Super semen could be one reason why primates evolved to masturbate

Despite centuries of taboo and titillation, masturbation in primates appears to serve an evolutionary purpose. A study published June 6 in the journal Proceedings of The Royal Society B, found that self-stimulating increases reproductive success and helps primates avoid sexually transmitted infections (STI), at least in males. 

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Self-pleasure is common across the animal kingdom, but is particularly frequent in primates including humans. The behavior was considered by some scientists to be either pathological or simply a by-product of sexual arousal. Recorded observations were also too fragmented to fully understand masturbation’s distribution, evolutionary history, or adaptive significance. 

In this new study, a team of researchers built a dataset on primate masturbation from close to 400 sources, including 246 published academic papers, and 150 questionnaires and personal communications from zookeepers and primatologists. To understand why and when the practice evolved in both females and males, the authors tracked the distribution of autosexual behavior across primates.

They found that masturbation has a long evolutionary history amongst primates, and was likely present in the common ancestry of all monkeys and apes, humans included. What was less clear is whether the common ancestor of other primates—lemurs, lorises and tarsiers—masturbated, largely because there was less data on these groups.

The team tested multiple hypotheses to better understand why this seemingly non-functional trait would evolve. According to the postcopulatory selection hypothesis, masturbation aids successful fertilization that can be achieved in various ways. 

Masturbation without ejaculation can increase arousal before sexual intercourse, which may be a useful tactic for low-ranking primate males that are likely to be interrupted during sex. 

Masturbation with ejaculation allows males to shed their more inferior semen, which leaves the fresh, high-quality semen available for mating. This super semen may be more likely to outcompete the semen of other males, which is necessary in primate communities with steep competition for mates. The study found support for this second hypothesis, namely that male masturbation co-evolved within multi-male mating systems where competition between males is high.

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According to the pathogen avoidance hypothesis, male masturbation reduces the chance of contracting an STI by cleansing the urethra with ejaculate. The team also found evidence to support this hypothesis, with the data revealing that masturbation in males co-evolved with high STI load across the primate tree of life.

The significance of female masturbation remains less clear. While it is frequent, fewer studies and reports describe female self-pleasure.The team argues that more data on female sexual behavior is needed before understanding masturbation’s evolutionary role in females. 

“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,” study co-author and University College London anthropologist Matilda Brindle said in a statement. “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.”