Geometric Morphometrics of Gorilla and Pan Scapulae

Understanding the morphology of the scapula of Gorilla and Pan is crucial to scientists who find fossil ape scapulae and need to assign them to taxa and interpret the functional significance of their morphology. While differences between the shapes of Gorilla and Pan scapulae are known, a morphological link between locomotor patterns and scapular shape has yet to be determined. Using geometric morphometrics and a large sample of Gorilla and Pan scapulae, the differences between the genera were investigated. Fifteen three-dimensional landmarks were collected from Gorilla (n=150) and Pan (n=210) adult scapulae and analyzed using Procrustes analysis and principle components analysis. An analysis of Pan vs. Gorilla showed that Pan has a narrow scapula with a cranially tilted glenoid fossa and acromion process, which coupled with its smaller size, are features that previous researchers have predicted to be adapted for arboreal behavior. Gorilla has large muscle attachment sites such as a large supraspinous fossa and has an acromion process that extends father laterally over the glenoid fossa, which are more suitable for stabilizing the shoulder joint when this larger animal climbs. Among the subspecies of Gorilla, Gorilla gorilla gorilla has the largest muscle attachment sites, which are consistent with with its more arboreal behavior. Gorilla beringei beringei, on the other hand, does not climb to find food and consequently was found to have smaller muscle attachment sites. Gorilla beringei graueri, being intermediate in habitat and behavior, was found to be intermediate in scapular shape. While previous research has searched for a continuum of scapular shape ranging from Pan paniscus, as the most arboreal African ape, to G. b. beringei, as the least arboreal African ape, this research showed that such a continuum does not exist, probably due to the great size dimophism between the two genera, which results in different biomechanical demands on each ape when climbing. Instead, researchers should focus on the relationship between morphology and locomotor behavior within each genus.