It's almost a given that, during any discussion about male infidelity, someone will throw out some variation of "men are biologically programmed to spread their seed."
Why is there this theory that men are more driven to cheat? Part of it has to do with the size of their gametes. If bigger is better, then men are pathetic, for their little sperm are 1/100th the size of a woman's egg. Because women have such a greater investment in each offspring right from the get-go, the assumption is that women are pickier when it comes to who they allow to fertilize their eggs. Men, since it doesn't cost them much anyway, are better off getting as many women to agree to let them fertilize. In mammals (like us), this difference in investment is further exacerbated by long, internal pregnancies and lactation, which place even more of the burden on the mother. It just makes so much sense - it's clear, simple, and like most biological explanations that are clear and simple, it's nowhere near the whole story.
Even still, we've internalized this supposed biological explanation so much so that it's almost an excuse. In an article about why men cheat, for example, it's carefully explained that "The biological urge to mate with many different partners is stronger in men than it is in women" and that "males mate with multiple females to ensure the survival of the species." Well, I guess if it's to ensure the survival of the species...
Perhaps, then, my female readers will appreciate a new study published in Current Biology which suggests that it's the women whose infidelity is important in ensuring species survival. Indeed, as they explain, females having multiple male partners may be vital in preventing extinction!
Ever since the "spreading the seed" explanation for why males are so sexually liberal was proposed, there have been a few issues with it. The most glaring one was that in many animals, women, too, have multiple partners. Some species seem to have the roles reversed, where women are dominant and have harems of male suitors. Even in species that seemed monogomous, it turned out the women were cheating - in many species of monogomous birds, for example, females actively seek "extra-pair copulations" (ornithologist speak for affairs), and up to 75% of the chicks were being raised by males that were not their dad!
Biologists began to realize that women, too, can benefit from sneaking around. In monogamous species, a female is stuck with whatever guy she gets to raise her young, even if isn't the biggest or the brightest. It's a shame, to her, that she can't have better babies, for surely her lackluster hubby's offspring will be less than ideal. Cheating, it was believed, gave a woman the best of both worlds. She gets to have a dedicated, loyal sub-par male take care of her young, but the young are born from better stock that she secured on the side.
The problem is, nothing explained polyandry - a mating system where women have multiple "husbands" or partners. Considering that there is a higher biological cost to the female to mate, why would she ever want to have a permanent array of suitors demanding her attention? It seemed like there was something missing. Biologists Tom Price, Greg Hurst, and Nina Wedell believed there was more to the story.
It turns out that the bigger picture may be genetic. Mutations in chromosomes can lead to what is called sex ratio distortion. There are certain alterations that, for whatever reason, cause the sperm containing either the X (female) or Y (male) chromosome to fail to fertilize. They are termed Sex-Ratio Distorter Genes or Chromosomes (SR genes or chromosomes) because they alter the balance of males to females in a population. Because mutations are constantly occurring, there is always a risk that all-male or all-female broods will be born, the result of which is potential extinction of a population or a species, if the altered gene becomes widespread.
The team hypothesized that female promiscuity may help protect against SR chromosomes that develop in males. They tested their hypothesis using a known SR chromosome in the the fruitfly Drosophila pseudoobscura. This particular mutation, when carried by a male, causes all of his sperm that carry a Y chromosome to die before they can fertilize, though it has no clear effect on eggs when carried by females. Because it doesn't detriment female gametes, this kind of mutation can persist and spread until, eventually, not enough males are being produced to maintain a population.
They created populations of fruitflies where 30% of the flies carried the SR mutation. They then had one population breed freely, where females mated with a number of males, while in the other, females were restricted to a male apiece. They bred these populations for several generations to see if there were any differences between the two.
In just fifteen generations, almost half of the monogamous populations became extinct because there weren't enough men around. Meanwhile, none of the populations with the promiscuous girls died out. In the monogamous populations that survived, the SR chromosome was far more prevalent than in the polyandrous ones.
Why did those populations fare so much better? It's likely the effect of sperm competition. When a female mates with many males, each of their sperm is vying to fertilize her eggs. Since the SR males produce half the sperm that normal males do, they've got an instant disadvantage in a system where they have to compete, meaning the damaging chromosome is less likely to spread.
This study is the first to suggest that a polyandrous mating system could have evolved as a means to protect against sex-ratio distortion-induced extinction. Of course, extrapolating these results to other species is more difficult, so it's impossible to say that this study has any grand relevance to humans. It does, however, plant the seed of possibility that female promiscuity is healthy and vital in a population.
So boys, next time I hear you say that men are dogs because it's 'biological', expect to hear a lecture about how girls can sleep with whomever they want because it will save the species. Jus' sayin'.
Price, T., Hurst, G., & Wedell, N. (2010). Polyandry Prevents Extinction Current Biology DOI: 10.1016/j.cub.2010.01.050
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