The Evolutionary Origins of Masturbation in Primates

A behavior often considered uniquely human may have emerged more than 40 million years ago and evolved through sexual selection, sperm competition, and pathogen avoidance.

Illustration of a chimpanzee. Photo by David Trinks on Unsplash

Masturbation is a form of self-stimulation practiced for sexual pleasure, and it represents a natural component of sexual behavior in humans and many other animals Yet, in many respects, it is still considered a taboo and often regarded as a uniquely human characteristic because it is associated with pleasure. In reality, it is a widespread practice throughout the animal kingdom, especially among primates, and it appears to be a very ancient behavior, inherited over millions of years from generation to generation.

Although sexuality and sex in general remain something of a taboo, both culturally and religiously, we are nevertheless dealing with a natural phenomenon that has accompanied us throughout our history. Indeed, researchers have traced the origin of masturbation back to approximately 40 million years ago. In other words, long before the appearance of our species and other ancient hominins, such as the australopithecines or the common ancestor shared by our lineage and that of chimpanzees, this autoerotic behavior was already present among primates.

Illustration of a woman wearing underwear. Photo by IFONNX Toys on Unsplash

Here, provided that Medium does not take offense and that I am not violating any guidelines by writing an article on this subject, we are nevertheless entering the field of evolutionary biology because, as with any trait (unless it is the result of a macromutation), this behavior did not appear “overnight”. Rather, it is linked to a series of biological functions that developed over time.

The research was made possible through the compilation of hundreds of scientific publications (and not only scientific publications). Researchers essentially reconstructed a primate family tree based on the presence of masturbatory behavior, and the result suggests that all primates engage in it. In short, it is a trait shared by all primates, including humans.

But what function could such a behavior serve?

Aside from making some religious extremists angry, there may be several very interesting evolutionary explanations for it. Here is what researchers suggest:

• It increases the chances of conception. This mainly concerns “non-dominant” males, which practiced (and still practice) this behavior to increase sexual arousal before encountering a female partner. From a reproductive perspective, this could reduce the effective duration of copulation, particularly in anticipation of a possible confrontation with dominant males. In other words: “the faster I finish, the greater my chances of avoiding detection by the alpha male and successfully reproducing.”
• It may help eliminate older sperm, making room for “fresher” sperm. Essentially, this behavior clears out older sperm reserves, allowing newer sperm to accumulate. These newer sperm cells may be more motile and better prepared for fertilization during subsequent mating opportunities.
• It helps maintain genital tract hygiene and may reduce the probability of sexually transmitted infections. It appears that the increasing prevalence of masturbation may have occurred alongside the progressive emergence and spread of sexually transmitted diseases. In this way, the behavior could help reduce the likelihood of infection within the host.

Illustration of a man lying on the bed. Photo by Haley Lawrence on Unsplash

As for female autoerotic behavior, less information is currently available, partly because it has been documented less extensively than its male counterpart. More detailed studies will undoubtedly be conducted in the future, but researchers suggest that female masturbation may have provided females with a degree of control over reproduction by influencing which male’s sperm was more likely to achieve fertilization.

From a biological perspective, this behavior appears to make the internal environment of the vagina less acidic, thereby creating more favorable conditions for sperm survival and transport.

Further Reading

In the more popular-science section, I referred only to the “functional” aspects of autoerotic behavior and to the potential fitness-related benefits associated with it. However, there are also “non-functional” hypotheses suggesting that this behavior may simply be a by-product of elevated sexual arousal. There was also a “pathological” hypothesis, although it does not seem to adequately explain the occurrence of masturbation in wild primates (and may be more relevant to our own species).

Illustration of a male gorilla individual. Photo by Christina Gillette on Unsplash

Overall, masturbation appears to be an adaptive trait that characterizes the entire order Primates following the divergence between tarsiers and haplorhines. The hypotheses involving postcopulatory selection are those mentioned in the popular-science section and can be divided into two additional hypotheses (or “sub-hypotheses”):

• The sexual arousal hypothesis. Non-ejaculatory male masturbation may accelerate subsequent ejaculation, thereby benefiting lower-ranking males that risk being displaced by dominant rivals. This trait is found in many animal species, including the marine iguana (Amblyrhynchus cristatus). In our species, it has also been associated with improvements in sperm quality.
• The sperm quality hypothesis. Male masturbation may expel lower-quality or older sperm, thereby improving the quality of the sperm available for subsequent mating opportunities.

As mentioned earlier, less is known about female masturbation, but an increase in vaginal pH appears to create a more hospitable environment for sperm. The vagina plays a role in filtering sperm and facilitating the transport of higher-quality sperm toward the uterus. In addition, the contractions associated with female orgasm may enhance sperm movement through the uterine cavity, while the associated prolactin secretion may further facilitate sperm retention and transport.

Stylized illustration of a vagina. Photo by Kate Bezzubets on Unsplash

All of this must also be considered in relation to sperm longevity within the female reproductive tract. Little is known about this process in non-human primates, but in humans, sperm can remain viable for more than five days. Therefore, it seems plausible that females may use masturbation (either before and/or after copulation) as a strategy to increase the likelihood of being fertilized by a particular male. In other words, it could facilitate mate choice or favor the sperm of a more desirable individual. Since episodes of forced copulation can occur among primates, such mechanisms may potentially reduce the likelihood of fertilization by unwanted males. Another possibility is that masturbation may function as a precopulatory display or as a form of courtship behavior in both sexes, analogous to the “penile display” performed by male chimpanzees prior to copulation.

A further hypothesis involves pathogen avoidance. According to this interpretation, masturbation may serve as a form of postcopulatory genital cleaning that helps reduce the risk of sexually transmitted infections. This strategy is also observed in several mammalian species, which engage in masturbation after mating and may cleanse the reproductive tract through ejaculation.

Illustration of some protections created by humans to prevent sexually transmitted diseases. Photo by Felicia Montenegro on Unsplash

More generally, masturbation and the presence of pathogens in males (but not females) may be viewed as a case of coevolution. In other words, there appears to be a relationship between the increasing prevalence of masturbatory behavior and the increasing presence of pathogens. Under this hypothesis, masturbation may provide an antibacterial defense for sperm before or during their passage through the female reproductive tract, while also potentially reducing pathogen transmission and protecting the hosts themselves.

Source: Brindle, M., Ferguson-Gow, H., Williamson, J., Thomsen, R., & Sommer, V. (2023). The evolution of masturbation is associated with postcopulatory selection and pathogen avoidance in primates. Proceedings of the Royal Society B: Biological Sciences, 290(1995).