The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Pelvic sexual dimorphism among species monomorphic in body size: commonalities and relationship to newborn size


Geography and Anthropology, Louisiana State University

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This study evaluates pelvic sexual dimorphism as an obstetrical adaptation among eight species – three primates (Aotus azarae, Hylobates lar, and Saguinus geoffroyi) and five others (Castor canadensis, Dasypus novemcinctus, Sciurus carolinensis, Sylvilagus floridanus, and Urocyon cinereoargenteus). As nonpelvic sexual size dimorphism (SSD) influences pelvic SSD, all eight species are sexually monomorphic in body size. The study addresses two issues. First, do the species show commonality in pelvic SSD and fusion of pelvic joints? Second, do the species that give birth to relatively large newborns have a higher magnitude of pelvic SSD and lower prevalence of pelvic joint fusion than those giving birth to relatively small newborns? Based on published data for 372 species, the three primates are characterized as giving birth to relatively large newborns and the five others as giving birth to relatively small newborns. Nine measures of the pelvis were taken; fusion of interpubic and sacroiliac joints was observed. Results are the following. First, the species show no commonality in pelvic SSD or prevalence of pelvic joint fusion. Second, based on relative newborn size, the two groups of species do not differ in magnitude of pelvic SSD or prevalence of sacroiliac joint fusion. The primates do have a lower prevalence of interpubic joint fusion. The following interpretations are presented: (1) there are multiple pathways to achieve an obstetrically sufficient pelvis; (2) pelvic and nonpelvic sizes are not related in some species; and (3) an unfused interpubic joint is an important obstetrical adaptation.

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