Sperm competition theory does seem to bear out in the natural world. Testis size, for example, correlates with female promiscuity across species as diverse as insects and primates. That makes perfect sense: If several males are competing to impregnate the same partner, then the one with the biggest balls (and thus the most sperm) would have an important advantage. So what might sperm competition have to say about masturbation? One natural prediction would be that "clearing the chamber" from time to time increases fertility by expelling old, ineffective sperm from the genital tract. (That idea was first proposed to explain human masturbation in 1965.) Another prediction might hold that the rate of masturbation is related to the degree of sperm competition—being higher during mating season, for example, and more prevalent among males that lacked exclusive access to a sexual partner. Indeed, both predictions seem to hold true for Japanese macaques (PDF).
Still, neither the fresh-sperm hypothesis nor its discredited cousin, the kamikaze-sperm hypothesis, can account for more than a small subset of animal masturbation. Reloading might explain the behavior of bucks, bulls, and male primates, all of which tend to ejaculate at the end of an autoerotic episode. But many other animals never reach that point. Horses rarely climax, despite masturbating dozens of times per day—so what motivates the dalliance of a stallion or, for that matter, a mare? Can evolution account for female masturbation in the animal kingdom?
We don't really know. The problem is, scientists haven't paid that much attention to the issue. The systematic study of animal masturbation didn't really emerge until the end of the 19th century, when sexologists like Havelock Ellis began compiling evidence of autoerotic behavior among elephants, camels, ferrets, bears, dogs, sheep, and many other animals. For those researchers, though, the phenomenon was most interesting for what it said about humans—and the great terror of masturbation that had blanketed Europe for almost 200 years.
Doctors of the era blamed masturbation for numerous ailments, including tuberculosis, blindness, and insanity. According to Thomas Laqueur, author of Solitary Sex: A Cultural History of Masturbation, the early crusaders against onanism saw the problem as one that was essentially human—a disorder of the imagination, and a movement away from the social world. The discovery that animals, too, could masturbate provided one means of challenging this long-held view. "Since masturbation appears to be universal among the higher animals," wrote one contemporary of Ellis, "we are not entitled to regard it as a vice."
In the 20th century, sexologists used animal masturbation to address more specific social anxieties. Alfred Kinsey brought the discussion to women, arguing that "the inclination to stimulate her own genitalia" is one the human female shares with rats, chinchillas, rabbits, porcupines, squirrels, ferrets, horses, cows, elephants, dogs, baboons, monkeys, and chimpanzees. He also cited evidence of self-fellation in nonhuman primates to conclude that oral sex is "a biologically normal aspect of sexuality."
These days, the popular debate over animal sexuality tends to gloss over examples of masturbation. Clearly we have more pressing cultural matters in mind: In the late 1990s, the primatologist Frans de Waal introduced the idea of animal homosexuality—among bonobos and chimpanzees, at least—to a wide audience, and Bruce Bagemihl published a compendium of evidence for homosexual behavior in hundreds of different species, including giraffes, dolphins, and penguins. (The propensity for tuxedoed birds to enact something like a gay marriage has since provided a memorable skirmish in the culture wars.)
What about masturbation, then? We know that autoeroticism is widespread among mammals and that it turns up in birds and reptiles, too. Yet we don't know whether or why the behavior might have evolved for the majority of animals that engage in it. Are we all such happy wankers that the science no longer moves us?
Click here for a slideshow of animal-masturbation videos from YouTube. (Note: This slideshow contains graphic images that may not be suitable for all viewers.)