Photo/Illutration Four male chimpanzees in Uki, Kumamoto Prefecture (Provided by the Kyoto University Wildlife Research Center Kumamoto Sanctuary)

A study in Japan suggests that peeing is contagious in chimpanzee troops. When one goes, they all tend to go. The question is why.

The finding is based on more than 600 hours of chimp observation in Kumamoto Prefecture, according to the research team at Kyoto University.

Team members found that chimpanzees with a lower status in the pecking order were more likely to engage in copycat behavior, “highlighting the role of social influence in the physiological phenomenon of urinating.”

Other known examples of socially contagious behavior in animals include yawning and sudden mass movements of herds.

The urination study marks a world first.

The behavior is far from uniform, however.

Researchers noted that the behavior has social dimensions, as exemplified by the way yawning is more likely to spread among mutually close individuals. The same goes for animal herds that suddenly make a mad dash in the veld. In the latter case, individuals with a higher social status, as well as those with central positions in the hierarchy, trigger the exodus.

Ena Onishi, a doctoral program student with the Kyoto University graduate school, was in charge of observing the chimps in the latest study.

Over a period of 604 hours, Onishi and her colleagues monitored the behaviors of 20 captive chimpanzees in four groups of five, aged between 28 and 53, at the Kyoto University Wildlife Research Center Kumamoto Sanctuary in Uki, Kumamoto Prefecture, and took notes of 1,328 urination events.

They began with calculating urination rates by individual and by time slot.

Comparison with repeated computer simulations showed the observed urination behaviors of the chimpanzees synchronized, or occurred within 60 seconds of another urination event, at a rate of more than double what it would have been if the events had occurred randomly.

That, however, does not offer definitive proof for social contagion because the tendency for synchrony may be caused by meals being taken at identical hours, said a report on the team’s observations.

For this reason, the team members checked the physical distance between the urinating individuals to study if the behavior is contagious.

Categorization into the three classes of “within arm’s reach” (50-60 centimeters), “within three meters” and “over three meters” showed that contagion of urination was more likely to occur between spatially closer individuals.

Further studies on links with social closeness between individuals, position in the pecking order and centrality in social networks showed that individuals with lower dominance status in the group were more likely to follow the urination of others.

The rate of contagious urination, however, was found unrelated to social closeness or centrality.

Why urination is contagious is a separate issue. Onishi said answering that question is beyond the scope of the latest study.

But she cited a number of candidate rationales: strengthening social cohesion among individuals; facilitating group behavior; and reducing the risk of being detected by predators, who may smell out the urine, by aligning urination in time.

“Behaviors that may appear casual at first glance may contain clues to complicated aspects of social evolution, such as mechanisms for maintaining a group,” Onishi said.

The research results were published on Jan. 20 in Current Biology, a U.S. science journal, at: (https://doi.org/10.1016/j.cub.2024.11.052).

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