A Comparative and Phylogenetic Analysis of the Relationship Between Facial Orientation and Circumorbital Breadths in the Hominoids
Authors: Leslie Erin , Mehta Yash 
Published: November 2023

This is the first study to comprehensively test various suggestions that the dorsally oriented face observed in the large‐bodied southeast Asian orangutans (Pongo) is structurally correlated with relatively narrower circumorbital breadth dimensions documented in this taxon. The orangutan craniofacial pattern is generally contrasted with that of the large‐bodied African apes (Gorilla and Pan) which display more ventrally oriented faces with broader upper faces. However, suggestions that the orangutan facial orientation is structurally correlated with its circumorbital morphology are based only on comparisons with Gorilla and Pan and overlook the craniofacial morphology of the small‐bodied hylobatid apes (Hylobatidae), the sister taxon to the large apes (Hominidae). The current study considers the variation in several circumorbital breadths across and within the extant hominoids, including both the large‐ and small‐bodied apes, and statistically analyses the relationship between these measurements and facial orientation in these taxa. We test the hypothesis that relative circumorbital breadth dimensions will be negatively correlated with increasing degrees of dorsal facial orientation across and within the hominoids.

Circumorbital and anterior skull base breadth measurements were collected on basal and anterior view radiographs of crania or on dry skulls of mixed sex, adult representatives of the extant hominoid taxa, including Hylobates, Nomascus, Pongo, Gorilla, and Pan. Body size effects were controlled for by scaling the linear measurements relative to a size surrogate. Facial orientation was quantified on lateral view radiographs of the same specimens by measuring the dihedral angle formed between the midline hard palate plane and a perpendicular plane to the Frankfort horizontal. The strength and direction of the relationships between the craniofacial breadth measurements and facial orientation angles were assessed by calculating the value and significance level for Pearson's product‐moment correlation coefficient between these variables for each taxon.

Significant correlations were noted across the hominoid taxa between decreasing circumorbital breadth measurements and increasing degrees of dorsal facial orientation across the hominoid taxa. However, there were no significant correlations between the variables within each of the included hominoid taxa with limited exceptions. Relative cribriform plate breath was found to moderately increase with increasing dorsal facial orientation within Pongo, which is opposite the predication. Also, relative postorbital breadth was moderately positively correlated with increasing dorsal facial orientation in Gorilla. Pongo also displays a significant degree of sexual dimorphism in some of the linear measurements.

The association between dorsal facial orientation and narrower circumorbital breadths in Pongo versus the condition observed in Gorilla and Pan which drives any interspecific hominoid contrasts. However, the relationship between these variables does not appear to be causal as this pattern does not map intraspecifically within any of the hominoid taxa, nor do hylobatids of comparable degrees of dorsal facial orientation exhibit relatively narrower circumorbital breadths. These results have implications for evaluating scenarios of hominoid skull form evolution as well as for broad comparative primate studies.

Support or Funding Information

Yash Mehta was supported by the Midwestern University Federal Work‐Study Program.