Nearly everyone does it, but we still don't know exactly why masturbation exists. Could rubbing one out have an adaptive function or it is simply a lucky byproduct of natural selection? Research on why and how monkeys get themselves off is helping scientists zero in on answers—and remove the stigma around one of our most beloved activities at the same time.
In the 1940s, the Kinsey Reports scientifically verified what humans had anecdotally known for a long time—that masturbation is widespread among both men and women. This finding created an evolutionary puzzle; it's still not immediately clear how masturbation improves the reproductive success of males or females, in fact at first it seems more likely that self-love gets in the way of the real thing. Trying to explain masturbation in terms of evolution has continuously created a sticky mess for scientists.
Humans have been preoccupied with diddling themselves since the beginning of civilization, a reality brought to light by our ancestors' explicit cave art showing exactly what got them off. Despite it being one of our most popular past-times, the social stigma around masturbation has prevented intensive scientific research into the evolutionary factors that have shaped how we masturbate, Cambridge primatologist Constance Dubuc tells me. This is now changing.
Although masturbation was once thought to be uniquely human, extensive field studies have shown that it is widespread among other animal species. From wild dolphins to sea otters and iguanas, many animals have been caught "self-comforting." These observations have allowed for the systematic study of masturbation in both captive and wild non-human animals that scientists hope can may provide valuable insight into human sexual health and behavior.
An older albeit widely accepted scientific explanation for the existence of masturbation in males is that it provides those who rarely mate with the opportunity to wash out old sperm and improve ejaculate quality. This seems to make evolutionary sense—better sperm quality equals more offspring.
In 2016, Dubuc and two of her colleagues conducted a study exploring how masturbation affects the mating and reproductive success of Rhesus macaque monkeys. The group found that as expected, males were most likely to masturbate on days they didn't have sex and that monkeys with a low social status and little access to females were more likely to masturbate than high-ranking individuals—depressing yet revelatory. The puzzle, for males species at least, seemed to be coming together.
However, as Dubuc explains, they "were surprised to observe that in the vast majority of cases, males did not ejaculate as the result of masturbation. Yet ejaculation is at the center of both adaptive and non-adaptive explanations for the occurrence of masturbation." It appears that masturbation may be "aimed at increasing sexual arousal rather than at releasing it." Practice makes perfect?
For these monkeys, Dubuc says, ejaculation is reached at the end of mating, which can last from several minutes to an hour. Masturbation could be used by males to decrease the length of matings, too. It is possible that speedy sex "could be instrumental to allow low-ranked males to sneak copulations, since a long mating series would increase the probability of a high-ranked males interrupting them before ejaculation was reached," she adds.
Human males—unlike their macaque cousins—don't appear to be masturbating to reduce the time spent having sex, though. In fact, studies have shown that masturbation is often used as a non-medically prescribed method of preventing premature ejaculation (whether this is effective or not has yet to be tested scientifically). But that doesn't mean that the monkey findings don't have implications for humans. As Dubuc explains, studying masturbation in monkeys has shown that touching ourselves is "well-rooted in our evolutionary heritage" and studying masturbation in humans could help take the shame or embarrassment out of it altogether.
When it comes to women, things are even less clear. Current theories fail to account for or explain the existence of female masturbation; meaning that it could be an evolutionary byproduct of the success of wanker men or an adaptive behavior in its own right. Examples of female masturbation are harder to find among non-human animals. "In many species, there is no external clitoris, making masturbation difficult. Moreover, in most species, females are only sexually aroused in short periods of time around ovulation, decreasing the probability to observe it," Dubuc says.
Dubuc explains that "primates—including humans—are unique because the clitoris is external and periods of sexual receptivity are extended. But still, from my personal experience, masturbation is much less common in females than in males in macaques, and the low rate at which I observed it would actually prevent me to study it quantitatively."
Having an external clitoris dramatically changes how members of a species interact with each other and their own bodies. External genitals allow female bonobos, for example, to engage in genito-genital rubbing, in which pairs of females rub their genitals together in a behavior thought to be vital for tension reduction and social cohesion. How and why we masturbate is intimately linked to our anatomy, and Dubuc believes that as we come to better understand the adaptive functions of the female orgasm and clitoris we may shed light onto the adaptive functions of female masturbation.
Unlike other primates though, humans can articulate their emotions, sexual arousal and habits to scientists providing data for future research. Until recently, stigma surrounding female sexuality, orgasm and masturbation has prevented research and slowed down understanding. Science is thankfully shifting to be more representative of female sexuality, and we may soon know more.
While many animals masturbate, it has been proposed that humans are unique in their ability to pair tactile genital stimulation with vivid sexual thoughts. From imagined fetishes to daydreamed ménages à trois; big brains set us apart in our ability for self-stimulation. In fact, the science writer Jesse Bering has postulated that imagination-aided me time is what makes humans unique.
"One thing that studies of masturbatory behavior in monkeys and apes has done is to make it clear that the behavior is widespread, perfectly normal and harmless," says Alan Dixson, a researcher in evolutionary biology and anthropology who has written extensively about primate sexuality. "It used to be thought that masturbation harmed human sexual and moral development...things are more enlightened nowadays, but it was not always so, and guilt concerning sexual matters is still an issue in many human societies. Monkeys and apes are, presumably, free of such feelings of guilt." If we can learn anything from our hairier relatives, it's that we should be less uptight about our solo play.
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