Did Human Female Pelvis Evolution Create Another Biological Clock?

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.

Via John Hawks.

Everyone know that women can only have children from puberty to menopause, and most people who have lived in the social world of college students know that female fertility declines markedly with age before menopause.  All of this, however, is basically directly hormonal and biochemical, not mechanical and morphological according to hormonal cues.

The new paper suggests that shifts in women's hips over their life cycle provides an independent biological clock that influences fertility and provides a biological bias towards having children in your 20s, rather than your 30s or later.  Of course, medically safe C-sections can now bypass the limits of the birth canal on safe births later in life.  But, this study ought to give pause to women who think that vaginal births for mothers of advanced maternal age are desirable because they are more "natural."

The issue of large-headed/large-bodies babies is a big one because many evolutionary anthropologists see large headed babies as a critical piece of evolution that has facilitated higher IQ in modern humans than in other primates.  And, it is widely accepted that higher IQ is a key factor in modern human selective fitness.

Given the ongoing relevance of IQ and the existence of safe C-sections, is it conceivable that we could evolve to a state where safe vaginal births are no longer possible because selection for high IQ leads to selection for large headed babies and hip size is not longer a meaningful constraint on that tendency?

This paper is also a welcome reminder that genetically driven phenotypes are not necessarily an all or nothing thing.  People's bodies change over their lifespans and our genes are clever enough to adapt one way at one age and another at a later age for maximum selective fitness.