pestulens OK, read through the summary and findings, I probably won't have time to dig into the methodology so I might have missed something but here are my takeaways.
The central question they were looking at is whether masturbation has an evolutionary benefit or is just a byproduct of a high sex drive.
This study was done on the species level, not individuals, and they only measured whether members of a particular species masturbate at all, not how prevalent it is. They were mostly looking for correlations between masturbation being observed in a particular species and that species living in conditions where it has previously been theorized it might be beneficial.
They found strong correlations between male masturbation and 2 different factors in which masturbation has been theorized to be beneficial. The first is 'postcopulatory competition' (ie when multiple males mate with a single female during one mating cycle). The second is the prevalence of pathogens that infect the reproductive system (ie STDs). They conclude that there is strong evidence that male masturbation in primates is an evolved trait.
They did not find a correlation between either of these factors and female masturbation. The data on female masturbation in primates is much less complete, so the researchers don't draw any conclusions from this other than "we need more data." Looking at the data they do have, however, it does appear that primate species where males masturbate but females don't are relatively common but ones where the reverse is true are pretty rare (though a few do exist).
Also, for anyone who is wondering, yes humans are included in the data set.
TLDR: evidence masturbation is beneficial in male primates, insufficient evidence to draw a conclusion in females.
