Energy, Not Hips, Determines Length of Human Gestation

Hillary Duff: I feel like a “walking zombie”

Wow! That was the first thought on hearing about this new paper.

Science is so cool, came a close second.

Damn, gotta change my lecture next week, that was the third thought.

Lead author Holly Dunsworth got that right!

“All these fascinating phenomena in human evolution—bipedalism, difficult childbirth, wide female hips, big brains, relatively helpless babies—have traditionally been tied together with the obstetric dilemma. It’s been taught in anthropology courses for decades, but when I looked for hard evidence that it’s actually true, I struck out.”

The paper “Metabolic hypothesis for human altriciality” is being published in the Proceedings of the National Academy of Sciences, and puts forth a strong argument that energy balance is the main driver of length of pregnancy in women, and not the long-assumed obstetrical dilemma, where babies’ heads were assumed to get too big for women’s hips and women’s hips were narrow because that helped bipedal walking.

As Medical Xpress puts it, Research refutes long-held theory: Mother’s metabolism, not birth canal size, limits gestation.

Since anthropologist Holly Dunsworth (blog, twitter) writes about her own work so well, I am going to direct you over to what she has to say!

This post just today on human birth, not human exceptionalism, outlines the research with co-authors Anna Warrener, Terry Deacon, Peter Ellison, and Herman Pontzer.

Notice how fetal energy demands increase exponentially as the end of a normal human gestation period approaches. To keep it in any longer, mother would have to burst through her normal metabolic ceiling. Instead, she gives birth and remains in a safe and possible (!) metabolic zone.

And an earlier post, That [obstetrical dilemma] really tied the [human evolution] together, gets into some of the previous research and previous assumptions behind her new co-authored paper. As she says there:

It’s all energetics. A mom gives birth when she does because she can’t possibly give any more energy into growing that fetus.

Both posts together give a real sense of how the research came together as well, which is great to see – more science in the open!

I can’t resist finishing with some lyrics from Kenny Loggins’ song “Birth Energy”:

I am opening
I am opening up
Birth energy, be my teacher

Photo Credit: Celebrity-Goosip.net

Citation: Dunsworth HM, Warrener A, Deacon T, Ellison P, & Pontzer H (2012) Metabolic hypothesis for human altriciality. Proceedings of the National Academy of Sciences.

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5 Responses to Energy, Not Hips, Determines Length of Human Gestation

  1. Janis says:

    Wonder how exclusive breastfeeding ties into this, because if a woman is the exclusive source of food for a postnatal baby, then … whether the kid is insider of her or outside, she’s still pretty much giving it everything it needs from her own metabolism.

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  2. Karen says:

    I’m no metabolic scientist, but I have a feeling that it has to do with making placenta-crossing, bloodstream-ready, no-digestion-needed energy rather than breast milk. It certainly could be that the latter is far more efficient than the former.

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    • Janis says:

      I’d like to see that accounted for … I’m still not entirely sure of this even though it seems like an interesting way to view the issue. The fact that humans are born with mooshy skulls seems a pretty solid indicator that it’s more a mechanical issue than a metabolic one.

      The only real differences I can think of is that the kid breathes for itself and regulates its own body temperature.

      Hm, there’s also the issue that the kid’s waste products don’t have to go through the mother anymore, too.

      Yeah, this is getting complicated. :-)

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  3. Kathy Dettwyler says:

    I don’t see how this is new. This is in Peter Ellison’s 2001 book “On Fertile Ground,” and I’ve been teaching it for years. The mother can provide basically only glucose through the placenta to meet fetal energy needs, but breast milk contains protein and fats as well. It’s a much more efficient way to grow a brain once the baby is born.

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  4. haha a walking zombie, hillarious.

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