CoM and lumbar lordosis during pregnancy

CoM stands for center of mass. Lumbar Lordosis, I understand, is an inward curvature of the lower back. Pharyngula explains the connection between the two:

… 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.

Pharygula’s post is about a recent paper published in Nature titled Fetal load and the evolution of lumbar lordosis in bipedal hominins by Whitcome, Shapiro and Lieberman:

As predicted by Darwin, bipedal posture and locomotion are key distinguishing features of the earliest known hominins. Hominin axial skeletons show many derived adaptations for bipedalism, including an elongated lumbar region, both in the number of vertebrae and their lengths, as well as a marked posterior concavity of wedged lumbar vertebrae, known as a lordosis. The lordosis stabilizes the upper body over the lower limbs in bipeds by positioning the trunk’s centre of mass (COM) above the hips. However, bipedalism poses a unique challenge to pregnant females because the changing body shape and the extra mass associated with pregnancy shift the trunk’s COM anterior to the hips. Here we show that human females have evolved a derived curvature and reinforcement of the lumbar vertebrae to compensate for this bipedal obstetric load. Similarly dimorphic morphologies in fossil vertebrae of Australopithecus suggest that this adaptation to fetal load preceded the evolution of Homo.

Pharyngula goes on to supply some alternative hypotheses and a few criticisms of the results reported in the paper:

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.

Take a look; and, while you are at it, do not miss the comments section too!

Tags: centre of mass, Lumbar lordosis, pregnancy