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 look at the history of mankind will show that humans have enjoyed masturbating for many thousands years. This activity is often immortalized in writings and art. 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.
A team of scientists, led by Matilda Brindle from University College London’s evolutionary biology department, tracked the evolution paths of primate masturbation over time using phylogenetic comparison methods. According to a study that was published in Proceedings of the Royal Society on Wednesday , the results show that masturbation is an old trait and has a high phylogenetic significance. This implies that masturbation could be an adaptive behavior that functions at macroevolutionary level.
“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 increase the chances of survival and is done at the expense of reproduction partners. This also costs time, energy and attention. 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. Many other studies suggest that masturbation can offer fitness benefits, which are more easily observed in men.
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 results presented in this study highlight that the Pathology Hypothesis is insufficient to explain autosexuality among primates, as masturbation was reported by over a third (of studies) of wild females and two-thirds (2/3) of wild males.
“We demonstrate that masturbation does not only occur as a result of sexual stimulation. “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. The team stressed the importance of more research on the female masturbation mechanisms, as they are less clear due to the lack of unambiguous observation in captive and wild primates.
“Female primate could be using pre- and postcopulatory masturbation to improve their chance of fertilization by a male,” Brindle, her coworkers 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.
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