Masturbation is relatively common in various species of the animal kingdom and appears to be particularly common among primates, including humans. A study led by researcher Matilda Brindle (of the Department of Anthropology at University College London) now proposes a new hypothesis about the origins and possible evolutionary advantages of this autosexual behavior in primates. The results of this research, published in an article in the Proceedings of the Royal Society B journal (June 7, 2023), highlight that "masturbation is not simply pathological behavior, and it is unlikely that it is solely a by-product of high sexual arousal". Instead, the authors note, with this new study "we provide the first evidence that both postcopulatory selection pressure and pathogen avoidance can influence this common, but poorly understood, sexual behavior on a macroevolutionary scale."
Masturbation has been and is considered, in some cultures and religions, as a negative, aberrant, pathological and unnatural behavior. There are no exhaustive studies on this behavior and to date no solid explanations have been put forward about its origins and evolutionary implications.
The scientists who signed the new study (two women and three men) have reviewed data from almost 400 sources (246 scientific publications and 150 questionnaires to experts in primatology and zoo keepers of primates) referring to more than a hundred species of primates, from both genders. Documented cases of masturbation are more frequent in captive animals but, as the authors indicate, this data is not particularly relevant because it is precisely captive animals that are most studied and observed by humans in these matters.
To understand why evolution would produce this apparently non-functional trait, the team raised several hypotheses, as explained by UCL in a disclosure note on the work of its anthropologists.
The "postcopulatory selection hypothesis" proposes that masturbation contributes to successful fertilization, which can be accomplished in a number of ways.
First, masturbation without ejaculation can increase arousal before intercourse and be an especially useful tactic for low-ranking males susceptible to interruption during copulation by helping them ejaculate more quickly.
On the other hand, masturbation with ejaculation allows males to get rid of lower-quality semen, leaving fresh, high-quality sperm available for mating, which is more likely to outperform that of other males.
The researchers corroborated these hypotheses by showing that male masturbation has coevolved with multiple mating systems in which competition between males is high.
Another hypothesis put forward by the team to understand the origin of masturbation is "pathogen avoidance", which proposes that the male reduces the possibility of contracting an STI after intercourse, by cleaning the ejaculate from the urethra.
The team also found evidence in support of this hypothesis, showing that male masturbation co-evolved with a high STI burden in the primate tree of life.
The importance of female masturbation remains less clear, because although it is frequent, there are fewer reports describing it, which diminishes 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.
These results, according to the study's lead author, Matilda Brindle, "help shed light on very common, but poorly understood sexual behavior, and represent a significant advance in our understanding of the functions of masturbation."
That autosexual behavior "can fulfill an adaptive function, is ubiquitous throughout the order of primates, and is practiced by members of both sexes in captivity and in the wild, demonstrates that masturbation is part of a healthy repertoire of sexual behaviors."
