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Masturbation’s Evolutionary Purpose Identified in Primate Study

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Biology has developed billion-dollar cancer atlases and sophisticated genomic techniques. It has answered countless questions about the beauty and mystery of human experience. But one key conundrum remains unsolved: why do we masturbate? A new study, published in the scientific journal Proceedings of The Royal Society B, goes some way toward answering this question by exploring the evolution of self-love.

Monkeying around

Masturbation is common across the animal kingdom but is particularly prevalent in primates – including humans. Masturbation frequency varies between country, culture and age. There’s also an onanism sex gap that means masturbation is much more commonly reported by men than women.

Dr. Matilda Brindle gave an exclusive interview with Technology Networks Junior Science Editor Rhianna-lily Smith to talk about her research.

Survey reports of masturbation frequency and function in humans always have a caveat – shame and morality might limit how much people share with self-service surveys. Luckily for science, we don’t have to study masturbation in humans to find out why it evolved – we can just record monkeys doing it!

Researchers, it turns out, have devoted an exceedingly large amount of time to doing just that. This left Dr. Matilda Brindle, an anthropologist at University College London, with plenty of data to sift through in her masturbation mega-review. Brindle, in a truly epic display of devotion to the academic cause, read through over 400 data sources detailing how primates masturbate. This includes nearly 250 published papers and dozens of questionnaires and personal communications from primatologists and zookeepers.

What’s the point of pleasure?

Armed with this trove of data, Brindle was able to detail some of the evolutionary background to what experts call autosexual behavior. Her team showed that it is, in genomic terms, an ancient practice. The common ancestor of all monkeys and apes, who lived during the Miocene era tens of millions of years ago, was most likely masturbating as the world cooled into a series of ice ages. A lack of data prevented Brindle from adding more clarity to the fossil record, as she was unable to define the habits of the last common ancestor of other primates like lemurs or lorises.

With this lengthy timeline in place, Brindle now challenged her data with a series of hypotheses that have tried to explain how this behavior – which on the surface seems likely to reduce the chances of passing on genes – may have evolved.

The “postcopulatory selection” hypothesis suggests that masturbation in males could actually increase the chance of successful fertilization. In species with high competition between males, low-ranking individuals may only have a short amount of time to copulate before a higher-ranking male intervenes to chase them off. In this scenario, suggest the researchers, masturbation without completion could help low-ranking males reach arousal and increase their chance of ejaculating in a limited time window.

The researchers showed that masturbation in males had co-evolved alongside higher male–male competition, which supports this hypothesis.

Beating STIs

Another theory investigated was the “pathogen avoidance hypothesis” – suggesting that a post-coital self-love session could cleanse the urethra of any sexually transmitted infections (STI). Again, the evolutionary tree gave credence to this idea, suggesting that male masturbation was more common in primates species that had a higher STI load.

Female masturbation is a far more poorly understood activity – we only properly mapped the clitoris in 1998 – and primatology has the same issue. There wasn’t enough available data for the team to draw firm conclusions about how it may have evolved – something they say needs to change.

Brindle said, “Our findings help shed light on a very common, but little understood, sexual behavior and represent a significant advance in our understanding of the functions of masturbation. The fact that autosexual behavior may serve an adaptive function, is ubiquitous throughout the primate order and is practiced by captive and wild-living members of both sexes, demonstrates that masturbation is part of a repertoire of healthy sexual behaviors.”

Reference: Brindle M, Ferguson-Gow H, Williamson J, Thomsen R, Sommer V. The evolution of masturbation is associated with postcopulatory selection and pathogen avoidance in primates. Proc. Royal Soc. B: Bio Sci. 2023;290(2000):20230061. doi:10.1098/rspb.2023.0061c

This article is a rework of a press release issued by University College London. Material has been edited for length and content.