Pharyngula

Load-bearing adaptation of women’s spines

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Blogging on Peer-Reviewed Research

Those of you who have been pregnant, or have been a partner to someone who has been pregnant, are familiar with one among many common consequences: lower back pain. It’s not surprising—pregnant women are carrying this low-slung 7kg (15lb) weight, and the closest we males can come to the experience would be pressing a bowling ball to our bellybutton and hauling it around with us everywhere we go. This is the kind of load that can put someone seriously out of balance, and one way we compensate for a forward-projecting load is to increase the curvature of our spines (especially the lumbar spine, or lower back), and throw our shoulders back to move our center of mass (COM) back.

Here’s the interesting part: women have changed the shape of individual vertebrae to better enable maintenance of this increased curvature, called lordosis, and fossil australopithecines show a similar variation.

The first step is to document the phenomenon. The posture and gait of pregnant women were studied kinematically through their pregnancy, and yes, they do extend their lower spines to shift the COM. The diagrams at the bottom, below, show the situation: on the left (c) is a non-pregnant woman with the center of mass marked. It’s located at the base of the spine, roughly centered over the hip joint. In (d), the woman is braced to hold her spine in the non-pregnant posture, and the weight of the fetus pushes the COM forward, to a place that would put her out of balance. In (e), you can see the preferred posture in pregnancy: the s-curve of the back is increased, moving the COM back over the hip joint.

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COM and lumbar lordosis during pregnancy.
a, Quadrupedal chimpanzee, non-pregnant. b, Quadrupedal chimpanzee, pregnant with no change in sagittal position of the COM with respect to the postural support base. c, Bipedal human female with typical lumbar lordosis and COM in approximate sagittal alignment with the hip. At a given 0.005-m COM distance from the hip, a 409-N upper body generates 2 N m torque at the hip. d, Pregnant human female with anteriorly translated COM, lacking positional adjustment of lumbar lordosis. The force of gravity, when more distant from the hip, generates a larger hip moment and an unstable upper body. With pregnancy, a 511-N upper body and a COM at 0.032 m from the hip increases the torque to 16 N m. e, Typical pregnant human female with naturally extended back and recovered COM by means of increased lumbar lordosis, a stable positional alignment with reduced hip torque (1.5 N m) but with exacerbated spinal shearing load. Open circle with cross hairs, COM in sagittal plane; filled circle, hip position in sagittal plane; arrow, direction of gravitational force.

Now let’s take a look at the anatomy. These are not measurements from the same women in the kinematic study, but from collections of skeletal remains where age and sex of the individuals were well-established from post-mortem records. The qualitative observation is that the three caudalmost lumbar vertebrae, and especially L3, show a characteristic sex difference. L4 and L5 in both sexes are typically wedge-shaped, to bend the spine forward; L3 in men tends to be more columnar, while L3 in women is also wedge-shaped, promoting a greater curvature.

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(click for larger image)

Sex differences in the lumbar vertebral column of human males and females.
Female values are shown by filled bars, male values by open bars. a, Wedging angle of vertebral bodies, angles greater than 0u are kyphotic (thoracic-type wedging), whereas angles less than 0u are lordotic (lumbar-type wedging). Females present a longer series of dorsally wedged vertebrae; L3, L4 and L5, whereas males are lordotic at only two levels, L4 and L5. b, Prezygapophyseal (prezyg.) area, adjusted by geometric mean for overall vertebral size. The female area is significantly larger than the male area at L2, L3, L4 and L5, indicating that females bear a greater proportion of spinal load along the dorsal pillar, which is consistent with fetal loading patterns identified during pregnancy. c, Prezygapophyseal angle. The female facets are significantly more oblique at L2, L3 and L5, conferring greater resistance to forward displacement of lumbar vertebrae. In a-c, n 5 59 males, 54 females. Data are means and s.d. d, Diagram of lumbar region in males and females, showing contrasting mean wedging patterns and anatomical structures within the dorsal pillar (including zygapophyses) and ventral pillar (vertebral bodies). e, Difference in vertebral body shape in males and females at L3. There are equivalent angles of excursion yet there is greater upper body extension in the female spine. The inherent dorsal wedging shape of the female L3 relative to the non-wedged male L3 generates less shearing force when the upper body is repositioned by means of lower back extension, as occurs during fetal loading.

The quantitative measurements back the observation up. The charts on the left above show the shape of the vertebra, and the area and angle of the zygapophyseal process. Just look at (a), though. That chart illustrates the degree of wedging of individual vertebrae; a bar that goes downward means the vertebra is trapezoidal, with the narrow end to the back, bending the spine foward; bars that extend upwards means the vertebra is wedge-shaped in the other direction; and bars that are close to the baseline mean the vertebra is columnar. Women’s vertebrae are black, men’s are white. Look in particular at L3; women’s L3 has a bar that is downward, while the men’s L3 is more columnar.

The variation is very large, though, which I’ll come back to later.

Now here are some measurements from a pair of australopithecine spines. It doesn’t quite have the same distribution as the above graph, but keep in mind that this one is from two individuals, while the modern human skeletal data is from 59 males and 54 females. Basically, though, the data show that one individual has more columnal vertebrae, while the other is more wedge-shaped. There is no independent determination of the sex of these two individuals, though, so all we can really say is that we see a morphological difference in the lumbar vertebrae that parallels a sex difference we see in modern populations.

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Australopithecine lumbar lordosis and prezygapophyseal angle.
a, Angle of lumbar vertebral wedging for Australopithecus africanus specimens Sts 14 (red) and Stw 431 (blue). Sts 14 shows a wedging pattern similar to that in modern human females, comprising the three caudalmost lumbar vertebrae, L4, L5 and L6. Although the preserved lumbar column of Stw 431 is less complete than that of Sts 14, the caudalmost levels are preserved well enough to identify a different wedging pattern. The dorsal wedging sequence of Stw 431 includes only one lumbar vertebra, at the last lumbar level. In this manner, Stw 431 is unlike Sts 14 and modern human females and is more similar to modern human males in having a shorter region of lordotic lumbar vertebrae. b, The prezygapophyseal angle of the preserved lumbar region for Sts 14 and Stw 431. The larger angles of Sts 14 relative to those of Stw 431 mirror the modern human female-male pattern in that Sts 14 presents more oblique angles and therefore greater coronal orientation of the prezygapophyseal facets than Stw 431.

So far, so good. I can believe that the authors have identified a statistical difference in the anatomy of lumbar vertebrae between males and females. However, I have some significant disagreements with the evolutionary interpretations of the paper. They claim to have identified evidence of an evolutionary novelty, but they haven’t tested the alternative hypothesis, that this is not an evolutionary adaptation, but a physiological one, and they haven’t adequately distinguished cause and effect.

My first thought on reading the results was that this is an example of developmental plasticity. Bones are flexible; they respond to stress with changes in shape and size that accommodate them to the pattern of activity they experience. This is an indirect evolutionary adaptation, of course — that bones have this response is a product of their genetic and developmental potential. However, the shape of an individual vertebra may not be so precisely specified, but may emerge as a product of the strains put upon it.

I’d make an alternative hypothesis. The female L3 vertebra is not wedge-shaped because women need to bear a fetal load, but instead, because women bear a fetal load, the L3 vertebra is wedge-shaped. In particular because their own data shows a significant amount of variability in vertebral shape, I’d be hesitant to assign a direct genetic cause on the pattern.

Unfortunately, the data in this paper do not touch on this possibility. All of it is from either pregnant women, or from skeletal remains of adults of child-bearing age. What I’d like to see is some developmental information, especially measurements of lumbar vertebrae in pre-pubertal children. If the difference precedes the child-bearing experience, then I’d agree that they’ve found a sexual dimorphism that could have an evolutionary cause.

Other data I’d like to see: is there a difference in vertebral morphology between women who have had children and those who have not? Another sex difference that could generate variation in vertebral morphology besides pregnancy is breast size; like carrying a fetus, women have another forward projecting weight that can shift the center of mass. Do large-breasted women have a consistent change in vertebral morphology that isn’t found in small-breasted women or men? How does obesity affect vertebral shape?

The authors have identified an interesting sexual dimorphism, but I think the paper was far too quick in assigning an evolutionary selective cause for the difference, and that it did not adequately examine the more likely (to my mind, at least) explanation of physiological adaptation.


Whitcome KK, Shapiro LJ, Lieberman DE (2007) Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature 450(7172):1075-8.

Comments

  1. #1 Coturnix
    December 15, 2007

    That is exactly how I understood it, as a physiological adaptation, and so I was a little dismayed when I saw others write about it as an evolutionary adaptation in the “hard” sense, i.e., not just predisposition for the physiological adaptation.

  2. #2 Russell
    December 15, 2007

    PZ, you’re spending too much time with squid. Most guys begin to notice lordosis in their female friends somewhat before any of the latter have become pregnant, and that notice oft is a preliminary step on the way to pregnancy. Of course, that raises the spectre that sexual selection also enters into the equation. 😉

  3. #3 Colugo
    December 15, 2007

    PZ Myers “an evolutionary selective cause for the difference, and that it did not adequately examine the more likely (to my mind, at least) explanation of physiological adaptation.”

    Could be genetic assimilation – developmental anatomical adaptation evolutionarily resulting in differential estrogenic effect on vertebrae morphology. This acts in concert with plastic developmental accommodation. (Prediction: lordosis less marked in small-breasted never-pregnant women, but still more prominent than in males.)

    Genetic assimilation is likely a major part of the evolution of bipedalism in general (Jon Marks, others).

  4. #4 PZ Myers
    December 15, 2007

    But Russell, I’ve always assumed that that feminine lordosis was a tightly selected adaptation for women to want me. Me, specifically.

  5. #5 Sven DiMilo
    December 15, 2007

    According to this guy:

    The article doesn’t mention that there are numerous other reports about sexual dimorphism in the human lumbar lordosis – about half of the studies found no statistical difference between males and females and the other half found a small difference.

    Hence the idea of sexual difference here is not new and although the wedging measure used by Whitcome et al produced a statistically significant difference, it is not clear the wedgeing of bones equals real differences in lordosis and it is not clear that the sexual difference will hold up in larger studies. They don’t tell us why the other studies showed no sexual difference.

  6. #6 Colugo
    December 15, 2007

    Then again, pronounced female lordosis could simply be a physiological adaptation (independent of pregnancy and breast size, although these increase it) of differences in leg, pelvic, and trunk morphology, which are the direct result of differential sex steroid action (or their downstream effects on other hormones and cytokines).

  7. #7 DiscoveredJoys
    December 15, 2007

    There are just too many alternative explanations possible to make the evolutionary ‘just so’ story stand up (pun stumbled upon).

    Certainly you could investigate the spinal curve of nuns to isolate the non-childbearing factors (or the no sexual relations explanation).

    Alternatively the female spinal shape may be the default genetically determined phenotype, and it is men whose spines become more columnar to aid running and walking as hunter gatherers.

    Similarly is the impact of pregnancy on the body (the truly heavy stage lasts for just a few months, every few years in the hunter gatherer culture) sufficiently great to affect fitness (i.e. enable natural selection to produce an adaptive response)?

    Perhaps the curve to spine was crucial to four footed and knuckle walking apes for some reason, but has not been lost in the move to upright posture?

    I think all we have is interesting data and more questions – but no definite answers.

  8. #8 LM
    December 15, 2007

    Sweet.

    Incidentally, I gave birth to a daughter a month ago. Squee!

  9. #9 raven
    December 15, 2007

    This could be partially due to relaxin. To this day, relaxin is not well understood but it has something to do with pregnancy. It facilitates pelvic changes associated with child birth. The effects on collagen metabolism might be involved in bone remodeling as collagen is the organic matrix for bone.

    wikipedia Relaxin:

    In humans
    In women relaxin levels rise after ovulation as a result of its production by the corpus luteum. In the absence of pregnancy its level declines at menstruation. During the first trimester of pregnancy levels rise and additional relaxin is produced by the decidua.

    Relaxin’s role or necessity in human pregnancy remains under investigation, as in humans its peak is reached during the first trimester, not toward the end of pregnancy.

    [edit] In animals
    In animals relaxin widens the pubic bone and facilitates labor, it also softens the cervix (cervical ripening), and relaxes the uterine musculature. Thus, for a long time, relaxin was looked at as a pregnancy hormone. However, its significance may reach much further. Relaxin affects collagen metabolism, inhibiting collagen synthesis and enhancing its breakdown by increasing matrix metalloproteinases.[3] It also enhances angiogenesis and is a potent renal vasodilator.

  10. #10 Beaglelady
    December 15, 2007

    Another wedge document!

  11. #11 Zara
    December 15, 2007

    As I would expect, PZ uses much more appropriate language than I would regarding alternative hypotheses:

    PZ: “How does obesity affect vertebral shape?”

    Me: “Where are the data on fat guys?!?”

  12. #12 Helioprogenus
    December 15, 2007

    As a further comparative study, I wonder what one would see if they compared the spine of Female and Male Kangaroos? After all, they are bipeds, and more importantly, front bearing weight without spinal adaptation can dramatically alter their ambulatory gait, probably throwing their jumping ability completely off balance.

    I agree with PZ that concluding an evolutionary significance as opposed to the physiological adaptiation is a bit of a far reach without further evidence of the methodology. Perhaps comparing the lumbar vertebrae of small breasted nuns, versus large breasted nuns (taking into account these nuns having never borne children) along with small breasted pregnant women, with large breasted pregnant women. Perhaps doing studies at 5 year increments starting at the age of 20 or so. That should be a very interesting study.

  13. #13 Jenbug
    December 15, 2007

    I fully intend to point my mom to this article the next time she calls me a ‘swayback’ because of the exaggerated S-curve of my spine.

    Although it does cause my already comically round ass to stick out further, which helped attract my current mate of 6+ years. The backaches when I periodically gain weight are almost worth it.

  14. #14 Richard Carnes
    December 15, 2007

    At least one IDer has already tagged this paper as an example of what he terms “just so natural selection stories” that he believes makes evolution look silly. What do you think, does he have a point?

  15. #15 Rjaye
    December 15, 2007

    I wondered if the wedge shape was a biological response as well…and whether if the researchers found a group of beer bellied boys of a certain age if they would display a similar spinal formation compared to men with no pot bellies.

    Since there was variation of the wedge shaped vertabrae in women, I wonder if that variation is similar in the amount of variation in pelvic shape.

  16. #16 David Marjanovi?, OM
    December 15, 2007

    Perhaps the curve to spine was crucial to four footed and knuckle walking apes for some reason, but has not been lost in the move to upright posture?

    No. Look at the chimp again (at the top of the first figure): it doesn’t have the curvature.

  17. #17 David Marjanovi?, OM
    December 15, 2007

    Perhaps the curve to spine was crucial to four footed and knuckle walking apes for some reason, but has not been lost in the move to upright posture?

    No. Look at the chimp again (at the top of the first figure): it doesn’t have the curvature.

  18. #18 Jud
    December 15, 2007

    At least one IDer has already tagged this paper as an example of what he terms “just so natural selection stories” that he believes makes evolution look silly. What do you think, does he have a point?

    What, that this paper out of tens of thousands may have claimed too much for evolutionary adaptation, and has been criticized by evolutionary biologists for that reason? What is the percentage, out of the handful of peer-reviewed ID papers ever published, of those that claimed too much for the design thesis? And what percentage of those, in turn, were crticized by other ID proponents? I’d say the former number is right up at 100, and the latter sits at zero.

    Shorter version: My irony meter’s pegged.

  19. #19 Richard Carnes
    December 15, 2007

    Actually I rather doubt that our ID friend has reached the level of sophistication necessary to argue that the paper claims too much for natural selection; I suspect he is simply using “just-so story” in the technical ID sense of “an evolutionary hypothesis.”

  20. #20 Stingray
    December 15, 2007

    @#12: I cannot believe you wrote “small breasted nuns versus large breasted nuns”. If anything proves the depravity of this blog and all who read it, surely it is this!

  21. #21 Bride of Shrek
    December 15, 2007

    I have ankylosing spondylitis and with that, and generally laziness, I have pretty poor posture. Interestingly during my three pregnancies I had the best posture I’ve ever had due to that compensating factor of throwing your shoulders back. As an aside, my AS was always great during pregnancy but pretty much post partum ( around four weeks) it would flare up something shocking. My Rheumatologist said it was pretty common to see that in women who have children but apart from speculating about the relaxin playing a part they don’t really have a concrete answer as to why.

  22. #22 DiscoveredJoys
    December 15, 2007

    No. Look at the chimp again (at the top of the first figure): it doesn’t have the curvature.

    Yes, good point from the article as posted, but see http://anthropology.net/2007/12/13/evolution-of-lordosis-and-pregnancy/

    These findings further support the point that position that bipedal walking – full time bipedalism – may be very ancient in the hominoids. It also may explain why various hominoid lineages such as those leading to modern orangutans, chimps and gorillas abandoned upright bipedalism in favor of a re-engineered lumbar spine that could better resist the weight of the late term fetus during long distance travel in horizontal quadrupedal and diagonal postures.

  23. #23 Monado
    December 15, 2007

    My impression is that the usual response of bone to force is to get denser, not necessarily to change shape.

    The data I’d like to see are measurements of adult women of equivalent age and nourishment, comparing those who have had children from those who haven’t. The comparison group could be men with basketball-bellies or “bay windows.” Do they develop a wedge-shaped L3 or any similar adjustments. The next set would be large-breasted vs. small-breasted.

    With imaging technology, we don’t have to wait until people die to look at the shape of their vertebrae.

    Until we get a definitive answer, looking at the L3 vertebra wouldn’t hurt when a pathologist is trying to determine the sex of a skeleton.

  24. #24 Monado
    December 15, 2007

    Forgot to mention: “back up the observations” is easier to understand than “back the observations up” because the reader doesn’t have to remember back to “back”. I hope that’s clear. :-)

  25. #25 CapitalistImperialistPig
    December 15, 2007

    Fifteen pounds? I think PZ’s partner must be a squirrel. What fraction of women gain fifteen pounds in pregnancy? Fifteen to twenty kilos is more like it, but most women gain more.

  26. #26 Tulse
    December 16, 2007

    Do large-breasted women have a consistent change in vertebral morphology that isn’t found in small-breasted women or men?

    I volunteer to be a research assistant for this study. I have a set of calipers.

  27. #27 Monado
    December 16, 2007

    Healthy women are supervised by their doctors to eat healthy foods and gain no more than 30 pounds (28 is the ideal). That’s, um, 12.5 kilos. PZ, are you mixing up pounds and kilos? There’s the fetus, fluids, a 40% increase in blood volume, enlargement of breast tissue, estrogen encouraging the laying down of fat, and I don’t know what else.

    Perhaps you’re remembering that women lose about half that weight at childbirth; but the rest is dropped gradually afterwards. Breastfeeding helps the uterus return to normal size and probably helps to use up extra fat as well.

  28. #28 Monado in Toronto
    December 16, 2007

    Oh, I get it. That’s the weight that’s dropped at birth, most easily measured and up-front in the body, not spread around. I imagine the actual moment arm is somewhat greater due to the uterus and surrounding flesh being thrust out ahead of the fetus. Now I want to know how they measured the centre of mass.

  29. #29 Eileen
    December 16, 2007

    28 lbs. may be the healthy ideal weight gain, but I overheard a new mother brag about the 10 lbs. she gained. Eight of which were the baby.

    Anyway, back to lordosis.

  30. #30 windy
    December 16, 2007

    The authors have identified an interesting sexual dimorphism, but I think the paper was far too quick in assigning an evolutionary selective cause for the difference, and that it did not adequately examine the more likely (to my mind, at least) explanation of physiological adaptation.

    Tomato, tomahto? Instead of one or the other, it could be both – a developmentally plastic response that arose as an adaptation in upright hominids.

    DiscoveredJoys:

    Similarly is the impact of pregnancy on the body (the truly heavy stage lasts for just a few months, every few years in the hunter gatherer culture) sufficiently great to affect fitness (i.e. enable natural selection to produce an adaptive response)?

    It’s not going to be much of a consolation that pregnancies are spaced by a few years, if one of them causes permanent damage to the back.

  31. #31 David Marjanovi?, OM
    December 16, 2007

    Fifteen pounds? I think PZ’s partner must be a squirrel. What fraction of women gain fifteen pounds in pregnancy? Fifteen to twenty kilos is more like it, but most women gain more.

    My mother says 10 kg all-inclusive, and I’m the first of four children, so I dare say she speaks from experience… My youngest sister had 4.39 kg at birth. She was rather globular.

  32. #32 David Marjanovi?, OM
    December 16, 2007

    Fifteen pounds? I think PZ’s partner must be a squirrel. What fraction of women gain fifteen pounds in pregnancy? Fifteen to twenty kilos is more like it, but most women gain more.

    My mother says 10 kg all-inclusive, and I’m the first of four children, so I dare say she speaks from experience… My youngest sister had 4.39 kg at birth. She was rather globular.

  33. #33 LM
    December 16, 2007

    I’m a tiny, tiny woman and I gained 30 lbs. Fortunately, 22 of those fell off in the two weeks post partum. Now, how the hell am I going to get rid of the extra ten???

  34. #34 marc
    December 17, 2007

    I think this proves that God made a really half-assed correction to His obviously flawed design rather than taking the effort to redesign it correctly from scratch. What a lazy Jerk.

  35. #35 SabrinaW
    December 17, 2007

    I am really surprised that differences in those vertebrae have not previously been observed between men and nulliparous women, as an observable difference would support their conclusion that this is developmental and not adaptive. Surely, there is access to the skeletons of childless women somewhere?

  36. #36 thalarctos
    December 17, 2007

    Oh, be fair now, marc; He was operating under a 6-day deadline that couldn’t slip, so of course He had to ship a faulty beta.

  37. #37 marc
    December 17, 2007

    True, thalarctos, He was having a busy week. Plus, if He were burdened with perfection in His creations, He wouldn’t have had time to write the 100% logically and historically consistent Word of God so seamlessly.

  38. #38 MememicBottleneck
    December 17, 2007

    Although it happens in males and older women as well, all the cases of Scoliosis that I’d ever known was in adolescent girls, and runs in families. Although I have no data outside of personal experience, I would not be surprised if Scoliosis was the natural (genetic cause) curving of the spine gone awry.

  39. #39 truth machine
    December 17, 2007

    The female L3 vertebra is not wedge-shaped because women need to bear a fetal load, but instead, because women bear a fetal load, the L3 vertebra is wedge-shaped.

    Um, “X because Y” and “because Y, X” are semantically identical.

  40. #40 truth machine
    December 17, 2007

    At least one IDer has already tagged this paper as an example of what he terms “just so natural selection stories” that he believes makes evolution look silly. What do you think, does he have a point?

  41. #41 PZ Myers
    December 17, 2007

    Um, “X because Y” and “because Y, X” are semantically identical.

    No, they’re not. There are two different statement Ys there that you’ve conflated. Read ’em again.

  42. #42 OmegaMom
    December 17, 2007

    Truth Machine: The two statements *sound* sementically similar, but what he means is: Is the L3 vertebra wedge-shaped in general in women because they *might* need it when pregnant and carrying the extra weight (e.g., evolution and applicable to all women) or is the L3 vertebra wedge-shaped only in women who have given birth (e.g., wedge-shaped because of physiological needs when gravid and thus only seen in women who have had children).

  43. #43 Brian Macker
    December 17, 2007

    Well this would explain why I find a nice lordosis to be sexually attractive in females. I noticed this a long time ago about myself but didn’t have a reason, even if this presumed sexual dimorphism might be just a rationalization if not true.

  44. #44 MememicBottleneck
    December 17, 2007

    Well this would explain why I find a nice lordosis to be sexually attractive in females. I noticed this a long time ago about myself but didn’t have a reason…

    I don’t think you are alone on this. Leaf through a few men’s magazines (or any number of internet web sites) and you’ll find the most common pose has a “nice lordosis”.

    Like evolution, Capitalism is survival of the fittest. These magazines have adopted what works. Although, I don’t think you’ll be hearing cat calls like “nice lordosis baby” from construction workers anytime soon.

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