Can anyone confirm for me if monkeys and gorillas actually masturbate?

When I was quite young, I went with my parents to the zoo in San Francisco, and as we stood watching a gorilla, he began to masturbate. I asked my parents what he was doing... Mom grabbed my hand and we left the primate exhibit area immediately. I don't remember how much later I realized what he had been doing, but I remember the sudden realization and how hard I laughed.