Sexual dimorphism in relative sacral breadth among catarrhine primates
Article first published online: 16 OCT 2013
Copyright © 2013 Wiley Periodicals, Inc.
American Journal of Physical Anthropology
Volume 152, Issue 4, pages 435–446, December 2013
How to Cite
Moffett, E. A., Maddux, S. D. and Ward, C. V. (2013), Sexual dimorphism in relative sacral breadth among catarrhine primates. Am. J. Phys. Anthropol., 152: 435–446. doi: 10.1002/ajpa.22372
- Issue published online: 9 NOV 2013
- Article first published online: 16 OCT 2013
- Manuscript Accepted: 23 AUG 2013
- Manuscript Received: 12 DEC 2012
- National Science Foundation. Grant Number: BCS 0716244
- University of Missouri Life Sciences Fellowship and University of Missouri Department of Pathology and Anatomical Sciences
- cephalopelvic proportions;
As the sacrum contributes to the size and shape of the birth canal, the sexually dimorphic sacrum of humans is frequently interpreted within obstetric contexts. However, while the human sacrum has been extensively studied, comparatively little is known about sacral morphology in nonhuman primates. Thus, it remains unclear whether sacral sexual dimorphism exists in other primates, and whether potential dimorphism is primarily related to obstetrics or other factors such as body size dimorphism. In this study, sacra of Homo sapiens, Hylobates lar, Nasalis larvatus, Gorilla gorilla, Pongo pygmaeus, Pan troglodytes, and Pan paniscus were evaluated for sexual dimorphism in relative sacral breadth (i.e., the ratio of overall sacral breadth to first sacral vertebral body breadth). Homo sapiens, H. lar, N. larvatus, and G. gorilla exhibit dimorphism in this ratio. Of these, the first three species have large cephalopelvic proportions, whereas G. gorilla has small cephalopelvic proportions. P. pygmaeus, P. troglodytes, and P. paniscus, which all have small cephalopelvic proportions, were not dimorphic for relative sacral breadth. We argue that among species with large cephalopelvic proportions, wide sacral alae in females facilitate birth by increasing the pelvic inlet's transverse diameter. However, given the small cephalopelvic proportions among gorillas, an obstetric basis for dimorphism in relative sacral breadth appears unlikely. This raises the possibility that sacral dimorphism in gorillas is attributable to selection for relatively narrow sacra in males rather than relatively broad sacra in females. Accordingly, these results have implications for interpreting pelvic dimorphism among fossil primates, including hominins. Am J Phys Anthropol 152:435–446, 2013. © 2013 Wiley Periodicals, Inc.