The primary objective of this research is to clarify the role of an anatomical structure known as the postorbital septum. This is a thin sheet of bone which forms the lateral aspect of the rear of the orbit. It is apparently a recently evolved structure which is only found in the cranium of the anthropoids (the higher primates) and the tarsier. While the postorbital septum has been used as a diagnostic criterion of the higher primates for many years, there have been virtually no tests of the various hypotheses which have been put forward to account for its existence. The PIs will undertake a variety of experiments in an attempt to account for the trait's evolution. One explanation which has been advanced is that the sheet of bone serves to isolate muscles which are involved in mastication (those in back of the septum) from the anterior muscles which are involved in vision. This would presumably be of significance to highly visually-oriented animals such as the anthropoids. The PIs will undertake a number of dissections of animals, including anthropoids, to determine the placement of various masticatory muscles relative to the orbit. Results will be plotted and examined as a predictive tool. Another explanation which has been advanced relates the septum to the reduction of strain in the face during mastication. Measurements of bone stress can be carried out using a series of animals already at the Yerkes Primate Center. This research will help to test and clarify the forces which brought the bony septum into existence millions of years ago and understand its functioning in animals today.