• Supraorbital torus;
  • Biomechanical and spatial models;
  • Allometry;
  • Old World monkeys


Theoretical discussions of primate browridge formation have resulted in several interpretations of its form and function. Lateral radiographs of adult Old World monkeys, representing most cercopithecine and colobine genera, were examined to address whether both biomechanical and spatial factors influence the development of a supraorbital torus.

A linear measurement of browridge size was compared with a series of measures related to each model. Partial correlations were used to ascertain the relative independence of biomechanical (model I or II) and spatial effects upon torus formation. Allometric (size-related) shape changes were evaluated with log-linear bivariate regression analysis; subfamily differences in scaling patterns, with an analysis of covariance. When spatial and biomechanical (I or II) factors were both significantly related to brow size, additive and interactive multiple regression models were used to further assess the manner by which each set mutually affects variation in browridge dimensions. Correlation analyses were repeated with size-corrected antilogged residuals to eliminate a potentially spurious effect of skull size.

Old World monkeys provide support of the spatial model. Also of interest is that skull size emerges as a primary influence on torus formation. Several alternative explanations are also put forward to account for browridge development in each subfamily.