Origin and Benefits of Masturbation Explained through Study on Evolution

Masturbation is common throughout the animal kingdom but is especially prevalent among primates, including humans. Historically, this behavior was considered either pathological or a by-product of sexual arousal, and recorded observations were too fragmentary to understand its distribution, evolutionary history, or adaptive significance. New research has been published in Proceedings of the Royal Society b He sees that this behavior, perhaps surprisingly, appears to serve an evolutionary purpose.

The findings suggest that masturbation is an ancient trait in primates, and that — at least in males — it increases reproductive success and helps stave off sexually transmitted infections (STIs).

Dr. Matilda Brindle (UCL Anthropology) and her colleagues have compiled the largest-ever data set of primate masturbation, collecting information from nearly 400 sources, including 246 published academic papers, 150 questionnaires and personal communications from primate scientists and zookeepers. From this data, the authors traced the distribution of subjective sexual behavior across primates, to understand when and why it evolved in both females and males.

The team found that masturbation has a long evolutionary history among primates and was likely present in the common ancestor of all monkeys and apes (including humans). It was unclear whether the ancestors of other primates (lemurs, lorises, and tarsiers) masturbated, in large part because data was more scarce for these groups.

To understand why evolution produces this seemingly non-functional trait, Dr. Brindel and his colleagues tested several hypotheses.

The “postnatal selection hypothesis” proposes that masturbation contributes to successful fertilization. This can be achieved in different ways. First, masturbation (without ejaculation) can increase arousal before sex. This may be a particularly useful tactic for lower-ranking males who are 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 outpace other males. The researchers found support for this hypothesis, showing that male masturbation co-evolved with poly-male mating systems where competition between males and females is high.

The “pathogen avoidance hypothesis” proposes that male masturbation reduces the chance of contracting an STI after intercourse, by clearing the urethra (a major site of infection for many STDs) with ejaculate accessed by masturbation. The team also found evidence to support this hypothesis, which shows that male masturbation co-evolved with a higher STD load across the main tree of life.

The importance of female masturbation is still less clear. Although frequent, there are fewer reports describing it, which reduces the analytical power of the statistics. The team argues that more data on female sexual behavior is needed to better understand the evolutionary role of female masturbation.

Lead researcher Dr Brindel said: “Our findings help shed light on a very common, yet poorly understood, sexual behavior and represent a major advance in our understanding of the functions of masturbation. The fact that subjective sexual behavior may serve an adaptive function is ubiquitous in throughout the primate order, and is practiced by captive and wild animal members of both sexes, demonstrating that masturbation is part of the repertoire of healthy sexual behaviors.”