behavior.”
Read More: Humans Aren’t the Only Primates Who “Share for Sharing’s Sake”
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
