Many animals indulge themselves, but primates, including humans, do so with particular frequency, and there is an evolutionary purpose behind this.
Male masturbation appears to increase reproductive success and reduce the risk of STD 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 at University College London. The results of their exhaustive studies have been published by the six participating researchers in the current edition of the journal Proceedings of the Royal Society B.
This behavior, also known as masturbation, is widespread throughout the animal kingdom, but 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, recorded observations were too fragmented to understand their prevalence, their evolutionary history, or their precise meaning.
The team led by anthropologist Matilda Brindle has now compiled the largest dataset 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 researchers and primate caretakers. Using this data, the authors tracked the spread of autosexual, or self-directed, behavior among primates to understand when and why it evolved in females and males.
Scientists have discovered that the common ancestors of all apes (including humans) probably already masturbated. They also tested several hypotheses to elucidate why this behavior evolved evolutionarily.
The “postcopulatory selection hypothesis,” for example, assumes that male masturbation favors successful fertilization. This can be done in several ways. First, masturbation (without ejaculation) can increase arousal before intercourse and can be a useful tactic for lower-level men who may be troubled 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 showing that male masturbation occurs mainly in mating systems where competition between males is high.
According to the “pathogen prevention 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 STDs. The team found circumstantial evidence to suggest that male masturbation evolved along with STDs in the primate family tree.
Even more data on female sexual behavior are needed to be able to study the evolutionary role of female masturbation. On the other hand, the importance of female masturbation remains unclear. While it is also common, there are far fewer solid reports to draw clear conclusions about its evolutionary purpose. The team argues that more data on female sexual behavior is needed before the role of female masturbation can be studied.
“Our findings help shed light on very common but poorly understood sexual behaviors and represent a significant advance in our understanding of the functions of masturbation,” said Matilda Brindle, according to a UCL release. “The fact that autosexual behavior is ubiquitous throughout the primate order and is practiced by captive and wild members of both sexes demonstrates that masturbation is part of a repertoire of healthy sexual behaviors.”