Adult female human bony pelvis obstetric morphology differences with respect to age are not due to selection

Recent studies call into question whether human pelvic morphology reflects evolutionary tradeoffs suggested in the obstetrical dilemma, in which responses to competing selection pressures for obstetrical sufficiency and locomotor efficiency shape the pelvis. Nevertheless, dimensions of the true pelvis likely evolved in response to selection pressures for parturition of large fetuses. Thus, females with narrow bony birth canals may have encountered difficulties in childbirth in the past, even though recent research shows that fetal size and maternal size covary, lessening the possible selection pressure that might result from mismatches in fetal and maternal size. Moreover, the dimensions of the birth canal do not scale isometrically among humans, so groups with smaller body size have proportionally larger dimensions in the true pelvis than groups with larger body sizes. In studies of skeletal remains from multiple archaeological sites, age‐at‐death in females is correlated with dimensions of the true pelvis, with younger females exhibiting narrower dimensions, a pattern not observed in males. These studies proposed selective pressures as leading to this discrepancy, though continued growth of the pelvis in adulthood could also be a cause for the pattern. Here, I examine whether selection motivated the relationship between female age‐at‐death and true pelvis dimensions. Fourteen linear pelvic dimensions were measured from the skeletons of 327 adults (188 females, 139 males) associated with six late Holocene Native American archaeological sites. Individuals were aged into two categories: “young” (approximately 25 years old and younger), and “not young” (approximately older than 25 years). Measurements were mean‐scaled within sex‐and‐ age groups. If selection were acting against young females with narrow dimensions, the variance for younger females should be greater than within not young females. Comparisons show no differences in variance between age groups. Thus, there is no evidence to support selection as driving the differences in pelvic dimensions between age groups. Further analyses, based on studies of integration and age‐group comparisons, indicates that the pelvis continues to grow throughout early adulthood. This growth is not driven by any single region of the pelvis, but rather appears to be the result of continued remodeling of the entire true pelvis. These results caution against making assumptions about the roles of evolutionary forces in shaping morphological variation. Support or Funding Information Funding provided by the National Science Foundation, BCS Division Collaborative Research Grant #0962752.