MASTURBATION IS COMMON across the animal kingdom but it seems to be an evolutionary paradox. Why would an animal waste time, energy and reproductive resources on self-pleasure instead of copulating with a partner?
The evolutionary history of masturbation in primates extends back at least 40 million years, and the behavior appears to help male primates be ready to mate when they get the chance, and also stay free of disease.
To determine when masturbation first evolved in primates, evolutionary biologists sifted through the scientific literature to find records of which primates masturbate and which do not, whether in the wild or in captivity.
To fill gaps in the data due to masturbation not always being easy to observe by biologists, questionnaires were sent out to researchers to ask about their observations of primate masturbation. This was especially important for information on female masturbation, which tends to go unnoticed because of a lack of an easy-to-spot erection. The team then used computer analyses to determine where in the primate lineage the behavior most likely originated.
Because of missing data, they can’t definitely say whether the first ever primates masturbated but that from around 40 million years ago, “the ancestors of all monkeys and apes” seemed to have masturbated. This would be around when simians — apes and monkeys — split from the tarsiers, diminutive, bug-eyed primates that live in Southeast Asia.
They also looked at whether primates that masturbate tend to mate with multiple partners. This mating system would lead to evolutionary pressure for both males and females to gain more control over the mating process, like by being able to copulate more quickly, being readily aroused for preferred partners or by improving male sperm quality.
Scientists found that multiple mating partners and pathogen prevalence are associated with masturbation in male primates, but not in females. Masturbation can help males be ready to mate quickly with fresh sperm while also purging their reproductive tract of pathogens. But for females, the two hypotheses don’t mix. Normally, the vagina is mildly acidic to keep pathogens at bay, but it becomes less so when the female primate is aroused so that sperm aren’t killed on arrival. Making the vaginal environment safer for sperm also makes it safer for pathogens.
While the study found no correlation between female primate masturbation and having multiple mating partners, the team suspects that there probably is a relationship that would come to light with more data. The lack of data for females may be driven in part by the historical tendency to think of female animals as passive recipients of male behavior.
The stigma around studying masturbation and sexual behavior has started to ease up, which makes it hopeful that more discoveries will be made soon. But given that other mammals, as well as birds and reptiles, also masturbate, if we want to understand the bigger evolutionary picture of this behavior, we need to look beyond primates. Homo sapiens, for instance? By Manny Palomar, PhD (EV Mail FEB. 10-16, 2025 issue)
