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7th June 2023
10:12am BST
New research has found that masturbation dates back to monkeys and apes, tens of millions of years before the first humans walked the Earth.
Evolutionary biologists compiled what is believed to be the largest ever dataset on masturbation by gathering information from nearly 400 sources, in the first ever study of its kind.
For decades, scientists have been baffled by why primates take part in an activity that doesn't involve a sexual partner, and therefore creates no offspring.
But they've now discovered masturbation dates back much earlier than previously believed, and has evolutionary benefits such as increasing reproductive success and protecting against sexually transmitted infections.
Dr Matilda Brindle, the lead author of the study - which was published in Proceedings of the Royal Society B - said: "Our findings help shed light on a very common, but little understood, sexual behaviour and represent a significant advance in our understanding of the functions of masturbation."
Dr Brindle said: "Historically, masturbation was considered to be either pathological or a by-product of sexual arousal.
"Recorded observations were too fragmented to understand its distribution, evolutionary history or adaptive significance.
"Perhaps surprisingly, it seems to serve an evolutionary purpose."
Dr Brindle's team gathered information from 246 published academic papers and 150 questionnaires and personal communications from primatologists and zookeepers.
They then mapped the information on to primate evolutionary trees - identifying when and why it evolved in both females and males.
Dr Brindle said: "We find support for two potential adaptive functions of masturbation in male primates, suggesting it may increase reproductive success and reduce the chance of contracting STIs by cleansing the genital tract."
In terms of how masturbation could aid successful fertilisation among primates millions of years ago, Dr Brindle explained: "First, masturbation without ejaculation can increase arousal before sex.
"This may be a particularly useful tactic for low-ranking males likely to be interrupted during copulation, by helping them to ejaculate faster.
"Second, masturbation with ejaculation allows males to shed inferior semen - leaving fresh, high-quality sperm available for mating, which are more likely to outcompete those of other males."
The study concluded that male masturbation has co-evolved with multi-male mating systems where competition is high.
Previous research into masturbation has suggested that it makes men better lovers and boosts their orgasms.
It has also been shown to relieve stress and could potentially help reduce the risk of prostate cancer by flushing out cancerous cells.
Studies into female masturbation have found it can improve mood and libido, along with being able to sooth cramps.
But the significance of female masturbation remained much more of a mystery to Dr Brindle and her team.
Biological studies from the past tended to ignore females, so the team had far less information and data to go off.
She said: "More data on female sexual behaviour are needed to better understand the evolutionary role of masturbation."
One idea put forward is that masturbation before sex gave female primates more control over which male gets them pregnant. The Guardian reports masturbating would make the female's vagina less acidic, and therefore more hospitable to the chosen male's sperm.
Explaining why she had decided to take on this study, Dr Brindle said she found it "absolutely baffling that nobody has researched such a common behaviour across the animal kingdom."
She added: “For people who think masturbation is wrong, or unnatural in some way, this is perfectly natural behaviour. It’s part of our healthy repertoire of sexual behaviours.”
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