Masturbation occurs widely throughout the animal kingdom, particularly among primates like humans. In the past, this behavior was considered abnormal or simply a result of sexual arousal, and our knowledge was insufficient to understand its distribution, evolutionary history, or purpose. However, a recent study published in Proceedings of The Royal Society B has revealed surprising findings, suggesting that masturbation may actually serve an evolutionary function.
The research indicates that masturbation is an ancient trait among primates and likely existed in the common ancestor of all monkeys and apes, including humans. The available data for other primate groups, such as lemurs, lorises, and tarsiers, is more limited, making it unclear if they engage in masturbation as well.
To unravel the evolutionary significance of this seemingly non-functional behavior, the researchers put forward several hypotheses. One hypothesis suggests that masturbation aids in successful fertilization. For instance, it can increase arousal before sexual activity, which may be particularly advantageous for males of lower rank who are prone to interruptions during mating. Additionally, male masturbation with ejaculation allows for the removal of inferior semen, leaving behind fresh, high-quality sperm that have a better chance of outcompeting those from other males. The researchers found supporting evidence for this hypothesis, particularly in primate species where competition between males is high.
Another hypothesis proposes that male masturbation serves as a protective measure against sexually transmitted infections (STIs) by cleansing the urethra, a common site of infection. The team also discovered evidence supporting this hypothesis, demonstrating a co-evolution between male masturbation and a higher prevalence of STIs across various primate species.
The significance of female masturbation, however, remains less clear due to fewer reports and limited statistical data. The researchers emphasize the need for more comprehensive information on female sexual behavior in order to gain a better understanding of the evolutionary role of female masturbation.
Lead researcher Dr. Matilda Brindle highlights that these findings contribute to our comprehension of a widely prevalent but poorly understood sexual behavior. The fact that masturbation appears to serve an adaptive function, exists among different primate species, and is practiced by both captive and wild individuals of both sexes underscores its natural place within a repertoire of healthy sexual behaviors.
This study, supported by the Natural Environment Research Council, was conducted by researchers from UCL Anthropology, the UCL Centre for Biodiversity & Environment Research, and Queen Mary University of London.