The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Sexual dimorphism in catarrhine sacra: Obstetrics versus body size dimorphism


Pathology and Anatomical Sciences, University of Missouri

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As part of the bony pelvis, the sacrum contributes to the size and shape of the pelvic inlet and outlet, and could be an important tool for interpreting the evolution of cephalopelvic proportions in fossil primates. However, while patterns of sexual dimorphism in the human sacrum are relatively well-understood, comparatively little is known about sacral dimorphism in extant non-human primates. In this study, sacra of adult male and female Homo sapiens (n=73), Hylobates lar (n=70), Gorilla gorilla (n=31), Nasalis larvatus (n=21), Pan troglodytes (n=20), Pongo pygmaeus (n=15), and Pan paniscus (n=13) were evaluated for dimorphism in relative sacral breadth (i.e. the ratio of overall sacral breadth to S1 vertebral body breadth). Only Homo sapiens, Hylobates lar, and Nasalis larvatus were found to be sexually dimorphic in relative sacral breadth. While these three species exhibit variability in overall body size dimorphism (monomorphic to highly dimorphic), all three species bear relatively large neonates. The four remaining species were not dimorphic for relative sacral breadth. These four species also vary in levels of overall body size dimorphism, but all give birth to relatively small neonates. These results indicate that the existence of sexual dimorphism in relative sacral breadth among non-human primates is primarily influenced by cephalopelvic proportions rather than body size dimorphism. Thus, it appears that among species bearing relatively large-brained neonates, lateral expansion of the female sacrum functions to increase the transverse diameter of the pelvic inlet. Accordingly, these results have important implications for interpreting cephalopelvic relationships among fossil primates, including hominins.

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