Masturbation in the animal kingdom.

Isn't it wonderful when science and religion come together? My Slate colleague William Saletan points out that a recent paper has laid the groundwork for a pro-life defense of onanism. According to obstetrician David Greening, a rigorous program of daily masturbation can actually improve sperm quality in men with fertility problems. (Samples collected at the end of the program showed less DNA damage and higher sperm motility than samples from control subjects.) Since masturbation can help you have babies, Saletan argues, it must also serve the "procreative and unitive purposes" described in the Catechism.

Let's take this one step further. If we've redeemed this dangerous supplement for man, what about the fowl of the air and the beasts of the field? Surely what works for God will work for Nature, too: Since masturbation improves fertility, then it ought to be a prime target for natural selection. That is to say, any animal that evolves the ability or inclination for self-pleasure will end up with healthier sperm, and more offspring, than its competitors. Indeed, if you take the theory of evolution seriously—as the Catholic Church has since February—then you might expect that all animals masturbate, or at least all animals with a reproductive system sufficiently like our own.

Sure enough, hairy palms abound in the animal kingdom. (Wikipedia offers a good summary of the evidence.) Dogs, cats, lions, bears, and a number of other mammals self-stimulate with their front paws; randy walruses use their flippers. Horses and donkeys, whose masturbatory habits have been particularly well-studied, engage in "rhythmic bouncing, pressing, or sliding of the erect penis against the abdomen" (PDF); male deer do the same. The 19th-century physiologist Karl Friedrich Burdach has even described something like female ejaculation among solitary mares, which "rub themselves against whatever obstacles they find, often spurting a white, viscous mucus." A bull, meanwhile, stimulates itself by alternately protruding its penis from a genital sheath, while some moose can ejaculate simply by rubbing their antlers on bits of vegetation. According to observations made at the University of Buffalo in the 1940s, both male and female porcupines manipulate their genitals with inanimate objects—they're also known to "seize, straddle, and ride sticks about the cage."

Needless to say, many animals engage in self-directed oral sex.

Our fellow apes are among the most ardent and industrious masturbators: Female orangutans have been observed to fashion primitive dildos from sticks or pieces of liana, while males stimulate themselves with pieces of fruit, leaves, or other objects. Although it's sometimes said that only mammals masturbate, we have clear examples of autoeroticism among birds, which rub their cloacae on whatever's handy. Turtles have also been observed in the act.

Despite this bestiary of autoeroticism, scientists have spent relatively little time on the question of why animals might have evolved to masturbate. At first glance, the behavior would seem to be maladaptive. First, there's all the energy that's wasted on the production of spilled seed—macaques, for example, are thought to devote between 1 percent and 6 percent of their daily metabolism to the production of ejaculate. Second, it distracts the animal from the more important work of finding food and evading predators, let alone mating. According to the literature on horses, a masturbating stallion sometimes takes on "a trance-like, glazed-eye appearance." What could be more inviting to a hungry bear?

The recent finding that masturbation improves the quality of human sperm supports the notion that it's an evolved trait and not merely a byproduct of our physiology. According to a branch of evolutionary theory called "sperm competition" that developed in the late-1960s, natural selection can produce just such a change in reproductive behavior. The theory focuses on polyandrous species—i.e., those in which a single female takes multiple partners and the sperm from several potential fathers might end up competing to fertilize the same egg. Under those conditions, the relative quality of male ejaculate very clearly determines whose genes are passed on to the next generation.

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?