If you thought that only some human freaks can reach orgasm through urine (a sexual deviation called urolagnia, urophilia or undinism), you'd better find out that our evolutionary relatives are 'better' than us: in capuchin monkeys, this is so for the entire species!

These monkeys splash their feet and hands in urine to achieve comfort or have sex. There are many species of monkeys which "recycle" their urine to wash their feet and hands, often taking a whiz on their hands and rubbing their feet. This affinity for the product of their own kidneys has been found even on chimps.

Researchers have tried to explain this odd behavior from being a cooling mechanism to increasing their grip on branches.

The team led by primatologist Kimran Miller at the University of Northern Iowa in Cedar Falls investigated tufted capuchin monkeys (Cebus apella) for 10 months at the National Institutes of Health Animal Center in Maryland. The dominant male of the group made up of 24 monkeys doubled the urination when he solicited sex services from 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. Or they could be doing it because they're excited." said Miller.

The team also discovered that when monkeys faced aggression, even if less dangerous, they ended this aggressive encounter briefly in 87 % of the cases through urine-washing, like an appeasing gesture, "a sort of 'Hey, whatever happened, I'm sorry,'" explained Miller.

Moreover, frequent urine-washing was linked to lower levels of stress hormone cortisol, thus the habit "might help the capuchins soothe themselves. That could make sense with the monkeys wanting to calm down when solicited by females or faced with aggression." said Miller.

The urine-washing was employed no matter the temperature or humidity of the air, thus the capuchin monkeys did not use it as a cooling behavior. The presence of another group of monkeys did not rise the frequency of the urine-washing, thus monkeys did not mark their territory off with urine.

"The leading explanations for urine-washing were either keeping cool or territoriality. Our findings suggest we should rethink why urine-washing happens." said Miller.

About the gripping hypothesis, "studies have not found a lot of support for that idea," said Miller.