The developmental obstetrical dilemma

Alik Huseynov and colleagues have a data-rich paper in the Proceedings of the National Academy of Sciences examining age-related changes in the human pelvis: “Developmental evidence for obstetric adaptation of the human female pelvis”. In the paper, Huseynov and colleagues present a new hypothesis for the evolution of sexual dimorphism in the hominin pelvis: the “Developmental Obstetrical Dilemma” hypothesis.

Here’s the paper’s abstract:

The obstetrical dilemma hypothesis states that the human female pelvis represents a compromise between designs most suitable for childbirth and bipedal locomotion, respectively. This hypothesis has been challenged recently on biomechanical, metabolic, and biocultural grounds. Here we provide evidence for the pelvis’ developmental adaptation to the problem of birthing large-headed/large-bodied babies. We show that the female pelvis reaches its obstetrically most adequate morphology around the time of maximum fertility but later reverts to a mode of development similar to that of males, which significantly reduces the dimensions of the birth canal. These developmental changes are likely mediated by hormonal changes during puberty and menopause, indicating “on-demand” adjustment of pelvic shape to the needs of childbirth.

I really like this paper and the Developmental Obstetrical Dilemma hypothesis. I think the idea makes a great deal of sense. What I see as the most important result of the paper is that the authors find that the female pelvis achieves its maximal inlet dimensions only around age 25-30. As the authors indicate, this is when female fertility is at a maximum. But from an evolutionary point of view, this timing reflects a serious trade-off that has previously not been clearly recognized. Females mature sexually, in many societies marry, and begin having offspring at substantially younger ages than 25.

If a woman is still developing her pelvic inlet at these younger ages, then infants born at those ages face a higher risk of fetal-pelvic disproportion, elevating the chances of a difficult birth and infant mortality, and slightly increasing the mortality risk of young mothers. All this is concordant with conventional wisdom about births to young mothers.

But from an evolutionary point of view, this seems paradoxical. The force of selection is maximal on individuals just at the beginning of their reproductive lives, when they have survived infant and juvenile mortality, and have all their adult reproduction ahead of them. The force of selection on 25-year-old mothers is actually lower than that on 18-year-old mothers. Why do younger women not develop their pelves faster, so that their early offspring will have the lower risk of birth complications enjoyed by children they have in their late twenties?

I don’t think the “pelvic shape on demand” aspect of the Huseynov paper answers this question. But I think the paper’s results point to a reason for the delayed pelvic development. The early component of a woman’s reproductive life, from menarche to maximal pelvic development is essentially a compromise between the fitness advantage of early reproduction and the fitness advantage of a large (and large-brained) offspring. The answer to “why don’t women develop faster” is that if they developed their skeleton faster, they would move first birth even younger than it already is. Natural selection would favor even younger births just to the point that the mortality cost balances the fertility advantage.

When we look at the average age of male first reproduction in traditional societies, it is substantially later than female first reproduction, but much more similar to the time that women gain their maximal pelvic dimensions. From this point of view, the component of a woman’s reproductive lifespan before this time is a roll of the evolutionary dice. Early children contribute very strongly to the growth of populations, so there must have been a strong selective pressure for early reproduction during the origin of modern humans, and probably at many earlier times in our evolutionary history.

The press coverage of this paper has largely focused on one aspect of the analysis; the observation that female pelves continue to change in shape as women age, so that post-menopausal women have slightly more constricted pelvic inlets than younger women. This is a bit unfortunate for two reasons. First, the amount of change in older women is relatively slight. Speculations about the selective value of pelvic changes in older women are probably without much merit, considering that the force of selection on older individuals of both sexes is very small, and the amount of change posited here is so small.

Second, the sample examined here is not a longitudinal sample, it is a cross-sectional sample. The human individuals are a wonderful resource and irreplaceable, and I sincerely hope that CT-scans of these will be made available on an open access basis. But they do not document age-related changes in the same individuals over time, they document the way that dead people of different ages vary. This means that the 80-year-olds do not represent the same set of birth cohorts as the 40-year-olds and the 20-year-olds. This difference is not such a factor when looking at the initial development of the pelvis through childhood and early adulthood, but it does make a big difference when comparing mature and senescent samples. I do not doubt that some age-related changes documented here in this sample are really characteristic of the European population. But older individuals represent a very different early childhood nutritional and disease history than younger individuals in this sample.

In other words, the study does not address, and does not present data to reject, the hypothesis that the 80-year-olds may have already had pelvic morphology at age 40 that was different from the 40-year-olds in the sample. That being said, I do think the comparison with males, which do not exhibit the same pattern of age-related changes, is sufficient to lead us to think that a female-specific pattern exists.

But at any rate, this part of the study is not central to the evolutionary history. Changes to pelvic morphology associated with very elevated ages must be a very recent phenomenon within human populations, and selection associated with them must have been very slight. What the study shows about the development of the pelvis in early adulthood is comparatively profound.

References

Huseynov A, Zollikofer CPE, Coudyzer W, Gaschoc D, Kellenberger C, Hinzpeter R, Ponce de León MS. 2016. Developmental evidence for obstetric adaptation of the human female pelvis. Proceedings of the National Academy of Sciences, USA (online) doi:10.1073/pnas.1517085113