Editor’s note: This pair of articles— Anat Rec (New Anat) 261:173–175 (Ravosa et al.) and 170–172 (Prossinger et al.), 2000—were originally submitted as a Letter to the Editor commenting on the article by Bookstein et al. [Anat Rec (New Anat) 257:217-224, 1999] and a Response by the Bookstein et al. (1999) authors. The Editorial Board concluded that the length and depth of this scientific discussion on one aspect of the referenced article—viz., possible relationships between browridge morphology and masticatory stress in ancient vs. modern hominids—warranted more space than is typically offered to Letters to the Editor. We therefore decided to publish these articles back to back, as brief Point/Counterpoint articles, in order to let the researcher present well-developed arguments on both sides of this intriguing debate. The authors of Bookstein et al. (1999) recently employed a suite of imaging techniques and morphometric analyses to describe evolutionary changes in circumorbital form among Pleistocene hominids. In discussing the broader implications of their study, they claim that, contrary to specific in vivo (Hylander et al., 1991a) and comparative (Ravosa, 1991b) work, “the external form of the browridges is related to the need to resolve the stresses in the face. . . induced by mastication” (Bookstein et al., 1999; p. 222). This conclusion is based on “the inner morphology of the frontal sinuses in the CT scans” of various hominoid crania, specifically the thin “anterior and posterior walls of the sinuses” (Bookstein et al., 1999; p. 222). According to the authors, “thinning minimizes bone mass without compromising the necessary strength, suggesting that. . .externally enormous supraorbital structures do relate to masticatory stresses” (Bookstein et al., 1999; p. 222). The purpose of our response is threefold. First, we address their argument regarding the functional significance of thin-walled browridges. Second, we indicate that recent in vivo bone-strain analyses provide no support for any masticatory-stress hypothesis of circumorbital form. Third, based on experimental and morphological data, we show that, rather than being adapted to counter masticatory stresses, variation in browridge proportions and frontal sinus pneumatization is supportive of the spatial model of supraorbital torus formation (Moss and Young, 1960; Shea, 1986; Ravosa, 1988, 1991a,b; Hylander and Ravosa, 1992). If Bookstein et al. (1999) insist that a thin-walled supraorbital torus in humans is especially designed for resisting masticatory stresses, we should then ask what kind of evidence would support a refutation of this hypothesis. Presumably the presence of a thicker-walled supraorbital torus would constitute such a refutation. But this does not seem reasonable since we could just as easily argue that relatively thicker walls also represent an adaptation to larger routine masticatory forces. This dilemma highlights the inherent difficulty in evaluating the functional significance of skull form in the absence of experimental data. This is perhaps best underscored by the lesson of the macaque zygomatic arch. In this primate, the thin cortical bone of the posterior region of the arch experiences very low strains, whereas the thick cortical bone of the anterior portion is a high strain area (Hylander and Johnson, 1997). Such a pattern is opposite to what would be predicted if Bookstein et al.’s (1999) morphological criterion were the sole means of inferring browridge function. Thus, we argue that the data employed by Bookstein et al. can provide neither support for, nor a refutation of, their claim regarding the masticatory determinants of variation in the thickDr. Ravosa is an associate professor in the Dept. of Cell and Molecular Biology at Northwestern University Medical School, and a research associate in the Dept. of Zoology, Mammals Division, at the Field Museum of Natural History. He is interested in the biomechanics, development, and ecomorphology of the mammalian skull, particularly as it relates to major morphological transformations during the origin of higher clades. Dr. Vinyard is a research associate, and Dr. Hylander is a professor, in the Dept. of Biological Anthropology and Anatomy at Duke University Medical Center. Dr. Vinyard is interested in morphological integration in the primate skull, and how covariation in craniofacial structures is related to the functional morphology and evolutionary history of this anatomical complex. Dr. Hylander studies the functional morphology and biomechanics of the vertebrate cranium, especially in primates. Over the past 25 years his laboratory has pioneered the development and application of in vivo bone-strain and jawmuscle EMG investigations of the masticatory apparatus. *Correspondence to: Dr. Matthew J. Ravosa, Department of Cell and Molecular Biology, Northwestern University Medical School, 303 East Chicago Ave, Chicago IL 60611-3008 USA. Fax: 312-503-7912; E-mail: m-ravosa@northwestern.edu THE ANATOMICAL RECORD (NEW ANAT.) 261:173–175, 2000