Jacking Off Has Evolutionary Benefits Going Back 40 Million Years, Study Says

Primates have likely been masturbating for at least 40 million years for a variety of adaptive reasons, according to a new study that probed the evolutionary origins of self-pleasuring behaviors that are widespread among many animals.

Scientists traced the evolution of masturbation through time, and across species, by analyzing more than 400 publications and observations of autosexual behaviors in primates, in what is the largest comparative dataset on masturbation ever amassed. The results suggest that masturbation confers evolutionary advantages in males, such as disease prevention and enhanced reproductive success, though its relevance to females will require more research to unpack. 

A cursory glance at human history will reveal that people really enjoy masturbating and have for thousands of years, often enshrining this activity in art and writings. Autosexual behaviors have also been observed in plenty of other animals—including cats, dogs, dolphins, horses, boars, otters, and a variety of primates—but the roots and potential adaptive benefits of self-arousal remains a mystery.

In the study, scientists led by Matilda Brindle, an evolutionary biologist at University College London, used phylogenetic comparative methods to track the evolutionary pathways of masturbation in primates across time. The results “demonstrate that masturbation has a strong phylogenetic signal and is an ancient trait within the primate order” which imply that “masturbation may be an adaptive trait, functioning at a macroevolutionary scale,” according to a study published on Wednesday in Proceedings of the Royal Society B. 

“Autosexual behavior, or masturbation, is common across the animal kingdom, but appears to be particularly prevalent in the primates,” Brindle and her colleagues said in the study. “There is little systematic comparative research into this behavior, and its evolutionary history is unclear.” 

“At a superficial level, masturbation poses a problem for evolutionary theory,” the team continued. “It does not directly increase survival prospects and, by definition, occurs to the exclusion of reproductive partners, while incurring costs in terms of time, attention and energy. Consequently, masturbation has historically been considered, at worst, a pathological behavior carried out by aberrant, typically captive, individuals and, at best, a sexual outlet necessitated by high libido.”

Those proposed drivers of masturbation, known as the Pathology hypothesis and the Outlet hypothesis, view autosexual behaviors as derivatives of sexual libido without any clear evolutionary purpose. But many other studies have suggested that masturbation may offer a range of fitness benefits that are most easily discerned in males. 

The study zeroed in on two alternate hypotheses that became apparent after tracking the roots of masturbation back through time using the researchers’ sweeping dataset. The Postcopulatory Selection hypothesis suggests that male primates use masturbation to ejaculate faster, and with fresher high-quality sperm, in preparation for copulation with females, which may boost their odds of successful fertilization. In addition, self-induced ejaculations in males may prevent the spread of sexually transmitted infections by frequently cleansing the urethra and removing pathogens, which is known as the Pathogen Avoidance hypothesis.

Brindle and her colleagues’s research produced evidence to support both the Postcopulatory Selection and Pathogen Avoidance hypotheses in males, but not for females. The results also suggest that our primate ancestors have probably masturbated since the divergence of monkeys and apes from tarsiers, which occurred more than 40 million years ago.

“The data presented here highlight the inadequacy of the Pathology Hypothesis to explain autosexuality in primates since, in our dataset, masturbation is reported in over a third of studies on wild females, and over two-thirds of reports on wild males,” the researchers said.

“We illustrate that masturbation is not simply a pathological behavior, and is unlikely to solely be a byproduct of high sexual arousal,” they added. “We provide the first evidence that both postcopulatory selection pressure and pathogen avoidance may influence this common, but little understood, sexual behavior at a macroevolutionary scale.” 

The study provides a foundation for future work on the emergence and function of masturbation in animals, which is a somewhat overlooked behavior given its prevalence in the natural world. In particular, the team emphasized the need for more research into the mechanisms that drive female masturbation, which are murkier in part because there are fewer unambiguous observations in wild and captive primates.

“Female primates could use pre- or postcopulatory masturbation as a strategy to increase their chances of being fertilized by a given male,” Brindle and her colleagues speculated. “Alternatively, masturbation could also serve as a form of precopulatory display or courtship behavior in both sexes, similar to precopulatory ‘penile displays' in chimpanzees.” 

“Considering the behavioral and socioecological complexity of primate societies, it is likely that primates employ masturbation as a flexible strategy according to the circumstances they find themselves in,” they concluded.