For chimpanzees, the need to pee appears to be contagious. A study published January 20 in the journal Current Biology finds that when one chimpanzee urinates, the others in a group are more likely to follow. The phenomenon is called “contagious urination,” and could have deep evolutionary roots in humans and chimpanzees–our closest relatives.
“In humans, urinating together can be seen as a social phenomenon,” study co-author and Kyoto University evolutionary biologist Ena Onishi said in a statement. “An Italian proverb states, ‘Whoever doesn’t pee in company is either a thief or a spy,’ while in Japanese, the act of urinating with others is referred to as ‘Tsureshon.’ This behavior is represented in art across centuries and cultures and continues to appear in modern social contexts.”
Onishi and colleagues decided to study this behavior when they observed that the chimpanzees in the sanctuary appeared to pee at roughly the same time. They were curious to see if urination could be similar to contagious yawning seen in humans.
In the study, they documented peeing behaviors in 20 chimpanzees at the Kumamoto Sanctuary. In more than 600 hours, they saw 1,328 instances of urination. They then analyzed the observational data to see if peeing was significantly synchronized over time and looked to see if urination was potentially influenced by individuals nearby or shaped by social factors.
They found that urination events were significantly more synchronized than would be expected if the animals were simply peeing at random times. The likelihood of contagious urination also increased if a chimpanzee was physically closer to the initial urinator.
Individuals with lower ranks in the group were also more likely to pee when others were urinating. The team believes this suggests that urination patterns are influenced by social hierarchy. There could also be a tendency for the behavior to “flow down” the dominance structure.
“We were surprised to discover that the contagion pattern was influenced by social rank. Since there were no prior studies on contagious urination in any species, we drew parallels to contagious yawning, another semi-voluntary physiological behavior,” Onishi said. “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, the results indicated that social closeness did not have any effect on this action. Instead, social rank appeared to have the greatest influence, with lower-ranking individuals being more likely to follow.
[ Related: Like humans, chimps often perform tasks differently when crowds are watching. ]
This unexpected finding related to social hierarchy and ranking could reflect some kind of hidden leadership that synchronizes group activities, reinforcing social bonds, or an attention bias among lower-ranking individuals. The results may also have some implications for understanding and exploring the role that contagious urination may have in keeping a group together, facilitating coordination, or reinforcing social bonds within a group. According to the team, it also shows how some seemingly mundane–yet absolutely necessary–behaviors may have an overlooked social significance.
Further study could better understand the specific functions and mechanisms that underlie contagious urination in chimpanzees and if it exists in other species.
