title: Shape variation in the human pelvis and limb skeleton: Implications for obstetric adaptation.
abstract: OBJECTIVE
  Under the obstetrical dilemma (OD) hypothesis, selection acts on the human female pelvis to ensure a sufficiently sized obstetric canal for birthing a large-brained, broad shouldered neonate, while bipedal locomotion selects for a narrower and smaller pelvis. Despite this female-specific stabilizing selection, variability of linear dimensions of the pelvic canal and overall size are not reduced in females, suggesting shape may instead be variable among females of a population. Female canal shape has been shown to vary among populations, while male canal shape does not. Within this context, we examine within-population canal shape variation in comparison with that of noncanal aspects of the pelvis and the limbs.
  
  
  METHODS
  Nine skeletal samples (total female n = 101, male n = 117) representing diverse body sizes and shapes were included. Principal components analysis was applied to size-adjusted variables of each skeletal region. A multivariate variance was calculated using the weighted PC scores for all components in each model and F-ratios used to assess differences in within-population variances between sexes and skeletal regions.
  
  
  RESULTS
  Within both sexes, multivariate canal shape variance is significantly greater than noncanal pelvis and limb variances, while limb variance is greater than noncanal pelvis variance in some populations. Multivariate shape variation is not consistently different between the sexes in any of the skeletal regions.
  
  
  CONCLUSIONS
  Diverse selective pressures, including obstetrics, locomotion, load carrying, and others may act on canal shape, as well as genetic drift and plasticity, thus increasing variation in morphospace while protecting obstetric sufficiency.
authors.affiliation: Department of Anthropology, University of Victoria, Victoria, BC, V8P 5C2, Canada.
authors.forename: Helen K
authors.initials: HK
authors.lastname: Kurki
authors.affiliation: Division of Biological Anthropology, Department of Archaeology and Anthropology, University of Cambridge, Cambridge, CB2 3QG.
authors.forename: Sarah-Louise
authors.initials: SL
authors.lastname: Decrausaz
mesh_terms: D000885:Anthropology, Physical; D005121:Extremities / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D008297:Male; D015999:Multivariate Analysis; D010384:Pelvic Bones / Q000033:anatomy & histology* / Q000502:physiology*; D010388:Pelvis / Q000502:physiology*; D011247:Pregnancy / Q000502:physiology*; D044469:Racial Groups / Q000706:statistics & numerical data
keywords: multivariate variance; obstetric dilemma; obstetric selection; plasticity