If you have been to a zoo and been around our primate evolutionary cousins with kids, you may have had an awkward moment or two. They are going to masturbate, and don't care who's watching.

A new study says more is better. The Postcopulatory Selection Hypothesis believes it helps shed low-quality semen while the The Pathogen Avoidance Hypothesis believes it may reduce the risk of contracting sexually-transmitted infections. Primates have been doing it for at least 40 million years so something keeps it going.

The authors used a Monte Carlo analysis on survey data of 150 expert witnesses and studies to stratify with 105 primate species. Masturbation occurred in 75 percent of studies with captive females and 87 of those on captive males. 

The results were in support of the Postcopulatory Selection Hypothesis - coevolution between masturbation presence and mating system in male, but not female, primates. The explanation is that inferior primates could finish quicker and give themselves a competitive advantage over more dominant ones. 

Some studies also correlate masturbation to lower risk of prostate cancer but if you don't believe epidemiological correlation on anything else until science confirms it, and you should not, then don't believe that in this instance either.