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Thread: New idea about the explanations for timing of human birth

  1. #1 New idea about the explanations for timing of human birth 
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    Interesting item on Science Daily about a topic that comes up here from time to time.
    Long-held theory on human gestation refuted: Mother’s metabolism, not birth canal size, limits gestation


    Big brains and the big heads that encase them are hard to push through the human birth canal, but a wider pelvis might compromise bipedal walking. Scientists have long posited that nature's solution to this problem, which is known as the "obstetric dilemma," was to shorten the duration of gestation so that babies are born before their heads get too big. As a result, human babies are relatively helpless and seemingly underdeveloped in terms of motor and cognitive ability compared to other primates.
    So far, so good. We've seen plenty of discussions here where this has been raised as conventional wisdom. Then we get to this part.

    The first problem with the theory is that there is no evidence that hips wide enough to deliver a more developed baby would be a detriment to walking, .....

    Then Dunsworth looked for evidence that human pregnancy is shortened compared to other primates and mammals. She found well-established research to the contrary. "Controlling for mother's body size, human gestation is a bit longer than expected compared to other primates, not shorter," she said. "And babies are a bit larger than expected, not smaller. Although babies behave like it, they're not born early." .....

    Using metabolic data on pregnant women, the researchers show that women give birth just as they are about to cross into a metabolic danger zone.
    "There is a limit to the number of calories our bodies can burn each day," says Pontzer. "During pregnancy, women approach that energetic ceiling and give birth right before they reach it. That suggests there is an energetic limit to human gestation length and fetal growth."
    My emphasis.

    Read the article rather than just these extracts. The paper itself is, unfortunately, published by PNAS so it will be behind a paywall, only people with a subscription or university library access will be able to get the original.


    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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  3. #2  
    SHF
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    Haven't read the article yet, but couldn't the "energentic limit" be the result of the physical limits (i.e. the physical limits determine size/gestation duration, and the metabolic limits result from this/ tailor themselves to it)?


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    I suggest you read the article. As for duration of gestation, despite the fact that human babies are so much more helpless than chimp infants, if you compare the two development paths human gestation is the equivalent of 16 months chimp gestation.

    The metabolic "limit" for the mother is looking like a pretty good explanation.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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    SHF
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    Have read the paper (it’s free by the way [Metabolic hypothesis for human altriciality]). Thanks for posting it, was interesting. It’s not my area but some thoughts come to mind.

    They argue female pelvis size results from baby brain size at birth which results from maternal metabolic threshold. They say pelvic size does not determine birth timing as there is no reduced locomotive efficiency or increased locomotor cost to increased pelvic size. Which begets the question of why the pelvic size would be just big enough for the size of the baby at birth, why not a safe size. They say there must be some other selection pressure preventing the pelvic size increasing….but I thought their premise for rejecting the OD hypothesis was that the pelvis could be wider with no costs? They even say that there are other locomotive parameters such as speed that have not been tested and give other reasons why pelvic size cannot increase (such as increased likelihood of injury).

    If narrower hips are not better for locomotion then why do males have them? (don’t female runners have narrower hips?)

    Would be interesting to see some detailed metabolic analysis, especially with regard to why the threshold is reached at 9 months as opposed to 6 or 12 months.

    Would be interesting to know if people’s metabolic thresholds differ significantly and whether this provides any correlation with pelvic size.

    I’m under the impression that we were a lot healthier 20,000 years ago so perhaps had higher metabolic danger zones, as such this would not be breached by the 9 month baby size of today thus if the maternal metabolic threshold was the trigger for birth then surely they would end up with much bigger babies at term, yet as far as I know hip morphology hasn’t changed much in 20,000 years.

    I suppose it would be useful to know exactly what triggers child birth (physiologically) so it could be traced back from who/what triggers the birth to why they would benefit (e.g. is the baby calling time? Is the mother? The father’s genes? Given that hip morphology will have a complex gene regulation does widening female hips effect male hips, such that male genes will ‘pull’ in the other direction?)
    Last edited by SHF; July 2nd, 2013 at 06:04 AM. Reason: typo
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