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Ryan And Jetha On Monogamy And Infertility

November 15, 2013

Excerpted from Sex at Dawn, by Christopher Ryan and Cacilda Jetha:

Use it or lose it is one of the basic tenets of natural selection. With its relentless economizing, evolution rarely equips an organism for a task not performed. If contemporary levels of sperm and semen production were typical of our ancestors, it is unlikely our species would have evolved so much surplus capacity. Contemporary men have far more potential than they use. But if it’s true that modern human testicles are mere shadows of their former selves, what happened?

Since the infertile leave no descendants, it’s a truism in evolutionary theory that infertility can’t be inherited. But low fertility can be passed along under certain conditions. As discussed above, chromosomes in humans, chimps, and bonobos associated with sperm-producing tissue respond very rapidly to adaptive pressures—far more rapidly than other parts of the genome or the corresponding chromosomes in gorillas, for example.

In the reproductive environment we envision, characterized by frequent sexual interaction, females would typically have mated with multiple males during each ovulatory period, as do female chimps and bonobos. Thus, men with impaired fertility would have been unlikely to father children, as their sperm cells would have been overwhelmed by those of other sexual partners. The genes for robust sperm production are strongly favored in such an environment, while mutations resulting in decreased male fertility would have been filtered out of the gene pool, as they still are in chimps and bonobos. But now consider the repercussions of culturally
imposed sexual monogamy—even if enforced only for women, as was often the case until recently. In a monogamous mating system where a woman has sex with only one man, there is no sperm competition with other males. Sex becomes like an election in a dictatorship: just one candidate can win, no matter how few votes are cast. So, even a man with impaired sperm production is likely to ring the bell eventually, thus conceiving sons (and perhaps daughters) with increased potential for weakened fertility. In this scenario, genes associated with reduced fertility would no longer be removed from the gene pool. They’d spread, resulting in a steady reduction in overall male fertility and generalized atrophy of human sperm-production tissue.

Just as eyeglasses have allowed survival and reproduction for people with visual incapacities that would have doomed them (and their genes) in ancestral environments, sexual monogamy permits fertility-reducing mutations to proliferate, causing testicular diminishments that would never have lasted among our nonmonogamous ancestors. The most recent estimates show that sperm dysfunction affects about one in twenty men around the world, being the single most common cause of subfertilility in couples
(defined as no pregnancy after a year of trying). Every indication is that the problem is growing steadily worse. Nobody’s maintaining the spare fridge much anymore, so it’s breaking down.

If our paradigm of prehistoric human sexuality is correct, in addition to environmental toxins and food additives, sexual monogamy may be a significant factor in the contemporary infertility crisis. Widespread monogamy may also help explain why, despite our promiscuous past, the testicles of contemporary Homo sapiens are smaller than those of chimps and bonobos and, as indicated by our excess sperm-production capacity, than those of our own ancestors. Sexual monogamy itself may be shrinking
men’s balls.

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