All this talk about choice makes me want to clobber certain blurb writers

Why Girls Sleep Around: The Evolutionary Case for Female Promiscuity

In a study of mouse-like marsupials, “survival of babies with promiscuous mothers was almost three times as high as those in the monogamous group.” Key reasons: 1) “The sperm of some males were far more successful than others.” 2) “Babies fathered by these males were twice as likely to survive.”

Takeaway for women: “Polyandry improves female lifetime fitness.” Takeaway for men: “Males with more competitive ejaculates sire more viable offspring.” Fine print: “Males usually died after a short and intense single mating season due to exhaustion and aggressive encounters with other males.

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Let’s think about the biological reasons why it’s inaccurate to imply this finding can be extrapolated across species (and thus suggest all females have somehow selected to be more promiscuous). Most of my rationale has to do with the idea there are…well, ridiculously huge differences in the life history traits and mating strategies employed by humans and small marsupials, to name just two animals.  Polyandry, the mating with multiple males by females, improves female lifetime fitness for semelparous species—organisms with high adult mortality that blast the world with offspring shortly before they die, like cicadas, salmon and the antechinuses mentioned here. That’s the only chance at reproductive success they get, and more importantly there’s no parental care involved. In contrast, humans are iteroparous and invest much more energy in the survival of a few periodic offspring. So popping out 3 times as many kids at once would actually decrease lifetime fitness, because it’s likely to kill off or otherwise take an energetic toll on the mother on which the young depend. You have your zillion offspring, but they won’t survive. The relationship between increased mating and healthier babies doesn’t hold.

On the bright side, the fine print doesn’t apply to humans either, because sperm competition in iteroparous species is comparatively less fierce. In other words, guys don’t spend so much energy in their one mating session that they die right after it. (Because, we hope, they get more than a single try.) Not so the case for these marsupials: “Male antechinuses copulate for 5–14 consecutive hours with each female, and ejaculate around 3 h after mating starts. This extraordinary male reproductive biology could subject sperm to extreme physiological and epigenetic stress, resulting in the marked relationship between male sperm competitive ability and offspring viability” (Fisher et al. 2006, yes I actually went and found the article and am citing it in a USP blog post).

So HA, Slate! You and your sensationalist, insinuating headlines can take a backseat to breeding biology!

Although antechinuses are still really cute.

—Irene L.