Lack of Sagittal Crest of Skull

The sagittal crest is a prominent ridge of bone that projects superiorly (upwards) from the cranial vault along its midline, most commonly seen in adult male gorillas and orangutans. Sagittal crests are rare in adult male chimpanzees and female gorillas, and are unknown in female chimpanzees, female orangutans, and humans and bonobos of both sexes (and are also absent in juveniles of all species). The crest provides a surface for the attachment of the large chewing muscle, temporalis. In humans, who have large brains (and hence large cranial vaults) relative to their body size, the temporal muscles occupy a position on the lateral walls of the cranial vault, and extend only about halfway up the vault surface. In apes the superior extent of the temporalis muscles is positioned higher on the cranial vault (because brain size is smaller, and the muscles of mastication are larger than those found in humans), such that the right- and left-side superior temporal lines approach one another at midline. In male gorillas and orangutans (and some species of fossil hominin), in which very large chewing muscles are anchored to a relatively small cranial vault, the right and left superior temporal lines not only converge at the midline of the top of the cranial vault (along the sagittal suture), but also require the development of a sagittal crest to provide sufficient attachment area for the temporalis muscles.