Yes monkeys can (and do) masturbate.
Especially Banobos. (Sp?)
Erhm.. Bonobo's are like, fucking monkey's, Thei're like.. even stupider Gouravs =D
They fuck all day long, and even masturbate all day long.. Just for fun ;D, All they miss is the hentai
Banobos are famous for having all kinds of sex. Sometimes not even with females. They establish relationships by having sex, just like dolphins.