Pelvic breadth, obstetric dimensions, and morphological constraint: the paradox of birthing hips?

Pelvic breadth in humans is thought to reflect competing selection factors, namely thermoregulation, obstetrics, and efficient locomotion. Wider bodies are associated with lower surface area‐to‐volume ratios, and therefore humans in colder climates. The balance of the selection pressures—if any—exerted by these variables has not been extensively investigated. Indigenous humans from the Americas exhibit wider pelvic breadths than humans living in similar environments in Africa and Europe. This morphology is possibly the result of shared ancestry from a cold‐adapted ancestral population. It is therefore possible that obstetric dimensions have constrained the narrowing of the pelvis among indigenous New World humans. If obstetric dimensions covary with body breadth, one would hypothesize that narrower obstetric dimensions might be associated with younger age‐at‐death, and therefore narrower females within ancient populations. Examining skeletal remains from eight locations representing high and low latitudes, this study demonstrates that there is a strong association between age‐at‐death and important obstetric dimensions. Females who died in their twenties or before have mediolaterally narrower pelvic inlets and more shallow pelvic outlets than females that died at older ages. Some of these dimensions correlate with overall pelvic breadth, likely due to developmental integration of the pelvis. Males do not demonstrate the differential mortality, but reflect the covariance of dimensions. These results indicate that multiple, non‐independent factors influence pelvic morphology. This research was supported by a Collaborative Research Grant from the National Science Foundation (BCS‐0962752) .