Both Twisty and Amanda seem a bit weirded out by this news that the fetus can be viewed as a kind of parasite. This story has been around long enough that a lot of us just take it for granted—I wrote about the example of preeclampsia a while back.

There are worse feminist-troubling theories out there, though. In particular, there is the idea of intersexual evolutionary conflict and male-induced harm. In species where there is some level of promiscuity, it can be to the male’s evolutionary advantage to compel his mate to a) invest more effort in his immediate progeny, b) increase her short-term reproduction rate, and c) suppress her ability to mate with other males. After all, his optimal strategy is to flit from female to female, copulate, and put her to work producing his offspring. The female’s preferred strategy, on the other hand, is to take her time, maximize her lifetime reproduction rate, and select the best genetic endowment for her children.

This sets up a cycle of counter-adaptations in the population. If a male acquires a mutation that increases his fitness at the expense of his mate’s—for instance, if some component of his semen works on her brain to suppress her interest in remating—it will spread through the population due to its positive effect on male fitness, even though it reduces female fitness. Subsequently, a female who acquired a counter-adaptive resistance to the male’s hormonal sabotage would have an advantage, and that gene would spread through the population, reducing male fitness by making them less capable of controlling female reproduction. Then, of course, males could evolve some other sneaky way of maximizing their reproduction rate—vaginal plugs, secretions that make the mated female unattractive to other males, proteins that put her ovaries into overdrive to produce more eggs now at the expense of the female’s long term survival.

It all sounds improbable and dystopian, but all of these mechanisms and more have been observed in that exceptionally promiscuous species, Drosophila. Drosophila seminal fluid has the property of reducing the female’s interest in remating, increasing her rate of egg-laying, and is also mildly toxic. Artificial selection in the lab can produce females that are resistant to the effects, and males that produce more and more potent semen to overcome their resistance, to the point where the line of “super potent” males, when crossed to unselected females, kill their partners with their ejaculations. There is literally a battle of the sexes in these species.

To speak up in my defense, though, not all males are evil exploitive pigs. The logic of this pattern of sexual competitiveness vanishes as species exhibit greater and greater monogamy—if you have only one mate, it is to your advantage to take good care of him or her, because a loss diminishes your reproductive fitness.

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Rice WR (2000) Dangerous Liaisons. Proc. Nat. Acad. Sci. USA 97(24):12953-12955.

Rice WR (1996) Sexually antagonistic male adaptation triggered by experimental arrest of female evolution. Nature 381(6579):189-90.