Peeing is contagious in chimpanzees, study suggests

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While watching a group of captive chimpanzees for her graduate research, Ena Onishi noticed something odd about the primates.

"They seemed to have a tendency to urinate around the same time," said Onishi, a wildlife researcher at Kyoto University. "It reminded me of some human behaviors of going to the bathroom together, and got me thinking 'could this be one of those contagious behaviors?' "

Contagious behaviors are actions that can be sparked by the detection of that action in another individual. Yawning is perhaps the most famous example — see someone yawn and you're very likely to get the urge yourself. But to Onishi's knowledge, no one had studied urination as a potentially contagious behavior.

So Onishi spent over 600 hours watching twenty captive chimps at an animal sanctuary, noting each time a chimp peed and where it was relative to other chimps. "Obviously urination makes a sound," she said, explaining that it was sometimes easier to hear the chimps going than to see them.

Chimps who observed other chimps urinating were slightly more likely to go themselves, the researchers reported this week in Current Biology. Proximity seemed to matter, as chimps who were closer to each other were more likely to contagiously pee, the study found. Social rank factored in too, with lower-ranking chimps taking a whiz more often after seeing their higher-ranking counterparts do the same.

Urination is only slightly contagious behavior in these chimps. Only about 10% of urination events appeared to be caught from other chimps within arms reach, Onishi told NPR.

Still, "this is a very interesting paper," says Andrew Gallup, a behavioral biologist at Johns Hopkins University who wasn't involved in the study. "I think that they demonstrate that urination is socially influenced in the chimpanzees that they studied, but the mechanisms that drive the contagious response remain to be uncovered."

Peeing together could be a way of syncing up individuals' internal states, which could facilitate group behaviors, like keeping watch for predators. Or it could be a form of social bonding, forging specific connections between chimps. In the wild, whizzing as a group could be a predator-avoidance strategy, says Onishi. "By keeping urination localized, the group can reduce the risk of predators tracking them through the scattered urine scents in their territory."

There could be a non-adaptive explanation for the behavior, too. Simply hearing or seeing another chimp urinate might prompt the need to pee, says Gallup, similar to how hearing running water can spur humans to hit the head.

Onishi hopes to answer these questions by continuing to study contagious urination. "It'd be fascinating to study this behavior in the wild setting to see if it has any role in natural context," she says. "Maybe it's preparation for long distance travelling."