When nature calls: Peeing is ‘contagious’ among chimps — and for good reason

600 hours of chimpanzee observation uncovers fascinating urine synchronization

KYOTO, Japan — Most of us have experienced that curious moment when one person’s bathroom break seems to trigger a chain reaction in a group. Now, scientists have discovered this isn’t just a human quirk – our closest animal relatives, chimpanzees, demonstrate the same behavior in a way that reveals surprising insights about social dynamics.

The study published in Current Biology documents what researchers call “socially contagious urination” in chimpanzees for the first time in any species. This means that when one chimpanzee urinates, others often follow suit within 60 seconds, particularly if they’re physically close to each other. While this might sound amusing, the behavior appears deeply rooted in social structures and may have evolved for important group purposes.

The phenomenon has long been observed in human societies across cultures. Lead researcher Ena Onishi of Kyoto University points out that an Italian proverb considers solitary urination suspicious, while Japanese culture has a specific term “Tsureshon” for the act of urinating with others. However, until now, no one had scientifically studied this behavior in humans or other animals.

The research team’s curiosity was sparked by patterns they noticed among chimpanzees at Japan’s Kumamoto Sanctuary. To investigate, they conducted extensive observations of 20 captive chimpanzees (16 males and 4 females), documenting 1,328 urination events over 604 hours. Think of this as watching chimpanzees for about 25 full days, noting every time any of them urinated and recording who else was nearby.

To determine whether these bathroom breaks were truly synchronized rather than random coincidence, researchers compared the actual timing of urination events to computer simulations. The results were striking: chimpanzees were more than twice as likely to urinate within 60 seconds of each other compared to random chance. The real-world synchronization rate was 12%, while simulations predicted only about 5%.

Physical proximity played a crucial role. Imagine three zones around each urinating chimpanzee: within arm’s reach, within about 10 feet, and farther away. The closer other chimps were to the one urinating, the more likely they were to follow suit. This pattern held true even when accounting for other factors like drinking times or outdoor access.

Perhaps most intriguingly, social status influenced this behavior in unexpected ways. “We were surprised to discover that the contagion pattern was influenced by social rank,” Onishi explains. Unlike other socially contagious behaviors such as yawning, which often occurs more frequently between close friends, urination patterns seemed to “flow downward” through the social hierarchy. Lower-ranking chimps were more likely to follow the lead of others, regardless of their friendship bonds.

This finding opens up fascinating possibilities about the behavior’s purpose. “For instance, it could reflect hidden leadership in synchronizing group activities, the reinforcement of social bonds, or attention bias among lower-ranking individuals,” says study co-author Shinya Yamamoto, in a statement. “These findings raise intriguing questions about the social functions of this behavior.”

The behavior might help maintain group cohesion among chimpanzees or coordinate movements, similar to how human groups often take bathroom breaks together before long trips.

The research suggests that basic biological functions can serve sophisticated social purposes. As scientists continue studying this phenomenon, they hope to discover whether similar patterns exist in other social species and what role this synchronization plays in group dynamics. Nature may call at random times, but as this research shows, answering that call might be more socially coordinated than we ever imagined.