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.

The ilium and ischiopubic bones of the pelvis arise from different regulatory pathways, and as a result, they may be modular in their organization such that features on one bone may be morphologically integrated with each other, but not with features on the other pelvic bone. Modularity at this gross level of organization can act to increase the ability of these structures to respond to selection pressures (i.e., their evolvability). Furthermore, recent work has suggested that the evolution of the human pelvis was facilitated by low levels of integration and high levels of evolvability relative to other African apes. However, the extent of morphological integration and modularity of the bones of the pelvic girdle is not well understood, especially across the entire order of primates. Therefore, the hypothesis that the ilium and ischiopubis constitute separate modules was tested using threedimensional landmark data that were collected from 752 pelves from 35 primate species. In addition, the hypothesis that the human pelvis demonstrates greatest evolvability was tested by comparing it to all other primates. The results demonstrate that regardless of phylogeny and locomotor function, the primate pelvis as a whole is characterized by low levels of overall integration and high levels of evolvability. In addition, the results support the developmental hypothesis of separate ilium and ischiopubis modular units. Finally, all primates, including humans, apparently share a common pattern of integration, modularity, and evolvability in the pelvis.

Identification of positional behavior adaptation in the pelvis of primates is complicated by possible confounding effects of body size and phylogeny. Previous work on primate pelvic allometry has focused primarily on sexual dimorphism and its relationship to obstetric constraints in species with large fetal size relative to maternal size. This study investigates patterns of pelvic scaling with a specific aim to understand how pelvic scaling relates to locomotor function. Patterns of scaling of nine pelvic dimensions were examined in a broad comparative sample of 40 species of primates, covering both haplorhines and strepsirrhines, while accounting for phylogenetic nonindependence. Phylogenetic reduced major axis regressions on pelvic scaling patterns suggest that primate-wide patterns are reflected in haplorhine- and strepsirrhine-specific analyses. Many measures scale isometrically with pelvis size, but notably, features of the ilium tend to scale allometrically. As predicted, ilium width and lower ilium cross-sectional area scale with positive allometry, while lower iliac height scales with negative allometry. Further regression analyses by locomotor group suggest that these ilium measures, as well as pubic symphysis and ischium lengths, differ in their scaling patterns according to locomotor mode. These results suggest that scaling differences within primates, when present, are related to functional differences in locomotor behavior and mechanics. This study supports recent work that identifies adaptations to locomotor loading in the ilium and highlights the need for a better understanding of the relationship between pelvic structural mechanics and the mechanical requirements of primate locomotion.

The pelvic girdle is a complex structure with a critical role in locomotion, but efforts to model the mechanical effects of locomotion on its shape remain difficult. Traditional approaches to understanding form and function include univariate adaptive hypothesis-testing derived from mechanical models. Geometric morphometric (GM) methods can yield novel insight into overall three-dimensional shape similarities and differences across groups, although the utility of GM in assessing functional differences has been questioned. This study evaluates the contributions of both univariate and GM approaches to unraveling the trait-function associations between pelvic form and locomotion. Three-dimensional landmarks were collected on a phylogenetically-broad sample of 180 pelves from nine primate taxa. Euclidean interlandmark distances were calculated to facilitate testing of biomechanical hypotheses, and a principal components (PC) analysis was performed on Procrustes coordinates to examine overall shape differences. Both linear dimensions and PC scores were subjected to phylogenetic ANOVA. Many of the null hypotheses relating linear dimensions to locomotor loading were not rejected. Although both analytical approaches suggest that ilium width and robusticity differ among locomotor groups, the GM analysis also suggests that ischiopubic shape differentiates groups. Although GM provides additional quantitative results beyond the univariate analyses, this study highlights the need for new GM methods to more specifically address functional shape differences among species. Until these methods are developed, it would be prudent to accompany tests of directional biomechanical hypotheses with current GM methods for a more nuanced understanding of shape and function. Anat Rec, 298:230-248, 2015. V C 2014 Wiley Periodicals, Inc.Key words: adaptation; geometric morphometrics; pelvis; primates; locomotion; functionThe identification of adaptation to biological function has been a goal of evolutionary biology since the time of Darwin. Within the field of biological anthropology, adaptations to diet and/or locomotion have been examined via hypothesis-testing of craniodental anatomy (Lieberman and Crompton, 2000;Ravosa et al., 2000;Daegling and McGraw, 2001;Spencer, 2003;Vinyard et al., 2003;Terhune, 2013), shoulder anatomy (Taylor, 1997), long bone morphology (Fleagle, 1976;Polk, 2002), and hand and foot anatomy (Orr, 2005;Patel, 2010;Tocheri et al., 2011). However, systematic hypothesistesting of pelvic adaptations has been relatively limited because the mechanical rules that govern pelvic shape are not well understood.Among other requirements, identifying morphological adaptations necessitates a clear association between a morphological trait and its presumed function, as well as evidence that the trait performs the function (e.g., Kay and Cartmill, 1977;Anthony and Kay, 1993). The comparative method is used to distinguish form-function correlations in broad samples of extant taxa (Felsenstein, 1985;Harvey and Pagel, 1991;Ross...