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.