Male Primate Masturbation May Have Evolved to Prevent STIs

Biologists in London have traced back the history of masturbation in primates to at least 40 million years ago, according to a new study published Wednesday in Proceedings of the Royal Society B.

In an evolutionary sense, the behavior could seem counter-intuitive, writes Darren Incorvaia for Science News—spending time and energy on self-pleasure doesn’t appear to serve a reproductive purpose when compared to mating with another animal. But the new research suggests masturbation may benefit males by boosting reproductive success and helping avoid sexually transmitted infections. 

“Historically, masturbation was considered to be either pathological or a byproduct of sexual arousal,” Matilda Brindle, an evolutionary biologist at the University College London, tells Jake Meeus-Jones of the South West News Service (SWNS). “Recorded observations were too fragmented to understand its distribution, evolutionary history or adaptive significance. Perhaps surprisingly, it seems to serve an evolutionary purpose.”

In the study, Brindle and her colleagues describe how they compiled the “largest ever dataset of primate masturbation” from 400 sources, including nearly 250 published papers and 150 responses from primatologists and zookeepers via questionnaires and personal communications, per a statement. 

The data suggested that this autosexual behavior most likely occurred in the common ancestor of all monkeys and apes (including humans). But a scarcity of data for other primate groups, like lemurs and tarsiers, makes it unclear whether self-pleasuring behavior was also present in their ancestors, per the statement. Through modeling, the researchers sought to detail when the practice came about.

“This is a very interesting article that sheds light on the evolutionary history of behaviors that leave no trace in the fossil record,” Kit Opie, an anthropologist at the University of Bristol in England, tells New Scientist’s Soumya Sagar.

To find more clues about why masturbation arose, the team used a pair of hypotheses. The first, the “postcopulatory selection hypothesis,” suggests the behavior helps to fertilize an egg. For example, masturbation without ejaculation may increase arousal before sex. This could lead lower-ranking male monkeys to ejaculate faster and therefore have higher breeding success. Masturbation with ejaculation, on the other hand, might allow males to get rid of inferior semen, leading to higher-quality sperm that might outcompete those of other males, reports SWNS.

Another potential explanation—the “pathogen avoidance hypothesis”—suggests that male ejaculation after sex cleanses the urethra, reducing the chance of contracting an STI.

The authors are “the first to use a cross-species approach” to explore the purpose of masturbation, Lateefah Roth, a biologist at the Institute of Forensic Psychiatry and Sex Research at the University of Duisburg-Essen in Germany, tells Science News, adding that the paper is “a great starting point.”

The researchers found that male masturbation evolved alongside mating systems where competition between males is high. They did not find a similar trend for females—but Brindle tells Science News this may be because of a lack of data rather than this link not existing at all. Research on female sexual behavior has historically been sparse because of past beliefs that females are “passive recipients of male behavior,” she tells the publication.

Brindle says to WION that she finds it “absolutely baffling that nobody has researched such a common behavior across the animal kingdom.” 

“For people who think masturbation is wrong, or unnatural in some way, this is perfectly natural behavior,” she tells the publication. “It’s part of our healthy repertoire of sexual behaviors.”

Margaret Osborne | | READ MORE

Margaret Osborne is a freelance journalist based in the southwestern U.S. Her work has appeared in the Sag Harbor Express and has aired on WSHU Public Radio.