Ena Onishi, a doctoral student at Kyoto University, has spent over 600 hours watching chimpanzees urinating. She has a good reason for all that peeping, though. She is part of a team of researchers that recently discovered that the primates tend to tinkle when they see nearby chimps do the same.
In a study published Monday in the journal Current Biology, Ms. Onishi and her colleagues described this phenomenon, which they call contagious urination. Their discovery raises questions about the role peeing might play in the social lives of chimps and other primates.
Ms. Onishi first spotted contagious urination in 2019 while observing chimps at the Kumamoto Sanctuary in Kyoto, Japan. “I was observing a group of captive chimpanzees for a different research project, and I noticed that they tended to urinate at the same time,” Ms. Onishi said. “It got me thinking, Could this be one of those contagious behaviors like contagious yawning?” she explained, referring to our innate tendency to yawn upon seeing or hearing others do it.
To find out, Ms. Onishi studied the sanctuary’s 20 chimpanzees, observing them peeing together over 1,300 times. After crunching the numbers, Ms. Onishi and her colleagues realized that the chimps were indeed urinating in rapid succession. They found that the nearer a chimp was to the initial urinator, the more likely it was to join the party. They also discovered that chimps lower on the social ladder were more likely to go when others were going.
“This result was surprising for us,” Ms. Onishi said. “It raised intriguing questions about the social function of this behavior, which has been overlooked for a long time.”
Why the chimps do this remains a mystery, but Ms. Onishi and her colleagues have several hypotheses. “Contagious urination might help reinforce group connections, boosting overall social cohesion,” she said. “It could promote a shared readiness for collective behaviors. There are so many possibilities.”
Although the study was limited to captive chimpanzees, many of them rescued from the biomedical research industry, the chances that this behavior is unique to this group are low.
“If you walk with great apes in the wild, you often see that group members really coordinate what they’re doing,” said Martin Surbeck, an evolutionary biologist at Harvard University who studies the behavioral ecology of chimps and bonobos and was not involved in the research.
Dr. Surbeck said that he wasn’t surprised to learn that the Kumamoto chimps were engaging in contagious urination and that the behavior wouldn’t be unexpected in the wild. “We might even see it in other social species,” he said.
While more research is needed on contagious urination and its evolutionary function, Ms. Onishi and her colleagues were delighted that they had learned so much through simple observation.
“There is a myriad of things to be discovered from the daily activities of animals,” Ms. Onishi said.
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