Capuchin monkeys wash their feet and hands in urine to get comfort or sex, research now suggests.
Many species of monkeys rinse their feet and hands in their own waste by urinating on their hands and rubbing their feet. Explanations put forward for such urine-washing have included everything from helping the primates cool down to improving their grip on branches.
Primatologist Kimran Miller at the University of Northern Iowa in Cedar Falls and her colleagues focused on tufted capuchin monkeys (Cebus apella) for 10 months at the National Institutes of Health Animal Center in Maryland.
The alpha male of the group of roughly two dozen monkeys doubled how often he washed in urine when solicited by females.
"So we think the alpha males might use urine-washing to convey warm, fuzzy feelings to females, that their solicitation is working and that there's no need to run away," Miller said. "Or they could be doing it because they're excited."
In addition, Miller and her colleagues found that when monkeys were confronted with aggression — even by something as subtle as a glance — after they urine-washed, 87 percent of the time the aggression ended. This suggests it might help appease aggressors, "a sort of 'Hey, whatever happened, I'm sorry,'" she explained.
The researchers also found monkeys that urine-washed frequently had lower levels of the stress hormone cortisol, suggesting pee rinses "might help the capuchins soothe themselves," Miller said. "That could make sense with the monkeys wanting to calm down when solicited by females or faced with aggression."
The monkeys urine-washed regardless of how warm or cool or humid it was in their environment, indicating they were not using it to control their body temperature. They also did not urine-wash more when surrounded by groups of other monkeys, suggesting pee rinses were not used to help mark territory off against strangers.
"The leading explanations for urine-washing were either keeping cool or territoriality," Miller said. "Our findings suggest we should rethink why urine-washing happens."
As to whether these monkeys use their own pee to improve their grip, "studies have not found a lot of support for that idea," Miller said.
The researchers will detail their findings in a forthcoming issue of the American Journal of Primatology.