"P. boisei" is the most robust of the robust australopithecines, whereas the South African "P. robustus" is smaller with comparatively more gracile features. The "P. boisei" skull is heavily built, and features a defined brow ridge, receding forehead, rounded bottom margins of the eye sockets, inflated and concave cheek bones, a thick palate, and a robust and deep jawbone. This is generally interpreted as having allowed "P. boisei" to resist high stresses while chewing, though the thick palate could instead be a byproduct of facial lengthening. The skull features large rough patches (rugosities) on the cheek and jawbones, and males have pronounced sagittal (on the midline) and temporonuchal (on the back) crests, which indicate a massive masseter muscle (used in biting down) placed near the front of the head (increasing mechanical advantage). This is typically considered to be evidence of a high bite force.