The adaptive significance of male masturbation


Rama Ratnam

January 18, 2011

I chanced upon an article in Newsweek1 on male masturbation. Given my relentless pursuit of anything that is quirky or bizarre in animal behavior, I was immediately intrigued. It has long been a puzzling question as to why or how masturbation ties in with sexual selection. After all, ejaculation without copulation reduces the amount of available sperm. And this has predictable consequences for reproductive fitness. The behavior is all the more puzzling considering that in several species the males that seem to masturbate most, are also the males most successful in gaining mates. Current theories offer hope for understanding this most fascinating and pleasurable of animal behaviors. Let us therefore take a good hard look at “spanking the monkey.” Something that Dr. John Kellogg, the creator of cornflakes, disapproved of rather severely. He railed against the pernicious "habit of Onanism", and created cornflakes to curb this vile propensity in children. Sadly for him, children enjoy their cornflakes but continue to indulge in this revolting behavior. His product has sometimes been referred to as “pornflakes”.

The case of the Cape ground squirrel (Xerus inauris) of Namibia is illuminating. The original article2 authored by Waterman in PLoS One, observes that the male sits with his erect penis in his mouth, and stimulates himself orally and with his fore paws. He is really having a go at it. But oddly, he masturbates when females are most fertile and when he has just mated. Squirrels are far removed from primate species, but along with a number of other species including dogs, cats, horses, walruses, and so on, masturbation seems to be fairly widespread. They are cheerfully at it. It is a wonder that males across the mammalian taxa have not been struck blind. 

What is the adaptive value if any of masturbation? Is it indeed a good thing? According to Newsweek there are four theories of male masturbation. 

1) It increases the fitness of sperm. Although the quantum of ejaculate reduces with frequent masturbation, it has been shown that the older and more broken-down sperm are removed. Overall the number of viable sperm in the female reproductive tract is unchanged because the sperm are young, more motile, and therefore more “fit”. In other words, and as a hint to those inclined towards the practice, not abstaining is better than abstaining.

Hm! Maybe. I don’t really buy it although papers have been written on it. My argument is that the signal-to-noise ratio (quantum of fit sperm versus unfit sperm) is a meaningless parameter because the unfit ones are less likely to succeed anyway. They are futile swimmers.

2) The display hypothesis states that showing off capacity to ejaculate will buy the male something. It is a sign of quality. Sperm can be wasted, the sin of Onan notwithstanding, because it is a sign of prodigious sexual capacity. In fact, it must be wasted as often as possible, with complete disregard for the Levirate union. However, this conjures an image of men converging to bars and other public places, and masturbating openly to improve their chances. One shudders at this image. It could be the end of civilization as we know it.

But it is possible and I reserve judgment.

3) It is a display of victory. It shows that the male has conquered and will conquer yet again. Other females will observe this behavior and queue up. Hence the male will be more successful

I was initially skeptical of this hypothesis until I read that some sub-dominant males, those who are not successful in finding mates, masturbate with greater frequency. This could be satellite behavior, a move to corner females even though the male is not dominant.

4) It serves a hygienic function. It keeps the reproductive tract clean, particularly after mating with a female who has mated with other males. It could reduce the chance of STD. This increases fitness.

This seems to be the theory adopted by Waterman when she studied the Cape ground squirrel. When males stimulate themselves orally, the anti-bacterial properties of their saliva can serve to combat infection. And if the activity gives pleasure then it is likely to be reinforced. It is elegant and I find it to be a worthy hypothesis.

Research into animal behavior favour strong “theories”. These are not mathematical theories, but theoretical ideas rooted in the principles of evolution and sexual selection. They provide a framework for ethology. Whenever you find a behavior, look for an answer in terms of fitness. It is in this regard that the idea of hygienic function is strong. It addresses a more selfish motive. One motivated by protection of the self, and the goal of not catching a disease. Because it is a pleasurable activity it forms a positively reinforcing loop. For a male who is successful in garnering mates, especially when females are most fertile, masturbating more often is an optimal strategy. The loss of sperm is secondary because there will still be enough fit sperm to impregnate an ovum. The benefit is that it increases reproductive success. But we would be hasty to dismiss the other theories. The proximate and ultimate causes of the evolution of sexual behaviors, and their links, are still open problems.

I have devoted an inordinate amount of space to male masturbation. What about female masturbation? What function does that serve? In a telling answer Newsweek devoted nearly two pages to male masturbation and dismissed female masturbation (purely from a scientific perspective), condensing it to a paragraph. This behavior in females has been studied mostly in Bonobo chimpanzees (Pan paniscus) where individuals are noted for trading sexual favors, including mutual masturbation to soothe and calm members of the group (the contact may be heterosexual or homosexual). But female masturbation is much more of a mystery than male masturbation. It does not seem to serve any clear sexual function. There are no strong theories of it. Most researchers conclude that it is done simply for pleasure. I have always believed that females are smarter.

The wonderful thing about behavior is how much of it is shared across taxa. We can understand so much about our behavior by observing it in other species. In modern times we have become quite accepting of behaviors like masturbation, if it is done in private, although historically it was not always the case. This is largely due to our understanding of animal behavior at large, and in understanding instinct. But it is still unacceptable as a public display, and we hope that it remains that way. We have civilized some of our deepest instincts in more sublime ways. But the monkeys in my research colony are quite unabashed about it, and masturbate with abandon even though I stare at them. This is the meaning of taxonomic distance. Enjoy your cornflakes, and may I paraphrase Leviticus 15:16-18? Please do go and wash. Scrub your hands with carbolic soap.

1Begley S (2010). The scientific case for masturbation. Newsweek, October 7, 2010.
2Waterman JM (2010). The adaptive function of masturbation in a promiscuous African ground squirrel. PLoS ONE, 5(9): e13060. doi:10.1371/journal.pone.0013060.