Variation in obstetric dimensions of the human bony pelvis in relation to age-at-death and latitude

Previous studies associate females who died in young adulthood with narrower obstetric pelvic dimensions, presumably in association with obstetric insufficiency (though this causal relationship is unresolved). In this study, we examine whether females within groups living at higher latitudes present this pattern, as high-latitude groups have larger pelvic dimensions than groups previously examined. These patterns are compared with males. We assess whether there is evidence for younger ages-at-death in females to have been in response to natural selection against narrower true pelvis dimensions.

We measured 14 pelvic dimensions in 327 adults (188 females, 139 males), representing archaeological sites from mid-latitude and high-latitude North America. Individuals were placed into a "young" or "not young" age-at-death category. Latitude, sex, and age-at-death groups were compared using ANOVAs and scaled variance, and evidence for selection was examined with F-tests.

Pelvic dimensions were larger in high-latitude females and males. Females but not males who died at younger ages had smaller pelvic canals than older individuals, especially in the mediolateral inlet and anteroposterior outlet dimensions. Variance in all pelvic dimensions is equal between the two female age-at-death groups.

We found narrower obstetrical dimensions in the female pelvis among individuals who died at younger ages. However, statistically equivalent variances in the two female age-at-death groups does not support natural selection on pelvic dimensions as leading to younger ages at death. We instead argue that this difference is result of continued growth due to remodeling in the pelvis occurring in females, but not males, after early adulthood.