Monkeys, Too, Will Pay for Sex

Dec. 19, 2007 -- People aren't the only primates who will pay for sex, new research shows. Male longtailed macaques exchange grooming for the right to mate with females whose fur they cleaned.

The findings, which have been accepted for publication in the journal Animal Behavior, present the first evidence that a "social market" influences sexual interaction in a non-human primate.

"I found that the amount of grooming a male performs on a female during a sexual interaction is related to the supply/demand ratio of females per male around the male-female pair at the time of the grooming," explained Michael Gumert, who conducted the research.

Put another way, male monkeys -- especially lower status ones -- have to groom more to get more action when fewer females are around. Grooming in macaques involves using the teeth and hands to pick through the fur of the recipient to remove dirt, tangles and parasites. The activity often sexually excites the monkeys, particularly the males, so many scientists suspect it evolved into foreplay in humans.

Gumert, a researcher in the Division of Psychology at Nanyang Technological University, Singapore, analyzed a wild population of longtailed macaques at Tanjung Puting National Park, Indonesia, from 2003 to 2005. During this period he documented 243 male-to-female grooming sessions, most of which were directed at females who were receptive to mating.

The "grooming before sex" bouts lasted anywhere from a few seconds to a half hour or more, with the durations frequently linked to either the number of potential other partners or to the status of the groomer or recipient. According to Gumert, "rank does not remove the market, it only skews it."

"Powerful individuals can take more and give less than low-ranked individuals can," he said, suggesting that such corruption of the fair trade ideal appears to be an inherent facet of primate social life that can apply to everything from monkey sex to human politics. High-ranking females can also skew the system because, in the case of macaques, they demand more attention before they agree to mate.

Since males often have their work cut out for them, they also try to first "flirt" with females, using facial gestures before they approach.