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.
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