Why monkeys and apes don’t masturbate as much as human, Jesse Bering believes:
The answer for this cross-species difference, I’m convinced, lies in our uniquely evolved mental representational abilities—we alone have the power to conjure up at will erotic, orgasm-inducing scenes in our theater-like heads … internal, salacious fantasies completely disconnected from our immediate external realities.
And why men masturbate more than women:
And, perhaps not surprisingly, men seem to entertain more visitors in their heads than do women. In a 1990 study published in the Journal of Sex Research, evolutionary psychologists Bruce Ellis and Donald Symons found that 32 percent of men said that they’d had sexual encounters in their imagination with more than 1,000 different people, compared to only 8 percent of women. Men also reported rotating in from their imaginary rosters one imagined partner for another during the course of a single fantasy more often than women did.