The 89th Annual Meeting of the American Association of Physical Anthropologists (2020)

The correlation between bi-iliac breadth and birth canal size in humans: implications for the obstetric dilemma


Anthropology and The Center for the Study of Human Origins, New York University, Anthropology, The New York Consortium in Evolutionary Primatology

April 17, 2020 , Diamond 10 Add to calendar

The obstetric dilemma posits that an antagonistic selection pressure prevented the modern human female pelvis from responding to selection acting to increase the size of the birth canal (BC) to decrease the incidence of obstructed labor. Selection for effective thermoregulation may be antagonistic to pressures acting to increase the size of the BC because, based on Bergmann’s Rule, narrower-bodied humans should be better at dissipating heat. If humans evolved in hot climates there may have been selection to reduce body breadth and thus bi-iliac breadth (BIB). However, this hypothesis presupposes that BIB is highly correlated with birth canal capacity.

To test this, three-dimensional scans were created of adult female pelves from skeletal collections representing geographically varied populations from a variety of climates (n=102). The scans were processed in Geomagic Studio. BIB and the areas of the three BC planes (inlet, midplane, and outlet) were measured in ImageJ. The correlation between the area of each plane and BIB was tested using an ordinary least squares regression analysis.

BIB and inlet area are significantly, but weakly, correlated (r2=0.25, p=0.0047,). BIB is not correlated with midplane (r2=0.088, p=0.15) or outlet area (r2=0.090, p=0.14). The outlet is generally the smallest of the three planes (84% of cases). Therefore, while BIB might be a factor in determining the size of the inlet, the inlet is rarely the most constrained in size. The results suggest that while selection on BIB might play a role in the obstetric dilemma, it is unlikely to play a large one.

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1650895.

Slides/Poster (pdf)