A recent study published in Current Biology on January 20 provides an intriguing look at an unexpected behavior among chimpanzees: “contagious urination.” The study, which observed a group of 20 captive chimpanzees living at the Kumamoto Sanctuary in Japan, reveals that when one chimpanzee urinates, others in the group are more likely to follow suit. This behavior is reminiscent of similar phenomena observed in humans and may carry deeper evolutionary significance than previously realized.
Human Parallels: The Social Act of Peeing Together
For humans, urinating in the presence of others can carry both practical and social connotations. In various cultures, the act of urinating together is even depicted as an indication of camaraderie or social connection. A well-known Italian proverb states, “Whoever doesn’t pee in company is either a thief or a spy” (Chi non piscia in compagnia o è un ladro o è una spia), while in Japan, the term Tsureshon (連れション) refers to peeing in the company of others, a practice viewed almost as a bonding experience.
Ena Onishi, a researcher at Kyoto University, one of the authors of the study, draws direct parallels between these human behaviors and those observed in chimpanzees. “Our research suggests that this phenomenon may have deep evolutionary roots,” Onishi explains. “We found that chimpanzees, our closest relatives, tend to urinate in response to the urination of nearby individuals.”
Uncovering the Phenomenon of Contagious Urination
The researchers at the Kumamoto Sanctuary decided to study this behavior after noticing that the chimpanzees appeared to urinate at around the same time. This coordinated timing reminded them of human social behaviors, such as the synchronized yawning that humans and other animals sometimes exhibit. Inspired by this, the researchers set out to gather data and investigate whether chimpanzee urination could similarly be a socially driven phenomenon.
Over the course of more than 600 hours of observation, the researchers recorded 1,328 individual urination events across the sanctuary’s group of chimpanzees. By analyzing this data, they were able to determine whether the chimps’ urination events showed any synchronization, especially in relation to the presence and actions of nearby individuals. The goal was to uncover whether this behavior mirrored human tendencies in certain social situations, such as contagious yawning.
The results were striking: the data revealed that urination events among the chimpanzees were significantly synchronized, suggesting that their peeing patterns were not entirely random. In fact, the likelihood that one chimpanzee would urinate after observing another was significantly higher when the two were in close physical proximity.
Influence of Social Rank and Unexpected Patterns
Beyond the immediate observations of synchronized urination, the study uncovered another fascinating detail: the contagion effect was influenced by social rank. Lower-ranking chimpanzees—those with less authority or dominance within the group—were more likely to urinate when higher-ranking individuals did. This observation leads to the hypothesis that chimpanzees’ urination behaviors could be shaped by hierarchical dynamics, which echo patterns seen in other social behaviors among primates and other animals.
“We were surprised to discover that the contagion pattern was influenced by social rank,” says Onishi. “Since there were no prior studies on contagious urination in any species, we drew parallels to contagious yawning, another semi-voluntary physiological behavior. Based on this, we initially expected that any social influences might resemble those seen in yawning—such as stronger contagion between socially close pairs. However, our results showed no evidence of effects related to social closeness. Instead, we observed a clear influence of social rank, with lower-ranking individuals being more likely to follow the urination of others.”
Shinya Yamamoto, a fellow researcher at Kyoto University, finds this result particularly compelling. “This was an unexpected and fascinating result, as it opens up multiple possibilities for interpretation,” says Yamamoto. “For instance, it could reflect hidden leadership in synchronizing group activities, the reinforcement of social bonds, or attention bias among lower-ranking individuals.”
Social Functions and Group Cohesion
The discovery that urination behaviors are not only synchronized but also influenced by social hierarchy raises intriguing questions about the social functions of this seemingly mundane activity. The chimpanzees’ synchronized urination might serve a greater social purpose, such as promoting group cohesion or facilitating the synchronization of other group activities.
The researchers suggest that the act of urination could help solidify social bonds, especially in larger groups where coordination of movements and activities can be beneficial. It’s possible that a collective behavior such as urination might play an overlooked but important role in maintaining the stability and function of chimpanzee social systems. Much like grooming and other social activities that serve to reinforce bonds among individuals, urination patterns might act as a social glue that aids group integration.
The influence of rank on the contagion pattern could suggest that leadership or power dynamics have a deeper significance in organizing how certain behaviors manifest across the group. Just as in human societies, chimpanzees may look to those higher in the social hierarchy for cues about acceptable behaviors.
The Role of Evolution
The researchers’ findings indicate that these behaviors might have roots deeper in evolutionary biology. Just as contagious yawning in humans is thought to be related to empathy and social bonding, contagious urination could be a sign of the chimps’ shared evolutionary history with humans. As a highly social species, chimpanzees use subtle cues to maintain unity within the group, and synchronized behaviors like urination might serve this purpose in ways not immediately obvious.
“We believe this discovery points to the possibility that many subtle, everyday behaviors in non-human primates may have complex social roles that are not immediately apparent,” Onishi explains. “In much the same way that yawning might act as an unconscious social cue, contagious urination could also signal subtle forms of coordination and bonding within groups.”
The study highlights how behavior that might otherwise be considered purely physiological or irrelevant can carry deep social significance, affecting both individual and group dynamics. The research opens up new avenues for studying animal behavior from an evolutionary perspective, where even the most routine actions might be connected to broader social functions.
Future Research Directions
While this study lays the groundwork for understanding the social functions of contagious urination, the researchers acknowledge that much remains to be explored. The mechanisms underlying this behavior, as well as its implications for social coordination, are still not fully understood. Questions about how other species might display similar behaviors remain unanswered, and future studies will be necessary to explore whether contagious urination exists in other primates or animal species.
“We’re very curious whether this phenomenon exists in other species, both within the primate family and beyond,” Onishi remarks. “It’s possible that contagious urination might be a feature of certain highly social or territorial species that rely on coordinated group behavior, but further study is needed to confirm these hypotheses.”
The team’s findings represent an exciting and previously overlooked avenue for animal behavior research. More comprehensive studies, particularly those exploring the neurobiological mechanisms and environmental triggers for such behaviors, will be necessary to build a clearer picture of how and why such “contagions” occur.
Conclusion
The study of “contagious urination” in chimpanzees presents an intriguing link between social behaviors and biological actions, adding new layers of complexity to our understanding of chimpanzee social structures and offering a glimpse into the evolutionary roots of human-like behaviors. Through their research, Onishi, Yamamoto, and colleagues have not only shed light on a previously unknown phenomenon but also challenged scientists to consider how even the most ordinary of behaviors can have profound social significance. As more research is conducted in this field, it’s likely that scientists will uncover additional hidden layers of social interaction that were previously overlooked in animals and, by extension, in our closest evolutionary relatives.
Reference: Socially contagious urination in chimpanzees, Current Biology (2025). DOI: 10.1016/j.cub.2024.11.052. www.cell.com/current-biology/f … 0960-9822(24)01594-X