Many animals satisfy themselves, but primates, including humans, do so with particular frequency, and there is an evolutionary purpose behind it.
Male masturbation appears to increase reproductive success and reduce the risk of STI infection. The importance of female masturbation remains unclear due to lack of data. Sexual masturbation appears to be an ancient primate trait that—at least in males—increases reproductive success and helps prevent sexually transmitted infections. This is the conclusion of a British research group from University College London. The results of their extensive studies have been published by the six participating researchers in the current issue of the Proceedings of the Royal Society B .
This behavior, also known as masturbation, is widespread throughout the animal kingdom, but it is especially common in primates, among which humans are phylogenetically. In the past, sexual masturbation used to be considered pathological or an expression of an exaggerated libido. As a result, the recorded observations were too fragmentary to understand their prevalence, their evolutionary history, or their precise significance.
The team led by anthropologist Matilda Brindle has now compiled the largest data set on primate masturbation to date. The group collected information from nearly 400 sources, including 246 published scientific articles, as well as 150 questionnaires and personal communications from primate researchers and keepers. Using these data, the authors tracked the spread of autosexual, or self-directed, behavior among primates to understand when and why it evolved in both females and males.
The scientists found that the common ancestors of all apes (including humans) most likely already masturbated. They also tested various hypotheses to understand the evolution of this behavior.
The “post-copulatory selection hypothesis,” for example, assumes that male masturbation favors successful fertilization. This can be accomplished in several ways. First, masturbation (without ejaculation) can increase arousal before intercourse and can be a useful tactic for lower-ranking males who might be disturbed during intercourse, as it allows them to ejaculate more quickly. Second, masturbation (with ejaculation) allows males to initially expel low-quality sperm in order to have fresh, high-quality sperm available for subsequent mating. The researchers corroborated this hypothesis by demonstrating that male masturbation occurs mainly in mating systems where competition between males is high.
According to the “pathogen avoidance hypothesis,” male masturbation reduces the likelihood of contracting an STI during mating by flushing the urethra with ejaculation. The urinary tract is the main site of infection for many STIs. The team found circumstantial evidence to suggest that male masturbation evolved alongside STIs in the primate family tree.
Even more data on female sexual behavior is needed to be able to study the evolutionary role of female masturbation. On the other hand, the importance of female masturbation remains unclear. Although it is also common, there are far fewer solid reports that allow clear conclusions to be drawn about its evolutionary purpose. The team argues that more data on female sexual behavior is needed first before the role of female masturbation can be studied.
“Our findings help to shed light on very common but poorly understood sexual behavior and represent a significant advance in our understanding of the functions of masturbation,” said Matilda Brindle, according to a UCL statement. “The fact that autosexual behavior is ubiquitous throughout the order of primates and is practiced by captive and feral members of both sexes demonstrates that masturbation is part of a repertoire of healthy sexual behavior’s.”