The science of masturbation: New study traces origins of self-pleasuring in primates

But then the research team thought that since self-pleasuring is so common throughout the animal kingdom, there has to be a bigger and clearer explanation.

The team collated information from over 400 sources, including 246 published academic papers, 150 questionnaires, and personal communications from primatologists and zookeepers, as per a press release by UCL. From this data set, the researchers tracked the distribution of self-pleasuring behavior across primates to understand when and why it evolved in both females and males.

The team tested two theses: Postcopulatory Selection and Pathogen Avoidance Hypotheses.

The Postcopulatory Selection hypothesis was further divided into two constituents – the Sexual Arousal hypothesis and the Sperm Quality hypothesis. The Sexual Arousal hypothesis revealed that non-ejaculatory masturbation in males speeds up subsequent ejaculation or increases ejaculate quantity which could help in fertilization. In females, masturbation increases vaginal pH, thus creating a more hospitable environment for the sperm. Vaginal excretions have also been known to filter out inferior sperm and facilitate the movement of high-quality sperm toward the uterus.

The Sperm Quality hypothesis states masturbation in males before a sexual encounter improves sperm quality by expelling inferior sperm. 

The other thesis, Pathogen Avoidance, predicted that masturbation after a sexual encounter, in both males and females, helps prevent sexually transmitted infections (STIs). These postcopulatory genital grooming strategies include oral self-cleaning, urination, and masturbation to clean the reproductive tract with ejaculation.

The significance of female masturbation remains less clear, noted the researchers. The team says that more data on female masturbation and sexual behavior needs to be done.

Lead researcher Dr. Matilda Brindle said, “Our findings help shed light on a very common, but little understood, sexual behavior and represent a significant advance in our understanding of the functions of masturbation. The fact that autosexual behavior may serve an adaptive function, is ubiquitous throughout the primate order, and is practiced by captive and wild-living members of both sexes, demonstrates that masturbation is part of a repertoire of healthy sexual behaviors.”

The study was published in The Royal Society Publishing.

Study abstract:

Masturbation occurs throughout the animal kingdom. At first glance, however, the fitness benefits of this self-directed behaviour are unclear. Regardless, several drivers have been proposed. Non-functional hypotheses posit that masturbation is either a pathology, or a byproduct of high underlying sexual arousal, whereas functional hypotheses argue an adaptive benefit. The Postcopulatory Selection Hypothesis states that masturbation aids the chances of fertilization, while the Pathogen Avoidance Hypothesis states that masturbation helps reduce host infection by flushing pathogens from the genital tract. Here, we present comprehensive new data documenting masturbation across the primate order and use these, in conjunction with phylogenetic comparative methods, to reconstruct the evolutionary pathways and correlates of masturbation. We find that masturbation is an ancient trait within the primate order, becoming a more common aspect of the haplorrhine behavioural repertoire after the split from tarsiers. Our analyses provide support for both the Postcopulatory Selection and Pathogen Avoidance Hypotheses in male primates, suggesting that masturbation may be an adaptive trait, functioning at a macroevolutionary scale.