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Dating Mating Sex and Reproduction

Competition’s Good For You. If You’re a Sperm.

One of the linchpin arguments against the predominant monogamy/mild polygamy model of human evolution involves sperm competition.  Physiologically, humans appear to be “designed” for rather intense sperm competition.  While we don’t have the degree of specialization seen in some invertebrates (such as Drosophila), it’s hard to deny that both human sperm and the female reproductive tract seem specialized for competition.

Proponents of the mono/mild poly model have been objecting vehemently for years by pointing to the creeping pace of evolution.  They say that humans are essentially the same as we have been for the past 10 to 15,000 years, and while we do show signs of sperm competition, they are not clear enough to make the leap to a more promiscuous model, like that of the Bonobo.

This claim, at least, faces demolition in light of recent research on mice.  In only 8 generations, the sperm of polygamous mice evolved a huge advantage over the sperm of monogamous competitors.

When looking at sperm competition in mice, “males evolving with sperm competition (polygamy) had a significant paternity bias over males evolving without sperm competition (monogamy),” the researchers wrote in the Jan. 20 issue of the journal BMC Evolutionary Biology. So those mice with multiple mates, over several generations, were more likely to father children than the monogamous males, the first time this has been seen in vertebrates.  LINK

This is interesting by itself, but there’s speculation that females might subconsciously “prefer” polygamous mice:

This “cryptic female choice” isn’t well understood, though it’s possible that in addition to males manipulating their sperm, the females may also be able to preferentially implant eggs fertilized by males from the polygamous lines.

So what does all of this mean?  Well, there are several things.  Perhaps most importantly, it gives us a model in mammals to explain why true monogamy is almost nonexistent.  Simply put, polygamy creates stronger and faster sperm than monogamy, so when the two systems compete, polygamy will have an advantage out of the gate.  And all things being equal, that’s probably enough in most cases.

Second, and more pertinent to humans, it makes us rethink the “obvious” conclusion that humans behave now as they behaved ten thousand years ago.  Even allowing double the time it took for mice, we’re still talking about under 500 years for human sperm to evolve significantly enough to favor one system over another.

Finally, if researchers are correct about the female preference for polygamous males, we might have to rethink a lot of things about human sexuality.  Off the top of my head, I’m wondering:  Are all those girls who can’t give up the “bad boys” and “players” succumbing to an ancient preference for polygamous males?  Do their bodies secretly know that the sperm from polygamous males is probably better?

It’s fascinating stuff.  And it seems that every new piece of research these days is bolstering the human non-monogamous model.  I’ll be interested to see where this leads.





4 thoughts on “Competition’s Good For You. If You’re a Sperm.

  1. It seems like faster sperm would only have an evolutionary benefit if the FEMALE had multiple sexual partners, so that the sperm of more than 1 man is competing for the same egg within her. A society in which men have multiple mates but women are controlled to have only one wouldn’t seem to have this effect.

    If a man is known to have knocked up several women, especially in an environment where women are very promiscuous, this should be especially strong proof of “fast sperm.”

    Posted by Rhen | February 16, 2011, 3:13 pm
  2. Rhen, yes. Faster sperm being an advantage is contingent on females having multiple sex partners. Interestingly, human sperm is probably not as competitive as it used to be. Our sperm doesn’t match our outward sexual physiology, which suggests that the period of 5000 years or so of enforced monogamy on women has had an effect on it.

    Posted by hambydammit | February 16, 2011, 3:48 pm
  3. There is some evidence that men can still respond to sperm competition, even if women have been historically monogamous.

    One study found that men viewing sexually explicit images depicting sperm competition (showing 2 men and 1 woman) led to the man having more motile sperm in his ejaculate than when he looked at pictures with only women in them. (

    Posted by blaflamme44 | April 26, 2011, 11:48 pm
  4. Thanks for the link. That’s definitely an interesting result, though the authors admit it’s difficult to interpret without more information. Without saying it explicitly, they are hinting at the interpretation that increased sperm count is consistent with the poly model for both male/female and female/female situations. That is, if you’re going to be inseminating two females, better to ramp up production. And if you’re competing with other males, better to ramp up production.

    However, you’re correct that this result could also support the poly male/mono female model. It would need a lot of corroboration to do so, since so much recent science points away from the mono female model.

    Posted by Living Life Without a Net | April 27, 2011, 10:18 am

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