Japanese researchers uncovered a surprising behavior among chimpanzees that may shed light on the social dynamics of these primates. A team led by Professor Shinya Yamamoto from Kyoto University discovered that chimpanzees display "contagious urination" behavior, where individuals tend to urinate in response to others urinating nearby. Their findings, published in the journal Current Biology, suggest that urinating together is a social phenomenon in both chimpanzees and humans.
Over 600 hours of observation at the Kumamoto Sanctuary in Japan, the researchers recorded 1,328 urination events among a group of 20 chimpanzees. They found that the probability of chimpanzees urinating at nearly the same time was higher than random chance, indicating a clear pattern of synchronization. The data revealed that the likelihood of other chimpanzees urinating increased with their spatial proximity to the first urinator, particularly when individuals were within a few feet of each other, as reported by Scientific American.
"The result of the study was unexpected and fascinating, as it opens multiple possibilities for interpretation," said Shinya Yamamoto, co-author of the study and Associate Professor at the Kyoto University Institute for Advanced Study, according to Phys.org. The study revealed that lower-ranking chimpanzees were more likely to be influenced by the urination of higher-ranking individuals. Social rank, rather than affiliative relationships, played a decisive role in this behavior.
"We were surprised when we discovered that the contagion pattern was influenced by social rank," said Ena Onishi, the first author of the study and a researcher at Kyoto University. "Initially, I expected the influence of intimacy to be significant like yawning, but such an effect was not observed; instead, the influence of social status was clearly observed," she said.
The researchers compared this contagious urination to other semi-voluntary physiological behaviors like contagious yawning. "Since there were no prior studies on contagious urination in any species, we drew parallels to contagious yawning," Onishi added. However, unlike yawning, social proximity was not the determining factor in which chimpanzee followed another in urination.
The term "Tsureshon" in Japanese refers to the act of urinating in a group, reflecting similar social behaviors observed across cultures. "Tsureshon has been represented in art across centuries and cultures and continues to appear in modern social contexts," Onishi adds.
The team hypothesizes that synchronized urination may serve important social functions within chimpanzee groups. "Hidden leadership or social bonds may be reflected in the synchronization of such group behaviors," Professor Yamamoto explained. Lower-ranking individuals may pay more attention to higher-ranking members, possibly due to an attention bias or heightened stress, leading them to mimic the behaviors of dominant individuals.
While the exact causes of synchronized urination behavior among chimpanzees remain unclear, the researchers believe it may facilitate communication and reinforce social bonds in their fusion-fission social system, where individuals temporarily separate and reunite. This behavior could have deep evolutionary roots shared with humans. "This potentially widespread facet of social behavior in animals has been completely overlooked so far," Onishi stated.
"Our research suggests that this phenomenon may have deep evolutionary roots," Onishi concluded. "I hope my study inspires others to look at urination in different animals or explore this phenomenon in greater detail," she said. The researchers expressed curiosity about whether contagious urination exists in other species and hope to investigate it from a broader comparative perspective, including studying other ape species such as bonobos.
Matthew Campbell, a psychologist at California State University Channel Islands who was not involved in the research, found the study "clever and novel," noting it leads to many interesting questions. "How this is working and what it means for the mental life of a chimpanzee, that's really the intriguing part to me," Campbell said, according to Scientific American. "On the surface, it may seem like a silly topic, but it actually gets at something that's rather fundamental."
The researchers analyzed urination events based on the chimpanzees' spatial proximity, examining instances when the animals were within arm's reach, within three meters (10 feet), or further apart. They compared the synchronization rate—the proportion of urinations within 60 seconds of another—to 1,000 sessions of randomized computer simulations. The results showed an increase in synchronized urination compared to random chance.
Contagious behaviors like yawning and laughter have long been observed in both humans and other primates. While yawning tends to occur more among socially close individuals, urination synchronization among chimpanzees appears to be dictated by social rank rather than affiliative relationships. According to Live Science, social proximity was not the determining factor in which chimpanzee followed another in urination, unlike grooming and yawning.
"This is the first study to investigate contagious urination in animals, including humans," Dr. Yamamoto stated. "The results open up various possibilities for interpretation and raise interesting questions about the social functions of contagious urination."
This article was written in collaboration with generative AI company Alchemiq