When Ena Onishi first noticed chimpanzees urinating in sync, she didn’t think she was observing the beginnings of a scientific discovery. A doctoral student at Kyoto University, Onishi was observing 20 chimpanzees in a Japanese wildlife sanctuary when a peculiar pattern emerged. “I noticed a tendency for individuals to urinate at the same time,” she recalled during an interview with Science News. Intrigued, she began to wonder if this behavior might share similarities with other known contagious actions, like yawning or laughter.

Her curiosity culminated in a study recently published in Current Biology, making it the first investigation into “contagious urination.” The findings? Peeing is, indeed, catching — at least for chimpanzees.

A Social Signal in the Stream

Chimpanzees don’t just follow the crowd; sometimes, they quite literally go with the flow. Over 600 hours of observation and 1,300 documented urination events revealed a pattern: if one chimpanzee urinated, others nearby were more likely to do the same. The closer they were physically, the stronger the effect. Within a few feet of a peeing chimp, the likelihood skyrocketed.

What made this behavior even more intriguing was the role of social hierarchy. Dominant individuals appeared to set the tone, while lower-ranking chimps were more likely to mimic their superiors. “This was an unexpected and fascinating result,” said Shinya Yamamoto, a co-author of the study.

The phenomenon may be more than a simple case of copycatting. “It could reflect hidden leadership in synchronizing group activities, the reinforcement of social bonds, or attention bias among lower-ranking individuals,” Yamamoto told Ars Technica.

Such insights align with broader themes in animal behavior. Social animals often mirror each other’s actions, from synchronized grooming to coordinated movements. But urination — a physiological need — adds a new dimension to understanding how social cues can permeate even the most basic behaviors.

More Than a Curious Quirk

By comparing chimpanzee behavior with computer simulations of randomized urination, the researchers’ observations ruled out randomness. The findings show that sometimes social dynamics shape not just actions but bodily functions. For Yamamoto, this strengthened his perception of chimpanzees as deeply social creatures: “This study shows that even their physiological aspects are influenced by their social contexts,” he told Scientific American.

Yet questions remain. Does this behavior serve a practical purpose? Perhaps it’s a form of group coordination — a way to reinforce bonds. Or could it simply be an unconscious response? Future research might uncover links between contagious urination and territorial behavior, ranging patterns, or even stress management.

Onishi and her colleagues are also eager to expand their scope. Studying wild chimpanzees or even closely related species like bonobos could reveal whether this behavior is unique to captive environments. Additionally, exploring sensory triggers and the mental processes behind the behavior might provide a window into how chimpanzees perceive their own actions.

As Onishi noted, “It’s a promising and preliminary step.” For now, though, one thing is clear: even in the animal kingdom, when you’ve got to go, you might not go alone.