It has long been noted that the human female birth canal is well adapted to giving birth to large-brained neonates. However, several species of nonhuman primates give birth to large-headed neonates compared to the maternal birth canal. The presence of such large cephalopelvic proportions in nonhuman primates presents the question of whether dimorphism in the birth canals of these other species is related to obstetric demand, as such dimorphism is presumed to be in humans. In this study, the hypothesis that either the presence or magnitude of dimorphism in the birth canal is related to large cephalopelvic proportions among anthropoid primates is directly tested. This study shows that birth canal dimorphism is common among anthropoids regardless of cephalopelvic proportions, but taxa with large cephalopelvic proportions have a higher magnitude of dimorphism than those that give birth to relatively small-headed neonates. Furthermore, humans have exceptionally high levels of dimorphism that cannot be explained based on our large cephalopelvic proportions alone. Anat Rec, 300:870-889, 2017. V C 2017 Wiley Periodicals, Inc.Key words: pelvis; pelvic; hip; anthropoids; birth Human females give birth to large neonates relative to maternal body and birth canal size (Schultz, 1949;Leutenegger, 1970), and the hypothesis that the female pelvis is adapted to the selective pressures of giving birth to such large neonates appears well-supported (Washburn, 1948;Tague, 1989Tague, , 1992 Lovejoy, 2005). Although females are smaller than males for both stature and body mass, females have a more spacious true pelvis than males (Turner, 1885; Caldwell and Moloy, 1933;Greulich, 1938;Thoms and Greulich, 1940;Young and Ince, 1940;Coleman, 1969;Krogman and Işcan, 1986;Hager, 1989;Tague, 1989Tague, , 1992Walrath and Glantz, 1996;Walrath, 1997;Correia et al., 2005;Kurki, 2007;Brown, 2011;Kurki, 2011). Across human populations, females have larger circumferences of the pelvic inlet, midplane, and outlet than males, both absolute and relative to body size (Tague, 1992). Females also have mediolaterally wider midplanes (Holland et al., 1982;Hager, 1989;Tague, 1991Tague, , 1992Correia, 2005;Brown, 2011) and outlets (Holland et al., 1982;Hager, 1989;Tague, 1992;Correia, 2005;Brown, 2011; but see Krogman and Işcan, 1986) than males relative to body size.Selection for obstetric imperatives in females should increase the size of the maternal birth canal (Berge et al

During the evolution of hominins, it is generally accepted that there was a shift in postcranial morphology between Australopithecus and the genus Homo. Given the scarcity of associated remains of early Homo, however, relatively little is known about early Homo postcranial morphology. There are hints of postcranial diversity among species, but our knowledge of the nature and extent of potential differences is limited. Here we present a new associated partial ilium and femur from Koobi Fora, Kenya, dating to 1.9 Ma (millions of years ago) that is clearly attributable to the genus Homo but documents a pattern of morphology not seen in eastern African early Homo erectus. The ilium and proximal femur share distinctive anatomy found only in Homo. However, the geometry of the femoral midshaft and contour of the pelvic inlet do not resemble that of any specimens attributed to H. erectus from eastern Africa. This new fossil confirms the presence of at least two postcranial morphotypes within early Homo, and documents diversity in postcranial morphology among early Homo species that may reflect underlying body form and/or adaptive differences.

The anterior fontanelle (AF) is an integral element of the developing human infant craniofacial system. Consideration of the AF is crucial for assessing craniofacial growth, as altered development of this feature may indicate abnormal growth. Moreover, prolonged patency of the AF may represent a derived hominin feature. The AF is regarded as essential for fetal head molding during birth in humans, with deformation of the head during birth often necessary for successful delivery. However, the function of a patent AF among fossil hominins is unclear. Because the AF represents an important structure in both a clinical and evolutionary context, techniques for estimating the size of the AF must be accurate and reproducible. Therefore, we have developed a novel method for assessing surface area of the AF with the goal of creating a more accurate measure of this feature. In this study, we test the accuracy and repeatability of a novel three-dimensional (3D) method for assessing the size of the AF in human infants and compare the results obtained for surface area of the AF using the conventional and 3D methods. Anat Rec, 297:234-239, 2014. V C 2013 Wiley Periodicals, Inc.

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.