Bony pelvic canal size and shape in relation to body proportionality in humans

Abstract

Obstetric selection acts on the female pelvic canal to accommodate the human neonate and contributes to pelvic sexual dimorphism. There is a complex relationship between selection for obstetric sufficiency and for overall body size in humans. The relationship between selective pressures may differ among populations of different body sizes and proportions, as pelvic canal dimensions vary among populations. Size and shape of the pelvic canal in relation to body size and shape were examined using nine skeletal samples (total female n = 57; male n = 84) from diverse geographical regions. Pelvic, vertebral, and lower limb bone measurements were collected. Principal component analyses demonstrate pelvic canal size and shape differences among the samples. Male multivariate variance in pelvic shape is greater than female variance for North and South Africans. High-latitude samples have larger and broader bodies, and pelvic canals of larger size and, among females, relatively broader medio-lateral dimensions relative to low-latitude samples, which tend to display relatively expanded inlet antero-posterior (A-P) and posterior canal dimensions. Differences in canal shape exist among samples that are not associated with latitude or body size, suggesting independence of some canal shape characteristics from body size and shape. The South Africans are distinctive with very narrow bodies and small pelvic inlets relative to an elongated lower canal in A-P and posterior lengths. Variation in pelvic canal geometry among populations is consistent with a high degree of evolvability in the human pelvis.

Assessment of the variability in the dimensions of the intact pelvic canal in South Africans: A pilot study

“…Kurki's (2013) findings on an archaeological collection were in line with those we report here. South African females displayed small pelvic inlets relative to a larger lower canal in anteroposterior diameters.…”

“…On the other hand, Kurki (2013) relates population specific variations in the pelvic canal to genetic reasons. Kurki's (2013) findings on an archaeological collection were in line with those we report here.…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…Evolutionary theory would suggest that because 'selection intensity and phenotypic variability are inversely related' ( [143], p. 59), there should be sexual dimorphism in the variability of pelvic morphology, with variation in the size and shape of the female birth canal limited by obstetric demands. However, the nature and extent of sexual dimorphism in the modern pelvis are particularly complex, and a large number of studies have produced no clear indication that obstetric constraints produced between-sex differences in variability [3,18,138,139,[141][142][143]. To further complicate matters, work by Betti and co-workers [3,18,141] has suggested that among modern humans, neutral evolutionary processes reflecting demographic history have also been important in determining variation in pelvic form (see also [17]).…”

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

“…The shape of the human pelvis is assumed to be a compromise solution. Human pelvises are shaped for upright walking, but at the same time, they must remain wide enough for giving birth to large-headed neonates (1,2,(9)(10)(11)(12). Upright walking evolved at least 4-5 million years ago and required major skeletal adjustments (9,13,14).…”