Masturbation dates back more than 10 million years to the common ancestor of monkeys, apes and humans, according to new research.
Flying solo serves an evolutionary purpose because in men it promotes fertility and protects against sexually transmitted diseases.
It is common across the animal kingdom but especially prevalent amongst primates – a phenomenon that has baffled anthropologists for decades.
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The first study of its kind has found it is an ancient trait within the primate order, beginning much earlier than previously believed.
Lead author Dr Matilda Brindle, of University College London, 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."
They support the idea it increases reproductive success and protects against STIs.
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."
Her team collected the largest ever dataset on masturbation, pooling information from nearly 400 sources.
They included 246 published academic papers and 150 questionnaires and personal communications from primatologists and zookeepers.
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It enabled the British team to track the distribution of onanism across primates – 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."
Previous research has suggested masturbating more makes men better lovers – and boosts their orgasms.
It has also been shown to relieve stress and flush out potentially cancerous prostate cells.
Female masturbation is said to improve mood and libido and sooth period cramps.
Dr Brindle said: "Masturbation has a long evolutionary history amongst primates.
"It was most likely present in the common ancestor of all monkeys and apes – including humans."
Much remains unknown about it – popularly dubbed 'the missing link.' Recent evidence suggests it resembled a gibbon but a definitive fossil is yet to be unearthed.
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Less clear is whether the ancestor of the other primates – lemurs, lorises and tarsiers – masturbated because data were more scarce for these groups.
The researchers tested several theories including the 'postcopulatory selection hypothesis' which proposes masturbation aids successful fertilisation.
Indeed, the study showed male masturbation has co-evolved with multi-male mating systems where competition is high.
The 'pathogen avoidance hypothesis' also suggests male masturbation protects against catching an STI after copulation.
It washes out the urethra – a tube leading to the bladder which is a primary site of infection.
But the significance of female masturbation remains more of a mystery. While frequent, there are fewer reports describing it.
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