Male masturbation in primates evolved to cut chance of catching STIs

Primates have been masturbating for millions of years and the practice seems to have evolved to boost reproductive success and reduce the chances of contracting a sexually transmitted infection (STI), in males at least.

Masturbation is widely seen in mammals and is nearly ubiquitous in primates, which include apes like chimpanzees and gorillas, as well as smaller monkeys and lemurs. “Masturbation seems like an evolutionary conundrum,” says Matilda Brindle at University College London. “Why would evolution produce this seemingly non-functional and costly sexual behaviour that’s very much a solo activity?”

Brindle and her team focused on two ideas. The postcopulatory selection hypothesis says that male masturbation improves the quality of ejaculate and makes superior sperm available for fertilisation, increasing the chance of reproductive success. The pathogen avoidance hypothesis suggests that masturbation is a post-sex cleansing activity.

To investigate, Brindle and her colleagues used nearly 400 data sources, including scientific papers and questionnaires they sent to zookeepers and primatologists, and created what they say is the largest data set of masturbation in primates.

They coupled this with the evolutionary tree of primates and the frequency of sexually transmitted pathogens in wild primates from the Global Primate Parasite Database. Where data was scant, the researchers used environmental proxies to help estimate pathogen load, because many parasites require hot, humid conditions or water to complete their life cycles.

Statistical analysis revealed that the males and females of the most recent common ancestor of all the New World monkeys, Old World monkeys and apes were likely to be masturbators. But going further back, to the common ancestor of this line and of tarsiers, which don’t masturbate, the authors conclude that the female of this common ancestor may have masturbated but aren’t sure about the male.

“It’s important to realise that masturbation is ubiquitous throughout the primate order, in both females and males, and is practised by captive and wild-living individuals. We hope that highlighting this helps to negate damaging attitudes towards masturbation, by demonstrating that it is part of a repertoire of healthy sexual behaviours,” says Brindle.

The evidence backed both hypotheses for why masturbation evolved in male primates, finding that it gives them a better chance at reproduction and helps them prevent STIs.

On the disease front, this could be because seminal fluid has antibacterial molecules and the force of ejaculation expels pathogens from the male urethra, say the researchers.

Smaller primates, like sifakas and marmosets, clean their genitals by licking and urinating after mating. But larger primates like chimpanzees, gorillas and bonobos lack the flexibility to lick their genitals, say the researchers, so they might have adopted masturbation to lower their STI risk.

The data was inconclusive for females, however, says Brindle, and it is possible that female masturbation doesn’t fit either hypothesis tested here. Masturbation in female primates doesn’t forcefully flush out pathogens. Sexual stimulation also raises the vaginal pH – which is usually in the acidic range to prevent pathogen growth – to make the environment more hospitable to expected sperm, weakening the chemical defence of the vagina.

“This is a very interesting article that sheds light on the evolutionary history of behaviours that leave no trace in the fossil record,” says Kit Opie at the University of Bristol, UK. “It’s amazing how widespread masturbation was across primates, especially monkeys and apes.”

Not knowing whether the masturbation is happening before or after sex is a limiting factor of the study, says Cyril Grueter at the University of Western Australia. If it happens to mitigate STI risk, then it should consistently happen after mating. If it serves to improve reproductive success in promiscuous groups, then it should consistently happen before mating, he says.

He suggests two other possible reasons for the practice. “Masturbation can lead to relaxation and reduced aggression, which may contribute to the maintenance of cohesion in these primate groups,” he says. “Second, those primates that have more time on their hands, maybe because they don’t have to worry about foraging, like those in zoos, may indulge more in pleasurable activities such as masturbation.”

The study did find that masturbation was more common in captive than in wild individuals.