Do Chimps Who Urinate Together Stay Together?

Scientists suspect that contagious urination, a behavior they observed among a troop of apes in Japan, may play an important role in primate social life.

Ena Onishi, a doctoral student at Kyoto University, has spent over 600 hours watching chimpanzees urinating. She has a good reason for all that peeping, though. She is part of a team of researchers that recently discovered that the primates tend to tinkle when they see nearby chimps do the same.

In a study published Monday in the journal Current Biology, Ms. Onishi and her colleagues described this phenomenon, which they call contagious urination. Their discovery raises questions about the role peeing might play in the social lives of chimps and other primates.

Ms. Onishi first spotted contagious urination in 2019 while observing chimps at the Kumamoto Sanctuary in Kyoto, Japan. “I was observing a group of captive chimpanzees for a different research project, and I noticed that they tended to urinate at the same time,” Ms. Onishi said. “It got me thinking, Could this be one of those contagious behaviors like contagious yawning?” she explained, referring to our innate tendency to yawn upon seeing or hearing others do it.

To find out, Ms. Onishi studied the sanctuary’s 20 chimpanzees, observing them peeing together over 1,300 times. After crunching the numbers, Ms. Onishi and her colleagues realized that the chimps were indeed urinating in rapid succession. They found that the nearer a chimp was to the initial urinator, the more likely it was to join the party. They also discovered that chimps lower on the social ladder were more likely to go when others were going.

“This result was surprising for us,” Ms. Onishi said. “It raised intriguing questions about the social function of this behavior, which has been overlooked for a long time.”

Why the chimps do this remains a mystery, but Ms. Onishi and her colleagues have several hypotheses. “Contagious urination might help reinforce group connections, boosting overall social cohesion,” she said. “It could promote a shared readiness for collective behaviors. There are so many possibilities.”