Self love: Why masturbation evolved

Though masturbation is common across the animal kingdom, it seems, at its face, to be an evolutionary paradox: Why would an animal waste time, energy and reproductive resources on self-pleasure instead of copulating with a partner?

Studies on individual species have found some explanations. Low-ranking Japanese macaques (Macaca fuscata), for example, masturbate to keep their sperm fresh for when they get a rare chance to mate. But the questions of when and why masturbation evolved in the first place have remained unsolved.

A new study, published in the June 7 Proceedings of the Royal Society B, suggests that the evolutionary history of masturbation in primates extends back at least 40 million years, and the behavior might indeed help male primates be ready to mate when they get the chance, and also stay free of disease.

[Researcher Matilda] Brindle 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,” Brindle says, 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.

This is an excerpt. Read the full article here