A wider pelvis does not increase locomotor cost in humans, with implications for the evolution of childbirth

PLoS One. 2015 Mar 11;10(3):e0118903. doi: 10.1371/journal.pone.0118903. eCollection 2015.

Abstract

The shape of the human female pelvis is thought to reflect an evolutionary trade-off between two competing demands: a pelvis wide enough to permit the birth of large-brained infants, and narrow enough for efficient bipedal locomotion. This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women. The basis for the obstetrical dilemma is a standard static biomechanical model that predicts wider pelves in females increase the metabolic cost of locomotion by decreasing the effective mechanical advantage of the hip abductor muscles for pelvic stabilization during the single-leg support phase of walking and running, requiring these muscles to produce more force. Here we experimentally test this model against a more accurate dynamic model of hip abductor mechanics in men and women. The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running. Since a wider birth canal does not increase a woman's locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adult
  • Anthropology, Physical
  • Biological Evolution
  • Biomechanical Phenomena
  • Female
  • Humans
  • Male
  • Muscle Strength
  • Muscle, Skeletal / physiology
  • Pelvis / anatomy & histology*
  • Pelvis / physiology
  • Running / physiology*
  • Sex Characteristics
  • Young Adult

Grant support

This research was supported by grants from the National Science Foundation (www.nsf.gov), BSN # 0850841, The Leakey Foundation (www.leakeyfoundation.org) and The Wenner-Gren Foundation (www.wennergren.org). Grants were obtained by AGW. Institutional support was provided by Washington University in St. Louis and obtained by AGW and HP. Institutional support for Harvard University was obtained by DEL and KLL. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.