Doctoral Dissertation Improvement: Bipedal Obstetric Load and the Evolution of Human Lumbopelvic Sexual Dimorphism

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A developing fetus modifies maternal body proportions and mass distribution introducing instability to the vertebral column and pelvis. This study hypothesizes that human sexual dimorphism of lumbar vertebral and pelvic anatomy represents, in part, unique female adaptations to structural risks associated with biomechanics of fetal load. The hypothesis differs from the prevailing view that lumbopelvic sexual dimorphism is entirely related to parturitional selection favoring a maternal birth canal spacious enough for delivery of a large-brained or large bodied neonate. As a framework for interpreting the scope of human lumbopelvic sexual dimorphism, current conclusions may be limited by focusing only on pelvic morphology directly related to the birthing event and by failure to consider the attendant stresses of reproductive load. Persistent interpretation of sexual dimorphism in lumbar and sacral vertebrae as compliant to the pelvis obscures the possibility that the vertebrae perform a more immediate function in trunk biomechanics. Although pregnancy is intermittent, the duration and recurrence of fetal load exert marked stress on the postural and locomotor skeleton, holding implications not only for modern humans but for earlier bipeds as well. Sexual dimorphism of the lumbopelvic skeleton will be examined morphometrically in 70 human females and 70 human males and in a comparative sample of 600 individuals of 13 species of nonhuman primates, including habitually erect taxa, to examine the prediction that lumbopelvic sexual dimorphism is greater in humans than in other primates. To investigate the biomechanical impact of human fetal load on the lumbopelvic skeleton, spinal loading patterns of twenty pregnant females will be kinematically identified for position of the maternal center of mass, curve of the lower back and angle of anterior pelvic tilt. The experimental approach will test hypotheses on weight redistribution and loading of the human female spine and pelvis during pregnancy; while the comparative morphometric approach will place these results into broader phylogenetic and evolutionary contexts. The wider effects of this project are threefold. First, reconstruction of adaptive patterns in bipedal obstetric load will provide evolutionary depth to a human experience currently defined by clinical practice. Second, the study will provide hands-on research experience for undergraduate assistants who will gain both academic and practical training in project management. Third, as a result of volunteer participation in this study, interactions between the female public and the academic community will help to advance women's access to current research and will ultimately inform the life education of women.