Obstetrical implications of sexual dimorphism in hominoid os coxae
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Many studies have examined pelvic sexual dimorphism and its functional implications in linear analyses, but few have used three-dimensional (3D) methods, which are ideal for complex structures such as the pelvis. This study examines human and ape os coxae using 3D geometric morphometrics (GM) to determine if patterns of sexual dimorphism relate to obstetrical and/or biomechanical requirements. Twenty-five 3D landmarks were collected on 35 human and 116 hominoid os coxae. Generalized Procrustes and principal component analyses were performed on combined and sex-specific species samples. Tests of male and female human os coxae demonstrate dimorphism in several pelvic traits related to increasing birth canal dimensions in females; females have a wider subpubic angle and greater sciatic notch than males, as well as a short ischial spine. Hominoids, however, did not exhibit sexual dimorphism in pelvic shape. These results confirm previous studies showing that human pelvic shape dimorphism is related to the obstetrical requirements of birthing a large fetus, rather than sexual dimorphism in locomotor biomechanics. Future 3D GM research on other primate species that have large neonates relative to maternal size may determine whether the primate pelvis exhibits similar patterns of response to obstetrical selection pressures as in humans.