Dienstag, 21. April 2015

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma; Barbara Fischer and Philipp Mitteroecker (2015)


Because of the tight fit of the large human neonate through the narrow maternal birth canal, childbirth is remarkably difficult. In this study we show that the dimensions of head, stature, and pelvis in a human body are linked in a complex way that was not recognized before and that contributes to ameliorate this tight fit. We show that females with a large head possess a birth canal that can better accommodate large-headed neonates. Because mothers with large heads usually give birth to neonates with large heads, the detected pattern of covariation contributes to ease childbirth and has likely evolved in response to strong selection.


Compared with other primates, childbirth is remarkably difficult in humans because the head of a human neonate is large relative to the birth-relevant dimensions of the maternal pelvis. It seems puzzling that females have not evolved wider pelvises despite the high maternal mortality and morbidity risk connected to childbirth. Despite this seeming lack of change in average pelvic morphology, we show that humans have evolved a complex link between pelvis shape, stature, and head circumference that was not recognized before. The identified covariance patterns contribute to ameliorate the “obstetric dilemma.” Females with a large head, who are likely to give birth to neonates with a large head, possess birth canals that are shaped to better accommodate large-headed neonates. Short females with an increased risk of cephalopelvic mismatch possess a rounder inlet, which is beneficial for obstetrics. We suggest that these covariances have evolved by the strong correlational selection resulting from childbirth. Although males are not subject to obstetric selection, they also show part of these association patterns, indicating a genetic–developmental origin of integration.

[via Steve Stuart Williams]

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