Looks like people aren’t the only ones who share the bathroom sometimes. A study this week showed that pee can sometimes be contagious in chimpanzees, too.

Scientists at Kyoto University in Japan led the research, namely PUBLISHED Monday in the journal Current Biology. They observed the captive chimpanzees living at the Kumamoto Sanctuary for hundreds of hours, noting that when one chimp decided to urinate, the others always followed suit. This phenomenon may shape the social hierarchy of our primate cousins, the researchers say.

As is often the case in science, lead researcher Ena Onishi’s inspiration for conducting this study was unexpected.

“While observing a group of captive chimpanzees as part of a different research project, I noticed a tendency for individuals to urinate at the same time. This similarity to some human behaviors piqued my curiosity,” Onishi, a doctoral student at the Kyoto University Wildlife Research Center, told Gizmodo. “In Japan, there’s a specific term called ‘Tsureshon’ (生るます), which refers to the act of urinating with others. From there, I developed my research question, wondering if it was one of the contagious phenomena, such as contagious yawning or contagious scratching, and decided to check whether ‘contagious urination’ existed. “

Onishi and his team studied chimpanzees at the Kumamoto Sanctuary for more than 600 hours, ultimately documenting 1,328 “events” of urination. They found that chimps usually urinate more often after a fellow chimp has recently peed. But there are some patterns in this contagious urination.

For one, a chimp is more likely to urinate if they are close to the original leaker (the researchers classified three levels of proximity: within arm’s reach, within 3 meters, and more than 3 meters). But social class also appears to influence the likelihood of one chimp urinating after another.

“(We) found that low-ranking individuals were more likely to follow others in urinating. This further supports the idea that urination has an infectious and socially influenced component in chimpanzees,” said Onishi. In contrast, the researchers found no evidence that the social closeness of one chimp to another affected their likelihood of infectious urination.

The findings are based on a small sample size of one chimp population, so more research is needed to confirm if this is a widespread behavior among chimps everywhere, including those in the wild. And at this point, researchers only have guesses as to why infectious urination might be common among chimps.

It’s possible that the behavior promotes group cohesion by ensuring that chimps are more likely to have the same shared physiological state, for example, Onishi said. The act of positively imitating a fellow chimp can also strengthen the bonds between them. Or group urination may be an anti-predator strategy, as it may reduce the risk of other animals tracking chimps through the diffuse scent of urine.

Whatever the cause, researchers have many mysteries related to urination left to unravel.

“There are so many things we want to learn in the future. For example, we want to observe other groups of chimpanzees to examine how social factors such as sex, familiarity, and age may influence infectious urination,” Onishi said. “We are also curious about how this phenomenon occurs in natural conditions – such as whether it serves as a preparation for long-distance travel.”

Because humans sometimes urinate together because of their closeness to each other, there seem to be some inherent differences in group urination behavior between humans and chimpanzees, the researchers said. And that raises the question of whether other monkeys have their own unique version of contagious urination—another question the researchers hope they and others can explore.

“Monkeys have very different social structures and dynamics,” study researcher Shinya Yamamoto, who studies the evolution of sociality in humans and other animals at Kyoto University, told Gizmodo. “These differences may also influence their decision to urinate and its contagious nature in a social context, which should be one of the targets of future studies.”