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Foul Play: Monkeys wash with urine to woo mates

March 4, 2011

Capuchin monkeys, found in the jungles of Costa Rica and other parts of Central and South America, have the strange and seemingly filthy practice of washing themselves in urine.

But a new study found that urine washing might actually be a turn-on, at least when it comes to female capuchin monkeys.

A study, published in the American Journal of Primatology, scanned the brains of female tufted capuchin monkeys after they had sniffed urine of adult male and juvenile monkeys. The scans showed that the female monkeys became aroused after smelling the sexually mature adult males’ urine. 

Kimberly Phillips, a primatologist at Trinity University in San Antonio, Texas, U.S., said the quirky discovery has generated more attention than she ever could have predicted. She’s had interviews with the BBC in London and National Geographic magazine.

 “The interest it’s generated has got us thinking of what other kind of studies we can do [about urine washing],” Phillips said.

Urine washing is a common act among monkeys in Central and South America, but the reason why monkeys apply urine to their feet and hindquarters like a layer of aftershave had remained a mystery to scientists.

The sexually mature male capuchin has higher levels of testosterone in its urine that would signal to females that the animal is fertile. Juvenile capuchins lack the amount of testosterone that would increase action in the female’s brain. Past hypotheses suspected that urine washing was a method for keeping warm or a method of visual communication.

Other monkey species found in Costa Rica, such as the squirrel monkey and the endangered Howler, also practice urine washing. However, Phillips cautioned that the same scans would have to be performed on other species of monkeys to know for sure if they urine wash for the same reason as the capuchin.

Questions also persist about the capuchin’s soiled manner of seduction.

“I don’t think we’ve fully explained why they still do this in the [study],” Phillips said. “I think we’ve just kind of found one reason. But it does not further explain this behavior. Females engage in this urine washing behavior. Do males get any sort of olfactory info from females? Infant monkeys urine wash, too.”

Phillips was familiar with Costa Rica’s capuchin monkeys as a professor. She taught biology fieldwork here and used to observe the capuchins at Corcovado National Park in the Southern Zone’s Osa Peninsula. But the idea to research urine washing came from Courtney Buzzell, co-author of the study. Buzzell wrote a paper on urine washing while a research technician under Phillips at Ohio University. They never realized such an unsexy finding would garner global attention.

Of course, the discovery also includes its share of tongue-in-cheek headlines like “Sexy Monkeys Wash With Own Urine” and “Sex-Crazed Monkeys Wash Their Hands With Urine.” But the study’s authors don’t seem to mind the foul humor.

Explained Phillips: “At least people are thinking about science.”

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