The evolutionary origins and advantages of masturbation

Newswise — Masturbation, a widely observed behavior in the animal kingdom, is particularly prominent among primates, including humans. In the past, this activity was often regarded as either abnormal or a consequence of sexual arousal. Due to limited and fragmented recorded observations, its distribution, evolutionary origins, and adaptive significance remained poorly understood. However, recent findings published in the Proceedings of The Royal Society B reveal a rather unexpected revelation: masturbation appears to have an evolutionary function.

The recent findings shed light on the ancient nature of masturbation in primates, suggesting that it has been a prevalent behavior for a significant period. Interestingly, these findings also highlight its potential benefits, particularly for males. Masturbation has been found to contribute to increased reproductive success by aiding in the avoidance of sexually transmitted infections (STIs). This suggests that the act of masturbation serves as a protective mechanism, ensuring the overall reproductive fitness and health of individuals within primate populations.

Dr. Matilda Brindle, along with her colleagues from UCL Anthropology, undertook an extensive research effort to compile the largest dataset on primate masturbation to date. Their comprehensive study involved gathering information from nearly 400 sources, including 246 published academic papers, as well as 150 questionnaires and personal communications from primatologists and zookeepers. By analyzing this vast amount of data, the researchers were able to trace the prevalence and distribution of autosexual behavior across various primate species. The objective was to gain insights into the evolutionary origins and reasons behind the development of masturbation in both female and male primates.

The research conducted by Dr. Matilda Brindle and her team revealed that masturbation has a significant evolutionary history among primates. Their findings suggest that this behavior most likely originated in the common ancestor of monkeys and apes, including humans. However, the data for other primate groups such as lemurs, lorises, and tarsiers were relatively scarce, making it less conclusive whether these primates also engaged in masturbation. Nonetheless, the study provides valuable insights into the presence and evolutionary development of masturbation across various primate species, highlighting its potential significance in the broader context of primate behavior.

To understand why evolution would produce this seemingly non-functional trait, Dr Brindle and colleagues tested several hypotheses.

The researchers put forward the "postcopulatory selection hypothesis" as an explanation for the role of masturbation in aiding successful fertilization. According to this hypothesis, masturbation serves several purposes in this context. Firstly, engaging in masturbation without ejaculation can increase arousal levels before engaging in sexual intercourse. This can be particularly advantageous for lower-ranking males who may face interruptions during copulation, as it helps them achieve ejaculation more quickly. Secondly, masturbation with ejaculation allows males to expel lower-quality semen, thus ensuring that fresh and high-quality sperm are available for subsequent mating attempts. This increases the chances of outcompeting the sperm of other males. The findings of the study supported this hypothesis, indicating that male masturbation has co-evolved with multi-male mating systems characterized by high levels of male-male competition.

The concept known as the "pathogen avoidance hypothesis" suggests that male masturbation serves as a protective measure against sexually transmitted infections (STIs) by effectively clearing the urethra, which is a common site of infection for many STIs. This cleansing mechanism is achieved through the expulsion of ejaculate obtained through masturbation. Additionally, the research team discovered compelling evidence supporting this hypothesis, indicating that male masturbation has co-evolved alongside the prevalence of STIs throughout the primate evolutionary tree.

The concept known as the "pathogen avoidance hypothesis" suggests that male masturbation serves as a protective measure against sexually transmitted infections (STIs) by effectively clearing the urethra, which is a common site of infection for many STIs. This cleansing mechanism is achieved through the expulsion of ejaculate obtained through masturbation. Additionally, the research team discovered compelling evidence supporting this hypothesis, indicating that male masturbation has co-evolved alongside the prevalence of STIs throughout the primate evolutionary tree.

The significance of female masturbation, in comparison to male masturbation, is currently less well-defined. Although it is known to occur frequently, there is a limited amount of reported data on female masturbation, which hinders the statistical analysis and reduces our understanding of its potential implications. The research team emphasizes the necessity for gathering more comprehensive data on female sexual behavior in order to gain deeper insights into the evolutionary significance of female masturbation.

According to lead researcher Dr. Brindle, the study's findings contribute to our understanding of a widespread but inadequately comprehended sexual behavior and mark a significant advancement in our knowledge of the functions of masturbation. The discovery that autosexual behavior, referring to self-stimulation, can serve an adaptive purpose and is observed across the primate spectrum, both in captivity and in the wild, highlights that masturbation is a natural component of a diverse range of healthy sexual behaviors.

The study received support from the Natural Environment Research Council and involved collaboration between researchers from UCL Anthropology, the UCL Centre for Biodiversity & Environment Research, and Queen Mary University of London. These institutions combined their expertise to contribute to the research and its findings.